Luận văn thạc sĩ Kỹ thuật hóa học: Metallic nanoparticles supported on zeolite-added TiO2 and plasma-modified TiO2: Synthesis and photocatalytic oxidation applications

VIETNAM NATIONAL UNIVERSITY - HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

-o0o -

LE NGUYEN QUANG TU

METALLIC NANOPARTICLES SUPPORTED ON

PHOTOCATALYTIC OXIDATION APPLICATIONS

Major: Chemical Engineering No.: 8520301

MASTER THESIS

HO CHI MINH CITY, 30 October 2019

CÔNG TRÌNH ĐƯỢC HOÀN THÀNH TẠI TRƯỜNG ĐẠI HỌC BÁCH KHOA –ĐHQG -HCM

Cán bộ hướng dẫn khoa học : PGS TS Nguyễn Quang Long TS Cù Thành Sơn

Cán bộ chấm nhận xét 1 : PGS TS Hồ Thị Thanh Vân

Cán bộ chấm nhận xét 2 : PGS TS Nguyễn Thị Phương Phong

Luận văn thạc sĩ được bảo vệ tại Trường Đại học Bách Khoa, ĐHQG Tp HCM ngày 25 tháng 10 năm 2019

Thành phần Hội đồng đánh giá luận văn thạc sĩ gồm: 1 GS TS Phan Thanh Sơn Nam

ĐẠI HỌC QUỐC GIA TP.HCM

TRƯỜNG ĐẠI HỌC BÁCH KHOA

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM Độc lập - Tự do - Hạnh phúc

Tp HCM, ngày … tháng … năm 20… CÁN BỘ HƯỚNG DẪN CHỦ NHIỆM BỘ MÔN ĐÀO TẠO (Họ tên và chữ ký) (Họ tên và chữ ký) TRƯỞNG KHOA ………

(Họ tên và chữ ký)

I would like to send my deepest gratitude to Associate Professor-Dr Nguyen Quang Long and Dr Cu Thanh Son, who had spent a lot of time, their great effort and dedication to communicate their valuable knowledge and experience in the process of completing this thesis Without their guidance and persistence help, this theisis would not have been possible

I would like to express my appreciation to the teachers of Department of Chemical Engineering - Faculty of Chemical Engineering of Ho Chi Minh University of Technology, who have communicated valuable knowledge to me while studying at the university This is a very important foundation for me to complete my thesis

Physico-I would like to thank the review committee, for spending time reading and giving valuable comments on my thesis

Finally, I would like to send my sincere thanks to my family and friends, who have encouraged and helped me through the years at Ho Chi Minh University of Technology

Best regards

Ho Chi Minh City, Oct 2019

ABSTRACT

This thesis studied the preparation of nanoparticles Au/TiO2-ZY and treated Au/TiO2 as catalysts for photocatalytic reaction which is a treatment of air pollutants VOCs The physicochemical properties of catalyst had been characterzied by various methods, including XRD, SEM, TEM, FTIR, ICP, BET surface area

plasma-Within the thesis, toluene vapor was used as a typical VOC to evaluate the catalytic activity under UV light The toluene concentrations before and after the reaction were analyzed by gas chromatography (GC) The effects of moisture concentration abd reaction temperature were investigated In each reaction, only one parameter was changed, while the other parameters were fixed

photo-The catalytic performance of zeolite-added TiO2 was enhanced due to the support of high surface area in zeolite and the surface plasmon resonance of Au nanoparticles The photo-catalytic oxidation process was stable throughout the whole experiment and was able to degrade 70% of toluene in the gas mixture

The presence of –OH functional group in plasma-treated samples enhance the toluene removal efficiency under low humid condition Furthermore, water content in gas mixture are less likely to effect the catalytic activity compared to non-treated TiO2

TÓM TẮT

Luận văn này nghiên cứu quá trình tổng hợp nano Au/TiO2-ZY và Au/TiO2 biến tính plasma làm chất xúc tác cho phản ứng quang hoát xử lý các chất gây ô nhiễm không khí VOCs Các tính chất hóa lý của chất xúc tác đã được xác định bằng nhiều phương pháp khác nhau, bao gồm XRD, SEM, TEM, FTIR, ICP, diện tích bề mặt BET

