Structural Evolution and Dynamics of an In2O3 Catalyst for CO2 Hydrogenation to Methanol: An Operando XAS-XRD and In Situ TEM Study

Abstract

We report an operando examination of a model nanocrystalline In₂O₃ catalyst for methanol synthesis via CO₂ 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 CO₂ hydrogenation: activation, stable performance, and deactivation. The structural evolution of In₂O₃ 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 In₂O₃ due to the formation of oxygen vacancy sites (i.e., In₂O_3-x). As the reaction proceeds, a reductive amorphization of In₂O₃ NPs takes place, characterized by decreasing In-O and In-In coordination numbers and intensities of the In₂O₃ Bragg peaks. A multivariate analysis of the XANES data confirms the formation of In₂O_3-x and, with TOS, metallic In. Notably, the appearance of molten In⁰ 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 In₂O₃ nanoparticles into a dynamic structure in which crystalline and amorphous phases coexist and continuously interconvert. The regeneration of the deactivated In⁰/In₂O_3-x catalyst by reoxidation was critically assessed revealing that the spent catalyst can be reoxidized only partially in a CO₂ atmosphere or air yielding an average crystallite size of the resultant In₂O₃ that is approximately an order of magnitude larger than the initial one.