LITERATURE REVIEW
Overview of volatile organic compounds
VOCs, or volatile organic compounds, are organic chemicals that readily evaporate at typical indoor temperatures and pressures Their definition varies and is primarily based on their vapor pressure.
Volatile Organic Compounds (VOCs) are defined in the USA as organic compounds with a vapor pressure exceeding 13.3 Pa at 25°C, based on ASTM test method D3960–90 The US Environmental Protection Agency (EPA) broadly classifies VOCs as a diverse group of organic chemicals that encompass any carbon compound, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides, carbonates, and ammonium carbonate, that engages in atmospheric photochemical reactions.
In the European Union, a volatile organic compound (VOC) is defined as any organic compound with an initial boiling point of 250°C or lower, measured at a standard atmospheric pressure of 101.3 kPa This classification aligns with guidelines set by the World Health Organization (WHO).
Health Organization) [2] as shown in Tab 1.1:
Table 1.1 Definition of volatile organic compounds (VOCs)
No Classification Boiling point range, ( o C) Examples
1 Very volatile organic compounds (VVOCs)
< 0 to 50-100 Propane, Butane, Methyl chloride
50-100 to 240-260 Formaldehyde, d-Limonene, toluene, acetone, ethanol (ethyl alcohol) 2-propanol (isopropyl alcohol), hexane
3 Semi volatile organic compounds (SVOCs)
Volatile Organic Compounds (VOCs) are classified into several categories based on their structure, including halogenated compounds, aldehydes, aromatic compounds, polycyclic aromatic hydrocarbons (PAHs), alcohols, ketones, and miscellaneous VOCs Halogenated VOCs, such as chlorobenzene and dichloromethane, are commonly used as solvents and cleaning agents in chemical processing industries, but they significantly contribute to ozone layer depletion and are linked to cancer Aldehydes, prevalent in treated wood resins, cosmetics, and plastic adhesives, can also harm the ozone layer and pose chronic toxicity risks Aromatic compounds like toluene, benzene, and xylene are found in various domestic and industrial products, including petrochemicals and paints, and are known to damage the ozone layer and contribute to photochemical smog and carcinogenic effects on human health PAHs, produced from incomplete combustion processes, are associated with various cancers, including skin and lung cancers Alcohols and ketones, commonly found in cosmetics and personal care products, can lead to increased aldehyde formation in the atmosphere, further threatening human health Miscellaneous VOCs, such as propylene, ethylene, and methyl tert-butyl ether, emitted from petrochemical processes, contribute significantly to photochemical ozone creation potential (POCP).
BTEX compounds, which include benzene, toluene, ethylbenzene, and xylene, are prevalent solvents utilized in both industrial applications and everyday life Due to regulatory measures, toluene has gained popularity as a solvent in products such as paints, paint thinners, silicone sealants, various chemical reactants, rubber, printing inks, adhesives, lacquers, leather tanning agents, and disinfectants Additionally, toluene is a by-product of coal coke production, contributing to its industrial significance Emission data for toluene from various industrial sources can be found in Table 1.2.
Table 1.2 Summary of toluene emission factors (Source: https://www3.epa.gov/ttnchie1/le/toluene.pdf)
No Source Description Emission factors
Organic Chemical Storage 0.66 lb/1000-gallon throughput
3 Petroleum Refining Petroleum Refining 21 lb/ton total hydrocarbon
4 Cyclic Organic Crudes and Intermediates
Styrene Production-General 1.52 g/kg styrene produced
Toluene Production-General 0.104 kg/hr/source
Petroleum Marketing- Service Stations-Stage I-No Control
Inhalation of toluene, even at low to moderate levels, can lead to symptoms such as tiredness, confusion, weakness, memory loss, nausea, and loss of appetite, with some effects subsiding after exposure ends However, high levels of toluene inhalation in a short period can result in severe consequences, including light-headedness, unconsciousness, and potentially death.
Toluene, along with other volatile organic compounds (VOCs), contributes to the formation of photochemical smog, which has several detrimental effects on health and the environment When toluene combines with hydrocarbons, it produces molecules that can irritate the eyes Additionally, air radicals disrupt the nitrogen cycle by inhibiting the breakdown of ground-level ozone, leading to reduced visibility and respiratory issues.
Overview of VOCs treatment technologies
Volatile Organic Compounds (VOCs) pose significant risks to both human health and the environment, making their control and treatment essential Various methods for managing VOCs exist, which can be categorized into two main groups, as illustrated in Fig 1.2.
Effective management of VOC emissions involves modifying process equipment, changing raw materials, or altering processes While this approach is the most efficient and impactful, its implementation can be challenging due to limitations related to equipment, materials, and technology.
(ii) Treatment technology: Many techniques have been applied to remove
VOCs, such as adsorption, condensation, membrane, biology, and oxidation methods
Figure 1.2 VOCs emission control technologies
Oxidation is a common method to treat fuel gas that contains VOCs in the industrial processes Basing on the productions, oxidation is divided into two types:
- Complete oxidation that includes only CO2 and H2O in the productions (A.1)
- Incomplete oxidation that includes some other substances than CO2 and
However, oxidation of VOCs needs specific activation energy to start the reaction The activation energy depends on how strong the chemical bonds between hydrogen (H), Carbon (C), and other possible atoms
Depending on the exits of catalysts on reaction, the oxidation process can classify into the thermal oxidation (Fig.1.3 a) and the catalytic oxidation (Fig 1.3.b)
By using a catalyst, the activation energy is much lower than the energy without catalysts a Thermal oxidation technology for treatment of VOCs
Exhaust gas heat exchanger Fuel
11 | P a g e b Catalytic oxidation technology for treatment of VOCs
Figure 1.3 Catalytic oxidation technology for treatment of VOCs
The complete oxidation of volatile organic compounds (VOCs) typically requires higher temperatures than those used in combustion However, the use of catalysts can significantly lower the oxidation temperature This reduction is influenced by various factors, including the type of catalyst, flow rate, VOC concentration, and the presence of other gases.
Table 1.3 The temperature required for complete oxidation of VOCs
No VOCs The temperature required, ( o C)
Exhaust gas heat exchanger Fuel
Table 1.4 The required temperature for catalytic oxidation of VOCs
The temperature requires over catalysts, ( o C)
Catalytic oxidation offers several advantages, but it also presents challenges, including high costs, complex synthesis processes, and susceptibility to deactivation by acid gases These factors limit the practical applications of this technology.
Certain microorganisms can naturally transform volatile organic compounds (VOCs) into harmless byproducts, making this process valuable for VOC removal through microbiological methods Popular biotechnologies employed for this purpose include biofiltration, biotrickling filters, bioscrubbers, and biomembranes.
Bio-filtration is a process that utilizes a filter containing immobilized microorganisms on a porous medium, creating an optimal environment with moisture, temperature, oxygen, nutrients, and pH for these organisms As contaminated air flows through the filter, pollutants are absorbed by the bio-film on the packing material, effectively removing contaminants from the air stream.
The bio-trickling filter is a highly favored biological oxidation technique, known for its stable operation and impressive removal rates It offers low capital expenditure and superior pH control, making it an optimal choice for various applications in wastewater treatment.
Bio-scrubbers: Bio-scrubber unit consists of two subunits, namely an absorption unit and a bioreactor unit At the absorption column, VOCs transfer
The liquid waste is processed in a bioreactor, where volatile organic compounds (VOCs) are decomposed Following this gaseous treatment, the resulting solution can be recycled for further use.
