Preparation And Modification Of CeO₂ Catalyst For Catalytic Oxidation Of Toluene At Low Temperature

The rapidly increasing emission of volatile organic compounds(VOCs)by various sources and industries is posing a serio’us threat to the environment and human health as they are biggest source of air pollution.At present,the abatement of volatile organic compounds(VOCs)from the atmosphere has become a challenging task globally.Recently,catalytic oxidation is considered an efficient approach for the abatement of VOCs as they can convert VOCs Into harmless substances.The catalyst used in catalytic oxidation approach is of prime importance as it is the core of catalytic oxidation technique.Transition metal oxide catalyst has gained much attention due to its cheap nature,resistance to poisoning,coking and good thermal stability in catalytic oxidation of VOCs.Among transition metal oxide catalyst cerium oxide based catalyst are considered as a promising catalyst in catalytic oxidation of VOCs.In this thesis,cerium was selected as the main active components of the catalyst and modified with different metal and different morphology to enhance the catalytic performance.A series of CoxCe1-xO2-δ(x=0.05-0.6),MnxCe1-xO2(x=0.05-0.2)oxide catalysts and different morphologies of cerium oxide catalysts were prepared for catalytic oxidation of toluene at low temperature.Different characterization methods such as XRD,BET,Raman,SEM,TEM,XPS,H2-TPR,O2TPD and In Situ DRIFTS were used to evaluate the interaction effect between different metal oxides and different morphologies of cerium oxide on the surface properties,structure and catalytic performance of catalyst.CeO2 catalysts with different morphologies exhibit different oxygen vacancies content,which plays a vital role in oxidation reaction.Herein,three distinct morphologies of CeO2 i.e,shuttle(CeO2(S)),nanorod(CeO2(R)),and nanoparticle(CeO2(P))were successfully fabricated by the SEM and TEM results,and investigated for toluene catalytic oxidation.The various characterizations showed that the CeO2(S)catalyst exhibited a larger surface area along with higher surface oxygen vacancies in contrast to CeO2(R)and CeO2(P),which is responsible for its excellent toluene catalytic oxidation.The 90%toluene conversion temperature at 225℃ over CeO2(S)was lesser from that over CeO2(R)(283℃)and CeO2(P)(360℃).In addition,CeO2(S)showed a greater reaction rate(14.37 ×10-2 μmol·g-1·s-1),TOFov(4.8 × 104·s-1)at 190℃ and lower activation energy value(67.4 kJ/mol).Furthermore,the CeO2(S)also displayed good recyclability,long-term activity stability,and good tolerance to water.As a result,CeO2(S)is considered a good candidate to remove toluene.A series of CoxCe1-xO2-δ(x=0.05,0.1,0.2,0.3,0.4,0.6)composite oxides were synthesized through non-ionic surfactant hydrothermal method and applied for catalytic toluene oxidation.Various characterizations were performed to investigate the relation between structure and activity of the catalysts.CoxCe1-xO2-δ showed higher catalytic performance than pure Co3O4 and CeO2,meanwhile Co0.2Ce0.802-δ exhibited the best catalytic activity for toluene oxidation in the presence and absence of water,moreover,no obvious difference in activity among three successive testing runs.This good activity can be mainly attributed to the strong interaction between Co and Ce oxides,leading to abundant surface active oxygen species.In situ DRIFTS was used to probe the catalytic reaction process,which revealed that toluene can be rapidly adsorbed and transferred to benzoate species,finally oxidized into CO2 and H2O.A series of doped MnxCe1-xO2 catalysts(x=0.05-0.2)with oxygen vacancy defects were synthesized by doping low-valent Mn in a CeO2 lattice.Different characterization techniques were utilized to inspect the effect of doping on oxygen vacancy defect generation.The characterization results revealed that the Mn0.15Ce0.85O2 catalyst had the maximum oxygen vacancy concentration,leading to increased active oxygen species and enhanced oxygen mobility.Thus,Mn0.15Ce0.85O2 catalyst showed an excellent toluene oxidation activity with 90%toluene conversion temperature(T90)of 197℃ at a weight hourly space velocity of 40,000 mL g-1 h-1 as compared to undoped CeO2(T90=225℃).In addition,the Mn0.15Ce0.85O2 catalyst displayed strong recyclability,water resistant ability and long-time stability.The in situ DRIFTS results showed that the Mn0.15Ce0.85O2 catalyst had a robust oxidation capability as toluene was quickly adsorbed and actuated as compared to CeO2.Thus,the present work lays the foundation for designing a highly active catalyst for toluene elimination from the environment.