Necessity of the study
Soil and groundwater pollution pose significant challenges in our country, primarily due to unregulated economic development This growth leads to the contamination of water sources with heavy metals and toxic organic compounds, including phenol and its derivatives Key contributors to phenol pollution include the production of synthetic plastics, insecticides, paints, and petroleum products.
The textile industry is a major source of harmful chemical emissions, particularly azo-based dyes like methyl orange Consequently, addressing the contamination caused by phenol and methyl orange has become a critical issue both nationally and internationally.
Historically, water pollution remediation has relied on physicochemical and biological treatment methods, with adsorption being a popular choice due to its simplicity and versatility in using various adsorbents Biological treatment can effectively remove up to 90% of organic matter, but struggles with challenging compounds like phenol and methyl orange Extensive research has explored alternative processing techniques for these substances, including electrochemical methods, ion exchange, ozone treatment, and activated carbon adsorption.
Despite their potential benefits, these approaches are seldom implemented in practice due to several constraints, such as the need for heavy equipment, intricate operational techniques, and significant initial and ongoing costs Additionally, without a sludge post-treatment step, the efficiency of these methods remains subpar, leading to unsatisfactory outcomes.
Photocatalysts offer an eco-friendly solution for treating polluted water by harnessing natural solar energy to effectively degrade challenging organic contaminants without the need for additional chemicals or sludge accumulation Among various semiconductor materials, titanium dioxide (TiO2) stands out as the most extensively researched photocatalyst due to its exceptional properties, including environmental safety, chemical and physiological inertness, self-cleaning capabilities, and minimal byproduct generation during production Other semiconductor materials like ZnO and Fe2O3 are also being explored for their potential as photocatalysts.
TiO2 nanoparticles are a leading photocatalyst for the photodegradation of organic pollutants due to their low cost, stability, abundance, and strong oxidizing capabilities However, their effectiveness is hindered by a large band gap of 3.2 eV, which restricts their activation to UV light and complicates catalyst separation and electron-hole recombination, leading to reduced photocatalytic efficiency Despite these limitations, TiO2 can effectively decompose various organic, inorganic, and toxic compounds in both liquid and gas phases To enhance its photocatalytic performance, various methods have been explored, including non-metal and transition metal doping, dye sensitization, spatial structuring, and rare earth metal doping, as well as the integration of microwave or ultrasonic radiation in photoreaction systems Additionally, using activated carbon as a support may help address the shortcomings of TiO2.
The recent discovery highlights the promising synergy between photocatalytic activity and the adsorptivity of activated carbon, making it a powerful solution for organic pollutant removal from liquids While commercial activated carbon is recognized for its effectiveness as an adsorbent, its high cost limits widespread use However, activated carbon can be produced from agricultural and industrial waste, offering a sustainable alternative that reduces environmental waste disposal Additionally, one of the main challenges in utilizing TiO2 as a catalyst is the difficulty in separating it from high-concentration effluents, which can lead to coagulation and aggregation issues.
The combination of activated carbon and TiO2 offers several advantages, including high porosity, significant surface area, and stability at room temperature, making it an effective photocatalytic solution Unlike other materials such as clays, zeolites, and silica, which have shown limited effectiveness, the synergistic effect of activated carbon and TiO2 enhances photocatalytic efficacy However, interactions between TiO2 and specific pollutants can lead to coagulation, reducing UV or solar radiation exposure to the catalyst's active core and diminishing its photocatalytic activity Activated carbon serves as an efficient adsorption trap for organic pollutants, facilitating their transfer to the catalyst's surface where photoreactions occur, ultimately resulting in improved pollutant elimination due to enhanced substrate adsorption on activated carbon.
To enhance the TiO2 catalyst's ability to absorb visible light, which constitutes about 45% of solar energy, it is essential to incorporate metal or nonmetallic modifications into its structure This approach is crucial, given that solar energy is a renewable and inexhaustible resource Recently, researchers have started utilizing graphene oxide to improve the performance of TiO2 photocatalysts, leveraging its numerous advantages in boosting catalytic efficiency under visible light conditions.
Objectives of the study
This study aims to develop TiO2 catalysts modified with activated carbon and graphene oxide, applied to various substrates for the degradation of organic pollutants in wastewater The focus is on methyl orange and phenol, two harmful substances commonly found in textile and other industrial processes in Vietnam and globally.
This study aims to explore the parameters involved in catalyst synthesis, identify the optimal catalysts for each synthesis method, and develop thin films of catalysts on various substrates to effectively degrade methyl orange Additionally, the research focuses on modifying catalysts to enhance their performance under a full spectrum of light conditions.
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 using sol-gel, co-precipitation, and hydrothermal methods Following synthesis, the catalysts were modified with activated carbon, graphene oxide, and silica gel The characterization of these catalysts was conducted through physical adsorption analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-Vis spectroscopy.
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
This literature review consolidates data from previous research on catalyst composites involving activated carbon and graphene oxide, focusing on their preparation and coating methods, as well as their effectiveness in the photodegradation of methyl orange and phenol The catalysts were synthesized using sol-gel, co-precipitation, and hydrothermal methods, followed by characterization through techniques such as BET physical adsorption, SEM, and XRD The photodegradation performance of these catalysts was assessed in specific reactor systems, employing UV-Vis and HPLC methods for evaluation.
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.
Scientific and practical meanings
This thesis offers a scientific foundation for synthesizing the photocatalyst TiO2 to effectively degrade challenging compounds like methyl orange and phenol in laboratory conditions Given that methyl orange and phenol are widely recognized as difficult-to-degrade aromatic pollutants, developing a catalyst with proven efficiency for their degradation suggests its potential effectiveness in photodegrading other environmental contaminants.
Thin films of catalysts were synthesized using dip coating and chemical vapor deposition (CVD) methods The study focused on optimizing the parameters and techniques for creating these thin films, with the potential application of treating industrial wastewater.
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 is structured into four key chapters The first chapter reviews existing literature on methyl orange (MO) and phenol contamination, along with methods to enhance titanium dioxide (TiO2) for improved photocatalytic degradation of these pollutants The second chapter details the synthesis methods for various catalysts, explaining the fundamental principles of the physico-chemical techniques employed and the experimental setup used in the research The third chapter evaluates the properties of the synthesized catalysts and examines how different synthesis methods affect their catalytic performance in the photodegradation of methyl orange and 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
Vietnam's textiles and garment industry plays a vital role in the country's economy, employing over 3 million people and operating more than 7,000 factories nationwide Given its significant reliance on water resources and the generation of wastewater, it is essential for industry stakeholders to comprehend the water-related challenges they face, the impacts of these issues, and the potential solutions available to address them.
The textile and dye industry generates significant wastewater during various processes, including sizing, cooking, bleaching, dyeing, and finishing, with the washing procedure after each cycle being the primary contributor Water usage in textile manufacturing is notably high, varying by product, with specialists estimating that 72.3% of total water consumption occurs during production, predominantly during dyeing and finishing Approximately 12 to 65 liters of water are needed to produce one meter of fabric, resulting in wastewater discharge of 10 to 40 liters Water pollution remains the most pressing environmental challenge for the textile sector, with the dyeing industry identified as the most polluting, based on both the volume of wastewater produced and the harmful pollutants it contains.
Textile dyeing wastewater is primarily contaminated by persistent organic chemicals, dyes, surfactants, organic halogen compounds, and neutral salts, which increase total solids content and elevate temperature The high alkalinity results in elevated pH levels, with azo dyes being particularly problematic, constituting 60-70% of the dye industry During dyeing, a significant portion of the dye pigments fails to bond with the fabric, leaving up to 50% of the original dye in the wastewater Consequently, the effluent is characterized by its intense color and high concentration of pollutants.
Methyl orange (MO) is an anionic azo dye widely utilized in textiles, printing, pharmaceuticals, food, and leather industries However, it contributes significantly to environmental pollution and poses health risks, including cancer and genetic mutations This water-soluble dye exhibits remarkable stability and distinct color properties, appearing orange in basic conditions and red in acidic environments The reductive cleavage of the azo bond (N=N) by the azo reductase enzyme in the liver produces aromatic amines, which may increase the risk of intestinal cancer in humans.
Fig 1.1: Chemical structure of MO molecule [33,34]
Methyl orange (C14H14N3SO3Na) is a model pollutant widely used in the industrial sector as a stable azo-dye, which constitutes up to 70% of today's dyes Approximately 10 to 15 percent of the dye used in textile production is wasted and released as effluent, contributing to "non-aesthetic pollution" with concentrations often below 1 part per million in water sources This dye wastewater can generate harmful byproducts through chemical processes like oxidation and hydrolysis Despite the primary concern being the degradation of methyl orange, its high stability due to significant aromatic content poses challenges for biological treatments, which may only alter the color of the effluents without effectively degrading them.
Phenol in industry and its impact to the health
Phenol (C6H6OH), first discovered in 1834 during coal distillation, was originally known as carbolic acid, as coal was the main source for its synthesis until the rise of the petrochemical industry Today, various chemical processes are used to produce phenol, with many steel plants releasing wastewater that contains phenolic compounds In its pure form, phenol appears as colorless or white solid crystals that can persist in the air Partial oxidation can give phenol a pink hue, and it decomposes upon contact with water vapor The odor of phenol becomes detectable at concentrations as low as 0.04 ppm, characterized as mildly pungent Phenol is crucial in industry, serving as a raw material for the production of plastics, chemical silk, agricultural pharmaceuticals, antiseptics, fungicides, pharmaceuticals, dyes, and explosives, and is a key ingredient in many plastic manufacturing processes.
Phenol can enter the human body through inhalation, skin contact, and mucous membranes, leading to severe health risks, including convulsions, coma, and respiratory issues when ingested It is highly toxic, primarily affecting the liver and heart, and prolonged exposure can result in muscle pain and liver enlargement Skin contact with phenol can cause burns and irregular heartbeats The legal limit for phenol in the human body is set at 0.6 milligrams per kilogram of body weight While there is limited research on the effects of low concentrations of phenol, experts suggest that chronic exposure may hinder growth, cause genetic abnormalities, and increase premature birth rates in pregnant women.
Phenolic compounds are widely utilized in the manufacturing industry but pose significant toxicity risks to both humans and the environment if not properly managed The discharge of wastewater containing high levels of phenol from the Formosa - Ha Tinh factory in Vietnam has resulted in severe ecological damage, including the death of numerous fish in the coastal provinces This highlights the urgent need for effective treatment measures to address phenol contamination Given the persistence of phenol and its derivatives in wastewater, their degradation is crucial not only in Vietnam but globally.
Titanium Dioxide (TiO2), also known as titania, is a versatile compound recognized for its stable chemical structure, biocompatibility, and unique physical, optical, and electrical properties This multipurpose substance is widely utilized in products such as paint pigments, sunscreen, electrochemical electrodes, capacitors, solar cells, and even as a food coloring agent in toothpaste Over the past two decades, TiO2 has been developed primarily to eliminate harmful chemical pollutants from the environment, particularly in air and water Its photocatalytic properties, activated by sunlight, enable TiO2 to effectively break down and remove hazardous chemical substances, including volatile organic compounds, thereby contributing to cleaner air.
