Electron deficient catalytic site

The catalytic effect of various surfaces was also investigated and showed that electron-deficient sites were responsible for promoting the condensation. Basic substances and small amounts of water were found to considerably reduce the rate of resinification. 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

From the results published on the hydrogenation of benzene (29, 30, 31), it appears that ruthenium and rhodium are more active than palladium. By adapting the scheme proposed by Dalla Betta and Boudart (9), we could suppose that the electron-deficient character of palladium on oxidizing sites leads to an electronic configuration very similar to that of rhodium, and, thus, to an increase in catalytic activity. 

Catalytic superactivity of electron-deficient Pd for neopentane conversion was recently verified for Pd/NaHY (157, 170). The reaction rate was positively correlated with the proton content of the catalyst. Samples that contained all the protons generated during H2 reduction of the catalysts were two orders of magnitude more active than silica-supported Pd. Samples prepared by reduction of Pd(NH3)2+NaY displayed on intermediate activity. It was suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaY samples deactivated rapidly and coke was deposited. Two types of coke were found (by temperature-programmed oxidation), one of... 

For peroxidase biocatalysis, the relevant redox couples are Compound I and Compound II, the intermediates present during the catalytic cycle, as described in Chap. 5. However, Fe(III)/Fe(II) redox potential could still be a useful indicator of the oxidizing character of peroxidases. Millis et al. [54] suggested for the first time that the noncatalytic Fe(III)/Fe(II) redox potential could be used to predict the oxidative capacity of a heme peroxidase during turnover. In this work, it was suggested that a more positive Fe(III)/Fe(II) redox potential indicates a higher electron deficiency within the active site, and thus the existence of enzymatic... 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

Here, we shall focus on ruthenium-catalyzed nucleophilic additions to alkynes. These additions have the potential to give a direct access to unsaturated functional molecules - the key intermediates for fine chemicals and also the monomers for polymer synthesis and molecular multifunctional materials. Ruthenium-catalyzed nucleophilic additions to alkynes are possible via three different basic activation pathways (Scheme 8.1). For some time, Lewis acid activation type (i), leading to Mar-kovnikov addition, was the main possible addition until the first anfi-Markovnikov catalytic addition was pointed out for the first time in 1986 [6, 7]. This regioselectiv-ity was then explained by the formation of a ruthenium vinylidene species with an electron-deficient Ru=C carbon site (ii). Although currently this methodology is the most often employed, nucleophilic additions involving ruthenium allenylidene species also take place (iii). These complexes allow multiple synthetic possibilities as their cumulenic backbone offers two electrophilic sites (hi). 

Another widely accepted viewpoint on the catalytically active sites in the silica-alumina is that they are aprotonic acids, viz., electrophilic A1 atoms with unfilled p-shell, the electron density being shifted from them towards the three surrounding 0 atoms (72). Such an electron deficiency confers on the A1 atom an afiinity towards an unshared electron pair of the basic adsorbed molecule, i.e., the properties of a Lewis acid. In fact, the addition compound, obtained on sorption of aniline vapor by a sublimed AICI3 film in a high vacuum exhibits the same shift towards the spectrum of benzene, as is the case with a protonic acid (73). 

It has been considered up to now that Cu on the ZnO in oxidized or electron deficient state is stabilized by certain chemical or structural causes and then becomes an adsorption site for COj and CO at the initial stage of the reaction. As a structure of the active sites, a two-dimensional epitaxial monolayer of Cu over ZnO and small Cu clusters which are not crystallized have been directly observed by XPS and EXAFS analyses. It has been proposed that the active sites are generated by the mild reoxidation of the surface of Cu-Zn alloy by the feed gas containing COj. The relation between the catalytic activity of various Cu-ZnO catalysts and the Cu surface area is measured and the effects of support oxides are classified in a different way from preceding work. The specific activity is found to be controlled by the oxygen coverage. 

It has been suggested [23] that the choice of the platinum precursor, when one of them contains chlorine, may play a role in the electron-deficiency of platinum site atoms and thus in catalytic activity and/or selectivity. Our results do not show differences between the two precursors used. 

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