(729e) Screening Cobalt-Based Single Atom Alloy Catalysts for Low-Temperature Fischer-Tropsch Synthesis

To reach net-zero CO₂ emissions by 2050, the installed carbon capture, utilization, and sequestration capacity needs to increase ~15 times by 2030¹. Among the CO₂ utilization applications, the production of synthetic fuel (also known as synfuel) has the highest CO­₂ use capacity due to the vast worldwide energy demands². Moreover, synfuels can supply low-carbon alternatives to sectors not yet ready for electrification, such as aviation and maritime³. One of the main CO₂-synfuel production routes consists of sequentially pairing the Reverse Water-Gas Shift (RWGS, CO₂ + H₂ ↔ CO + H₂O) with Fischer-Tropsch Synthesis (FTS, CO + 2H₂ → -(CH₂)- + H₂O)⁴. One drawback of this pathway is the separation of the CO₂ effluent from the RWGS step, as typical FTS catalysts are also active for the Sabatier reaction (CO₂ + 4H₂ → CH₄ + 2H₂O)⁵. Considering CO₂ activation requires high temperatures, usually above 300 °C ⁶, low-temperature (LT) FTS (~200 °C) can reactively separate the CO/CO₂ inlet.

Cobalt is the most popular LT-FTS catalyst, often enhanced by doping (or promoting) with precious metals⁷. In our previous work, Ru-doped Co Single Atom Alloys (SAAs), in which isolated Ru atoms are dispersed on the Co surface, were investigated for LT-FTS. The Ru-Co SAA catalyst showed enhanced reducibility and improved efficiency in the use of Ru (Co:Ru atomic ratio decreased ~20 fold compared to co-impregnated catalysts)⁸. In this work, we search for cost-effective alternatives for Ru that result in active LT-FTS catalysts by using density functional theory to screen various Co-based SAAs. The overall catalytic performance is evaluated by comparing the energetics of three FTS elementary steps: 1) _­CH₂ coupling (2CH₂ → C₂H₄) to probe the formation of long-chain hydrocarbons; 2) CH₃ hydrogenation (CH₃ + H → CH₄) to evaluate the formation of undesired methane; and 3) CH₃ dissociation (CH₃ → CH₂ + H) to gauge the likelihood of finding low-saturated carbon chains. Among the tested dopants, Re is identified as a cost-efficient alternative to Ru, with similar reaction energetics, whereas, V is found to have the most favorable energetics for long-chain hydrocarbons (unfavorable CH₄ formation and low CH₂ coupling barriers). The selectivity predictions on these two SAAs are experimentally validated by temperature-programmed reactor studies. Finally, fitted linear scaling relationships using common (de)hydrogenation descriptors show deviations with varying adsorption configurations, suggesting the importance of site-sensitive descriptors. Overall, this work introduces a framework for the effective screening of SAA candidates in complex multi-step reactions.

References

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2. International Energy Agency Putting CO2 to Use; 2019.

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