Titanium dioxide (TiO2) exhibits three crystal structures: the stable Rutile (tetragonal) phase and two meta-stable phases, Anatase (tetragonal) and Brookite (orthorhombic), which can transform into Rutile under specific temperature conditions Rutile has a higher recombination rate of surplus charged carriers compared to Anatase, making it more suitable for paint formulations despite Anatase's greater efficiency in charge transfer The Anatase phase, with a band gap of 3.2 eV, is recognized as TiO2's most active crystal structure due to its favorable energy band positions and high surface area In contrast, Rutile, with a band gap of 3.0 eV, is predominantly used in the pigment industry, while their corresponding wavelengths are 388 nm for Anatase and 410 nm for Rutile.
Titanium dioxide (TiO2) acts as a semiconductor that can be photo-activated, generating a redox environment capable of degrading both organic and inorganic pollutants The photocatalytic reaction process on irradiated TiO2 involves several key steps, as outlined in Table 1.1.
The photodegradation of pollutants using TiO2 is initiated when UV radiation is absorbed by TiO2 particles, which have a band gap of 3.2 eV for Anatase and 3.0 eV for Rutile This absorption generates electron-hole pairs in the semiconductor's valence and conduction bands While both Anatase and Rutile TiO2 can absorb UV light, Rutile can also capture radiation closer to the visible spectrum However, Anatase exhibits superior photocatalytic activity due to its conduction band positioning, which enhances its reducing power The energy absorbed, along with the kinetic energy from recombining holes and electrons, facilitates redox processes Electron donors and acceptors, either adsorbed on the semiconductor surface or within the surrounding double layer, engage in these redox reactions The mechanism of dye photodegradation by TiO2 under UV light is detailed by Koustantinou [49].
TiO 2 + hv(UV) → TiO 2 (e CB − + h VB +) (1)
TiO 2 (h VB +) + OH− → TiO 2 + OH• (3) TiO 2 (e CB −) + O 2 → TiO 2 + O2•− (4)
O 2 •− + H+ → HO 2 • (5) Dye + OH• → degradation products (6) Dye + h VB + → oxidation products (7) Dye + e CB − → reduction products (8)
Fig 1.3: The mechanism of photocatalytic activity of TiO 2 [50]
The junction between the semiconductor and liquid generates an electrical field that separates energized hole-electron pairs, preventing their recombination This separation allows holes to migrate to the illuminated areas of TiO2, while electrons move to the unlit regions The failure to recombine leads to the formation of highly reactive, short-lived hydroxyl radicals (OH-), marking the initial step in photocatalytic degradation This process is widely accepted, with OH- formation occurring either through a highly hydroxylated semiconductor surface or the direct oxidation of pollutant molecules under UV radiation, potentially happening simultaneously in some cases This occurs following the reduction of adsorbed oxygen species, which can originate from dissolved oxygen in the aqueous system or other available electron acceptors.
This research investigates how free radicals generated by photocatalytic activity can target organic contaminants in polluted water, specifically using model chemicals like MO and phenol to assess the effectiveness of TiO2 produced via sol-gel and other methods TiO2 can be utilized in photocatalytic processes either as a suspended particle in aqueous solutions or immobilized on various support materials such as quartz sand, glass, activated carbon, zeolites, and noble metals Different reactor designs, including fluidized bed and fixed bed reactors, can be employed for these processes Notably, a study by Matthews and McEvoy in 1992 found that photocatalytic reactors utilizing immobilized photocatalysts exhibited lower efficiency compared to those with dispersed titania particles.
The lower efficiencies observed with immobilized photocatalysts can be attributed to two main factors: a reduced number of activated sites in the photoactivated volume compared to freely suspended catalysts, and mass transfer limitations that may hinder reaction rates at low flow rates This issue is particularly significant under intense illumination, where mass transport may fail to match the reaction rate at the photoactivated surface, resulting in mass-transfer limitations Consequently, increased photon intensity may not lead to a noticeable acceleration in the reaction process.
Akpan and Hameed investigated how operational settings affect the photocatalytic degradation of textile dyes using TiO2-based photocatalysts Their research highlights the diverse processes involved in producing TiO2 photocatalysts, with the Sol Gel process being particularly favored for its ability to create high-purity, nanometer-sized crystalline TiO2 powder at low temperatures.
1.4 Principles of Precipitation, sol-gel and hydrothermal synthesis methods
The process begins with thoroughly dissolving precursors in water to create a homogeneous solution, followed by the addition of a precipitant to form solid precipitates The solids are then extracted, washed to eliminate contaminants, dried in an oven, and calcined at high temperatures to produce the final materials This method allows for atomic-level diffusion of reactants, enhancing their interaction capacity However, a significant drawback of this technique is the inability to consistently achieve the desired elemental ratios in the final product.
The sol-gel method has gained popularity in recent years for producing catalytic supports, beginning with the hydrolysis of metal alkoxides in an organic medium This process is followed by the polymerization of hydrolyzed alkoxides through the condensation of hydroxyl and alkoxy groups, leading to the formation of a solid gel as polymerization and cross-linking increase The gel's porosity, surface area, pore volume, pore size distribution, and thermal stability after calcination are significantly influenced by the size and branching degree of the inorganic polymer, along with the extent of cross-linking Gels with highly branched and cross-linked polymeric chains tend to have extensive voids and a rigid structure, resulting in macropores and mesopores in the final oxide Conversely, gels with minimal branching and cross-linking exhibit fewer voids and weaker structures, often leading to collapse during calcination and producing oxides with predominantly micropores and reduced surface area Nonetheless, the sol-gel process can effectively produce nanomaterials by combining reactants at the atomic level.
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
The use of liquid solutions instead of traditional methods for combining raw components allows for rapid homogenization at the molecular level, thanks to the low viscosity of the liquids involved.
The thorough mixing of precursors in the solutions ensures that they will be uniformly distributed at the molecular level during gel formation Consequently, this uniformity facilitates straightforward chemical reactions that can occur at lower temperatures when the gel is subsequently heated.
(4) It is possible to change the physical features of the material, such as the pore 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
Principles of Precipitation, sol -gel and hydrothermal synthesis methods 12 1 Preparation of photocatalyst using sol -gel method
The process begins by thoroughly dissolving precursors in water to create a homogeneous solution, followed by the addition of a precipitant to form solid precipitates The resulting solids are then extracted, washed to eliminate contaminants, dried in an oven, and calcined at high temperatures to produce the final materials This method allows for atomic-level diffusion of reactants, enhancing their interaction However, a significant drawback is the inability to ensure the desired elemental ratio in the final product.
The sol-gel method has gained popularity in recent years for producing catalytic supports This process begins with the hydrolysis of a metal alkoxide through the addition of water, followed by the polymerization of hydrolyzed alkoxides via the condensation of hydroxyl and alkoxy groups As polymerization and cross-linking increase, the solution transforms into a solid gel The gel's porosity, surface area, pore volume, pore size distribution, and thermal stability after calcination are significantly influenced by the size and branching of the inorganic polymer and the extent of cross-linking Gels with high branching and cross-linking tend to have extensive voids, resulting in stiff structures that yield macropores and mesopores upon calcination Conversely, gels with minimal branching and cross-linking exhibit fewer voids, leading to weaker structures that collapse easily during calcination, producing oxides primarily composed of micropores with reduced surface areas Nevertheless, the sol-gel process allows for the synthesis of nano-materials by combining reactants at the atomic level.
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
The use of liquid solutions in lieu of traditional methods for combining raw components allows for a rapid homogenization process Due to the low viscosity of the liquids involved, this process can be completed efficiently and effectively at the molecular level.
Well-mixed precursors in the solutions are anticipated to achieve uniform molecular distribution during gel formation, leading to simplified chemical reactions that occur at lower temperatures when the gel is subsequently heated.
(4) It is possible to change the physical features of the material, such as the pore 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 hydrothermal processing method offers a unique approach to producing nanocrystalline inorganic materials This technique allows for precise tuning of the synthesis of various materials through a direct correlation between precursors and products, eliminating the need for additional structure-guiding agents.
In a controlled hydrothermal environment, precursor substances are consistently dissolved in the fluid at optimal temperatures of approximately 300 degrees Celsius and pressures around one kilobar, facilitating synthesis Notably, even with alumosilicate materials, gel production is absent throughout the process, as larger molecular units undergo hydrolysis due to the elevated temperature and pressure conditions.
In aqueous solutions under autogeneous pressure below the critical point, various dissolution states can coexist, including basic structural units and colloidal forms This phenomenon occurs because the critical point signifies where autogeneous pressure becomes critical During high-pressure hydrothermal synthesis, larger, unstable macromolecular units are first broken down through chemical reactions These units may appear as colloidal solutions, precipitated colloids (such as crystalline, partially crystalline, gel, glassy, or amorphous forms), or solid-state precursors Consequently, it is anticipated that a true solution will form, allowing the smallest structural building blocks and their associated cation hydration spheres to be effectively transported.
1.4.1 Preparation of photocatalyst using sol-gel method
Titanium dioxide (TiO2) can be produced in various forms, including powder, crystals, and thin films, offering versatility for multiple applications Nanosized crystallites, which range from a few nanometers to several micrometers, can aggregate into larger structures, but distinct nanoparticles can be created using advanced techniques that eliminate the need for additional deagglomeration The production of nano-TiO2 typically involves hydrolysis of titanium precursors followed by methods such as annealing, flame synthesis, hydrothermal, and sol-gel processes The sol-gel method is particularly favored due to its ability to synthesize nanoparticles under ambient conditions, simplicity of setup, and advantages in purity, homogeneity, and stoichiometry control, making it easier to introduce dopants in large concentrations.
The synthesis of solid materials in a liquid medium often occurs at low temperatures, involving particles in a dispersed state within a colloidal suspension (Sol) These colloidal particles aggregate to form a three-dimensional open grid structure known as a Gel Common molecular precursors for this process include metallo-organic compounds like alkoxides M(OR)n, where M represents metals such as Si or Ti, and R denotes an alkyl group (e.g., CH3 or C2H5) For instance, Ti(iOC3H7)4 is utilized in the manufacturing process of titanium dioxide (TiO2).
The Sol-Gel synthesis of TiO2-based products involves the reaction of Ti(OCH(CH3)2)4 with water, leading to the formation of TiO2 and (CH3)2CHOH This process typically starts with the addition of water to a dissolved alkoxide in alcohol, where factors like the presence of additives (e.g., acetic acid), water quantity, and mixing speed influence the final inorganic characteristics Notably, titanium (IV) is essential for the Sharpless epoxidation process, which produces chiral epoxides using isoproxide During Sol-Gel synthesis, a water-soluble precursor undergoes hydrolysis, creating a colloidal particle dispersion (the sol), which evolves into a gel through particle bonding Heating the gel subsequently yields the desired material This synthesis method offers significant advantages, including the ability to produce high-purity materials at lower temperatures compared to traditional synthetic methods.