Membrane bioreactors (MBRs) are an advanced filtration technique that offers a larger gas-liquid interface for effective treatment of volatile organic compounds (VOCs) In this process, VOCs are transferred through a membrane and degraded by a biofilm, allowing for the targeted removal of specific pollutants One of the key advantages of MBRs over traditional biological reactors is their ability to degrade VOCs that are poorly soluble in water, making them an efficient solution for environmental remediation.
Biotechnology offers a promising approach for the removal of volatile organic compounds (VOCs) at low concentrations, boasting several advantages However, it faces challenges, including microbial control and the maintenance of suitable living conditions The restricted application of biotechnology in VOC treatment systems is illustrated in Table 1.5 [7].
Table 1.5 Performance evaluation of bioreactors for VOCs and odor control
Capital cost Op cost Bioprocess control
Low conc of VOCs/ odors
High conc of VOCs/ odors
Bio-filter High Low High Low Low Low Low Low Low
Bio-trickling filter High Low High Low Low Low Low Low Low
Bio-scrubber High High High Low High Very low Medium Medium High
Membrane reactor High High High High Need long term evaluation
High High Need long term evaluation
This method involves the interaction between exhausted gases and a liquid, leading to the dissolution of gases or their conversion into less harmful substances Its effectiveness is influenced by factors such as the surface area of the gas-liquid interface, contact time, absorbing concentration, and the reaction rate between the absorber and the gas Previous studies have demonstrated that certain solutions can efficiently absorb organic solvent vapors, as illustrated in Table 1.6.
Table 1.6 The absorption solutions can absorb the organic solvent vapor
No The pollutant The absorption solutions The equipment The efficiency,
VOCs Oil The hollow tower 95
3 Methyl ethyl ketone (MEK) Silicone oil 95
5 Toluene Silicone oil The packed tower 99
6 n-Decane The packed tower and water The strayed tower 70
Adsorption is a widely used technique for capturing volatile organic compounds (VOCs) due to its effectiveness in removing pollutants at low concentrations However, its efficiency diminishes at high contaminant levels, as it quickly reaches adsorption equilibrium A significant challenge in this method is the necessity for secondary treatment of VOCs Once the adsorbent becomes saturated, it requires either replacement or regeneration to continue functioning effectively.
The regeneration process involves removing the adsorbate from the adsorbent's surface for reuse, necessitating significant adjustments in conditions such as temperature, pressure, inert gases, or additional chemicals.
Condensation of volatile organic compounds (VOCs) involves converting VOCs from a gaseous state to a liquid state, a widely used industrial method This process can be achieved by either increasing the gas phase's pressure at a fixed temperature or lowering the gas-phase temperature while maintaining constant pressure Condensation occurs at the dew point, where the partial pressure of VOCs in the gas phase matches their vapor pressure The correlation between temperature and vapor pressure for various common VOCs is illustrated in Figure 1.4.
Figure 1.4 The relationship between temperature and vapor pressure of the most common VOCs
Depending on the composition and concentration of VOCs in the exhaust gas, there are many cold agents used, such as water, saline solution, NH3 solution, and chlorofluorocarbons.
Catalytic oxidation of VOCs
1.3.1 Mechanisms and kinetics of catalytic oxidation of VOCs
Various mechanisms have been proposed for the complete catalytic oxidation of volatile organic compounds (VOCs), with their effectiveness largely influenced by the catalyst's properties—specifically the active metal and support—as well as the characteristics of the VOCs themselves These mechanisms can be broadly classified into three main categories: Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen.
Figure 1.5 The mechanisms of VOCs oxidation over catalysts
(Source: https://www.researchgate.net/publication/312493700)
The Langmuir-Hinshelwood (L-H) mechanism posits that the reaction takes place between adsorbed volatile organic compounds (VOCs) and adsorbed oxygen on the catalyst's surface For this process to occur, both VOCs and oxygen must be adsorbed, which can happen on either the same active sites in the single site L-H model or on distinct active sites in the dual site L-H model.
The Eleye Rideal (E-R) mechanism describes a reaction that takes place between adsorbed species and gas-phase reactant molecules, with the key step being the interaction between an adsorbed molecule and a gas-phase molecule.
The Mars-van Krevelen (MVK) model posits that the reaction between volatile organic compounds (VOCs) and the catalyst primarily involves the lattice oxygen, rather than oxygen from the gas phase This model outlines a two-step oxidation process: initially, the adsorbed VOCs interact with the catalyst's oxygen, leading to the reduction of the metal oxide Subsequently, the reduced metal oxide undergoes a regeneration process, completing the reaction cycle.
The redox mechanism describes the process where a catalyst is first reduced and then re-oxidized by the gas phase oxygen in the feed This model is extensively utilized for kinetic modeling of hydrocarbon oxidation reactions over metal oxide catalysts.
1.3.2 Catalysts for oxidation of VOCs
Catalysts used for the oxidation of VOCs can be classified into three major groups [9]: (i) noble metals catalysts; (ii) non-noble metal oxide catalysts; and (iii) mixed-metal oxides catalysts
Noble metals such as platinum (Pt), palladium (Pd), rhodium (Rh), and gold (Au) are highly effective catalysts for the removal of volatile organic compounds (VOCs) at low temperatures, achieving over 90% conversion rates at temperatures below 200°C Among various techniques for loading noble metals onto support materials, wetness impregnation is the most widely used method.
Noble-metal-based catalysts, while effective, are costly and can lose efficiency due to sintering or poisoning, making them less selective on their own Their performance is influenced by factors such as the preparation method, type of precursor, specific noble metal used, and the variety and concentration of volatile organic compounds (VOCs) present, including alkanes, acetone, aromatic hydrocarbons, and alcohols.
Previous studies indicate that platinum (Pt) is the most effective catalyst for the oxidation of volatile organic compounds (VOCs) at low temperatures Research by Rui et al demonstrated that Pt supported on Al2O3, prepared via the wetness impregnation method, achieved a 95% conversion of toluene at 200 °C Similarly, Sedjame et al investigated the oxidation of m-butanol over a Pt/Al2O3 catalyst, also prepared by the same method, finding that butanol with an initial concentration of 1000 ppm was decomposed at 165 °C with a 95% conversion rate The study further revealed that the oxidation of butanol over a Pt/CeO2 catalyst occurred at a temperature 30 °C lower than that of Pt/Al2O3, attributed to cerium's capacity to store and release lattice oxygen Additionally, Joung et al examined Pt on activated carbon, reporting complete oxidation of benzene, toluene, ethylbenzene, and xylene at temperatures of 112 °C, 109 °C, 106 °C, and 104 °C, respectively.
19 | P a g e of Pt on aluminosilicate materials are captured by some authors For example, Uson et al [21] studied in the catalyst of Pt/SBA 15 to oxidize n-Hexane, while Zang et al
[22] studied in the catalyst of Pt/ZSM 5 to oxidize propane Several previous investigations on catalysts of Pt on different supports have been summarized and reported in Tab.1.6
Furthermore, Pd is an excellent catalyst for oxidizing the BTEX group
Wang et al and Huang et al investigated the oxidation of xylene using a Pd catalyst, revealing varying results due to differences in support materials and loading techniques Notably, the Pd/Co3O4 catalyst was prepared using a precipitation method, which influenced the outcomes of the oxidation process.
Studies have shown that both post-impregnation and pre-impregnation methods achieve a conversion rate of 90% at temperatures of 249 °C and 254 °C Notably, Huang's research demonstrated that using Al2O3 as a support and the wetness impregnation method could lower the required temperature to 145 °C Additionally, the oxidation of toluene over palladium-supported catalysts was investigated by Rooke et al and Bendahou et al., with complete conversion temperatures recorded at 190 °C, 220 °C, and 400 °C for Al2O3 and SBA supports.
