OVERVIEW
Introduction
With the rapid development of society, people are living with life that is more comfortable However, along with the positive aspects, the process of industrialization
- modernization has many negative effects Environmental pollution has become a global problem, especially air and water pollution
Vietnam is a developing country with a breakthrough in industry The emergence of many industrial parks, factories is promoting the strong economic development of the country However, they were built near urban areas and not all factories are equipped with efficient waste treatment systems, especially gas and wastewater, before releasing into the environment Thus, the industrial activity is one of the major causes of air pollution
The development of the industry, especially the chemical industry, also contributes great values to human life Household products provide comfort and convenience to users Nevertheless, most products of this industry are using chemical solvents and these substances are the source of indoor pollution and one of which is volatile organic compounds (or VOCs) They appear in detergents, paints, etc in small quantities However, the accumulation of VOCs over time can harmfully affect human health, especially sensitive people such as children and the elderly Therefore, it is necessary to eliminate this source of indoor pollution.
Volatile organic compounds
Volatile organic compounds (VOCs) do not have a common definition It is used to refer to all organic compounds that exist in the atmosphere All activities, including cutting grass, making a fire, cooking and even breathing are also sources of VOCs such as carbonyls, alcohols, alkanes, alkenes, esters, aromatics, ethers and amides However, fuel combustion and industrial processes generate most of the VOCs released into the environment [1]
Table 1.1 Overview of important sources and global annual emission rates of selected groups of VOC per year (2007).[1]
3 Sum of anthropogenic and oceanic emissions
The atmospheric lifetime of a VOC species can be determined using the overall rate of removal of that species from the atmosphere This rate can be derived by summing the reaction rates with radical species, rates of photolysis and the wet and dry deposition rates
Table 1.2 Overview of average tropospheric lifetimes of VOC compound groups and some selected VOCs as examples
Today, human are more and more concerned about the quality of air indoor In recent reports, indoor air pollutants mainly contain nitrogen oxides (NOx), carbon oxides (CO and CO2), volatile organic compounds (VOCs) and particulates.
Treatment of VOCs in the gas phase
The VOCs treatment methods are mainly divided into two major directions: decomposition technology and recovery technology
Thermal oxidation method (also known as combustion method) uses single chambers with ceramic blanket refractory lining the oxidizers and it is equipped with a propane or natural gas burner In the chamber, burner capacities range between 0.4 to 2 (mil BTUs)/hr with high temperatures range from 760 to 870°C and a maximum gas residence time of 1 sec The modern heat oxidation equipment can treat 95% to 99% of VOCs with a flow rate of 1000-500,000 ft 3 /min, concentrations from 100 to 2000 ppm Non-combustible compounds or low input concentration, which requires the higher heat and longer residence time in the chamber However, when the combustion temperature is close to 870 o C, nitrogen in the air can be converted into secondary oxide - pollutants, so further secondary treatment is required
Catalytic oxidation method is similar to thermal oxidation method However, due to the presence of the catalyst, the system is operated at a lower temperature of about 370-480°C This method is suitable for low concentrations of 100-2000 ppm VOCs, with a flow rate of 1000-100000 ft 3 /min, whereby the maximum efficiency is 95% In spite of its outstanding advantage in reducing the energy costs, catalytic oxidation method still has many limitations such as the high cost of catalysts, more byproducts, catalyst after using can be poisoned if not properly treated and it is highly sensitive to non-VOCs (water, halogen and sulfur compounds) Therefore, current thermal oxidation methods are more commonly used than catalytic oxidation method
