Structural Evolution and Dynamics of an In2O3Catalyst for CO2Hydrogenation to Methanol: An Operando XAS-XRD and In Situ TEM Study

Abstract

We report an operando examination of a model nanocrystalline In 2 O 3 catalyst for methanol synthesis via CO 2 hydrogenation (300 °C, 20 bar) by combining X-ray absorption spectroscopy (XAS), X-ray powder diffraction (XRD), and in situ transmission electron microscopy (TEM). Three distinct catalytic regimes are identified during CO 2 hydrogenation: activation, stable performance, and deactivation. The structural evolution of In 2 O 3 nanoparticles (NPs) with time on stream (TOS) followed by XANES-EXAFS-XRD associates the activation stage with a minor decrease of the In–O coordination number and a partial reduction of In 2 O 3 due to the formation of oxygen vacancy sites (i.e., In 2 O 3– x ). As the reaction proceeds, a reductive amorphization of In 2 O 3 NPs takes place, characterized by decreasing In–O and In–In coordination numbers and intensities of the In 2 O 3 Bragg peaks. A multivariate analysis of the XANES data confirms the formation of In 2 O 3– x and, with TOS, metallic In. Notably, the appearance of molten In 0 coincides with the onset of catalyst deactivation. This phase transition is also visualized by in situ TEM, acquired under reactive conditions at 800 mbar pressure. In situ TEM revealed an electron beam assisted transformation of In 2 O 3 nanoparticles into a dynamic structure in which crystalline and amorphous phases coexist and continuously interconvert. The regeneration of the deactivated In 0 /In 2 O 3– x catalyst by reoxidation was critically assessed revealing that the spent catalyst can be reoxidized only partially in a CO 2 atmosphere or air yielding an average crystallite size of the resultant In 2 O 3 that is approximately an order of magnitude larger than the initial one.