Supported metals, small particles structure sensitivity

Thus, recent experimental evidence seems to support the idea that growing small particles have maximum chemical reactivities, and certain sized/shaped small particles may have the highest reactivities. What size and/or shape varies with the metal in question and the reaction in question This information strongly supports three ideas (1) structure sensitivity in chemical reactions on metal surfaces is very important, (2) more than one atom is necessary to carry out at least some bond breaking processes, and (3) defect sites on growing small particles are extremely reactive (see Fig. 9). It has also been possible by pulsed laser vaporization to produce many types of gas phase metal clusters. Particularly interesting have been reactivity studies of niobium clusters Nb where X = 5-20. A definite cluster size dependence on reactivity was observed. Exposure... 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

To answer these questions requires some understanding of the properties of small metal particles, both structural and electronic. In this review we shall examine first the evidence relating to metal particles prepared by direct methods, e.g., vapour deposition or condensation in the gas phase. Then we shall consider whether this information can be applied to the case of supported metals where both precursor decomposition and support effects may add to the complexity of the total system. We shall then consider whether further changes in catalytic properties occur after preparation, i.e., during the catalytic reaction. Finally, we shall summarize some of the more recent evidence concerning the nature of structure sensitivity. 

Gas-induced morphological changes have been reported, and there is growing evidence that this may be a common occurrence with supported metal catalysts. There is further evidence that a small metal particle may consists of a solid core having a fluid-like surface layer of metal atoms. This raises the possibility that in addition to having catalytic reactions which are structure sensitive it may be necessary to allow that structures are sensitive to catalytic reactions, i.e., reaction-sensitive structures. It is possible that during the initial adsorption of the reactants a small particle will change its surface structure into one which best suits those particular reactants. 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

One of the key issues of supported model catalysts is to prepare collection of metal particles having a well-defined morphology. Indeed, if a catalytic reaction is structure-sensitive [54], it will depend on the nature of the facets present on the particles. Moreover, the presence of edges, the proportion of which is increasing rapidly below about 5 nm, can affect the reactivity by their intrinsic low coordination and also by their role as boundary between the different facets. In this section I first discuss the theoretical predictions of the shape of small particles and clusters, then I briefly describe the available experimental techniques to study the morphology, and finally I discuss from selected examples how it is possible to understand and control the morphology of supported model catalysts. 

Structure-sensitive reactions are extensively discussed in the catalytic literature, but careful examination of the published work reveals that on the atomic scale the catalytic materials used in these studies are in general poorly characterized with respect to particle size and structure. Extended X-ray absorption fine structure (EXAFS) has been successfully applied to the study of small particles on supportsand small metal molecules in matrices subject to the caveat that samples of these materials consist of a distribution of particle sizes. Information thus obtained is an average over the entire distribution. Supported, monosized clusters have not yet been used in catalytic studies. However, Woste and coworkers demonstrated in the first experiment where monosized clusters were deposited that Ag4 is the critical cluster... 

The formation of PO over Au-based catalysts is a structure-sensitive reaction. Only hemispherical Au particles with a suitable size (2-5 nm) will selectively produce PO [167,168,403] and 2.2 to 2.4 nm particle size seemed to be optimum in the early experiments [31]. The most effective type of Au nanoparticles is prepared by the DP technique, which brings them in strong contact with the support. Gold particles smaller than 2 nm show a shift in selectivity from PO to propane [7,169-171,403]. This switch of epoxidation to hydrogenation for particles under 2 nm size indicates that small Au clusters exhibit different behaviour in surface properties from that of metallic Au [171] (see Fig. 6.17). 

Studies on both supported small metal particles and naked metal clusters indicate that the TOF tends to zero as the number of atoms per particle tends to unity. This amounts to saying that, in the limit of the smallest particles, all systems show antipathetic structure sensitivity. We have used particles" as a general term. They may be crystallites, but as the number of atoms per particle decreases their organization departs in general from that of a macroscopic crystal. These small particles (<1000 atoms d = 2.5 nm) are called clusters in the literature on naked metal particles (347). However, organometallic chemists may reserve the term cluster to describe a structure of metal atoms at least partly terminated by various ligands. 

High surface area since a noble metal is used in a reaction at low temperature, a very high dispersion of the active metal is required, with a support that allows the stabilization of small metal particles. Nevertheless, there are also some structure-sensitive half-reactions (e.g. ORR) involved, that implies poorer kinetics at lower metal particles size, due to geometric effects. Additionally, the morphology of the metal particle also plays an important role in determining catalytic activity, as well as the formation of alloys between metals, and this is not always obtained by increasing the surface area. Practical applications will use a support of 150-250 m g" ... 

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