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

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