Trong luận án, hơi toluene được sử dụng làm nguồn VOC điển hình để đánh giá hoạt động xúc tác quang dưới ánh sáng tia cực tím Nồng độ toluene trước và sau phản ứng được phân tích bằng sắc ký khí (GC) Ảnh hưởng của nồng độ ẩm, nhiệt độ phản ứng đã được nghiên cứu Trong mỗi phản ứng, chỉ có một tham số thay đổi, trong khi các tham số khác được cố định

Đối với Au/TiO2-ZY, hiệu suất xúc tác được tăng cường nhờ sự hỗ trợ của diện tích bề mặt cao trong zeolite và cộng hưởng plasmon bề mặt của hạt nano Au Quá trình oxy hóa xúc tác quang hóa ổn định trong toàn bộ thí nghiệm và có thể làm giảm 70% lượng toluene trong hỗn hợp khí

Đối với Au/TiO2 biến tính plasma, sự hiện diện của nhóm chức -OH tăng cường hiệu quả loại bỏ toluene trong điều kiện độ ẩm thấp Hơn nữa, hàm lượng nước trong hỗn hợp khí ít có khả năng ảnh hưởng đến hoạt động xúc tác so với TiO2 không được

xử lý

LỜI CAM ĐOAN CỦA TÁC GIẢ LUẬN VĂN

Tôi xin cam đoan những kết quả được trình bày trong Luận văn Thạc sĩ này là do chính tôi thực hiện từ kiên thức của chính mình Tôi không nộp luận văn này cho bất kỳ Trường, Viện nào để được cấp bằng

CONTENT

Page

NHIỆM VỤ LUẬN VĂN THẠC SĨ II ACKNOWLEDGEMENT III ABSTRACT IV ACKNOWLEDGMENT OF AUTHORIZATION VI LIST OF FIGURES X LIST OF TABLES XI LIST OF ABBRIVIATIONS XII

CHAPTER 1: OVERVIEW 1

1.1 Introduction 1

1.2 Volatile organic compounds 1

1.3 Treatment of VOCs in the gas phase 4

2.5 Investigation of the effects of reaction’s conditions 33

CHAPTER 3: RESULT AND DISCUSSION 34

3.1 Characterization of catalysts 34

3.1.1 X-ray diffraction pattern of catalysts 34

3.1.2 BET specific area - ICP analysis 35

3.1.3 FTIR analysis of plasma treated catalysts 36

3.1.4 Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) 38

3.2 Catalytic performance of modified TiO2 40

3.2.1 The effect of UV light on the degradation of toluene 40

3.2.2 The effect of gold nanoparticles on the photo-catalytic performance of TiO2 41

3.2.3 The catalytic performance of hydro-plasma-treated TiO2 43

3.2.3.1 The adsorption efficiency of plasma treated TiO2 samples 43

3.2.3.2 The effect of plasma treating time on the photo-catalytic activity of plasma-treated TiO2 44 3.2.3.3 The effect of relative humidity on the photo-catalytic activity of

3.2.3.4 The effect of reactor temperature on the catalytic activity 47

3.2.4 The catalytic performance of zeolite-added TiO2 48

3.2.4.1 The adsorption efficiency of zeolite-added TiO2 samples 48

3.2.4.2 The effect of zeolite ratio on the photo-catalytic activity of Au/TiO2samples 49

3.2.4.3 The effect of relative humidity on the photo-catalytic activity of added TiO2 sample 51

zeolite-3.2.4.4 The effect of temperature on the catalytic activity of Au/TiO2-ZY 52

3.2.4.5 Summary on the photo-catalytic activity of zeolite-added TiO2 samples 53

CHAPTER 4: CONCLUSION AND SUGGESTIONs 54

4.1 Conclusions 54

4.2 Suggestions 54

REFERENCES 56

LIST OF FIGURES

Page

Figure 1.1 Flow chart of bio-filtration system 6

Figure 1.2 Schematic diagram of equipment used for condensing VOCs 6

Figure 1.3 Structure of activated carbon 7

Figure 1.4 The diagram illustrates the photocatalytic mechanism of TiO2 10

Figure 1.5 Lattice structure of Rutile 10

Figure 1.6 Lattice structure of Anatase 11

Figure 1.7 Photo-catalyst investigation route 12

Figure 1.8 Au face-centered cubic lattice structure 15

Figure 1.9 Illustration of the excitation of localized surface plasmon resonance 16