Homogeneous multi-component systems can be created by combining precursor solutions, facilitating chemical doping of the prepared materials The rheological properties of sol and gel are crucial for various processing techniques, such as dip coating thin films and spinning fibers By integrating Sol-Gel synthesis principles with template fabrication for nanomaterials, a novel method for producing semiconductor nanostructures and other inorganic materials is established This synthesis can occur within the pores of microporous and nanoporous membranes, yielding monodisperse tubules and fibrils of the desired material In the sol-gel synthesis of nano TiO2, a high water ratio is maintained to enhance the nucleophilic attack on titanium (IV) isopropoxide, preventing rapid condensation and facilitating the formation of TiO2 nanocrystals.
Xu presented a novel approach to the synthesis of titanium dioxide in the year 1991
This study utilized a sol-gel technique, cellulose membrane, and heat peptization to effectively separate by-products from alcohol solvents The results indicated that sols subjected to thermal peptization exhibited significantly denser aggregation compared to those that were dialyzed Dialysis facilitates the gradual removal of protons from highly charged particles, leading to their aggregation This process highlights the key qualitative feature of aggregation and deposition, where charges are effectively screened by electrolyte ions and diffusion-driven forces Notably, TiO2 with fewer particles demonstrated a greater surface area compared to its densely packed counterpart.
Sol-gel processing is a widely utilized method for producing photocatalyst TiO2 in both coatings and powder forms, enabling the generation of nanoparticles at room temperature and normal atmospheric pressure without the need for complex equipment This straightforward technique allows for effective synthesis, as demonstrated by Venkatchalam's research, which examined the effects of hydrolyzing agents and water quantity when using Titanium (IV) Isopropoxide as a precursor The study found that employing acetic acid as the hydrolyzing agent led to smaller particle sizes of TiO2, facilitating the rapid synthesis of Titanium Hydroxide and its subsequent condensation into TiO2 nanoparticles.
Fig 1.4: Nanocrystalline Metal Oxide Preparation using Sol-Gel method
Excessive acetate anion adsorption on TiO2 surfaces can hinder TiO2 development, potentially reducing crystallite size during gel synthesis Utilizing a high water ratio enhances nucleophilic attacks on titanium (IV) isopropoxide and slows its condensation, facilitating the production of TiO2 nanocrystals However, residual alkoxy groups may significantly delay TiO2 crystallization, promoting the exclusive formation of the less dense anatase phase Low hydrolysis rates, coupled with excess titanium alkoxide, encourage Ti–O Ti chain formation through alcoxolation, resulting in tightly packed three-dimensional polymeric skeletons and a higher ratio of the rutile phase, as each titanium atom coordinates with four oxygen atoms.
Calcination is crucial for removing organic molecules and completing the crystallization process in catalyst synthesis, significantly influencing the physicochemical properties of the final product Research indicates that the calcination temperature of TiO2 films directly affects their photocatalytic activity Specifically, studies by Porkodi and Arokiamary revealed that calcining TiO2 at temperatures below 520 degrees Celsius leads to the formation of anatase, while temperatures above this threshold result in a phase transition to rutile Additionally, the calcination temperature impacts the films' mechanical stability, making it essential to understand the relationship between calcination temperature, photocatalytic performance, and mechanical resilience when designing nano-photocatalysts, which must endure high temperatures.
Support and thin films
Cordierite is a ceramic material composed of three key components: magnesium oxide (MgO), aluminum oxide (Al2O3), and silicon dioxide (SiO2) Its chemical formula is 2MgO·2Al2O3·5SiO2, featuring a composition of 13.78% MgO, 34.86% Al2O3, and 51.36% SiO2 Due to its specific composition, cordierite is classified within the ceramic group that has a high content of mullite (3Al2O3·2SiO2).
Cordierite crystals possess beneficial properties such as low thermal expansion and minimal thermal loss, making cordierite ceramic ideal for applications in industries facing rapid temperature changes, including motor filter manufacturing, catalyst carriers, and arc welding linings Despite its advantages, cordierite ceramic is challenging to work with due to its narrow firing temperature range The calcination process is influenced by factors like maximum temperature, heating rate, retention time, particle size, composition, and the presence of impurities, complicating the equilibration process Typically, preparation involves heating to the required temperature, tailored to specific usage needs High mechanical strength, porosity, and low water absorption characterize the resulting materials, which generally require heating to at least 0.8T (where T is the melting temperature), exceeding 1200°C for optimal solidification.
C for the intended result, which is cordierite [68]
1.5.2 Mesoporous TiO 2 and coating techniques
Porous materials are crucial for modern civilization, finding applications in areas such as catalysis, adsorbents, optics, sensors, insulating lacquers, and ultralow-density materials Their widespread use in catalysis significantly influences the global economy by enabling chemical reactions to occur under lower energy conditions A notable example is the petroleum refining process, where various microporous zeolites are essential for catalytic cracking reactions.
Microporous materials, such as zeolites, have limited pore sizes that restrict their use in high-demand applications, including oil refining, highlighting a major limitation of these materials.
Stucky and colleagues [73] have documented the synthesis of large-pored, mesoporous metal oxide powders and films utilizing P123 Their research incorporates metal chloride salts as inorganic starting materials.
To delay the crystallization of titanium, a non-hydrolytic method that breaks carbon-oxygen bonds has been developed, which is crucial for the controlled creation of mesostructures.
Sanchez and colleagues conducted an in-depth study on the role of water in the process using TBT, TET, or TPT as precursors They found that condensation does not occur before the mesostructured hybrid stage due to low condensation rates at low water content Conversely, significant amounts of water facilitate condensation reactions that lead to the formation of oxo clusters prior to hybrid processes However, excessive water addition resulted in gels that lacked periodicity.
The synthesis of mesoporous titanium using the EISA solution involved CTAB and TET, with NBB self-assembling around micelles in a process termed "titaniatropic." This assembly includes interactions between Ti – OH and CTAB bromide anions or HCl ions Yan and his research team found that varying solvents and co-solvents could yield different titanium phases By using TiCl4, P123, or F127 and changing solvents from methanol to ethanol, 1-butanol, and 1-octanol, they produced a mix of anatase, rutile, and pure rutile The study highlights how the retention of chlorine atoms in alcohol moieties increases with longer carbon chains, leading to greater obstruction Typically, anatase forms under low acidity, while rutile is produced in high acidity conditions.
A well-ordered TiO2 mesostructure was synthesized using TiCl4 and TBT in conjunction with P123, resulting in pore walls composed of both rutiles and anatases This mesoporous material boasts a surface area of 244 m² per gram, achieved at a P123/TBT mole ratio of 0.2 Additionally, citric acid was incorporated into the mixture with TPT and F127 to enhance the hydrophilicity of the titanium nanoparticles, promoting stronger binding to the F127 ethylene oxide units The introduction of ethylenediamine led to the formation of a thermally stable mesoporous anatase structure Calcination at temperatures reaching 700 degrees Celsius facilitated the bonding of ethylenediamine molecules to the titanium nanoparticle surface, effectively preventing pore collapse and inhibiting the conversion of anatase to rutiles.
TiO2 catalyst powder, commonly suspended in water for various applications, presents challenges due to the high costs and time involved in recovering the catalyst While the wide surface area of suspended TiO2 enhances its effectiveness in reactions, it also obstructs ultraviolet light, reducing catalytic efficiency To address these issues, researchers are exploring the immobilization of catalyst particles onto substrates, a method increasingly vital for photocatalytic treatment of organic pollutants This approach represents one of several potential solutions to improve catalyst recovery and performance.
The immobilized form of TiO2 photocatalyst is cost-effective, highly stable, and resistant to photocorrosion, making it an ideal choice for large-scale wastewater treatment Recent advancements have enabled various methods for immobilizing TiO2 on substrates, including anodization, sol-gel processes, reactive sputtering, chemical vapor deposition, electrostatic sol-spray deposition, and aerosol pyrolysis When choosing a catalyst immobilization strategy, factors such as the substrate type, pollutant nature, and environmental conditions must be considered While loading TiO2 onto a support can enhance photocatalytic properties, it may also lead to a reduction in the support's surface area Techniques like spraying, electrophoresis, inkjet printing, dip-coating, and spin coating are commonly employed in the sol-gel process for effective sol deposition on substrates.
Spin coating effectively produces a uniform coating, with the deposition of TiO2 particles under vacuum resulting in a hard finish and complete elimination of residual air While TiO2 can be homogeneously applied to AC through vacuum rotation, dip coating offers significant advantages due to its use of simple equipment Various substrates, including glass, silicon, stainless steel, and titanium, have been utilized in this process The incorporation of binders into the TiO2 suspension, along with post-deposition annealing, enhances adhesion However, the direct creation method typically yields crystals of inferior quality compared to the binding approach.
TiO 2 /AC Materials
In 1972, Fujishima and Honda discovered that water can be photocatalytically split using TiO2 electrodes, marking a significant advancement in heterogeneous photocatalysis Recent studies have highlighted TiO2's potential as a photocatalyst for degrading organic pollutants, which pose risks to environmental and human health Ongoing research focuses on optimizing the transformation of these contaminants into less harmful substances by controlling factors such as calcination temperatures, pH, and aging times Additionally, incorporating supporting materials containing titania can enhance photocatalytic efficiency Key criteria for selecting an appropriate catalyst load include ensuring the composite material is transparent or allows UV radiation to pass through, is chemically inert towards pollutants, adheres well to TiO2 without hindering its reactivity, possesses a large surface area with high adsorption affinity for contaminants, and enables easy recovery and reuse of photocatalysts.
Activated carbon is commonly combined with TiO2 to create composite catalysts due to its exceptional adsorptive properties With a high surface area and a favorable microporous structure, activated carbon enhances adsorption capabilities, making it a preferred adsorbent in various industrial applications Its ability to modify surface chemistry and porous structure during preparation further distinguishes it from other oxide-based adsorbents Activated carbons are widely used for diverse purposes, including purification, decolorization, deodorization, dechlorination, detoxification, filtering, and the separation and concentration of materials for recovery.
Combining Titania with activated carbon enhances its effectiveness as an adsorbent for removing water contaminants During photocatalysis, activated carbon facilitates the diffusion and breakdown of pollutants into water and carbon dioxide A study by Wang et al (2007) utilized titanium dioxide and activated carbon composites, created using the Sol-gel technique and calcined at various temperatures The research revealed that a composite catalyst calcined at 450°C exhibited superior properties compared to those processed at lower or higher temperatures Thermogravimetric analysis indicated that the carbon content remained stable, preserving the composite's surface area, while higher calcination temperatures led to carbon loss and reduced surface area due to gasification Scanning electron microscopy showed that increased temperatures strengthened interphase contact as TiO2 entered the activated carbon pores, further explaining the surface area reduction Activated carbon proved effective as an adsorbent below 450°C, particularly at 300°C, but a significant interphase reaction occurred at 450°C, enhancing photodegradation of the chromotrope 2R pollutant The composite catalyst demonstrated superior photoactivity compared to TiO2, with the 80%-TiOAC 2-450 variant achieving optimal performance across all pollutant concentrations.
