(603f) Insights Into Mechanism of Syngas Reaction On Mo2C Catalysts --- Identifying “Descriptor” Elementary Steps for Complex Reaction Systems

With the limited fossil fuel resources, extensive research efforts have been devoted to find alternative building blocks in the chemical industry. Syngas (CO + H₂) is potentially promising because it can be derived from both conventional and renewable energy sources and produce a range of different downstream products.

As a syngas reaction catalyst, molybdenum carbides have been reported in applications towards various products such as Fischer-Tropsch synthesis, water gas shift, and higher alcohol synthesis. It was also found experimentally that selectivity could be further modified by the addition of alkali promoter. In principle, finding the “descriptor” controlling product selectivity could eventually help us designing catalyst selective to specific product. However, little progress has been made so far due to the complexity of the syngas reaction mechanism.   

In this work, we seek to provide a broad perspective of syngas reaction network on Mo₂C and determine individual elementary steps most sensitive to product selectivity. We first built a kinetic model containing steps relevant in Fischer-Tropsch synthesis, methanol synthesis, ethanol synthesis and water gas shift reaction. Reaction energies were then calculated from Density Functional Theory while activation energies were approximated from Bronsted-Evans-Polanyi (BEP) relations. With these energies as inputs, reaction in the kinetic model was simulated. Our computed selectivity was consistent with experimental selectivity without any data fitting.  Sensitivity analysis was applied to determine steps sensitivity to the overall model. Our results suggested CH_x intermediates on the surface were mostly produced through H-assisted CO dissociation although it might not be the rate determining step due to surface coverage effect. We hope this work could give insights into the reaction mechanism of syngas reaction systems and serve as a guideline of applying a complex kinetic model to determine “descriptor” elementary steps.