Role of Hydroxyl Groups in the Preferential Oxidation of CO over Copper Oxide-Cerium Oxide Catalysts

Model CuO/Ce0.8X0.2O delta catalysts (with X = Ce, Zr, La, Pr, or Nd) have been prepared in order to obtain CuO/ceria materials with different chemical features and have been characterized by X-ray diffraction, Raman spectroscopy, N-2 adsorption, and H-2 temperature-programmed reduction. CO-PROX experiments have been performed in a fixed-bed reactor and in an operando DRIFTS cell coupled to a mass spectrometer. The CO oxidation rate over CuO/ceria catalysts correlates with the formation of the Cu+-CO carbonyl above a critical temperature (90 degrees C for the experimental conditions in this study) because copper-carbonyl formation is the rate-limiting step. Above this temperature, CO oxidation capacity depends on the redox properties of the catalyst. However, decomposition of adsorbed intermediates is the slowest step below this threshold temperature. The hydroxyl groups on the catalyst surface play a key role in determining the nature of the carbon-based intermediates formed upon CO chemisorption and oxidation. Hydroxyls favor the formation of bicarbonates with respect to carbonates, and catalysts forming more bicarbonates produce faster CO oxidation rates than those which favor carbonates.