Tunning valence state of cobalt centers in Cu/Co-CoO1-x for significantly boosting water-gas shift reaction

Abstract

Dual active sites with synergistic valence state regulation under oxidizing and reducing conditions are essential for catalytic reactions with step-wise mechanisms to modulate the complex adsorption sites of reactant molecules on the surfaces of heterogeneous catalysts with maximized catalytic performances, but it has been rarely explored. In this work, uniformly dispersed CuCo alloy and CoO nanosheet composite catalysts with dual active sites are constructed, which shows huge boost in activity for catalyzing water-gas shift reaction (WGSR), with a record high reaction rate reaching 204.2 μmol_CO g_cat.^-1 s^-1 at 300 °C for Cu₁Co₉O_x amongst the reported Cu-based and Co-based catalysts. A synergistic mechanism is proposed that Co^δ+ species can be easily reduced by CO adsorbed on Cu and Co⁰ can be oxidized by H₂O. Systematic in situ characterization results reveal that the addition of Cu can regulate the redox properties of Co species and thus modulate the adsorption properties of catalysts. Particularly, doping of Cu⁰ sites weakens the affinity of the surface to CO or CO₂ to a moderate level. Moreover, it also promotes the oxidation of *CO to *COOH and the desorption of the product CO₂, reducing the carbon poisoning of the catalyst and thus increasing the reactivity. The results would provide guidance for the construction of novel heterogeneous catalyst with dual active sites and clarify its underlying reactivity enhancement mechanism induced by the tunning of valence state of metal centers for heterogeneous catalytic reactions.

Fig. 1. Catalytic performance of the Cu_aCo_bO_x catalysts.

a Temperature-dependent activities of the catalysts (reaction atmosphere was 2%CO/10%H₂O/88%N₂). b Comparison of reaction rates with other catalysts of transition metals. c The long-term stability at a high GHSV (Cu₁Co₉O_x: 240 °C, GHSV = 140,000 mL g⁻¹ h⁻¹; Co₃O₄: 240 °C, GHSV = 42,000 mL g⁻¹ h⁻¹). d Arrhenius plots for activation energy, e CO reaction order and f H₂O reaction order of Cu₁Co₉O_x and Co₃O₄ catalysts.

Fig. 2. Characterizations of the Cu_aCo_bO_x catalysts.

The aberration-corrected HAADF-STEM images and corresponding EDS elemental maps of a–c Cu₁Co₉O_x and d–f Co₃O₄ after WGSR.

Fig. 3. Structural evolution of the Cu_aCo_bO_x catalysts.

a XRD patterns of the catalysts after WGSR. In situ XRD pattern of b Cu₁Co₉O_x and c Co₃O₄ catalysts. d H₂-TPR, e H₂-TPR after 5%H₂/Ar reduction at 300 °C and f H₂-TPR after WGSR of Cu_aCo_bO_x catalysts. g CO-TPSR after WGSR of Cu₁Co₉O_x catalysts. h Schematic illustration of structure evolution of Cu₁Co₉O_x catalysts during WGSR.

Fig. 4. Identification of electronic structures of the Cu_aCo_bO_x catalysts under WGSR.

a Quasi in situ XANES and b EXAFS results of the Co₃O₄ and Cu₁Co₉O_x catalysts after WGSR. Quasi in situ XPS spectra of c Cu LMM, d Co 2p and e O 1s of the Co₃O₄ and Cu₁Co₉O_x catalysts after WGSR. f In situ Raman spectra of the Co₃O₄ and Cu₁Co₉O_x catalysts under WGSR condition (250 °C, 2%CO/3%H₂O/Ar).

Fig. 5. Investigation of actual active sites and mechanism of the Cu_aCo_bO_x catalysts under WGSR.

In situ DRIFTS results of a–c Co₃O₄ and d–f Cu₁Co₉O_x catalysts under 2%CO/3%H₂O/He reaction atmosphere at different temperatures. g Gibbs Free energy diagram for COOR processes on the surface of Co-Cu alloy(100), Co(100) and CoO(110). Orange, blue, gray, green and purple atoms are corresponding to Co, Cu, C, O and H sites, respectively.

Fig. 6. Investigation of mechanism and the valence shift between Co⁰ and Co^δ+ to facilitate the H₂O dissociation.

a CO surface reaction results of the Cu₁Co₉O_x catalyst. In situ DRIFTS results of b Co₃O₄ and c Cu₁Co₉O_x catalysts after switching gas mixture from 2%CO/3%H₂O/He to 3%H₂O/He at 240 °C. d NAP-XPS of Cu₁Co₉O_x catalysts under 2 mbar 10%CO/Ar and 2 mbar 5%H₂O/Ar at 300 °C. In situ Raman spectra of e Cu₁Co₉O_x and f Co₃O₄ catalysts under 2%CO/Ar and 3%H₂O/Ar at 300 °C. g Schematic illustration of the dual active centers for WGSR.