Single crystals, metallic Subject

Growing ultrathin metallic layers on a (chemically different) single crystal metal surface allows us to explore the changes in morphology and electronic structure that occurs as strain relief processes develop and relate them to the changes in reactivity, a subject of immense importance in catalysis. 

RAIRS has proved to be a powerful vibrational spectroscopic technique for the study of adsorbates on metal surfaces, allowing not only the identification of the surface species, but also information concerning molecular geometry and chemical environment. The application of RAIRS to measiu-ements on single crystal metal surfaces has been the subject of a number of reviews [1, 3-8, 31, 32], and both the theoretical and experimental aspects have been discussed extensively for measurements on metals. The extension of RAIRS to oxide surfaces requires foremost a consideration of the difference in optical response of the substrate. This aspect had already been examined in attempts to extend RAIRS to measurements on semi-conducting surfaces [7, 32-35], which have a similar optical response to many metal oxide surfaces through much of the IR. 

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

Cu(II) impurity complexes in amino acid single crystals have been the subject of several EPR studies181-183. Since nitrogen and proton hf structures are only partially resolved in the EPR spectra, no detailed information about the electronic properties of the complex in the neighborhood of the metal ion can be evaluated. ENDOR spectroscopy has therefore been applied58,63 to draw detailed pictures of the positions and the molecular environment of Cu(II) impurities in amino acid crystals. 

The fused salt electrolysis technique was successfully applied to the preparation, also as single crystals, of several transition metal compounds. A review on this subject was published by Wold and Bellavance (1972). A systematic treatment of several reactions and processes, was presented possibly somewhat obsolete now and with a drawback due to the presence of several impurities in the synthesis products. The preparation of the following compounds was especially discussed. 

Electrodes modified by underpotential deposition of metal were subjected as electrocatalysts to reduction of oxygen,oxidation of formic acid, and other processes in which polycrystalline metal substrates were used (see review in Ref. 151). Electrocatalysis of single-crystal electrodes modified by underpotential deposition was also investigated, as reviewed by Ad2iC. ... 

Reconstruction of all the three low-index faces of Au single crystal under elevated temperature has been described in several papers. This subject has been discussed in detail by Dakkouri and Kolb [335], as well as (also for other metals) by Kolb [336], and Gao et al. [337, 338]. A recent concise review of Au reconstruction, including experimental details of the surface preparation, has been published by Trasatti and Lust [4]. 

UPD of various metals on different gold surfaces is one of the most intensively studied subjects. Abruna and coworkers have reviewed [380] the UPD deposition at single-crystal surfaces of Au, Pt, Ag, and other materials. More recently, Mag-nussen [381] has described ordered anion adlayers on metal electrode surface, which can affect the UPD process. 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

The siting of cations in mordenite is generally less well understood than that in the zeolites described above. Smith and co-workers (12) have, however, in recent years carried out a number of single-crystal X-ray analyses on various cation-exchanged forms of mordenite. These workers correctly emphasize, however, that the cation population densities are subject to unknown errors due to pseudosymmetry. The alkali metal ions are distributed over four major sites, namely ... 

The weak band at 14.4 kK (e = 800) in the essentially low-spin cyto-chrome-c has been the subject of much discussion. Its sensitivity to conformation, temperature and pH (52, 53) were considered by Williams (4, 5) to be consistent with electron transfer from a distal oxygen or sulphur atom to the metal. This assignment is also compatible with single-crystal spectra (90) which show that the band is z-polarized. However, it is equally possible to assign the band to charge transfer from the porphyrin a2 orbital to the dzz orbital (90). 

Unfortunately the study of three faces of a germanium single crystal is the only one which has been made on the influence of crystal orientation on the oxidation of semiconductors. The authors feel that results on semiconductors will probably be similar to those reported here for other metals. It has not been possible in this paper to discuss many subjects such as adsorption, work function of the different faces, and surface diffusion, all of which are important for a complete understanding of the influence of orientation on oxidation. There is a scarcity of experimental data on these subjects and it is hoped that future experimentation will alleviate this situation. 

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