Originally, biological filtration method was used for reducing the odor of exhaust gases but then proved to be an effective and economical method for removing VOCs in the industry This technology is based on the ability of microorganisms (usually bacteria) to convert organic pollutants into water, carbon dioxide and biomass in anaerobic conditions Contaminated air flows through a capillary buffer which is the living medium of the bacteria However, in order for the oxidation process to take place completely, it is necessary to maintain a moderate and suitable environment for the growth of bacteria, most importantly moisture and pH Although bio-filtration has been
6 shown to be effective in treating low levels of VOCs (few ppm) but with only simple organic compounds
Figure 1.1 Flow chart of bio-filtration system
Based on the principle of refrigeration or/and pressure compression, this method is only suitable for organic compounds boiling above 38°C with a high concentration of 5000 ppm It allows large amounts of organic compounds to be separated and recovered, which can then be purchased and reused However, this is a method that requires many bulky and modern equipments to ensure the safety In addition, water after condensation needs to be treated, so the cost of condensation method is relatively higher than the others
Figure 1.2 Schematic diagram of equipment used for condensing VOCs
Through direct contact with liquid solvents in the tower and tray, soluble organic compounds are absorbed Absorption systems can handle gas flow with rate of 2000 to
100000 ft 3 /min at concentrations of 500 to 5000 ppm Depending on the need for treatment, the characteristic of the pollution and processing conditions, the material is very variety in structure, size, surface as well as cost This method is particularly suited for high-moisture pollution (more than 50%) Due to the use of buffer material, the pressure in the absorption tower should remain low, especially in the absence of dust or impurities that may clog the column
Adsorption is categorized into two types: physical adsorption and chemical adsorption In VOCs recovery applications, physical adsorption exhibits superior performance The two most widely used materials for VOCs treatment are activated carbon and zeolite
Due to its large surface area, flexibility, low cost and high recyclability, activated carbon is most commonly used in VOCs recovery technology in particular and adsorption in general This method is suitable for non-selective adsorption at room temperature with molecular weight of about 40 - 150 g/mol, boiling point of 38 o C to
260 o C However, the limitations of activated carbon include flammable, poor selectivity, low polymerization and low humidity
Figure 1.3 Structure of activated carbon
8 Many reports have shown that zeolite has many outstanding advantages that can replace activated carbon such as heat resistance, high selectivity and good moisture As the Si/Al ratio increases, the heat resistance increases as well The zeolite with SiO2 is 100% resistant to 850°C Zeolite is also called a molecular sieve due to uniform capillary size, which allows the adsorption to be very selective In addition, the presence of water can reduce the adsorption capacity of zeolite, but the water vapor is almost unaffected The zeolite can withstand the gas flow, which has humidity up to 90% Although there are many precious properties but the cost of zeolite is still high, the application is still limited, only considered when activated carbon is not suitable.
Photocatalytic oxidation of volatile organic compounds (VOCs)
Decomposition of VOCs has become one of the major directions studied by scientists around the world Commonly used methods have their disadvantages Adsorption using activated carbon merely transfers pollutants from gaseous phase to solid phase instead of decomposing them Bio-filtration method is slow and has no obvious effect Thermal oxidation destruction requires high temperatures of 200˚C - 1200˚C for efficient operation and expensive Moreover, all of these methods are ineffective with low and medium levels of processing or with a large number of different organic substances, recycling and re-using are not economically feasible Therefore, this is great demand for scientists to find a more cost-effective, more efficient and environmentally benign technology Photocatalytic oxidation (PCO) does not have such limitations and are cost-effective for treating low concentration pollutions It has been demonstrated that organics can be oxidized to harmless carbon dioxide, and water which makes PCO especially attractive for treating air indoor pollutants.