Figure 1.10 Spatial structure of zeolite Y (a) and frame structure of zeolite (b) 18

Figure 1.11 The angle of incidence and angle of reflection in XRD method 20

Figure 1.12 Diagram of principle XRD analysis equipment 21

Figure 1.13 Gas chromatography system 24

Figure 2.1 Hydrogen plasma system 27

Figure 2.2 The process of preparing Au/TiO2 28

Figure 2.3 The process of synthesis zeolite Y 29

Figure 2.4 The reaction system 31

Figure 3.1 XRD pattern of TiO2, Au/TiO2, TiO2-Xand zeolite Y 34

Figure 3.2 FTIR spectrum of plasma treated TiO2 samples 37

Figure 3.3 FTIR spectrum of plasma treated Au/TiO2 samples 37

Figure 3.4 The SEM images of Au/TiO2 samples 38

Figure 3.5 The TEM images of Au/TiO2 39

Figure 3.6 Au nanoparticles size distribution in Au/TiO2 samples 39

Figure 3.7 Energy distribution spectrum of Sankyo Denki F10T8BLB (10W) 40

Figure 3.8 The effect of UV light on toluene removal 40 Figure 3.9 The effect of gold nanoparticles on the photo-catalytic activity of TiO2 41

Figure 3.10 a) Diagram illustrating the light absorption length/minority carrier

diffusion length mismatch in TiO2 b) Diagram illustrating the local field enhancement

Figure 3.11 An alternative route of photo-electron under the presence of AuNPs [13].

43

Figure 3.12 Toluene removal by adsorption of TiO2, plasma TiO2 and p-Au/TiO2 43

Figure 3.13 The toluene removal efficiency of TiO2-X samples with different plasma durations 44

Figure 3.14 The toluene removal efficiency of H-Au/TiO2 samples with different plasma durations 45

Figure 3.15 The effect of humidity on the toluene removal efficiency of TiO2, Au/TiO2 p-TiO2 and p-Au/TiO2 46

Figure 3.16 The effect of temperature on the toluene removal efficiency of TiO2 and Au/TiO2-ZY: 39oC (line) and 50oC (dash) 47

Figure 3.17 Toluene removal by adsorption of TiO2, Au/TiO2 and Au/TiO2-ZY 49

Figure 3.18 The effect of Au/TiO2:zeolite ratio on the toluene removal efficiency 50

Figure 3.19 The effect of humidity on the toluene removal efficiency of TiO2 and Au/TiO2-ZY: 60% (blue) and 15% (red) 51

Figure 3.20 The effect of temperature on the toluene removal efficiency 52

LIST OF TABLES Page Table 1.1 Overview of important sources and global annual emission rates of selected groups of VOC per year (2007).[1] 2

Table 1.2 Overview of average tropospheric lifetimes of VOC compound groups and some selected VOCs as examples 3

Table 1.3 Specific mass and energy of the restricted area of TiO2 11

Table 1.4 Some methods of modified metal and application 13

Table 1.5 Some methods of preparing hydrogenated TiO2 17

Table 2.1 List of chemicals 25

Table 2.2 Investigation conditions 33

Table 3 Catalyst’s properties 35

LIST OF ABBRIVIATIONS

CHAPTER 1: OVERVIEW 1.1 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 1.2 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]

Emission rate (mg/min)

Uncertainty range

Fossil fuel use

Sum of anthropogenic and oceanic emissions

Table 1.2 Overview of average tropospheric lifetimes of VOC compound groups

and some selected VOCs as examples

1.3 Treatment of VOCs in the gas phase

The VOCs treatment methods are mainly divided into two major directions: decomposition technology and recovery technology

1.3.1 Decomposition technology 1.3.1.1 Oxidation 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 ft3/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 870oC, 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 ft3/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

1.3.1.2 Biological filtration 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

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 1.3.2 Recovery technology

1.3.2.1 VOCs condensation

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

1.3.2.2 Absorption technology

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 ft3/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

