Catalytic synthesis of methanol from CO/H2IV. The effects of carbon dioxide

The effects of carbon dioxide on the catalytic synthesis of methanol over the copper-zinc oxide catalysts were investigated for CO2/CO/H2 ratios between 0/30/70 and 30/0/70. A maximum synthesis rate was observed-at CO2/CO/H2 = 2/28/70. At lower concentrations of CO2 the catalyst is deactivated by overreduction and at higher concentrations of CO2 the synthesis is retarded by a strong adsorption of this gas. A kinetic model is presented which quantitatively describes the observed patterns in the indicated range of synthesis gas compositions and at temperatures between 225 and 250 °C. This model is consistent with all physical characteristics of the CuZnO catalysts and corroborates earlier findings that an intermediate oxidation state of the catalyst is its active state. The adsorption enthalpies and entropies for the reactants indicate that carbon dioxide is strongly bound and immobile while carbon monoxide and hydrogen are chemisorbed with intermediate strength and experience a considerable mobility in the adsorbed layer. At concentrations of CO2 greater than 10%, methane is a side product. Mechanistic implications of this finding are that there is a nonselective pathway parallel to the CO hydrogenation; this pathway may involve formate and methoxy intermediates.