Titanium dioxide
The development of new materials and their potential application form an important part of today's scientific research efforts Indeed, many areas of research and develop programs related to nanostructured materials Furthermore, it is estimated that nanotechnology has invested millions of dollars in development and become the dominant independent technology in the 21st century [2]
9 Among the photocatalyst potential, TiO2 is the most popular photocatalyst currently employed due to the hydrophilic properties of TiO2, which has ability to degrade organic compounds under irradiation of UV or near UV-light In addition, TiO2 is especially used for on PCO reaction because of its outstanding properties such as cheap cost, safety, high photoactivity and stability TiO2 is applied in such areas as: self- cleaning surface material, gas treatment, water treatment, microbial treatment, eliminating fog phenomenon [3]
The reaction mechanism of photocatalytic removal of toluene, a typical VOC compound, using the common TiO2 photocatalyst has been proposed [4-8]:
H2Oads + h + → H + ads + OH ads (3) e - + O2 ads → O2 - ads (4)
H2O2 ads + O2 - ads → OH - + OH ads + O2 (7)
OH ads + C6H5-CH3 ads → H2O + C6H5CH2 ads (8)
C6H5CH2 ads + O2 ads → C6H5CH2OO ads (9)
C6H5CH2OO ads + e - → C6H5CHO ads + OH - (10)
C6H5CHO ads + (mOH ads + nO2 ads)→ oxidized compounds (11) oxidized compounds + (xOH ads + yO2 ads)→ (CO2, H2O) (12)
It can be seen that the hydroxyl radicals (OH ), which are highly chemical active species for the toluene decomposition, are mainly produced from the reaction of the photo-generated holes (h + ) and the OH - (surface) or the adsorbed H2O Therefore, the high water adsorption capacity of the photocatalysts should be desired to stably decompose the organic pollutants
Figure 1.4 The diagram illustrates the photocatalytic mechanism of TiO2
TiO2 has three forms of crystalline structure: rutile, anatase and brookite; only two of the forms of TiO2 commonly used in photochemical catalysis are rutile and anatase, in which the anatase form has a higher catalytic activity
In both structures, each base unit is composed of titanium atoms surrounded by six oxygen atoms forming the orbital distortion Within each structure, the two bonds between the titanium and oxygen at the symmetry axis of the orbit are longer All three TiO2 structures are made up of octahedrons [9]
Figure 1.5 Lattice structure of Rutile
Figure 1.6 Lattice structure of Anatase
The difference between the Ti-O and Ti-Ti linkages, together with the order of octahedrons, leads to differing mass ratios and energies (Ebangap) for the Rutile and Anatase networks
Table 1.3 Specific mass and energy of the restricted area of TiO2
In spite of being the most efficient catalyst for PCO processes, TiO2 still exhibits some major disadvantages Its limited photocatalytic region (λ99.99%) divided into three streams Toluene was fed using the first nitrogen flow Water vapor was introduced
32 using the second nitrogen flow The third nitrogen stream was used to dilute the concentration of VOC and water vapor The final nitrogen stream enabled easy control over the concentration of toluene or water by changing the ratio between streams An oxygen stream (>99.99%) was supplied Each flow was controlled by a needle valve and monitored using a flow meter The mixing chamber collected all streams and stabilized the concentration of the inlet flow A 3-way valve, which was connected with the outlet of the chamber, separated the flow into 2 streams, a bypass stream and a stream to the reactor Once the mixing chamber was fully loaded, the 3-way valve was adjusted to let the gas mixture to pass through to the reactor Toluene was flown through the reactor and to the GC system The sample will be injected into the GC system using a 6-way valve