In Liu and colleagues' research, activated carbon fibers (ACF) were combined with titanium dioxide (TiO2), which formed a fractured film rather than a compact coating on the ACFs This deposition and subsequent calcination did not damage the micropore structure or significantly reduce the specific surface area of the ACFs The TiO2/ACFs system effectively degraded low molecular weight organic contaminants in wastewater, with the TiO2 photocatalyst and ACFs working synergistically to inhibit the formation of intermediate species during photocatalysis High photocatalytic activity and regeneration capacity were demonstrated by the TiO2/ACFs catalyst XRD analysis revealed the presence of anatase and minor rutile, while BET analysis showed a reduction in surface area from 1065 m²/g for ACFs to 845 m²/g for TiO2/ACFs, maintaining a mesoporous structure Scanning electron micrographs confirmed consistent surface morphology across the composite materials, and the TiO2/ACFs catalyst achieved a 94% degradation efficiency of methylene blue (MB) after just a short duration.
After 40 minutes of reaction, the TiO2/ACFs catalyst achieved complete degradation of organic contaminants within three hours, outperforming both uncovered ACFs and pure TiO2 This enhanced breakdown activity is attributed to the ability of ACFs to concentrate organic pollutants in proximity to TiO2, facilitating their effective degradation.
Calcination temperatures significantly impact catalyst structure, particularly in the removal of phenol from water using TiO2-mounted activated carbon Research indicated that heat treatment between 600 and 900 degrees Celsius affected the composite material, prepared through hydrolytic precipitation The TiO2 mounting obstructed pore entrances, decreasing the activated carbon's surface area As the heat treatment increased, TiO2 particle size grew, leading to reduced efficiency due to pore clogging Optimal phenol removal was observed at 900 degrees Celsius, compared to activated carbon alone However, composites calcined above 700 degrees Celsius, especially at 800 degrees, showed diminished photocatalytic activity due to the transformation of TiO2's crystalline structure from anatase to rutile.
In a 2007 study by Li et al., both TiO2-coated activated carbon composites and pure TiO2 were synthesized using the sol-gel method, with calcination temperatures ranging from 300 to 700 degrees Celsius Characterization techniques revealed that the AC matrix influenced the TiO2 phase transformation, crystalline growth, morphology, and surface area of the composites XRD analysis indicated that at temperatures of 300 to 500 degrees Celsius, both the composite and pure TiO2 exhibited anatase crystallites, while a phase transformation to rutile occurred at 600 and 700 degrees Celsius The composite catalyst demonstrated slower crystallite growth due to the high surface area of the AC (435 m2/g), which hindered the anatase to rutile transformation by creating anti-calcination effects BET studies showed that the surface area of the composite increased with higher calcination temperatures, in contrast to the pure TiO2, which decreased Furthermore, photocatalytic tests using methylene blue indicated that the composite catalyst significantly outperformed pure TiO2, removing nearly all of the substrate compared to the 61% removal achieved by pure TiO2, attributed to the AC's large surface area enhancing the concentration of organic compounds near TiO2.
Graphene oxide (GO)
Graphene, a single-layer carbon sheet with a two-dimensional sp2-hybridized structure, has garnered significant research interest since its discovery by Novoselov et al due to its remarkable properties, including a high specific surface area of 2630 m²/g, excellent optical transparency of 97.7%, and outstanding thermal conductivity As a fundamental component in constructing various carbon materials like C60, graphite, and carbon nanotubes (CNTs), graphene's versatility is further demonstrated by the numerous synthesis techniques developed, such as chemical vapor deposition (CVD) and the chemical reduction of graphene oxide (GO) Graphene-based films, tailored for specific thicknesses and chemical compositions, find applications across diverse fields, including fuel cells, supercapacitors, hydrogen storage, lithium-ion batteries, and solar cells However, due to its hydrophobic characteristics, the application of pure graphene in water and wastewater treatment remains limited.
Graphene oxide (GO) is a crucial derivative of graphene, produced through the chemical oxidation of natural graphite The most widely used method for synthesizing GO is the Hummers technique, which utilizes flake graphite, potassium permanganate (KMnO4), and concentrated sulfuric acid (H2SO4) GO's structure is characterized by a high density of oxygen-containing functional groups, such as hydroxyl and carboxyl groups, which impart hydrophilicity and enhance its capability as a support for inorganic nanoparticles.
Fig 1.5: Structures of graphene, C60, CNT and graphite [109]
A substantial amount of G/GO-based materials, including composites with metals, metal oxides, and polymers, has been synthesized The production of these composites can be categorized into two main synthetic processes: ex-situ hybridization and in-situ crystallization The in-situ crystallization technique encompasses various methods such as chemical reduction, electroless deposition, sol-gel, hydrothermal, electrochemical deposition, and thermal evaporation.
To enhance the electrochemical and analytical properties of pure metals, G/GO sheets have been combined with various metals, including silver, gold, platinum, palladium, nickel, and copper For example, Liu and colleagues developed a graphene-Pt composite by integrating graphene sheets with Pt nanoparticles, resulting in superior oxygen reduction activity compared to commercial catalysts due to an increased electrochemical surface area Additionally, graphene-gold composites were created through the in-situ chemical reduction of chloroauric acid, leading to the deposition of gold nanoparticles on RGO sheets These composites exhibited remarkable photodegradation capabilities for RhB dye under visible light, attributed to their high adsorption capacity for organic dyes, reduced charge recombination rates, and strong interactions with dye chromophores.
TiO 2 /GO Materials
Over the past twenty years, advanced oxidation processes (AOP), particularly photocatalysis, have gained significant interest due to their diverse applications in both environmental and non-environmental fields, including energy storage and processing Despite their potential, the limited effectiveness of photoactivated catalysts has restricted their widespread use, prompting a surge in research aimed at improving these technologies.
Graphene oxide (GO) features active sites due to the presence of functional epoxide and hydroxyl groups on its surface, complemented by carboxylic acid, quinoidal, ketone, and lactone groups around vacancies These oxygen-rich functional groups enhance GO's structural and chemical complexity through further modification, allowing for the customization of its physical and chemical properties GO is renowned for its exceptional optical and mechanical properties, making it suitable for a variety of applications However, the reduction process of GO can introduce residual defects and holes, which may impair the electrical performance of reduced graphene oxide (r-GO) Despite this, GO and its composites hold significant promise for use in energy storage, conversion, and environmental conservation.
Funded photocatalysts play a vital role in industrial catalytic technology by enhancing catalyst access to reactants, with photocatalyst-support interactions being essential for long-term efficiency TiO2 powders, known for their high surface area of 30 to 300 m²/g, effectively capture sunlight and exhibit strong photocatalytic activity However, the inherent quick recombination of electrons and holes in TiO2 limits its photocatalytic effectiveness Increasing catalyst loading can lead to minor transport limitations, while the removal of microscopic TiO2 particles from water post-remediation poses additional challenges To address these issues, adhering catalyst particles to a surface can improve mass transfer and oxidation capabilities, overcoming the limitations associated with the reduced surface-volume ratio and the degradation of light-absorbing substrates.
Photocatalysts can be effectively repaired using various materials, with graphene oxide (GO) emerging as an ideal substrate due to its specialized surface and exceptional electron mobility Efforts to immobilize TiO2 photocatalysts on support structures with a high surface-to-volume ratio aim to enhance photocatalytic oxidation efficiency, contingent upon sufficient light absorption The synthesis of GO-based material nanocomposites is crucial, employing techniques such as hydrothermal, electrochemical, in-situ polymerization, microwave-assisted methods, and sol-gel processes In these nanocomposites, GO serves as either a functional component or a support for immobilizing other elements.
Fujishima and Honda's pioneering research initiated significant scientific exploration into photocatalysis, demonstrating the activation of semiconductor materials through radiation to dissociate water Since their groundbreaking work, various photocatalysts have been extensively studied, including the role of graphene-metal oxide composites in water treatment as photocatalysts, adsorbents, and disinfectants Among these, titanium dioxide (TiO2) has emerged as the most crucial binary transition metal oxide, known for its chemical stability and insolubility in aqueous media, which facilitates easier separation after the desired reactions.
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
TiO 2 - GR 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
Recent research has explored the potential of graphene oxide (GO) as an electron acceptor in the production of TiO2 composites Key factors such as particle size, GO content, and targeted pollutants are summarized in Table 1.2 The remarkable electron transport properties of reduced graphene oxide enhance the conduction of photo-generated electrons while preventing the recombination of these electrons and holes This process boosts the generation of hydroxyl radicals and superoxide ions, which are effective in degrading dye molecules Additionally, reduced graphene oxide's superior adsorption capacity increases the likelihood of contaminants interacting with active compounds Consequently, the modification of TiO2 with GO has gained significant attention in recent studies.
MO photocatalytic degradation
Significant research has focused on the degradation of methyl orange dye, particularly using Ag+ ion-modified TiO2 as a catalyst, which proved more effective than other ions like Cu2+, Co2+, Fe3+, and Ce4+ The decolorization rate increased with pH, peaking at 8.75 before declining due to Ag+ precipitation Al-Qaradawi and Salman demonstrated that titanium dioxide could also degrade methyl orange under sunlight, with optimal degradation occurring at pH 3 and a concentration of 4 x 10^-5 M, while higher concentrations slowed the process Silver-modified TiO2 thin films showed three times the catalytic efficiency compared to unmodified versions, although excessive Ag+ concentration reduced effectiveness due to shading of the semiconductor surface Additionally, the catalytic activity remained stable over multiple trials Guettai and Amar found no degradation of methyl orange in the dark using TiO2, highlighting the importance of light in the degradation process, while also examining the effects of pH, substrate concentration, and catalyst dosage on degradation rates.
The maximum degradation rate of methyl orange (MO) was achieved at an initial dye concentration of 50 mg/L According to Chen et al [128], pelagite from the East Pacific Ocean proved to be an effective and low-cost catalyst for the complete breakdown and decolorization of methyl orange.
The catalytic breakdown of methyl orange under UV light exposure for 120 minutes has been thoroughly studied, revealing that pelagite demonstrates significant catalytic effectiveness in degrading organic molecules Research by Li et al utilized TiO2 coated activated carbon (TiO2/AC) as a catalyst in an aqueous solution under UV irradiation, confirming that the degradation of methyl orange adheres to a pseudo first-order kinetic pattern.
The presence of AC significantly enhances the photoefficiency of titanium dioxide catalysts, as noted in recent studies A modified Langmuir-Hinshelwood model can effectively describe the kinetic behavior of these catalysts Sharma et al reported the successful production of pure and nickel-doped TiO2 thin films on soda glass substrates using a sol-gel dip coating technique, which were evaluated for catalytic activity by monitoring the degradation of methyl orange in UV light Liu and Sun synthesized Fe2O3-CeO2-TiO2/-Al2O3 using various characterization techniques, revealing that this catalyst could degrade 98.09% of methyl orange and 96.08% of total organic carbon in synthetic wastewater within 2.5 hours at ambient conditions Additionally, Ma et al documented the effective degradation of methyl orange and methylene blue at lower temperatures using a CuO-MoO3-P2O5 catalyst, achieving a 99.65% removal rate for MB and 55% for methyl orange Rashed and Al-Amin explored the catalytic oxidation of methyl orange with a TiO2 catalyst under different light sources, finding that sunlight exposure resulted in faster dye degradation compared to halogen and fluorescent lights, with the degradation process following pseudo-first order kinetics.