1.3.2.3 Adsorption technology

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 38oC to 260oC However, the limitations of activated carbon include flammable, poor

selectivity, low polymerization and low humidity

Figure 1.3 Structure of activated carbon

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

1.4 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 1.5 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]

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]:

C6H5CH2OO ads + e- → C6H5CHO ads + OH- (10) C6H5CHO ads + (mOH ads + nO2ads)→ oxidized compounds (11) oxidized compounds + (xOH ads + yO2ads)→ (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

However, the disadvantage of these pairs is their existence Before reaching the surface to interact with oxygen and water molecule, most of the electrons and holes recombine on the surface or in the bulk of the semiconducting material This process may occur faster than the reaction to produce radicals [12, 13] Thus, it is possible to enhance the photocatalytic efficiency of TiO2 if the existing time of the electron-hole pair is longer To overcome those limitations of TiO2-based photocatalyst, the following solutions have been adopted in previous studies: (1) modification of TiO2 catalyst, (2) enhancing surface area of TiO2 catalyst, (3) doping on the additional adsorbents, etc

This thesis will study the photo-catalytic performance of modified TiO2 The process of enhancing TiO2 will consist of doping gold nanoparticles, mechanical mixing with zeolite Y or plasma-treating TiO2 (Figure 1.7.) Gold nanoparticles was chosen due to its ability to oxidize CO under low-temperature condition and its optical property The low surface area will be enhanced using zeolite Y Plasma treating is used to enhance the surface of TiO2 with either Ti3+ or OH functional group.

Figure 1.7 Photo-catalyst investigation route

1.6 Noble metal doping 1.6.1 Previous studies

Photochemical activity in the visible light region of metal doped TiO2 is explained as the new energy level generated in the forbidden zone of TiO2 due to nanoparticle dispersion in the TiO2 network

Table 1.4 Some methods of modified metal and application

Ag [14]

AgNO3 mixed with sodium citrate tribasic dehydrate, temperature reaction reached 80oC, continued to stir TTIP and HNO3

were then added, maintaining the temperature reaction at 50 °C for 24 hours Sol was dried at 105oC for 24 hours and heated at 300oC

Decompose nitrophenol in water

phase

Fe [15]

Magnetron method: 99.99% titanium beer and 99.99% iron samples were made in the reactor, argon and oxygen were added to the tank during the exchange

Decolorize in waste water

V [16]

Sol-gel Method: Solution 1 (vanadium acetylaceton dissolved in n-butanol) was mixed with solution 2 (acetic acid in titanium butoxide) hydrolyzed for 24 hours by water formed between the esterification of acetic and butano The suspension was

Decolorize in waste water

dried at 150oC, milling and heating at 400oC for 0.5 hours

Au [17]

Titanium (IV) butoxide is dissolved in absolute alcohol, then was added to a solution containing tetrachloroauric acid (HAuCl4.4H2O), acetic acid and ethanol The suspension was aged for 2 days, vacuum drying, milling and heating at 650oC

Decolorize in waste water

Pt [18]

Optical reduction process: TiO2 emulsified in a mixture of hexachloroplatinic acid in methanol The suspension is irradiated with a mercury lamp for 1 hour Pt-TiO2 was separated by filtration, washed with distilled water and dried at 100 °C for 24 hours

Decolorize in waste water

Ce [19]

A proper amount of Ce(NO3)3 and the titania precursor of nanotubular titanic acid (NTA) were dissolved in deionized water and mixed by stirring Then the resultant mixed suspension was transferred to the autoclave reactor and kept at 120∘C for 4h The resultant mixture was transferred to a round bottom flask to evaporate water by a vacuum distillation and then the production was calcinated at 400∘C for 2h

Decompose organic compounds

1.6.2 Gold nanoparticles (AuNPs)

Nowadays, with the development of science and technology, many significant characteristics of gold under nano-scale have been discovered along with their practical applications Gold nanoparticles (AuNPs) catalysts began to be applied in 1991, when Hutching and his colleagues conducted hydrochlorination of acetylene [20]

