Necessity of the study
Soil and groundwater resource pollution are serious concerns in our nation One of the unavoidable effects of uncoordinated economic zone growth is the contamination of water sources with heavy metals and harmful, persistent organic compounds such as phenol and its derivatives The primary sources of phenol and phenol polluting compounds are the manufacture of synthetic plastics, insecticides, paints, and petroleum [1] Additionally, the textile sector emits a significant number of harmful chemical compounds into the atmosphere, including azo-based dyes, one of which is methyl orange As a result, the remediation of contaminated environments with two chemical compounds as phenol and methyl orange, is a hot topic not only in the nation, but also globally.
Historically, remediation of polluted water has been mostly dependent on physicochemical and biological treatment approaches Among these, adsorption is one of the most frequently used strategies for treating chemical contaminants in water due to its ease of use and the broad application of a variety of adsorbents Another workable solution is biological treatment, which may eliminate around 90% of organic debris entirely However, this procedure is less efficient for compounds that are difficult to decompose, such as phenol and methyl orange Numerous extensive research studies have been undertaken to process the aforementioned chemicals, which include electrochemical methods, ion exchange, ozone, and adsorption on activated carbon [2, 3] In the other hand, these approaches are rarely used in reality due to their inherent constraints, which include heavy equipment systems, complex operation techniques, high initial and ongoing expenditures, and birth abnormalities It must include a sludge post-treatment step, otherwise the efficiency will remain poor results. Using photocatalysts to treat polluted water is one of the most environmentally friendly green treatment methods available, since it employs natural solar energy and is capable of degrading organic contaminants that are difficult to decompose Without the addition of extra chemicals or sludge buildup in the treatment system [4] TiO2, ZnO, and Fe2O3 are among the semiconductor materials that researchers are interested in as potential photocatalysts Due of titanium dioxide's (TiO2) outstanding characteristics, it is the most investigated material It is ecologically safe, chemically and physiologically inert, self-cleaning, and produces minimal byproducts during production [5].
TiO2 nanoparticles have played a key role in the photodegradation of organic pollutants; it seems to be the most investigated photocatalyst due to its cheap cost, photostability, abundance, and high oxidizing power against a wide range of organic pollutants [6,7] Despite these positive qualities, its use is limited due to its large band gap (3.2 eV), the difficulty in separating the catalyst TiO2 from the solution, and the recombination of the photogenerated electron-hole pairs, which results in low photocatalytic reaction efficiency [8] TiO2 is effective in decomposing a vast array of organic, inorganic, and toxic compounds in liquid and gas phase environments. However, the 3,2 eV energy band gap of pure TiO2 limits its use to UV light (387 nm, or about 4% of solar radiation) Numerous approaches have been used to improve the photocatalytic activity of TiO2, including two major classes of chemical treatments, including doping with non-metals, transition metals, dye sensitization, spatial structuring, and doping with rare earth metals [9] Alternative methods include infusing microwave or ultrasonic radiation into TiO2 photoreaction systems [10]. Activated carbon may be an ideal substrate for evaluating the drawbacks of TiO2 when supported by activated carbon.
This new discovery has a great deal of potential as a result of the synergy between the photocatalytic activity of the catalyst and the adsorptivity of the activated carbon The use of commercial activated carbon as an effective adsorbent for the removal of organic pollutants from liquid phase [11] is well established However, due to the prohibitive cost involved, its use is severely limited Activated carbon may also be produced from waste products derived from agricultural by-products [12] and the wood industry, as well as non-conventional waste items from municipal and industrial operations The use of waste materials in the manufacturing of activated carbon may be of significant benefit in minimizing waste disposal in the environment, which may have further impacts Activated carbon may be produced by using waste materials.When using TiO2, one of the most important obstacles that must be overcome is separating the powder catalyst from the effluent at high concentrations, which might result in the coagulation of the catalyst as well as the creation of aggregates [13].Activated carbon's high porosity, high surface area, high photostability, and appropriateness for use at room temperature are some of the benefits that accrue from using TiO2 in conjunction with activated carbon Other advantages include the ease with which the catalyst can be extracted from the bulk solution Other materials than activated carbon, such as clays [14], zeolite [15], silica [18], alumina [16], and glass, were used in an attempt to boost the photocatalytic efficacy of the catalyst; however, these other materials did not make a significant contribution It's possible that the synergistic effect of activated carbon and TiO2 is what's responsible for the promising nature of the combo Sometimes the reaction between TiO2 and a specific pollutant can result in coagulation, which will prevent a significant amount of UV or solar radiation from reaching the catalyst's active core This can happen in a number of different ways This resulted in the reduction in the surface area of the catalyst, which in turn led in a decrease in its photocatalytic activity [17] Because the activated carbon at the surface of the TiO2 functions as an efficient adsorption trap for the organic pollutant, this results in the mass transfer of the pollutant to the surface of the catalyst, which is where the photoreaction takes place It has been shown that the higher adsorption of the substrate onto the surface of the carbon in activated carbon contributes to the enhanced photocatalytic elimination of pollutants [19,20] This effect is attributed to activated carbon.
In order to broaden the absorption spectrum of the TiO2 catalyst to include the visible light area, which accounts for about 45 percent of solar energy, it is necessary to incorporate metal or nonmetallic alteration methods into the structure of the TiO2 material Since solar energy is a renewable and endless source of energy, this step is necessary because it is required to broaden the absorption spectrum of the TiO2 catalyst.
Recently, a number of researchers have begun using graphene oxide in an effort to enhance the performance of TiO2 photocatalysts This is owing to the multiple advantages that graphene oxide offers in terms of enhancing catalytic performance under circumstances of visible light [21-25].
Objectives of the study
The general objective of this study is to produce catalysts TiO2 modified with activated carbon and graphene oxide, coated on various materials to degrade organic pollutants in wastewater, which is represented by methyl orange and phenol as two and in the world.
The other aims are to investigate the process parameter in catalyst synthesis, to find out the optimum catalysts of each synthesis methods, to make the thin films of catalyst on various substrates to degrade methyl orange, to modify catalyst to have positive results in full range light condition.
Content of the thesis
Firstly, literature review on previous studies will be investigated to select the preparation methods of the catalysts, materials to modify catalyst, coating techniques and model pollutants to conduct research
The photocatalyst TiO2 was synthesized by sol-gel, co-precipitation and hydrothermal methods After that, catalysts were modified with activated carbon and graphene oxide, silica gel then the catalysts were characterized by physical adsorption, SEM, XRD, UV-Vis.
The catalytic activities of these catalysts were conducted for methyl orange and phenol, one stable organic compound, in UV-C and full range light condition.
The main process parameters in phenol photodegradation of the optimum catalysts were evaluated and do kinetics study this process.
Methodologies of the study
Literature review: it is a general section to collect related data from previous researches such as the catalyst composites with activated carbon and graphene oxide, the preparation methods, coating methods, methyl orange and phenol photodegradation.
Experiments: the catalysts were prepared by sol-gel, co-precipitation, and hydrothermal methods, then characterized by various techniques such as BET physical adsorption, SEM, XRD Finally, the photodegradation performances of these catalysts were evaluated using specific reactor systems combined with UV-Vis and HPLC methods.
Data analysis and processing: the method is used to gather and determine the concentration of methyl orange and phenol based on the calibration curve of these substances.
Scope of the study
Organic pollutants: Methyl orange and phenol were chosen to evaluate the catalyst performance since they are popular pollutants in wastewater.
Catalyst thin films: Catalyst thin films made by dip coating and CVD methods on various substrates as cordierite, glass and aluminum are studied.
The thesis can provide a scientific background to synthesize the photocatalyst TiO2 in methyl orange and phenol in laboratory condition Since methyl orange and phenol are popular and difficult compounds to be degraded with aromatic compound, a catalyst with positive efficiency to degrade them will be certainly possible to photodegrade other pollutants.
The catalysts were synthesized in thin films by dip coating and CVD methods. The parameters and method in making thin films were investigated which can further apply to treat industrial wastewater.
The main innovations of this research include:
1 Process parameter optimization for catalysts synthesized via co-precipitation, hydrothermal, and sol-gel methods.
2 Catalytic film formation optimization on various substrates using CVD (chemical vapor deposition) and dip coating.
3 Creating a catalytic thin film and successfully testing the activity with methyl orange on the surface of an environmentally friendly carrier as synthesized cordierite ceramic from natural source.
4 Research on the modification of catalysts synthesized by sol-gel and hydrothermal methods on activated carbon and graphene oxide carriers appliedd in the treatment of methyl orange and phenol.
The thesis consists of four main chapters The first chapter summarizes the literature on methyl orange (MO) and phenol contamination, and the methods for preparing titanium dioxide (TiO2) to improve its performance in the photocatalytic degradation of MO and phenol The second chapter describes the synthesis method to prepare the various catalysts, introduce basic principles of the physico-chemical methods used as well as the experimental set-up utilized in the thesis The third chapter focuses on evaluating the properties of the prepared catalysts, and the influence of different synthesis methods on the catalytic performance of the catalysts in the photodegradation of methyl orange and methyl orange phenol.
Finally, the fourth chapter summarizes the main points of the thesis and gives some recommendations for future works.
This chapter presents the previous research that are related to this study as TiO2 catalyst, preparation method of photocatalyst, photodegradation for MO and phenol, TiO2/AC and TiO2/GO materials, coating techniques et al.
1.1 Textile industry and Methyl Orange dye
The textiles and garment industry of Vietnam has been a critical area for the Vietnamese economy for a long time The industry employs over 3 million employees and has over 7,000 factories throughout the country As a sector relies heavily on water supply for its development and produces wastewater as a result, it is crucial for stakeholders in the sector to better understand the water threats they pose, their impacts and the possible approaches they provide to these challenges [26].
In the textile and dye business, wastewater is produced throughout the steps of sizing, cooking, bleaching, dying, and finishing These processes may be broken down into their individual stages here The quantity of wastewater produced is mostly attributable to the washing procedure that occurs after each cycle The amount of water that is required in a textile industry is quite high, although the amount varies greatly from item to item The examination of specialists indicates that the quantity of water used in the manufacturing stages amounts for 72.3% of the total, with the majority of this water coming from the dyeing and finishing stages of the goods One may do a rough calculation that places the water need for one meter of fabric anywhere in the range of 12–65 liters and the amount of water discharged somewhere between 10–40 liters Water contamination is the most significant environmental issue that the textile sector faces The textile dyeing business is regarded to be the most polluting of all industries when two parameters, namely the volume of wastewater and the types of pollutants that are included within the wastewater, are taken into consideration [27,28].
The primary contaminants in textile dyeing wastewater include persistent organic chemicals, dyes, surfactants, organic halogen compounds, neutral salts that enhance the total solids content, and temperature Due to the high alkalinity, the effluent pH is also high Among these, dyes are the most complex to process, particularly azo dyes,which account for 60-70 percent of the dye industry [29-32] During the dyeing process, the pigments in the dyes do not normally attach themselves to the fibers of the cloth; nonetheless, a certain quantity of the pigments still stays in the wastewater.There may be as much as fifty percent of the original quantity of color left in the has been dyed [29-30] Because of this, the wastewater that is produced from the textile dyeing process has a strong color and a significant concentration of contaminants.
Methyl orange, often known as MO, is an anionic azo dye that has found widespread use in a variety of different sectors, including those dealing with textiles, printing, paper, pharmaceuticals, food, photography, and leather Methyl orange and the various compounds that come from it are responsible for significant amounts of pollution that are released into the environment It has been shown that this coloring agent may cause cancer as well as genetic mutations [33] In addition to being a dye that is soluble in water, methyl orange is characterized by a high degree of stability as well as unique color qualities This compound has an orange appearance when it is in a basic medium, but it has a red appearance when it is in an acidic media It was discovered that the reductive breakage of the azo bond (–N=N–) by the azo reductase enzyme that is present in liver creates aromatic amines, and that these aromatic amines may potentially contribute to intestinal cancer if they are taken by human humans [34].
Fig 1.1: Chemical structure of MO molecule [33,34]
Methyl orange, also known as (C14H14N3SO3Na), served as the model pollutant for the purpose of this investigation Methyl Orange is a typical kind of azo-dye that is used in the industrial sector It is prized for the stability that it has Up to 70% of today's dyes are made up of azo-compounds, which are synthetic inorganic chemical chemicals These compounds are used to make colors It is believed that somewhere between 10 and 15 percent of the dye that is used in the production of textiles is wasted and emitted as effluent The discharge of this effluent is referred to as "non- aesthetic pollution" since the concentrations that are visible in water sources are lower than 1 parts per million Although this is the major reason for degrading methyl orange, the dye waste water may also create harmful byproducts through other chemical processes such as oxidation and hydrolysis [35-37] Although this is the primary motive for degrading methyl orange, it is not the only motivation These azo- compounds are quite stable, as was previously said, and this is because the dye contains a significant amount of aromatics Biological treatments may not be able to degrade the dye effluents; instead, they could only change the color of the effluents.
1.2 Phenol in industry and its impact to the health
The chemical compound known as phenol (C6H6OH) was found for the first time in 1834 during the distillation of coal It was first referred to as a carbolic acid since coal distillation was the primary source of phenol synthesis up to the advent of the petrochemical industry At the moment, quite a few different chemical processes have been discovered that may be used to generate phenol In particular, a large number of steel plants discharge wastewater that contains phenol chemicals Pure phenol is either colorless or white in appearance In this state, phenols are solid crystals that may persist in air for an extended period of time Partial oxidation of phenol causes the material to take on a pink hue and causes it to break down when it comes into contact with water vapor The concentration of phenol at which an odor can be detected begins at 0.04 ppm; at this level, the phenol has an odor that may be described as mildly pungent and pungent Phenol plays a very important part in industry; it is the raw material that many factories use to produce plastics, chemical silk, agricultural pharmaceuticals, antiseptics, fungicides, pharmaceuticals, dyes, and explosives [38,39].
In addition, phenol is the source material for many other industries that produce plastics.
Phenol may enter the human body by inhalation as well as through contact with the skin, eyes, and mucous membranes When ingested, substances with a high phenol content will lead to a fatal phenomenon with symptoms such as convulsions, inability to control, coma leads to respiratory disorders, blood changes in the body leading to a drop in blood pressure Phenol is considered to be extremely toxic to humans when it enters the body of a human through the mouth When a person is poisoned by phenol, it first affects their liver, and then it goes on to attack their heart When individuals were subjected to phenol for extended periods of time in a number of different trials, it was found that they experienced pain in their muscles and an enlargement of their livers Burns to the skin and irregular heartbeats are also side effects of phenol's contact with the skin The amount of phenol that may legally be present in a human body is capped at 0.6 milligrams per kilogram of total body weight There are no studies on the effect of phenol at low concentrations on the development of the body at this time; however, many scientists believe that chronic exposure to phenol can lead to growth retardation, cause abnormal changes in the next generation, and increase the rate of premature birth in a pregnant woman [40-42].
In a nutshell, phenolic compounds are one of the most commonly used chemical compounds in the manufacturing industry On the other hand, they are also toxic compounds that can be extremely hazardous to both organisms and humans if they are not treated properly before being released into the environment Because the wastewater from the Formosa - Ha Tinh factory in Vietnam, which has been discharged into the marine environment, contains high levels of phenol, which has caused the death of a large number of fish in the coastal provinces in the central part of our country, it is necessary to take measures to thoroughly treat this phenomenon The level of chemical contaminants in waste water is rather high, particularly those that are long-lasting organic chemicals like phenol and its derivatives Because of this, phenol degradation is considered to be of critical significance not just in Vietnam but also across the whole globe [43].
Titanium Dioxide, often known as TiO2 or titania, is a substance that has received a significant amount of attention and study owing to the stability of its chemical structure, biocompatibility, as well as its physical, optical, and electrical characteristics TiO2 is a multipurpose substance that may be used in a wide variety of goods, including pigments for paint, sunscreen lotions, electrochemical electrodes, capacitors, solar cells, and even as a food coloring additive in toothpaste [45] TiO 2 was created and put to use in the previous two decades with the primary goal of ridding the environment of harmful chemical compounds, particularly those found in air and water TiO2 may be used to lessen the amount of pollutant substances in the air, such as volatile organic compounds, or perhaps get rid of them entirely In the presence of sunshine, the photocatalysis of titanium dioxide (TiO 2 ) has the potential to break down and eliminate hazardous chemical molecules [46].
