Single-crystal surfaces, metal complexes

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

Rigby J, Kondratenkov M (2004) Arene Complexes as Catalysts. 7 181-204 Risse T, Freund H-J (2005) Spectroscopic Characterization of Organometallic Centers on Insulator Single Crystal Surfaces From Metal Carbonyls to Ziegler-Natta Catalysts. 16 117-149... 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

Overall perspectives of the results from ethene and the higher alkenes have been attempted in Sections VI.B.6 and VI.G. What has become clear, particularly in the context of hydrocarbon adsorption, is that the study of spectra on single-crystal surfaces is of great assistance in finding the correct interpretation of the more complex multispecies spectra obtained from finely divided metal catalysts. This has only become possible by the development of VEELS and RAIRS, the latter allied with the Fourier-transform methods that have also transformed the quality of the spectra from metal-particle catalysts obtained by transmission infrared spectroscopy. The use of RAIRS in turn has emphasized the general significance of the MSSR. 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

Considering that under reaction conditions identical to those stated just above the Pd(l 11) single-crystal surface remained metallic, palladium nanoparticles are apparently easier to oxidize than bulk palladium, possibly because of the higher abundance of surface defects. The palladium oxide phase may be located not only on the particle surface but also at the palladium/alumina interface (515). Partial oxidation of palladium particles has been observed previously for combustion reactions on technological catalysts and may lead to complex hysteresis phenomena (see Refs (514,516) and references therein). 

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. 

The following describes results of three, relatively simple chemical reactions involving hydrocarbons on model single crystal metal catalysts that illustrate this general approach, namely, acetylene cyclotrimerization and the hydrogenation of acetylene and ethylene, all catalyzed by palladium. The selected reactions fulfdl the above conditions since they occur in ultrahigh vacuum, while the measured catalytic reaction kinetics on single crystal surfaces mimic those on reahstic supported catalysts. While these are all chemically relatively simple reactions, their apparent simplicity belies rather complex surface chemistry. 

The application of the reflection technique to adsorption on single crystals will possibly offer a new lease on life to the use of infrared in fundamental gas-metal adsorption studies. Infrared studies that have contributed so much to CO adsorption studies, particularly those closely allied to catalytic systems, are likely, by application of the reflection technique, to be unique in their designation of adsorption complexes on single crystal surfaces. 

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