UCLA

Heterogeneous catalysis plays a crucial role in the production of industrially important chemicals. Conventional Pt-group metals have been widely used in various reactions for their high activity. However, these catalysts often favor undesired side-reactions, resulting in low reaction selectivity. To tackle this issue, a new class of catalytic materials named single-atom alloys (SAA) has been established in the past decade. Typically, SAAs are synthesized by dispersing a small amount of active elements into less reactive host metals, forming isolated dopant atoms on the surface. In this dissertation, we use density functional theory (DFT) and microkinetic modeling to unravel the reasons behind the improved activity and selectivity of the SAAs in different reactions.DFT calculations and microkinetic modeling were first performed to investigate the selective hydrogenation of 1-hexyne to produce 1-hexene on dilute Pd-in-Au alloy catalysts. It is demonstrated that the high selectivity of isolated Pd atoms in Au(111) is attributed to the difficult H2 dissociation and H-spillover steps, which help impede the supply of H atoms for further hydrogenation of 1-hexene. Although larger Pd ensembles could facilitate H2 dissociation, and hence supposedly enhance the reaction rates, they tend to be poisoned by the more strongly bound alkyne molecules, resulting in even lower reaction reactivity than isolated Pd atoms. Besides alkyne hydrogenation, the challenging selective hydrogenation of α,β-unsaturated aldehydes to form unsaturated alcohols was also addressed in this dissertation. In this work, mid-transition metals in Cu(111) appear to be the optimal systems for this reaction as they favor acrolein adsorption via the C=O bond but with a moderate binding strength, compatible with catalysis. In addition, there is a large barrier for reactant migration from the C=O to the C=C binding mode, which help impede C=C bond hydrogenation from happening and enhance the reaction selectivity. Finally, the potential of Cu-based dilute alloys for alkane dehydrogenation was explored. Specifically, our results reveal that isolated Hf and Ir atoms in Cu(111) could selectively and reactively dehydrogenate propane. The former, in particular, even demonstrates higher reaction reactivity than the widely used Pt-based catalysts. It is shown in this study that for alkane dehydrogenation on single-atom alloys, C-H bond breaking might not be the sole rate-limiting step. The migration of H atoms away from the active sites, which has not been discussed in literature, is also somewhat rate-controlling.