Active bridging hydride species in ZnO nanorods originated from hydroxyl and oxygen vacancy.

The bridging hydride species (Zn-H-Zn) formed via H2 dissociation on ZnO surface play crucial roles in hydrogenation of unsaturated hydrocarbon to industrial production. Here, we find that the migration of surface hydroxyl in ZnO nanorods to nearby oxygen vacancy can also lead to the formation of this Zn-H-Zn species that are reactive to CO2 hydrogenation to methanol using solid-state NMR spectroscopy. Below 100 °C, bridging Zn-H-Zn species show no activity toward CO2 activation, while formate species are formed via the reaction of CO2 with surface hydroxyl groups. At 150-200 °C, Zn-H-Zn species hydrogenate formate to methoxy species. At 250 °C, methanol is produced and desorbs from the ZnO surface. These results confirm the methanol formation mechanism via formate and methoxy intermediates in the presence of active bridging Zn-H-Zn species. This work reveals a new source of active hydrogen species in ZnO nanorods without introducing H2, which is highly significant for heterogeneous hydrogenation reactions.