1.6.2.1 Overview

Gold is the 79th element in the periodic table, denoted by Au, belonging to group IB, with configuration [Xe]5d96s2. The energy at 5d and 6s subclasses of Au are approximately the same, so there is competition between these two subclasses Thus, electrons are very flexible, able to move between both states, which creates a special plasticity in Au and the complexity of optical spectrum

Au exhibits the metal characteristics which are soft, ductile and easy to laminate The golden color of this metal can covert to black, ruby or purple when finely cut Gold is inert with most chemicals but dissolves in Aqua Regia forming cloroauric acid, as well as react with cyanide solutions of alkali metals

Different from bulk state, when in nano-scale, golden exhibits many special properties These include the ability to change color, change from golden to red or light purple This is explained by the fact that in gold nanoparticles absorb and scatter light with wavelength spectral region differ from normal solid gold In addition, gold catalyst is difficult to be poisoned, even under high sulfur condition

Gold (Au) has a face-centered cubic structure when in crystalline form Each Au atom is linked to 12 Au atoms around and has a network constant 𝑎 = 4,0786 Å

Figure 1.8 Au face-centered cubic lattice structure

1.6.2.2 AuNPs optical characteristic

Optical properties are one of the outstanding properties of Au nanoparticles These nanoparticles absorb and scatter light with exceptional efficiency This phenomenon is explained by the oscillation of conduction electrons on metal surfaces when they are excited by light at a specific wavelength This oscillation is also known as "Surface plasmon resonance" and it causes Au nanoparticles to absorb and radiate light much better than non-plasmon nanoparticles of the same size This phenomenon can be tuned by controlling the particle size, shape and the local refractive index near the particle surface [21, 22]

Figure 1.9 Illustration of the excitation of localized surface plasmon resonance

1.7 Hydrogenated TiO2

1.7.1 Previous studies

Recently, hydrogen TiO2 modification processes have received a lot of attention thanks to the ability to expand the light absorption spectrum of TiO2 and enhance the existence of photoelectron and holes [23, 24] Hydrogenated TiO2 can be prepared through many methods such as hydrogen thermal treatment [25], chemical reduction and oxidation [26], electrochemical reduction [27], etc

Table 1.5 Some methods of preparing hydrogenated TiO2

Hydrogen reduction

Hydrogenation or hydrogen reduction has become a powerful tool to synthesize black TiO2

nanomaterials A variety of parameters, such as source of raw TiO2, hydrogenation time and temperature, H2 pressure, exposed crystal facet of TiO2, and even reactor materials, will affect the colorization, surface structure and groups, and photocatalytic performance of hydrogenated TiO2

nanomaterials

[25, 28-31]

Hydrogen plasma

Hydrogen plasma technology has attracted increasing interest owing to its effectiveness in engineering surface-disordered TiO2 nanomaterials with a typical crystalline/amorphous core/shell structure

[32-35]

Chemical reduction

Black TiO2 can be prepared using different reducing agent such as aluminum, CaH2, magnesium, NaBH4, lithium, etc This process is called chemical reduction

[26, 36-41]

Chemical oxidation

Most of these studies prepared black TiO2 by oxidizing TiH2 in H2O2, followed by calcination in inert gas

[42-44]

chemical reduction

Electro-The electrochemically reduced black TiO2 often possessed abundant Ti3+ species and oxygen vacancies

[27, 45, 46]

Other methods

These methods include: assisted; nitrogen doping; electrochemical oxidation; ionothermal synthesis; laser irradiation; proton implantation

water-plasma-[47-52]

1.7.2 Plasma surface modification

In spite of the remarkable findings of this material, the equipment and the general conditions lead to high costs Therefore, it is necessary to develop a simple method to effectively synthesize this advanced TiO2 material The hydrogenation TiO2 technology by plasma is known for its ability to modify TiO2 surfaces without heat or high pressure and improving photocatalytic activity in the treatment of organic compound in the liquid phase [53]

1.8 Zeolite

The zeolite is an open-cell, alumino-silicate crystal with a frame formed by a three-dimensional network of TO4 tetrahedrons (T is Si or Al) The crystal structure and surface properties of the zeolite can be accurately determined Zeolite Y has a crystalline structure similar to zeolite faujazite in nature