Figure 1.2 illustrates the arrangement of the various crystal structures of titanium dioxide There is one stable phase of TiO2 known as Rutile (tetragonal), as well as two meta-stable phases known as Anatase (tetragonal) and Brookite (orthorhombic), both of which have the potential to transform into Rutile when exposed to temperatures outside of their normal range When compared to the Anatase form, the recombination rate of the surplus charged carriers in the Rutile structure is often seen to be greater In addition to this, it is known to occupy the charged transfer that occurs between the catalyst and the potential reactants This provides a fundamental explanation for why rutile form of titanium dioxide (TiO2) is used in paint formulation rather than the anatase form, which has a much slower recombining rate but is far more efficient in charged transfer TiO2's anatase phase, which has a band gap of 3.2eV, has been shown to be TiO2's most active crystal structure This is largely due to the fact that it has favorable energy band positions and a high surface area [47, 48] Rutile, which has a band gap of 3.0, is used in a wide variety of applications, the majority of which are in the pigment industry Anatase has a band gap of 3.2eV and another thing to take into account is the wavelengths that correspond to the anatase and rutile phases of titanium dioxide, which are 388 for anatase and 410 for rutile, respectively.
As a semiconductor, TiO2 may be photo-activated to create a redox environment that can destroy organic and inorganic contaminants In Table 1.1, we can see the overarching steps that occur throughout the photocatalytic reaction process on irradiated TiO 2
Novelty of the study
The main innovations of this research include:
1 Process parameter optimization for catalysts synthesized via co-precipitation, hydrothermal, and sol-gel methods.
2 Catalytic film formation optimization on various substrates using CVD (chemical vapor deposition) and dip coating.
3 Creating a catalytic thin film and successfully testing the activity with methyl orange on the surface of an environmentally friendly carrier as synthesized cordierite ceramic from natural source.
4 Research on the modification of catalysts synthesized by sol-gel and hydrothermal methods on activated carbon and graphene oxide carriers appliedd in the treatment of methyl orange and phenol.
The thesis consists of four main chapters The first chapter summarizes the literature on methyl orange (MO) and phenol contamination, and the methods for preparing titanium dioxide (TiO2) to improve its performance in the photocatalytic degradation of MO and phenol The second chapter describes the synthesis method to prepare the various catalysts, introduce basic principles of the physico-chemical methods used as well as the experimental set-up utilized in the thesis The third chapter focuses on evaluating the properties of the prepared catalysts, and the influence of different synthesis methods on the catalytic performance of the catalysts in the photodegradation of methyl orange and methyl orange phenol.
Finally, the fourth chapter summarizes the main points of the thesis and gives some recommendations for future works.
LITERATURE REVIEW
Textile industry and Methyl Orange dye
The textiles and garment industry of Vietnam has been a critical area for the Vietnamese economy for a long time The industry employs over 3 million employees and has over 7,000 factories throughout the country As a sector relies heavily on water supply for its development and produces wastewater as a result, it is crucial for stakeholders in the sector to better understand the water threats they pose, their impacts and the possible approaches they provide to these challenges [26].
In the textile and dye business, wastewater is produced throughout the steps of sizing, cooking, bleaching, dying, and finishing These processes may be broken down into their individual stages here The quantity of wastewater produced is mostly attributable to the washing procedure that occurs after each cycle The amount of water that is required in a textile industry is quite high, although the amount varies greatly from item to item The examination of specialists indicates that the quantity of water used in the manufacturing stages amounts for 72.3% of the total, with the majority of this water coming from the dyeing and finishing stages of the goods One may do a rough calculation that places the water need for one meter of fabric anywhere in the range of 12–65 liters and the amount of water discharged somewhere between 10–40 liters Water contamination is the most significant environmental issue that the textile sector faces The textile dyeing business is regarded to be the most polluting of all industries when two parameters, namely the volume of wastewater and the types of pollutants that are included within the wastewater, are taken into consideration [27,28].
The primary contaminants in textile dyeing wastewater include persistent organic chemicals, dyes, surfactants, organic halogen compounds, neutral salts that enhance the total solids content, and temperature Due to the high alkalinity, the effluent pH is also high Among these, dyes are the most complex to process, particularly azo dyes,which account for 60-70 percent of the dye industry [29-32] During the dyeing process, the pigments in the dyes do not normally attach themselves to the fibers of the cloth; nonetheless, a certain quantity of the pigments still stays in the wastewater.There may be as much as fifty percent of the original quantity of color left in the has been dyed [29-30] Because of this, the wastewater that is produced from the textile dyeing process has a strong color and a significant concentration of contaminants.
Methyl orange, often known as MO, is an anionic azo dye that has found widespread use in a variety of different sectors, including those dealing with textiles, printing, paper, pharmaceuticals, food, photography, and leather Methyl orange and the various compounds that come from it are responsible for significant amounts of pollution that are released into the environment It has been shown that this coloring agent may cause cancer as well as genetic mutations [33] In addition to being a dye that is soluble in water, methyl orange is characterized by a high degree of stability as well as unique color qualities This compound has an orange appearance when it is in a basic medium, but it has a red appearance when it is in an acidic media It was discovered that the reductive breakage of the azo bond (–N=N–) by the azo reductase enzyme that is present in liver creates aromatic amines, and that these aromatic amines may potentially contribute to intestinal cancer if they are taken by human humans [34].
Fig 1.1: Chemical structure of MO molecule [33,34]
Methyl orange, also known as (C14H14N3SO3Na), served as the model pollutant for the purpose of this investigation Methyl Orange is a typical kind of azo-dye that is used in the industrial sector It is prized for the stability that it has Up to 70% of today's dyes are made up of azo-compounds, which are synthetic inorganic chemical chemicals These compounds are used to make colors It is believed that somewhere between 10 and 15 percent of the dye that is used in the production of textiles is wasted and emitted as effluent The discharge of this effluent is referred to as "non- aesthetic pollution" since the concentrations that are visible in water sources are lower than 1 parts per million Although this is the major reason for degrading methyl orange, the dye waste water may also create harmful byproducts through other chemical processes such as oxidation and hydrolysis [35-37] Although this is the primary motive for degrading methyl orange, it is not the only motivation These azo- compounds are quite stable, as was previously said, and this is because the dye contains a significant amount of aromatics Biological treatments may not be able to degrade the dye effluents; instead, they could only change the color of the effluents.
Phenol in industry and its impact to the health
The chemical compound known as phenol (C6H6OH) was found for the first time in 1834 during the distillation of coal It was first referred to as a carbolic acid since coal distillation was the primary source of phenol synthesis up to the advent of the petrochemical industry At the moment, quite a few different chemical processes have been discovered that may be used to generate phenol In particular, a large number of steel plants discharge wastewater that contains phenol chemicals Pure phenol is either colorless or white in appearance In this state, phenols are solid crystals that may persist in air for an extended period of time Partial oxidation of phenol causes the material to take on a pink hue and causes it to break down when it comes into contact with water vapor The concentration of phenol at which an odor can be detected begins at 0.04 ppm; at this level, the phenol has an odor that may be described as mildly pungent and pungent Phenol plays a very important part in industry; it is the raw material that many factories use to produce plastics, chemical silk, agricultural pharmaceuticals, antiseptics, fungicides, pharmaceuticals, dyes, and explosives [38,39].
In addition, phenol is the source material for many other industries that produce plastics.
Phenol may enter the human body by inhalation as well as through contact with the skin, eyes, and mucous membranes When ingested, substances with a high phenol content will lead to a fatal phenomenon with symptoms such as convulsions, inability to control, coma leads to respiratory disorders, blood changes in the body leading to a drop in blood pressure Phenol is considered to be extremely toxic to humans when it enters the body of a human through the mouth When a person is poisoned by phenol, it first affects their liver, and then it goes on to attack their heart When individuals were subjected to phenol for extended periods of time in a number of different trials, it was found that they experienced pain in their muscles and an enlargement of their livers Burns to the skin and irregular heartbeats are also side effects of phenol's contact with the skin The amount of phenol that may legally be present in a human body is capped at 0.6 milligrams per kilogram of total body weight There are no studies on the effect of phenol at low concentrations on the development of the body at this time; however, many scientists believe that chronic exposure to phenol can lead to growth retardation, cause abnormal changes in the next generation, and increase the rate of premature birth in a pregnant woman [40-42].
In a nutshell, phenolic compounds are one of the most commonly used chemical compounds in the manufacturing industry On the other hand, they are also toxic compounds that can be extremely hazardous to both organisms and humans if they are not treated properly before being released into the environment Because the wastewater from the Formosa - Ha Tinh factory in Vietnam, which has been discharged into the marine environment, contains high levels of phenol, which has caused the death of a large number of fish in the coastal provinces in the central part of our country, it is necessary to take measures to thoroughly treat this phenomenon The level of chemical contaminants in waste water is rather high, particularly those that are long-lasting organic chemicals like phenol and its derivatives Because of this, phenol degradation is considered to be of critical significance not just in Vietnam but also across the whole globe [43].
Titanium dioxide, TiO 2
Titanium Dioxide, often known as TiO2 or titania, is a substance that has received a significant amount of attention and study owing to the stability of its chemical structure, biocompatibility, as well as its physical, optical, and electrical characteristics TiO2 is a multipurpose substance that may be used in a wide variety of goods, including pigments for paint, sunscreen lotions, electrochemical electrodes, capacitors, solar cells, and even as a food coloring additive in toothpaste [45] TiO 2 was created and put to use in the previous two decades with the primary goal of ridding the environment of harmful chemical compounds, particularly those found in air and water TiO2 may be used to lessen the amount of pollutant substances in the air, such as volatile organic compounds, or perhaps get rid of them entirely In the presence of sunshine, the photocatalysis of titanium dioxide (TiO 2 ) has the potential to break down and eliminate hazardous chemical molecules [46].
Figure 1.2 illustrates the arrangement of the various crystal structures of titanium dioxide There is one stable phase of TiO2 known as Rutile (tetragonal), as well as two meta-stable phases known as Anatase (tetragonal) and Brookite (orthorhombic), both of which have the potential to transform into Rutile when exposed to temperatures outside of their normal range When compared to the Anatase form, the recombination rate of the surplus charged carriers in the Rutile structure is often seen to be greater In addition to this, it is known to occupy the charged transfer that occurs between the catalyst and the potential reactants This provides a fundamental explanation for why rutile form of titanium dioxide (TiO2) is used in paint formulation rather than the anatase form, which has a much slower recombining rate but is far more efficient in charged transfer TiO2's anatase phase, which has a band gap of 3.2eV, has been shown to be TiO2's most active crystal structure This is largely due to the fact that it has favorable energy band positions and a high surface area [47, 48] Rutile, which has a band gap of 3.0, is used in a wide variety of applications, the majority of which are in the pigment industry Anatase has a band gap of 3.2eV and another thing to take into account is the wavelengths that correspond to the anatase and rutile phases of titanium dioxide, which are 388 for anatase and 410 for rutile, respectively.
As a semiconductor, TiO2 may be photo-activated to create a redox environment that can destroy organic and inorganic contaminants In Table 1.1, we can see the overarching steps that occur throughout the photocatalytic reaction process on irradiated TiO 2
Photodegradation of pollutants by TiO2 begins with UV radiation absorbed by the TiO2 particles, with a band gap value of 3.2eV for Anatase and 3.0eV for Rutile By doing so, holes and electron pairs are generated in the valence band (hole) and conduction band (electrons) of the semiconductor, respectively To clarify, although both Anatase and Rutile type TiO2 absorb UV radiation, Rutile type TiO2 may also absorb radiation that is closer to visible light In contrast to Rutile-type TiO2, Anatase- type TiO2 has more photocatalytic activity because of its conduction band location, which reveals better reducing power Both the absorbed energy and the kinetic energy of the recombining holes and electrons may be used in the redox processes Electron donors and acceptors adsorbed on the semiconductor surface or even just close the double layer encircling the particle will participate in the redox reactions The mechanism of the relevant reactions in dye photodegradation by TiO2 under UV light is stated by Koustantinou [49].
TiO2 + hv(UV) → TiO2 (eCB− + hVB+) (1) TiO2 (hVB+) + H2O → TiO2 + H+ + OH• (2)
TiO2 (hVB+) + OH− → TiO2 + OH• (3) TiO2(eCB−) + O2 → TiO2 + O2•− ( 4 )
Dye + OH• → degradation products (6) Dye + hVB+ → oxidation products (7) Dye + eCB− → reduction products (8)
Fig 1.3: The mechanism of photocatalytic activity of TiO 2 [50]
The solid side of the junction between the semiconductor and the liquid creates an electrical field that separates the energized hole and electron pairs that are unable to recombine This allows the holes to migrate to the illuminated part of the TiO2 and the electrons to migrate to the unlit part of the TiO2 particle surface The failure of the pairs to recombine results in the separation of the pairs The creation of a very reactive but short-lived hydroxyl radical (OH - ) via hole-trapping is generally considered to be the first step in the process of photocatalytic degradation This theory is generally accepted Either the highly hydroxylated surface of the semiconductor or the direct oxidation of the pollutant molecules under the influence of UV radiation might result in the formation of OH It is also possible that both of these ways of producing OH - occur concurrently in certain situations This is another option This action takes place immediately after the reduction of adsorbed oxygen species, which may be generated either from dissolved oxygen molecules (in the aqueous system) or from other electron acceptors that are accessible in the aqueous system [50].
In the course of this research, the free radicals that are generated as a result of the photocatalytic activity will assault the organic component that is present in the polluted water MO and phenol will be used in the evaluation of the ability of TiO2 to be manufactured by sol-gel and other ways to play the role of a model chemical found in waste water TiO2 may be used in photocatalytic processes in one of two different ways: either it can be suspended in aqueous fluids or it can be immobilized on support materials Quartz sand, glass, activated carbon, zeolites, and noble metals are the materials that are used for the supports The fluidized bed reactor [40] and the fixed bed reactor [40,43] are two examples of various reactor designs that are possible. Matthews and McEvoy discovered in 1992 that photocatalytic reactors using immobilized photocatalysts had a reduced efficiency compared to those that used dispersed titania particles [48].
It has been proposed that the lower efficiencies that can be achieved with immobilized photocatalyst can be attributed to the following: first, the decreased number of activated sites in a given photoactivated volume that are available when the catalyst is immobilized as compared to the same weight of catalyst that is freely suspended; and second, the mass transfer limitation that may become rate controlling at low flow rates This latter issue is especially problematic in the presence of intense illuminations because it is possible that the mass transport will be unable to keep up with the reaction occurring at the photoactivated surface, leading to the possibility that the reaction will become entirely mass-transfer limited When this occurs, the growing photon intensity will not create a discernible change in the pace at which the reaction occurs.
Akpan and Hameed conducted research on the influence of operational settings on the photocatalytic degradation of textile colors using TiO 2 -based photocatalysts[50] In addition to this, it has been shown that there are a variety of processes involved in the manufacturing of photocatalysts based on TiO 2 The Sol–Gel process is quite popular because it makes it possible to produce nanometer-sized crystalline TiO2-based catalysts powder with a high degree of purity at a temperature that is very low.
Principles of Precipitation, sol-gel and hydrothermal synthesis methods 12 1 Preparation of photocatalyst using sol-gel method
After thoroughly dissolving the precursors in water to make the homogeneous solution, the precipitant is added to the solution to precipitate the solid.This completes the process of making the homogenous solution After that, the solids are extracted from the solution, washed to remove any contaminants, dried in an oven, possible to diffuse the reactant on an atomic size, which ultimately leads to an increase in the reactants' capacity to make contact with one another The fact that the desired ratio of elements in the product cannot be guaranteed to be achieved using this method is, however, one of the method's major drawbacks [51].
