Mechanistic Insights into the Selectivity of Catalytic CO2 Hydrogenation to High-Value Chemicals over Fe–Co Bimetallic Catalysts

Carbon dioxide hydrogenation has captured significant attention as a key reaction in CO2 valorization. This reaction has proved valuable as it is considered as both a mitigating solution to harmful greenhouse gas emissions and a production method of high-value chemicals and fuels. Due to the thermal stability of CO2, an intermediate C1 building block is required to initiate the formation of valuable C2+ hydrocarbon products. As such, considerable previous research has sought to identify possible precursor-mediated pathways for the formation of C2+ products, revealing two as the most prominent: (1) methanol-synthesis route, where methanol is the C1 precursor, and (2) Fischer–Tropsch synthesis (FTS) route, which utilizes CO as the C1 precursor. For the FTS route, Fe and Co catalysts have proven to be the most promising. However, the wide range of products formed with such catalysts imposes a challenge on understanding the underlying mechanism of FTS, and thus selectivity control limitations. Here, we explore the active surface of an Fe–Co bimetallic catalyst under experimentally optimized reaction conditions using a theoretical approach. We model the catalyst in CO2 hydrogenation conditions as Co doped χ-Fe5C2 carbide. We show that the mechanism initiated by the C–O bond cleavage in CO2 is preferred and we identify two main chain-lengthening schemes involving carbon coupling of: (1) CHx* species, and (2) active oxygen-containing species (HCO*). The energetic span approximation shows that both schemes are comparatively active for CO2–FTS under the experimental conditions, however, we establish that through the CHx* species coupling route, more undesired side reactions producing light products, such as methane and methanol, arise. In contrast, the active oxygen-containing intermediate route (via HCO*) shows a more direct pathway to desired C2+ and C5+ products (including higher alcohols) with minimal undesired side reactions.