Due to climate change concerns, there is an increasing interest in reducing CO2 emissions. A promising process to mitigate emissions is the hydrogenation of CO2 (CO2-FTS) into value-added products, where the required H2 can be obtained from renewable sources. Fe-based catalysts stand out as CO2-FTS catalysts since they are highly active for the reverse water-gas shift reaction and the Fischer-Tropsch-Synthesis, which presumably are involved in the CO2-FTS overall mechanism. However, there is still uncertainty about the impact of several factors on the catalyst performance. It has been reported that metal-support interactions play a crucial role in the CO2-FTS. At the same time, several studies have shown that also the nanoparticle size affects the selectivity in the CO2-FTS. Moreover, Fe carbide is frequently pointed out as the active phase during CO2 hydrogenation. However, these findings have typically been reported for non-supported materials characterized by techniques unable to disentangle the contribution from surface and bulk sites. In this work, we explored the chemical evolution of oxide-supported Fe nanoparticles under activation and CO2-FTS reaction conditions. A systematic study of well-defined model and real catalysts by near ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy allowed us to shed light on the nature of the Fe under reaction conditions and its dependency with the nanoparticle size and the support used. Our findings indicate a correlation between the reduction degree of the Fe and the nanoparticle size, which directly impacts on the chemical nature of the active phase under reaction conditions by hindering Fe carbide formation. Moreover, the interaction Fe-support also impacts on the Fe chemical state. Overall, the results suggest the formation of a core (metal-rich)-shell (oxide-rich) structure during the reaction and highlight the role played by the support and nanoparticle size of Fe-based catalysts for CO2-FTS.

Unravelling the chemical state of Fe-based catalysts for CO2 hydrogenation by NAP XPS and Operando XAS: substrate and size effects

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