Synthesis gas (CO + H2) is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels. Processes such as Fischer-Tropsch synthesis have been demonstrated as viable commercial routes to higher hydrocarbons; however, to date there is no industrial-scale process for conversion of syngas to higher alcohols or oxygenates. Rhodium has long been studied as it is the only elemental catalyst that has demonstrated selectivity to ethanol and other C2+ oxygenates, particularly with proper choice of promoter and support. However, despite numerous experimental and theoretical studies, the fundamentals of syngas conversion over rhodium are still debated. Obtaining an atomic-scale understanding of the mechanism and active site of syngas conversion to oxygenates on rhodium is a scientific challenge which may provide insight into rational design of higher oxygenate catalysts based on rhodium or more abundant metals.
In this work a state-of-the-art microkinetic model is demonstrated for conversion of CO and H2 into methane, ethanol, and acetaldehyde on the Rh(111) surface. The model is based on density functional theory (DFT) calculations using the BEEF-vdW surface-science functional, providing an atomic-scale model of syngas conversion over Rh(111). The mean-field kinetic model includes lateral adsorbate-adsorbate interactions, and the BEEF-vdW error estimation ensemble is used to propagate error from the DFT calculations to the predicted rates. The theoretical model is combined with experimental results from a variety of Rh/SiO2 catalysts to demonstrate that the Rh(111) surface is intrinsically selective toward acetaldehyde. Furthermore, an inverse correlation between catalytic activity and oxygenate selectivity is observed, supporting the structure-sensitivity of oxygenate production over rhodium catalysts. The experimental and theoretical results provide new insight into the mechanism, active site, and intrinsic selectivity of syngas conversion over rhodium catalysts.
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