Metal-support interaction apparent

Thus not only promotion and electrochemical promotion is catalysis in the presence of a double layer but apparently the same applies for supported catalysts undergoing metal-support interactions (Chapter 11). 

At this point we note that the overall form of the absorption, fluorescence emission and excitation profiles for Agx, Ag + and AgP+ for AgxNaX and AgxNaY is superficially reminiscent of those observed for Ag°, Ag2 0, and Ag3 ° entrapped in rare gas solids (4-10). However, a number of important differences are also apparent. These details are discussed for each silver guest as a necessary prelude to the subject of metal-support interactions. 

It is apparent from the discussion above that metal-support interactions of the electronic type have been proposed to explain a large variety of catalytic phenomena. The exact nature and mechanism of this interaction depend on the particular catalytic system, i.e., the support material, the nature of the active phase, the size of the metal cluster or particle, the gaseous atmosphere under which the catalyst operates, and possibly other parameters. 

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

Taking into account the difficulty to control strong metal support interactions (SMSI), it is easy to imagine that the performances of these systems are very changeable and depend on a vast number of factors intervening during the preparation of the catalysts and even during the reaction. That is the reason why apparent discrepancy exists in the literature with respect to selectivity values on... 

In summary, apparent metal-support interactions may arise through the operation of a specific particle size effect, or of bifunctionality or spillover, or through the support acting as a source or sink of a catalytic poison. Deliberately added promoters constitute an additional complication. Real interactions not due to these or similar causes may be attributed either to electronic or geometric effects, the latter embracing possible differences in crystal habit, or to the creation of phases which contain the active component in some form but which are hard to reduce. 

Torr) [32-36], Alternatively, higher H2 pressures at 300 K can be used to both form the hydride and saturate the Pd surface with H atoms, then an evacuation step is used to rapidly decompose the bulk hydride, and this is followed by obtaining a second isotherm. Similar to the situation in Figure 3.3, the difference, a, represents the irreversible H adsorption on the Pd surface. This approach may be preferred because it provides additional information about the Pd crystallites, i.e., once the surface Pds atoms are counted by the irreversible uptake, the remainder of the atoms can be attributed to bulk (Pdb) atoms, i.e., Pdb = Pdt Pds, and the apparent bulk hydride ratio can be determined [32]. Values near PdHo b are typically attained with large clean Pd crystalhtes, but on small Pd crystallites this apparent hydride ratio can become larger than 0.6 because reversible chemisorption on the Pds atoms can dominate the second isotherm as the Pd dispersion approaches unity. Consequently, valuable information can be obtained regarding surface cleanliness and metal-support interactions (MSI) [32,37]. An example of such an effort is provided by Illustration 3.1. 

A in the CITEP crystal structure. These are essentially the same, due to the resolution of the experimental structure. These n interactions are not optimal. Our optimizations of metal ion ti complexes with both the thiazolothiazepines and the salicylhydrazines result in distances of less than 3 A. The apparent weakness of this interaction may demonstrate why our studies of inhibitor complexes with metal ions did not support a direct metal ion interaction. Studies are under way to dock the CITEP inhibitor to the integrase catalytic core to evaluate whether the scoring methods used to rank the salicylhydrazine complexes are able to accurately provide a consensus that the metal ion site is more favorable for this inhibitor. The result of this evaluation will determine whether our apparent consensus favoring the site above the catalytic loop for the salicylhydrazines does indeed represent the preferred binding site for this class of inhibitors. 

To further test this hypothesis freshly-reduced catalysts were reacted with high-pressure steam (5 atm). A significant loss of BET surface area (from 215 to 188 mVg) is observed after Co/Davisil was reduced at 1 atm and reacted with 5 atm steam for 24 h (see Table 3). Increasing the space velocity by a factor of four also increases the rate of BET surface area loss (from 12.5 % / 24 h to 39.0 % / 24 h). Extents of reduction of cobalt oxide to cobalt metal before and after steam treatment are shown in Table 3. After steam treatment the cobalt oxide-support interaction is apparently substantially increased, i.e., the fraction of cobalt reduced to the metal at 400°C decreases from 89 to 4% moreover, the amount of cobalt-silicates (as inferred from TPR spectra shown elsewhere [22, 23]) also increases after steam treatment. This latter observation is consistent with the substantially higher extent of reduction of these catalysts (71-72%) at 750 C, a temperature at which a significant fraction of cobalt silicate can be reduced to the metal. 

Catalano et al. reported the synthesis and characterization of a new series of Pd°-based metallocrypates that bind Tl1 ion in the absence of attractive ligand interactions through metal-lophilic connections. The cationic species have been characterized by a variety of methods and have considerable stability. From the solid-state structural data it is apparent that interaction of the metal atoms with one another is the dominant bonding interaction within the metallocryptate cavity. The characterization of complexes supports the concept of metallophilic behavior as a fundamental component of bonding in closed-shell systems. These materials may ultimately serve as prototypical systems for detection of closed-shell ions 946... 

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