The water-gas shift activity for metal-promoted ceria catalysts has recently been explained by researchers in terms of a redox mechanism, whereby CO adsorbed on a metal reduces ceria surface to generate CO2, and water reoxidizes the ceria surface to CeO2, liberating hydrogen in the process. in contrast, a number of results (steady-state and transient diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), XANES, and kinetic isotope effect studies) support a formate mechanism. In this study, a reduction promoter loaded actinide, Pt/thoria, is utilized and it is found to be an interesting analog to its lower molecular weight counterpart, ceria. Utilizing the urea precipitation-decomposition method, a surface area of 165 m(2)/g was achieved. The Pt/thoria catalyst exhibited a higher concentration of formate, bands upon CO addition than Pt/ceria (105 m(2)/g), indicating higher active site densities on a per gram catalyst basis, and the catalyst exhibited slightly higher conversion on a fixed weight basis than ceria. As with ceria, the Pt was a necessary component for catalyzing the removal of surface oxygen atoms and possibly carbonates to activate the OH groups. A normal kinetic isotope effect was observed (1.6) which is similar to the one obtained with Pt-ceria. The data support a formate mechanism for the thoria catalyst. (C) 2004 Elsevier B.V. All rights reserved.