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 Oads was not significantly influenced. This suggests that the enhancement of the oxidation of CHX,ads by increasing the concentration of Oads 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/2H2, was not affected by potassium below the threshold coverage of ΘK=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=r0(1−ΘK)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 ΘK>0.4 is ascribed to the physical blockage of the nickel ensemble by potassium.