(605h) Tuning Ni-Catalyzed CO2 Hydrogenation Pathways Via Ni-Ceria Support Interactions and Ni-Fe Bimetallic Formation

Conversion of CO₂ to CO for use as a Fischer-Tropsch feedstock provides an attractive option for offsetting CO₂ emissions.^1,2 Non-precious Ni catalysts are active for the reduction of CO₂ by H₂, but the catalysts are highly selective to CH₄ production.^1,3,4 We investigated the coordinated mechanistic contributions of the CeO₂ support and Ni admetal. We then modified Ni catalysts with Fe in order to tune the CO selectivity while maintaining high activity.

CO₂ hydrogenation over Fe-modified Ni/CeO₂ catalysts was investigated in a batch reactor using time-resolved in situ FTIR spectroscopy and in a pulse reactor using mass spectrometry. Low loading of Ni/CeO₂ was associated with high selectivity to CO over CH₄, while higher Ni loading improved CO₂ hydrogenation activity but reduced CO selectivity. In situ XANES and AP-XPS revealed Ni to be metallic for all catalysts including the CO-selective low loading 0.5% Ni catalyst, suggesting that the selectivity trend is due to structural rather than oxidation state effects. Isotope studies using C¹⁸O₂ revealed that oxygen exchange with the CeO₂ support occurs beyond the surface layer. Subsurface oxygen participates in the reaction, and oxygen exchange is fast with respect to the reaction time. The results suggest that H₂ dissociation by metallic Ni promotes C-O bond breaking of CO₂ adsorbed on reduced ceria, and that oxygen vacancies are filled by oxygen from the adsorbed CO₂.⁵

Bimetallic Ni-Fe catalysts demonstrated improved selectivity towards CO without significantly compromising activity, coupling the high activity of Ni catalysts and the high CO selectivity of Fe. TPR results indicated bimetallic interactions between Ni and Fe, and XANES analysis showed that the majority of Fe in the bimetallic catalysts was oxidized.⁶

The trends in bimetallic modification and support effects should guide efforts to develop non-precious metal catalysts for the selective production of CO from CO₂ hydrogenation.

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6. Winter, L. R., Gomez, E., Yan, B., Yao, S. & Chen, J. G. Tuning Ni-catalyzed CO2 hydrogenation selectivity via Ni-ceria support interactions and Ni-Fe bimetallic formation. Appl. Catal. B Environ. 224, 442–450 (2018).