Effect of Modifying NiNbO Catalyst with Tetravalent (Sn, Ti) and Pentavalent (Sb, Ta) Cations on Its Ethane Oxidative Dehydrogenation Performance

The purpose of this study was to investigate the effect of the addition of a third cation, M (M = Ti, Sn, Sb and Ta) on the physicochemical properties and catalytic performance of the Nb-containing NiO catalyst (Ni_0.85Nb_0.15O) during ethane oxidative dehydrogenation to ethylene. Hydrothermal and solvent evaporation methods were used for the preparation of tricationic oxides of type Ni_0.765Nb_0.135M_0.1O, which were then calcined at 450 °C. The catalysts were characterized by XRD, SEM-EDX, H₂-TPR, MicroRaman, and XPS, and evaluated via in situ electrical conductivity measurements under varying conditions. The findings indicate that the addition of a third cation substantially changes the structural, electronic and redox properties of the NiNbO system, with considerable effects on its catalytic activity in the oxidative dehydrogenation of ethane. Among all the catalysts tested, the Ta(5)–NiNbO-8 sample─a 5 at % Ta-doped NiNbO catalyst prepared under alkaline conditions (pH 8)─exhibited the best performance: 40% ethane conversion and 75% ODH selectivity at 350 °C. It outperforms the undoped NiNbO system in terms of both ethane conversion and ethylene selectivity over the whole temperature range studied. Its superior behavior is attributed to an optimal balance between redox ability and surface composition, particularly a reduced density of nonselective active species and enhanced lattice oxygen exchangeability under reaction conditions. However, none of the catalysts, including Ta(5)–NiNbO-8, demonstrated sustained stability at 400 °C. Progressive deactivation was linked to a gradual loss of p-type conductivity and diminished reoxidation capacity, consistent with a decrease in the density of active lattice O^– species. Surface compositional changes together with structural changes were also associated with catalyst deactivation. These results demonstrate how important the type of dopant is in adjusting the physicochemical characteristics and catalytic activity of Nb-promoted NiO systems for ethane ODH. Future efforts will focus on exploring additional high-valence dopants and surface modifications to improve long-term stability and ODH selectivity.