Research has shown that the degradation of methyl orange is significantly enhanced when using Pt-TiO2–SiO2 compared to TiO2 alone, attributed to a 16 charge-transfer on the TiO2–SiO2 composites and improved electron trapping on the Pt-modified TiO2 surface Huang et al synthesized Pt-TiO2/zeolites through a sol-gel process and photo reductive deposition, achieving an 86.2% decolorization rate of methyl orange under UV light with optimal Pt doping at 1.5 weight percent Factors such as calcination temperature, catalyst concentration, H2O2 amount, and pH were found to influence catalytic activity, with satisfactory repeatability observed across five trials Sohn et al further explored the breakdown of methyl orange using TiO2 nanotubes, which exhibited superior activity when produced via ultrasonic methods and annealed in nitrogen, especially under an external bias, enhanced by the presence of oxidants like oxygen and hydrogen peroxide.
Fig 1.7: Possible mechanism of MO with TiO 2 [138]
Galindo et al identified 18 intermediates in the degradation of methyl orange, including aniline, N,N-dimethyl aniline, hydroxy anilines, and various acids Spadaro et al suggested that the oxidation of aminoazobenzene dyes begins with a hydroxyl radical adding to the carbon atom of the azo link, leading to the breakdown of the adduct This process can yield compounds like benzenesulfonic acid and N,N-dimethylaniline The presence of the electron-withdrawing sulphonate group reduces the reactivity of the ring it occupies, making the amino group-containing ring the primary target for hydroxyl radicals.
Sun et al [139] demonstrated the effective production of Sb2S3 as a novel catalyst, achieving a remarkable 97% degradation of methyl orange in aqueous solution under visible light after just 30 minutes, outperforming CdS and TiO2 The study utilized liquid chromatography and mass spectrometry to explore the underlying catalytic mechanisms In another study, Zhao and Zhu [140] synthesized gold and platinum-loaded titania nanotubes, which exhibited degradation rates of 96.1% and 95.1% for methyl orange using Au/TiNT -500 and Pt/TiNT -500 catalysts, respectively, with light deposition facilitating the metal loading Additionally, Zhu and colleagues [141] developed Polythiophene/titanium dioxide (PT/TiO2) catalysts via in situ chemical oxidative polymerization, utilizing various analytical techniques to assess their properties The research indicated that Pt/TiO2 composites demonstrated strong adsorption capabilities due to electrostatic interactions with methyl orange, enhancing their catalytic efficiency.
Recent studies have demonstrated the enhanced degradation of Rhodamine B (RhB) and Methyl Orange (MO) in aqueous solutions under simulated solar light, utilizing one-dimensional TiO2 nanostructures, including nanotubes and nanowires These nanostructures were synthesized through a hydrothermal method using Degussa P25 TiO2 as a precursor Notably, the TiO2 nanocatalysts exhibited significantly greater photocatalytic activity compared to the conventional P25, highlighting their potential for effective dye degradation under UV and visible light exposure.
The pH level significantly influences the photodegradation rate of dyes, as it affects the surface charge of TiO2 particles and alters catalytic process potentials Changes in pH can modify dye adsorption on the catalyst surface, impacting reaction rates The point of zero charge (PZC) of TiO2, approximately pH 6.8, is crucial for assessing the adsorption efficiency of organic contaminants In acidic conditions, dye solutions are expected to exhibit higher adsorption on high PZC catalysts, while pH levels above 6.8 result in a negatively charged surface, further influencing the degradation process.
The reaction between TiOH and OH results in the production of H2O and TiO When the pH level is below the point of zero charge (PZC), the surface of the TiO2 molecule acquires a positive charge.
The structure of a catalyst is crucial for enhancing photocatalytic activity, with titanium dioxide (TiO2) existing in three phases: anatase, rutile, and brookite Among these, anatase is favored for photocatalysis due to its stability, conduction band position, hydroxylation degree, and adsorption capacity Additionally, the morphology of the catalyst significantly influences degradation efficiency Nanomaterials, particularly nanosized titanium dioxide, demonstrate superior photocatalytic effectiveness compared to bulk materials, attributed to their larger surface area and smaller size Reducing the catalyst size increases the surface area-to-volume ratio, facilitating charge carrier transfer and creating more active sites Since the photocatalytic redox reaction predominantly occurs on the catalyst surface, its surface characteristics are vital for optimizing catalyst performance in applications like water filtration and recycling.
The incorporation of activated carbon (AC) significantly enhances the photocatalytic activity of TiO2, primarily due to improved adsorption of methyl orange (MO) on the TiO2/AC composite Although AC itself lacks photocatalytic properties, it facilitates a higher concentration of MO near TiO2, allowing for effective degradation upon irradiation This synergistic effect illustrates how AC aids in transporting adsorbed MO molecules to TiO2, where they are efficiently destroyed The increased photocatalytic degradation of MO is largely attributed to AC's superior adsorption capacity, which is crucial for optimal photocatalytic performance AC's well-developed pore structure and large surface area make it an excellent adsorbent and catalytic support, enabling it to act as a site for organic molecule adsorption before they reach the illuminated TiO2 breakdown center This mechanism underscores the enhanced activity of the TiO2/AC catalyst system.
Fig 1.8: Role of activated carbon [145]
Phenol photocatalysis degradation
Researchers have explored various methods to reduce phenol levels in wastewater, including polymerization using enzymes and hydrogen peroxide, though this method can be costly Biological techniques, such as activated sludge in membrane bioreactors, have proven effective but come with high cleaning costs due to fouling Other investigated processes include electrocoagulation, extraction, adsorption, ion exchange, and photodecomposition, all contributing to the overall effort to manage phenol contamination in wastewater.
Titanium dioxide (TiO2) is an affordable and widely available photocatalyst known for its ability to decompose and mineralize various organic pollutants, making it valuable for environmental cleanup, particularly in phenol degradation in water However, its photocatalytic methods are limited by low quantum efficiency and the requirement for UV light, which restricts its practical applications To enhance its effectiveness, TiO2 is often combined with other materials; for example, activated carbon has been shown to boost its photocatalytic activity Recently, graphene oxide has gained attention as a promising alternative due to its advantageous electrical, adsorption, thermal, and mechanical properties.
The use of ultraviolet (UV) radiation for breaking down large quantities of wastewater is not a viable solution Researchers are continuously seeking environmentally friendly, cost-effective methods to degrade phenol and eliminate it from water This thesis explores the photocatalytic degradation of phenol using titanium dioxide modified catalysts illuminated by both visible and ultraviolet light, aiming to revolutionize industrial wastewater treatment In heterogeneous photocatalytic systems, molecular transformations occur at the catalyst's surface, initiated by photon absorption, which generates a highly reactive state Titanium dioxide (TiO2), with a band gap of 3.2 eV, requires UV light for excitation, as visible light is insufficient Upon exposure to UV light (wavelengths shorter than 390 nm), electron transfer occurs, reducing oxygen to O2 while generating hydroxyl radicals (OH) that drive chemical reactions in the photocatalytic process The deexcitation process is significantly slower than the initial photon absorption, with photochemical reactions occurring on the timescale of 10^-12 to 10^-9 seconds.
The promotion of an electron generates an electron-hole pair, leading to two potential pathways: the electron can either transfer to adsorbed organic or inorganic species or to the solvent Electron-hole recombination, an unfavorable mechanism, competes with the charge transfer process, negatively impacting the efficiency of photocatalytic reactions This efficiency is inversely related to the combined rates of charge transfer and electron-hole recombination, emphasizing the need to minimize recombination to enhance charge transfer effectiveness on the catalyst surface Strategies to reduce recombination rates include modifying semiconductor surfaces with metals or integrating them with other semiconductors.
To enhance the efficiency of the photocatalytic process, two main challenges must be addressed Firstly, the light photons must possess sufficient energy to promote electrons across the band gap; for titanium dioxide (TiO2), this necessitates ultraviolet (UV) light, as visible light is inadequate However, using UV light for large-scale wastewater treatment is impractical, given that it constitutes less than 5% of the solar spectrum that reaches the Earth's surface Secondly, it is crucial to significantly reduce the electron-hole recombination process to facilitate effective reactions.
Fig 1.9: Mechanism of Phenol Decomposition Reaction over nonmetal TiO 2 [151]
Titanium dioxide (TiO2) is an effective catalyst for the photocatalytic degradation of phenol, facilitating its interaction with hydroxyl radical ions to produce several intermediate compounds These intermediates include hydroquinone, pyrocatechol, 1,2,4-benzenetriol, pyrogallol, 2-hydroxy-1,4-benzoquinone, and 1,4-benzoquinone Through further photocatalytic oxidation, these intermediates are transformed into highly polar substances such as carboxylic acids and aldehydes, ultimately leading to the formation of carbon dioxide and water.
Nonmetal doping of TiO2 catalysts has been found to reduce band gap energy, enhancing photocatalytic activity across a wider spectrum, including visible light This modification allows for more efficient utilization of solar energy In this study, nitrogen doping of TiO2 was chosen to investigate its potential to improve photocatalytic performance when exposed to visible light.
Ki netics study of phenol photodegradation
Heterogeneous photocatalytic degradation reactions predominantly adhere to Langmuir-Hinshelwood (LH) kinetics, a conclusion supported by numerous studies demonstrating that the photocatalytic oxidation rates of various pollutants on irradiated TiO2 align with this model The photocatalytic process primarily occurs on the surface of solid semiconductor photocatalysts, making the adsorption of organic compounds on this surface crucial to the reaction's efficiency, as higher adsorption capacity typically enhances reaction rates Consequently, adsorption-kinetic models, particularly the Langmuir-Hinshelwood model, are widely utilized to characterize photocatalytic mineralization reactions.
This study focuses solely on phenol concentration as the variable, utilizing the Langmuir-Hinshelwood (LH) model to describe the photocatalytic reaction The resulting rate equation indicates that a plot of ln (C0/Ct) against time yields a straight line, confirming the model's applicability.
(1) Where r = rate of reaction [mg/L min]
= 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:
= initial reaction rate [mg/L min]
= rate constant for photocatalysis, [mg/L min]
= equilibrium dye concentration in a solution after the completion of dark experiment [mg/L]
1/ is plotted against 1/ and the kinetic parameters and K are estimated by the slope and intercept respectively.
Summary
The environmental status remains one of the most pressing challenges we face today, with various types of pollution threatening human life This article explores solutions for reducing organic pollutants, specifically methyl orange and phenol, which are harmful to both humans and ecosystems Enhancing the photocatalytic activity of TiO2 through the loading of nanotitanium with carbon and graphene oxide (GO) is essential for accelerating the degradation of these pollutants under UV-C and visible light Research in Vietnam has demonstrated advancements in modified TiO2 materials, particularly through the sol-gel process, which successfully synthesized Fe-modified TiO2 on silica gel beads Results showed that Fe-modified TiO2 exhibited significantly higher methylene blue decomposition efficiency compared to unmodified samples Furthermore, the Fe-TiO2 catalysts displayed two to three times greater decomposition activity for p-xylene, particularly with 2% Fe2O3, highlighting the effectiveness of combining ultraviolet and visible light in reducing pollutant levels.
