CO DISPROPORTIONATION ON SILICA-SUPPORTED NICKEL AND NICKEL-COPPER CATALYSTS

The carbon formation/gasification equilibrium on silica-supported nickel and nickel-copper catalysts in CO + CO2 gas mixtures and the steady state kinetics of carbon formation from CO on the same catalysts were studied in a thermogravimetric flow system. The equilibrium gas composition was found to be the same for the nickel and the nickel-copper catalysts except at the highest copper concentrations (greater-than-or-equal-to 25 at.%), where larger deviations from graphite equilibrium were found at the lowest temperature (673 K). The deviations at equilibrium of the free energy, DELTAG(c), from the value calculated for graphite equilibrium were larger than found previously for carbon formation in CH4 + H-2 gas mixtures. The steady-state kinetic results have been modelled by using a simple Langmuir model with one type of sites. In contrast to some previous studies of carbon formation from CO the present kinetic results are well described by a model in which the surface reaction of two CO molecules to form carbon and CO2 is the rate-limiting step at temperatures below about 680 K and CO pressures in the range 15-50 kPa. At higher temperatures and pressures, the experimental rates fall below the model rates, probably due to diffusion limitations and to the partial coverage of the active surface by graphitic carbon. The conflict whereby a model with CO dissociation as the rate-limiting step gives a better description of the results of some previous kinetic studies of carbon formation from CO is suggested to be resolved by assuming that the latter model is valid when the CO pressure is low or when the gas contains CO2. (C) 1994 Academic Press, Inc.