Properties catalytic character

Although the Ira and Ir clusters catalyze the same reactions as metallic iridium particles, their catalytic character is different, even for structure-insensitive hydrogenation reactions. It is inferred [15] that the clusters are metal-like but not metallic consistent with the structural inferences stated above, we refer to them as quasi molecular. Thus these data show the limit of the concept of structure insensitivity it pertains to catalysis by surfaces of structures that might be described as metallic, i.e., present in three-dimensional particles about 1 nm in diameter or larger. This conclusion suggests that supported metal clusters may be found to have catalytic properties superior to those of conventional supported metals for some reactions. The suggestion finds some support in the results observed for platinum clusters in zeolite LTL, as summarized below. 

It is surprising that the confusion over fundamental solid state properties such as phase composition and cationic oxidation states should have remained unclarified for so long, and it is disturbing that so many contradictory results and interpretations have been reported from supposedly similar materials. The lack of unanimity over the extent of antimony solubility in tin(IV) oxide is particularly surprising since many workers 9-11, 23, 25) have suggested that the formation of a solid solution is an important factor in the catalytic character of these materials. Hence, it is clear from these early studies that the extent and conditions for solid solution formation are completely uncertain. 

There have been several studies of the catalytic character of tin-antimony oxides, and in this section an attempt will be made to relate the results to the solid state properties of the catalyst which have already been considered. 

Studies of the selective oxidation of propylene, which is an important application of the tin-antimony oxide catalyst, have resulted in the description of several mechanisms and the subject was recently reviewed by Keulks et al. (7). It is not the purpose of this article to give a similar in depth consideration of this aspect of the catalytic properties of tin-antimony oxides. However, it is important that the improved knowledge of the bulk and surface properties of the catalyst and their relationship with the catalytic character should be considered in terms of the formulation of reaction mechanisms. 

Electro)catalytic properties Undoubtedly, the most frequently emphasized feature of CNT- and IL-based carbon pastes is their (self)catalytic character... 

The electron acceptor or electron donor character of adsorbates plays a very important role in their catalytic properties. It also plays a crucial role in their electrochemical promotion behaviour. This is to be expected since electrochemical promotion is catalysis in presence of a controllable double layer which interacts strongly with the adsorbate dipoles. 

The functionalized polymers have catalytic properties similar to those of their soluble analogues.1 A solution-like character is characteristic of polymer gels. As polymers become more highly cross-linked, they lose the solution-like character and their properties approach of those of inorganic solids. 

The study of the two (MPMol2)b and (MPMol2)b series salts (M Fe, Ni, Co) shows that their physic-chemical properties (XRD and BET) were different. The catalytic test of isopropanol decomposition at 150°C, shows the strong acidic character of the (MPMol2)b series and the acid and redox character of the (MPMol2)a. 

The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

Various astoichiometric components (hydrogen, carbon, and others, for example, silicium and aluminum) present may interact with localized and nearly free electrons to differing extents. According to the localized free electron interplay model of metal catalysts developed by Knor 163, 164) the ratio of the two types of electrons may influence the catalytic properties considerably. For example, a subsurface proton attracts nearly free electrons and thus uncovers some localized orbitals. Carbon may interact first with localized electrons 164). This may be one of the reasons why their effects are of opposite character. The collective efforts of catalytic and surface chemists are necessary to bring some clarity to the multitude of problems arising here. 

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