Surface-insensitive reactions

Vannice and co-workers (176) found that methanation over Pd catalysts is essentially a surface-insensitive reaction however, the activity depends upon the support used. Alumina-supported and silica/alumina-supported catalysts were 10 times more active than the silica-supported ones, resembling very much the corresponding relation reported for benzene... 

However, it was observed that the surface of these nanoparticles was contaminated by residual Ag ions. When these different shapes of nanoparlicles catalyzed ethylene hydrogenation, which is a surface-insensitive reaction, no difference in the activity was expected for the various shaped nanoparticles. However, the cubes had a turnover frequency of 8.6 s", whereas the turnover frequency of the octahedra was 0.02 s" [22]. The Ag residing on the Pt nanoparticles hindered the catalytic reaction. The catalytic reaction on Pt octahedra with the greatest residual Ag ion concentration showed the poorest activity. Although beautifully shaped nanocrystals were obtained, the effect of the surface crystalline structure on catalytic activity could not be observed because of surface contamination. 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

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

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

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 split peaks can be used for estimation of electron-transfer coefficient as well as for precise determination of the formal potential of the surface electrode reaction. The potential separation between split peaks is insensitive to the electron-transfer coefficient. However, the relative ratio of the heights of the split peaks depends on the electron-transfer coefficient according to the following function ... 

In contrast, structure-insensitive reactions are those for which turnover frequency under fixed conditions does not depend or depends slightly on the surface crystalline anisotropy of clusters of varying size or of single crystals exposing different faces. For these kinds of reactions, all accessible surface atoms can be considered as equally active sites (Boudart, 1981 and 1995). 

In order to avoid any confusion, the surface structure used in sensitive and insensitive reaction analysis has nothing to do with the surface arrangement used in the catalyst level rates analysis—the first refers to the microscopic level of the active site, whereas the latter to the catalyst level. 

The activity of a catalyst for a particular reaction may be strongly dependent on the surface structure. Reactions of this type are called structure sensitive or demanding, whereas with structure insensitive or facile reactions this effect is of minor importance (206). With real catalysts this distinction is usually obtained by varying the mean particle size (and thereby... 

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. 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Many years ago Taylor1 noted that the amount of a surface which is catalytically active is determined by the reaction catalyzed . More recently, Boudart2 proposed dividing reactions catalyzed by metals into two groups -structure sensitive and structure insensitive reactions. Sensitive reactions were those which for a particular metal showed a marked variation in activity with method of preparation. (Earlier Boudart et al had used the terms facile and demanding .)... 

Quite different site densities are obtained if these assumptions are changed. Perez et al.13 have calculated the surface site statistics using a computer model which can simulate incomplete layers by removing atoms from complete shells. The atoms removed are those which have the smallest number of first and second nearest neighbours. Many more types of site are considered in the models used by Perez et al. However, one of the most interesting results of their calculations is to demonstrate that for all sites, apart from B2 sites, there are very pronounced oscillations in number as the particle size is increased. Figure 2 shows the variation in the number of B2, B3, and B4 sites, and Figure 3 shows the ratio of B3/B4 sites as a function of particle size. Any reaction which is controlled by this ratio will show activity maxima for particle diameters of 0.8 and 2.0 nm. On the other hand B2 and B2 sites are the ones most likely to catalyse structure insensitive reactions. 

Two structure insensitive reactions have been selected cyclohexene hydrogenation [6] oil surface Pt sites upon silica and but-l-ene isomerisation on acidic sites in bentonite. Both reactions were studied in differential reactors The former was investigated at 273-313K and lOlkPa Samples (5-lGmg) of catalyst were flushed with N2, pre-reduced in H at 423K for lh, flushed with N2 and then the reactant stream (lOlkPa total pressure cyclohexene N2 H2 = 1,7 89.5 10 1 200cm3.min 1 total flow rale) was introduced and analysed... 

The higher dehydrogenation activity keeps the 3-methylpiperidine concentration on the metal surface at lower levels thus suppressing the condensation reaction leading to the dimer. The increased (de)hydrogenation activity at higher dispersion can be explained in different ways. The rate of a structure insensitive reaction can be linearly correlated to the number of active sites thus to the dispersion. On the other hand metal particles with different shapes and dimensions could interact with molecules in the gas phase in a different way or could display different resistance against deposition of coke precursors. 

As shown in Table 4.1, formation of the mixed adduct is favored over homodimerization of 8a with the simple styrene 13a, but this selectivity is inverted for the case of the more bulky dienophile tra 5-[3-methylstyrene 13b, presumably due to steric effects. Although the overall reaction is highly exothermic on the radical cation surface, the reaction is not insensitive to steric effects. Chemoselectivity in the radical cation cycloaddition is largely a consequence of a substrate s ability to stabilize the radical cation of the oxidized species through the formation of a weakly bound ion-molecule complex. Such complexes have been known for a long time in gas-phase... 

Steady-State rates are almost similar on Pd(lll), (100), (110), and (210) and polycrystaUine Pd surfaces (,1 ). Boudart (62) concluded that under similar experimental conditions the rate per Pd atom is equal on small Pd clusters ( 5 nm) and Pd(lll). Hence, data observed with a number of both single-crystal and supported metal catalysts indicate that the reaction is essentially surface insensitive. 

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. 

These three aspects in catalysis by metals enter in the general frame of structure-activity relationships. They have been the subject of reviews dealing with the (1) particle-size and plane-structure sensitivities [10], (2) ensemble-size sensitivity [11], and (3) metal-support interaction [9]. Depending on whether or not the turnover frequency (TOF), or rate per unit surface area or per accessible metal atom, is affected by the structure of the particle surface, the reactions have been called structure-sensitive or structure-insensitive [12]. The structure-activity relationships... 

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