Revealing Hydrogenation Reaction Pathways on Naked Gold Nanoparticles

Gold nanoparticles (AuNPs) display distinct characteristics as hydrogenation catalysts, with higher selectivity and lower catalytic activity than group 8-10 metals. The ability of AuNPs to chemisorb/activate simple molecules is limited by the low coordination number of the surface sites. Understanding the distinct pathways involved in the hydrogenation reactions promoted by supported AuNPs is crucial for broadening their potential catalytic applications. In this study, we demonstrate that the mechanism of the hydrogenation reactions catalyzed by AuNPs with clean surfaces may proceed via homolytic or heterolytic hydrogen activation depending on the nature of the support. The synthesis of naked AuNPs employing gamma-Al2O3 and ionic liquid (IL)-hybrid gamma-Al2O3 supports was accomplished by sputtering deposition using ultrapure gold foils. This highly reproducible and straightforward procedure furnishes small (similar to 6.6 nm) and well-distributed metallic gold nanoparticles (Au(0)NPs) that are found to be active catalysts for the partial and selective hydrogenation of substituted conjugated dienes, alkynes, and alpha,beta-unsaturated carbonyl compounds (aldehydes and ketones). Kinetic and deuterium labeling studies indicate that heterolytic hydrogen activation is the primary pathway occurring on the AuNPs imprinted directly on gamma-Al2O3. In contrast, AuNPs supported on IL-hybrid gamma-Al2O3 materials cause the reaction to proceed via a homolytic hydrogen activation pathway. The IL layer surrounds the AuNPs and acts as a cage, influencing the frequency of the interaction of the catalytically active species and the metal surface and, consequently, the catalytic performance of the AuNPs. The IL layer is shown to improve the product selectivity by the enhancement of the substrate/product discrimination, and to decrease the catalytic activity by shifting the rate-determining step to the H-2 and substrate competitive adsorption/activation on the same active sites. A series of kinetic experiments suggest that AuNPs imprinted on an IL-hybrid gamma-Al2O3 support are more efficient (lower activation energy, E-a) than group 8-10 metal based catalysts for hydrogenation reactions at moderate to high temperatures (75-150 degrees C).