The sol-gel method has seen a lot of action in recent years when it comes to the production of catalytic supports In preparations that begin with a metal alkoxide, the alkoxide is first hydrolyzed by the addition of water to an organic medium This is followed by polymerization of the hydrolyzed alkoxides by condensation of hydroxyl and/or alkoxy groups in the alkoxides The whole solution will become hard and a solid gel will be created when the level of polymerization and cross-linking of polymeric molecules becomes considerable The porosity of this gel, as well as the surface area, pore volume, pore size distribution, and thermal stability of the final oxide following calcinations, are all strongly impacted by the size and degree of branching of the inorganic polymer, as well as the level of cross-linking If the gel comprises polymeric chains that have a high amount of branching and cross-linking, the gel will, in general, have extensive void areas, will be structurally extremely stiff, and the oxide that will arise from calcinations will mostly have macropores and mesopores If the gel comprises polymeric chains that do not branch out much or have many cross-links, then the gel will have fewer void regions, will be structurally weak, and will thus easily collapse when calcinations are performed The oxide that is produced as a consequence consists mostly of micropores and has a small surface area.
In spite of this, it is feasible to manufacture nano-material by employing the sol-gel process [52], which involves combining the reactants at the atomic range.
The following are some of the many benefits that come with using the sol-gel process [53]:
(1) High-purity materials may be created by using synthetic chemicals rather than minerals, which makes this process possible.
(2) It requires the use of liquid solutions in place of more traditional methods of combining raw components Because the liquids being mixed have a low viscosity, the process of homogenization may be completed in a very short amount of time and at the molecular level.
(3) Because the precursors are well-mixed in the solutions, it is expected that they will be as well-mixed at the molecular level when the gel is created; as a result,chemical reaction will be simple and occur at a low temperature when the gel is heated. size distribution and the pore volume.
(5) It is possible to include many different components into a single process step.
(6) It is possible to produce a variety of various samples in their physical shapes.
The production of nanocrystalline inorganic materials by the use of hydrothermal processing is an unorthodox approach It is possible to tune the synthesis of nearly any material thanks to the existence of a direct precursor-product correlation, which does not need the inclusion of any additional structure guiding agents.
The precursor substance is continually dissolved in the hydrothermal fluid while the hydrothermal environment is maintained at a certain temperature and pressure that is suitable for the synthesis (for example, temperatures of around 300 degrees Celsius and water pressures of one kilobar) Even when alumosilicate materials are used, the production of gels is not noticed at any point throughout the process This is due to the fact that larger molecular units are hydrolyzed when the temperature and pressure are increased.
In an aqueous solution, under autogeneous pressure conditions that are well below the critical point, different states of dissolution may be existent, and most importantly, not only the basic structural building units, but also colloidal states may be present This is because the critical point is the point at which autogeneous pressure becomes critical Because larger units that exceed the size that is present in true solutions are not stable under high pressure hydrothermal conditions, high pressure hydrothermal synthesis implements a first step of crackdown of possibly present
"macromolecular" units by chemical reaction These "macromolecular" units could be present, for example, as a colloidal solution, as a precipitated colloidal solution (crystalline, partially crystalline (e.g gel), glassy and amorphous) or as solid state precursor materials Therefore, it is believed that a real solution will emerge, in which the smallest feasible structural building blocks, in addition to cations with their associated hydration spheres, will be transported [54].
1.4.1 Preparation of photocatalyst using sol-gel method
According to O Carp [55], TiO2 may be manufactured into powder, crystals, or thin films This versatility allows it to be used in a variety of applications Crystallites may range in size from a few nanometers to several micrometers, and they can be used to generate powders, films, or both It is important to highlight that nanosized crystallites have a propensity to aggregate into larger structures A procedure known as
"deagglomeration" is often required in order to achieve the desired result of having nanoparticles that are distinct from one another Nanoparticles may be produced using a variety of cutting-edge techniques that do not need an extra deagglomeration phase. The nanosize titanium dioxide, also known as nano-TiO2, was produced using a procedure that began with the hydrolysis of titanium precursors and ended with annealing, flame synthesis, hydrothermal, and sol-gel processes The sol-gel approach has seen widespread use because it makes the synthesis of nanoparticles in conditions similar to those found in nature—that is, at room temperature and under the pressure of the atmosphere In addition to this, the set-up for this method is not very complex Sol- gel has all the advantages over other preparation techniques in terms of purity, homogeneity, flexibility in introducing dopants in large concentrations for stoichiometry control, and ease of processing and composition control since this method is a solution process Sol-gel also has all the advantages over other preparation techniques in terms of stoichiometry control.
Synthesis of solid materials carried out in a liquid medium and often carried out at low temperatures Particles in a dispersed condition in the solvent independent colloidal suspension (Sol) the colloidal particles are connected together to create a three- dimensional open grid (Gel) The typical molecular precursors are metallo- organic compounds such as alkoxides M(OR)n, where M is a metal such as Si, Ti, or another similar element R represents an alkyl group (R may be CH3, C2H5, or any other combination of these) In a similarl way Ti(iOC3H7)4 is used in the manufacturing process for titanium dioxide the preparation of TiO2.
Ti(OCH(CH3)2)4 + 2 H2O → TiO2 + 4 (CH3)2CHOH The Sol-Gel synthesis of TiO2-based products makes use of this reaction as one of its building blocks In most cases, water is added to a solution containing an alkoxide that has been dissolved in an alcohol The presence of additives (such as acetic acid, for example), the quantity of water, and the mixing speed all have a role in determining the characteristics of the inorganic result Titanium (IV) (IV) The formation of chiral epoxides may be accomplished by a process known as the Sharpless epoxidation, which requires the use of isoproxide as an essential ingredient.
A water-soluble precursor molecule is hydrolyzed during the Sol-Gel synthesis, which results in the formation of a dispersion of colloidal particles (the sol) The continuation of the reaction results in the formation of bonds between the sol particles, producing an endless network of particles (the gel) After that, the gel is heated in order to produce the required substance in most cases This approach to the synthesis of inorganic materials provides a number of benefits that are not shared by other, more traditional methods of synthetic synthesis For instance, very pure materials are able to
In addition to this, homogenous multi-component systems may be generated by combining precursor solutions; this makes it possible for the materials that have been prepared to have simple access to chemical doping In conclusion, the rheological characteristics of the sol and the gel may be employed in the processing of the material in a variety of different ways, including the dip coating of thin films, the spinning of fibers, etc In order to provide a new generic method for the manufacture of nanostructures of semiconductors and other inorganic materials, the principles of Sol- Gel synthesis and the fabrication of templates for nano-materials are merged It is possible to do this by carrying out a Sol-Gel synthesis inside the pores of a variety of microporous and nanoporous membranes in order to generate mono-disperse tubules and fibrils of the material that is needed During the sol–gel synthesis of nano TiO2, a high water ratio was maintained This increased the nucleophilic attack of water on titanium (IV) isopropoxide and prevented the quick condensation of titanium (IV) isopropoxide species, which resulted in the production of TiO2 nanocrystals [56].
Xu presented a novel approach to the synthesis of titanium dioxide in the year
1991 [57] This approach included the use of a sol-gel technique, a cellulose membrane, and heat peptization in order to separate the by-product from the solvent, both of which were alcohol Compared to those that were dialyzed, the aggregation of the sols that underwent thermal peptization was much more dense The slow removal of protons from highly charged particles is accomplished by the process of dialysis, which ultimately results in the aggregation of the particles This provides an explanation for the primary qualitative characteristic of aggregation and deposition, which is that the charges are completely screened by electrolyte ions and those driven by diffusion TiO2 that contains fewer particles reveals a greater surface area than does TiO2 that is densely packed.
Support and thin films
Cordierite is a material made of 3 components MgO–Al2O3–SiO2 The chemical composition of cordierite is 2MgO.2Al2O3.5SiO2 Cordierite contains 13.78% MgO, 34.86% Al2O3 and 51.36% SiO2, with this composition Cordierite belongs to the ceramic group with high mullite (3Al2O3.2SiO2) composition.
Crystals of cordierite offer a number of beneficial features, including a low coefficient of thermal expansion and a low thermal loss Cordierite ceramic is a form of ceramic whose primary crystal is cordierite Because cordierite ceramic has great thermal stability and it is simple to generate porosity, it is frequently employed in sectors that experience fast temperature fluctuations, such as the manufacturing of filters for motors mechanical, as a catalyst carrier, and as a lining material in arc welding (Mig- Mag) technology mechanical, as a catalyst carrier, and as a lining material in arc welding (Mig-Mag) technology Because of its very limited temperature range during the firing process, cordierite ceramic is one of the most challenging types of ceramic to work with The reactions that take place during calcination are dependent on the maximum calcination temperature, the heating rate, the amount of time that is retained at the maximum calcination temperature, as well as the particle size and composition During the calcination process, another factor that plays a significant impact is the number of impurities present The reaction never quite reaches its destination since equilibration is such a difficult process to do In most cases, the only preparation that is necessary before use is heating to the desired temperature These preparations are carried out in accordance with the needs and objectives of the usage.Objects that are clumped together and have a high mechanical strength, porosity, and a low water absorption capacity or bulk mass will be the biggest In general, materials that wish to solidify properly under normal circumstances must be heated to a temperature of not less than 0.8T (where T is the melting temperature) This means that the heating temperature must be over 1200° C for the intended result, which is cordierite [68].
1.5.2 Mesoporous TiO 2 and coating techniques
It is essential for modern civilization to make advantage of porous materials in a variety of applications [69-71], including catalysis, adsorbents, optics, sensors, insulating lacquers, and ultralow-density materials, among others Porous materials are widely used in the field of catalysis and have a direct impact on the economy of the world This is because porous materials make it possible for reactions to take place under conditions that require less energy One example of this is the refining of petroleum, in which diverse microporous zeoliths play a prominent role in catalytic cracking reactions [72] Microporous materials, like zeolites, have restricted apertures, which prevents them from being used in demanding applications like oil refining This is one of the most significant limitations associated with these types of materials.
Documentation has been provided by Stucky and colleagues [73] about the synthesis of large-pored, mesoporous metal oxide powders and films using P123 As a kind of inorganic starting material, metal chloride salts have been used in their studies.
In order to postpone the crystallization of titanium, a non-hydrolytic approach that involves the breakage of carbon-oxygen bonds was developed This seems to be an essential step when creating the mesostructures in a controlled manner.
Using TBT, TET, or TPT as precursors, Sanchez and colleagues conducted an in- depth study on the function that water plays in the process [74] Condensation does not take place before the formation of the mesostructured hybrid stage because the condensation rate is very low when the water content is low On the other hand, condensation reactions, which take place in the presence of considerable amounts of water and contribute to the formation of oxo clusters, come before the formation of hybrid processes In spite of this, the addition of an excessive amount of water resulted in the production of gels that lacked periodicity.
For the synthesis of the mesoporous titanium that was employed in the EISA solution, CTAB and TET were used NBB, which self-assembles all around the micelles, is the component of the hybrid process referred to as "titaniatropic." This assembly might be thought of as having interactions of the type Ti – OH + –X– CTAB + , where X represents the CTAB bromide anion and/or HCl chloride ions that are involved in the synthesis Altering the solvents and co-solvents used allowed Yan and his team of researchers to discover that distinct phases of titanium could be produced The use ofTiCl4, P123, or F127, together with changing the solvent from methanol to ethanol, 1- butanol, and 1-octanol, resulted in the production of a combination of anatase, rulia,and pure rutile anatase Clarification is provided on the variances in the ability to release atoms of chlorine As the length of the carbon chain increases, a greater quantity of chlorine is retained in the moieties of color alcohol [TiCl4x(OR)x], which results in an increase in the level of obstruction In phases, anatase normally forms when the acidity is low, while rutile forms when the acidity is high [75].
A well-ordered TiO2 mesostructure was produced by using TiCl4 and TBT in conjunction with P123, as stated in reference number [70] Intriguingly, our investigation has produced pore walls that include a combination of rutiles and anatases in their phase composition The mesoporous material has a surface area of
244 m 2 per gram and was generated at a ratio of 0.2 to P123 / TBT mole [76] Citric acid was added to the mixture that included TPT and F127 in order to produce mesoporous titania [77].
The hydrophilic titanium surface of the nanoparticles is functionalized by the citric acid, which also increases the binding of the nanoparticles to the F127 ethylene oxide units The production of a titania-P123 mesostructure was followed by the addition of ethylenediamine, which resulted in the development of a mesoporous anatase that is thermally stable Following calcination at temperatures as high as 700 degrees Celsius, the en molecules bond to the surface of titanium nanoparticles This prevents the pores from collapsing and even prevents anatase from converting into rutiles [78].
TiO2 catalyst powder has been suspended in water for use in a wide variety of applications Due to the high cost of retrieving the catalyst, researchers have been looking at several ways that may immobilize catalyst particles in a substrate In spite of the fact that suspended TiO2 powder is effective because of the wide surface area of catalyst that is readily accessible for reaction, the procedure of recovering the catalyst is time consuming in addition to being a costly one In addition to this, suspended catalyst impeded the passage of ultraviolet light, which in turn decreased the effectiveness of the catalyst [79] Immobilizing the catalyst on a substrate, which is becoming more important in photocatalytic treatment of organic pollutants, is one approach to resolving this issue This is only one of several potential solutions.
It is well known that the immobilized form of the TiO2 photocatalyst is inexpensive, has great stability, and does not show any signs of photocorrosion.Additionally, immobilized TiO2 has great surface characteristics, making it an excellent candidate for the treatment of wastewater on a wider scale [80-82].Immobilizing TiO2 in a substrate has been possible via the use of a variety of different methods recently Anodization [83-84], the sol-gel method [85-86], reactive direct current magnitude 27 sputtering [87], chemical vapor deposition [81], electrostatic sol- spray deposition [86], and aerosol pyrolysis [87] are some of the processes that fall under this category When selecting a strategy for catalyst immobilization, several aspects, such as the nature of the catalyst substrate or support, the nature of the pollutant, and the surrounding environment, are taken into account (liquid or gas) When TiO2 is loaded onto a support, the photocatalytic characteristics of the support are altered, with the primary disadvantage being a decrease in the surface area of the support [88] Spraying, electrophoresis, inject printing, dip-coating, and spin coating are some of the techniques that may be used in the sol-gel process for the purpose of sol deposition on the substrate [89] In spin coating, a uniform coating was seen; the deposition of TiO2 particles under vacuum led to a hard coating, and the procedure eliminated residual air completely [90] Spin coating was successful in producing a uniform coating Even if it was discovered that TiO 2 particles can be placed homogeneously on AC by utilizing vacuum and rotating, the dip coating process still provides numerous benefits over alternative deposition procedures owing to the fact that it uses extremely basic equipment [91] Glass, silicon, stainless steel, and titanium are just few of the substrates that have been used in the past When TiO 2 was utilized as the starting material, various binders were added as additions to the suspension, and post- deposition annealing was also performed with the intention of improving the adherence [92] When compared to the binding approach, the direct creation method often produces crystals of worse quality [93-99].
TiO 2 /AC Materials
Fujishima and Honda made the discovery in 1972 that photocatalytic splitting of water may be accomplished on TiO2 electrodes Because of this occurrence, a new stage for heterogeneous photocatalysis was established [100] that involves the exploitation of TiO2 as a semiconductor Recent research has shown that TiO2 may be used as a photocatalyst for the photodegradation of organic pollutants, which are substances that are thought to have a harmful influence on both the environment and human health The transformation of organic contaminants into less hazardous compounds is nonetheless still ongoing and contributes to the ongoing enhancement of its characteristics Controlling key factors, such as calcination temperatures, pH, and aging times, is one way to go in the right direction On the other hand, the inclusion of supporting material that contains titania would be an excellent strategy to improve the photocatalytic effectiveness of it As a result, on the basis of the assessment of the relevant literature, a number of fundamental criteria for choosing an appropriate catalyst load have been defined, including [94, 101]: a The composite material should be transparent, or at the very least, should allow some ultraviolet radiation to flow through it Additionally, the material should be chemically inert or non-reactive to pollutant molecules, their intermediates, and the surrounding aquatic system. b The composite material should be able to attach to the TiO2 in an adequate manner, either physically or chemically, without inhibiting the reactivity of the TiO2. c The composite material should have a large surface area as well as a significant adsorption affinity towards the contaminants (organic or inorganic substances) that need to be degraded This criteria decreases or eliminates the intermediates that are formed during the photocatalytic degradation, while simultaneously improving the mass transfer rates and processes for an effective photo- degradation. d The composite material should make it possible for photocatalysts to be recovered and reused in a quick and simple manner, with or without the need for regeneration.
