Cyclohexene as a chemical probe of the low-temperature hydrogenation activity of Pt/Ni( [formula omitted]) bimetallic surfaces

Abstract

The reactions of H 2 and cyclohexene on Pt/Ni(1 1 1) surfaces were investigated using temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Hydrogen desorbs from Pt/Ni(1 1 1) bimetallic surfaces at temperatures approximately 40–50 K lower than the clean Ni(1 1 1) surface. The reaction pathways of cyclohexene are also significantly modified on the Pt/Ni(1 1 1) surfaces. On Ni(1 1 1), cyclohexene undergoes dehydrogenation and decomposition to produce benzene (c-C 6H 6), atomic C (a) and H 2(g). HREELS results also reveal that cyclohexene (c-C 6H 10) is strongly bonded to the Ni(1 1 1) surface. However, when Pt is added to the Ni(1 1 1) surface at coverages from 0.5 to 3 ML, a fraction of the cyclohexene undergoes self-hydrogenation to produce cyclohexane (c-C 6H 12) at 237 K. HREELS results indicate that cyclohexene is more weakly bonded to the Pt/Ni(1 1 1) surface, with the ν(CC) stretching frequency (∼1638 cm −1) and skeletal distortions (649 cm −1) still present at 200 K. When a submonolayer coverage (40–50% saturation) of hydrogen is pre-adsorbed on the Pt/Ni(1 1 1) surfaces, the cyclohexene hydrogenation activity is enhanced by 10-fold, with a lower onset temperature of desorption of the cyclohexane product at ∼180 K.