A gas mixture of toluene, oxygen, and water vapor, and nitrogen was introduced to the annular reactor The toluene and water vapor in the stream was generated by bubbling method In this report, by controlling the mass-controllers and the liquid baths temperatures, the oxygen concentration was fixed at 20 vol.%, while the water vapor concentration can be varied in order to investigate the effect of these concentrations The temperature of the gas in the annular reactor was monitored by a thermocouple located at the outlet of the reactor In addition, before starting the photo-catalytic experiment with light, the feed stream was flowed in the reactor in dark condition for saturating adsorption After an adsorption equilibrium was reached, UV light was irradiated by the four lamps The gas stream was analyzed on-line by a Flame Ionization Detector (FID) in gas chromatography (Hewlett Packard 5890plus) which equipped with a 6-way valve for online injection Toluene removal efficiency ψ (%) was calculated by the following equation:
Where: 𝐴 𝑖 : area of toluene peak at time i
𝐴 0 : area of toluene peak at initial time
Investigation of the effects of reaction’s conditions
Catalytic activity (zeolite ratio; plasma treating duration) 𝐹 = 50𝑚𝑙/𝑚𝑖𝑛 , 𝐶 𝑇𝑜𝑙 = 314𝑝𝑝𝑚 𝑣 , 𝑅𝐻 = 60% , 𝑇 𝑟𝑒𝑎𝑐𝑡𝑜𝑟 = 39℃ , 𝑚 𝑐𝑎𝑡 = 0.2 𝑔
RESULT AND DISCUSSION
Characterization of catalysts
3.1.1 X-ray diffraction pattern of catalysts
Figure 3.1 XRD pattern of TiO2, Au/TiO2, TiO2-Xand zeolite Y
The XRD patterns of the samples are shown in Figure 3.1 The XRD patterns of TiO2, Au/TiO2 and TiO2-X exhibited peak at 25.2 o ; 36.8 o ; 37.7 o ; 38.5 o ; 48 o ; 53.7 o ; 55 o ; 62.6 o ; 69 o which is characteristic of the anatase form in TiO2 (JCPDS Card no 21-1272) and at 27.8 o ; 36.2 o ; 39.8 o ; 41.6 o ; 44.8 o ; 55 o ; 57.5 o ; 63 o ; 65.2 o which indicated the rutile form of TiO2 (JCPDS Card no 21-1276) The peaks have the same intensity as the standard sample TiO2 (P25 Degussa) in [64-67], indicating no denaturation after p-TiO 2
35 synthesis No peak was detected for Au, which may be due to the small content of Au and its well-dispersion on the TiO2 surface In addition, it can be concluded that hydrogen-plasma-modification did not alter the phase of TiO2 From the JCPDS data (JCPDS Card no 39-1380), we observe that the samples of zeolite Y obtained can be classified as faujasite - type cubic Y zeolite
3.1.2 BET specific area - ICP analysis
The process of plasma treatment may altered the surface of TiO2, from which it coould possibly alter toluene adsorption capacity as well as the competitiveness of water molecules in the environment In the case of TiO2-zeolite, the large surface area (Table 3) and highly hydrophilic property of zeolite Y may lead to the high dispersion of Au/TiO2 on the coated catalyst layer, which enhanced the absorption of UV light, as well as a large amount of adsorbed water molecules which were ready for reacting with photo-holes to produce hydroxyl radical (OH ) Also, the electric field of the zeolite framework prevents the electron-hole recombination from happening by capturing the free electron [68].To preliminary examine these hypotheses, the specific surface area was determined using NOVA 2200e, Quantachrome Instruments and the result are given in the following Table 3
BET (m 2 /g) Au (wt.%) TiO2 (wt.%) Zeolite
(a) : Obtained by ICP analysis ; (b) : Calculated values
36 From N2 adsorption result, the surface area of TiO2 was not much affected by plasma treatment The surface area of Au/TiO2, p-TiO2 and TiO2 were almost the same
In addition, by incorporating with zeolite, the surface area of the sample increased nearly 4 times This could potentially lead to a better toluene and water adsorption property
The result of Au composition on TiO2 using ICP method was 0.49wt% which is close to the theoretical hypothesis (0.5wt%) set forth by conducting the sample synthesis using metal-sol method Thus, this method had a high efficiency in preparing metallic nanoparticles
3.1.3 FTIR analysis of plasma treated catalysts
FTIR spectrum for samples of hydrogen plasma-treated TiO2 are shown in Figure 3.2 FTIR spectrums of all p-TiO2 samples had a characteristic peak at about 400-800 cm -1 This is a sign for bridging stretching modes of Ti-O and Ti-O-Ti in structure [69]
