H2 Adsorption on Wurtzite ZnO and on ZnO/M(111) (M=Cu, Ag and Au) Bilayer Films

The dissociative adsorption of H-2 on the wurtzite ZnO (101?0) surface and on ZnO/M(111) (M=Cu, Ag and Au) bilayer films has been investigated by mean of DFT+U calculations. On ZnO wurtzite and free-standing ZnO bilayer, H-2 prefers to dissociate heterolytically, forming a proton adsorbed on an O atom and an hydride atom sitting on a Zn cation. While this adsorption mechanism is exothermic on the wurtzite ZnO (101?0) surface, an endothermic process is observed on a free-standing, unsupported ZnO bilayer. The trend is completely reversed when H-2 is adsorbed and dissociated on a metal supported ZnO bilayer. Here the homolytic dissociation mechanism is exothermic and preferred over the heterolytic dissociation. This is due to the effect of the metal support which acts as an electron reservoir to accept extra electrons accompanied to homolytic dissociation of H-2 molecule (two electrons are transferred to the metal support with formation of two H+ ions). The study of the reaction pathway for dissociative adsorption of H-2 on ZnO/Cu(111) shows that the reaction occurs via (a) heterolytic splitting and (b) the displacement of an H atom from a Zn site to the neighboring O site, forming the final product of homolytic dissociation.