Effect of Zn on the selectivity of Ru in benzene partial hydrogenation from density functional theory investigations

Catalytic hydrogenation of benzene to cyclohexene is of great importance in the petroleum industry. The introduction of Zn into Ru catalyst can significantly improve the selectivity toward cyclohexene. To elucidate the effect of Zn on the enhanced selectivity, the adsorption of all the possible reaction intermediates involved is investigated by performing density functional theory calculations, and the transition states for the hydrogenation steps are identified by using the constrained Broyden minimization method. With the introduction of Zn into Ru(0001) at high H surface coverage, the adsorption energies of the intermediates are much lower than those on bare Ru(0001), which indicates that the desorption of cyclohexene is promoted on the alloyed surface. The calculated energy barriers for the hydrogenation steps from benzene to cyclohexene are in the region of 0.58–1.04eV, and the hydrogenation of benzene on Zn/Ru(0001) follows the Horiuti-Polanyi mechanism with the successive hydrogenation of the neighboring C atoms in the hexagonal carbon ring. The deep hydrogenation of cyclohexene to produce cyclohexane is kinetically hindered with the introduction of Zn, which also gives rise to the improvement of the selectivity toward cyclohexene.