Abnormal subsurface hydrogen diffusion behaviors in heterogeneous hydrogenation reactions.

Abstract

Hydrogen adsorption and diffusion behaviors on noble metal model catalyst surfaces and into the subsurfaces are of paramount significance in the exploration of novel heterogenous catalytic hydrogenation reactions. We present an in-depth study of hydrogen adsorption on and diffusion into the subsurfaces of three typical 5d noble metals from three-dimensional electronically adiabatic potential energy surfaces (PESs) by interpolating plenty of ab initio density functional theory (DFT) configuration-energy points. The surfaces and subsurfaces regions of the relaxed Ir(100) and (111), Pt(100) and (111), and Au(100) and (111) surfaces, are, respectively, taken into account. For hydrogen adsorption on the (100) surfaces, the lowest adsorption energy site is the Bridge site, instead of the traditional Hollow site. Hydrogen prefers to follow the indirect pathway with a lower diffusion barrier, in the competition with the direct pathway with much higher diffusion barrier. For hydrogen diffusion on the (111) surfaces, hydrogen follows the pathway from Top site to fcc site on the surface and prefers an up-down direct pathway into the subsurface. Importantly, the nudged elastic band (NEB) based on the PESs can reproduce those results calculated from the NEB(DFT) very well. The developed highly-accurate and efficient approach based on the PESs helps us to further investigate the more complex reactant diffusion dynamics at surfaces.