Catalytic mechanism and selectivity prediction for syngas conversion over pure and K-promoted Mo2C catalysts

Abstract

Density functional theory (DFT) calculations were performed to reveal the intricate pathways of syngas conversion over Mo2C catalysts. Our computational results show that CO activation occurs preferably via its hydrogenation to HCO on the Mo2C (001) surface. The Mo-Mo bridge site is found to be the active site, and ethanol formation involves the HCO*, H2CO, CH2, CH3, CH3CHO, and CH3CH2O intermediates. Product selectivity in this reaction is mainly affected by reaction steps involving CH2, and ethanol yield is limited by the high energy barriers of CH3+CHO coupling and C2H5O hydrogenation. Since K has been experimentally shown to be an outstanding promoter for increasing alcohol selectivity, we further investigated the effect of the K promoter to the catalytic performance of the Mo2C catalyst. Compared to the pure Mo2C (001) surface, K-Mo2C (001) promotes the activity of the CH3+CHO coupling. To give a semi-quantitative estimate for the product selectivity, we used the effective energy barrier difference (ΔEeff) between CH2+CH2 coupling and CH3+CHO coupling as an energy descriptor. The ΔEeff value of the K-Mo2C (001) surface is significantly greater than that of the pristine surface, indicating higher selectivity of ethanol over the K-promoted surface, which is consistent with the experimental observation. Furthermore, comparing the Mo2C(001) and (100) surfaces, we find that the Mo2C (001) surface exhibits better activity toward CO activation, whereas the Mo2C (100) surface favors the CC coupling reaction.