(52a) Reaction Pathways and Intermediates for CO2 Methanation over Ni-Ce Mixed Metal Oxides

Ni-Ce mixed metal oxides have emerged as a single-component dual-function material for the direct air capture and conversion of CO₂ to methane. Although these materials clearly outperform other sorbent-catalyst systems for the capture and conversion of CO₂, the nature of the active sites involved in the conversion of CO₂, as well as the identity of reaction intermediates mediating methanation turnovers, especially at low pressures, remain unclear. More specifically, proposed mechanisms primarily fall into two categories: unassisted activation routes in which CO₂ dissociates unimolecularly to form CO*[1], followed by subsequent hydrogenation steps, and H-assisted routes involving formate species (Figure 1a). In this study, we use kinetic and in-situ spectroscopic measurements to evidence the participation of unassisted CO₂ activation routes in which formate species formation precedes C-O bond scission. CO₂ methanation reversibilities (Z₃) remain larger than CO methanation reversibilities (Z₁) over a wide range of CO co-feed pressures (Figure 1c), pointing to the prevalence of unassisted methanation routes over Ni-Ce oxides. [2] These unassisted methanation routes circumvent the need for the formation of CO* as an intermediate, with in-situ infrared spectra measured under CO₂ methanation conditions confirming the presence of HCOO* species that increase in coverage with reaction temperature (Figure 1b). Inhibition in methane formation rates is only observed at high CO co-feed pressures, implying that such high CO* coverages are never reached under CO₂ methanation conditions, even at very high CO₂ pressures. Overall, our work provides a mechanistic basis for kinetic observations critical to the understanding and development of catalysts for the conversion of CO₂ from dilute sources to value-added chemical products such as methane and methanol.

References:[1] O. Mohan, et. al., Chemcatchem 13 (2021) 2420−2433, 120−125. [2] T.C. Lin, A. Bhan, J. Catal., 429 (2024) 11521.