Theoretical Study on the Effect of Pd/Zn Ratio on Benzene Hydrogenation Catalytic Activity and Selectivity

Partial hydrogenation of benzene is the main approach to cyclohexene synthesis in industry. Here, the reaction mechanisms of benzene hydrogenation on Pd-Zn bimetallic catalysts were studied using density functional theory, with the aim of understanding the effect of different Pd/Zn ratios on catalytic activity and cyclohexene selectivity. Three different surfaces, Pd(111), Pd4Zn1(111), and Pd2Zn1(111), were considered as catalyst models. It was found that increasing the Zn concentration decreases the hydrogenation energy barriers while also hindering the reverse reactions. These findings are corroborated by microkinetic simulations and also indicate that cyclohexene selectivity increases with higher Zn concentration but at the expense of reaction activity, which decreases due to the weaker C6H6* and H* adsorption strength in systems with high Zn concentration. The hydrogen coverage has a significant effect on the reaction activity, degree of rate control coefficient, and apparent activation energy as well. For the high hydrogen coverage situations, C6H9 hydrogenation is the rate-controlling step on H1.0/Pd(111) at all considered temperatures, but the degree of rate control for the C6H11 hydrogenation step significantly increases at high temperatures. For H0.8/Pd4Zn1(111), the rate-controlling step changes from C6H7 hydrogenation to C6H9 hydrogenation with increasing temperature, and for H0.67/Pd2Zn1(111), it changes from C6H7 and C6H8 hydrogenation to C6H10 hydrogenation.