The Effect of Structural Change during the Activation Process on the Catalysis of In/SiO2 Nonoxidative Coupling of Methane: An Operando XAFS Study

A SiO2-supported indium catalyst (In/SiO2) has shown high performance for the nonoxidative coupling of CH4 (NOCM). Previous operando X-ray absorption fine structure (XAFS) studies revealed that the In/SiO2 catalyst 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/SiO2 catalyst during the activation process under various gas flows (CH4, H2, and He) using operando XAFS. Fresh In/SiO2 catalysts 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/SiO2 was reduced to liquid-metal In. By contrast, in the In/SiO2 under the H2 flow, the In–O bonds were removed at ∼600 K. In the spectra of In/SiO2 under the CH4 flow, an In–X peak 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–X peak was assigned to either In–CH3 formed via the reaction with CH4 or to In–O of surviving InOx species. The In–X peaks 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.