Metal single crystal surfaces, reactions

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

Reaction Kinetics and Mechanism on Metal Single Crystal Surfaces... 

Figure 1 Schematic of the atomic beam dosing source used with REMPI detection by Murphy et al. to study the recombination of H [36, 37] and N [38] at metal single crystal surfaces. A single crystal surface is supported on a manipulator in the path of a collimated molecular beam. The beam supplies reactant molecules or atoms, produced using a microwave discharge in the glass nozzle, which react and recombine at the surface. The reaction products are ionised by the laser, which is focused in front of the surface (inset), and the resulting ions are timed into a microchannel plate detector.

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

Oscillations have been found in numerous catalytic systems. At least 24 different oscillating reactions are reported in the literature, involving catalysts ranging from noble metal single-crystal surfaces to zeolites. Table I lists the reactions that have been observed to oscillate and also briefly indicates the catalyst, the pressure regime, and the type of the reactor in which the oscillations have occurred. 

Somorjai GA, McCrea KR (2000) Sum frequency generation Surface vibrational spectroscopy studies of catalytic reactions on metal single-crystal surfaces. Adv Catal 45 385... 

Oudar et al. (112) have studied the influence of sulfur on the hydrogenation of 1,3-butadiene and H2-D2 equilibration over Pt(110). The rates decayed linearly with sulfur coverage, so that each sulfur atom poisons one dissociation site for hydrogen without influencing the activation energies or mechanism. The authors established the first isotherm for sulfur adsorption under actual catalytic reaction conditions. The adsorbed hydrocarbons influenced the equilibrium coverage of sulfur on the Pt surface. The thermodynamics of adsorbed sulfur on several metal single crystal surfaces have been presented by Bernard et al. (114). 

Another interesting surface reaction involves the direct synthesis of a metal-coordinated porphyrin array. From the perspective of surface science, surlace-supported 2-D metal-oiganic frameworks (MOFs) with open-spaced adlayers have been extensively investigated under UHV on a variety of metal single crystal surfaces, such as Au, Ag, and Cu [16-18]. The 2-D stmctures of these MOFs, including, for example, the 2-D supramolecular coordination of not only zinc(II) telrapyridyl porphyrin (ZnTPyP) with Au [17] but also the bicomponents composed of ZnTPyP and 4, 4""-(l,4-phenylene)bis(2,2 6, 2"-terpyridine)... 

Kita H, Ye S, Gao Y. Mass transfer effect in hydrogen evolution reaction on Pt single-crystal electrodes in acid solution. J Electroanal Chem 1992 334 1—2) 351—7. Protopopoff E, Marcus P. Effect of chemisorbed sulfur on the hydrogen adsorption and evolution on metal single-crystal surface. J Chim Phys 1991 88 1423-52. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

Reaction Kinetics and Mechanisms on Metal Single Crystal Electrode Surfaces Recent Developments in Faradaic Rectification Studies... 

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

While experiment and theory have made tremendous advances over the past few decades in elucidating the molecular processes and transformations that occur over ideal single-crystal surfaces, the application to aqueous phase catalytic systems has been quite limited owing to the challenges associated with following the stmcture and dynamics of the solution phase over metal substrates. Even in the case of a submersed ideal single-crystal surface, there are a number of important issues that have obscured our ability to elucidate the important surface intermediates and follow the elementary physicochemical surface processes. The ability to spectroscopically isolate and resolve reaction intermediates at the aqueous/metal interface has made it difficult to experimentally estabhsh the surface chemistry. In addition, theoretical advances and CPU limitations have restricted ab initio efforts to very small and idealized model systems. 

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