A wide band at 3200-3600 cm -1 is the primary O-H stretching of the hydroxyl functional group At the same time, the band at about 1600 cm -1 is the contribution of bending vibration of the H-OH group Therefore, plasma treatment introduced hydroxyl (-OH) functional group into TiO2, leading to the appearance of characteristic peaks of –OH It also be seen that processing time contributed to the number of functional groups appearing on the material In the first 30 minutes, the longer the treatment time, the more active site are modified From 30 minutes to 60 minutes, the increase in time does not change the –OH functional group From 60 minutes onwards, the –OH groups will be separated from the material, leading to the peak intensity of –OH vibration of TiO2-
90 samples lower than the previous samples This phenomenon occurred due to the elimination of internal hydroxyl from within the TiO2 shell and also reported in previous study of hydrogenated TiO2 using different methods [34, 70, 71]
Figure 3.2 FTIR spectrum of plasma treated TiO2 samples
Based on the FTIR result of p-TiO2, the Au/TiO2 samples were chosen to modify under the same plasma condition in 15 and 30 minutes and their FTIR spectrum can be seen in Figure 3.3 Similarly to p-TiO2, the longer the time was, the clearer adsorption peak at the –OH stretching position was obtained
Figure 3.3 FTIR spectrum of plasma treated Au/TiO2 samples
3.1.4 Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM)
Figure 3.4 The SEM images of Au/TiO2 samples
Figure 3.4 illustrated the SEM images of Au/TiO2 samples It can be seen that there was no difference in the morphology of TiO2 and Au/TiO2 samples The TiO2 material blocks were relatively uniform in size However, it is not possible to see the
Au nanoparticles due to the small particle size, which cannot be displayed on SEM images
Figure 3.5 The TEM images of Au/TiO2
Figure 3.5 illustrated the TEM images of Au/TiO2 samples The mean particles size and particle size distribution were obtained by measuring more than 300 individual
Au particles from TEM images using Image J software The average particle size and the size range of Au NPs were ~12 nm and 7.5-35 nm, respectively
Figure 3.6 Au nanoparticles size distribution in Au/TiO2 samples
Catalytic performance of modified TiO 2
3.2.1 The effect of UV light on the degradation of toluene
The experiment was conducted to evaluate the effects of light on toluene vapor deposition Total flow rate is 3L/h The toluene vapor concentration was 314 ppmv, the humidity was 60%, the reaction temperature was 39 ±2 o C No catalyst was used
Figure 3.7 Energy distribution spectrum of Sankyo Denki F10T8BLB (10W)
Figure 3.8 The effect of UV light on toluene removal
(C tol 14 ppmv, RH`%, F= 50 mL/min, C O2 v%, T9 o C)
T ol uene re m ova l per c e nt a ge (C re /C o ) (% )
It can be concluded that non-catalytic treatment with UV light only changed 7% of the toluene concentration compared to the initial concentration Hence, the toluene degradation in the following experiments was due to the photochemical reactions with catalysts
3.2.2 The effect of gold nanoparticles on the photo-catalytic performance of TiO 2
Figure 3.9 The effect of gold nanoparticles on the photo-catalytic activity of TiO2
(C tol 14 ppmv, RH`%, F= 50 mL/min, C O2 v%, T9 o C)
When Au NPs were introduced to the surface of the catalyst, the efficiency and lifetime were significantly improved This can happen because Au NPs have improved the ability to create e-hole pairs of TiO2 To explain the mechanism of this process, we look at each feature of TiO2 catalysis and the oxidation catalysis Au The PCO catalyst process uses TiO2 catalysts to generate electron-hole pairs This pair is the agent that produces the free hydroxyl radicals OH from the water molecules adsorbed on the catalytic surface [72, 73] However, the disadvantage of these pairs is their existence Since the majority of holes are far from the neighbor water molecules, greater than the free electrons excited by light Therefore, instead of combining with the water molecules on the surface, the hole will quickly recombine with free electrons To overcome this shortcoming, noble metals (Pt, Au, etc.) are introduced onto the surface
