Hydrogen spillover through hydride transfer: the reaction of ZnO and ZrO2 with strong hydride donors

Abstract

Hydrogen spillover, transfer of H₂ from a metal surface to a support (often metal oxides), is pivotal for many heterogeneous catalytic processes, including Cu/ZnO and Cu/ZrO₂ catalyzed methanol synthesis. Little is known about hydrogen spillover on ZnO or ZrO₂, due to the high complexity of the metal–metal oxide interface. Here, we model hydrogen spillover on ZnO and ZrO₂ by reacting them with molecular metal hydrides to see how the properties of the hydrides affect hydrogen spillover. While the good H· donors HV(CO)₄dppe (1) and CpCr(CO)₃H (2) do not react with the metal oxide surfaces, the strong hydride donors iBu₂AlH (3), Cp₂ZrHCl (4), and [HCu(PPh₃)]₆ (5) do reduce ZnO and ZrO₂ to give defect sites with the same EPR signatures as obtained via hydrogen spillover. We also observe new M–O bonds to the surface using X-ray absorption spectroscopy (XAS). We propose that these metal oxides undergo hydrogen spillover via initial hydride transfer followed by tautomerization of the surface hydride, giving reduced sites and OH bonds. This mechanism is in contrast to the traditional spillover mechanism involving discrete proton- and electron transfer steps. We also observe that ZnO is easier to reduce than ZrO₂, explaining the difficulty observing spillover on Cu/ZrO₂.