15 and activated carbon, which was reported by Kim et al [26], Bendahou et al [25] and Bedia et al [27] respectively
Recent studies indicate that gold exhibits promising performance in the oxidation of volatile organic compounds (VOCs), influenced by synthesis methods, support characteristics, and the shape and size of gold on supports For instance, Ali et al demonstrated that 50% propane oxidation occurred at 360 °C using an Au/CeO2-ZrO2-TiO2 catalyst prepared via deposition-precipitation Additionally, Carabinerio et al found that toluene oxidation over various metal oxide-supported gold catalysts ranked Au/CuO as the most active, followed by Au/NiO, Au/Fe2O3, Au/MgO, and Au/La2O3 Liu et al reported that Au/Co3O4 achieved a remarkable 90% conversion of toluene at 138 °C, and the same catalyst effectively oxidized benzene and xylene with 90% conversion at 189 °C and 162 °C, respectively A summary of several investigations on noble-metal catalysts supported by different materials is presented in Table 1.7.
Table 1.7 The noble metal catalysts for VOCs oxidation
Simple ultrasonic-aided incipient wetness
Pt CeO2 Wetness impregnation n-Butanol 1,000 60,000h -1 167 90
Pd Al2O3 Wet impregnation Xylene 100 100 mL/min 145 90 15
Aqueous impregnation Toluene 1,000 100 ml/min 440 90 16
Au CuO The double impregnation method
Pt SBA-15 Wet impregnation Hexane 200 31,000 h −1 186 90 21
Pd Co3O4 Post-Impregnation Xylene 150 60,000 mlh −1 g −1 254 90 24
Pd SBA-15 Wetness impregnation Toluene 1,000 10,000h -1 440 50 25
Au Co3O4 PVA-protected colloidal deposition method
Polyvinyl alcohol (PVA)- protected reduction method
Non-noble metal oxides, including transition and rare earth metals, demonstrate effective performance in the oxidation of VOCs due to their advantages such as high active component dispersion, availability, long lifespan, tolerance to masking, regeneration capability, and cost-effectiveness While their activity is generally lower than that of noble-metal catalysts, non-noble metal oxides are favored in industrial applications for VOC oxidation The performance of these metal-oxide catalysts is significantly influenced by support materials and preparation methods, with porous materials being preferred for their high surface area and large pores that enhance metal dispersion and catalytic activity Commonly utilized metal-oxide catalysts encompass copper oxide, manganese dioxide, iron oxide, nickel oxide, chromium oxide, and cobalt oxide.
Co3O4 is the most common non-noble metal catalyst in VOCs oxidation [30-
Co3O4 demonstrates exceptional catalytic performance in the oxidation of various volatile organic compounds (VOCs), including acetylene, propylene, propane, and the BTEX group Notably, toluene is identified as the VOC for which Co3O4 serves as the most efficient catalyst, according to research conducted by Jiang S The presence of mobile oxygen and a high concentration of electrophilic oxide species contributes to the effectiveness of Co3O4 in these oxidation processes.
Co3O4 on CNT support can totally decompose toluene at 257 o C < Co3O4/Beta (317 o C)
In a comparative study, Co3O4/ZSM5 demonstrated superior catalytic performance at 335 °C, outperforming Co3O4/SBA-15, which required a higher temperature of 363 °C Phung Thi Lan et al introduced an innovative adsorption-catalysis technique for the oxidation of m-xylene using a Co3O4/activated carbon catalyst Their findings revealed that this catalyst effectively fulfilled dual roles of adsorption and oxidation, achieving significant m-xylene oxidation at 180 °C with a 5% Co/AC catalyst when the adsorption time was minimized In contrast, direct oxidation techniques yielded different m-xylene conversion results.
Copper oxide is an effective catalyst for the complete oxidation of volatile organic compounds (VOCs) Among various single metal oxide catalysts, the CuO/alumina support demonstrates superior performance in the total oxidation of toluene Additionally, the CuO/activated carbon catalyst has also been investigated for its catalytic properties.
EXPERIMENT AND REARCH METHODS
Catalyst preparation
Two nitrate salts of copper and cobalt were employed as the precursors, which were supplied by Xilong Chemical Co., Ltd (China) and Sigma (Germany)
Table 2.1 Properties of chemicals using to prepare catalysts
1 Formula Cu(NO3)2.3H2O Co(NO3)2.6H2O
Activated carbon (AC) from Tra Bac JSC, MCM-41 supplied by Sud Chemie (Germany), and silica gel distributed by Sigma were selected as supports for catalyst synthesis The catalysts were prepared using two methods: wet impregnation and solid-solid blending.
This study employs the wet impregnation method, previously utilized by Nguyen Thi Lan and Nguyen Hoang Hao to synthesize single metal oxides of copper (Cu) and cobalt (Co) on activated carbon.
The preparation process involves two key steps: first, two nitrate salts, Cu(NO3)2.3H2O and Co(NO3)2.6H2O, are diluted with distilled water to create 0.2M solutions Next, the support material is thoroughly cleaned and heated in an oven.
120 o C for 24 hours, was added The mixture was stirred for 2 hours at 70 o C, then heated for 24 hours at 120 o C Finally, the collected solids were calcined at 180 o C for
2 hours, then labeled and stored in closed bottles The process of the wet impregnation method can be described in Fig 2.1
Figure 2.1 Procedure of wet impregnation method
The list of catalysts, which were prepared by the wet impregnation method, are summarized in Tab 2.2
Solution of Co(NO3)2 and Cu(NO3)2 with concentration of 0.2M
30 rounds per minute at 70 o C for 2 hours
O2 (Air) with flow rate of 2 ml/min
Table 2.2 List of catalysts prepared by wet impregnation method
Metallic weight components, (%) Support Labeling
1 7 3 AC WI-AC7Cu3Co
2 5 5 AC WI-AC5Cu5Co
3 3 7 AC WI-AC3Cu7Co
9 5 5 Silica gel WI-S5Cu5Co
In this study, two nitrate salts were combined and melted at 180 °C A support material, preheated at 120 °C for 24 hours, was then mixed with the molten salts and heated for 5 minutes at 180 °C This procedure was repeated five times using a mixer operating at 15 rounds per minute, followed by calcination of the solid at 450 °C for 2 hours with a heating rate of 2 °C/min Previous research has demonstrated the effectiveness of the solid-solid blending method in oxidizing organic compounds, as illustrated in Fig 2.2.
Figure 2.2 Procedure of solid-solid blending method
Bulk of combined nitrate salts
Melting at 180 o C to become liquid
Calcinations at 450 o C with rate of 2 o C/minute
The list of catalysts, which were synthesized by solid-solid bleeding method, are summarized in Tab 2.3
Table 2.3 List of catalysts prepared by solid-solid blending method
Metallic weight components, (%) Support Labeling
11 5 5 Silica gel SS-S5Cu5Co
13 70 30 Without carrier SS-70Cu30Co
14 50 50 Without carrier SS-50Cu50Co
15 30 70 Without carrier SS-30Cu70Co
Catalyst characterization
Thermal analysis encompasses various techniques that monitor a sample's properties over time or temperature under controlled conditions Simultaneous thermal analysis specifically involves the concurrent use of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) on the same sample within a single instrument, ensuring that the test conditions for both analyses are identical.
Table 2.4 Technique of thermal analysis
No Technique Abbreviation Property Use
2 Differential thermal analysis DGA Temperature difference
Differential thermal analysis (DTA) measures the temperature difference between a sample and an inert reference during a controlled temperature change, while thermogravimetric analysis (TGA) monitors changes in the sample's weight due to reactions, vaporization, or decomposition under similar temperature conditions.
