Structure Sensitive and Insensitive Reactions

Figure 3.2 Activity— Structure sensitive and insensitive reactions.

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. 

Important catalytic reaction concepts include structure sensitivity and insensitivity of reactions, mechanistic classifications (Langmuir-Hinshelwood, Eley-Rideal), the compensation effect, the presence of strongly chemisorbed overlayer, and the roles of structure and bonding modifier additives (promoters). 

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. 

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. 

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 .)... 

A new concept has emerged that distinguishes between structure-sensitive and structure-insensitive reactions the specific rate of the latter is independent of particle size. This is a useful concept with many implications. A complete independence from crystal size, if applicable to the transition from solid to atom would, however, seem incongruous in the light of any electronic theory-physical or chemical. We must realize that the metal particles, even at the low end of the range investigated, are still relatively large a 20-A metal crystallite contains 300 to 400 atoms. 

Nevertheless, it is necessary to return to the main point of interest here, which is the effect of deactivat ion and the accumulation of carbonaceous species on the Pt surface on the structure sensitivity or insensitivity exhibited (as inferred from the dependence of K on the platinum surface area)- Turnover numbers so calculated after 10 min reaction time decrease rapidly with increasing platinum surface area in a hyperbolic manner (and increase almost linearly with the mean platinum particle size) for reactions followed at both 3L3K and 295K, in contrast to the results obtained by Boudart over a narrower range of Pt surface area, Such behaviour very unusual but has been reported for structure sensitivity in cyclopentadiene hydrogenation on supported copper. However, the turnover numhers were almost independent of platinum surface area and... 

TABLE 7.42. Structure-Sensitive and Structure-Insensitive Catalytic Reactions... 

In a catalytic reaction, all steps do not equally depend on the surface structure. So, for example, the rate of simple desorption processes is often not markedly affected by the structure of the surface. In catalysis, therefore, reactions are classified into "structure sensitive" and "structure insensitive" [5], usually on the basis of the variation of reactivity with particle size. As an example, the electrocatalytic oxygen reduction at platinum (which is of importance for fuel cells) will be mentioned, where a decrease of specific activity with increasing particle size was reported [6,7]. In a theoretical analysis [8], the kinetics was treated on the (111), (10 0), and (211) facets of several transition metals, and the results were combined with simple models for the geometries of catalytic nanoparticles. Thus, the experimentally observed trend could be well reproduced. 

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. 

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