Weakly-bound hydrogen on defected Pt(111)

Abstract

Step edges and kinks, abundant on multi-faceted nanoparticles, are catalytically active sites. Weakly-bound atomic H, at either topmost surface or subsurface sites, would be important for low-temperature hydrogenation in platinum-based catalysts. Here we report experimental results for such H atoms on Pt(111). Saturation-adsorbed atomic H from molecular H-2 on the defect-free Pt(111) surface indeed gave only a single-peaked H-2 desorption (beta(2)) at 285 K. Instead, defected Pt(111) surfaces rendered triple peaks (beta(1) to beta(3)) including a prominent feature (beta(1)) at as low as 205 K in addition to another desorption (beta(3)) at 360 K. This beta(1)-H state was inhibited and created by pre- and post-adsorbed CO, respectively. We attribute the beta(1)-H desorption to H atoms trapped at interstitial sites beneath surface defects on the basis of: (1) its desorption at a very low temperature in addition to two other peaks from terrace- and defect-adsorbed H; (2) its and total H uptakes by far larger than the surface defect density; (3) its desorption amount up to similar to 3.6 times that of the beta(3) desorption from defects; (4) its complete inhibition by a small pre-coverage of CO; and (5) the complete beta(3)-to-beta(1) H conversion, while the beta(1)-H state remaining intact, by postdosed co. Our proposed mechanism is that the derelaxation (upward lifting) of the H- or CO-bound Pt lattice atoms at (step) defects, as a result of strong H-H and even stronger H-CO lateral repulsions under (near) saturation surface coverages, opens a low-barrier path for H diffusion into the subsurface. (C) 2014 Elsevier B.V. All rights reserved.