Structure-insensitive reactions over

Kinetics of Structure Insensitive Reactions Over Clean Single Crystal Surfaces... 

The data shown in Fig. 6 for cyclohexane dehydrogenation imply that this reaction should be structure-sensitive. Some results from the older literature [e.g., Cusumano et al. (128)] seem to indicate that this is a structure-insensitive reaction over Pt/Al203 and Pt/silica-alumina. A... 

Methanol carbonylation over nickel- activated carbon was shown to be a structure-insensitive reaction. 

The data in Table IV and Figure 8 show that the dispersion of nickel decreases with increasing nickel loading from 90% (1 wt%) to 22% (10wt%). However, the turn over frequency of the carbonylation is independent of the Ni loading, indicating that this reaction is a structure insensitive reaction. 

These results indicate that, perhaps, a better definition of structure sensitive reactions would be those that occur over ensembles of surface atoms while structure insensitive reactions are those that are promoted by single atom active sites. 

Results that illustrate three examples of structure sensitivity are presented in Fig. 7. Curve 1 represents the TOF for the structure-insensitive reaction of benzene hydrogenation on Pt/SiC>2 (136). The TOF is about 0.22 s l and is not a function of particle size over the range studied. Also plotted for this system is the atomic rate AR, calculated from TOF by Eq. (2). As already mentioned, the atomic rate is an approximate measure of... 

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... 

For some reactions on metal surfaces, the activity of the catalyst depends only on the total number of surface metal atoms, Ms, available, and these are termed structure-insensitive reactions consequently, the TOF is essentially independent of metal dispersion or crystal plane and varies over a very small range (within a factor of 5, for example). For other reactions, the TOF is much greater on certain surface sites, thus the activity can be dependent on metal dispersion, crystal plane, or defect structures. These are termed structure-sensitive reactions. Preceding these terms, such reactions have been referred to as facile and demanding reactions, respectively [3]. 

This review covers the personal view of the authors deduced from the literature starting in the middle of the Nineties with special emphasis on the very last years former examples of structure-sensitive reactions up to this date comprise, for example, the Pd-catalyzed hydrogenation of butyne, butadiene, isoprene [11], aromatic nitro compounds [12], and of acetylene to ethylene [13], In contrast, benzene hydrogenation over Pt catalysts is considered to be structure insensitive [14] the same holds true for acetonitrile hydrogenation over Fe/MgO [15], CO hydrogenation over Pd [16], and benzene hydrogenation over Ni [17]. For earlier reviews on this field we refer to Coq [18], Che and Bennett [9], Bond [7], as well as Ponec and Bond [20]. 

In 2006, Iida et a/.497,498 examined low temperature water-gas shift over Pt/Ti02 rutile phase catalysts. The authors tried a number of different Pt precursors, and concluded that the reaction over Pt/Ti02 was structurally insensitive relative to Pt, as a linear relationship was observed between dispersion and activity, as shown in Table 112. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Karpinski et al. (189) found that the reaction of neopentane with H2 over a series of Pd/Si02 catalysts is structure insensitive and exhibits very mild changes in activity and selectivity. The same reaction performed by... 

Oyama and Somorjai have studied the oxidation of ethanol and ethane over vanadia supported on silica.33 Ethane oxidation yielded ethylene and acetylene with considerable CO2 for highly dispersed catalysts. Ethanol oxidation to acetaldehyde, on the other hand was shown to be structure insensitive. Conversions and selectivities for reaction of ethanol are shown in Table III.33... 

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