In this thesis, TG-DSC spectra were obtained using a NETZSCH STA 449 F3 instrument from the Leibniz Institute for Catalysis in Germany, aiming to assess the influence of temperature on the thermal stability of the catalysts.
Physical adsorption of gas molecules on solid surfaces is a fundamental technique for measuring the specific surface area of materials, grounded in the BET theory (Brunauer, Emmett, and Teller) This theory is based on several key assumptions that facilitate accurate analysis.
- Gas molecules physically adsorb on a solid in layers infinitely;
- Gas molecules only interact with adjacent layers; and
- The enthalpy of adsorption for the first layer is constant and greater than the second (and higher)
- The enthalpy of adsorption for the second (and higher) layers is the same as the enthalpy of liquefaction
In this thesis, physical adsorptions of the catalysts were tested by the Gemini VII Micrometrics equipment, in Advanced Institute for Science and Technology, Hanoi, Vietnam
X-ray diffraction is a common technique to determine the phrase and crystallite sizes utilizing lattice structural parameters
X-ray diffraction is the elastic scattering of X-ray photon by atoms in a periodic lattice X-rays scattered by atoms in an ordered lattice interfere constructively in directions given by Bragg’s law:
The equation \( n\lambda = 2d \sin \theta \) describes the relationship between the wavelength of X-rays (\( \lambda \)), the distance between lattice planes (\( d \)), the angle of incidence (\( \theta \)), and the order of reflection (\( n \)) Here, \( n \) represents an integer value that indicates the order of the reflected X-rays, where \( n \) can take values of 1, 2, and so on This fundamental equation is crucial in understanding the diffraction of X-rays by crystal lattices.
(https://www.doitpoms.ac.uk/tlplib/xray-diffraction/HTML5/bragg2.html)
It can calculate the size of particles from Scherrer formula given:
𝛽𝑐𝑜𝑠𝜃 Eq 2.2 where Dp is the average crystallite size, β is line broadening in radians, θ is Bragg angle, and λ is X-Ray wavelength
XRD patterns were primarily obtained using a D8 Advance Bruker device at the Faculty of Chemistry, Hanoi University of Science, Vietnam The diffractometer utilizes a copper source emitting Cu K radiation (λ = 0.154 nm) with a step scan rate of 0.03°/sec.
Scanning electron microscopy (SEM) is a type of electron microscopy that effectively determines the size and shape of supported particles This technique also provides valuable insights into the composition and internal structure of these particles.
Figure 2.4 Schematic diagram of the core components of an SEM microscope (https://www.scimed.co.uk/education/sem-scanning-electron-microscopy/)
Scanning Electron Microscopy (SEM) operates by directing a focused electron beam across a surface, allowing for the detection of secondary or backscattered electrons based on the primary beam's position.
In this thesis, SEM images were captured by using JSM-7600F Schottky Field Emission Scanning Electron Microscope (Advanced Institute for Science and Technology, Hanoi, Vietnam)
2.2.5 Chemical and temperature programmed desorption
Isothermal chemisorption analyses are obtained by two chemisorption techniques: a) static volumetric chemisorption, and b) dynamic (flowing gas) chemisorption
- The volumetric technique is convenient for obtaining a high-resolution measurement of the chemisorption isotherm from very low pressure to atmospheric pressure at essentially any temperature from near ambient to
Commercial applications of this technique operate at temperatures around 1000 °C and are predominantly automated Achieving a high-resolution isotherm necessitates numerous precise dosing and pressure steps to reach the equilibrium point, making automation essential to avoid the time-consuming and error-prone nature of manual procedures.
The dynamic gas technique functions under ambient pressure, where the sample is first cleaned before small, precise doses of adsorptive quantities are injected in pulses This process continues until the sample reaches saturation, which is why it is referred to as 'pulse chemisorption.'
The chemisorption technique demonstrates remarkable versatility, providing extensive information about materials This versatility has been highlighted in previous discussions, and further capabilities will be explored in the upcoming sections For clarity, the examples presented will focus solely on catalysts made up of a single active species, as mixed metal catalysts would complicate the mathematical analysis by necessitating the summation of multiple terms, each weighted by the fractional contribution of the respective adsorbing species.
Temperature programmed reaction methods are techniques used to monitor chemical reactions as temperature increases linearly over time These methods are applicable to both real catalysts and single crystallites, offering experimental simplicity and cost-effectiveness compared to other spectroscopic techniques While qualitative interpretation is relatively straightforward, extracting quantitative reaction parameters like activation energies and pre-exponential factors from TP methods can be more complex.
42 | P a g e methods is a complicated matter The instrumentation for TP investigations is relatively simple; the set-ups for the temperature programmed desorption (TPD) studies of catalysts are shown in Fig 2.5
Figure 2.5 Experimental for temperature programmed reduction, oxidation and desorption
CO pulse analysis was utilized to evaluate metal dispersion, surface area, and activated particle diameter in the Autochem II 2920 at the School of Chemical Engineering, Hanoi University of Science and Technology A 100 mg catalyst underwent a pre-treatment with a Helium flow of 50 ml/min, where the temperature was increased to 300 °C at a rate of 10 °C/min and held for 60 minutes Subsequently, the catalysts were reduced using a 5% H2/Argon flow within a temperature range of 30 °C to 300 °C, followed by cooling to room temperature The CO pulse process was carried out with a CO/He flow of 50 ml/min and concluded when CO was no longer adsorbed on the catalyst Data were collected every 0.5 seconds using a TCD detector Metal dispersion (D) is defined as the percentage of metal atoms available to the probe molecule.
43 | P a g e where VAds is the amount of chemisorbed carbon monoxide, MW it the molecular weight of the metal, and SF is the stoichiometric factor;
The average particle size can be estimated using this formula:
𝑉 × 𝜌 Eq 2.4 where A is the area of the particle, V is the volume of the particle; and ρ is the density of the metal (Cu: 8.96 g/cm 3 or Co: 8.9 g/cm 3 )
O2-TPD profiles were measured with the AutoChem II 2920: 100mg catalyst was pre-treated by He flow while temperature increased to 400 o C at the rate of
Oxygen was chemisorbed onto the catalyst at a rate of 20 °C/min for 90 minutes Following this, a helium flow was employed for 30 minutes to purge the oxygen Desorption was conducted using helium flow across a temperature range of 50 °C to 700 °C at a rate of 10 °C/min, maintaining each temperature for 30 minutes Results were captured every 0.5 seconds using a TCD detector.
CH4-TPD profiles were measured with the AutoChem II 2920: 100mg catalyst was pre-treated by He flow while temperature increased to 400 o C at the rate of
10 o C/min, then keep the temperature stable for 30 minutes Then the temperature is cooled to 100 o C, and CH4 was chemisorbed over the catalyst at that temperature for
Adsorption and catalytic activity measurement
2.3.1 Adsorption and nitrogen desorption measurement
The adsorption and desorption of the catalysts are evaluated in the micro- reactor systems, which is shown in Fig 2 6
1 N2 cylinder, 2 O2 cylinder, 3 N2 mass flow controller, 4 O2 mass flow controller,
5 Toluene generators, 6 Reactor, 7 Oven, 8 Temperature controller, 9 Gas Chromatography with TCD detector, 10 Computer, V Valves
Figure 2.6 Adsorption and desorption experiment systems
In the adsorption-desorption experiment, the O2 cylinder and the O2 mass flow controller, along with valve V8, were securely closed All nitrogen components were precisely metered and regulated using mass flow controllers (MFC), which could also be shut off when the gas was not in use.
