METHANOL SYNTHESIS AND REVERSE WATER-GAS SHIFT KINETICS OVER CLEAN POLYCRYSTALLINE COPPER

The kinetics of simultaneous methanol synthesis and reverse water-gas shift from CO2/H-2 mixtures have been measured at low conversions over a clean polycrystalline Cu foil at pressures of 5 bar. An absolute rate of 1.2 x 10(-3) methanol molecules produced per second per Cu surface atom was observed at 510 K, with an activation energy of 77 +/- 10 kJ/mol. The rate of CO production was 0.12 molecules per second per Cu surface atom at this temperature, with an activation energy of 135 +/- 5 kJ/mol. The rates, normalized to the metallic Cu surface area, are equal to those measured over real, high-area Cu/ZnO catalysts. The surface after reaction was examined by XPS and TPD. It was covered by almost a full monolayer of adsorbed formate, but no other species like carbon or oxygen in measurable amounts. These results prove that a highly active site for methanal synthesis an real Cu/ZnO catalysts is metallic Cu, and suggest that the rate-determining step in methanol synthesis is one of the several steps in the further hydrogenation of adsorbed formate to methanol.