Hydrogen Spillover to Nonreducible Supports

Density functional theory (DFT) calculations show that the interaction between a hydrogen atom and the surface of MgO is weak, the interaction between a hydrogen atom and the surface of SiO2 is repulsive, and the migration of H atoms from a metal particle to a nonreducible support is endothermic. As a consequence, transition-state theory estimates that the migration of a hydrogen atom from a metal particle to the surface of a nonreducible support is too slow to explain the observed hydrogenation of molecules adsorbed on the support by spillover of H atoms. On the other hand, H atoms bind strongly to the surface of WO3, because the H atoms decompose into electrons, which reduce the W6+ cations, and protons, which bind strongly to the oxygen anions. Hydrogen spillover to defect-free surfaces of nonreducible metal oxides cannot take place, but spillover to defects is possible, as is spillover from a metal particle to an oxygen-containing group on a carbon surface. Defects and carbonaceous deposits may therefore be responsible for observations that have been ascribed to hydrogen spillover.