The experiment adsorption and desorption process were performed as following:
The GC Thermo Focus (Italia) was set up according to the parameters outlined in Table 2.5 A 0.2-gram catalyst was placed in a 1/8-inch diameter reactor and heated in an oven set at 180°C, regulated by a temperature controller To clean the catalyst and system, nitrogen (N2) was flowed from the cylinder for 15 minutes, with specific valves opened to control the N2 flow at 9.5 ml/min Additionally, 200 ml of toluene was introduced into the generator.
Table 2.5 Operating factors of GC
Block temperature ( o C) 180 Oven run time (min) 8,5
Flow of N2 (ml/phút) 35 Initial Time (min) 1
Flow of He (ml/phút) 20 Ramp ( o C/min) 60
Final temperature ( o C) 210 Final Time (min) 5
The analysis of initial toluene concentration involved opening specific valves (V1, V3, V4, and V5) while keeping others (V2, V6, V7, and V8) closed Nitrogen (N2) from cylinder (1) facilitated the transport of evaporated toluene from generator (5) to gas chromatography (GC) system (9), where the initial concentration of toluene was accurately monitored The N2 flow rate was maintained at 9.5 ml/min for precise measurement.
N2 controller (3), corresponding to initial toluene concentration of 9000 ppm
The adsorption process involved maintaining a stable initial toluene concentration of approximately 9000 ppm, with a temperature of 180°C and a nitrogen flow rate of 9.5 ml/min During this phase, valves V1, V3, V4, V6, and V7 were opened while valves V2, V5, and V8 remained closed The nitrogen and toluene flowed through the catalyst in reactor (6), where toluene was captured Continuous analysis of the outlet toluene concentration was conducted using gas chromatography (GC) The adsorption process concluded when the outlet concentration matched the initial toluene concentration of 9000 ppm.
The desorption process commenced after the completion of adsorption, utilizing a continuous flow of nitrogen (N2) at the same temperature and flow rate as during adsorption During this phase, valves V2, V3, V4, and V8 were closed while the remaining valves remained open As the N2 flowed through the catalyst, it effectively removed toluene from the catalyst's surface, which then traveled along with the nitrogen stream The concentration of toluene at the outlet was continuously monitored and analyzed using gas chromatography (GC).
46 | P a g e desorption finished when outlet toluene concentration was not recorded at the outlet flow
For the adsorption process, the adsorption capacity was calculated by throughout curve as the equation:
The adsorption capacity (AAd) is defined by the equation Eq 2.5, which relates to the total flow rate of the adsorption process (QAd) measured in ml/min It incorporates the inlet toluene concentration (C o Tol) in ppm and the outlet toluene concentration at time ti (C i Tol,t) in ppm Additionally, the equilibrium time (te) in minutes and the weight of the catalyst (mC) in grams are essential components of this equation.
For desorption process, the desorption amount was also calculated basing on throughout curve as the following equation
The desorption amount (ADe) is calculated using Eq 2.6, where it is influenced by the total flow rate of nitrogen (QDe) measured in milliliters per minute (ml/min), the outlet toluene concentration (C i Tol,t) at time ti expressed in parts per million (ppm), the desorption time (td) in minutes, and the weight of the catalyst (mC) in grams.
2.3.2 Catalytic activity measurement for complete oxidation of toluene
To evaluate the toluene oxidation, two experiment techniques are applied in the study:
The toluene oxidation over catalyst in desorption process:
The adsorption technique effectively treats toluene, but requires additional high-temperature processes like desorption and incineration to recycle adsorbents During desorption, the adsorbed toluene is released, which poses a challenge if recovery is not needed Consequently, to eliminate the adsorbed toluene, oxidation methods should be employed Initially, volatile organic compounds (VOCs) are adsorbed, setting the stage for effective treatment.
To effectively remove toluene from desorbed gas flows containing volatile organic compounds (VOCs) on porous materials like activated carbon, MCM-41, and silica gel, it is essential to introduce sufficient oxygen The presence of activated catalyst centers within the adsorbents facilitates the immediate oxidation of desorbed toluene, converting it into carbon dioxide (CO2) and water This process ensures efficient VOC removal and contributes to cleaner air quality.
This technology evaluates oxidized toluene over catalysts by facilitating the desorption of adsorbed toluene using a flow of oxygen This process occurs after the initial adsorption of toluene, as illustrated in the experimental system shown in Fig 2.7.
1 N2 cylinder, 2 O2 cylinder, 3 N2 mass flow controller, 4 O2 mass flow controller,
5 Toluene generators, 6 Reactor, 7 Oven, 8 Temperature controller, 9 Gas Chromatography with TCD detector, 10 Computer, V Valves
Figure 2.7 The toluene adsorption – desorption oxidation experiment systems
The experiment toluene oxidation over catalyst in desorption process were performed as followings:
The GC Thermo Focus (Italia) apparatus was operated under specified conditions, with a 0.2-gram catalyst placed in a 1/8-inch diameter reactor This setup was then placed in an oven set to 180°C, regulated by a temperature controller, while nitrogen gas flowed from a cylinder.
To clean the catalyst and the system, valves V1, V2, V6, and V7 were opened while the other valves remained closed for a duration of 15 minutes During this process, nitrogen (N2) flow was maintained at 9.5 ml/min using the N2 controller, and 200 ml of toluene was introduced into the generator.
After the preparation phase, the initial toluene concentration was analyzed by opening specific valves (V1, V3, V4, and V5) while keeping others (V2, V6, V7, and V8) closed Nitrogen gas (N2) from cylinder (1) facilitated the transport of evaporated toluene from generator (5) through the gas chromatograph (GC) (9) for monitoring The N2 flow rate was precisely controlled at 9.5 ml/min to ensure accurate measurement of the initial toluene concentration.
N2 controller (3), corresponding to initial toluene concentration of 9000 ppm
The adsorption process was conducted with a stable initial toluene concentration of approximately 9000 ppm, maintaining a temperature of 180°C and a nitrogen flow rate of 9.5 ml/min During this process, valves V1, V3, V4, V6, and V7 were opened while valves V2, V5, and V8 remained closed The nitrogen and toluene mixture flowed through the catalyst in reactor 6, where toluene was adsorbed onto the catalyst Continuous analysis of the outlet toluene concentration was performed using GC (9), and the adsorption concluded when the outlet concentration matched the initial toluene concentration of 9000 ppm.
During the desorption process, oxidation occurs after the completion of adsorption At this stage, the N2 mass flow controller is turned off, and valves V1, V2, and V5 are closed An O2 flow of 9.5 ml/min is introduced from cylinder (2) and regulated by the O2 mass flow controller (4), while maintaining a temperature of 180 °C Valves V8, V3, V4, V6, and V7 are activated to allow the O2 flow to pass through the catalyst in reactor (6), facilitating oxidation on the catalyst's surface.
RESULTS AND DISCUSSIONS
Characterizations of supports and catalysts
To ensure the stability of catalysts based on activated carbon (AC) in high-temperature environments, thermal analysis in static air is conducted This analysis assesses the thermal instability of activated carbon and its representative catalysts, as illustrated in Figure 3.1, which displays the TG and DSC curves.
Figure 3.1 Thermal analysis in static air of catalyst on AC
AC SS-AC7Cu3Co SS-AC5Cu5Co
SS-AC3Cu7Co WI-AC5Cu5Co
AC SS-AC7Cu3Co SS-AC5Cu5Co
SS-AC3Cu7Co WI-AC5Cu5Co
In static air, the mass of the activated carbon (AC) sample remained stable between 50 and 270 °C, while a notable mass decrease was observed in the catalysts starting at 200 °C Specifically, at temperatures exceeding 200 °C, the weights of SS-AC3Cu7Co, SS-AC5Cu5Co, and WI-AC5Cu5Co decreased by approximately 15% This mass loss at elevated temperatures, coupled with the endothermic effects indicated in the DSC curves, suggests that the activated carbon samples undergo combustion at high temperatures.
