부산대학교 도서관

A SiO2-supported indium catalyst (In/SiO2) has shown high performance for the nonoxidative coupling of CH4(NOCM). Previous operandoX-ray absorption fine structure (XAFS) studies revealed that the In/SiO2catalyst undergoes dynamic structural changes during the activation process under flowing CH4, such as melting, new bond formation, and regeneration of the metallic form. To understand the effect of these dynamic changes, especially the formation of new bonds, on the catalytic properties, we examined the structures of an In/SiO2catalyst during the activation process under various gas flows (CH4, H2, and He) using operandoXAFS. Fresh In/SiO2catalysts are composed of metallic In particles covered with InOx. All of the samples melted at 430 K. In the XAFS spectrum of the sample under the He flow, peaks associated with the In–O bonds on the surface became visible at ∼800 K. In was partially evaporated at 1100 K because of the formation of volatile In2O. When the gas was switched from He to CH4, the In/SiO2was reduced to liquid-metal In. By contrast, in the In/SiO2under the H2flow, the In–O bonds were removed at ∼600 K. In the spectra of In/SiO2under the CH4flow, an In–Xpeak appeared at 800 K and then increased in intensity as the temperature was increased to 1100 K; this latter peak-increasing process was not observed under the He flow. The In–Xpeak was assigned to either In–CH3formed via the reaction with CH4or to In–O of surviving InOxspecies. The In–Xpeaks completely disappeared at 1100 K, and the catalyst became active toward the NOCM. Under all three gas-flow conditions, the liquid-metal In served as the active site during the reaction. The remarkable activity and long lifetime of the active site stem from the exceptional mobility and flexibility of the In atoms within the liquid metal.