Role of potassium in carbon-free CO2 reforming of methane on K-promoted Ni/Al2O3 catalysts

In order to reveal the role of alkali in the carbon-free CO2 reforming of methane, the kinetics of the individual steps involved in the reforming were examined on Ni/Al2O3 catalysts with 0, 1, 5, and 10 wt% K. Although the adsorption of CO2 was enhanced by the presence of potassium, the dissociation of CO2 to CO and O-ads was not significantly influenced. This suggests that the enhancement of the oxidation of CHX,ads by increasing the concentration of O-ads is not the cause for the carbon-free CO2 reforming. The carbon-free reforming was mainly ascribed to the ensemble control; i.e., potassium plays a role in dividing the nickel surface into the smaller ensembles and, thereby, the carbon deposition is suppressed. On the other hand, the rate-determining step, ascribed to the dissociation of CHXOads to CO and x/2H(2), was not affected by potassium below the threshold coverage of Theta (K) = ca. 0.4, but above it, the rate became slow. The number of surface nickel atoms (nickel ensemble) required for the reforming was estimated from a simple Langmuir form, r = r(0)(1 - Theta (K))(n), to be ca. 2.9. The number was similar to that obtained on sulfur-passivated Ni catalysts in H2O reforming of methane, suggesting that the retardation of the rate-determining step at Theta (K) > 0.4 is ascribed to the physical blockage of the nickel ensemble by potassium. (C) 2001 Academic Press.