Insights by in-situ studies on the nature of highly-active hydrotalcite-based Ni-Fe catalysts for CO2 methanation

Designing CO2 methanation catalysts that meet industrial requirements is still challenging. We report Ni-Fe hydrotalcite-derived catalysts with a wide range of Ni and Mg loadings showing that an optimised composition with Ni0.4 gives a very high CO2 conversion rate of 0.37 mmol/gcat/s at 300 degrees C. This catalyst is studied by in-situ APXPS and NEXAFS spectro- scopies and compared with the other synthesised samples to obtain new mechanistic in- sights on methanation catalysts active for low-temperature (300 degrees C) methanation, which is an industrial requirement. Under methanation conditions, in-situ investigations revealed the presence of metallic Ni sites and low nuclearity Ni-Fe species at xN; L (Ni loading)=21.2 mol%. These sites are oxidised on the low Ni-loaded catalyst(xN;L=9.2 mol%). The best CO2 conversion rate and CH4 selectivity are shown at intermediate xN;L (21.2 mol%), in the presence of Mg. These superior performances are related to the high metallic surface area, dispersion, and optimal density of basic sites. The TOFCO2(turnover frequency of CO2 con- version) increases exponentially with the fractional density of basic to metallic sites (XB) from 1.1 s-1(xN;L=29.2 mol%) to 9.1 s-1(xN;L=7.6 mol%). It follows the opposite trend of the CO2 conversion rate. In-situ DRIFTS data under methanation conditions evidence that the TOFCO2at high XB is related to the presence of a formate route which is not predominant at low XB (high xN; L ). A synergistic interplay of basic and metallic sites is present. This con- tribution provides a rationale for designing industrially competitive CO2 methanation cat- alysts with high catalytic activity while maintaining low Ni loading.(c) 2023 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved.