Subsurface Hydrogen Diffusion into Pd Nanoparticles: Role of Low-Coordinated Surface Sites and Facilitation by Carbon

Hydrogen species absorbed in the volume of Pd nanoparticles play a crucial role in the hydrogenation of unsaturated hydrocarbons. Recent evidence suggests that the rate of diffusion of hydrogen from the surface of Pd nanoparticles to the subsurface region critically affects the hydrogenation rate, as this rate dictates whether or not the nanoparticles can maintain a sufficient concentration of subsurface hydrogen species under operating conditions. Recent theoretical calculations predicted pronounced facilitation of subsurface hydrogen diffusion by coadsorbed carbon and identified the conceptual importance of atomically flexible low-coordinated surface sites on small metal clusters for the diffusion process. In this study, we experimentally probe the kinetics of hydrogen diffusion into the subsurface by performing pulsed molecular beam experiments on carbon-free and carbon-containing Pd nanopartides and on Pd(111). We provide experimental proof that low-coordinated surface sites on Pd particles play a crucial role in the diffusion process and that their selective modification with carbon results in marked facilitation of subsurface hydrogen diffusion, in line with the theoretical predictions.