Role of potassium on reaction pathways in CO₂ hydrogenation

Abstract

Potassium (K) has been widely employed as an alkali promoter for Fe-based catalysts in the reverse water gas shift (RWGS) reaction, Fischer-Trospch synthesis (FTS) and the integration of RWGS and FTS for direct CO₂ hydrogenation to higher hydrocarbons, commonly referred to as the CO₂-FTS process. K is recognized as both a structural promoter of the Fe phase and an electronic promoter that modifies the adsorption and activation behavior of reactants on the catalyst surface. However, its role in reaction pathways and the effect of K loading in each reaction remains largely unexplored. In this study, a series of K-promoted carbon-supported Fe-based catalysts was prepared to investigate the effect of K/Fe molar ratios from 0.02 to 0.5 on the RWGS and FTS reactions in a fixed-bed reactor at 300 °C, 11 bar, H₂/CO₂/Ar=3/1/1, 1000–375000 mL·g_cat⁻¹·h⁻¹. Quasi in situ Mössbauer spectroscopy shows that sufficient K promotion (K/Fe ≥ 0.1) on carbon-supported Fe-based catalysts led to complete carburization following a reduction-carburization activation procedure. The resulting Fe carbides remained stable after both RWGS and FTS conditions, suggesting that they are responsible for catalyzing both reactions. This finding contrasts with literature reports that attribute RWGS and FTS activity to Fe oxides and Fe carbides, respectively. Increasing the K/Fe ratio progressively suppressed CO₂ methanation as a primary reaction, ultimately rendering primary methanation insignificant and leaving RWGS as the only primary reaction. Both RWGS and FTS activities reached their optimum at a K/Fe ratio of 0.1. However, a higher K/Fe ratio led to higher CO selectivity in the RWGS reaction, whereas an optimal K/Fe ratio of 0.1 was found for FTS to promote the production of higher hydrocarbons.