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

DOI: 10.1039/D0CP00622J DOI Help

Authors: Gianfranco Giorgianni (University of Calabria) , Chalachew Mebrahtu (RWTH Aachen University) , Manfred E. Schuster (Johnson Matthey Technology Centre) , Alexander I. Large (University of Reading) , Georg Held (University of Reading; Diamond Light Source) , Pilar Ferrer (Diamond Light Source) , Federica Venturini (Diamond Light Source) , David Grinter (Diamond Light Source) , Regina Palkovits (RWTH Aachen University) , Siglinda Perathoner (Farmaceutiche ed Ambientali-ChiBioFarAm) , Gabriele Centi (University of Messina) , Salvatore Abate (Farmaceutiche ed Ambientali-ChiBioFarAm) , Rosa Arrigo (Diamond Light Source; University of Salford)
Co-authored by industrial partner: Yes

Type: Journal Paper
Journal: Physical Chemistry Chemical Physics

State: Published (Approved)
Published: April 2020
Diamond Proposal Number(s): 19472

Abstract: 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. Our results highlight that Fe(II) exerts a beneficial effect on maintaining Ni in a metallic state, whereas the extension of the Fe oxidation front from the surface towards the bulk is accompanied by a more extended Ni surface hydroxylation with a negative impact on the selectivity towards methane.

Subject Areas: Chemistry, Energy

Diamond Offline Facilities: Electron Physical Sciences Imaging Centre (ePSIC)
Instruments: B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS , E01-JEM ARM 200CF