Researchers [161 164] explored the enhancement of TiO2's photocatalytic capabilities by modifying it with metals like Nd, F, Ag, and N Their findings could lead to effective strategies for reducing water contaminants in the environment and may also pave the way for innovative methods to limit other harmful organic pollutants This study serves as a valuable resource for fellow researchers investigating improvements in catalysts or related topics.
Recent research focuses on enhancing the activity of TiO2 for environmental applications, particularly in wastewater treatment, reflecting its scientific and practical significance The study aims to synthesize and optimize catalysts and catalyst carriers using various methods, integrating activated carbon and graphene oxide, and dispersing them on thin films across different carriers to improve photodegradation efficiency in wastewater treatment.
The research findings will be utilized in further studies under the Rohan Catalyst Program, concentrating on wastewater treatment and environmental preservation.
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
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.
To prepare the catalyst, dissolve 1.7 g of CTAB in 18 ml of ethanol to create solution 1 In a separate container, mix 9 ml of TTIP with 17 ml of ethanol and 3.9 ml of HCl to form solution 2 Combine solution 1 with solution 2 and stir vigorously Next, add 6 ml of distilled water and continue stirring for 15 minutes Dry the mixture at 80 °C for three days, followed by refluxing with 0.1M NaOH solution for two days Finally, filter the catalyst from the solution and separate it for further use.
1.7 gam of CTAB 9 ml of TTIP 15 mL of ethanol
The meso-TiO2 was filtered, washed, and dried at 90°C for 11 hours, then divided into two portions One portion was dried at 80°C for 2 hours (referred to as CTAB-NE), while the other portion was mixed with a small amount of ethanol and dried at 80°C for 2 hours (designated as CTAB-E) Both catalysts underwent calcination at 450°C for two hours with a heating rate of 2°C per minute.
The catalyst was synthesized through a hydrothermal process using CTAB surfactant, similar to the CTAB-NE method This involved dissolving the catalyst after drying at 80°C in a 0.1M NaOH solution, followed by heating for 11 hours The final steps included filtering and heating the material at 450°C for 2 hours to produce 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
To prepare the solution, dissolve 4g of P123 in 100ml of distilled water at 55°C, then add 0.8ml of sulfuric acid to create solution 1 In a separate mixture, combine 12.5ml of TTIP with 8.687g of citric acid to form solution 2 Next, incorporate solution 2 into solution 1 and incubate the combined mixture for 3 hours at 55°C.
4g of 12.5 ml of TTIP 8.687 gam of citric acid (99%)
0.8 ml of concentrated sulfuric acid
100ml of distilled water, 55°C Mixing
The P123-C100-450 catalyst undergoes a hydrothermal treatment lasting 10 hours, followed by calcination at 450°C for 2 hours The quantity of citric acid is adjusted across various samples to assess its impact on the catalyst's performance.
Several catalysts were synthesized using a similar procedure, differing in the presence of citric acid and calcination conditions The P123-300 catalyst was prepared without citric acid, undergoing calcination at 300°C for 8 hours at a rate of 10°C/min Another catalyst was created with citric acid, incubated at 55°C for 1.5 hours, followed by hydrothermal treatment for 11 hours, then filtered, washed, dried, and calcined at 450°C for 2 hours, resulting in the P123-450 catalyst Additionally, varying amounts of citric acid (25%, 50%, and 75% of the P123-C100-450 benchmark) were used, yielding catalysts labeled P123-C25-450, P123-C50-450, and P123-C75-450.
The catalyst synthesized as P123-C25-450 undergoes hydrothermal treatment for 11 hours, resulting in its division into two parts One half is rinsed in ethanol five times and subsequently calcined at 450°C, yielding the P123-C25-450-RE catalyst Meanwhile, the other half is processed in an autoclave filled with ethanol.
70 o C for 24 hours then calcined sample at 450 o C obtaining sample P123-C25-450- -HT
Cataly synthesis was carried out at the Gevicat catalyst center, Hanoi st 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- -300 C0 Hydrothermal using P123, no citric acid, calcined at 300 o C
3 P123- -450 C0 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 was synthesized using the cost-effective sol-gel method, which is ideal for producing nano TiO2 This procedure follows the established methodology outlined by Venkatachalam et al.
In 2007, a modified method for preparing titanium dioxide (TiO2) sol involved the introduction of activated carbon (AC) into the solution The process began with the preparation of titanium isopropoxide (126 ml) as a precursor and glacial acetic acid (250 ml) as a hydrolyzing agent in a 5-L beaker kept in an ice bath at 0°C Double distilled water (2.45 L) was added dropwise while stirring at 1050 rpm Afterward, the solution was sonicated for 30 minutes and stirred for 5 hours to achieve a clear TiO2 solution AC was then incorporated into the TiO2 sol at a weight ratio of 1:18 (AC/TiO2), with corresponding theoretical percentages of AC detailed in Table 2.4 The resulting TiO2 and AC mixture was stirred for an hour and sonicated for an additional 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 aged in an oven at 70 °C for 12 hours, followed by drying the gel at 100 °C for 72 hours After crushing the dried gel into powder, it was calcined at 400 °C for 5 hours with a heating rate of 100 °C/min The final catalysts were then finely ground Additionally, bare TiO2 was prepared using a similar method, resulting in the formation of TiO2 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 the TiO2–GO composite, graphene oxide (GO) was synthesized from graphite powder using the Modified Hummers method Initially, 2 g of graphite powder was combined with 100 ml of 98% sulfuric acid in an ice bath for 2 hours Afterward, 8 g of potassium permanganate (KMnO4) was gradually added while mixing for 3 hours Water was then added dropwise to the mixture under stirring to oxidize the graphite for 1 hour, followed by the addition of 300 ml of water and 30 ml of additional reagents.
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 in Fig 2.3
Fig 2.3: Flowchart of GO synthesis
The TiO2–GO composite is synthesized using the sol-gel method, following procedures similar to those outlined in section 2.1.2 for the TiO2/AC catalyst, but with graphene oxide (GO) replacing activated carbon (AC) The amount of GO is determined based on the weight ratio of TiO2 to AC in the final composite The resulting GO-TiO2 composites are designated as SG GO1/4, SG GO1/18, and SG GO1/24.
The TiO2/GO composite was synthesized using a hydrothermal method Initially, 2 mg of graphene oxide (GO) was dissolved in a water and ethanol solution with a 2:1 volume ratio, followed by ultrasonic treatment for one hour and stirring for two hours Subsequently, 200 mg of TiO2, also prepared via hydrothermal synthesis, was added to the slurry This mixture was then placed in a Teflon-lined autoclave, where it underwent a hydrothermal process at 120°C for three hours, resulting in the reduction of GO to graphene and the deposition of TiO2.
GO was achieved The composite was obtained after centrifuging, rinsing with
2 g of graphite powder 100ml of H2SO4, 98%
Mixing, maintained in a ice bath for 2h
300ml of H2O and 30ml of H2O2
Cleaned by HCl 5%, washed by deinonized water
GO deionized water, and drying at 60°C under vacuum, which was denoted as GO TiO– 2
To evaluate the photocatalytic activity of the synthesized GO–TiO2 composite, a comparison was made with the GO–ZnO composite, which was prepared using the same methodology The ZnO used in this study, with a purity of 99%, was sourced from Merck.
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 undergoes a process where it is ground and soaked in water for six hours, then settled for two days and filtered to obtain cleaned kaolin In the next step, 50g of cleaned kaolin is combined with 5.12g of MgO, 3.01g of Al(OH)3 from Tan Binh, and 11.75g of dolomite, which are finely ground and filtered through an 88-mesh screen before being thoroughly mixed Finally, sufficient distilled water is added to the mixture and mixed well.
Characterization of the catalysts
Surface morphology imaging techniques are essential for characterizing nanostructures, as traditional optical microscopes lack the resolution to observe submicron to nanoscale features Scanning Electron Microscopy (SEM) is frequently employed to analyze the surface morphology of micro and nano-structured materials Due to the inherent properties of electrons compared to photons, electron microscopes offer significantly better resolution and depth of focus.
Scanning Electron Microscopy (SEM) utilizes two main types of electron sources: thermionic guns and field emission guns (FEGs) Thermionic guns operate by heating filaments, such as tungsten, to release electrons, but they suffer from thermal drift and low brightness In contrast, field emission guns, or cold cathode field emitters, eliminate the need for heating by using a polished filament with a pointed tip that enhances the electric field, allowing for a greater electrical potential gradient that effectively draws and accelerates electrons This results in FEGs achieving up to three times the electron density compared to standard SEM Furthermore, Field Emission Scanning Electron Microscopy (FESEM) offers superior spatial resolution and reduced electrostatic distortions, making it a preferred choice for high-precision imaging.
Fig 2.7: Simplified internal structure of FESEM
FE cannons utilize tungsten cathodes with precise 0.1 m points and are equipped with two anodes—one for current restriction and the other for focusing and accelerating the electron stream Electromagnetic lenses horizontally focus the electron beams, while a scanning coil deflects the beam toward the sample by adjusting the current through radially oriented coils The sample atoms emit signals ranging from a few hundred eV to fifty keV, which are quantified by a detector that converts these impulses into digital images.
FESEM results are derived from the interactions between an electron beam and sample atoms, leading to both elastic and inelastic scattering This process generates X-rays, Auger electrons, secondary electrons, and backscattered electrons, with surface structure influencing the angle and velocity of secondary electrons The electron detector typically captures secondary electrons from within the material, and the findings are displayed on a computer monitor For effective sample preparation, drop casting onto a Si/SiO2 substrate using ethanol dispersion is recommended It is essential that the sample is clean, dry, and properly mounted in a sample holder before being placed on the specimen stage.
This thesis presents SEM images obtained using the JSM-7600F Schottky Field Emission Scanning Electron Microscope at the Advanced Institute for Science and Technology in Hanoi, Vietnam, as well as the FEI Nova NanoSEM 400 field emission scanning electron microscope at Hanoi University of Science, VNU Hanoi.
2.3.2 Elemental surface composition and traces of impurities
EDX analysis is a technique employed to identify the elemental composition of a specimen or targeted area This method is an essential part of a scanning electron microscope (SEM) and relies on it for operation.
During EDX Analysis, a scanning electron microscope bombards the specimen, causing electrons to be ejected from the atoms This process involves a higher-energy outer shell electron replacing a lower-energy inner shell electron, which results in the emission of X-rays as the outer electron transfers energy.
When an electron is transported, it emits energy from both its source and destination shells During this process, the atoms of each element release unique X-rays, which can be measured to identify the atom that emitted the X-ray due to electron beam bombardment.
The EDX spectrum displays X-ray frequencies across various energy levels, with peaks indicating the energy levels where the highest number of X-rays are detected Each peak corresponds to distinct elements, as every atom has unique characteristics Furthermore, the concentration of elements within a specimen is directly related to the height of the spectrum peaks.