Figure 1.10 Spatial structure of zeolite Y (a) and frame structure of zeolite (b)

The zeolite Y crystals formed during the crystallization of SiO4 and AlO4

tetrahedrons Each of the four adjacent tetrahedrons is connected by joining together atoms of oxygen at the top In tetrahedral AlO4, there is a coordinated number of 4, so the Al3+ ion makes the AlO4- tetrahedron had negative charge, so that the negative charge will be compensated by a 1-cation metal cation which is usually a alkaline metal Therefore, the number of cations in the chemical element of the zeolite is equal to the number of aluminum atoms (Al) Then the crystal network of the zeolite will be balanced

Compensation cations (Na+, NH4+, H+, ) are located inside the conduits, or cavities of the zeolite to balance negative charges in the frame

Zeolite Y is usually made in the form NaY, mainly made from three sources: Silicon (Sodium Silicate), Aluminum (Sodium Aluminate) and Sodium Hydroxyl The NaY crystallization process lasts from 8 hours to several days, depending on the material, the crystallization conditions, the chemical composition of the zeolite NaY have good SiO2 / Al2O3 ≥ 5, with a surface area of about 800m2/g, high crystallinity, negligible impurities (using XRD test method)

Zeolites are widely used as catalysts for many reactions due to the following four properties [54]

Ion exchange capacity: Due to the crystal structure in the three-dimensional space, the zeolite frame does not swell during the ion exchange This is a precious property that other inorganic exchangers do not have

The zeolite after exchange with H+ ion becomes a solid acid and it is capable of catalyzing the transformation process

The volume of porous hole in the zeolite is very large, allowing them to absorb large amounts of reactants As a result, the concentration of molecules around the active center will be greater than at the outer surface

With a uniform capillary structure of less than 10 Å in diameter, zeolites exhibit very high selectivity The diffusion of the reaction agents and the porous materials of the zeolite plays an important role in the catalytic reaction, affecting the rate of reaction as well as the distribution of the product

In these four properties, the activity and selectivity of the catalyst are determined by the surface acidity and the geometric selectivity of the zeolite This is also the basic condition for selecting the appropriate catalyst for each reaction process to achieve the highest efficiency

In the process of reacting the diffusion capacity of molecules greatly influences Diffusion ability both depends on the molecular nature and depends on the size of the capillary in the zeolite, which represents the geometric selectivity of the zeolite With a special and very uniform structure of the capillaries, zeolite allows only smaller molecules to enter and exit its capillaries Therefore, the selective shape in the capillary

is more important than the catalytic adsorption surface 1.9 Method of analyzing

1.9.1 X-ray Diffraction (XRD)

The principle of operation of the method is based on the phenomenon of X-ray diffraction on the crystal lattice due to the periodicity of the structure to create the maximum and minimum diffraction At that point, each lattice node becomes a diffraction center The incoming rays and diffraction rays interfered with each other, forming alternating light and dark veins

Figure 1.11 The angle of incidence and angle of reflection in XRD method

The law Vulf-Bragg: n λ = 2d.sinθ Where in:

d: Distance between two atomic planes

θ: The angle between the X-ray beam and the reflecting plane

n: diffraction degree (n is an integer) λ: wavelength of the X-ray beam

Each crystal phase corresponding to the characteristic distance d gives the diffraction peaks at certain angular values, so the X-ray diffraction spectrum is the dependence of the diffraction intensity

Figure 1.12 Diagram of principle XRD analysis equipment

1.9.2 Analysis of surface area by adsorption N2 (BET)

According to the BET theory, adsorption molecules do not move freely on the surface of the adsorbent and do not interact with each other, at different points may form multiple adsorbent layers but the overall is constant

Equation BET:

P⁄ − 1)

VmC+C − 1

VmC (PP0) Where in:

P is the equilibrium pressure P0 is the saturation pressure

V is the amount of adsorbed N2 Vm is the amount of mono-adsorbed N2 C is the BET constant

