Performance And Mechanism For The Catalytic Combustion Of Methane Over The SnO₂-Based Catalysts

Methane, an economical and clean alternative to fuel, is widely used in power plants and new energy vehicles. However, the emission of the unburned methane can cause huge greenhouse effect. The catalytic combustion technology is a popular strategy to solve this problem. In this work, the Sn0.7M0.3O2(M=Zr and Zn) mixed oxides catalysts were prepared by the co-precipitation method to evaluate the performance for the catalytic combustion of methane. Doping with Zr and Zn cations decreases the crystallite sizes and increases the surface areas of the mixed oxides, as compared with the SnO2. Kinetics results suggest that the reaction pathways follow the Mars-van Krevelen model and the dissociation of the C–H bond is the rate determining step during the methane combustion process. The activation energy(Ea) and pre-exponential factor(A) are determined by the oxidizability and the area-specific quantity of the surface Sn4+ cations, respectively. Upon the methane combustion reaction, the surface Sn4+ cations are active sites, and the surface lattice oxygen directly involves the CH4 dissociation. EXAFS results indicate that doping with Zr weakens the Sn–O bond and thus increases the oxidizability of the surface Sn4+(H2-TPR results), while doping with Zn shortens the Sn–O bond and decreases the oxidizability of the surface Sn4+(H2-TPR results). Therefore, the Zr doping decreases its Ea values and promoted its area-specific rate.To further increase the activity of the SnO2-based catalyst, Sn1-xCexO2 catalysts were prepared by the co-precipitation method. Similarly, the formation of the Sn1-xCexO2 solid solution decreases the crystallite sizes and increases the surface areas. The Raman, XANES and XPS results demonstrate the coexistence of the surface and bulk oxygen vacancies on the Sn-rich catalysts(SnO2 phase); while for the Ce-rich catalysts(CeO2 phase), the oxygen vacancies mainly exist on the surface. Thus, the former catalysts exhibit the better oxygen mobility than the latter ones. The CH4-TPSR results provided the direct evidence that methane was dissociated by the surface Sn4+ and lattice oxygen. The oxidizability of the surface Sn4+ upon the formation of the solid solution was enhanced due to the interaction between the Sn4+/Sn2+ and Ce4+/Ce3+ couples. The Sn0.7Ce0.3O2 catalyst exhibits the highest area-specific rate because of its lowest Ea and relatively bigger A values. Its turnover frequency is five times higher, as compared with the SnO2. The reaction pathways upon the Sn-rich catalysts(SnO2 phase) follow the Mars-van Krevelen model, while they become more complex upon the Ce-rich ones(CeO2 phase). Additionally, these Sn1-xCexO2 catalysts display the high water resistance.To substitute the noble metal catalyst, we synthesized the binary metal oxide CuO/SnO2 catalysts. XRD results show that when the CuO molar proportion in the CuO/SnO2 catalysts are greater than or equal to 60 %, the CuO/SnO2 catalysts are consisted with the separated CuO and SnO2 phases without phase incorporations. We observed the obvious crystal interface between the(002) and(110) planes of the CuO phase and the(110) plane of the SnO2 phase from the TEM images. EXAFS results indicate that this interface structure lengthens the Cu–O bond and shortens the Sn–O bond. XPS results show that the binding energies of Cu 2p and Sn 3d5/2 over the CuO(60) catalyst are lower than that of the pure CuO Cu 2p and SnO2 Sn 3d5/2, respectively; while in the O 1s region, the binding energies of the CuO(60) catalyst locate between the BE peak of the CuO and SnO2 catalysts. H2-TPR results show that the oxidizability of the metal cations is greatly enhanced due to the electron interaction. In situ Raman and XANES results point out that the oxidizability of the Cu2+ is higher than that of the Sn4+. Thus, the interface Cu2+ is the active site upon the methane combustion reaction. The Ea values of the CuO/SnO2 catalysts were greatly decreased compared with the pure SnO2 and CuO catalyst. Among the CuO(x) catalysts, the CuO(60) catalyst, which owned the largest amount interface Cu2+ exhibit the highest area-specific rate, which is even higher than that of the 2 % Pd/CeO2. Moreover, the CuO(60) catalyst exhibit the more excellent H2 O and SO2 resistance ability with respect to the 2 % Pd/CeO2 catalyst. Thus, the CuO(60) catalyst is a potential catalyst for the industrial application.Moreover, we investigate the effect of the preparation method on the catalytic activity of the CuO(60) catalyst. XRD results indicate that no doping occurrs in spite of the different preparation methods. TEM results show that the CuO(60)-PM catalyst prepared by the impregnation method doesn’t contain the interface structure, while the interface structure exists in the CuO(60)-IM catalyst prepared by the impregnation method. H2-TPR results show that the peak area ascribed to the reduction of the interface Cu2+ decreases as follows: CuO(60) > CuO(60)-IM > CuO(60)-PM; meanwhile, the area-specific rates of the catalysts prepared by different methods simultaneously decrease in the above order.