Catalysis supported metal catalysts

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. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Davda, R. R. Shabaker, J. W. Huber, G. W. Cortright, R. D. Dumesic, J. A., A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueous-phase reforming of oxygenated hydrocarbons over supported metal catalysts. Applied Catalysis B 2005,56,171. 

Grenoble, D.C., Estadt, M.M., and Ollis, D.F. 1981. The chemistry and catalysis of the water gas shift reaction. 1. The kinetics over supported metal catalysts. J. Catal. 67 90-102. 

D.C. Meier, X. Lai, and D.W. Goodman, Surface chemistry of model oxide-supported metal catalysts An overview of gold on Titania, in Surface Chemistry and Catalysis, eds. A.F. Carley et al. Kluwer, New York, 2002, pp. 147-189. 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Inelastic electron tunneling spectroscopy has been shown to be a useful method for the study of chemisorption and catalysis on model oxide and supported metal catalyst systems. There are in addition a number of proven and potential applications in the fields of lubrication, adhesion (48), electron beam damage (49,50), and electrochemistry for the experimentalist who appreciates the advantages and limitations of the technique. 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

Without a doubt, a complete picture of the dynamics of dissociative chemisorption and the relevant parameters which govern these mechanisms would be incredibly useful in studying and improving industrially relevant catalysis and surface reaction processes. For example, the dissociation of methane on a supported metal catalyst surface is the rate limiting step in the steam reforming of natural gas, an initial step in the production of many different industrial chemicals [1]. Precursor-mediated dissociation has been shown to play a dominant role in epitaxial silicon growth from disilane, a process employed to produce transistors and various microelectronic devices [2]. An examination of the Boltzmann distribution of kinetic energies for a gas at typical industrial catalytic reactor conditions (T 1000 K)... 

The recent accomplishments of near-edge X-ray absorption spectroscopy in catalysis studies are already quite impressive, in particular if one considers the limited availability of suitable X-ray spectrometers. Developments of catalytic interest have concerned the Shell Higher Olefin process, size effects, metal-support interaction, mono- and bimetallic catalysts (in particular the PtRe/Al203 system), the reactivity of supported metal catalysts, dynamical and in situ catalyst studies, and a variety of oxide and sulfide catalysts. Other catalytic problems are now coming within easy experimental reach, such as the study of sulfur poisoning and the nature of coking. 

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