First-principles microkinetic analysis of dehydrogenation of cyclohexene on the Pt/Cu/Pt (111) surface.

Abstract

Three kinds of Pt-Cu bimetallic catalysts (Cu/Pt (111), Pt/Cu/Pt (111), and Pt4Cu5/Pt (111)) have been researched employing density functional theory (DFT) calculation, using dehydrogenation of cyclohexene to benzene as a probe reaction. The adsorption energies are basically in the sequence: Pt4Cu5/Pt (111) > Cu/Pt (111) ≈ Pt/Cu/Pt (111). The key step is C6H9 → C6H8 on Cu/Pt (111) (0.85eV) and Pt/Cu/Pt (111) (0.87eV). On Pt4Cu5/Pt (111), the key step is C6H7 → C6H6 (1.17eV). The selectivity for gas phase benzene is in the order of Cu/Pt(111) > Pt/Cu/Pt(111) > Pt4Cu5/Pt(111), according to the energy difference between the barrier of benzene dehydrogenated to phenyl and benzene desorption. The co-adsorbed hydrogen atoms lead to improved selectivity for gas phase benzene on Cu/Pt (111) and Pt/Cu/Pt (111), by making benzene desorption easy but dehydrogenation difficult. However, the barrier of benzene dehydrogenation decreases with the increase of H coverage on the Pt4Cu5/Pt (111) due to obvious destabilized benzene, and thus the effect on Pt4Cu5/Pt (111) is closely related to the concentration of surface H. Attributed to thermodynamic stability, high activity, and selectivity for gas benzene, the Pt/Cu/Pt (111) structure is suggested as reasonable dehydrogenation catalyst, and the dehydrogenation process on Pt/Cu/Pt(111) has been further studied by microkinetic modeling. A volcano-like relationship is found between the adsorption of cyclohexene and the TOF (turnover frequency) of gas phase benzene. Secondly, two apparent activation energies are obtained: 0.77eV (250~350K) and 0.45eV (350~650K), implying the RDS (rate-determined step) changes with temperature. Graphical abstract The influence of temperature and desorption barrier of cyclohexene on the TOF of C6H6(g).