| Comparison of oxygen chemisorption studies on the co-precipitated, bi-phase, Cu/ZnO methanol synthesis catalysts at 78 K with those for carbon monoxide at 293 K show that reversible CO capacities increase linearly with irreversible O(,2) capacities, and that these gases titrate only copper surface area in the Cu/ZnO binary catalysts. The specific adsorption of oxygen on pure copper metal is used as a standard to calculate copper surface areas for the Cu/ZnO compositional range 0/100-100/0. Reversible H(,2) capacities are also proportional to copper surface area in these catalysts.; In addition, an enhancement in the carbon monoxide and^hydrogen capacities in the coprecipitated catalysts, in comparison^to the pure components, is observed. The enhancement in CO^capacities, as irreversible CO adsorption, is a linear function of the^solute copper concentration in zinc oxide which was previously^determined. The higher specific adsorption of CO on the Cu/ZnO^= 40/60 - 67/33 catalysts in which the basal (0001) plane of ZnO^is predominantly exposed, over the Cu/ZnO = 10/90 - 30/70^catalysts in which the prism (1010) plane is the major exposed(' )^plane, indicates that the solute copper species is charged. Themethanol synthesis rates correlate with a second to third powerdependence of the solute copper site concentration, indicatingthat several copper centers are involved in the synthesis of onemethanol molecule.; Carbon dioxide is adsorbed strongly on a Cu/ZnO = 30/70 catalyst. A catalyst that is reduced by H(,2)/N(,2) mixture only at 523 K, does not convert CO(,2) to CO, but is reduced by CO at 523 K to give CO(,2). This CO-reduced catalyst is reoxidized by CO(,2). The CO/CO(,2) concentration ratio controls the redox state of the catalyst by the general reaction; A(,red) + CO(,2(g)) (DBLHARR) A(,ox) + CO(,(g)); The active site in methanol synthesis, A(,ox), is most likely an intermediate oxidation state of the solute copper. A model of the synthesis involves activation of CO on solute Cu sites, followed by hydrogenation by hydrogen activated on neighboring Cu sites. |
