Ethanol Synthesis From Syn-Gas: How Surface Diffusion of Intermediates Impacts the Product Distributions Predicted for Bimetallic Catalysts

A major challenge associated with the synthesis of ethanol from syn-gas is an inability to find a low-cost catalyst that promotes the proper combination of CO dissociation and CO insertion steps, so as to yield ethanol as the primary reaction product and inhibit the formation of methane, longer chain alkanes, and other coking reaction products. For this purpose, quantum mechanical simulations were used to evaluate the catalytic activity of a bimetallic cobalt-palladium cluster, Co₇Pd₆ . DFT simulations and Bronsted-Evans-Polanyi (BEP) relations were used to map out the full reaction mechanism from syn-gas to ethanol. Microkinetic models were built, considering the reaction steps as well as the diffusion of intermediate species between different metal surface sites: Co₃, Pd₃, and mixed CoPd sites. For this system, CO insertion into the metal-carbon bonds of adsorbed methyl groups is the primary pathway for C₂ oxygenate production. Due to most CoPd and Pd sites being blocked by adsorbed CO, a significant fraction of product forming reactions occur on the Co sites, which leads to methane being the major product (90%). However, the diffusion of CH₄ and CH₃CO between Co and CoPd sites facilitates CO insertion reactions, which ultimately leads to some ethanol (10%) being produced on the Co sites. These results provide a basis for future studies focused on identifying optimal bimetallic catalysts for ethanol production.