Showing papers in "Advances in Catalysis in 1986"

Spillover of sorbed species

Abstract: Publisher Summary This chapter focuses on spillover of sorbed species. The exchange of species from one position to another, either on the surface or through the bulk, has been well established. More unique is the mobilization of a sorbed species from one phase onto another phase where it does not directly adsorb. This has been defined as “spillover. Spillover may result in a spectrum of changes in the nonmetallic phase onto which it occurs. In the weakest sense, the spiltover species is transported across the surface of this phase as a two-dimensional gas. It may exchange with similar surface species.The spiltover species may react with the surface, which can result in the creation of surface defects and/or active sites. Further, the bulk of the solid may be transformed into a different structure. In each of these cases, the second phase is no longer an inert. It is not serving to promote the inherent activity on the first phase. The second phase is participating directly in the transport, exchange, and reaction with the spiltover species. In some cases it is able to become catalytically active on its own and thereby to participate directly in subsequent catalysis. The chapter discusses the types of phenomena associated with spillover, the spillover of species other than hydrogen, the aspect of spillover with the most significant catalytic implications, the implication of spillover to catalysis and other heterogeneous processes, the mechanism of spillover, and the nature of the surface and spiltover species.

Quantum-Chemical Cluster Models of Acid-Base Sites of Oxide Catalysts

Abstract: Publisher Summary This chapter focuses on the quantum-chemical cluster models of acid-base sites of oxide catalysts It is desirable, on the one hand, that the size of such a cluster should be as small as possible to permit a sufficiently rigorous quantum-chemical description of its electron properties and the interaction with adsorbed molecules On the other hand, the cluster should be large enough to produce both the real geometry of the whole crystal lattice of a catalyst and its electron structure, which are important for chemisorption The numerous quantum-chemical computations carried out for such clusters have demonstrated that chemisorptions interactions are indeed sufficiently localized, even in the case of metals where maximal cooperative effects should be anticipated Thus quantum chemistry quite unambiguously advocates the use of the local approach in describing the catalytic phenomena The cluster approach opens the way for a direct application of quantumchemical methods to the problems of chemisorption and catalysis Quantum chemistry has a rich experience in describing the local chemical interactions in molecular systems Its methods are continuously refined and improved, and the computational techniques have become more and more accurate and predictive

Mechanistic Aspects of Transition-Metal-Catalyzed Alcohol Carbonylations

Abstract: Publisher Summary This chapter focuses on the nature of the transformations at the metal center, especially with regard to oxidation state and formation of the initial alkyl-, alkoxy-, or carboalkoxy-metal bond from saturated precursors, and discusses the mechanistic aspects of transition-metal-catalyzed alcohol carbonylations. Prerequisite to any catalytic activity is the ability of the metal center to interact effectively with alcohols or alcohol-derived precursors. There are several ways in which this can occur, and most of these are observed or postulated in at least one catalytic scheme. In order to understand the specific reactivities, though, the reader should be familiar with some fundamental aspects of organo-transition-metal chemistry. Homologation reactions, which are believed to usually proceed by way of aldehyde intermediates, are also discussed in the chapter but only as they pertain to the incorporation of the CO into the metal-carbon bonds. A frequent theme in alcohol carbonylations by transition metals is the use of a halide or pseudo-halide promoters or cocatalysts. Despite major problems of corrosion associated with its use, iodide is almost always found to be most effective in this capacity. This is because the halide serves several purposes, for each of which iodide is ideally suited.

Near-Edge X-Ray Absorption Spectroscopy in Catalysis

Abstract: Publisher Summary Near-edge X-ray absorption (XAS) spectroscopy and techniques related to it by an extension of the photoelectron energy range or a modification in the experimental detection mode have all matured in the past years. The contributions of near-edge X-ray absorption spectroscopy to catalysis and surface science have grown considerably, essentially since the development of synchrotron radiation sources. On a low interpretative level the application of threshold fine-structure effects to catalyst diagnostics has rapidly taken off as atoms in different structural or bonding conditions result in fingerprints that can readily be used for analysis of their structural and chemical state. The near-edge spectroscopy is at its best as an adjunct to other physicochemical characterization methods, such as other XAS techniques, and in combination with XRD, STEM, Auger electron spectroscopy (AES), thermal desorption, and catalytic activity measurements, can expect to bring new insights into the nature of basic catalytic reaction processes. In situ X-ray absorption near edge experiments have the considerable advantage over others that they can be applied to real catalytic problems, sensing changes in the bonding electron distribution and coordination of the environment of the absorber and resulting in more closely matching models of structure and reaction processes.