Interfacial Ni active sites strike solid solutional counterpart in CO2 hydrogenation

Ni enhanced CeO2 based catalysts were tested in CO2 hydrogenation reaction with the objective to investigate the importance and role of Ni0 and Ni0-NiO-CeO2−x interfaces. Two materials were prepared: a 4 mol% NiO/CeO2 interfacial oxide where CeO2 supports NiO nanoparticles was produced by wet impregnation method and a Ce0.96Ni0.04O2 mixed oxide was obtained by co-precipitation of Ce3+ and Ni2+ ions. Besides these materials, pure NiO and CeO2 were also synthesized and tested, serving as reference materials. HRTEM, TPR, XRD, BET, EDS, Raman spectroscopy, and XPS were used for the characterization of the catalysts. The activity and selectivity strongly depended on metallic Ni content, which is tunable by the temperature of hydrogen pretreatment. The impregnated catalyst with significant metallic content exhibited high activity and high methane selectivity. The mixed oxide was more resistant to reduction, in the case of both the Ni2+ and Ce4+ species, thus it has low activity with high CO selectivity. The activity and methane selectivity increased considerably by increasing the metal content or higher hydrogen pretreatment temperature. Operando DRIFTS experiments suggested that bicarbonate and formate intermediates play important role in CO2 hydrogenation in both types of catalysts. The reduced Ni particles help to produce the high amount of hydrogen needed for the hydrogenation of formate to methane. It turned out that the metallic Ni not only catalyzes the methane production from formate but also the bicarbonate-formate transformation. The most important factor in the high performance of these catalysts is the Ni0/NiO ratio in the Ni0-NiO-CeO2−x interface.