To make a composite catalyst, activated carbon and TiO2 are often combined. This is done because activated carbon has excellent adsorptive characteristics Because of its high surface area, good microporous structure, increased adsorption capabilities, and the ability to adjust its surface chemistry and porous structure during the preparation or activation steps, activated carbon is a popular adsorbent that is used in the vast majority of industrial applications Because of this, when compared to adsorbents based on other oxides, it has a number of advantages that make it stand out
[101] Activated carbons are extensively employed as an adsorbent in a broad variety of applications, including purifying, decolorizing, deodorizing, dechlorinating, detoxifying, filtering, removing or altering salts, separating, and concentrating for the purpose of recovery.
Titania's effectiveness as an adsorbent might be improved by combining it with activated carbon, which is known for its ability to remove contaminants from water. During the process of photocatalysis, activated carbon allows for the migration of contaminants by diffusion; these pollutants are then further broken down into water and carbon dioxide Activated carbon and titanium dioxide were used as composite materials in a research project that was carried out by Wang et al in 2007 [102] In the course of this research, TiO2 was manufactured using the Sol-gel technique, and composite material was manufactured before being calcined at a variety of temperatures Various amounts of the composites, including 20% by weight, 50% by weight, and 80% by weight (representing the weight proportion of the starting carbon content to the final TiO2 produced), were also subjected to testing When calcined at
450 degrees Celsius, the composite catalyst was able to display superior qualities in comparison to those calcined at temperatures either much lower or significantly higher The TG analysis demonstrated that the carbon content of the composite did not vary over its whole, which resulted in the composite maintaining its identical surface area However, when the composite was heated to higher degrees, a lower amount of carbon was discovered inside it, which led to a reduction in the surface area of the composite This weight reduction may be attributed to the carbon gasification that took place In addition, when the calcination temperature increased, the interphase contact grew more robust owing to the entrance and agglomeration of TiO 2 in the pores of the activated carbon, as shown in the SEM pictures This was the case This provides an explanation for why the surface area of the composite catalysts was reduced In addition, AC was effective as an adsorbent at temperatures below 450°C, especially 300°C However, at 450°C, a substantial interphase reaction occurred in the composite material, which was extremely obvious in the photodegradation of chromotrope 2R water pollutant In addition, the photo-activity of the composite catalyst was much greater than that of TiO2 Catalyst 80-AC-TiO 2 -450, which has the maximum capabilities in all pollutant concentrations, was calcined at a temperature of 450 degrees Celsius and contains 80% of the weight percentage of the original carbon content converted to TiO2.
Together with titanium dioxide (TiO2), activated carbon fibers (ACF) were used in Liu and colleagues' research The TiO2 that was deposited on the outside surface of ACFs seemed to be in the form of a film with close fractures rather than a thin, compact coating The deposition of TiO 2 and the following calcination procedure did not cause any harm to the micropore structure of ACFs, nor did it affect the high specific surface area of these materials The TiO2/ACFs system performed quite well, particularly when it came to the breakdown of organic contaminants in wastewater that had a low molecular weight During the process of photocatalysis, the development of intermediate species was inhibited as a result of the synergistic action of the TiO2 photocatalyst and the ACFs Both photocatalytic activity and the capacity to regenerate are shown at high levels by the TiO 2 /ACFs catalyst Based on the findings of the XRD examination, it was determined that anatase was present, along with a little amount of rutile According to the findings of the BET analysis, the surface area of the ACFs was somewhat decreased as a consequence of the deposition of the catalyst, going from 1065 of ACFs to 845 of TiO 2 /ACFs In spite of this, the top layer of the film may still be defined as having a mesoporous structure The scanning electron micrographs (SEM) clearly demonstrated that the surface morphology of the composite materials were consistent across the board The TiO2/ACFs catalyst demonstrated a rather high capacity for the breakdown of MB The effectiveness of the breakdown reached 94% after just 40 minutes of reaction, and it reached 100% after demonstrated greater breakdown activity than uncovered ACFs and pure TiO2 This is due to the fact that ACFs helped concentrate organic contaminants near TiO2, which is where they were degraded after being concentrated [103].
Temperatures during calcination may have an effect on the structure of the catalyst [104] examined the removal of phenol from water using TiO2-mounted activated carbon The heat treatment ranged from 600 to 900 degrees Celsius The composite material was made by using hydrolytic precipitation as the preparation method The mounting of the TiO2, which blocked the pore entrances on the surface of the activated carbon, resulted in a reduction in the surface area of the activated carbon. However, as the heat treatment progressed, the particle size of the TiO2 grew, which led to a reduction in efficiency as a consequence of the clogging of pore entrances. Because of adsorption, the best phenol elimination occurred at a temperature of 900 degrees Celsius The use of activated carbon by itself served as a comparison for this. Additionally, composite material that was calcined at temperatures higher than 700 degrees Celsius, specifically at 800 degrees Celsius, exhibited lower photocatalytic activity in the degradation of phenol This was because the crystalline structure of the TiO2 changed into a form that was less active (anatase to rutile).
TiO2-coated activated carbon composites and unadulterated TiO2 were both used in a research that was conducted in 2007 by Li et al [105] The Sol-gel approach was used in the production of both the composite catalyst and the pure catalyst The calcination temperatures of the samples ranged from 300 degrees Celsius to 700 degrees Celsius, and a variety of characterization techniques were utilized in order to determine the impact that the AC matrix had on the TiO2 phase transformation, crystalline growth, morphology, and surface area of the composites The results of the XRD examination allowed for the determination of the ratio of anatase to rutile as well as the average crystal size After being calcined at temperatures ranging from 300 to
500 degrees Celsius, it was observed that the composite catalyst crystallites are anatase TiO2, and the same findings were reported for pure TiO2 At temperatures of 600 and
700 degrees Celsius, the anatase phase of the pure and composite catalysts underwent a phase transformation into rutile In addition to this, the development of the crystallites on the composite catalyst occurred at a slower rate in comparison to the growth on the pure TiO2 catalyst This is because the AC has a high surface area of
435 m2/g, which impedes the phase transformation from anatase to rutile This is because the large interfacial energy of the AC creates anti-calcination effects for the
AC matrix, which in turn controls the growth of TiO2 particles and prevents agglomeration According to the findings of the BET study, the surface area of the composite catalyst rose when the calcination temperature was raised, in contrast to the pure TiO2, which had a decreasing surface area as the temperature was raised In addition, methylene blue was used in the testing process to determine the photocatalytic activity of both the composite and the pure catalyst At the end of the 200-minute test, the pure TiO2 had only removed 61% of the substrate, whereas the composite catalyst had nearly completely removed all of the substrate This occurred because the AC in the composite has a large surface area, which concentrates the organic compound near theTiO2.
Graphene oxide (GO)
Graphene, which is a single layer carbon sheet with a perfect sp2-hybridized two- dimensional structure (Fig 1.5), has attracted a tremendous amount of research attention ever since it was first discovered by Novoselov et al [100] This is due to the fact that graphene possesses a number of unique properties, such as a large specific surface area (2630m2.g-1), good optical transparency ( 97.7%), excellent thermal conductivity Graphene is a key building component that is used in the construction of other carbon compounds such as C60, graphite, and carbon nanotubes (CNTs) [106,107] In addition, numerous techniques have been developed in order to synthesize graphene These techniques include chemical vapor deposition (CVD), epitaxial growth of graphene on silicon carbide, the arc discharge method, substrate- free gas-phase synthesis of graphene, chemical reduction of GO, electrochemical synthesis of graphene, unzipping CNTs for graphene nanoribbon, and many others
[108] In addition, graphene-based films with the appropriate thickness and chemical compositions have been utilized in a variety of research areas, such as fuel cells, supercapacitors, hydrogen storage, lithium ion (Li-ion) batteries, solar cells, electrochemical sensors, fluorescent sensors, and many others [109] Due to the hydrophobic nature of pure graphene, its use in the area of water and wastewater treatment is very uncommon.
GO is one of the most significant derivatives of graphene, which is created by the chemical oxidation of natural graphite [110,111] ( Figure 1.5) The Hummers technique, which is a powerful oxidation process that involves combining flake graphite, potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4)[112,113], is the method that has been most commonly used for the synthesis of graphite oxide (GO) The presence of a significant number of oxygen-containing functional groups in the structure of GO, such as hydroxyl and carboxyl groups,confers hydrophilicity on GO and makes it an ideal supporter of inorganic nanoparticles [114] These groups include hydroxyl and carboxyl groups.
Fig 1.5: Structures of graphene, C60, CNT and graphite [109]
Up to this point, a significant quantity of G/GO-based materials have been synthesized These materials include G/GO-metal composites, G/GO-metal oxide composites, G/GO-polymer composites, and so on and so forth [106] Ex-situ hybridization and in-situ crystallization are the two categories that may be used to describe the synthetic processes that are used to produce G/GO-based composites In addition, the chemical reduction, electroless deposition, sol-gel, hydrothermal, electrochemical deposition, thermal evaporation, and other processes are included in the in-situ crystallization technique [112].
In order to further improve the electrochemical and analytical characteristics of pure metals, G/GO sheets have been mixed with a variety of metals, such as silver,gold, platinum, palladium, nickel, copper, and others [108] For instance, in order to construct a graphene-Pt composite, Liu and colleagues [114] mixed graphene sheets with Pt nanoparticles to create the material In comparison to the commercial catalyst, the composites demonstrated increased oxygen reduction activity owing to the greater electrochemical surface area The synthesis of graphene-gold composites was accomplished using the in-situ chemical reduction of chloroauric acid, which resulted in the deposition of gold nanoparticles on RGO sheets [115] In addition, graphene- gold composites demonstrated an excellent photodegradation ability of RhB dye when exposed to visible light This was possible due to the unique properties of the composites, which included a high adsorption capacity for organic dyes, a slow charge recombination rate, and a strong interaction with dye chromophores.
TiO 2 /GO Materials
In the last two decades, advanced oxidation (AOP) techniques, including photocatalysis, have attracted substantial attention In a number of environmental and non-environmental applications, such as energy storage and processing, photocatalytic processes have an environmental system However, the relatively poor effectiveness of photoactivated catalysts limited their uses, resulting in an increase in research [116-
Active sites may be bonded because to the availability of functional epoxide and hydroxyl groups on graphene oxide (GO) surfaces, with carboxylic acid, quinoidal, ketone, and lactone groups decorating the margins and rim sites surrounding vacancies Fortunately, the oxygen-favoring classes will successfully enhance GO's structural / chemical complexity by further chemical modification or functionalization, giving an efficient means to tailor GO's physical and chemical characteristics to the desired degree In addition to possessing exceptional optical and mechanical qualities,
GO is useful for a broad range of applications Additionally, residual faults and holes resulting from GO's reduction process decrease the quality of the electrical components of r-GO Consequently, GO and GO-based composites have great potential in energy storage/conversion and environmental conservation applications
Funded photocatalysts are a prominent way in industrial catalytic technology for providing enhanced catalyst access to reactants In this setup, the essence of photocatalyst-support interactions is crucial However, the impact of the photocatalytic systems' linkages is recognized to be a good chemical connection for long-term overall efficiency Due to the fact that TiO2 powders in suspension have a high particulate- large surface area between 30 and 300m 2 /g, which helps to maintain the limits of mass transmission and produce strong photocatalytic activity, the application is successful at catching sunlight However, owing to the quick recombination of conductive band (CB) electrons and valence band (VB) holes, the photocatalytic activity of TiO 2 on its own was very poor With increasing catalyst loading, however, a minor transport limitation develops In addition, it is difficult to remove the microscopic TiO2 particles from the water following remediation To remedy this problem, catalyst particles may be adhered to a surface Moreover, due to the difficulty of mass transfer and a moderate limit to transport due to I a decreased catalyst surface-to - volume relation, (ii) the occurrence of substrates that absorb light and degrade their distributions, and (iii) the loss of particle movement, this can reduce the ability of oxidation to reflect volume of water compared to suspension of solid components in the system.
Photocatalysts may be repaired using many types of materials Due to a highly specialized surface and remarkable electron mobility, among its many supports, GO is an ideal substratum for a variety of applications[111] Several attempts have been made to immobilize TiO2 Photocatalyst on a variety of support structures with an enhanced surface-to-volume ratio, hence enhancing the efficacy of photocatalytic oxidation However, it will only be successful if the surface area allows for efficient light absorption The preparation techniques of GO-based material nanocomposites are accorded considerable significance GO-based material, nanocomposite compounds, including hydrothermic, electrochemical, in-situation, in-situ polymerization, microwave-assisted, vacuum, vacuum, and sol-gel, are required for synthesis through a variety of techniques and processes GO is either a functional component or a support for the immobilization of other components in GO-based nano-composites
The study series that Fujishima and Honda have been doing are the ones that have started the ball rolling in terms of scientific inquiry into the activation of photocatalysis Their first demonstration was based on the activation of a semiconductor particulate material by the action of radiation with an adequate wave length to catalyze the dissociation of water In the years that have passed since then, a number of other photocatalysts have been the focus of in-depth research In a previous article [123], the author examined the function that graphene-metal oxide composites play in the treatment of water as photocatalysts, adsorbents, and disinfectants In the realm of photocatalysis, titanium dioxide, also known as titanium oxide (TiO2), was the first and most significant binary transition metal oxide to be investigated TiO2 is distinguished by its chemical stability and non-solubility in the aqueous media,qualities that make the process of separation easier to do once the appropriate reactions have been carried out.
Titanium dioxide's lack of toxicity is another reason why it's a good choice for use in photocatalytic processes, which are mainly intended for use in environmental applications.
Table 1.1 Summary of TiO 2 and GO composites used as photocatalyst
Composites TiO 2 particle size GO content Pollutant Ref
Pt-GO-TiO 2 /GR 30 nm 0.5 wt% Dodecylbenzenesulfonate Neppolian et al
5-15 nm 90 wt% Methylene blue Ismail et al 2013
2013 TiO 2 /GO - 10 mg Methylene blue Min et al 2012
TiO 2 /GO 57 nm 3.3 wt% Diphenhydramine methyl Pastrana et al
TiO 2 /GO 50 nm thickness 10 wt% Methylene blue Zhang et al 2011
TiO 2 /GO 20-40 nm 4.6 wt% Methyl orange Jiang et al 2011
TiO 2 /GO 30 nm 10 wt% Methyl blue Nguyen et al 2011
TiO 2 /GO - GO:TiO 2 = 1.5 wt Methyl orange Pu et al 2013
TiO 2 /GO 10 nm 0.03 mg GO Methyl blue Yoo D-H et al
TiO 2 /GO 4-5 nm 3.3 - 4.0 wt% Diphenhydramine and methyl orange Pastrana et al
TiO 2 /GO 15 nm ~10% Rhodamine B Liang et al 2010
TiO 2 /GO 10 nm GO:TiO 2 = 3:2 wt Methyl orange Gao et al 2010
TiO 2 /GO 6-9 nm 1 wt% Methylene blue Kim et al 2014
Research has been done to investigate the potential of GO as an electron acceptor molecule for use in composite production with TiO2 The impacts of particle size, GO content, and targeted pollutants for various TiO2 and GO composites have been illustrated by examples in Table 1.2 It is due to the extremely strong electron transport ability of reduced graphene oxide, which can rapidly conduct photo-generated electrons and inhibit the compounding of photo-generated electrons and holes, thereby increasing the concentration of hydroxyl radicals and superoxide ion radicals, two highly reducing and oxidizing radicals that are capable of degrading dye molecules In addition, reduced graphene oxide can also inhibit the compounding of photo-generated electrons and holes, which on the other hand, reduced graphene oxide has an exceptional capacity for adsorption, which increases the likelihood that contaminants will come into contact with active compounds Because of this, the modification of TiO 2 with GO has been carried out very recently.
