No abstract

Different binding modes of the reactant on different catalysts determine the hydrodeoxygenation selectivity.

Density functional theory calculations have been used to explore the mechanism of dissociative adsorption of silane (SiH4) on the Si(100)-(2×1) surface. Two reaction paths are described that produce silyl (SiH3) and hydrogen atom fragments adsorbed on the dimer dangling bonds. The energy barrier on the lowest energy path is 12–14 kcal/mol (depending on the details of the theoretical method used), while the barrier on the other path is about 17 kcal/mol. The initial step in both mechanisms is abstraction of a hydrogen atom from silane by an electron-deficient surface atom. It is also possible for the surface to react by forming a bond between the more electron-rich surface atom and the silane Si atom. This latter reaction path has a prohibitively high barrier (39 kcal/mol), and it leads to different products (adsorbed SiH2 and elimination of H2). These results are discussed in the context of Si film growth kinetics, ultrahigh vacuum studies of silane adsorption and other theoretical studies of silicon surface chemistry.

HOCl has been implicated in the destruction of stratospheric ozone via a reaction catalyzed on ice surfaces in polar stratospheric clouds. Adsorption of HOCl on ice is a prototype of a simple ice surface reaction, and it has been the subject of several laboratory studies, making this a valuable case for testing theoretical approaches to ice surface reactions. The present paper describes a first-principles theoretical study of HOCl adsorption on ice, applying density functional theory (DFT) to cluster models of the (0001) surface of ice Ih. In the most stable binding configurations, HOCl acts as a proton donor in a hydrogen bond. The strength of HOCl binding on the surface depends not only on the hydrogen bonding interaction but also on electrostatic and many-body interactions with neighboring water molecules. The results of these DFT calculations are compared to post-Hartree−Fock calculations of the H2O−HOCl complex and to empirical potential simulations of HOCl on ice.

Pulsed molecular beam-surface infrared measurements of the kinetics of CO populating step sites on Ni(9,1,1) are reported and interpreted in terms of elementary surface rate processes. An analytic model is developed to describe the distribution of CO between step and terrace sites in the equilibrium limit, and refine our previous determination of the binding energy difference between these sites to ΔEs-t=0.6±0.2 kcal/mol. Time-resolved surface infrared measurements indicate that the equilibrium step coverage is reached within 100 ms of the chemisorption event. This rapid migration across the (100) terraces to step sites implies a barrier to surface hopping of <5.5 kcal/mol. On a longer time scale of minutes, the CO population at step sites increases further as the equilibrium point is shifted by the dissociative adsorption of residual hydrogen. These slower step filling rates are described with a kinetic model, in which hydrogen adsorption is the rate-limiting step.