Effect of manganese addition to Ni-Cu oxides on properties and activity in the oxidation of volatile organic compounds

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Transition metal oxides are suitable for the total oxidation of harmful volatile organic compounds. The effect of Mn addition to Ni-Cu oxides was investigated to determine the optimum cation composition for achieving high catalytic performance in the total oxidation of ethanol and toluene, which were chosen as model volatile organic compounds. Single component (Ni, Cu, Mn), binary Ni-Cu, and ternary (Ni-Cu-Mn) oxide catalysts with various Ni:Cu and Ni:Cu:Mn molar ratios were prepared by precipitation of aqueous nitrate solutions followed by calcination at 500 °C in air. The catalysts were characterized by AAS, powder XRD, Raman spectroscopy, N2 adsorption, H2-TPR, and XPS spectroscopy. The addition of Mn to Ni-Cu oxides led to the formation of Ni-Cu-Mn mixed oxides with spinel structure and Ni-Mn mixed oxides (Ni6MnO8, NiMnO3). On the other hand, the Ni-Cu catalysts contained a mixture of NiO and CuO. In the H2-TPR experiments, the Ni-Cu-Mn oxides were reduced at lower temperatures compared to the binary Ni-Cu catalysts. The catalytic performance of Ni-Cu-Mn oxide catalysts was higher than that of the Ni-Cu catalysts. As the Mn concentration increased, the concentration of metal-bound oxygen raised, while the concentration of oxygen vacancies slightly decreased. A linear correlation was revealed between the surface concentrations of oxygen vacancies determined by XPS and specific reaction rates in ethanol and toluene oxidation over the Ni-Cu and Ni-Cu-Mn catalysts. For the Ni-Cu-Mn mixed oxides and the mixture of Ni and Cu oxides, a synergistic effect was found in the oxidation of both model compounds.