Supported-metal catalysts chemisorption properties

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

The chemisorption of some selected probe molecules, typically H2 and CO, is a routine procedure for characterizing supported metal catalysts. In addition to providing basic information about the chemical properties of the dispersed metal phase, these studies are commonly applied to the estimate of metal dispersion data. 

Table 1 summarizes the information required for a detailed characterization of a supported metal catalyst for supported bimetallics there are additional questions, e.g., the distribution of atoms in bimetallic clusters and the surface composition of larger alloy crystallites. For the support and the prepared catalyst, the total surface area, pore size distribution, and surface acidity are routinely measured, if required, while other characteristics, e.g., thermal and chemical stability, will have been assessed when selecting the support. The surface structure of alumina, silica, charcoal, and other adsorbents used as catalyst supports has been reviewed. Undoubtedly, the most commonly measured property is the metal dispersion, often expressed in terms of the specific metal area and determined by selective chemisorption or titration but, as discussed (Section 2), there is the recurring problem of deciding the correct adsorption stoicheiometry. 

The characterization of supported metal catalysts is a matter of some complexity and supported bimetallic catalysts even more so. Nevertheless the development and application of methods for determining catalyst structure is essential for an understanding of why the performance of a selected combination of metal(s) and support varies as a function of preparative variable, activation procedure, reaction conditions, or time. Although some aspects of catalyst structure can be routinely determined, the basic measurement of absolute metal dispersion by selective chemisorption/gas titration is still the subject of many publications and the necessity of cross-checking by instrumental methods is generally appreciated. The characterization of supported metal catalysts also involves some less accessible properties, e.g., the sites available on crystallites as a function of size, high-temperature... 

Three main properties have come to characterize the SMSI effect (1). The first of these is a diminished activity toward the chemisorption of H2 and CO induced by the high temperature reduction of a supported metal catalyst (low temperature reductions are ineffective). The second is a significant alteration of the catalytic... 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

Adsorption is an exothermic process and the magnitude of the heat of adsorption is used to distinguish chemisorption from physisorption. Heats of adsorption greater than 10 kcal/mol are definitely associated with chemisorbed species. Small heats of adsorption (2-5 kcal/mol) do not always indicate physisorption, however. Therefore, it is best to look at more than one property when trying to distinguish between chemisorption and physisorption. Physical adsorption is used to measure the area of high-area oxide catalysts and oxide-supported metal catalysts. Physisorption isotherms and their use are discussed in Chapter 7. The discussion that follows treats only chemisorption. 

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. 

There is a one-point modification of a chemisorption method, which is widely used for measurements of Ac. In this case, only one adsorption point of a chemisorption isotherm is measured, and is compared with only one point on a chemisorption isotherm on a reference material (usually, powder [black] or foil). The identity of the chemisorption properties of the active components in supported and pure form is postulated, but very often does not fulfill, making one-point modification an inaccurate procedure, which can hardly be used in scientific studies. For example, studies of supported Rh catalysts by 02 and CO chemosorption have shown that three different blacks of Rh yield three different results [88], The multipoint comparison of chemisorption isotherms shown that only one black had a chemisorption isotherm that had affinity to the isotherm on a supported metal. 

Relatively few studies have focused on influence of the acid/base properties of the support on the chemisorption of reactants on supported metal clusters. A NMR study by Tong et al.23 showed that the stretching frequency of CO chemisorbed on zeolite supported Pt particles correlates with the surface local density of states (LDOS) of the Pt. The LDOS also showed a correlation with the faujasite framework acidity, but an explanation of this correlation is lacking. Several infrared studies on similar supported Pt catalysts show that the mode of CO... 

Section 4.3.2 will be devoted to the chemical characterisation studies. Because of the relationship existing between the support reduction degree and the occurrence of the deactivation phenomena mentioned above, we shall review first some of the major problems to be faced in relation to the redox characterisation of ceria and related oxide supports, sub-section 4.3.2.1. Then, we shall discuss the chemisorptive properties of these catalysts. In particular, section 4.3.2.2, will be devoted to the adsorption of H2 and CO, by far the two most commonly used probe molecules. Special attention will be paid to the relationship existing between chemisorptive behaviour and reduction temperature. We shall also report on some recent hydrogen chemisorption studies, in accordance with which, the sensitivity to the deactivation phenomena may vary from one noble metal to the other (97,117,235), being also influenced by the presence of chlorine in the support (163). 

With a few exceptions, most research applying the surface science approach to study ceria based systems has been performed in about the last five years. As evidence for this statement consider that in a 1995 book which presents a comprehesive review of surface science of oxide surfaces only a single reference to cerium oxide surfaces is cited or that in a 1997 review of structural, electronic and chemisorptive properties of metal films and particles on oxide surfaces there are six references to ceria surfaces or that in a 1998 review of surface studies of supported model catalysts there are only six references to ceria as a support. The... 

In identifying and difTerentiating the roles of the active metal and the ceria support in a model catalyst, it is helpful to study the reactivity and chemisorptive properties of the ceria surface by itself Generally, ceria and other oxide surfaces are less active than metals to most adsorbates, but substantial uptake occurs in some cases. Most Studies performed to date, and summarized below, reflect interest in molecular adsorbates related to emission control and redox type reactions, i.e. small molecule... 

Broadly speaking, promoters can be divided into structural promoters and electronic promoters. In the former case, they enhance and stabilize the dispersion of the nanoparticle-dispersed active phase on the catalyst support. In the latter case, they enhance the catalytic properties of the active phase itself. This stems from their ability to modify the chemisorptive properties of the catalyst surface and to significantly affect the chemisorptive bond strength of reactants and intermediates. At the molecular level this is the result of direct ( through the vacuum ) and indirect ( through the metal ) interactions. The term through the vacuum denotes direct electrostatic, Stark type, attractive or repulsive interactions between the adsorbed... 

The second phenomenon, i.e., the change in catalytic activity or selectivity of the active phase with varying catalyst support, is usually termed metal-support interaction. It manifests itself even when the active phase has the same dispersion or average crystallite size on different supports. Metal-support interactions can influence in a very pronounced way the catalytic and chemisorptive properties of metal and metal oxide catalysts. Typical and spectacular examples are... 

There is no single interpretation to explain the effects of particle size, alloying, and metal-support interaction on the chemisorption and catalytic properties of supported metal particles. Depending on the particle size, the nature of co-metal and support, and the nature of the reaction, the change of chemisorption and catalytic properties can be interpreted in terms of geometric features, electronic modifications, and/or mixed sites. This is due to the formation of various adsorbed species and intermediates. Moreover, in many cases, the promotion of catalytic properties will be directly related to the method of catalyst preparation, which affects the architecture of the active site, with respect to chemical and electronic states of components and topology. 

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