Partial Oxidation of Bio-methane over Nickel Supported on MgO-ZrO₂ Solid Solutions

Syngas can be produced from biomethane via Partial Oxidation of Methane (POM), being an attractive route since it is ecofriendly and sustainable. In this work, catalysts of Ni supported on MgO-ZrO₂ solid solutions, prepared by a one-step polymerization method, were characterized by HRTEM/EDX, XRD, XPS, H₂-TPR, and in situ XRD. All catalysts, including Ni/ZrO₂ and Ni/MgO as reference, were tested for POM (CH₄:O₂ molar ratio 2, 750 ºC, 1 atm). NiO/MgO/ZrO₂ contained two solid-solutions, MgO-ZrO₂ and NiO-MgO, as revealed by XRD and XPS. Ni (30 wt%) supported on MgO-ZrO₂ solid solution exhibited high methane conversion and hydrogen selectivity. However, depending on the MgO amount (0, 4, 20, 40, 100 molar percent) major differences in NiO reducibility, growth of Ni⁰ crystallite size during H₂ reduction and POM, and in carbon deposition rates were observed. Interestingly, catalysts with lower MgO content achieved the highest CH₄ conversion (~ 95%), high selectivity to H₂ (1.7) and CO (0.8), and low carbon deposition rates (0.024 g _carbon.g_cat^-1 h^-1) with Ni4MgZr (4 mol% MgO) turning out to be the best catalyst. In situ XRD during POM indicated metallic Ni nanoparticles (average crystallite size of 31 nm), supported by MgO-ZrO₂ solid solution, with small amounts of NiO-MgO being present as well. The presence of MgO also influenced the morphology of the carbon deposits, leading to filaments instead of amorphous carbon. A combustion-reforming mechanism is suggested and using a MgO-ZrO₂ solid solution support strongly improves catalytic performance, which is attributed to effective O₂, CO₂ and H₂O activation at the Ni/MgO-ZrO₂ interface.