2.3.3 Specific surface area, pore volume, and average pore size
BET analysis is a method used to evaluate the surface area, pore volume, average pore size, and pore size distribution of powdered or porous materials by calculating the amount of gas adsorbed This process measures all gas-accessible surfaces, both interior and exterior Due to Van der Waals interactions, solids tend to absorb gases poorly, necessitating the cooling of the material to the adsorbate's boiling point to achieve sufficient gas absorption for accurate surface area measurement In this analysis, nitrogen gas is commonly used as the adsorbate, with the solid being cooled using liquid nitrogen at a temperature of 77.35K.
Adsorption occurs until the amount of nitrogen (N2) adsorbed matches the gas phase concentration, achieving a monolayer coverage on the surface This method requires minimal sample quantities and features brief outgassing and analysis times As a result, it is considered the most advanced and dependable technique for rapid and precise BET surface area measurements.
In 1938, Brunauer, Emmett, and Teller introduced an adsorption model that proposed molecules could be adsorbed in multiple layers on an adsorbent Their equation, aligned with the Langmuir equation, assumed a homogeneous surface with isolated sites, indicating that adsorption at one site did not influence adjacent sites The energy of adsorption stabilized the initial monolayer, while the condensation energy of the adsorbate facilitated the absorption of subsequent 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
X-ray diffraction (XRD) temperature plays a crucial role in determining the crystalline structures of materials by analyzing small crystalline regions The three-dimensional arrangement of non-amorphous materials, such as minerals, is governed by regular, repeating atomic planes within their crystal lattices These atomic planes interact with a focused X-ray beam by transmitting, absorbing, diffracting, and scattering it, much like water droplets create a rainbow when diffracting light Each mineral exhibits unique X-ray diffraction patterns based on its atomic composition, lattice size, shape, and internal tension X-ray powder diffractometry employs vacuum-sealed tubes to generate X-rays, with electrons accelerated by high voltage—typically between 15 to 60 kilovolts—colliding with a copper target to produce X-rays These collimated X-rays are directed at finely powdered samples, usually less than 10 microns in size The resulting X-ray energy is detected and processed into a count rate, as the angle between the source, sample, and detector is gradually adjusted during the scan.
The crystal size is determined by the full width at half maximum (FWHM) of an X-ray diffraction (XRD) peak, where an increase in peak breadth indicates a decrease in crystallite size To calculate the average size of individual crystallites, Scherrer's formula can be applied, utilizing the FWHM of corresponding XRD 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 -width- -half-maximum of the (1 0 1) plane (in radians) full at
XRD patterns were primarily obtained using a D8 Advance Bruker device at the Faculty of Chemistry, Hanoi University of Science, Vietnam This diffractometer utilizes a copper source emitting Cu K radiation with a wavelength of 0.154 nm and employs a step scan rate of 0.030 seconds.
Experimental set up
The photodegradation of methyl orange (MO) was conducted in a cylindrical glass cell with an 800 ml capacity, utilizing a batch reactor setup A 100 W mercury lamp emitting UV-C light at 254 nm was vertically placed in a quartz tube equipped with a water-cooling jacket to maintain a constant temperature of 25 °C This configuration allowed the UV-C source to be centrally located within the glass photo-reactor, while a magnetic stirrer ensured uniform fluid dispersion throughout the process.
Fig 2.9a: Photocatalytic exerimental setup with UV-C lamp
The photocatalytic degradation of MO was conducted using a 300 W Xenon lamp system, where light is directed through a diagram box into a horizontally positioned quartz tube equipped with magnetic stirring to ensure continuous catalyst distribution in the solution After each run, an aliquot of the solution was collected using a pipette and filtered through a 0.22 m syringe filter, then kept in the dark for 30 minutes to establish adsorption-desorption equilibrium The concentration of MO was measured using an Avantes UV-VIS spectrometer.
Fig 2.9 b: Principle diagram of visible photocatalytic exerimental setup.
The study investigated the photodegradation of phenol using UV light at 254 nanometers and visible light Liquid samples were collected every 30 minutes and filtered through a 0.22 m syringe filter to remove catalyst particles The concentration of phenol was analyzed using High-Performance Liquid Chromatography (HPLC) (Jasco, Japan) For HPLC measurements, a mobile phase composed of acetonitrile and deionized water in a 1:1 volume ratio was utilized at a flow rate of 1.0 ml/min, with 10 µl of the sample injected for analysis at a wavelength of 280 nm.
2.5.1 Calibration curve of methyl orange solution
To determine the amount of methyl orange (MO) degraded during a reaction, it's essential to assess the concentration of the MO solution using the optical absorption titration curve According to Lambert-Beer law, the optical absorbance is linearly related to the concentration of the solution when using a consistent cuvette By measuring the absorbance of a solution with a known concentration, a linear curve can be established, allowing for the calculation of the unknown solution's concentration based on its optical absorbance and the standard curve.
To create a series of standard solutions with MO concentrations ranging from 0.5 to 20 ppm mg/l, utilize a volumetric flask and double distilled water as the diluent Analyze these solutions using a UV-Vis photometer, focusing on the intensity at the wavelength of 464 nm, which corresponds to the maximum absorption of the MO solution.
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:
Ad so rb an ce (a u )
A standard curve was established to illustrate the correlation between the concentration of MO and the intensity of absorption (Abs), along with a regression equation to accurately quantify the remaining MO content in the solution at the specified time.
Similarly, it is determined the phenol concentration with UV-Vis replaced by HPLC analysis instrument with equation between area peak and concentration
This chapter presents the results of experiments conducted with various catalysts, focusing on preliminary tests to identify the optimal catalyst for phenol phototesting under 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
Q ua nt ity A ds or be d (c m ³/g S TP )
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.
The nitrogen physisorption isotherms and pore size distributions for calcined P123 C25-450 and CTAB samples are illustrated in Figures 3.1 and 3.2 According to IUPAC classification, the TiO2 demonstrates type IV adsorption-desorption isotherms characterized by an H–2 hysteresis loop The pore size distribution curves reveal a broad range centered between 40 and 70 Å, indicating the properties of the catalyst P123 C25.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 dV /d lo g( w ) P or e Vo lu m e (c m ³/g ãÅ )
450 has smaller pore diameter (40 Å) compared to CTAB-NE Å) thus P123 C25-(70 450’s surface area is higher compared to CTAB-NE and is expected to also have a better adsorption and desorption capacity
The XRD patterns of TiO2 samples, prepared through hydrothermal methods using various templates, are illustrated in Figure 3.3 The selected catalysts were characterized using XRD, and the results are presented in the subsequent figures.
From the given figures above, the diffraction peaks at 2θ values of , 37.83, 4825 ,
The XRD results indicate that the peaks at 54° and 55° correspond to the anatase crystal planes (101), (004), (200), (150), and (211), confirming that the synthesized TiO2 possesses a high degree of crystallization in the anatase structure These findings align with the reported ASM data (Card No 96-900-9087) Additionally, the broadening of the TiO2 peaks suggests the presence of small crystal sizes.
XRD analysis revealed that all catalyst samples predominantly formed the Anatase phase of TiO2 Notably, the CTAB-NE catalyst exhibited not only the Anatase phase but also the presence of Rutile and Brookite forms, which are inactive photochemically, at varying intensities (2θ = 28; 36; 41; 54°) This indicates that the synthesis process involving CTAB surfactant led to the formation of Anatase TiO2, which was partially converted into different polymorphs in the presence of ethanol.
The XRD pattern of the CTAB- sample reveals the presence of both rutile and brookite phases, similar to the CTAB-NE sample, which exhibits superior photocatalytic efficiency However, the difference in performance is minimal This phenomenon can be attributed to references indicating that a synergistic effect between anatase and rutile phases, when present in an optimal ratio under specific conditions, enhances catalyst performance Notably, the XRD spectrum indicates that the rutile peaks in the CTAB-NE sample are more pronounced than those in the CTAB- sample.
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 analysis of anatase TiO2 crystal sizes using the Scherrer equation for the diffraction peak at 25.27° indicates that sample P123-C25-450 has the smallest crystal size, aligning with the surface area findings.
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
The SEM analyses revealed that the morphology of TiO2 samples remained consistent across different templates, with particle sizes measuring approximately 5 nm, which is smaller than the CTAB-H particles at 20 nm under the same magnification This indicates a uniform distribution of TiO2 particles.
The SEM micrograph of TiO2 reveals a non-uniform distribution of spherical particles, which can appear as either individual particles or clusters These spherical structures are composed of numerous small TiO2 crystals, likely resulting from agglomeration.
On the other hand, the FE SEM image of TiO 2 nanoparticle powder calcined at 450°C showed crystallinity with particle sizes that varies between 5.0 nm to 10 nm
Fig 3.4: -SEM images of CTAB-H (a) and P123 C25-450 (b)FE
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
RESULTS AND DISSCUSSIONS
Characterization and photocatalytic activity of T iO 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
Figure 3.9 illustrates a typical type IV nitrogen sorption isotherm, featuring a type 1 hysteresis loop indicative of cylindrical ordered channels This behavior suggests that capillary condensation of nitrogen within uniform mesopores occurs, resulting in a significant increase in nitrogen uptake at a characteristic relative pressure (P/P0) range of 0.6 to 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
Q ua nt ity A ds or be d (c m 3 /g S TP )
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 dV /d lo g( w) P or e Vo lu m e (c m 3 /g Å )
Table 3.3: Surface area of two samples by sol-gel synthesis
Ratio AC/TiO 2 in weight
Catalyst samples with AC/TiO2 ratios of 1/18 and 3/1 were analyzed using SEM-EDX SEM images for the 1/18 catalyst samples, illustrated in Figure 3.11, revealed uniform catalyst particles approximately 5 nm in size, with AC content being too low to distinguish from TiO2 Nevertheless, EDX analysis confirmed the presence of AC within the sample.
Fig 3.11: Morphology of SG AC-1200/Ti 1/18(a) and SG AC-1200/Ti 2/1 (b)
In samples with a low AC/TiO2 ratio of 1/18, the actual AC/TiO2 ratio aligns closely with theoretical calculations, indicating that the AC content remained stable during heating Conversely, in samples with a high AC/TiO2 ratio of 3/1, the actual ratio significantly decreased compared to the expected theoretical ratio, suggesting a substantial loss of AC during the heating process.
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
In samples with a low AC/TiO2 ratio of 1/18, the actual ratio aligns closely with theoretical calculations, indicating that the amount of AC mixed with TiO2 remains stable during heating Conversely, in samples with a high AC/TiO2 ratio of 3/1, the actual ratio significantly decreases compared to the expected theoretical ratio, demonstrating a notable loss of AC content.
During the heating of the catalyst in the air, activated carbon (AC) was burned Photo-degradation analysis revealed that the SG AC-1200/Ti 1/18 and SG AC-1200/Ti 3/1 samples exhibited the highest performance levels Consequently, these two samples were further analyzed using the XRD diffraction method, with the results illustrated in the figures below.