The impregnation of Cu and Co oxides significantly impacts the heat resistance of activated carbon due to their high catalytic activity for oxidation Specifically, samples SS-AC3Cu7Co, SS-AC5Cu5Co, and WI-AC5Cu5Co exhibited a greater mass loss during heating in static air, suggesting these catalysts enhance the combustion of activated carbon (AC) Consequently, to prevent the incineration of AC, the processes of adsorption, desorption, and oxidation with these catalysts must be limited to temperatures below 200 °C.
The N2 physical adsorption characteristics of activated carbon (AC), MCM-41, and silica gel are illustrated in Fig 3.2 The adsorption-desorption isotherms for both activated carbon and MCM-41 are classified as type IV, indicating a pore size range of 2-50 nm In contrast, the isotherm for silica gel is categorized as type II, which corresponds to a pore size greater than 50 nm, according to the IUPAC classification.
55 | P a g e a Isotherm linear plot of activated carbon (AC) b Isotherm linear plot of Silica gel
56 | P a g e c Isotherm linear plot of MCM-41
Figure 3.2 Isotherm linear plot of AC, silica gel and MCM-41
The findings align with the pore distribution results obtained from BJH desorption (Fig 3.3) and correspond with previously published data for activated carbon (AC), silica gel, and MCM-41.
Figure 3.3 Pore distribution of AC, silica gel and MCM-41
The BET surface, pore volumes and average pore sizes of these sorbents are presented in Tab 3.1
Table 3.1 The Surface characteristics of AC, silica gel and MCM-41
Activated carbon (AC) and MCM-41 exhibit significantly larger surface areas, exceeding 1000 m²/g, compared to silica gel's surface area of 295 m²/g However, silica gel has the largest average pore size at 96.2 Å, followed by AC at 39.43 Å and MCM-41 at 33.74 Å This suggests that AC and MCM-41 are more effective at adsorbing toluene due to their larger surface areas, despite silica gel's larger pore size, which is less favorable for toluene adsorption given its kinetic diameter of 6.7-8.7 Å.
The BET surface area analysis of catalysts derived from activated carbon (AC) and silica gel reveals that the incorporation of metallic oxides significantly decreases the surface area of AC by 40-58% In contrast, this modification has a minimal impact on the surface area of silica gel, which experiences a reduction of just over 10%.
AC decreased more when the composition of Co increased, thus, WI-AC3Cu7Co sample possessed a surface area of only 418 m 2 /g
Table 3.2 The surface characteristics of catalysts on AC and silica gel
The results of surface areas, BJH pore sizes, and volumes of the catalysts on MCM-41 are shown in Tab 3.3
Table 3.3 The surface characteristics of catalysts on MCM-41
The deposition of bimetallic oxides on MCM-41 surfaces significantly reduced the catalyst's surface area and porous volumes The highest surface area was achieved with the SS-M5Cu5Co sample, which maintained the pore size distribution of MCM-41, followed by SS-M10Cu, SS-M3Cu7Co, and SS-M10Co This phenomenon can be attributed to the larger particle size of single cobalt oxide compared to samples containing both Co and Cu, which led to a narrowing of MCM-41's surface Notably, the combination of 5% Cu and 5% Co resulted in smaller particles, thereby recording the largest surface area for the SS-M5Cu5Co catalyst.
The catalysts based on MCM-41 exhibit a type IV isotherm and pore distribution, while MCM-41 itself is classified as type II This indicates a noticeable alteration in the surface characteristics of MCM-41.
The incorporation of bimetallic oxides, specifically copper (Cu) and cobalt (Co), into MCM-41 significantly influences its pore size This effect is observed through both solid-solid blending and wet impregnation methods, as illustrated in Figure 3.4.
59 | P a g e a Prepared by solid-solid blending b Prepared by wet impregnation method
Figure 3.4 Pore distribution of catalyst on MCM-41
Loading bimetallic oxides onto MCM-41 significantly impacts pore size, with the exception of SS-M5Cu5Co This process obstructs the mesopores of MCM-41, resulting in the formation of larger pores.
SS-M7Cu3Co SS-M5Cu5Co SS-M3Cu7Co
WI-M7Cu3Co WI-M5Cu5Co
WI-M3Cu7Co WI-M10Co
60 | P a g e influence is only minor with SS-M5Cu5Co, which may be due to the smaller sizes of 5Cu5Co particles and it will be discussed later in XRD results
Wet impregnation enhances the incorporation of Cu-Co oxides into the pores of MCM-41, leading to a reduced surface area in the resulting catalysts, as shown in Table 3.3 This reduction in surface area and pore size is influenced by both the presence of bimetallic oxides and the synthesis methods employed.
The XRD patterns for the catalysts WI-AC5Cu5Co and AC, illustrated in Fig 3.5, reveal the absence of metal oxide peaks This is attributed to the amorphous nature of AC, which exhibits a high baseline in its XRD pattern, coupled with the minimal metallic content that prevents the detection of reflected beams.
A: WI-AC5Cu5Co; B: Activated carbon
Figure 3.5 XRD patterns of catalysts on AC
The XRD patterns for catalysts on silica gel, namely SS-S20Co, WI-S20Co and WI-S5Cu5Co were presented in Fig 3.6 There was the only structure of Co3O4
(ICSD-01-078-1969), which was detected on the samples of WI-S20Co and SS- S20Co, while the structures of both Co3O4 and CuO (ICSD-01-080-0076) were found in WI-S5Cu5Co
A: SS-S20Co; B: WI-S20Co; C: WI-S5Cu5Co
Figure 3.6 XRD patterns of catalysts on silica gel
The crystallite sizes of Co3O4 and CuO were determined by Scherer equation as shown in Tab 3.4
Table 3.4 Crystallite size and phase of Cu-Co/Silica gel
No Catalysts Crystallite sizes, (nm)
Catalysts prepared through the wet impregnation method resulted in smaller Co3O4 particle sizes on the surface of silica gel Additionally, an increase in cobalt (Co) content correlated with larger Co3O4 particle sizes, with samples containing 20% Co displaying greater particle sizes compared to those with 5% Co.
The XRD patterns of catalysts on MCM-41 base, prepared by solid-solid blending method and wet impregnation method, were shown in Fig 3.7 and Fig 3.8, respectively
A: SS-M10Cu; B: SS-M10Co; C: SS-M3Cu7Co; D: SS-M5Cu5Co; E: SS-M7Cu3Co
Figure 3.7 XRD patterns of 10% catalysts on MCM-41 prepared by solid-solid blending method
A: WI-M10Co; B: WI-M3Cu7Co; C: WI-M5Cu5Co
Figure 3.8 XRD patterns of 10% catalysts on MCM-41 prepared by wet impregnation method
The crystallite sizes of the metal oxides were determined by Scherer equation which showed in Tab 3.5
Table 3.5 Crystallite sizes and phases of 10% Cu-Co on MCM-41
The XRD pattern analysis of SS-M7Cu3Co reveals the presence of CuO and Co3O4, with average crystallite sizes of 22.87 nm and 9.26 nm, respectively In contrast, only Co3O4 was identified in the catalysts SS-M5Cu5Co and SS-M3Cu7Co due to their lower copper content, resulting in average crystallite sizes of 8.71 nm and 10.24 nm for Co3O4 This indicates that increasing copper content leads to a reduction in the crystallite size of cobalt particles, suggesting that the use of bimetallic oxides enhances the dispersion of metal oxide sites on carrier surfaces.
