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 °C. This catalyst is studied by in-situ APXPS and NEXAFS spectroscopies and compared with the other synthesised samples to obtain new mechanistic insights on methanation catalysts active for low-temperature (300 °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 xNiL (Ni loading) = 21.2 mol%. These sites are oxidised on the low Ni-loaded catalyst (xNiL= 9.2 mol%). The best CO2 conversion rate and CH4 selectivity are shown at intermediate xNiL (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 TOFCO(turnover frequency of CO2 conversion) increases exponentially with the fractional density of basic to metallic sites (XB) from 1.1 s-1 (xNiL= 29.2 mol%) to 9.1 s-1 (xNiL= 7.6 mol%). It follows the opposite trend of the CO2 conversion rate. In-situ DRIFTS data under methanation conditions evidence that the TOFCOat high XB is related to the presence of a formate route which is not predominant at low XB (high xNiL). A synergistic interplay of basic and metallic sites is present. This contribution provides a rationale for designing industrially competitive CO2 methanation catalysts with high catalytic activity while maintaining low Ni loading.