The addition of AI,O,, ZrO,, Ga, O, or ZnO to Cu/SiO, catalysts results in a definite increase in the rate of methanol synthesis per unit area of copper. Most apparent is the synergy between ZnO and Cu, which occurs even when particles of ZnO/SiO, and Cu/SiO, are physically mixed. If, after high-pressure testing, the Cu component of a physical mixture is isolated, its activity on re-testing is found to return to a value expected for Cu/SiO, . The synergy is not due to 'classical ' bifunctional catalysis, because no enhancement in activity is observed when the two phases are loaded as separate beds. However, a correlation is observed between the incidence of synergy and the spillover of hydrogen, which is detected during the temperature-programmed dissociation of intermediates on the Cu surface. It is proposed that ZnO can act as a reservoir for spillover hydrogen, and that reverse spillover may account for the higher rate of methanol synthesis on Cu when ZnO is present in the catalyst. The ZnO is perceived as playing the role of an enabling support, acting as a reservoir for atomic hydrogen and promoting hydrogen spillover.
ZnOOH(surf) + CO + HC02(a) HCO,(a) + 2H(a) + CH,O(a) + O(surf) cu ZnO CO, + 3H, + CH,OH + H, O CH,O(a) + H 2 0 + CH,OH + OH(surf) Scheme 1. Cu-ZnO bifunctional catalysis involving transfer of water.
