The Interplay between Homogeneous and Heterogeneous Phases of PdAu Catalysts for the Oxidation of Alcohols

Abstract

The relationship between the homogeneous and heterogeneous phases of a catalyst is widely neglected in part due to the inherent differences between the experimental and theoretical techniques employed to study them. It is well-known that, under reaction conditions, many homogeneous catalysts deactivate and generate black metals (i.e., nanoparticles). Simultaneously, heterogeneous catalysts tend to suffer of leaching processes under harsh conditions, which produce the formation of species in the homogeneous phase (i.e., volatile or organometallic species). To unravel the links between these two types of catalytic species, we have taken PdAu catalysts in the oxidation of crotyl alcohol to crotonaldehyde and investigated the reaction process for both homogeneous and heterogeneous phases. We show that the process is possible in both phases and, essentially, contains the same elementary steps. The results indicate that the homogeneous catalyst is slightly more active; however, the enhanced stability of the heterogeneous phase provides a better performance under relevant reaction conditions. Both catalytic systems are connected through two simple steps that can be computed: oxidative leaching and deposition. The oxidative leaching of the PdAu nanoparticles in the presence of dioxygen can produce Pd(II) monomeric species able to catalyze the alcohol oxidation in homogeneous conditions. After the reaction the reduced Pd(0) homogeneous catalyst is reabsorbed onto the PdAu nanoparticles, preventing the aggregation process. The present work shows that the full homogeneous/heterogeneous catalytic cycle can be analyzed in a holistic manner with computational techniques.