Active sites structure insensitive reactions

The catalytic activity of strained-layer Ni on W(llO) for methanation and ethane hydrogenolysis has been studied as a function of Ni coverage. The activity per Ni atom site for methanation, a structure-insensitive reaction, is independent of the Ni coverage and similar to the activity found for bulk Ni. The activation energy for this reaction is lower on the strained-metal overlayer, however, very likely reflecting the lower binding strength of CO on the bimetallic system. 

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). 

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. 

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. 

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. 

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. 

A comparative review on the evaluation of the catalysts Al, A2, B1 and B2 reveals that activity and dispersity are two closely related parameters which is influenced by the mode of preparation especially with respect to removal of chloride or to the final calcination temperature. Earlier works had identified benzene hydrogenation on supported platinum catalysts as a facile or structure insensitive reaction(5). A facile reaction may be defined as one for which the specific activity is practically independent of its mode of preparation(6). In other words all surface atoms are believed to be the active sites in a facile reaction without any dependence on the coordination number of site or on the collective properties of the crystallite. 

The difference between the two definitions is that the rates vary with time or position, while the TOF only depends on the availability of the sites present. The activity of the catalysts and the selectivity of the reaction depend on the characteristics of the material. Some important features of these reactions should be remembered. The first one is related to the sensitivity of the chemical reaction to the catalyst structure. According to Boudart s theory, the supported catalysts consist of metal particles of different and variable sizes. With the increase of particle diameter, the concentrations of metal atoms and exposed sites vary significantly, indicating that no changes in surface structure have happened. The structure sensitive reactions (SSR) are those in which the intrinsic reaction rate relative to the number of surface active sites, i.e., the frequency of the reaction varies with the particle sizes, which does not happen in structure insensitive reactions (SIR). This means that in the structure sensitive reactions, the frequency of... 

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. 

Dotted orange line No dependence of the fraction of surface atoms working as active sites on metal particle nanosize (structure-insensitive reactions). Full dark blue line Dependence of the fraction of surface atoms working as active sites on metal particle nanosize as in Fig. 3.90 (structure-sensitive reactions)... 

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]. 

Finally, one has the Madon-Boudart technique in which the number of active sites is varied by changing the metal loading or, for structure-insensitive reactions, by altering the metal dispersion or perhaps by poisoning sites. In contrast, the metal dispersion should be maintained constant while L is varied for structure-sensitive reactions. A slope of unity from a plot of In 3 vs. In L, i.e., a constant TOF, verifies the absence of ANY mass transfer limitations. 

Benzene is the sole product and no deactivation was observed. Both the dispersion and the rate decreased in the presence of a second metal however, the intrinsic activity (TOP) remained constant around 1 (s ), indicating that the rate is proportional to the surface active sites, and thus it is a structure-insensitive reaction. 

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. 

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. 

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