MO photocatalytic degradation
A significant amount of study has been done so far on the degradation of the methyl orange dye Decolorization of methyl orange was investigated by Chen and Chou [124] using an Ag+ ion modified TiO2 suspension solution as the catalyst It was shown that the ion with the charge of Ag+ was more effective in degrading methyl orange compared to other ions, such as Cu 2+ , Co 2+ , Fe 3+ , and Ce 4+ In the presence of
Ag + ion, the impact of bubbling oxygen and nitrogen on the decolorization of methyl orange was minimal The rate of decolorization rose with pH and reached its maximum value at a pH of 8.75 After reaching this point, the rate began to decline owing to the precipitation of Ag + cation with OH - anion, which occurred at higher pH values The apparent primary quantum yield was noticeably boosted by increasing the load of Ag + ions Under the influence of sunshine, Al-Qaradawi and Salman [125] found that methyl orange (MO) may be broken down by employing titanium dioxide as a catalyst According to the findings of the research, the pace at which the MO degrades is not directly proportional to the amount of solar radiation that is received,but a high rate of deterioration requires a small number of photons At a pH of 3, the rate of degradation was found to be highest The most effective concentration of MO was found to be 4 x 10 -5 M, and further increases in MO concentration led to a slower rate of deterioration After five hours, the model compound had been degraded to an entirely different state The catalytic breakdown of Methyl Orange (MO) was investigated using a silver-modified titanium dioxide thin film catalyst The changed materials demonstrated improved catalytic effectiveness and were able to break down the organic pollutant three times more quickly than the original films in their undoped form (Degussa P25) Due to the silver layer's shade of the accessible semiconductor surface, the catalytic efficiency decreased when further increases in Ag + concentration were made This was the cause of the drop in efficiency In addition to this, it was shown that the catalytic activity of the silver- modified films does not change after being subjected to six successive trials in which fresh amounts of pollutant were introduced The catalytic oxidation of a monoazo dye called methyl orange was studied by Guettai and Amar [127] in TiO2/UV aqueous solutions that were exposed to artificial UV-light It was discovered that there was no deterioration of MO when it was exposed to TiO2 in the dark The effects of a variety of factors, including pH,substrate concentration, and catalyst quantity, on the rate of degradation were
3, and an initial concentration of the dye that was 50 mg/L, the MO degradation rate that was attained was the maximum that it could be It was reported by Chen et al.
[128] that pelagite from the East Pacific Ocean was successfully used as a low-cost, highly high reserve catalyst for the entire breakdown and decolorization of methyl orange within 120 minutes when exposed to UV light Under the influence of UV light, several aspects of the catalytic breakdown of methyl orange have been investigated According to the findings, pelagite has an adequate level of catalytic effectiveness in the process of the degradation of organic molecules The decomposition of methyl orange was investigated by Li et al [129] by the use of TiO 2 coated activated carbon (AC) grain (TiO2/AC) as a catalyst in an aqueous solution that was subjected to
UV irradiation It was found that the breakdown of methyl orange followed a kinetic pattern known as pseudo first order It was discovered that the presence of the AC improved the photoefficiency of the titanium dioxide catalyst [Citation needed] A modified Langmuir-Hinshelwood model may be used to characterize the kinetic behavior, if one were to want to do so The production of pure and nickel-doped TiO 2 thin films on soda glass substrates was reported by Sharma et al [130] The thin films were created using a sol–gel dip coating technique Under the influence of ultraviolet light, the catalytic activity was evaluated by observing the breakdown of methyl orange in aqueous solution By performing several impregnations, Liu and Sun [132] were able to successfully synthesis Fe2O3- CeO2-TiO2/-Al2O3 Techniques such as BET, SEM, XRF, XPS, and chemical analysis were used in order to characterize the prepared catalyst At ambient temperature and at atmospheric pressure, a CWAO process using Fe2O3-CeO2-TiO2/-Al2O3 as a catalyst was reported to degrade an azo dye called methyl orange It was also discovered via research that the catalyst had remarkable catalytic activity when it comes to treating synthetic wastewater containing
500 mg/L of methyl orange In only 2.5 hours, it is possible to remove 98.09% of the color and 96.08% of the total organic carbon (TOC) The UV-Vis and FT-IR spectra were used to conduct an investigation of the methyl orange breakdown process Using a CuO-MoO3-P2O5 catalyst with high activity for catalytic wet oxidation (CWO) processing at lower temperatures (35°C) and atmospheric pressure, Ma et al [133] have documented the degradation of Methyl Orange (MO) and Methylene Blue (MB).
A solid-state reaction was used as the synthesis technique for the catalyst It has also been observed that the catalyst has a high catalytic activity for the degradation of methylene blue (MB), however this catalyst has less of an influence on methyl orange(the color removal was 99.65% for MB and 55% for methyl orange under the same circumstances) Rashed and Al-Amin [134] investigated the catalytic oxidation of methyl orange dye using a TiO2 catalyst while exposing it to a variety of irradiation sources (including a 1000-watt halogen lamp, a fluorescent lamp, and the light from the sun) Researchers looked at how starting dye concentrations, the length of irradiation time, and the intensity of the light affected degradation It has been shown that dyes degrade more rapidly when exposed to the light of the sun than when exposed to the light of halogen or fluorescent bulb sources.
As the amount of time spent using halogen and fluorescent lights as a light source increased, a rapid decline was noticed It was hypothesized that the breakdown of methyl orange followed pseudo-first order kinetics Under the irradiation of visible light, Zhang et al.
[135] have observed the degradation of methyl orange that was catalyzed by Pt-TiO2– SiO2 and TiO2 by itself It was discovered that the rate of deterioration was much higher for Pt-TiO2–SiO2 than it was for TiO2 The increased activity was determined to be the result of a 16 charge-transfer occurring on the TiO2–SiO2 composites as well as an improved capacity to trap photo-generated electrons on the Pt-derived states that were adsorbed on the TiO2 surface Huang et al [136] described the synthesis of Pt- TiO2/zeolites by loading it with Pt-modified TiO2 using the sol-gel process and the photo reductive deposition method Under the influence of ultraviolet light, the prepared catalyst was put to use in the process of decolorizing methyl orange solution It was discovered that Pt doping generated an increase in catalytic decolorization, and the best amount of Pt doping is around 1.5 weight percent, with 86.2% of the decolorization rate under 30 minutes of irradiation time Studies were done to determine how temperature of calcination, concentration of catalyst, amount of oxidant
H2O2, and pH affected the catalytic activity After five rounds, a satisfactory repeatability was shown in the data Researchers Sohn et al [137] investigated how methyl orange dye (MO) broke down on TiO 2 nanotubes in their study In comparison to the stirring approach, the TiO2 nanotubes that were produced in EG by using ultrasonic, and then annealed in an environment of nitrogen exhibited a much greater activity level toward the destruction of dye When carried out in the presence of an external bias, dye degradation demonstrated much higher levels of activity than when carried out in the absence of such a bias The improvement was shown to have occurred as a result of the addition of oxidants such as oxygen and hydrogen peroxide.
Fig 1.7: Possible mechanism of MO with TiO 2 [138]
Galindo et al stated there are 18 intermediates, including aniline, N,N-dimethyl aniline, hydroxy anilines, hydroxy analogues of AO 52, phenols, quinone, benzene sulfonic acid, demethylated analogues of AO 52, and other aliphatic and carboxylic acids, that are found in the process of degrading methyl orange In their paper, Spadaro et al postulated that the oxidation of aminoazobenzene dyes involves the addition of a hydroxyl radical to the carbon atom containing the azo link, followed by the breakup of the resulting adduct Such reactions might produce compounds such as benzenesulfonic acid, N,N-dimethylaniline, and 4-hydroxy-N,N-dimethylaniline The electron- withdrawing sulphonate group suppresses reactivity towards the •OH of the ring that carried it; hence, the ring containing the amino group is the primary target for hydroxyl radicals [138].
In their study [139], Sun et al reported the production of Sb2S3, a new catalyst,and investigated the degradation of methyl orange solution in aqueous solution when exposed to visible light It was found that after 30 minutes of irradiation, the rate of methyl orange breakdown reached up to 97%, which was much higher than the rates of degradation achieved by CdS and TiO2 under the same circumstances Liquid chromatography and mass spectrometry were both used in the investigation of the potential mechanism behind the catalytic process In their paper [140], Zhao and Zhu described the synthesis of gold (or platinum) loaded titania nanotubes as well as the catalytic activity of these nanotubes for the degradation of methyl orange They found that the degradation rate of methyl orange solution could reach 96.1% (or 95.1%) when employing the Au/TiNT -500 (or Pt/TiNT -500 ) as a catalyst The light deposition process was used to load gold (or platinum) onto TiO 2 nanotubes Zhu and colleagues ([141]) described the manufacture of Polythiophene/titanium dioxide (PT/TiO2) catalysts via in situ chemical oxidative polymerization The resulting composites were analyzed by transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy (DRS), and ultraviolet– visible (UV–Vis) diffuse reflectance spectroscopy (DRS) (TEM). Degradation of methyl orange was used as a test substance for determining the catalyst's catalytic activity It was also shown that Pt/TiO 2 composites had strong adsorption capabilities owing to the electrostatic attraction between the positively charged composite particles'surfaces and MO This was found in the research that was done It was discovered that adding Pt to the composites boosts the catalytic degrading activity for MO when exposed to either UV or visible light This was a finding that was made possible thanks to the aforementioned observation Degradation of Rhodamine B (RhB) and Methyl Orange (MO) in aqueous solutions under simulated solar light irradiation has been reported by Guo et al [142] using one- dimensional (1D) TiO2 nanostructures, nanotube and nanowires, which were synthesized by a hydrothermal method using Degussa P25 TiO2 as a precursor These nanostructures were created by using TiO2 It was discovered that the degrading activity of the TiO2 nanocatalysts was much greater than that of the P25.
One of the most important factors that may have an effect on how quickly dye photodegrades is the value of the pH When the pH of the solution changes, the surface charge of the TiO2 particles fluctuates, which causes the potentials of catalytic processes to alter as well This leads to a change in the adsorption of the dye on the surface, which in turn causes a change in the rate at which the reaction occurs [143] In the meanwhile, the PZC of the catalyst, which stands for point of zero charge, was used to assess how quickly the substrate could adsorb organic contaminants It is anticipated that a dye solution with an acidic pH may adsorb more in a high PZC catalyst Since it is known that the PZC of TiO 2 is somewhere around a pH of 6.8, this indicates that for pH values that are greater than 6.8, the surface will become negatively charged, as demonstrated in the following equation:
TiOH and OH lead to the formation of H2O and TiO, and when the pH of the environment is less than PZC, the surface of the TiO2 molecule becomes positively charged, as illustrated in the following equation [144]
When it comes to producing improved photocatalytic activity, the structure of the catalyst plays a critical role TiO2 may exist in three different phases: anatase, rutile, and brookite Because of its stability, placement within the conduction band, degree of hydroxylation, and adsorption capacity, anatase has the potential to perform photocatalysis As was previously observed, morphology is also a factor that determines the final efficiency of degradation The photocatalytic effectiveness of nanomaterials is much higher than that of bulk materials due to the nanomaterials' large surface area and small size When it comes to water filtration and recycling, nanosized titanium dioxide is superior than bulk titanium dioxide When a catalyst is reduced in size, more atoms are forced to congregate on its surface, which results in an increase in the ratio of surface area to total volume This characteristic boosts the pace of charge carrier transfer, which in turn leads to an increase in the number of active sites Due to the fact that the photocatalytic redox reaction takes place largely on the photocatalyst surface, surface features have an impact on the effectiveness of the catalyst.
Phenol photocatalysis degradation
In order to cut down on the amount of phenol that is present in wastewater streams, researchers have looked at a wide variety of different phenol breakdown procedures Polymerization of phenol takes place when it is exposed to enzymes as well as hydrogen peroxide Due to the enzymes that are required, while it is extremely successful, the cost may be rather high In addition, biological methods, such as the use of activated sludge in membrane bioreactors, have been applied in order to break down phenol This has also been demonstrated to be successful, despite the fact that large cleaning expenditures are required due to fouling In addition, electrocoagulation, extraction, adsorption and ion exchange, and photodecomposition are all processes that have been looked at.
Titanium dioxide, often known as TiO2, is a photocatalyst that may be purchased at an affordable price and is readily available in commercial quantities It has been shown that it has the potential to stimulate the decomposition and mineralization of a wide range of organic pollutants As a result, the phenol degradation that may be accomplished by the application of TiO2 photocatalysis in water can be useful for cleaning up the environment However, TiO2 photocatalyzed phenol methods are limited since they have a low quantum efficiency and need UV light [146] These constraints limit the process's potential applications As a direct result of this, titanium dioxide is often mixed with a broad range of other substances For instance, it has been shown that activated carbon increases the photocatalytic activity of TiO 2 Because of its favorable electrical, adsorption, thermal, and mechanical properties, graphene oxide has lately seen a surge in interest as a potential substitute [147].
However, the use of ultraviolet (UV) radiation to breakdown significant quantities of wastewater is not a feasible option In order to hasten the process of degrading phenol and removing it completely from water, researchers are always looking for environmentally friendly, cost-effective, and chemically or catalytically based solutions to the problem In addition, the purpose of this thesis is to investigate the deterioration of phenol via the process of photocatalysis by making use of titanium dioxide modified catalysts and illuminating the reaction mixture with both visible light and ultraviolet light It has the potential to completely transform the way that industrial wastewater is treated if photodegradation of phenol and other organic compounds can be carried out using just visible light In a heterogeneous photocatalytic system, photo- induced molecular transformations or reactions typically take place at the surface of the catalyst The first thing that has to be done is the system needs to be excited by employing photons of light The absorption of photons by molecules results in the production of a highly reactive and electrically excited state The photon has to have enough energy to push an electron over the band gap, also known as the empty area that may be found between the top of the full valence band and the bottom of the unoccupied band Because TiO 2 has a large band gap of 3.2 eV, visible light is insufficient for the reaction In order for the photoreaction to begin, then, TiO2 has to be exposed to light in the ultraviolet spectrum, which has a wavelength that is shorter than 390 nm After this initial excitation, an electron transfer occurs to the solvent or organic species, which then becomes absorbed on the surface of the catalyst In the case of TiO2, an excited electron is responsible for the reduction of an oxygen molecule to the gas O2 During this time, the positively charged electron hole, shown by the symbol h+, interacts with water to form a hydroxyl radical, denoted by the symbol OH Within the framework of the photocatalytic process, the hydroxyl radical serves as the catalyst for the beginning of the chemical processes [147] The following deexcitation happens at a significantly slower pace than the initial absorption of a photon of light, which happens extremely quickly (on the range of tens to hundreds of milliseconds) On a timescale of 10-12 to 10-9 seconds, the photochemical processes take place [148].
The promotion of an electron results in the creation of an electron-hole pair, which may lead to one of two different possible routes It is preferable for the photoinduced electron to either transfer to the organic or inorganic species that have been adsorbed, or it might transfer to the solvent The unfavorable mechanism known as electron-hole recombination is in direct rivalry with the charge transfer process The efficiency of a photocatalytic process is inversely proportional to the sum of the charge transfer rate and the electron-hole recombination rate The efficiency is related to the rate of the charge transfer process, which in turn is proportional to the rate of the charge transfer process As a result, the recombination of the electron-hole pair is something that has to be prevented in order to make the process of charge transfer on the catalyst surface more effective It has been suggested that decreasing the recombination rate may be accomplished by making modifications to the surfaces of semiconductors using metals or combining them with other semiconductors [149].
In order to maximize the effectiveness of the photocatalytic process, there are two obstacles that must be overcome To begin, the photon of light that reaches the catalyst has to have enough energy to encourage an electron to go over the band gap. When it comes to TiO2, visible light is not enough to activate the catalyst; instead, ultraviolet light is required It is not possible to use UV light on an industrial scale for the breakdown of wastewater due to the fact that the amount of UV light that reaches the surface of the Earth accounts for less than 5% of the solar spectrum [150] The second difficulty is that, in order to have an effective reaction, the electron-hole recombination process has to be slowed down significantly.
Fig 1.9: Mechanism of Phenol Decomposition Reaction over nonmetal TiO 2 [151]
TiO2 has been shown to be an effective catalyst for the photocatalytic degradation of phenol The interaction of phenol with OH- radical ions results in the formation of a number of intermediate products These products include hydroquinone (HQ), pyrocatechol (CC), 1,2,4-benzenetriol (HHQ), pyrogallol (PG), 2-hydroxy-1,4- benzoquinone (HBQ), and 1,4-benzoquinone (BQ) These intermediates are then subjected to further photocatalytic oxidation, which results in the production of highly polar intermediates such as carboxylic acids and aldehydes, and ultimately carbon dioxide and water [150,151].