42 of TiO2 to take advantage of the surface plasmon resonance (SPR) mechanism Basically, when a spherical metal nanoparticle is illuminated by a light source having a wavelength larger than the diameter of the particle, the charges on the surface being redistributed to form local electric fields on the particle surface A force of self-recovery (or a coulomb restoring force) will appear, this force affects the local electric fields, causing them to oscillate harmonically These oscillations will generate the electron- hole pairs at a rate greater than 1000 times that of the TiO2 excited by light [13] It is these pairs of e-holes that extend the hole's existence time, while increasing the density of holes in the surface of the catalyst, thereby enabling multiple holes to be able to react with water molecules to make free hydroxyl radicals Even if the doping of noble metals nanoparticles on the catalytic surface reduces the contact area with the reactant molecules, the SPR mechanism and the noble metal oxidation process are significantly effective in promoting and prolong the activity of the catalyst
Figure 3.10 a) Diagram illustrating the light absorption length/minority carrier diffusion length mismatch in TiO2 b) Diagram illustrating the local field enhancement of gold nanoparticles [13]
Figure 3.11 An alternative route of photo-electron under the presence of AuNPs [13]
In addition, Au NPs can catalyze the oxidation of oxygen-containing compounds by oxygen gas at low temperatures This is one of the key determinants of the increase in the efficiency of the PCO process In 2004, Haruta proposed the mechanism of oxidation of CO using Au/TiO2 catalyst in the presence of oxygen [74]
3.2.3 The catalytic performance of hydro-plasma-treated TiO 2
3.2.3.1 The adsorption efficiency of plasma treated TiO 2 samples
Figure 3.12 Toluene removal by adsorption of TiO2, plasma TiO2 and p-Au/TiO2
(C tol 14 ppmv, RH`%, F= 50 mL/min, C O2 v%, T9 o C)
T o luen e r emo v al p er ce n tage (Cre /Co ) (% )
TiO2 p-TiO2-15 p-TiO2-30 p-TiO2-60 p-Au/TiO2
44 The investigation of toluene adsorption capacity of the catalyst samples was carried out in two relative humidity of 60% (Figure 3.12) First, from N2 adsorption result, the surface area of TiO2 was not much affected by plasma treatment (Table 3.) Second, the toluene removal efficiency by adsorption between treated samples and TiO2 is similar However, with the appearance of Au nanoparticles on the material’s surface, the adsorption capacity dropped slightly
3.2.3.2 The effect of plasma treating time on the photo-catalytic activity of plasma-treated TiO 2
Figure 3.13 The toluene removal efficiency of TiO2-X samples with different plasma durations
((C tol 14 ppmv, RH`%, F= 50 mL/min, C O2 v%, T9 o C)
T o luen e r emo v al p er ce n tage (Cre /Co ) (% )
Irradiation time (minutes) p-TiO2-15 p-TiO2-30 p-TiO2-60
Figure 3.14 The toluene removal efficiency of H-Au/TiO2 samples with different plasma durations
(C tol 14 ppmv, RH`%, F= 50 mL/min, C O2 v%, T9 o C)
In the investigation of the effect of surface plasma processing time at the relative humidity 60% (Figure 3.13), all samples reached the highest conversion efficiency value after the first 10 minutes and these values differed The oxidation efficiency was 65.94%, 54.05% and 52.85%, corresponding to p-TiO2-15, p-TiO2-30 and p-TiO2-60, respectively It can be seen that the prolonged plasma time will reduce catalytic activity However, all catalytic samples lost their activity over time After 60 minutes of the experiment, only about 20% of toluene was decomposed due to photocatalytic oxidation From this result, hydrogen-plasma-treating durations of Au/TiO2 were determined to be 15 and 30 minutes This investigation’s condition was the same with p-TiO2 and the results were shown in Figure 3.14 It can be seen that, the effect of treating period remained the same with the presence of Au nanoparticles This phenomenon may explained by the fact that the catalyst had become more hydrophilic which caused the surface of the catalyst to be occupied by more water molecules Consequently, preventing the toluene from reaching an appropriate distance to react with free hydroxyl radicals
