Performance of nickel-manganese and nickel-cobalt-manganese mixed oxide catalysts in ethanol total oxidation

The mixed oxide catalysts containing Ni, Co, and Mn components in various molar ratios were examined in the total oxidation of ethanol and the effect of their composition on physical chemical properties and catalytic performance was studied. The catalysts were obtained by calcination (4 h at 500 °C in air) of the coprecipitated precursors. The chemical analysis (AAS), powder XRD, Raman spectroscopy, TEM, N2 adsorption, H2-TPR, and XPS were used for the catalysts characterization. Formation of mixed oxides was indicated by powder XRD and Raman spectroscopy: NiCo2O4 spinel together with NiO were detected in the Ni-Co catalysts, MnIV-containing mixed oxides Ni6MnO8 (murdochite) and NiMnO3 (ilmenite) were identified in the Ni-Mn samples, and Ni-Co-Mn mixed oxides with spinel structure were found in the Ni-Co-Mn catalysts. Combination of transition metal components in the catalysts resulted in a synergistic effect; easier reduction in H2-TPR measurements and enhanced catalytic performance compared to single-component Ni, Co, and Mn oxides were observed. Addition of Mn to the Ni and Ni-Co oxides increased their catalytic activity in the ethanol oxidation, which was higher than that of the Ni-Co catalysts. The Ni-Mn catalyst with Ni:Mn molar ratio of 1:1 showed the highest activity (50% conversion of ethanol was achieved at 91 °C) and good selectivity (acetaldehyde and carbon monoxide were detected as main byproducts of the ethanol oxidation). High activity of the Ni-Mn catalysts can be ascribed to the presence of MnIV-containing mixed oxides, namely NiMnO3. Activity of the Ni-Co-Mn catalysts, especially those with high (Co+Mn) contents, was comparable with that of the most active Ni-Mn catalyst with Ni:Mn molar ratio of 1:1. The high activity of Ni-Co-Mn catalysts can be explained by the formation of spinel-type mixed oxides with rather poor structure ordering and large surface area.