Metal-hydrogen bridge bonding of hydrocarbons on metal surfaces

Study of the competitive mechanisms of cyclohexane dehydrogenation by gas-phase Ni2(+) cationic dimer: one-face dehydrogenation versus flip dehydrogenation

The mechanism of cyclohexane dehydrogenation catalyzed by the cationic dimer Ni2 (+) has been investigated at the B3LYP level of density functional theory. The first dehydrogenation occurs readily (it is exothermic by 30 kcal/mol), whereas the second and third dehydrogenations show weaker exothermicity than the first (23 and 21 kcal/mol, respectively). These three hydrogenations corresponding to the total dehydrogenation of one face of cyclohexane mainly proceed in the doublet state due to the presence of significant minimum-energy crossing points (MECPs). In addition, because the elimination of non-negligible amounts of [H2,2D2] and [2H2,D2] in this reaction was also observed in a previous experiment, we calculated a flip mechanism which would yield results that agree with those experimental results. This flip process includes two MECPs, meaning that the reaction mainly proceeds along the doublet potential energy surface but finishes in the quartet state. The rate-limiting step ((2)IM9 → (2)TS9/10 → (2)IM10) of the flip process is endothermic by 3 kcal/mol and the barrier to this step is 33 kcal/mol. Our calculations indicate that one-face dehydrogenation is a more favorable channel than the flip one. We excluded the possibility that eliminations of [H2,2D2] or [D2,2H2] could proceed through a mechanism involving Ni2 (+) dissociation, or that [H-D] scrambling could occur through (2)TS11/13 ((4)TS12/15), due to the large amounts of energy required. In the dissociation of (2)IM19, (2)[(H2)Ni2(C6H6)](+), a molecule of hydrogen first dissociates, leaving a final product of (2)[Ni2(C6H6)](+). Neither C6H6 nor (H2)Ni2 (+) can easily dissociate from (2)IM19 due to π backdonation.

The relationship between adsorption energies of methyl on metals and the metallic electronic properties: A first-principles DFT study

A theoretical study of CH3 adsorbed on the (111) surface of some transition and noble metal surfaces M (M = Cu, Ni, Rh, Pt, Pd, Ag, Au) and on the Fe(100) is presented. We find that the hollow site is preferred more than the top one for Fe, Ni, Rh, and Cu, but it is the other way for Pt, Pd, Au, and Ag. In addition, a good linear relationship was observed between the chemisorption energy and d-band center for Group VIII metals or the square of the coupling matrix element for Group IB metals at the hollow site. Interestingly, with a detailed comparison of the adsorption energies at the top and hollow sites, we find that the adsorption energies among each group are very similar on the top site, which supports the theoretical model of Hammer and Norskov that the coupling between the HOMO of adsorbate and sp states of the metal is dominant and almost equal, and that the second coupling to the d-band contributes less but reflects the change of the adsorption energy. It confirms that the coupling to the d band comprises two opposite factors, that is, the d-band center was attractive and the square of the coupling matrix element was repulsive, such that the contributions from the two factors can counteract each other at the top site.

Chemistry of acetylene on platinum (111) and (100) surfaces

An ultra-high vacuum experimental study of acetylene chemisorption on Pt(111) and Pt(100) and of the reaction of hydrogen with the acetylene adsorbate has established distinguishing features of carbon-hydrogen bond breaking and making processes as a function of pressure, temperature, and surface crystallography. The rates for both processes are substantially higher on the Pt(100) surface. Net acetylene-hydrogen processes, in the temperature range of 20 degrees C to approximately 130 degrees C, are distinctly different on the two surfaces: on Pt(100) the net reaction is hydrogen exchange ((1)H-(2)H exchange) and on Pt(111) the only detectable reaction is hydrogenation. Stereochemical differences in the acetylene adsorbate structure are considered to be a contributing factor to the differences in acetylene chemistry on these two surfaces.