Supported structure insensitive reaction

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. 

In addition, the same studies that were carried out on the Pt(lll) crystal face result in reaction rates identical to those found on stepped crystal surfaces of platinum. These observations support the contention that well-defined crystal surfaces can be excellent models for polycrystalline supported metal catalysts. It also tends to verify Boudart s hypothesis that cyclopropane hydrogenolysis is an example of a structure-insensitive reaction. The initial specific reaction rates, which were reproducible.within 10%, are within a factor of two identical to published values for this reaction on highly dispersed platinum catalysts. The activation energies that were observed for this reaction, in addition to the turnover number, are similar enough on the various platinum surfaces so that we may call the agreement excellent. 

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). 

Catalytic properties of supported clusters identified as primarily Ira or Ir6 were reported by Xu et al [15], who investigated a structure-insensitive reaction, toluene hydrogenation. The support was NaY zeolite or, for comparison, MgO. EXAFS spectroscopy (Table 3) showed that the first-shell Ir-Ir coordination numbers characterizing both the fresh and used MgO-supported catalysts made by decarbonylation of supported [Ir4(CO)i2] or [HIr4(CO)ii] are indistinguishable fi om 3, the value for a tetrahedron, as in [Ir4(CO)i2] and [HIr4(CO)n]. The decarbonylated clusters retained or nearly retained this metal frame. EXAFS data show that the decarbonylated Ire clusters had metal fi ames indistinguishable from the octahedra of the precursor hexairidium carbonyls, indicated by the coordination number of approximately 4. 

Zeolite-supported metal clusters are a new class of catalyst made possible by syntheses involving organometallic chemistry and by precisely controlled treatment of metal complexes in zeolite cages. Elucidation of the preparation chemistry would not have been possible without the guidance of EXAFS spectroscopy. Clusters such as Ir4, Ir, and Pt (where n is about 6) are small enough to be considered quasi molecular rather than metallic. Their catalytic properties are distinct from those of metallic particles, even for structure-insensitive reactions. The zeolite pores seem to confer some properties on the clusters that are not yet well understood. 

A comparative review on the evaluation of the catalysts Al, A2, B1 and B2 reveals that activity and dispersity are two closely related parameters which is influenced by the mode of preparation especially with respect to removal of chloride or to the final calcination temperature. Earlier works had identified benzene hydrogenation on supported platinum catalysts as a facile or structure insensitive reaction(5). A facile reaction may be defined as one for which the specific activity is practically independent of its mode of preparation(6). In other words all surface atoms are believed to be the active sites in a facile reaction without any dependence on the coordination number of site or on the collective properties of the crystallite. 

The yield of citronellol is given in Figure 5 over catalysts with different metal loading. The hydrogenation rate of citrai and the selectivity to citronellol increased with increasing support surface area and metal dispersion (Table 4), characteristic for a structure insensitive reaction. The highest initial hydrogenation rate and 92% selectivity to citronellol (at maximum yield) was achieved over the... 

Structure sensitive reactions for which the specific rate is highly dependent on the metal dispersion [6]. Boudart suggested that specific sites with special geometrical requirements are involved in structure sensitive reactions while all metal atoms of the surface are available for structure insensitive reactions. Such results obtained on metal supported catalysts with a polycrystalline structure for the metal crystallites corresponded to the breakthrough of Surface Science and the use of single crystals where site identification began to be possible. 

TOF values for the hydrogenation of cyclohexene at 25 °C and 1 bar (supported catalysts, structure insensitive reaction Table 1-2) ... 

The difference between the two definitions is that the rates vary with time or position, while the TOF only depends on the availability of the sites present. The activity of the catalysts and the selectivity of the reaction depend on the characteristics of the material. Some important features of these reactions should be remembered. The first one is related to the sensitivity of the chemical reaction to the catalyst structure. According to Boudart s theory, the supported catalysts consist of metal particles of different and variable sizes. With the increase of particle diameter, the concentrations of metal atoms and exposed sites vary significantly, indicating that no changes in surface structure have happened. The structure sensitive reactions (SSR) are those in which the intrinsic reaction rate relative to the number of surface active sites, i.e., the frequency of the reaction varies with the particle sizes, which does not happen in structure insensitive reactions (SIR). This means that in the structure sensitive reactions, the frequency of... 

First, is the reaction structure sensitive or insensitive According to Boudart," supported catalysts have dispersed metallic particles of different sizes d. With increasing or decreasing particle sizes, the concentration of accessible atoms or sites varies significantly, indicating surface structural changes. Structure sensitive reactions (SSRs) are those reactions where the intrinsic reaction rate relative to the accessibility of surface sites or the turnover frequency (TOF) varies with particle sizes or diameters, but it does not vary for structure insensitive reactions (SIRs). It means, for SSRs, the TOF depends on particle sizes, or dispersion of particles or on the accessible surface sites of the catalyst. On the other hand, for SIRs, the activity or TOF is independent of the particle sizes. 

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