TPR and XPS study of cobalt-copper mixed oxide catalysts: evidence of a strong Co-Cu interaction

In this work the results of a TPR and XPS investigation of CoxOy-CuO mixed oxides in the range of composition Co:Cu=100:0-8:92 are reported and compared. The final catalysts were obtained by thermal decomposition in air and N-2 at 723 K for 24 h of single-phase cobalt-copper hydroxycarbonates prepared by coprecipitation at constant pH. The Co:Cu=100:0 specimen calcined in air formed the Co2+[Co3+](2)O-4 (Co3O4) spinel phase. The copper-containing catalysts (Co:Cu=85 : 15-8 : 92) showed mainly two phases: (i) spinels, like Co2+[Co3+](2)O-4, Co1-x2+Cux2+[Co3+](2)O-4 and (ii) pure CuO, the relative amount of each phase depending on the Co:Cu atomic ratio. The results of the XPS study are consistent with the bulk findings and revealed the presence of Co2+, Co3+ and Cu2+ species at the catalyst surface. Moreover, the surface quantitative analysis evidenced a cobalt enrichment, in particular for the most diluted cobalt samples. The TPR study showed that the catalyst reduction is affected by a strong mutual influence between cobalt and copper. The reducibility of the mixed oxide catalysts was always promoted with respect to that of the pure Co3O4 and CuO phases and the reduction of cobalt was markedly enhanced by the presence of copper. Cobalt and copper were both reduced to metals regardless of the catalyst composition. On the other hand, the Co:Cu=100:0 specimen calcined in N-2 formed, as expected, CoO. The initial addition of copper resulted in the formation of the Cu+Co3+O2 compound, besides CoO, up to a Co/Cu=1 atomic ratio at which the CuCoO2 phase was the main component. A further addition of copper led to the formation of CuCoO2 and CuO phases. The XPS results were in good agreement with these findings and the surface quantitative analysis revealed a less enrichment of cobalt with respect to the catalysts calcined in air. The TPR analysis confirmed that the reduction of the N-2-calcined catalysts was also remarkably promoted by the presence of copper. Also in this case cobalt and copper metal were the final products of reduction.