Active sites structure sensitive reactions

Furthermore, ir-arene complexes of transition metals are seldom formed by the direct reaction of benzene with metal complexes. More usually, the syntheses require the formation of (often unstable) metal aryl complexes and these are then converted to ir-arene complexes. The analogous formation of w-adsorbed benzene at a metal surface via the initial formation of ff-adsorbcd phenyl, merits more consideration than it has yet been given. It is to be hoped that the recognition and study of structure-sensitive reactions will allow more exact definition of the sites responsible for catalytic activity at metal surfaces. The reactions of benzene, using suitably labeled materials, may prove to be useful probes for such studies. 

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 transformation of methyloxirane on various Pt catalysts is a structure-sensitive reaction. The total TOF of the reaction exhibits a maximum curve as a function of dispersion. The structure sensitivity is caused by the change in the number of active sites. 

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. 

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 resulting CO complex can either desorb using its energy of formation for activation or survive due to insufficient activation energy. The amount of activation energy depends on the carbon skeleton onto which site C is bonded to. It is a structure-sensitive reaction [86,... 

For example, it is believed that defects in the crystal structure produce highly energetic and active sites for catalytic reactions. This may be true but the more crystalline the catalytic site the lower is the number of surface atoms and the lower is its catalytic surface area. All this being said there are reactions that favor certain catalyst crystalline sizes and are said to be structure sensitive. The above discussion points to the mystery of catalysis. The goal of finding a universal model describing the nature of the active catalytic site still eludes us today and will undoubtedly be the subject of fundamental research for years to come. 

The geometric structure of the intetface. Many catalytic reactions are surface sensitive and it is well known that the metal-support interactions are strongly textural and stochiometrically dependent [11,14]. Moreover, it is of particular interest to understand if the metal clusters introduce new active sites, where chemical reactions can occur on the oxide support. 

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. 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

Figure 2 demonstrates that the reaction rates did not decrease with time to any appreciable extent. For the samples of low Pd content, hydrogenation was completed in a remarkably short time. The Pd content of the samples of higher loadings formed aggregates which, as in the case of 1-octene, tend to block the interlamellar space and restrict conversion to the surface active sites. This particularly holds for 10.2% Pd-HDAM, which was the least effective sample in the reaction. Similarly to conventional Pd supported catalysts, styrene hydrogenation conducted on Pd-HDAM samples was found to be a moderately structure sensitive reaction. 

As will be discussed later, the continuous decrease in the hydrogenolysis rates is probably due to the rare earth exclusively blocking an active Co and Ni surface atom, resulting in a decrease in the number of specific sites available for structure-sensitive reactions such as alkane rearrangements. If cobalt and nickel are presumed to be active sites for the hydrogenolysis, the behavior is as expected. The Eu- and Yb-containing systems exhibit similar catalytic behavior. 

The difference between TOFchem and TOFitk for both structure-insensitive and structure-sensitive reactions, without a doubt, reflects the difference in the concentration of adsorption sites (for hydrogen or CO) and that of reaction sites occupied by the most active intermediates. The larger difference in the TOF s for structure-sensitive reactions is an outcome of ensemble size and geometric arrangement required for active reaction sites. 

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