In situ diffuse reflectance mode (DRIFTS) measurements for adsorption of CO and under the water-gas shift (WGS) reaction revealed that formates emerge on the surface of reduced ceria after the reaction of CO with geminal OH groups. These groups are formed after reduction of the ceria surface shell. X-ray absorption near edge spectroscopy (XANES) results demonstrated that the process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little, if at all, the catalysis of bulk ceria reduction. For 1% Pt/ceria, under steady state WGS at a high H2O/CO ratio, surface formate concentrations were strongly limited at high CO conversions, while Pt-CO was H2O/CO ratios, CO exhibits a first order rate dependency, and therefore, the active site is expected to move affected only slightly. Under high H to sparser coverages of CO, which suggested that the WGS mechanism likely proceeded via formates. Later, XANES work gave no evidence for the reoxidation of ceria surface by water under a hydrogen environment, which would be necessary to substantiate an alternate mechanism, referred to as the ceria-mediated redox process. Later, isotope switching from H2O to D2O was carried out to validate the possibility that decomposition of surface formates could be the rate limiting step for the mechanism, as was proposed and demonstrated earlier by Shido and Iwasawa. In agreement with their findings, we also observed a normal isotope effect, consistent with a link between the activation energy barrier of the rate limiting step to the breaking of the C-H bond of the formate. In this study, a variety of metals were screened to try to gain further insight into the role played by the metal in the catalysis of metal/ceria systems for WGS. One group of metals was selected on the basis of reduction temperature, since spillover from reduced metal likely catalyzes the reduction of ceria surface. Therefore, we tested the following metals, moving from lowest to highest reduction temperature: Pt < Ni < Co = Fe. These were found to catalyze reduction of ceria surface by the same trend, as well as the WGS rate. In each case, DRIFTS showed that as reduction of ceria surface occurred, marked increases in geminal OH intensities occurred, which yielded formates upon adsorption of CO. The second group of metals was a comparative study between Pt and Group 11 metals that have been purported to catalyze surface reduction at the same or even lower temperatures than Pt. Indeed, the surface reduction occurred at a lower temperature than for Pt with An and at a similar temperature with Cu. However, on an equivalent atomic basis, the depth of reduction of ceria was found to be higher when Pt was used over Au, and the WGS rate was about 20 times higher with Pt than by Group 11 metal promotion at 250degreesC and higher. WGS feed conditions were carefully chosen to mimic conditions found in a fuel cell reformer. (C) 2003 Elsevier B.V. All rights reserved.