In the range of 0.05 < P

P0 < 0.35 then 1

V(P0⁄ −1)P is the linear function of P

P0 Surface area of the material by formula: S =VmNA

1.9.3 Scanning electron microscope (SEM)

This method uses a narrow beam of electrons to scan the surface of the sample to produce a sample image, which can reach the desired magnification when it arrives at the fluorescence screen The electron beam generated from the cathode through the two caustics will converge onto the sample When the electron beam hits the sample, the surface of the sample emits secondary electron beam Each of these electrons is accelerated by the accelerating voltage into the receiver, which changes into a light signal, amplifying the signal, and bringing it into the control network to create a brightness on the sample surface

1.9.4 Transmitted electron microscope (TEM)

This method allows for the observation of many of the nanoscale elements of the sample: shape, particle size, particle boundaries, etc using high energy electron beam through thin solid specimens and use magnetic lenses to produce images with great magnification (up to a million times), images can be produced on fluorescents, or on optical film, or recorded using digital cameras

1.9.5 FTIR analysis

When exposed to infrared radiation, sample molecules selectively absorb radiation of specific wavelengths which causes the change of dipole moment of sample molecules Consequently, the vibrational energy levels of sample molecules transfer from ground state to excited state The frequency of the absorption peak is determined by the vibrational energy gap The number of absorption peaks is related to the number of vibrational freedom of the molecule The intensity of absorption peaks is related to the change of dipole moment and the possibility of the transition of energy levels Therefore, by analyzing the infrared spectrum, one can readily obtain abundant structure information of a molecule What makes infrared absorption spectroscopy even more useful is the fact that it is capable to analyze all gas, liquid and solid samples The common used region for infrared absorption spectroscopy is 4000 ~ 400 cm-1 because the absorption radiation of most organic compounds and inorganic ions is within this region

1.9.6 Inductive coupled plasma (ICP)

ICP (Inductively Coupled Plasma) is an analytical technique used to detect elements in the fields of environment, geology, mineral, etc ICP spectrum is the type of emission spectrum using high frequency induction plasma generator Disperse atoms/ions to emit electromagnetic waves at the characteristic wavelengths for each element The intensity of this emission represents the concentration of the element in the sample

1.9.7 Gas Chromatography (GC)

The basis for separation by gas chromatography is the distribution of the sample between the two phases: the static phase has a large contact surface, the mobile phase is the permeate gas through the entire static surface If the static phase is solid then it is called gas-solid chromatography Columnar solids are usually silica gel, molecular sieve or activated carbon This process is mainly adsorbed If the static phase is liquid, we have gas-liquid chromatography The liquid envelops the surface of an inert solid, called a carrier, to form a thin film The basis for the separation is the distribution of the sample in and out of this thin film[55]

Samples are injected in and follow the carrier gas (N2) to the chromatographic column (static phase) Sample through this column will be adsorbed on the static phase After that, the substances are separated from the column by the outflow of air that is detected by the probe From the received signals the computer will process and present the result by chromatogram Substances determined by the residence time value in the chromatogram[56]

The gas chromatography system consists of the components[55]:

Carrier gas supply: can be used gas cylinders or gas appliances (the device separates N2 from the air, H2 gas from the distilled water, )

Column furnace: used to control column temperature analysis

Sample Injection Unit: Used to put the sample into the analytic column with variable pump volume When injecting a sample into a column, split and splitless modes can be used There are 2 ways to put the sample into the column: by manual injection and auto ampler (Autosampler - with or without headspace)

Catalysis Column: There are two types of columns are packed and capillary Packed column: Static phase is packed into the column, the column is 2-4mm in diameter and 2-3m in length Capillary column: static phase is covered where in (thickness 0.2-0.5μm), the column is 0.1 - 0.5mm in diameter and 30 - 100m in length Probes: Use signaling to identify and quantify the substance to be analyzed There are different types of probes for different purposes such as FID-Flame Ionization Detector, TCD-Thermal Conductivity Detector, ECD- Electronic Capture Detector, FPD-Flame Photometric Detector, NPD - Nitrogen Phosphorus Detector, MS-Mass Spectrometry

Signal Recorder: This unit records the signal detected by the probe For modern systems, this part is software recorded in the system, save parameters, chromatography, parameters related to the peak as symmetry, resolution coefficient, etc processing of parameters related to analysis results

Figure 1.13 Gas chromatography system