Metals supported, catalysis

Volume 11 Metal-Support and Metal-Additive Effects in Catalysis. Proceedingsof an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik,C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau,... 

G.L. Haller, and D.E. Resasco, Metal-Support Interaction Group VIII Metals and Reducible Oxides, Advances in Catalysis 36, 173-235 (1989). 

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

INTERRELATION OF PROMOTION, ELECTROCHEMICAL PROMOTION AND METAL-SUPPORT INTERACTIONS THE DOUBLE-LAYER MODEL OF CATALYSIS... 

Thus not only promotion and electrochemical promotion is catalysis in the presence of a double layer but apparently the same applies for supported catalysts undergoing metal-support interactions (Chapter 11). 

Thus aside from the, very likely, forthcoming technological applications (Chapter 12) electrochemical promotion is a unique and efficient tool for studying the heart of classical catalysis, namely promotion and metal support interactions. 

Since then Electrochemical Promotion of Catalysis has been proven to be a general phenomenon at the interface of Catalysis and Electrochemistry. More than seventeen groups around the world have made important contributions in this area and this number is reasonably expected to grow further as the phenomenon of electrochemical promotion has very recently been found, as analyzed in this book, to be intimately related not only to chemical (classical) promotion and spillover, but also to the heart of industrial catalysis, i.e. metal-support interactions of classical supported catalysts. 

Quaternary ammonium bromides and hydroxides (quats) are applied as templates in the synthesis of zeolites with relatively high Si/Al ratio. Examples will be given of the use of mon-, di-, poly- and associated quats as templates in zeolite growth. Templated zeolites of the MFI-type can be grown in a lateral or in an axial way onto metal supports, providing promising composite systems, for separation and catalysis, respectively. 

Volume 11 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an... 

Sabatier s Principle is illustrated in Fig. 6.40 where the ammonia rate is plotted for similar conditions versus the type of transition metals supported on graphite. The theory outlined so far readily explains the observed trends metals to the left of the periodic table are perfectly capable of dissociating N2 but the resulting N atoms will be bound very strongly and are therefore less reactive. The metals to the right are unable to dissociate the N2 molecule. This leads to an optimum for metals such as Fe, Ru, and Os. This type of plot is common in catalysis and is usually referred to as a volcano plot. 

Imellk, B. et al., eds. "Metal-Support and Metal Additive Effects In Catalysis" Elsevier Amsterdam, 1982, and references cited therein. 

The interactions between metals and supports in conventional supported metal catalysts have been the focus of extensive research [12,30]. The subject is complex, and much attention has been focused on so-called strong metal-support interactions, which may involve reactions of the support with the metal particles, for example, leading to the formation of fragments of an oxide (e.g., Ti02) that creep onto the metal and partially cover it [31]. Such species on a metal may inhibit catalysis by covering sites, but they may also improve catalytic performance, perhaps playing a promoter-like role. 

Stevenson S, Dumesic JA, Baker RTK, Ruckenstein E (1987) (eds) Metal-support interactions in catalysis, sintering and redispersion. Van Nostrand Reinhold, New York Ferrari AM, Neyman KM, Mayer M, Staufer M, Gates BC, ROsch N (1999) J Pbys Chem B 103 5311... 

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

Metal oxides of variable oxidation state as supports or support modifiers [202] are well known in gold catalysis. In the previous section we have already indicated some metal-support interactions influencing the electronic state of gold nanoparticles as well as the metallic or ionic state of gold. Of the numerous literatures we have to mention Haruta and Date [169], Bond [195], as well as Goodman works [186,203]. Further results can be found on the iron oxide system in recent literatures [162,204]. 

Kalakkad, Datye, and coworkers—TEM indicates strong metal-support interactions in Pt/ceria and Pt/Ce/Al catalysts. Kalakkad et al 362,369 published TEM and probe reaction studies on 0.6%Pt/CeO2 catalysts relevant to ACC catalysis. The... 

Among the various types of composite systems, that of the metal-support ranks as one of the most important, because of its crucial role in catalysis. The situation under consideration is that of chemisorption on a thin metal him (the catalyst), which sits on the surface of a semiconductor (the support). The fundamental question concerns the thickness of the film needed to accurately mimic the chemisorption properties of the bulk metal, because metallization of inexpensive semiconductor materials provides a means of fabricating catalysts economically, even from such precious metals as Pt, Au and Ag. 

M. Boudart. Catalysis by supported metals. Advanced Catalysis 20 (1969) 153-166. 

Non-noble metals such as Ni, Co, Mo, W, Fe, Ag and Cu have been added to zeolites for use in catalysis. In addition to CO, nitric oxide (NO) has been shown to be a good adsorbate for probing the electronic environment of these metals. When NO chemisorbs on these metals, it can form mononitrosyl (M-NO) and dinitrosyl species (ON-M-NO). The monontrosyl species has a single absorption band and the dinitrosyl species has two bands due to asymmetric and symmetric vibrational modes of the (ON-M-NO) moiety. Again, there have been many studies reported in the literature on the use of NO and/or CO adsorption on non-noble metals supported on zeolites and they are too numerous to list here. Several examples have been selected and summarized to provide the reader with the type of information that can be provided by this method. 

Stevenson, S.A., Dumesic, J.A., Baker, R.T.K. and Ruckenstein, E. (1987) Metal-Support Interactions in Catalysis, Sintering and Redispersion, Van Nostrand Reinhold, New York. 

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc catalysts. 

I 8 Carbonyl Compounds as Metallic Precursors of Tailored Supported Catalysis... 

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