Effect op the preparation conditions on the reducibility of Cu-Co based oxides

The nature of the active sites of Cu-Co-Al Catalysts used to convert synthesis gas to methanol and higher alcohols is still a matter of debate. The main difficulty to discern the nature of the synergism responsible for alcohol formation is related to the Co reducibility under reaction conditions. In this paper we studied the influence of the preparation conditions on the reducibility of Co. Binary Cu-Co, Cu-Al and Co-Al, and ternary Cu-Co-Al samples were prepared by coprecipitation and characterized by temperature-programmed reduction. X-ray powder diffraction and diffuse reflectance spectroscopy techniques. Depending on the Co/Al ratio and calcination atmosphere, the Co ions were present in three different phases before H₂ activation: CuCoO₂, GO₃O₄ and partly inverse CoAl₂O₄ spinel. The Co reducibility was in the order CuCoO₂ > Co₃O₄ > Co_oh ions in CoAl₂O₄ > Co_td ions in CoAl₂O₄. In all the cases, the presence of Cu activated the reduction of Co²⁺ and Co³⁺ ions. The mixed oxides obtained in nitrogen did not contain CO₃O₄ and the CuCoO₂ phase was formed only when the Co/Al atomic ratio was greater than 0.5. The samples calcined in air contained CO₃O₄ but not CuCoO₂. All the samples contained CoAl₂O₄, spinel phase with finely dispersed CuO. It was found that the preparative procedures and the chemical composition of the samples greatly Influence the reducibility of Co ions, thereby determining the kind of Cu-Co interaction.