Structure-sensitive reactions over

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

In contrast, ethane hydrogenolysis, which is a structure sensitive reaction over bulk Ni, displayed marked structural effects on the Ni/W system (41). We have observed, as shown in Figure 5, that the specific rate, or rate per surface metal atom, but not the activation energy, is a strong function of metal coverage on the Ni/W(110) surface, suggesting that the critical... 

The carbides and nitrides were also found to be active in ammonia decomposition. The activities of vanadium carbides were 1-2 orders of magnitude lesser than that of Mo nitride catalyst, but 1-2 orders of magnitude higher than that of a Pt/C catalyst (138). Vanadium carbides were 1 order of magnitude more active than vanadium nitrides. Both the synthesis and decomposition of NH3 were found to be structure-sensitive reactions over carbide and nitride catalysts with activities depending strongly on the particle sizes of the catalysts. 

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]. 

An example for a non-structure-sensitive reaction is provided by Davis et al. [102], who investigated the liquid-phase hydrogenation of glucose over carbon and silica based ruthenium catalysts with particle sizes between 1.1 and 2.4 run. Depending on catalyst loading which was between 0.56 wt.% and 5 wt.%, dispersion decreased from 91% to 43%. At the same time, TOFs varied only insignificantly in a range between 0.21 1/s and 0.32 1/s. 

Several studies suggested that CH4 partial oxidation over Rh and other metals (that is, the combined activation of CH4 by O2. H2O and CO2) involves a network of structure-sensitive reaction steps (including C-H and C-0 bond breaking) [151—... 

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... 

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 application of the fusion process can lead to a control over structure-sensitive reactions for unsupported catalysts. The prototype example for such a catalyst is the multiply-promoted iron oxide precursor used for ammonia synthesis. In Section B.2.1.1 a detailed description is given of the necessity for oxide fusion and the consequences of the metastable oxide mixture for the catalytic action of the final metal catalyst. 

The catalytic reduction of Cu2+ by hydrogen is faster on small platinum particles since the turn over number increases from 0.37 Cu2+ ions reduced per accessible platinum atom per hour to 3.0 when the platinum dispersion increases from 11% to 54% [8], Thus the catalytic hydrogenation of Cu2+ ions is a structure-sensitive reaction occurring preferentially on small platinum particles. 

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. 

In this same series of reactions it was also found that C-C bond cleavage took place most readily over the 111 face catalyst (A in Fig. 3.2) and to a considerably less extent on the others. This reaction, which is stmcture sensitive, obviously teikes place over ensembles of face atoms. It was also found > 8 that the dehydrocyclization of hexane to benzene, another structure sensitive reaction, took place four times faster on the 111 face of a platinum crystal than on one cleaved to expose the 100 face. The hydrogenolysis of ethane, on the other hand, was found to be significantly faster on Ni (100) than on Ni (111).22 Thus, it is not sufficient merely to define a site as an ensemble of face atoms the orientation of the atoms must also be specified. 

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). 

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... 

The disproportionation of CO has been measured as a function of particle size for a number of supported metals Ni/mica (332, 333), Pd/mica (332), Pd/Si02 (102), and Fe/C (334). For all these systems the rate of CO desorption decreases with increasing d, and also the rate of carbon deposition decreases with increasing d. This sympathetic structure sensitivity extends over the range of d from 1 to about 7 nm. We recall from Fig. 18 that over Ni and Ru on several supports the methanation reaction shows extended antipathetic structure sensitivity. However, Pd is not such a... 

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