Dynamics over a Cu-graphite electrode during the gas-phase CO2 reduction investigated by APXPS.

The electrocatalytic conversion of CO2 to fuels and chemicals using renewable energy is a key decarbonization technology. From a technological viewpoint, the realization of such process in the gas phase and at room temperature is considered advantageous as it allows one to circumvent the limited CO2 solubility in liquid electrolytes and CO2 transport across the electrical double layer. Yet, electrocatalysts' performances reported so far are promising but not satisfactory. To inform the design of new materials, in this study, we apply ambient pressure X-ray photoelectron and absorption spectroscopies coupled with on-line gas detection via mass spectrometry to investigate in situ performance and interface chemistry of an electrodeposited Cu on graphitic carbon support under conditions of CO2 reduction. We use the ISISS beamline at the synchrotron facility BESSY II of the HZB and the electrochemical cell based on polymeric electrolyte membrane previously developed. We show that under cathodic potential in which methanol is formed, a fraction of the electrode with a predominantly Cu(I) electronic structure undergoes reduction to metallic Cu. The C speciation is characterized by C-O and sp3 CH3 species whereas no atomic C was formed under this condition. We also show the important role of water in the formation of methanol from accumulated surface CH3 species.

Elucidating the mechanism of the CO2 methanation reaction over Ni-Fe hydrotalcite-derived catalysts via surface-sensitive in situ XPS and NEXAFS.

Hydrotalcite-derived Ni and Fe-promoted hydrotalcite-derived Ni catalysts were found to outperform industrial catalysts in the CO2 methanation reaction, however the origin of the improved activity and selectivity of these catalysts is not clear. Here, we report a study of these systems by means of in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy elucidating the chemical nature of the catalysts surface under reaction conditions and revealing the mechanism by which Fe promotes activity and selectivity towards methane. We show that the increase of the conversion leads to hydroxylation of the Ni surface following the formation of water during the reaction. This excessive Ni surface hydroxylation has however a detrimental effect as shown by a controlled study. A dominant metallic Ni surface exists in conditions of higher selectivity towards methane whereas if an increase of the Ni surface hydroxylation occurs, a higher selectivity towards carbon monoxide is observed. The electronic structure analysis of the Fe species under reaction conditions reveals the existence of predominantly Fe(iii) species at the surface, whereas a mixture of Fe(ii)/Fe(iii) species is present underneath the surface when selectivity to methane is high. Our results highlight that Fe(ii) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.