Low temperature water–gas shift: Characterization and testing of binary mixed oxides of ceria and zirconia promoted with Pt

Abstract

A series of Pt promoted ceria–zirconia mixed oxides was prepared, characterized, and tested for the low temperature water–gas shift reaction. An enhancement in the water–gas shift rate was observed by doping zirconium atoms into ceria to form a binary oxide for Pt promoted catalysts. By characterization using TPR and XANES, doping zirconia to ceria decreased the temperature for the surface reduction step. However, the total number of bridging OH group defect sites decreased, as Zr remained to a great extent in the Zr 4+ oxidation state. This was confirmed by CO adsorption, whereby the density of total surface formates was found to decline with increased Zr concentrations. However, the formate forward turnover rate in steam was increased by zirconia addition, and was found to be higher than either Pt/ceria or Pt/zirconia alone. Both the overall rate of the formate decomposition and the water–gas shift rate, as measured by the CO conversion, passed through a maximum with increasing Zr content. Two types of formates were observed, those associated with a ceria-rich surface phase, and those associated with a zirconia-rich surface phase. The relative amounts of the two formates correlated with the Zr/Ce atomic ratios obtained by XPS. EXAFS results provided direct evidence that a solid solution was present in the mixed oxide, as a distinct peak in the Fourier transform magnitude corresponding to the Zr–Ce interaction was observed, increasing with increasing Ce/Zr ratio. The sensitivity to added carbon dioxide in the feed of the undoped and a Zr doped catalyst was also explored.