Hydrogen surface phases

Azimuthal scans obtained for tliree surface phases of Ni l 10] are shown in figure B 1.23.9 [38]. The minima observed for the clean and hydrogen-covered surfaces are due only to Ni atoms shadowing neighbouring Ni atoms, whereas for the oxygen-covered surface minima are observed due to both O and Ni atoms shadowing... 

Campbell and coworkers269 also published a kinetics study of the reverse water-gas shift over Cu(110) in 1992, and the results were cast in terms of the redox mechanism (reverse of Scheme 60, left side). A hydrogen-induced surface phase transition was suggested to impact the rate at high H2/C02 ratios, as the rate was found to exhibit a saturation-like behavior with increasing P(H2) when 5 Torr of C02 was used, but continued on a log-linear trend when 150 Torr of C02 was... 

The substrate in these studies was restricted to be rigid, and Morse functions were used for the hydrogen-surface and two-body interactions. The parameters in the Morse functions were determined for single hydrogen atoms adsorbed on the tungsten surface by fitting to extended Huckel molecular orbital (EHMO) results, and the H2 Morse parameters were fit to gas-phase data. The Sato parameter, which enters the many-body LEPS prescription, was varied to produce a potential barrier for the desorption of H2 from the surface which matched experimental results. 

At the polymer surface radicals are lost by reactions involving gaseous atomic hydrogen, gas phase free radicals, and adsorbed free radicals. The rate of surface termination can be expressed as... 

Monovalent Cation vs. Hydrogen Ion Competition in Surface Phase. 

We require that a given surface species only resides in one particular surface phase. For example, the properties of a hydrogen atom adsorbed on a step site might be different from a hydrogen atom adsorbed on a terrace site, so they could reasonably be considered different species (even though their elemental composition is the same). The number of species in surface phase n is termed Ks(n), and the species in that phase are numbered sequentially from the first species in the phase k (n) to the last species Kls(n). The total number of surface species in all surface phases is designated Ks. 

The constancy of the activity for benzene hydrogenation over a wide range of overall concentrations of copper and nickel was shown to be indeed due to a surface phase of constant concentration, in accord with the cherry model. 

The adsorption of thiophene on supported palladium has been studied (61, 62). The studies were performed under conditions used for industrial hydrogenations liquid phase, low temperature, and hydrogen pressure. The carrier is a special inert alumina with large pores (greater than 10 nm) and a surface area of less than 100 m2/g. 

The images in Fig. 13c and d show that when the density of the pits increases and the distance between adjacent pits is small compared to the size of the precipitates, the preferential nucleation is not obvious. However, the total amount of the hydrogen bronze phase produced still correlates with the total area of the pits. Both crystals in Fig. 13 c and d were reduced for 5 min at 400 °C in forming gas to form the pits. One was then oxidized in ambient air at 410 °C for 1 h so that the pits could grow, while the other was not. As a results, the pits shown in Fig. 13c are all less than 200 A deep while those on the surface that received the oxidation treatment are all greater than 500 A deep (see Fig. 13d). 

Fig. 22. Equilibrium between bulk and surface phases for a film of insoluble acid HA (e.g., octadecanoic acid) and its anion A , in the presence of a small amount of HOI. The hydrogen and chloride ions can penetrate into the surface phase, remaining in equilibrium with the bulk. The final equilibrium is given by the Donnan equations (xxv) and (xxvi), the former referring to the equilibrium of HCl between the surface and bulk phases, and the latter to the electrical neutrality of the surface phase.

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

One of the few cases in which hydrogenated surface complexes have been suggested to be active involves the decomposition of nitrous oxide.The reaction was suggested to involve hydrogen in the carbon, although the reaction mechanism was written in terms of gas phase hydrogen involved in a free radical chain reaction. As a result, the importance of hydrogenated complexes is open to question. 

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