Diving into the CO_<i>x</i> Methanation Mechanism of CO and CO₂ Mixtures Catalyzed by the Nanostructured NiO-CeO₂ Catalyst

The CO₂ methanation process in the presence of CO is a catalytic challenge toward the abatement of CO₂ emissions from industrial exhaust gases with the goal of producing CH₄ as a clean fuel. Herein, the CO _<i>x</i> methanation mechanisms of a NiO-CeO₂ (Np) catalyst constituted by nickel oxide-cerium oxide nanoparticles with efficient operation under CO₂ and CO + CO₂ gas mixtures were addressed. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) analyses and ¹³C¹⁸O₂ (49) pulse isotopic experiments reveal that solo CO₂ methanation and comethanation (CO + CO₂) present a common mechanism in which CO₂ is transformed into *CO, from where the reaction proceeds. According to our outputs, inlet CO interferes with the CO₂ methanation activity, delaying the reaction onset without selectivity impacts. Conversely, in the absence of CO₂, the solo CO methanation performance is remarkably limited with poor CH₄ selectivity. As demonstrated, the solo CO methanation mechanism is based on formates as key intermediates at low temperatures (<250 °C), while competitive CO₂ production takes place onward. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) analyses conducted in synchrotron revealed a higher degree of reduction in the catalyst when CO and H₂ are fed either alone or in combination with CO₂, attributed to the CO oxygen abstraction capacity from the CeO₂ phase. In the comethanation mixture, CO₂ and CO are balanced in oxidation-reduction processes, yielding the maximum CO _<i>x</i> conversion, while in the solo CO methanation, reductive processes prevail, limiting the CH₄ formation.