AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA1
A New Mechanism for Hydrocarbon Dehydrogenation on Pt under Conditions of High Hydrogen Coverage

Monday, October 2, 2000, 2:00 pm, Room 208

Session: Model Catalysts at High Pressures
Presenter: D.A. Perry, University of Illinois
Authors: J.C. Hemminger, University of California, Irvine
D.A. Perry, University of Illinois
Correspondent: Click to Email
In 1978 Demuth and Ibach published the HREELS vibrational spectrum of cyclohexane (C@sub 6@H@sub 12@) adsorbed on clean Pt(111) at low temperature showing strong softening of some C-H modes. Subsequently, data from several laboratories resulted in a consistent picture for the initial step in the dehydrogenation of cyclohexane on clean Pt. The mechanism of the dehydrogenation involves the weakening of one of the axial C-H bonds of the cyclohexane by interaction with a Pt 3-fold hollow site (leading to the soft C-H modes). This is followed by C-H bond breaking to form Pt-H and Pt-C bonds. The initial dehydrogenation product is cyclohexyl (C@sub 6@H@sub 11@). Work from our laboratory has shown that in the presence of high coverages of surface hydrogen the dehydrogenation actually occurs at a lower surface temperature, indicative of a 20% reduction in the activation energy for the initial step. In combination with our HREELS data, that shows no C-H mode softening, this reduction of the activation energy strongly indicates a different mechanism for the dehydrogenation in the presence of high surface coverages of hydrogen. The high surface coverages of hydrogen will effectively fill the 3-fold hollow sites on Pt(111). This leads to a more weakly adsorbed cyclohexane and the lack of softening of the C-H modes. Under these conditions we propose that the dehydrogenation occurs via a @sigma@ bond metathesis mechanism. In this mechanism, which is well established in metal-hydride organometallic chemistry, a hydrogen that is bonded to the Pt interacts strongly with a hydrogen of the cyclohexane. The metal-hydride hydrogen combines with the hydrogen from the cyclohexane to directly form H@sub 2@ at the same time that a C-Pt bond is formed. This concerted mechanism is consistent with our observations that the C-H modes are not softened when the 3-fold hollow sites are blocked with hydrogen and yet a lower activation barrier path to the initiation of the dehydrogenation exists.