T o luen e r emo v al p er ce n tage (Cre /Co ) (% )
3.2.3.3 The effect of relative humidity on the photo-catalytic activity of plasma-treated TiO 2
In the investigation of the effect of relative humidity on the catalytic activity of plasma-treated samples (Figure 3.15), all samples reached the highest conversion efficiency value after the first 10 minutes and these values differed
As mentioned above, TiO2’s catalytic performance suffered severely from the water content in the gas mixture At RH`%, TiO2 and Au/TiO2 could oxidized toluene more than 70% in the first 15 minutes and this number dropped gradually overtime However, when the humidity reduced to 15%, the efficiency of TiO2 and Au/TiO2 were only 30% and 61% Furthermore, the rate of catalyst deactivation was higher
Figure 3.15 The effect of humidity on the toluene removal efficiency of
TiO2, Au/TiO2 p-TiO2 and p-Au/TiO2
(C tol 14 ppmv, F= 50 mL/min, C O2 v%, T9 o C, m cat =0.2 g)
TiO2 p-TiO2-15 Au/TiO2 p-Au/TiO2
TiO2 p-TiO2-15 Au/TiO2 p-Au/TiO2
47 Surprisingly, the change in humidity has only a small effect on the oxidation rate and the trend of the hydrogen-plasma-treating samples For p-TiO2-15 (black), the only difference in the result lied in the first 5 minutes of the experiment This sample reached its highest performance after 10 minutes in both cases and ended at 20% after 60 minutes For p-Au/TiO2 (purple), although the highest removal percentage at RH% decreased by 5% comparing to that at RH`%, the toluene oxidation efficiency after
60 minutes actually improved and ended up at 20% which is double that at RH`%
This can be explained by the presence of the -OH group on the surface of the material These groups can easily form free hydroxyl radicals to react with toluene in the absence of water molecules in the surrounding environment In addition, under lower humidity, there are less water molecules near catalyst to compete with toluene
3.2.3.4 The effect of reactor temperature on the catalytic activity
Figure 3.16 The effect of temperature on the toluene removal efficiency of TiO2 and Au/TiO2-ZY: 39 o C (line) and 50 o C (dash)
(C tol 14 ppmv, F= 50 mL/min, C O2 v%, RH%, m cat =0.2 g)
48 Figure 3.16 illustrated the result of the experiment investigating the effect of the reaction temperature on the toluene decomposition reaction The experiment was carried out with a concentration of toluene of 314 ppmv, a flow of 3 L/h, a relative humidity of 60%
It can be seen that, changing the reactor temperature affected the catalytic performance of p-TiO2-15 sample At 39 o C, its highest and lowest removal percentage were 66.72% and 19.42%, respectively At, 50 o C these values were 60.52% and 12.47% The catalyst quickly lost its catalytic activity It is worth noting that at a higher temperature, the catalyst would reach it maximum activity sooner In this case, p-TiO2-
15 reached its peak at 5 minutes and maintained the efficiency for another 5 minutes
Unlike p-TiO2-15, p-Au/TiO2 did not seem to be effected by raising the temperature The trend of the photo-catalytic oxidation was the same at both 39 o C and
50 o C However, due to the presence of OH functional groups on the surface, p-Au/TiO2 exhibited a lower photo-catalytic activity than non-treated samples
3.2.3.5 Summary on the photo-catalytic of plasma treated TiO 2
The process of non-thermal atmospheric hydrogen plasma treating is simple and easy to implement and does not change the phase of the material By applying this process, -OH species were introduced to the surface of TiO2 FTIR spectrum had confirmed the existence of -OH species in TiO2 Toluene adsorption capacity between materials before and after modifying was the same Notably, the ability of toluene oxidation in low humidity conditions is significantly improved The results also showed that, under the selected conditions to perform plasma treatment, the p-TiO2-15 exhibited better result than non-hydrogenated TiO2 In spite of increasing catalytic activity, the catalyst was slowly deactivated over time Therefore, this material is inappropriate for photo-catalytic oxidation of toluene in this thesis
