Water-gas shift: in situ spectroscopic studies of noble metal promoted ceria catalysts for CO removal in fuel cell reformers and mechanistic implications

In situ, steady-state diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements for adsorption of CO and for water-gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, being formed by reaction with geminal OH groups that are present after reduction of the ceria surface shell. 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 reduction. Gold was found to reduce the surface ceria at a lower temperature than that of platinum, but platinum gave a slightly higher degree of surface shell reduction. Under steady-state WGS at a high H2O/CO ratio, the concentrations of surface formates are strongly limited at high CO conversions, while metal-CO was not. Since under these conditions, CO exhibits a first order rate dependency, the active site should move to sparser coverages of CO, indicating that a formate mechanism is more likely the correct one. At low temperatures and conversions, the formates were close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. In situ X-ray absorption near edge spectroscopy (XANES) directly links the metal to its ability to aid in catalyzing reduction of the surface shell of ceria. After surface shell reduction of ceria by hydrogen, addition of water to the hydrogen stream gave no indication of reoxidation whatsoever, as would be necessary under a ceria-mediated redox process. The reoxidation of ceria by water under helium alone was very slow, and only slight changes were recorded at 350 degreesC. Therefore, the results strongly favor a formate mechanistic scheme for low temperature water-gas shift. To date, most researchers have claimed a ceria-mediated redox process operating to describe the mechanism. Both mechanisms require reduction of the ceria surface. (C) 2003 Elsevier B.V. All rights reserved.