Ni-Ce mixed metal oxides have emerged as a single-component dual-function material for the direct air capture and conversion of CO
2 to methane. Although these materials clearly outperform other sorbent-catalyst systems for the capture and conversion of CO
2, the nature of the active sites involved in the conversion of CO
2, 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 CO
2 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 CO
2 activation routes in which formate species formation precedes C-O bond scission. CO
2 methanation reversibilities (Z
3) remain larger than CO methanation reversibilities (Z
1) 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 CO
2 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 CO
2 methanation conditions, even at very high CO
2 pressures. Overall, our work provides a mechanistic basis for kinetic observations critical to the understanding and development of catalysts for the conversion of CO
2 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.