It has been observed that nonmetal doping of TiO2 catalysts may lower the band gap energy, hence boosting photocatalytic activity under a broader spectrum, including the visible light spectrum This would result in a more efficient exploitation of the major component of the solar system [152,153] For the purpose of this investigation,nitrogen doping of TiO2 was selected as the method of investigation in order to determine whether or not the photocatalytic performance might be increased when exposed to visible light.
Kinetics study of phenol photodegradation
Most of the heterogeneous photocatalytic degradation reactions follow Langmuir– Hinshelwood (LH) kinetics [154,155 [ This statement was supported by a great number of academics, saying the destruction rates of photocatalytic oxidation of various pollutants over irradiated TiO2 fitted the Langmuir-Hinshelwood (LH) kinetics model According to the principles of photocatalytic reaction, the main steps in the process occur on the surface of a solid semiconductor photocatalyst, so the adsorption of organic compounds on the photocatalyst surface definitely affects the reaction and usually high adsorption capacity favors the reaction Adsorption-kinetic models, and specifically the Langmuir–Hinshelwood (LH) model, are the most commonly applied to describe photocatalytic mineralization reactions.
In this study phenol concentration is the only variable therefore, assuming the model Langmuir-Hinshelwood (LH) for the photocatalytic reaction, the rate equation is as follows where the straight line resulting from a plot of ln (C0/Ct) as a function of time.
1+𝐾𝐶 r = rate of reaction [mg/L min]
𝑘 𝑟 = rate constant for photocatalysis [mg/L min] K = rate constant for adsorption [L/mg]
C = dye concentration at time t [mg/L] t = time [minutes]
The rate of reaction r can also expressed as follow:
𝑟 0 = initial reaction rate [mg/L min]
𝑘 𝑟 = rate constant for photocatalysis, [mg/L min]
𝐶 𝑒 = equilibrium dye concentration in a solution after the completion of dark experiment [mg/L]
1/𝑟 0 is plotted against 1/𝐶 0 and the kinetic parameters 𝑘 𝑟 and K are estimated by the slope and intercept respectively.
Summary
The status of the environment has been and continues to be one of the most significant difficulties that we face on a daily basis, and it has been like this for some time There are a wide variety of unique types of pollution, each of which poses a risk to human life and is rather widespread This thesis focuses on the development of a solution to address the prevalence of organic pollutant compounds in our environment, including methyl orange and phenol, both of which are recognized as being detrimental to humans and other living things Studying the impacts of loading nanotitanium with carbon and GO supports and altering the circumstances of its synthesis in order to boost its photocatalytic capacity will result in a rise in TiO2's photocatalytic activity, which will be accomplished by improving its photocatalytic capacity [156-158] As a consequence of this, the rate at which methyl orange and phenol degrade when subjected to UV-C and visible light irradiation will be accelerated.Several authors have published research on the manufacturing of modified TiO2 materials and the ability to increase the activity of modified TiO2 materials The use of modified TiO2 photocatalyst materials has been extensively researched and developed in Vietnam Using the sol-gel process, the study [159] successfully synthesized TiO2 material that had been modified with Fe and coated on silicagel (SiO2) beads The results of the decomposition of methylene blue (MB) solution at a concentration of 10 ppm indicated that the sample TiO2/SiO2 modified with Fe had a higher MB decomposition efficiency than the TiO2/SiO2 sample without Fe This was indicated by the fact that the sampled TiO2/SiO2 modified with Fe had a higher
MB decomposition efficiency TiO2 was modified with Fe2O3 at concentrations of 0.025, 0.05, 0.10, 0.50, 1.00, and 2.00 percent using the sol-gel process, as described in the paper [160] As a result of the degradation of p-xylene, it was discovered that the Fe-TiO2 catalyst samples demonstrated a decomposition activity that was two to three times as high as that of unmodified TiO2 The combination of ultraviolet and visible light results in a significant reduction in the amount of p-xylene that can be converted by samples of TiO that include 2% Fe O In addition, the studies [161–
164] studied the possibility of enhancing TiO2's photocatalytic ability by modifying it with metals such as Nd, F, Ag, and N This research would contribute to the development of strategies for minimizing the presence of water contaminants in our environment by providing information on how to reduce their presence This study may also aid in the development of novel methods for limiting the presence of other potentially dangerous organic contaminants The findings may also serve as a useful source for other researchers who are looking into the enhancements that may be made to other catalysts or even on other studies that may focus on the same topic.
Recently, the research on improving the activity of TiO2 and its application in environmental treatment, especially wastewater, continues to be carried out by domestic and foreign scientists, which demonstrates the scientific and practical the topic of choice [165-170] The objective of research is to study, synthesize and optimize catalysts and catalyst carriers in each synthesis method, combining activated carbon and graphene oxide and dispersed on thin films on different carriers to improve photodegradation in wastewater treatment.
In addition, the information that was gathered during this research would be put to use in more research that would be conducted as part of the Rohan catalyst program.These additional studies would focus on topics like the treatment of wastewater and the preservation of the environment.
EXPERIMENTS
Materials and instruments
No Item Name/Specifi cations Remarks
1 Titanium isopropoxide TTIP 95% C 12 H 28 O 4 Ti Precursor
2 Glacial acetic acid 99% CH 3 COOH Hydrolyzing agent
3 Activated carbon Carbo Karn Support
14 Sulfuric acid 96% H 2 SO for pH adjustment
15 Sodium hydroxide 98% NaOH for pH adjustment
17 Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol)
19 Cetyl trimethyl ammonium bromide CTAB
Table 2.2: List of the main instruments
1 Xenon 300W/15A lamp Full range from185 – 2000 nm
3 HPLC Jasco at Lab of Catalyst Research
9 Soft pipe, air tight plugs
11 Quartz tube reactors for UV and full range
12 Aluminium paper, PE thin film
13 Glassware and common tool set
14 Tool set for grinding and screening
1.7 gam of CTAB 9 ml of TTIP 15 mL of ethanol
Stirred, maintained at 80°C, 3 days Refluxing with NaOH 0.1 M, 2 days Filtered, washed, dried at 90°C for 11 h
Catalyst preparation
2.2.1.1 Synthesis of mesoporous TiO 2 by precipitation method using CTAB surfactant
Fig 2.1 Flowchart of TiO 2 synthesis using CTAB.
Firstly, 1.7 g of CTAB is dissolved in 18 ml of ethanol (solution 1) 9 ml of TTIP is dissolved in 17 ml of ethanol and 3.9 ml of HCl (solution 2) Pour solution 1 into solution 2 and strongly stir the resulting solution Add 6 ml of distilled water, stir the solution strongly for 15 minutes and dry at 80 o C for 3 days Proceed reflux for 2 days with 0.1M of NaOH solution The catalyst is then filtered from the solution and divided
Calcined at 450°C, 2h Stirred vigorously, 15 min
6 mL of distilled meso-TiO 2
100ml of distilled water, 55°C Mixin
Solution 2 into two parts, one part is dried at 80 o C for 2 hours (CTAB-NE) and the other part was added a little of ethanol then mixed and dried at 80 o C for 2 hours (CTAB-E) Both catalysts were calcined at 450 o C for two hours with 2 o C/min.
The catalyst was also obtained by conducting hydrothermal synthesis using CTAB surfactant The process is like CTAB-NE synthesis but subjected to hydrothermal treatment dissolving the catalyst after drying at 80 o C in 0.1M NaOH solution and heating for 11 hours Filtering and heating were then conducted at 450 o C for 2 hours in order to obtain CTAB-H.
2.2.1.2 Synthesis of mesoporous TiO 2 by hydrothermal method using P123 surfactant
Fig 2.2 Flowchart of P123 C25-450 synthesis using P123.
Firstly, 4g of P123 was dissolved completely in 100ml of distilled water at 55 o C and 0.8 ml of Sulfuric acid (solution 1) Mix 12.5ml of TTIP with 8.687 g Citric acid (solution 2) Mix solution 2 in solution 1 and incubate for 3 hours at 55 o C After that,
0.8 ml of concentrat ed sulfuric acid
8.687 gam of citric acid (99%) hydrothermal treatment is conducted for 10 hours, the catalyst was then calcined for 2 hours at 450 o C and is denoted as P123-C100-450 catalyst The amount of Citric acid was varied for different samples.
Some other catalysts were also prepared using similar procedure but not adding Citric acid and the calcination was conducted at 300 o C for 8 hours at a rate of
10 o C/min, such as the P123-C0-300 catalyst The other catalyst was also synthesized using Citric acid but just incubated at 55 o C for 1.5 hours then followed by hydrothermal treatment for 11 hours The catalyst is filtered and washed with distilled water, dried, and then calcined at 450 o C for 2 hours obtaining P123-C0-450 Also, following similar procedure, with the amount of Citric acid added by 25%, 50% and 75% respectively compared to the amount of Citric acid in the P123-C100-450 (considered 100%) and samples obtained were denoted as P123-C25-450, P123-C50-
It is processed as P123-C25-450 synthesis but after 11 hours of hydrothermal treatment, the catalyst is divided into 2 parts The half of the catalyst is soaked in ethanol for 5 times and then the calcined sample at 450 o C obtained as P123-C25-450-
RE (rinsing in ethanol) The other half is processed in an autoclave containing ethanol at 70 o C for 24 hours then calcined sample at 450 o C obtaining sample P123-C25-450- HT- E (hydrothermal treatment by ethanol).
Catalyst synthesis was carried out at the Gevicat catalyst center, Hanoi University of Science and Technology by photocatalyst research group.
Table 2.3: Catalyst synthesized by hydrothermal and precipitation methods using surfactant.
Order Name code Synthesis Method Characteristics
Hydrothermal using P123 with maximum amount of citric acid, calcined at 450 o C
2 P123-C0-300 Hydrothermal using P123, no citric acid, calcined at 300 o C
3 P123-C0-450 Hydrothermal using P123, no citric acid, calcined at 450 o C
Hydrothermal using P123 and 0.25% citric acid compared to P123- C100-450, calcined at 450 o C
Hydrothermal using P123 and 0.50% citric acid compared to P123-C100-450, calcined at 450 o C
Hydrothermal using P123 and 0.75% citric acid compared to P123- C100-450, calcined at 450 o C
7 CTAB-NE Precipitation method using CTAB, no ethanol washing
8 CTAB-E Precipitation method using CTABl, ethanol washing
9 CTAB-H Hydrothermal method using CTAB
Hydrothermal using P123 and 0.25% citric acid compared to P123- C100-450, calcined at 450 o C P123 removal by ethanol treatment with the autoclave-container (Heating Ethanol)
Hydrothermal with P123 and 0.25% citric acid compared to P123- C100-450, calcined at 450 o C P123 removal by ethanol treatment through continuous rinsing (Rinsing Etanol)
2.2.2 Synthesis of TiO 2 and AC/TiO 2 by Sol-gel method
The AC/TiO2 composite catalyst used in this study was prepared via sol-gel method Sol-gel method was employed in producing nano TiO2 due to its relatively low cost The procedure for sol-gel was adopted from the work of Venkatachalam et al (2007) [58] The method was modified by introducing AC in the TiO2 sol Titanium isopropoxide (126 ml) used as a precursor and glacial acetic acid (250 ml) as hydrolyzing agent were prepared in a 5-L beaker placed in an ice bath maintained at a temperature at 0 o C Double distilled water (2.45 L) was added dropwise to the solution with stirring at a speed of 1050 revolutions per minute (rpm) Subsequently, the solution was placed in a sonicator for 30 minutes and stirred for 5 hours until a clear solution of TiO 2 was formed AC was then added to TiO2 sol The ratio of weight
AC (grams) and weight of TiO2 was varied (1:18 AC/TiO2) Weight of AC added inTiO2 solution with corresponding theoretical % wt AC in AC/TiO2 are shown in Table2.4 The TiO2 sol and AC mixture was stirred for 1 hour and sonicated for 30 minutes.
Table 2.4 : AC/TiO 2 catalyst with various AC to TiO 2 Ratio
Catalyst denoted Weight of AC (g) :
Weight AC in TiO 2 solution
The solution was heated in an oven maintained at 70 o C for 12 hours for aging. The gel produced was dried at 100 o C for 72 hours, crushed into powder and calcined at
400 o C for 5 hours at a heating rate 100 o C/min; Finally,the resulting catalysts after calcination were subjected to fine grounding Bare TiO2 was also prepared using a similar procedure TiO2 is created in this reaction
The respective theoretical % weight of AC in these catalysts are also presented in the Appendix A.
2.2.3 Synthesis of TiO 2 GO by sol-gel method
To prepare TiO2–GO composite, firstly, GO was prepared from graphite powder by Modified Hummers method [171, 172] 2 g of graphite powder was mixed with 100 ml of H2SO4 98% in a beaker, which was maintained in an ice bath for 2 h Then 8 g of KMnO4 was added under mixing for 3 h H2O was added dropwise to the resulting solution under stirring to oxidize graphite for 1 h Afterwards, 300 ml of H2O and 30 ml H2O2 were added into the solution under vigorous stirring for 5 h The slurry was cleaned by HCl 5% and then washed by deionized water to obtain GO as presented inFig 2.3
Cleaned by HCl 5%, washed by deinonized water
8g of Mixing, maintained in a ice bath for 2h
300ml of H 2 O and 30ml of H 2 O 2 Stirred, 1h
Added dropwise of H 2 O to solution
Fig 2.3: Flowchart of GO synthesis
For the TiO2–GO composite by sol-gel method: the next stage is to follow similar procedures in item 2.1.2 “Creating composite with activated carbon (TiO2/AC catalyst)’ where AC replaced by GO and its amount calculated via the ratio between
TiO2:AC in the weight of final composites GO-TiO2 is denoted as SG GO1/4, SG
The TiO2/GO composite was prepared by hydrothermal method in Fig 2.4 [172].
Brieftly, 2 mg of GO was dissolved in a solution of water and ethanol (the volume ratio of water to ethanol was 2:1) followed by ultrasonic treatment for 1 h and stirring for 2 h Afterwards, 200 mg of TiO2 synthesized by hydrothermal method as mentioned previously was added into the slurry The mixture was transferred into a Teflon-lined autoclave, and then a hydrothermal process was performed at 120°C for 3 h During this process, GO could be reduced to graphene and concurrently the deposition of TiO2 on GO was achieved The composite was obtained after centrifuging, rinsing with
2 g of graphite powder 100ml of H2SO4,
200mg of P123-C25-450/ZnO Ultra sonic treatment for 1h, stirred for 2h deionized water, and drying at 60°C under vacuum, which was denoted as GO–TiO2.
In order to compare the photocatalytic activity of as-synthesized GO–TiO2 with other composite, the GO–ZnO composite where ZnO (99%) was purchased from Merck and was also prepared by the same procedure with GO–TiO2.
Fig 2.4: Flowchart of GO-TiO 2 (GO-ZnO) synthesis
2.2.4 Synthesis of TiO 2 films on cordierite
2.2.4.1 TiO 2 thin film synthesis by dip coating on the surface of corerdierite
Kaolin is ground then soaked in water for 6 hours, settled for two days then filtered, dried to get cleaned kaolin Next, 50g cleaned kaolin, 5.12g MgO; 3.01g Al (OH)3 Tan Binh và 11.75g dolomite is finely ground then filtered via a 88àm screen then well mixed Thereafter, the mixture was added with sufficient distilled water and mixed in 12 hours The solution is settled, filtered, vacuum pressed in order to get a paste powder The powder was pressed and molded to get samples having dimensions of 25 x 15 x 5 mm All samples were dried at 120 o C in 3 hours, then calcined at
2mg of GO with a heating rate of 50 o C/min.
Put the Cordierite samples prepared above soaked in 36% HCl solution, at a temperature of 90 o C Change the acid solution every 1 hour Treat with 36% HCl within 8 hours.
After soaking in acid for 8 h, Cordierite tablets were washed with distilled water until the ion Cl- was gone (tested with AgNO3) Then proceed to dry at 120oC [173].
