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

Abstract

Model CuO/Ce 0.8 X 0.2 O δ 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 °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.

Improved CO-PROX Performance of CuO/CeO2 Catalysts by Using Nanometric Ceria as Support

“…In Table 2, the H2 uptake evaluated from the integration of TPR curves is reported. As reported in the literature, ceria reduction starts at temperatures higher than 350 • C [52,59,60]; however, a small copper amount supported on ceria significantly modifies the TPR pattern, showing two to four reduction peaks at temperatures much lower than those obtained on pure ceria [9,19,22,43,52,61,62].…”

“…However, it has been demonstrated that the same sites can shift from CO oxidation to H2 oxidation depending on their oxidation state. In particular, while the redox cycle between Cu 2+ and Cu + is suggested for CO oxidation [19], the formation of metallic copper All xCuCe catalysts exhibit CO conversion higher than that of 4% CuO/CeO2 at T > 90 °C and 100% CO selectivity up to 110 °C, thus highlighting the much better performance of catalysts supported on ceria with a double surface area. Surprisingly, catalytic performance both in terms of CO conversion (Figure 5a) and selectivity to CO2 (Figure 5b) appears to be independent of the copper content.…”

“…However, it has been demonstrated that the same sites can shift from CO oxidation to H2 oxidation depending on their oxidation state. In particular, while the redox cycle between Cu 2+ and Cu + is suggested for CO oxidation [19], the formation of metallic copper CuO/CeO2 at T > 90 °C and er performance of catalysts erformance both in terms of e independent of the copper ture, a full comparison of the ations can be drawn. Table 3 ia catalysts reported in the ure at which CO conversion times (τ') defined as number best catalysts.…”

“…However, it has been demonstrated that the same sites can shift from CO oxidation to H2 oxidation depending on their oxidation state. In particular, while the redox cycle between Cu 2+ and Cu + is suggested for CO oxidation [19], the formation of metallic copper 4%CuO/CeO 2 [15].…”

“…A proper catalyst should operate at quite low temperatures in order to reduce CO concentration to this level, limiting as much as possible the competitive H 2 oxidation activated at slightly higher temperatures [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”