Total oxidation ability of the catalysts for methane
Methane, a member of the alkane group, is known for its strong bonds, making complete oxidation challenging even at high temperatures Typically, methane undergoes partial oxidation to yield methanol, aldehyde, or carbon monoxide at temperatures exceeding 400°C In contrast, complete oxidation of methane requires temperatures above 850°C Thus, evaluating the oxidation capabilities of catalysts is essential for assessing their effectiveness in fully oxidizing methane.
75 | P a g e hardest compound – methane Thus, the synthesized catalysts were pre-examined by methane oxidation in this study
Methane (CH4) is challenging to adsorb onto porous materials, prompting the use of temperature program desorption (TPD) to evaluate the adsorption and oxidation capabilities of various catalysts.
Figure 3.16 CH 4 –TPD profiles of Cu-Co/MCM-41
CH4 –TPD profiles of the catalysts are presented in Fig 3.16, there are two desorbed peaks at low temperature (about 300 o C), and high temperature (about 500-
At temperatures of 600 °C, the adsorption capacity of CH4 was found to be greater on SS-M10Cu compared to SS-M10Co, as shown in Table 3.11 This difference can be attributed to the unique properties of CuO Catalysts with higher CuO content, such as SS-M5Cu5Co and SS-M7Cu3Co, demonstrated superior CH4 adsorption due to an increased number of active CuO centers Enhanced CH4 adsorption capabilities are likely to lead to improved catalytic activity for CH4 oxidation.
SS-M5Cu5Co SS-M10Co SS-M10Cu
SS-M7Cu3Co SS-M3Cu7Co
Table 3.11 CH 4 -TPD quantities of Cu-Co/MCM-41
The results of methane oxidation over the catalysts on silica gel base were introduced in Fig 3.17
Figure 3.17 Catalytic activity of Cu-Co/silica gel for the complete oxidation of methane
The experiment demonstrated that methane conversion does not occur below 200 °C, as previous studies indicate that silica gel lacks the capacity to absorb methane, resulting in low conversion rates at this temperature range Additionally, methane oxidation over the catalysts commenced at temperatures exceeding 200 °C.
The catalytic activities of the catalysts were ranked based on their performance at varying temperatures, showing a significant increase from 200 °C to 450 °C The order of effectiveness was identified as follows: SS-S20Co, WI-S20Co, WI-S3Cu7Co, SS-S5Cu5Co, WI-S5Cu5Co, and WI-S7Cu3Co.
The catalytic activity of bimetallic oxides on silica gel is influenced by both the cobalt content and the preparation method used Specifically, the solid-solid blending method yields a more active catalyst compared to the wet impregnation method This difference in activity can be attributed to the greater exposure of metallic oxides on the surface in the solid-solid blending process, while the wet impregnation method tends to trap metallic oxides within the pores.
The study found that the highest conversion rate of methane to CO2 was 83%, achieved with the SS-S20Co catalyst In contrast, catalysts containing both Cu and Co on silica gel demonstrated significantly lower conversion rates, remaining below 30% This suggests that the inclusion of Cu negatively impacts the catalyst's activity, specifically the performance of CuO.
Co3O4 is crucial for the oxidation of methane, although it struggles to fully convert methane to CO2 at 450°C In contrast, catalysts supported on silica gel show significantly lower methane conversion rates compared to unsupported catalysts, with a 20% loading demonstrating notably higher activity than a 10% loading.
The results of methane oxidation over MCM-41-supported catalysts, as shown in Fig 3.18, indicate that higher temperatures enhance methane conversion rates At 450 °C, the catalysts WI-M10Co and SS-M10Co achieved the highest conversions of 100% and 95%, respectively, outperforming unsupported catalysts This suggests that the fine dispersion of cobalt oxide on MCM-41 increases the availability of active sites for the reaction Conversely, catalysts containing only copper demonstrated poor activity, with significant improvement observed only when copper content was reduced to 5% in the WI-M3Cu7Co and WI-M5Cu5Co formulations.
79 | P a g e b Methane oxidation over Cu-Co/MCM-41 prepared by wet impregnation
Figure 3.18 Catalytic activity of Cu-Co/MCM-41 for the complete oxidation of methane
Figure 3.19 illustrates the comparison of catalyst activities prepared through various methods When the cobalt (Co) content exceeded 5%, the methane conversion rates for samples produced using the wet impregnation (WI) method surpassed those created via the solid-solid blending method, attributed to the larger particle size of the bimetallic oxides, as shown in Table 3.5.
Figure 3.19 Comparison of methane oxidation with different preparations at 450 o C
Bimetallic oxide compounds exhibited greater oxidation activities compared to single copper compounds (33%), although their performance was slightly inferior to that of single cobalt oxide This indicates that copper's impact on methane conversion is minimal, primarily serving to disperse the catalytic particles.
The results of methane oxidation over unsupported catalysts indicate that the catalytic activity of bimetallic oxides without MCM-41 follows the order: 100Co > 70Cu30Co > 50Cu50Co > 30Cu70Co > 100Cu While the differences in activity among 100Co, 70Cu30Co, 50Cu50Co, and 30Cu70Co are minimal, the activity of 100Cu is significantly lower This suggests that the Co3O4 oxide catalyst demonstrates superior oxidation properties compared to the CuO catalyst Additionally, the presence of CuO in the bimetallic oxide catalysts aids in reducing the particle size of Co3O4, resulting in more exposed active sites for the reaction Co3O4-based catalysts can achieve nearly 90% methane conversion at 450 °C.
Figure 3.20 Catalytic activity of unsupported Cu-Co catalysts for the complete oxidation of methane
The comparison of methane oxidation over the catalyst over supports and without supports was presented in Fig 3 21
The study found that utilizing supports resulted in a reduction of methane conversion Conversely, an increase in methane conversion was observed with higher cobalt (Co) content in the Cu-Co/MCM-41 catalyst.
Figure 3.21 Comparison of methane oxidation on Cu-Co with and without supports at 450 o C
The study reveals that cobalt (Co) catalysts demonstrate superior activity for methane oxidation compared to copper (Cu) catalysts, despite Cu catalysts showing a higher capacity for CH4 and O2 adsorption on MCM-41 This suggests that the nature of the oxide plays a more critical role in complete oxidation than the adsorption capabilities of the reactants Interestingly, when 5% Cu is incorporated into Co catalysts (SS-M5Cu5Co), the catalytic activity improves significantly, matching that of the Co catalysts This enhancement is attributed to the finer distribution of active sites, as the smaller particle size of Co3O4 in this composite exposes more Co sites for the reaction, while still maintaining higher adsorption abilities for CH4 and O2 than the Cu catalyst.
While supports enhance the surface area of catalysts, they often result in a lower number of active sites compared to unsupported catalysts, leading to decreased catalytic activity Specifically, catalysts supported on silica gel show significantly reduced activity compared to those on MCM-41, attributed to the latter's considerably larger surface area.
Toluene treatment
3.3.1 Toluene adsorption on catalysts/ sorbents
3.3.1.1 Toluene adsorption over Cu-Co/Activated carbon
The simulated isotherm for toluene on different component catalysts of Cu- Co/AC are shown in Fig 3.22
Figure 3.22 Toluene adsorption breakout curves on AC base
The adsorption capacity was determined using the breakthrough curve of toluene adsorption, as outlined in Eq 2.5 and presented in Table 3.12 The findings of toluene adsorption on the catalysts correlated well with the characteristics of the catalysts' surfaces and the sizes of the particles discussed earlier.
