Title: Reconstruction of Heterogeneous Catalysts on Atomic, Nanometer, and Micrometer Length Scales

Abstract

Heterogeneous catalysts play a crucial role in the production of over 80% of all manufactured products worldwide. However, the performance of these catalysts is highly dependent on their surface composition and structure, which can dynamically change under high temperature and pressure or in severely reducing or oxidizing environments typically used in industrial catalytic reactions. Catalyst reconstruction can occur at multiple length scales: the atomic scale, where bonds are formed to active sites; the nanometer scale, where metal nanoparticles change shape or composition; and the micrometer scale, where catalyst particles change size and surface area. This work encompasses catalyst reconstruction across these length scales under desorption and reaction environments

On the atomic scale, I examine the structure of Rh atoms supported on Al₂O₃ and ZSM-5 during the desorption of CO, a common catalyst poison, under UV light irradiation and thermal environments. While desorption is commonly assumed to be an elementary step on supported metal nanoparticles, I demonstrate that it is not an elementary step on Rh single atom sites, but instead proceeds through an associative ligand exchange reaction which is largely controlled by the local Rh environment. On the nanometer scale, I explore intra-particle rearrangement of supported bimetallic Pt-Ir nanoparticles which resulted in decreased butane hydrogenolysis activity. Using pilot plant kinetic data, coupled with in-situ DRIFTS characterization, I elucidate how the metal surface composition of a bimetallic alloy can be tailored and stabilized to optimize catalyst performance through pretreatments that deposit a low coverage of strongly bound adsorbates. Finally, on the nano-to-micrometer scale, I investigate the bulk reconstruction of Rh/TiO₂ under high-conversion CO₂ hydrogenation conditions. By combining reactivity studies with microscopy and spectroscopy, I identify the predominant mechanisms and timescales of Rh reconstruction which influence the catalyst activity and selectivity for CO production under these conditions. Overall, understanding the relationship between the function of catalyst materials and their dynamic structure on all length scales under reaction conditions is imperative for the rational design of stable catalyst materials for new catalytic processes.