PATH AND MECHANISM OF HYDROGEN ABSORPTION AT PD(100)

The absorption of hydrogen at Pd(100) during exposure to H-2, is studi ed by thermal desorption and high-resolution electron energy-loss spec troscopies. At the temperatures investigated (i.e. 105-200 K), absorpt ion occurs by a mechanism in which the hydrogen molecules impinging on the H-covered surface are dissociated and penetrate at surface defect sites. Neither the penetration of prechemisorbed hydrogen atoms nor t he direct absorption of gaseous hydrogen at terrace sites is observed under our experimental conditions. Below 120 K, H atoms penetrate via quantum tunneling and remain at the subsurface sites to form palladium hydride, which gives rise to a molecular hydrogen desorption peak at 155 K. The absorption energy for the subsurface site is estimated to b e 3.4 kcal mol-H-1. At higher temperatures, the hydrogen atoms diffuse thermally into the Pd bulk during exposure and desorb at around 300 K , exhibiting a broad structure. An isotopic difference is observed in the absorption state which is associated with the higher mobility of D . The activation energies of penetration into the subsurface sites are determined to be 1.1 kcal mol(-1) for H and 0.8 kcal mol(-1) for D. T he structure of the absorption site is discussed. (C) 1998 Elsevier Sc ience B.V. All rights reserved.