Isotopic mixing in carbon monoxide catalyzed by zinc oxide

Abstract

The rate of the isotopic mixing in CO has been studied at 300 [deg]C, for CO partial pressures from 6 to 100 Torr and a total pressure of 250 Torr on ZnO catalysts. Significant deviations from a first-order rate in pco were found. The rate of oxygen exchange between ZnO and gasphase CO was also measured and the results were employed to calculate the fraction of surface sites active for the CO isotopic mixing. Values on the order of 0.001 were found. The turnover rate and surface collision efficiency varied between 0.7 and 107 min-1 and 0.13 and 2.24 x 10-8, respectively. H2 additions to CO increased the rate of isotopic mixing, whereas the rate of H2 + D2 was decreased by the presence of CO. The H2 + D2 rate was faster than that of isotopic mixing in CO, but as the ratio pH2/pco decreased the rates became about equal.It is argued that on ZnO samples, in which the rate of CO isotopic mixing and the rate of ZnO---CO oxygen exchange were influenced in a similar manner by the CO pressure, the isotopic mixing in CO took place via the ZnO oxygen, while oxide oxygen participation was not kinetically significant for ZnO samples in which the two reactions had different kinetics. The crucial factor controlling the path followed by the isotopic mixing in CO seems to be the surface Zn/O ratio, since a close correlation was found between the former and the reaction kinetics of the CO isotopic mixing reaction. Solid-state conditions which may vary the Zn/O surface ratio (foreign additions) are indicated. The implications of these findings to the problem of product selectivity from CO---H2 mixtures reacting on metal oxide surfaces are discussed.