Structure sensitive/insensitive reactions

Catalysis of cyclohexene hydrogenation has been studied extensively both in the vapour and liquid phases on platinum ", palladium and other metallic surfaces. Here the kinetics of the cyclohexene hydrogenation on platinum have been considered lu terms of the specific activities of samples of silica-supported platinum, previously characterised by hydrogen chemisorption. Particular attention has been paid to the structure sensitivity-insensitivity of the reaction and how this varies as carbonaceous overlayers are built up on the catalysts with increasing reaction time. 

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. 

The systematic use of classical catalytic kinetics is always a useful approach in modeling (Boudart 1986). Even if these models do not reflect the true mechanism in the case of structure-sensitive catalysts, they are a formally correct representation of the observed facts. As Boudart sees it in the case of structure-insensitive reactions, it can also be the real thing. 

O showed a profound difference in CO2 formation rate [M.J.P. Hopstaken and J.W. Niemantsverdriet, J. Chem. Phys. 113 (2000) 5457]. Hence, care should be taken to interpret apparent structure sensitivity found under normal operating conditions of high pressure and coverage in terms of the intrinsic reactivity of sites. From the theory of chemisorption and reaction discussed in Chapter 6 it is hard to imagine how the concept of structure insensitivity can be maintained on the level of individual sites on surfaces, as atoms in different geometries always possess different bonding characteristics. 

These are examples of structure Insensitive (facile) (Reactions 1-4) and structure sensitive (demanding) reactions (Reactions 3-7). 

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]. 

Figure 2. Structure-insensitive reaction (1), sympathetic (2)/an-tipathetic structure sensitivity (3), and TOF reaching a maximum with varying particle size (4) [9].

Boudart [4] TOFs can either be independent of particle size (structure-insensitive reactions), increase (antipathetic structure sensitivity) or decrease (sympathetic structure sensitivity) with growing particle size, or cross a maximum (Figure 2). 

Thus, although CO disproportionation is structure-sensitive, the CO-O2 reaction appears to be structure-insensitive at both 445 K and 518 K, provided we define correctly the number of Pd sites available for reaction at both temperatures. It should also be noted that the turnover rate at 445 K is the same on metal particles supported on 1012 a-A O, 0001 a-A O, and... 

These reactions exemplify the two major types of catalytic processes, namely, those where the specific activity is sensitive to changes in the catalyst particle morphology (structure sensitive), and those where the specific activity is independent of the catalyst morphology (structure insensitive). Generally, reactions (1) and (2) are structure insensitive whereas reactions of type (3) are structure sensitive. 

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. 

Boudart (223) suggested that all reactions might not be equally sensitive to the geometric arrangements in various metal surfaces or to the differences in the electronic structure of sites in different geometric environments (coordination). Boudart divided the reactions into two groups (I) structure insensitive and (II) structure sensitive. The operational criterion of structure sensitivity is the specific activity (the rate per unit surface area) or, the turnover numbers (TONs) (the rate per site) TONs should differ by more than a factor of 5-10 when the dispersion D is varied sufficiently. Bond (224) formulated similar ideas and also suggested several reasons why the variations of TONs with D can monotonically decrease (antipathic), mono-tonically increase (sympathetic), or show a maximum. 

This classification reminds one of the classification of structure-insensitive (I) and structure-sensitive (II) reactions by Boudart (223). It is probably not true that all reactions of group II are structure-sensitive and vice versa, but it is to be expected that the possible candidates for structure sensitivity are indeed in group II rather than in group I of the above classification. 

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... 

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 ... 

When the catalytic properties of metals are examined, the importance of the non-uniformity of sites depends on the reaction under study. For some reactions, the activity of the metal catalyst depends only on the total number of sites available and these are termed structure-insensitive reactions. For other reactions, classified as structure-sensitive reactions, activity may be much greater on sites associated with a particular crystal face or even with some type of defect structure. The alternative names of facile or demanding have been used to describe structure-insensitive or structure-sensitive reactions, respectively. 

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