Monolayer dispersive supported CuO/γ-Al₂O₃ catalysts studied by XAFS

The local structures of supported CuO/γ-Al₂O₃ monolayer dispersive catalysts with different CuO loadings have been investigated by XAFS. For the CuO/γ-Al₂O₃ catalyst with CuO loading of 0.4 mmol/100 m², the XAFS results indicate that only one peak corresponding to the Cu-O first shell appears in the radial structural function of Cu atoms, and that the bond length R_Cu−O and coordination number N are 1.95 Å and 3.8, respectively. We consider that the CuO active species on the surface of γ-Al₂O₃ are in the form of isolated atomic clusters (CuO₄)_mⁿ⁺ with distorted structure for the CuO/γ-Al₂O₃ (0.4 mmol/100 m²) catalyst. It has been found that small amount of CuO nano-grains is formed on the surface of γ-Al₂O₃ for CuO/γ-Al₂O₃ (0.8 mmol/100 m²) catalyst while the CuO loading is just equal to the threshold 0.8 mmol/100 m². For the catalyst with CuO loading of 1.2 mmol/100 m², the local structure of Cu atoms in CuO/γ-Al₂O₃ is similar to that of CuO powder. On the basis of the XAFS results, a schematic monolayer dispersion model has been proposed for CuO/γ-Al₂O₃ catalysts.