Fig 3.13: XRD result of AC TiO 2 catalysts
The XRD patterns of the SG AC-1200/Ti 1/18 and SG AC-1200/Ti 3/1 samples reveal that both catalysts are composed entirely of the Anatase phase, indicated by peaks at 2θ values of 25, 38, 48, and 55 The inclusion of activated carbon (AC) does not significantly alter the phase composition due to its low quantity and amorphous nature However, the sample with a higher AC content (SG AC-1200/Ti 3/1) exhibits a more pronounced amorphous characteristic Additionally, calculations of crystal size from the XRD patterns show that the sample with increased AC has a slightly larger 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 types of activated carbon (AC) were utilized to adsorb Methyl Orange under dark conditions: Type 1, produced in Vietnam with a surface area of 300 g/m², and Type 2, manufactured in Thailand with a surface area of 1200 g/m² Prior to the experiment, both types of AC were meticulously cleaned and dried to eliminate moisture and impurities, with a usage amount of 0.4 g for each type.
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:
The catalytic synthesis components for two types of activated carbon (AC) with surface areas of 300 g/m² and 1200 g/m² were analyzed Photocatalytic activity results indicate that during the initial 20 minutes of dark adsorption, the TiO2/AC 1200 exhibited a conversion rate for methyl orange (MO) degradation that was over 1.5 times faster than that of TiO2/AC 300 Following this dark adsorption phase, the photocatalytic process was initiated using UV-C light at a wavelength of 245 nm.
Ab so rb an ce (% ))
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 TiO2 / AC 1200 can degrade up to 95% of MO, hence the AC1200 was selected for further study
De gr ad at io n (%)
SG AC-1200/Ti/1/1 Light OFF
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)
In this experiment, catalysts were synthesized using AC 1200 with varying ratios of activated carbon (AC) to titanium dioxide (TiO2) The specific weight ratios evaluated were 1:18, 1:4, 1:1, 2:1, and 3:1, resulting in the following catalyst designations: SG AC-1200/Ti/1/18, SG AC-1200/Ti/1/4, SG AC-1200/Ti/1/1, SG AC-1200/Ti/2/1, and SG AC-1200/Ti/3/1.
The experimental results demonstrated that the SG AC-1200/Ti 3 /1 sample achieved the highest degradation of more than 60% for MO, while the SG AC-1200/Ti/1/18 sample exhibited the lowest degradation rate at 40% Additionally, the SG AC-1200/Ti 1 / 1 sample also showed a performance of 40%.
The SG AC-1200/Ti 2/1 and SG AC-1200/Ti 1/4 exhibit nearly identical performance at 40% degradation, primarily due to the higher amounts of activated carbon (AC) facilitating greater methylene orange (MO) adsorption in the solution In subsequent photocatalytic tests, the SG AC-1200/Ti 1/18 achieved the highest performance, demonstrating over 80% MO degradation, followed by SG AC-1200/Ti 3/1, SG AC-1200/Ti 1/4, SG AC-1200/Ti 2/1, and SG AC-1200/Ti 1/1.
The results indicate that the degradation of methyl orange (MO) primarily occurs through a photocatalytic process rather than mere absorption During the 40 to 60-minute interval, the catalysts SG AC-1200/Ti 1/18, SG AC-1200/Ti 3/1, and SG AC-1200/Ti 1/4 achieved nearly 96% MO degradation, while SG AC-1200/Ti 2/1 reached about 90%, and SG AC-1200/Ti 1/1 achieved approximately 80% This suggests that the synergistic effects of activated carbon (AC) adsorption and TiO2 photoactivity contribute to enhanced performance, leading to rapid MO degradation From 60 to 80 minutes, MO degradation increased to 98%, with SG AC-1200/Ti 1/1 showing the lowest efficiency at 90%.
De gr ad at io n (%)
F ig 3.16: MO photodegradation via time of catalyst samples at pH=7.
(Condition: C MO 0ppm, catalsyt dosage =0.2g, solution volume 0ml, pH=7; dark adsorption in 20 minutes, UVC 254 nm with 100 W lamp)
Nanometer-sized TiO2-AC powder has been synthesized using Titanium Isopropoxide via the sol-gel method The composition and surface area of activated carbon significantly influence the photochemical activity of the resulting catalyst Activated carbon with a surface area of 1200 m²/g demonstrated superior efficiency compared to that with 300 m²/g in both dark adsorption and photocatalytic processes The catalyst SG AC 1200 3/1 achieved an adsorption effect exceeding 60%, while the SG TiO2 sample exhibited the lowest adsorption In the subsequent photochemical process lasting up to 80 minutes, the results remained consistent.
SG AC 1200 1/18 being the highest The additive effect of activated carbon supporting the photochemical process is shown [188,189 ].
The improved findings in dye removal can be attributed to the synergistic effects of adsorption and photodegradation Activated carbon serves as an effective adsorbent, capturing dye molecules from the solution, which then interact with TiO2 for photodegradation The TiO2/AC composite operates as both a semiconductor and an adsorbent, facilitating dye removal irrespective of the dye's characteristics This combination effectively enhances the overall efficiency of the degradation system The presence of activated carbon boosts the photocatalytic activity of TiO2 by increasing the adsorption of dye molecules, leading to a higher concentration of contaminants being degraded While activated carbon itself lacks photocatalytic properties, it significantly enhances the performance of TiO2 in dye removal applications.
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
TiO films
De gr ad at io n (% )
Fig 3.25 MO Photodegradation by SG GO Ti/ 1/18 various concentrationin
(Condition: catalsyt dosage =0.05g, solution volume Pml, pH=7; dark adsorption in 30 minutes,Full range light with Xenon 300 W lamp)
❖ Dip coating with low concentration of PEG
The sol-gel method produces a catalyst coated on cordierite with a more uniform distribution, while the co-precipitation method results in larger and uneven catalyst particles ranging from 20 to 100 nm, in contrast to the approximately 20 nm particles achieved through the sol-gel technique.
Fig 3.26 : a) SEM Low PEG Cor-gel-200 and (b) SEM 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
The reaction results of two samples, Low PEG-gel-50 and Low Cor-gel-200, were analyzed after combining them with 0.4 g of Powder-gel sample at a methyl orange concentration of 30 ppm.
De gr ad at io n %
Low PEG Cor-gel-50 Low PEG Cor-gel-200 Gel powder
Low PEG Cor -gel CTAB Light Off
To evaluate the efficiency of catalyst thin films, a dip coating method was employed using a low concentration of PEG The experimental conditions included a catalyst dosage of 0.05g, a solution volume of Pml, and a pH level of 7 The process involved a dark adsorption phase lasting 30 minutes, followed by exposure to full-range light using a Xenon 300 W lamp.
The catalyst amounts on low PEG Corgel 200 and low PEG Corgel 50 are nearly identical, as shown in Figure 3.27, which compares the photocatalytic performance of these two cordierite-coated samples against a powder-gel sample at an initial methyl orange concentration of 20 ppm The results indicate that while the photocatalytic efficiency of the cordierite-coated catalysts is similar, it remains inferior to that of the powder catalyst This is attributed to the greater amount of catalyst in the powder form, which disperses more evenly in solution, allowing for better light penetration and higher conversion rates Additionally, the limited UV exposure to the surface of the cordierite-coated catalysts reduces their reaction efficiency Furthermore, the molar ratio of TTIP to H2O significantly influences the photocatalytic efficiency of the catalyst mass coated on the cordierite surface.
When using Low PEG Cor-gel-350, reduced adhesion occurs if the solution is not concentrated Additionally, the catalyst mass is lower compared to both Low PEG Cor-gel-50 and Low PEG Cor-gel-350.
The performance of Low PEG Cor-gel-CTAB shows minimal improvement over Low PEG Cor-gel 200, primarily due to the low concentration of CTAB powder and the limited amount of PEG Consequently, future experiments in this research will involve increasing the PEG quantity to enhance performance.
❖ Dip coating with high concentration of PEG
In the precipitation method, the crystal sizes of Cor-CTAB catalysts are bigger at
The crystal size of cordierite dip-coated in a well-distributed solution measures approximately 20 nm, contrasting with Corgel 200, which has a crystal size of only 20 nm This difference arises because the cordierite is treated with a solution that yields a more uniform distribution of nano-sized crystals, unlike the paste form containing surfactants P123 or CTAB used in precipitation and hydrothermal methods, which results in larger, unevenly distributed crystals on the cordierite surface.
The SEM results indicate that TiO2 nanoparticles are well-separated and dispersed, appearing as free nanoparticles Additionally, the presence of TiO2 nanoparticles is confirmed by the EDX results, as illustrated 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
Table 3.8 illustrates that the amount of catalyst applied to the cordierite surface is largely unaffected by the water content in the TiO2 solution or the modification with activated carbon Notably, samples produced through the sol-gel method exhibited a greater coating of TiO2 on the cordierite surface compared to other catalyst types.
Table 3.8: Catalysts films coated cordierite
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 pp m.
The photocatalytic capacities of the High PEG Corgel-150AC and High PEG Corgel-200 catalysts are nearly equal to that of the powder catalyst, as illustrated in Fig 3.30 A significant performance difference was observed between High PEG Corgel-150 and High PEG Corgel-150AC up to the 160th minute of the reaction This can be attributed to the activated carbon adsorption on the surface of the High PEG Corgel-150AC sample, which leads to a rapid decrease in Methyl orange concentration during the initial reaction stage However, their performances converge once absorption reaches saturation.
Weight of Catalyst coated on cordierite, g
High PEG Corgel-150 High PEG Corgel-150AC Powder-gel
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
The High PEG Corgel-150 sample exhibits a significantly lighter color of methyl orange compared to the High PEG Corgel-150AC sample, suggesting that the addition of AC enhances the adsorption capacity of methyl orange onto the surface of sample 91.
Samples synthesized via the sol-gel method, even without activated carbon, demonstrated the ability to adsorb methyl orange, though performance varied between P123 and CTAB samples Additionally, TiO2 films with a smaller and uniform particle size of 20 nm exhibited superior methyl orange adsorption capacity compared to those with a larger and uneven particle size distribution ranging from 20 to 100 nm.
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)
In a comparison of High PEG Corgel-150AC and High PEG Corgel-200 samples, the methyl orange treatment was found to be slightly more effective with Corgel-200, despite a lower amount of catalyst coating The adsorption surfaces for methyl orange were similar, but the insufficient coverage of the catalyst on the cordierite tablet surface in the Corgel-200 sample hindered the regeneration of methyl orange within the cordierite Ultimately, the photocatalytic performance of High PEG Corgel-150AC remains superior to that of High PEG Corgel-200, indicating the significance of the molar ratio in these samples.
TTIP:H2O = 1:150 showed a better performance while the High PEG Corgel-150AC sample is the best
Despite the AC-Gel powder containing three times more catalyst mass than the High PEG Corgel-200 and High PEG Corgel-150AC samples, the performance of the Corgel samples is only slightly lower than that of the powder This outcome demonstrates the effectiveness of applying a TiO2 film on cordierite using the 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
De gr ad at io n (% )
High PEG Cor-P123 Powder P123 Light ON
Fig 3.33: Photocatalytic performance of-P123 and High PEG Cor-P123 samples (Condition: CMO ppm, solution volume 0ml, dark adsorption in 20 minutes,
While the quantity of catalysts applied to cordierite matches that of the catalyst powder, the latter demonstrates a higher conversion rate This difference is primarily due to the stirring of catalyst powder in the solution, which allows UV light to penetrate more effectively, thereby enhancing the reaction process.