Supported structure sensitive reaction

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... 

By using thermosensitive poly-acrylamides, it is possible to prepare cubic Pt nanocrystals (with predominant (1 0 0) facets) and tetrahedral Pt nanocrystals (rich in (111) facets). These Pt nanocrystals can be supported on oxide (alumina) and used as a catalyst in structure-sensitive reaction, NO reduction by CH4. The results proved that morphologically controlled metal nanoparticles supported on adequate support give us a novel tool to connect the worlds of surface science with that of real catalysis. 

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. 

As shown below, for structure-insensitive reactions the surface characteristics of the single crystal catalysts simulate the activity of supported catalysts in the same reactant environment. This proves to be most fortunate since the advantages of single crystals are retained along with the relevance of the measurements. Moreover, the use of single crystals allows the assessment of the crystallographic dependence of structure-sensitive reactions. 

Some support for the proposal that special surface sites are important for these mildly structure sensitive reactions comes from the work of Somorjai318... 

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. 

The particle-size effect is for both supports the largest for the selectivity towards the roll-over mechanism (via the di-G-T)1 intermediate, Figure ID), which is strongly increased with the larger particles. Hence, also the roll-over mechanism is a clearly structure-sensitive reaction. It is facilitated by large particles, and probably an ensemble of catalytically active, empty sites is needed for the formation of the di-G-r)1 intermediate. 

The rate of a structure sensitive reaction catalyzed by a metal single crystal is also a function of the exposed plane. As illustrated in Figure 5.1.3, the low-index planes of common crystal structures have different arratigctncnls of surface atoms. Thus, caution must be exercised when the rates of structure sensitive reactions measured on single crystals are compared to those reported for supported metal particles. 

Acidic forms of zeolites are well suited as supports for metal functions which are employed for hydrogenation, since they can also withstand the presence of traces of sulfur compounds frequently found in feedstocks of petrochemical industry. It should be noted, however, that hydrogenation is a structure insensitive reaction so it will primarily depend upon the concentration of the accessible metal particles and the adsorption constant of the unsaturated hydrocarbon. This may offer an explanation as to why Pt catalysts, for example, are still active for hydrogenation, when their activity for dehydrocyclization or hydrogenolysis (i.e., for structure sensitive reactions) is completely lost (e.g., by poisoning). 

While these results show a relationship between single crystals and supported metals when used for structure sensitive reactions, similar relations for those reactions promoted by a single atom site may not be as straightforward since there are more different types of single atom sites possible on a metal surface than there are groups of face atoms. 

Clearly, pmsi is expected to vary from one support to another even for facile reactions, while for structure-sensitive reactions pmsi can be reasonably expected to be a function not only of the support but also of the particle size of the active phase. 

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). 

Figure 2 demonstrates that the reaction rates did not decrease with time to any appreciable extent. For the samples of low Pd content, hydrogenation was completed in a remarkably short time. The Pd content of the samples of higher loadings formed aggregates which, as in the case of 1-octene, tend to block the interlamellar space and restrict conversion to the surface active sites. This particularly holds for 10.2% Pd-HDAM, which was the least effective sample in the reaction. Similarly to conventional Pd supported catalysts, styrene hydrogenation conducted on Pd-HDAM samples was found to be a moderately structure sensitive reaction. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

This latter was accepted as evidence for possible aggregation, since some literature data indicate that hydrogenolysis, being a structure-sensitive reaction, involves multiple adsorption (/58-161). However, two recent review articles on particle size effects on metal catalysts (162, 163) emphasize the inconsistency of the rather scarce data on nickel and warn of the difficulties connected with this problem. These data, although scarce, support the conclusion that the structure in the precrystallization state is the most favorable for catalytic activity, but detailed understanding of the phenomenon requires further clarification. 

The reason for this is the relative instability of Pt oxide and its tendency to decompose at temperatures <550 C even under oxidizing conditions. Platinum oxide does not penetrate into the subsurface of any support material. To achieve a high dispersion additives are used in which the oxygen ions are more reactive than in 7-AI2O3 as for instance CeC. Thus the addition of 2.6% ceria to 7-AI2O3 increases the surface concentration of PtO from 2.2 /imole Pt/m to 4.2 /imole Pt/m (BET) [3]. As will be shown below other more reactive metal oxide additives have a similar effect on Pt dispersion. This, in turn affects the behavior of the catalysts with respect to several structure sensitive reactions. 

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