Path and mechanism of hydrogen absorption at Pd(100)

The absorption of hydrogen at Pd(100) during exposure to H-2, is studied by thermal desorption and high-resolution electron energy-loss spectroscopies. At the temperatures investigated (i.e. 105-200 K), absorption 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 the 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 be 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. The structure of the absorption site is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.