Table 3.12 Adsorption amount of toluene on Cu-Co/Activated carbon
The fresh activated carbon (AC) exhibits the highest adsorption capacity of 0.28 g/g due to its extensive surface area The presence of metallic oxides on the surface and within the pores of the AC, as shown in SEM images, contributes to a reduction in available surface area The adsorption capacity is ranked as follows: AC180 > AC3Cu7Co > WI-AC5Cu5Co > WI-AC7Cu3Co Importantly, the impregnation of bimetallic oxide catalysts on the AC support, specifically WI-AC5Cu5Co and WI-AC3Cu7Co, does not significantly diminish the toluene adsorption ability of the material, indicating that the adsorption capacity remains largely unaffected and the sorbent continues to perform effectively during the toluene adsorption process.
3.3.1.2 Toluene adsorption over Cu-Co/Silica gel
The outlet concentration of toluene over Cu-Co/Silica gel is illustrated in Fig 3.23, revealing that the adsorption capacity of toluene on these catalysts is constrained, which correlates with the surface properties of silica gel.
Figure 3.23 Toluene adsorption breakout curves on silica gel base
The toluene adsorption capacity of Cu-Co/Silica gel, as shown in Table 3.13, was notably low due to the macro porous nature of silica gel, which limits its ability to adsorb toluene effectively This finding aligns with prior research on toluene adsorption using silica gel Consequently, the impregnation of bimetallic oxide onto silica gel is deemed unsuitable for toluene treatment through adsorption methods.
Table 3.13 Adsorption amount of toluene on Cu-Co/Silica gel
3.3.1.3 Toluene adsorption over Cu-Co/MCM-41
The break curve of toluene adsorption on catalysts over MCM-41 base were presented in Fig 3.24 and the adsorption amount were shown in Tab 3.14
Figure 3.24 Toluene adsorption breakout curves on MCM-41 base
The impregnation of bimetallic oxides on MCM-41 support significantly reduces the support's surface area, leading to decreased toluene adsorption In contrast, the impregnation of single metallic oxide (CuO) further diminishes adsorption due to larger particle sizes occupying the support's pores Additionally, the synthesis method plays a crucial role in adsorption efficiency; for instance, sample SS-M5Cu5Co demonstrates higher toluene adsorption compared to WI-M5Cu5Co, attributed to its greater surface area Notably, SS-M5Cu5Co achieves an adsorption amount nearly equivalent to that of MCM-41, indicating its potential effectiveness for toluene adsorption during the adsorption period.
Table 3.14 Adsorption amount of toluene on Cu-Co/MCM-41
The impact of surface area on toluene adsorption by various catalysts is illustrated in Fig 3.25, revealing that surface area alone does not determine adsorption capacity For instance, samples with high surface areas, such as activated carbon (AC) and MCM-41, do not demonstrate significantly higher adsorption abilities compared to bimetallic oxide catalysts on different supports This suggests that factors like pore size also play a crucial role in toluene adsorption Notably, the incorporation of bimetallic oxides did not diminish the toluene adsorption capacity of AC or MCM-41 Among the tested materials, catalysts on AC exhibited the highest toluene adsorption ability.
The study found that 41 demonstrated effective toluene adsorption capabilities and offered a significant advantage with an unrestricted temperature range for the desorption period, unlike activated carbon (AC), which is limited to a maximum desorption temperature of 200°C In contrast, catalysts supported on silica gel displayed considerably lower toluene adsorption performance, indicating that silica gel may not be an optimal choice for toluene treatment.
Figure 3.25 Effect of surface’s area of supports on toluene adsorption amount
3.3.2 Oxidation over catalysts in desorption process
3.3.2.1 Toluene oxidation over Cu-Co/Activated carbon in desorption process
The desorption process of Cu-Co/AC, depicted in Fig 3.26, reveals a significantly higher initial toluene concentration compared to the toluene inlet concentration These findings align with similar results reported in previous studies [69, 70].
89 | P a g e a Toluene generation by nitrogen flow in desorption process b Toluene generation by oxygen flow in desorption process
Figure 3.26 Generated toluene concentrations from heat desorption over
Table 3.15 presents the desorption amounts of previously adsorbed toluene using various carrier gases A notable difference in toluene concentration was observed when nitrogen (Fig 3.26a) was substituted with oxygen (Fig 3.26b) This variation occurs because the presence of oxygen causes the bimetallic oxide catalysts on activated carbon to oxidize the generated toluene, resulting in a decreased concentration of toluene in the outlet flow.
Figure 3.27 Formed CO 2 from heat desorption by oxygen flow over Cu-Co/AC
The outlet CO2 concentration during desorption with oxygen flow is illustrated in Fig 3.27 The CO2 yield was calculated using Eq 2.9 and subsequently compared to the theoretical oxidation of toluene, as detailed in Tab 3.16.
Table 3.15 Generated toluene by thermal desorption
Toluene conversion compared to the desorption amount, (%)
Toluene conversion compared to the adsorption amount, (%)
Table 3.16 Evaluation of total toluene oxidation over the catalysts on AC
No Samples Theoretical CO 2 amount, (mmol/g)
The findings indicate that the amount of toluene desorbed by nitrogen is significantly lower than the amount adsorbed, suggesting that toluene is not fully desorbed and that a portion remains trapped within the pores of activated carbon (AC).
The study concludes that while the catalysts can oxidize toluene at 180 °C, complete oxidation is not achieved The presence of metallic oxides on activated carbon (AC) and oxygen in the flow significantly reduces toluene concentration compared to nitrogen desorption; however, high toluene levels persist during the initial minutes of the outlet flow, and CO2 yield remains low, suggesting that toluene is being oxidized into other organic compounds Notably, the WI-AC5Cu5Co catalyst demonstrates exceptional performance, achieving a 100% CO2 yield, indicating its strong activity for oxidation, consistent with previous findings on methane oxidation.
Toluene is fully decomposed into CO2 over the WI-AC5Cu5Co catalyst at 180 °C; however, the conversion rate does not achieve 100% due to the high initial concentration of toluene during thermal regeneration Following this, the WI-AC3Cu7Co and WI-AC7Cu3Co catalysts show varying levels of performance Bimetallic oxides of cobalt and copper demonstrate superior activation in the oxidation of volatile organic compounds (VOCs), indicating their potential for further synthesis over different supports to enhance VOC oxidation evaluation.
Previous studies indicate that adsorbed oxygen is released at temperatures below 400 °C for Co3O4 and below 200 °C for CuO, while the oxidation temperature is only 180 °C This suggests that lattice oxygen is not responsible for the oxidation of toluene Consequently, the toluene oxidation mechanism over these catalysts is likely classified as either Langmuir-Hinshelwood (L-H) or Eley-Rideal (E-R), rather than the Mars-van Krevelen mechanism, which relies on lattice oxygen oxidation at higher temperatures Further investigation is needed to confirm the mechanism that aligns with this reaction.
3.3.2.2 Toluene oxidation over Cu-Co/ /Silica gel in desorption process
The thermal desorption of toluene on Cu-Co/Silica gel with nitrogen and oxygen flow resulted in low adsorption levels, as illustrated in Figures 3.28 and 3.29 This limited adsorption is attributed to the macro porous nature of the silica gel material.
93 | P a g e capacity of toluene It is in agreement with the previous studies in the toluene adsorption of silica gel
Figure 3.28 Toluene generation on Cu-Co/silica gel by N 2 in desorption
Figure 3.29 Toluene generation on Cu-Co/silica gel by O 2 in desorption