UCLA

Hydrogen dissociation is an essential step in hydrogenation catalysis and critical to catalyst performance. Recently, Au–Pd core-shell nanoparticle catalysts have emerged as a highly promising class of materials for hydrogenation catalysis. Yet, fundamental understanding on the origin of bimetallic synergy in these core-shell catalysts and their interaction with hydrogen is largely unknown. Here, we investigate the kinetics of hydrogen dissociation and recombination over Au–Pd core-shell catalysts using the hydrogen-deuterium (HD) exchange reaction. We employed colloidally prepared core-shell nanorod catalysts and used 4D scanning transmission electron microscopy to assess the degree and directionality of the strain in the Pd-shell. This revealed that even relatively thick Pd-shells are highly strained in the direction parallel to the surface. Catalytic testing showed that the HD exchange rate increases with increasing Pd-shell thickness and drastically decreases upon alloying. Interestingly, the core-shell nanorods showed temperature-dependent apparent activation energies and pre-exponential factors, indicating the presence of hydrides in the Pd-shell at the lower temperatures and gradual hydrogen release upon heating. Density functional theory calculations indicate that subsurface hydrogen plays a major role in accelerating the hydrogen-deuterium exchange reaction by providing a lower energy pathway for recombinative desorption of hydrogen and deuterium, which is the rate limiting step. Altogether, our combined experiment-theory approach revealed that lattice strain and hydride formation can significantly influence the hydrogenation behavior of Au–Pd core-shell nanoparticles and is relevant to further advance the design of core-shell materials for hydrogenation catalysis.