3.2.4 The catalytic performance of zeolite-added TiO 2
3.2.4.1 The adsorption efficiency of zeolite-added TiO 2 samples
The aim of this experiment was to determine saturation adsorption time to emphasis the effect of zeolite as addition adsorbent The initial conditions included the toluene concentration of 314 ppmv, the relative humidity of 60%, the flow rate of 3 L/h
49 The catalyst using for these experiments are TiO2, nano Au/TiO2 and nano Au/TiO2-
ZY Saturated adsorption time was applied to all other experiments The results are shown in Figure 3.17
Figure 3.17 Toluene removal by adsorption of TiO2, Au/TiO2 and Au/TiO2-ZY
(C tol 14 ppmv, RH`%, F= 50 mL/min, C O2 v%, T9 o C)
Au/TiO2 and TiO2 have almost the same adsorption time, about 30 minutes Due to the support of zeolite Y, the saturation adsorption time of Au/TiO2-ZY increased to
50 minutes The adsorption time increased, indicating that the content of toluene and water were adsorbed on the surface of ZY
3.2.4.2 The effect of zeolite ratio on the photo-catalytic activity of Au/TiO 2 samples
CONCLUSION AND SUGGESTIONs
Conclusions
Some conclusions from the research findings presented above are as follows: The Au/TiO2 catalyst and zeolite Y were successfully synthesized using methods which were reported in previous research The catalytic activity of toluene oxidation of catalyst was investigated based on the change of factors: humidity of the air stream and reaction temperature
The activity and lifetime of TiO2 had been enhanced due to the plasmon surface resonance of Au NPs Zeolite-added and plasma-treated TiO2 exhibited different results The OH functional groups on the surface of TiO2 enhanced the photo-catalytic activity under low humidity condition Specifically, the highest toluene removal percentage was over 67% under RH% which is 1.5 times that of non-treated TiO2 However, the existence of both OH functional groups on the surface and Au NPs caused the activity to drop However, the plasma-treated samples still exhibited deactivation overtime
In this study, the zeolite:catalyst ratio of 50:50 is the most efficient ratio The highest removal percentage of Au/TiO2-ZY is over 70% of VOC at RH= 60% and it was stable throughout the experiments Although the low humidity caused the efficiency to drop, the hydrophilic property and porosity of zeolite Y had contributed a significant role in reserving the activity of the photo-catalyst
In this thesis, Au/TiO2-ZY is the most efficient catalyst for photo-catalytic oxidation process of toluene.
Suggestions
Suggestions for further research are as follows:
The VOC source in this study was toluene only Hence, the study on catalytic behavior in a mixture of VOCs should be conducted to better evaluate the material in practical uses
The light intensity in the study was considered a constant to simplify the process of research However, this value could not be maintain the same in practical use over
55 time Therefore, the effect of light intensity on the ability of toluene treatment should be observed
- The surface plasmon resonance can be observed from other rare metals Therefore, the ability of toluene decomposition would be affected by the second rare metal doping in the catalyst This study should also be conducted to investigate this effect using catalyst from two methods: direct doping method and mixing method
- The Au doping ratio was 0.5 wt% for all experiments However, the study for different ratio should be considered in future for an optimum value of metal ratio
For p-TiO 2 and p-Au/TiO 2 :
- It is critical to research on regenerating the catalysts to save operating costs when applied to industrial scale
- The conversion rate of the reaction using the synthesized catalyst was unstable Different conditions of plasma treatment (voltage, pressure, temperature, etc.) may have different impact on the outcome Such conditions should be investigated to obtain the highest and stable toluene treatment result
- p-TiO2 is not suitable for PCO in this study It is necessary to investigate new application for this material (water-splitting, etc.)
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