The Active Sites And Kinetics Study Of CO Oxidation Over CuO-CeO₂ Catalysts

CuO-CeO₂ catalysts have attracted much attention due to its excellent performance in CO oxidation.Therefore,the nature of CO oxidation over CuO-CeO₂ catalysts has been widely investigated,and the high activity has been ascribed to synergistic effects between copper oxide and ceria.In this paper,a series of CuO/CeO₂ and inverse CeO₂/CuO catalysts were prepared using incipient wetness method and tested for CO oxidation.The crystalline structure of catalyst and active species on its surface were characterized by XRD,Raman,in-situ DRIFTS,H₂-TPR and CO-TPR.Commbined with catalytic properties,a relationship between the catalyst structure and CO oxidation activities was established,and verified by kinetics study.The role of oxygen vacancies in CO oxidation was also investigated.The main contents of the thesis are as follows:1.A series of CuO/CeO₂ and inverse CeO₂/CuO catalysts were prepared using incipient wetness method and tested for CO oxidation.Crystallite sizes of CuO and CeO₂ were evaluated by means of N₂O chemisorption and XRD.It was found that the crystallite sizes of CuO and CeO₂ increased with increasing loading and calcination temperature. Diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) results indicated that CO chemisorption on Cu⁺ was enhanced by CeO₂ due to its promotion of Cu²⁺/Cu⁺ redox couple.A higher activity was obtained on a CuO(5)/CeO₂-500 catalyst compared to a pure CeO₂ or CuO(5)/SiO₂-500 catalyst,suggesting the synergistic effects of CuO and CeO₂ in the reaction.Furthermore,by comparing the CuO(5)/CeO₂-500 catalyst with a CuO crystallite size of 4.1 nm and a CeO₂(5)/CuO-500 catalyst with a CeO₂ crystallite size of 4 nm,it was found that these catalysts had identical reactivities, implying that the reaction occurred at the interface of CuO-CeO₂.A detailed calculation of turnover frequency(TOF) based on the active sites located on the periphery of the CuO-CeO₂ interface shows that the reactivity was much higher on a single active site over larger CuO crystallite than over the smaller one,suggesting that the CO oxidation on CuO-CeO₂ catalyst is structure sensitive.The enhanced activity was ascribed to a higher density of chemisorbed CO on the active sites on the larger CuO crystallite,which in turn determines the reaction rate.2.The kinetics of CO oxidation over a CuO/CeO₂ catalyst was investigated in a differential reactor which was used to obtain kinetic data of CO oxidation.The reaction rate in form of power-rate law type indicated that the reaction rate was essentially dependent on CO but independent of O₂ partial pressure.In the CO oxidation at 70℃, the reaction orders were 0.7 and -0.14 with respect to the partial pressure of CO and O₂, respectively,with an activation energy of 63.8 kJ/mol.A Mars-van Krevelen type reaction model was proposed to describe the catalytic behavior of the catalysts.The model assumes that firstly CO adsorbs on the Cu⁺,then the adsorbed species migrates to the interface between the copper components and the ceria support,and reactes there with the activated oxygen supplied by the ceria support.And the oxygen vacancies on the support are replenshed by the oxygen in the reaction mixture,closing the catalytic cycle.3.The change of catalytic properties and structure before and after acid treatment was investigated on various CuO(5)/CeO₂ Catalysts calcined at different temperatures.The highest activity was obtained on a CuO(5)/CeO₂-600 catalyst,and the catalytic activity increased with increasing calcination temperature,in-situ DRIFTS results indicated that CuO(5)/CeO₂-400 provided more sites for CO chemisorption compared to the CuO(5)/CeO₂-600 catalyst;while the CuO(5)/CeO₂-600 catalyst had a higher concentration of oxygen vacancies than the CuO(5)/CeO₂-400,as evidenced by Raman spectroscopy.The acid treatment removed CO chemisorption sites,but had no influence on the oxygen vacancies.The higher reactivity obtained on the acid treated CuO(5)/CeO₂-600 catalyst compared to the CuO(5)/CeO₂-400 suggested that the oxygen vacancies played an important role in the reaction,by affecting the activation of oxygen species on the catalyst surface.