TiO 2 synthesis by precipitation method using CTAB template
Characterization of the catalysts
Surface morphology imaging techniques play a crucial role in the characterisation of nanostructures Due to its limited resolution, an optical microscope can no longer be used to see submicron to nanoscale features SEM is often used to examine the surface morphology of micro/nano-structured materials As a result of the inherent behavior of electrons in comparison to photons, electron microscopes provide superior resolution and depth of focus
SEM electron sources are thermionic and field emission guns Thermionic guns provide electrical current to warm filaments (e.g tungsten) When heated enough, the electron may exit the filament The thermionic cannon's thermal drift and low brightness are drawbacks Field emission guns or cold cathode field emitters may circumvent these concerns without heating the filament The field emission gun is a polished filament Its pointed tip concentrates the electric field to create a large electrical potential gradient that draws and accelerates electrons from the filament material Standard SEM and FESEM also differ in electron gun type FE weapons may triple electron density FESEM has better spatial resolution and less electrostatic distortions than SEM
Fig 2.7: Simplified internal structure of FESEM.
FE cannons use tungsten cathodes with sharp 0.1 m points The FE cannon features two anodes: one restricts current, the other focuses and accelerates the electron stream Electromagnetic lenses focus electron beams by applying magnetic field horizontally The scanning coil deflects the electron beam toward the sample by controlling current using radially oriented coils The sample atoms provide signals from a few hundred eV to fifty keV electron beam A detector quantifies and converts these impulses into digital images.
FESEM results result from electron beam interactions with sample atoms(electrons or nuclei) Elastic and inelastic scattering occurs when the electron beam hits the sample atom Elastic and inelastic scattering emitted X-rays, Auger electrons,secondary electrons, and backscattered electrons Surface structure affects secondary electron angle and velocity Repeat until the incident electron loses energy The electron detector usually catches secondary electrons from 200 within the material.After that, a computer monitor will display the samples Drop casting FESEM samples works Drop casting onto a Si/SiO2 substrate will follow ethanol dispersion Before putting the sample on the specimen stage, it must be clean, dry, and mounted in a sample holder.
In this thesis, SEM images were captured by using JSM-7600F Schottky Field Emission Scanning Electron Microscope (Advanced Institute for Science and Technology, Hanoi, Vietnam) and FEI Nova NanoSEM 400 field emission scanning electron microscope at Hanoi Unviversity of Science-VNU Hanoi.
2.3.2 Elemental surface composition and traces of impurities
EDX analysis is a method used to determine the elemental composition of a specimen or a specific region of interest The EDX analysis system operates as an integral component of a scanning electron microscope (SEM) and cannot function alone.
A scanning electron microscope bombards the object during EDX Analysis. Bombardment electrons knock electrons off specimen atoms A higher-energy outer shell electron replaces an inner shell electron Yet, the outer electron transferring energy must generate an X-ray.
A transporting electron releases energy from both its source and destination shells Each element's atoms release different-energy X-rays during transmission The atom that released an X-ray during electron beam bombardment may be identified by measuring its energy (SiliconFarEast.Com, 2004).
EDX spectrum shows X-ray frequency at each energy level EDX spectrum peaks usually correlate to energy levels where most X-rays were received As each atom is unique, these peaks belong to different elements A specimen's element concentration increases with a spectrum's peak height.
2.3.3 Specific surface area, pore volume, and average pore size
BET analysis measures surface area, pore volume, average pore size, and pore size distribution A powdered or porous material's surface area may be calculated from the quantity of gas adsorbed All gas-accessible surfaces—interior and exterior—are measured Van der Waals interactions make solids absorb gases poorly To absorb enough gas to measure surface area, the material must be cooled to the gas's boiling point The gas (adsorbate) is nitrogen, and liquid nitrogen chills the solid (77.35K).
Adsorption continues until N2 adsorbed equals gas phase concentration The quantity is equivalent to that needed to cover the surface in a monolayer BET, which requires minuscule quantities of sample and has short out gassing and analysis periods It is the most sophisticated and reliable method for fast and accurate BET surface area measurements.
Brunauer, Emmett, and Teller (1938) developed an adsorption based on the idea that molecules might be adsorbed in many layers on the adsorbent Consistent with the Langmuir equation, their equation assumed that the adsorbent surface had homogeneous, isolated sites and that adsorption at one site did not affect nearby sites. The energy of adsorption held the original monolayer in place, while the condensation energy of the adsorbate absorbed successive layers.
In this thesis, physical adsorptions of the catalysts were tested by the Gemini VII Micrometrics equipment, in Gevicat catalyst center, Hanoi university of science and technology.
2.3.4 Crystal structures formed and the crystallite diameter
XRD temperature determines material crystalline structures by measuring small crystalline regions Crystal lattices with regular, repeating atom planes govern the three- dimensional structure of non-amorphous materials like minerals These atomic planes transmit, absorb, diffract, and scatter a focused X-ray beam A crystalline solid diffracts X-rays as water droplets diffract light, creating a rainbow Depending on its atomic composition and arrangement, crystal lattice size, shape, and internal tension, each mineral diffracts X-rays differently X-ray powder diffractometry uses vacuum- sealed tubes to generate X-rays Electrons from the filament increase with current The tube often receives 15–60 kilovolts High voltage accelerates electrons, which hit a copper target Electron collisions produce X-rays X-ray wavelength defines this item. Collimated X-rays target finely powdered material (typically to produce particle sizes of less than 10microns) X-ray energy is detected by a detector, then electrically or microprocessor-processed into a count rate An X-ray scan includes slowly changing the angle between the source, sample, and detector within preset limits
In general, the crystal size equals to the whole width at half maximum of an XRD peak As the breadth increases, the size of crystallites decreases Scherrer's formula, described in Eq 2.1, may be used to compute the mean size of the single crystallite from the full-width at half-maximum of matching X-ray diffraction peaks:
D = Mean size of the single crystallite, nm. λ = Wavelength of the X-ray radiation (l = 0.15418), K = Scherrer constant (K 0.9) θ= Characteristic X-ray radiation (u = 12.78). β= the full-width-at-half-maximum of the (1 0 1) plane (in radians).
In this work, XRD patterns were mainly recorded using a D8 Advance Bruker device (Faculty of Chemistry, Hanoi University of Science, Vietnam) The diffract meter has a Cu source with Cu K radiation (λ = 0.154 nm), step scan=0.030/sec.
Experimental set up
The photodegradation of MO was carried out in a cylindrical 800 ml-capacity glass cell in a batch reactor Vertical placement of a 100 W mercury lamp as a 254 nmUV-C source in a quartz tube with a water-cooling jacket to maintain a constant temperature of 25 °C, which was then put into a glass photo-reactor The UV-C laser is positioned in the middle of the reactor, and a magnetic stirrer was used to evenly disperse the fluid (Fig 2.10 a).
Fig 2.9a: Photocatalytic exerimental setup with UV-C lamp
In MO photocatalytic degradation process with the full range 300 W Xenon lamp system, the light goes through diagram box to a Quartz tube horizontally with magnetic stirring to distribute catalyst continuously in solution An aliquot of solution was withdrawn using a pipette and filtered through a 0.22 m syringe filter after each run was kept in the dark for 30 minutes to achieve adsorption–desorption equilibrium.
A UV– VIS spectrometer by Avantes was used to measure the MO concentration.
Fig 2.9 b: Principle diagram of visible photocatalytic exerimental setup.
Both UV light with a wavelength of 254 nanometers and visible light were used for the photodegradation of phenol Every 30 minutes, liquid samples were taken and filtered using a 0.22 m syringe filter to eliminate any catalyst particles A HPLC was used to assess phenol concentration (Jasco, Japan) In HPLC measurement operation, a combination of acetonitrile and deionized water (volume ratio of 1:1) was employed as the mobile phase at a flow rate of 1.0 ml/min, and 10 l of sample was injected for analysis at a wavelength of 280 nm.
2.5.1 Calibration curve of methyl orange solution
To be able to determine the amount of MO degraded during the reaction, we need to determine the concentration of MO solution based on the graph of the optical absorption titration curve of the MO solution According to Lambert - Beer law applied to a type of cuvette containing the sample unchanged, the dependence of the optical absorbance on the solution concentration is a linear dependence So if we measure the absorbance of a solution of known concentration, we can construct a linear curve A = f(C) On this basis, the concentration of the solution to be analyzed is calculated by measuring the optical absorbance of the solution and determining the concentration on the standard curve.
Prepare a series of samples of standard solutions with MO concentrations decreasing from 0.5 to 20ppm mg/l using a volumetric flask Use double distilled water as diluent Analyze the above solutions by UV-Vis photometer, determine the intensity corresponding to the wavelength of 464 nm (which is the wavelength corresponding to the maximum absorption of the MO solution). y = 0.028x + 0.0051 R² = 0.9962 y = 15924x + 4393 R2 = 0.9998
2.5.2 Calculation the concentration via equation
From the corresponding pairs of A - C values of the standard solution samples, we build a standard curve in the A - C coordinate system, which is graphed as follows:
From there, a standard curve was built based on the relationship between MO concentration and absorption intensity (Abs) and regression equation to determine the remaining MO content in the solution at the moment determined.
Similarly, it is determined the phenol concentration with UV-Vis replaced by HPLC analysis instrument with equation between area peak and concentration.
Phenol concentration (ppm)A ds or ba nc e A
In this chapter, the content is displayed in the order of experiments conducted with the relevant catalysts The preliminary tests are to find the optimum catalyst to modify to do phenol phototesting in both UV and visible light irradiation.
3.1 Characterization and photocatalytic activity of mesoporous TiO 2 synthesized by precipitation and hydrothermal using CTAB and P123 surfactants
Table 3.1: The surface characteristics of catalysts synthesized by hydrothermal and precipitation methods
Fig 3.1: Nitrogen isotherm of CTAB-NE and P123 C25-450.
Fig 3.2: Pore size distribution of CTAB-NE and P123 C25-450.
Moreover, the nitrogen physisorption isotherms and corresponding pore size distributions of calcined P123 C25-450, CTAB-NE samples are shown in Fig 3.1 and Fig 3.2 The TiO2 exhibits type IV adsorption–desorption isotherms with H2 hysteresis loop according to the IUPAC classification The pore size distribution curves shows a broad pore size distribution centered at 40 – 70 Å Meanwhile, the catalyst P123 C25- dV /d lo g( w ) P or e V ol um e Q ua nt ity A ds or be d (c m ³/g
450 has smaller pore diameter (40 Å) compared to CTAB-NE (70 Å) thus P123 C25- 450’s surface area is higher compared to CTAB-NE and is expected to also have a better adsorption and desorption capacity.
The XRD pattern of TiO2 samples prepared by hydrothermal with different template methods are shown in Fig 3.3 The selected catalysts were characterized via the XRD method and the results are depicted in the following figures:
From the given figures above, the diffraction peaks at 2θ values of 25, 37.83, 48,
54 and 55° corresponds to the anatase (101), (004), (200), (150) (only in catalyst using P123) and (211) crystal planes, respectively These results confirmed that the prepared TiO2 are in anatase structure with high crystallization and all the diffraction peaks are basically in agreement with the reported ASM data (Card No 96-900-9087) [176] The broadening of TiO2 peaks is due to the presence of small crystal size.
The results of XRD analysis have shown that the catalyst samples all formed in Anatase phase of TiO2 however, the CTAB-NE catalyst sample contains not only Anatase phase but also both Rutile and Brookite forms (inactive photochemical) at various intensities (2θ= 28; 36; 41; 54 o ) [176-179 ] Thus, the synthesis process with CTAB surfactant resulted to Anatase TiO2 which was converted partially to the different polymorphs during the process in the presence of ethanol.
XRD pattern of CTAB-H showed both rutile and brokite phases similar to the CTAB-NE sample, the photocatalytic efficiency of this sample MO is better However, that difference is not too much This can be explained in some references stating that the synegestic effect between Anatase and Rutile phases with an optimal ratio under a certain condition will improve the performance of the catalyst The XRD spectrum shows that the Rutile peaks of the CTAB-NE sample are larger than that of the CTAB-
H sample and this may have caused the that difference in the MO removal efficiency of the two catalysts [180-182].
Fig 3.3: XRD paterns of catalysts synthesized with surfactants CTAB and P123
Table 3.2: Crystalline sizes of catalysts
Sample No Catalysts denoted Crystal size (nm)
The calculation of anatase TiO2 crystal sizes based on Scherre equation for the diffraction peak at 25.27 o in table 3.2 show that sample P123-C25-450 possesses the smallest crystal size, which is in an agreement with surface area results
Further, the FE-SEM analysis as presented in Figure 3.4 shows that both CTAB-
H and P123-C25-450 catalysts are nano-sized and P123-C 25-450 has a particle size
In te ns ity about 5 nm, smaller than CTAB-H (20 nm) under the same magnification SEM analyses were employed to investigate the morphology of the particles and the homogeneity of the TiO 2 distribution It was observed that the morphology of the TiO2 samples has no difference when compared to various templates.
The SEM micrograph for TiO2 displayed a non- uniform distribution of spherical particles and consisted of either single particle or cluster of particles The spheres observed consisted of many small crystals of TiO2 which can be due to agglomeration.
On the other hand, the FE SEM image of TiO2 nanoparticle powder calcined at 450°C showed crystallinity with particle sizes that varies between 5.0 nm to 10 nm
Fig 3.4: FE-SEM images of CTAB-H (a) and P123 C25-450 (b)
The parameters evaluated during the conduct of experiments at a dosage of 0.1g TiO2 and 800 ml of 20 ppm solution of methyl orange in a batch reactor with UV-C lamp.
Catalysts using CTAB surfactants for MO photodegradation
The MO photocatalytic degradation of catalysts synthesized with CTAB surfactant is shown in Fig.3.5 It is noticeable from the results that CTAB E and CTAB
RESULTS AND DISSCUSSIONS
Characterization and photocatalytic activity of TiO 2 /AC catalyst synthesized
The nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of SG AC -1200/Ti1/18 are shown in Fig 3.9 and Fig 3.10.
Fig 3.9: Nitrogen isotherm of SG TiO 2 and SG AC1200 TiO 2 1/18
Fig 3.9 displays a typical type IV nitrogen sorption isotherm with a type 1 hysteresis loop which is the characteristic of cylindrical ordered channels [187] As observed, the capillary condensation of nitrogen with uniform mesopores occurred, causing a sudden step increase in nitrogen uptake at a characteristic relative pressure (P/P0) range of 0.6 –0.9 for SG AC -1200/Ti 1/18
This suggests a typical mesoporous structure with uniform pore diameters [188]. The BET surface area was also determined to be 149.77 m 2 /g The pore size was shown to reach approximately 9 nm.
Fig 3.10: Pore size distribution of SG TiO 2 and SG AC1200 TiO 2 1/18. dV /d lo g( w ) P or e V ol um e Q ua nt ity A ds or be d (c m 3 /g
Table 3.3: Surface area of two samples by sol-gel synthesis
Ratio AC/TiO 2 in weight
Catalyst samples at a ratio of AC/TiO 2 as 1/18 and 3/1 is analyzed using SEM- EDX The SEM images of 1/18 catalyst samples were described in Figure 3.11 The results showed that the catalyst particles of about 5 nm in size is uniform and AC cannot be distinguished between TiO 2 as AC content in the catalyst is small However, EDX results show the presence of AC in the sample.
Fig 3.11: Morphology of SG AC-1200/Ti 1/18 (a) and SG AC-1200/Ti 2/1 (b)
For samples containing low amount of AC (AC/TiO2 = 1/18), the actualAC/TiO2 ratio is similar with theoretical calculations, proven that AC amount mixed with TiO2 did not decrease it content during heating However, for samples containing high amount of AC (AC/TiO2 = 3/1), the actual ratio AC/TiO2 is decreased significantly as compared to expected ratio based on theoretical calculations This showed that significant amounts AC burned during the heating of catalyst.
