Copper-based Composite Metal Oxide Catalysts For Toluene Oxidation:Catalytic Performance And Reaction Mechanism

Volatile organic compounds（VOCs）are important precursors to the formation of ozone（O₃）and fine particulate matter(PM_2.5),and their emission can cause serious harm to human health and atmospheric environment.Therefore,it is necessary to use scientific and effective technology to control the emission of VOCs.Catalytic oxidation is one of the most effective technologies for VOCs treatment at present.Herein,research and development of efficient,stable and low-cost catalysts is the key to this technology.Metal oxide catalysts have been applied to VOCs purification because of their lower cost and higher catalytic stability compared with noble metal catalysts,and composite metal oxides can effectively make up for the lack of their catalytic activity.Thus,a series of Cu-Mn,Cu-Ce composite metal oxide catalysts were prepared in this paper,and the effects of metal interaction on the physicochemical properties and catalytic activity of the catalysts were discussed.The toluene oxidation mechanism over the catalysts was further studied by in-situ infrared spectroscopy.The main contents and conclusions of this study are as follows:（1）By adjusting the catalytic activity and physicochemical properties of CuO-MnO₂catalysts with different Cu-Mn ratios,we found that Cu1Mn3 catalyst with a Cu-Mn mole ratio of 1:3 has the best catalytic performance for toluene oxidation,and its lowest apparent beneficial to the rapid oxidation of toluene.Through XRD,SEM,BET,XPS,O₂-TPD,Toluene-TPD characterization analysis,it was found that different ratios of copper and manganese in the catalyst will have different degrees of influence on the physical and chemical properties of the catalyst.The composite of Cu and Mn can effectively improve the specific surface area of the catalyst.Moreover,with the introduction of Cu into MnO₂,the active electron transfer between Cu-Mn resulted in the presence of more high-value Mn species(Mn³⁺,Mn⁴⁺),which is conducive to the degradation of VOCs,and produces abundant oxygen vacancies and promoted the migration of lattice oxygen in MnO₂.Hence,Cu-Mn composite can also enhance the adsorption capacity of toluene.Herein,the excellent catalytic activity of Cu1Mn3 is attributed to its large specific surface area,highest oxygen vacancy concentration,Mn₃₊content and toluene adsorption capacity.Furthermore,the lattice oxygen species in the catalyst desorbed at higher temperature can participate in the oxidation reaction of toluene together with the adsorbed oxygen species.（2）CuO particles were successfully in situ grown on CeO₂ nanorods by hydrothermal method to synthesize Cu1Ce2 composite metal oxide catalyst,and single component CeO₂ and CuO catalysts were synthesized.Through the catalytic activity test,it was found that the catalytic activity of Cu1Ce2 composite is far better than that of single component catalyst,and it has good catalytic stability and water resistance.The results of BET,TEM,SEM,XPS,UV-Raman,H₂-TPR,O₂-TPD characterization showed that the Cu-Ce interaction obviously affects the physicochemical properties of catalyst.Herein,Cu1Ce2 catalyst retained CeO₂fluorite structured,and part of CuO diffraction peak can be observed.Meanwhile it has the largest average pore size and average pore volume,which are conducive to the adsorption and diffusion of pollutant molecules.There are mainly finely dispersed CuO clusters and massive CuO on Cu1Ce2,and the highly dispersed CuO can have strong interaction with CeO₂.In addition,The Cu-Ce interaction functions through the reaction:Cu²⁺+Ce³⁺（?）Cu⁺+Ce⁴⁺.The enhanced electron transfer between CuO and CeO₂ leads to high redox properties,abundant oxygen vacancies and high mobility of lattice oxyen.Thus the Cu1Ce2 obtains excellent toluene catalytic performance.（3）According to the analysis of in situ DRIFTS,the reaction pathway of toluene oxidation is as follows:firstly,toluene is rapidly adsorbed and activated to form benzyl due to the fracture of C-H bond on methyl;The benzyl group is then further oxidized to benzaldehyde,benzoate,and then through the destruction of the aromatic ring to form short chain carbonate,which is finally deeply oxidized to CO₂ and H₂O.Under oxygen-poor conditions,reactive oxygen species on the catalyst could not be replenish by gas oxygen in time,and the cleavage of C=C bond of aromatic ring was inhibited,which would lead to the accumulation of phenolate species（C₆H₅OH）and the phenolates can react with lattice oxygen in the catalyst to produce maleic anhydride and other by-products.In addition,the main adsorption species of CuO-CeO₂catalyst and mono-component metal oxide catalyst are different,while the main adsorption species of CuO-MnO₂ catalyst and mono-component catalyst are the same.Herein,the C=C bond fracture of benzoate species is the rate-controlling step of the reaction.Cu-Ce,Cu-Mn interactions can lead to the generation of abundant oxygen vacancies and the high mobility of oxygen species,thus accelerating the transformation of benzyl species and the cleavage of the C=C bond of the aromatic ring on the catalyst,which is conducive to the rapid degradation of toluene.