Screening Co-Based Single-Atom Alloys for Fischer–Tropsch Synthesis: Identifying Linear Scaling Relationships

Abstract

CO2-derived fuels can advance carbon recycling efforts by providing drop-in replacements for hard-to-decarbonize sectors, such as aviation, while storing surplus renewable energy. A promising route for synfuel production consists of a two-step process, combining the reverse water gas shift reaction (CO2 + H2 ↔ CO + H2O) with Fischer–Tropsch synthesis of hydrocarbons (CO + 2H2 → -(CH2)- + H2O). To ensure a high yield of heavier synfuels, low-temperature Fischer–Tropsch synthesis (LT-FTS) is necessary. While Ru-doped Co single-atom alloys (SAAs) have shown promise as LT-FTS catalysts, the potential of other transition metal promoters remains largely unexplored. Herein, we employed density functional theory (DFT) calculations to screen various Co-based SAAs for LT-FTS. Catalytic trends were inferred from the reaction energetics of key FTS elementary steps. Activity descriptor analysis revealed poor correlations between the activation and reaction energies, likely rooted in the poor electronic mixing between host and promoter atoms in SAAs. To overcome this, the adsorption energies of small molecules were used as activity descriptors, maintaining configurational similarities between the binding modes of the probing species and the transition state geometries. To corroborate the DFT-predicted trends, temperature-programmed reactor studies were conducted on Re–Co and V–Co SAAs, the two most promising systems identified. Activity and selectivity deviations were observed experimentally compared to the DFT predictions, suggesting that catalyst deactivation might occur. To this end, reducibility trends were investigated both computationally (DFT-computed oxygen-vacancy formation energies) and experimentally (temperature-programmed reduction studies). Both approaches revealed differences in the reducibility trends previously reported on intermetallic systems. Overall, this work highlights how electronic and structural catalytic effects change at dilute promoter concentrations.