Fig 3.12: EDX analysis results of samples: SG AC-1200/Ti 1/18 (a); SG AC-1200/Ti
3/1 (b) For samples containing low amount of AC (AC/TiO2 = 1/18), the actual AC/TiO2 ratio is similar with theoretical calculations, proven that AC amount mixed with TiO2 did not decrease it content during heating However, for samples containing high amount of AC (AC/TiO2 = 3/1), the actual ratio AC/TiO2 is decreased significantly as compared to expected ratio based on theoretical calculations This showed that significant amounts AC burned during the heating of catalyst (in the air) With the results of the photo- degradation analysis, we found that the SG AC-1200/Ti 1/18 and
SG AC-1200/Ti 3/1 have the highest performances In view of these results, these two
(2) samples were analyzed by XRD diffraction method and figures below showing the results were obtained:
Fig 3.13: XRD result of AC TiO 2 catalysts
Figure 3.14 showed the XRD patterns of the SG AC-1200/Ti 1/18 and SG AC- 1200/Ti 3/1 samples Looking at the figure, we can see that the two (2) catalysts are 100% Anatase phase (2θ= 25; 38; 48; 55) and adding activated carbon have no significant effect on the phase composition since the amount of AC in the sample is low and AC is also in amorphous form In the sample with more AC ( SG AC-1200/Ti 3/1), the amorphous nature is clearer When calculating crystal size from XRD patterns (Table 3.4), it shows that the sample with more AC has a little bigger particle size.
Table 3.4: Crystalline sizes of catalysts
Sample No Catalysts denoted Crystal size (nm)
3.2.2 Photocatalytic activity of the synthesized samples
In this experiment, two different types of activated carbon were used to adsorb Methyl Orange performed under dark condition Type 1 is the AC produced in Vietnam (surface area of 300 g/m 2 ) and type 2 is the AC manufactured in Thailand (surface area of 1200 g/m 2 ) Both types of AC were thoroughly cleaned and dried in order to remove moisture and impurities prior to conducting the experiment The amount used is 0.4 g AC for 800 ml of 30ppm MO inside a reactor.
The adsorption of two kinds of AC with sonicator is within 30 minutes The adsorption result in Fig 3.14 show that AC 1200 exhibits better adsorption ability than
AC 300 due to higher surface area.
To investigate the influence of the AC category for photocatalytic activity, the two selected catalysts being synthesized with the mass ratio between the AC and TiO2 of 1: 1 in the catalytic synthesis components for two types of AC are shown above (surface area of 300 and 1200 g/m2) The photocatalytic activity results in Fig 3.15 show that in the first 20 minutes of dark adsorption, the conversion rate of TiO2 / AC
/ AC 1200 for MO degradation showed that TiO2 / AC 1200 is faster more than 1.5 times compared to TiO2/AC 300 After this dark adsorption period, the photocatalytic process with UV-C wavelength of 245nm is then conducted.
SG AC-300/Ti/1/1 SG AC-1200/Ti/1/1
Fig 3.14: MO dark adsorption of AC.
(Condition: C MO ppm, AC dosage =0.4g, solution volume 0ml, pH=7; dark adsorption in 30 minute with sonicator)
From 20 minutes to 120 minutes, the results between the two different types of catalysts indicated a clear difference The TiO2 / AC 1200 degraded and absorbed more than 90% of MO while the TiO2 / AC 300 only processed more than 40% of MO This can be explained by the impact of synergistic effect of AC adsorption with MO concentration focused on the surface supporting MO photochemical process of TiO2. The two types of AC having a significant difference in adsorption so that the efficiency during the photocatalytic process also varied considerably From 100 to 120 minutes, due to the adsorption of both AC increases, the photocatalytic process efficiency for MO degradation also increased The TiO 2 / AC 1200 can degrade up to 95% of MO, hence the AC1200 was selected for further study.
Fig 3.15: MO photodegration is affected by activated carbon category.
(Condition: CMO0ppm, catalyst dosage =0.2g, solution volume 0ml, pH=7; dark adsorption in 20 minutes, UVC 254 nm with 100 W lamp)
Ab so rb an ce D eg ra da tio n
SG AC-1200/Ti 1/18 SG AC-1200/Ti 1/4 SG AC-1200/Ti 1/1 SG AC-1200/Ti 2/1 SG AC-1200/Ti 3/1 SG TiO2
In this experiment, the AC 1200 was used to synthesize catalysts with different ratios of AC and TiO2 contents The catalyst prepared is used to evaluate the weight ratio AC / TiO2 respectively 1:18, 1: 4, 1: 1, 2: 1, 3: 1 (SG AC-1200/Ti/1/18, SG AC-
1200/Ti/1/4, SG AC-1200/Ti 1 / 1, SG AC-1200/Ti 2 /1, SG AC-1200/Ti 3 /1).
Experimental results of dark adsorption by SG AC-1200/Ti 3 /1exhibited the best results with more than 60% of MO degraded and the lowest was 40% for SG AC-
1200/Ti/1/18 In the sample for SG AC-1200/Ti 1 / 1 the performance is 40%, while for SG AC-1200/Ti 2/1 and SG AC-1200/Ti 1/ 4 performances are almost the same at
40% This result can be attributed to bigger amounts of AC resulting in greater MO adsorption in the solution [168,169] In the next photocatalytic process, the SG AC-
1200/Ti 1/18 displays the best performance of over 80% MO degradation then followed by SG AC- 1200/Ti 3/1, SG AC-1200/Ti 1 / 4, SG AC-1200/Ti 2/1 and SG
In general, the results can be interpreted as MO degradation that was mainly due to photocatalytic process rather than by pure absorption From minute 40 to 60, the three catalysts SG AC-1200/Ti 1 / 18, SG AC-1200/Ti 3/1, SG AC-1200/Ti 1/4 processed nearly 96% of MO while SG AC-1200/Ti 2/1 nearly 90% and SG AC-
1200/Ti 1/1 approximately 80% In this process, the synergetic effects of AC adsorption and TiO2 photoactivity is a possibility resulting to best performance Hence, the overall impact caused rapid MO degradation From 60 to 80 minutes, the MO degraded up to 98% and only SG AC-1200/Ti 1/1 has the lowest performance with
F ig 3.16: MO photodegradation via time of catalyst samples at pH=7.
(Condition: C 0ppm, catalsyt dosage =0.2g, solution volume 0ml, pH=7;
D eg ra da tio n dark adsorption in 20 minutes, UVC 254 nm with 100 W lamp)
Nanometer-sized TiO2-AC powder has been successfully prepared from the starting material TitaniumIzopropoxide by sol-gel method The type and content of components of activated carbon are the important factors affecting the photochemical activity of the synthesized catalyst Activated carbon with a surface area of 1200 m 2 /g showed higher efficiency than the type of surface area of 300 m 2 /g in both dark adsorption and photocatalytic photocatalytic processes The catalyst with SG AC 1200 3/1 had the highest adsorption effect of more than 60% and the lowest was the sample
SG TiO2 In the subsequent photochemical process up to 80 min, the results are almost the same with SG AC 1200 1/18 being the highest The additive effect of activated carbon supporting the photochemical process is shown [188,189].
These improved findings may be the consequence of a combination of adsorption and photodegradation As an adsorbent, activated carbon absorbs the dye molecules from the solution The molecules adsorbed to the surface of the activated carbon subsequently come into contact with TiO 2 , resulting in photodegradation TiO2/AC functions as both a semiconductor and an adsorbent to remove dye independent of the dye's properties In this instance, the combination of activated carbon adsorption and TiO2 photodegradation was effective for dye removal The dual effect was capable of enhancing the overall performance or efficiency of the degrading system When loaded with activated carbon, the photocatalytic activity of TiO 2 was increased This is due to the enhanced adsorption of MO on TiO2/AC Activated carbon has no photocatalytic activity, but it improves the photocatalytic activity of TiO2 Higher adsorption results in a greater concentration of MO in the vicinity of TiO2 The adsorbed MO molecules on AC are transported to TiO2, where they are destroyed by irradiation, demonstrating synergism It can be shown that AC significantly increased the photocatalytic degradation of MO catalyst activity As AC is very adsorptive and the photocatalytic studies were conducted under identical circumstances other than on bare TiO2, this increased activity may be attributable to AC adsorption Appropriate adsorption capacity is necessary for optimal photocatalytic activity The adsorption of reacting chemicals onto the surface of the catalyst is an essential stage in the photocatalytic process AC is often employed as an adsorbent and catalyst support because of its well-developed pore structure, extremely large surface area, and high adsorption capacity Because of the concentration difference, the AC in the TiO2/AC catalysts may operate as a center where organic molecules can adsorb before being transferred to the breakdown center, illuminated TiO , which is positioned on the AC surface
SG AC-1200/Ti/1/18 SG AC-1200/Ti/1/4 SG AC-1200/Ti/2/1 SG AC-1200/Ti/3/1
Therefore, catalyst SG AC 1200 1/18 is used to create thin film catalysts on the surface of Cordierite
TiO 2 films
Dip coating with low concentration of PEG
It can be seen that the catalyst coated on cordierite in the sol-gel method has a more uniform distribution, while in the co-precipitation method, the catalyst particle size is quite large and uneven, about 20-100 nm compared to about 20 nm in the sol- gel method.
Fig 3.26 : a) SEM Low PEG Cor-gel-200 and (b) SEM Low PEG Cor-gel-CTAB
Low PEG Cor-gel-50 Low PEG Cor-gel-200 Gel powder Low PEG Cor -gel CTAB
Table 3.7: Effect of ratio mol TTIP:H 2 O to the catalyst mass coated
Sample Ratio TTIP:H2O Initial cordierite mass, g
Below are the results of running the reaction of two samples Low PEG -gel-50 and Low Cor-gel-200 with 0.4 g of Powder-gel sample at a concentration of methyl orange of 30 ppm:
Fig 3.27: Investigate the efficiency of catalyst thin films by dip coating with low concentration of PEG (Condition: C MO ppm, catalsyt dosage =0.05g, solution volume Pml, pH=7; dark adsorption in 30 minutes,Full range light with Xenon 300 W lamp)
The amount of catalyst on low PEG Corgel 200 and low PEG Corgel 50 is approximately the same Figure 3.27 is the result of running the reaction of two samples Low PEG Cor-gel-50 and Low PEG Cor-gel-200 and Powder-gel sample (the initial methyl orange concentration is 20 ppm) Looking at the graph, we can see that the photocatalytic performance of the two cordierite-coated catalyst samples is quite similar, but compared to the powder catalyst, the efficiency is still poor It proves that when the amount of catalyst attached to cordierite is close together, the ability to perform photochemical reaction of the two catalyst samples coated on cordierite is equivalent, while the powder catalyst has a higher conversion due to the amount of the catalyst used is much larger and the powder form is more evenly dispersed in solution.
As for the catalyst carried on cordierite, UV rays cannot reach the entire surface of the catalyst film, so the efficiency of the reaction is not as high as when using a powder catalyst The ratio mol of TTIP: H 2 O has a impact to the photocatalytic efficiency in catalyst mass coated on cordierite surface.
When the solution is not concentrated for Low PEG Cor-gel-350 then the adhesion is reduced then the catalyst mass less then Low PEG Cor-gel-50 and Low PEG Cor-gel-
200 For Low PEG Cor-gel- CTAB the performance is not improved much compared to Low PEG Cor -gel 200, one reason is the CTAB powder is not high and the PEG amount is small That is why the PEG amount is increased in the next experiments in this research.
Dip coating with high concentration of PEG
In the precipitation method, the crystal sizes of Cor-CTAB catalysts are bigger at
20 – 100 nm and are not smooth compared to that of Corgel 200 with a crystal size of only 20 nm One reason is that the cordierite is dip-coated in a better-distributed solution with crystal nano-size which is different to that of dipped in paste form containing surfactants P123 or CTAB in precipitation and hydrothermal method resulting to crystal size which is big and not smoothly distributed on the surface of cordierite.
Based on the SEM results, it seems that the TiO2 nanoparticles are well separated and dispersed and is also observed as free TiO2 nanoparticles Meanwhile, TiO2 nanoparticles were also determined in the EDX results as shown in Fig 3.28.
Fig 3.28: SEM characterization: (a) High PEG Cor-CTAB, (b) High PEG Corgel
200 (c) High PEG Cor-P123 and pure cordierite
Fig 3.29: SEM characterization of 2 samples Corgel-150AC (a) and Cor-P123 (b) after the first reaction
Photocatalytic testing of the synthesized samples
It is depicted from Table 3.8 that the amount of catalyst coated on cordierite surface depends not much on the amount of water in TiO 2 solution as well as the modification by activated carbon However, the samples synthesized by sol-gel method were coated with more TiO 2 on the surface of cordierite compared to other catalysts.
Table 3.8: Catalysts films coated cordierite
Weight of Catalyst coated on cordierite, g
Photocatalytic performance of samples coated on cordierite by sol-gel method
Fig 3.30 shows the photocatalytic performance results of three High PEG Corgel-
150, High PEG Corgel-150AC, High PEG Corgel-200 samples and 0.4 g of AC-gel powder sample in degrading the methyl orange solution at 20 ppm.
It is shown in Fig 3.30 that the photocatalytic capacities of the two catalysts coated with cordierite High PEG Corgel-150AC, and High PEG Corgel-200 is nearly equal compared to the powder catalyst There was a significance difference in performance between the two samples of High PEG Corgel-150 and High PEG Corgel-150AC until the 160th minute of reaction The explanation of the results would be that the High PEG Corgel-150AC sample has an activated carbon adsorption to Methyl range on the surface (see SEM image) that makes Methyl orange concentration decreased rapidly in the early reaction stage However, their respective performances are the same as absorption reached its saturation point.
High PEG Corgel-150 High PEG Corgel-150AC Powder-gel High PEG Corgel-200
Fig 3.30: The photocatalytic degradation of four samples High PEG Corgel-150,
High PEG Corgel-150AC, High PEG Corgel-200 and AC-gel powder
(Condition: CMO ppm, solution volume 0ml, dark adsorption in 20 minutes,
Fig 3.31: Surface of High PEG Corgel-150 (left) and High PEG Corgel-150AC
It is apparent that the color of methyl orange on the surface of the High PEG
Corgel-150 sample is much lighter than that of the High PEG Corgel-150AC, indicating
D eg ra da tio n that the presence of AC increased the ability to adsorb methyl orange onto the surface of sample [191].
However, samples synthesized by solgel method without activated carbon also adsorbed methyl orange but there is a difference observed in performance of samples P123 and CTAB The results also showed that at small and uniform particle size (size 20nm), TiO2 films have better methyl orange adsorption capacity than TiO2 films with large and uneven particle size (from 20-100nm) distribution.
Fig.3.32: (a) Surface of High PEG Corgel 150AC (left) and High PEG Corgel-200 (right);(b)Inside view of High PEG Corgel-150AC (left) và High PEG Corgel-200 ( right)
Comparing High PEG Corgel-150AC and High PEG Corgel-200 samples, the methyl orange treatment by Corgel-200 is slightly better, in spite that the amount of catalyst coated is less Further, the methyl orange adsorption surface is rather equal It could be explained by the lower amount of catalysts coated on High PEG Corgel-200 sample, the insufficient covered film on the cordierite tablet surface, causing Methyl Orange inside cordierite samples covered insufficiently so the catalyst find it difficult to regenerate The photocatalytic performance of sample High PEG Corgel-150AC is still better than that of the High PEG Corgel-200 It is suggested that the molar ratio
TTIP:H2O = 1:150 showed a better performance while the High PEG Corgel-150AC sample is the best.
Although the mass of catalyst mass in AC-Gel powder is three (3) times to that of High PEG Corgel-200 and High PEG Corgel-150AC samples, the performance of the Corgel samples just slightly lower than the powder sample The result indicated the effectiveness of coating TiO2 film on cordierite by Sol-gel method.
Photocatalytic performance of samples coated on cordierite by hydrothermal method
Fig 3.33 shows the photocatalytic degradation of Methyl Orange in 0.087 g of TiO2 coated on cordierite compared with 0.1 g catalyst powder synthesized by hydrothermal method.
Fig 3.33: Photocatalytic performance of-P123 and High PEG Cor-P123 samples (Condition: CMO ppm, solution volume 0ml, dark adsorption in 20 minutes, UVC 254 nm with 100 W lamp)
Although the amount of catalysts coated on cordierite is nearly equal to the amount of catalyst powder, the catalyst powder has a greater conversion rate than that of catalyst coated on cordierite This can be ascribed to the fact that the catalyst powder is stirred
Hig PEG Cor-CTAB Powder-CTAB in the solution, so UV light has easily passed through the catalyst in order to stimulate the reaction to occur [192].