Hydrogen reduced transition metal

Another characteristic feature of the hydrogen-reduced transition metal zeolites is their acidic properties, as demonstrated by their catalytic behavior (7). Naccache and Ben Taarit (8) gave IR evidence of the subsequent formation of protons on hydrogen-reduced Cu(II)-Y zeolite. Furthermore, transition metal ions have various oxidation states. Owing to the shielding effect caused by the zeolite network and the electric fields, the transition metal ions may be stabilized in unusual oxidation states—i.e. Ni(I) (9). 

As exemplified in Figure 2, Type 1 mechanism, electron transfer from L to sens yields two radicals, the substrate radical, L", and the sensitizer radical anion (sens ). In the next step, the lipid radical may induce a chain peroxidation cascade involving propagation reactions -The sensitizer radical anion may also start a sequential one-electron reduction of 2 generating HO in the presence of reduced transition metals. As a result, this may lead to abstraction of a lipid allylic hydrogen with subsequent generation of a carbon-centered lipid radical, L, that is rapidly oxidized to a peroxyl radical (vide supra). 

Figure 2.9. Glucose can enolize and reduce transition metals thereby generating superoxide free radicals (02" ), hydroxyl radicals ( OH), hydrogen peroxide (H202) and reactive dicarbonyl compounds. Adapted with permission from Wolff, S. P. (1996). Free radicals and glycation theory. In The Maillard Reaction. Consequences for the Chemical and Life Sciences, Ikan, R., ed., John Wiley Sons, Chichester, UK, 73-88.

Although the activation of hydrogen peroxide by reduced transition metals has been known for almost 100 years as Fenton chemistry (eqnations 74-77) there is nncertainty as to whether this chemistry prodnces free hydroxyl radicals (HO-). There is clear kinetic evidence that free HO- is not the dominant reactive intermediate in Fenton chemistry, bnt rather an HOOH/Fe L addnct that resnlts from nncleophilic addition. ... 

Although the activation of hydrogen peroxide by reduced transition metals has been known for almost 100 years as Fenton chemistry,... 

The aim of this work was to develop and present a new method for the synthes selective platinum catalysts for the hydrogenation of unsaturated aldehydes to unsatui alcohols. The method of preparation presented above makes it possible to obtain cata possessing polar platinum surface, due to the presence of active centers near the intei metal - partially reduced transition metal oxide. This transition metal oxide adsorb carbonyl oxygen atom whereas adjacent platinum atom interacts with carbonyl ca atom. In such a situation, the C=C bond is quite far from the surface so its adsorption i favored. There are also some additional parameters which should be taken consideration. The first is the size of platinum crystallites located on the catalyst sur Large platinum crystallites cause planar adsorption of aldehyde molecule in such adsorption of both double bonds is highly probable. It is known that a more sur... 

The formation of hydrogen peroxide by photolysis of natural waters is discussed in Chapter 6. It is also formed by illumination of some sands and semiconductor oxides (Kormann et al., 1988 see also Section 6.E.3). Other sources of H2O2 include formation in the gas phase of the troposphere by the self-termination (dismutation) reaction of OOH and the autooxidation of reduced transition metals such as iron (Equation 4.4). The formation and fate of H2O2 in the atmosphere has been reviewed (Gunz and Hoffmann, 1990 Sakugawa et al., 1990). 

The resulting hydrogen is then desorbed. Various reducible transition metal oxides have been tested as promoters, and the following activity series was found ... 

Ascorbate and urate are considered to be antioxidants in biological systems due to dieir reaction widi potentially damaging free radicals. However, like ascorbate, urate is able to reduce transition metal ions making these more reactive towards hydroperoxides, e.g. hydrogen peroxide and Upid hydroperoxides. Therefore, the role of ascorbic acid and uric acid is somevdiat controversial and die effect of these two antioxidants will especially depend on the type of catalyst inducing the observed oxidative changes. 

Figures 17 Surface reflectivity of Pd(111) and Pd(IOO) with and without adsorbed H. The reflectivity is on log scale. The addition of hydrogen shifts and intensifies the lowest energy surface resonance on Pd(111) (5.5 eV). The sharp drop in reflectivity at 8 eV corresponds to the emergence of a surface diffraction beam, and opens a new channel for electron interaction with the surface. The image potential states are just below this emergence threshold. On Pd(IOO) the curves are similar, but the energy scale is reduced due to the different crystal structure of the surface and different-sized surface Brillouin Zone. Reprinted from Surface Science, 178, M.E. Kordesch, Surface resonances in vibrational spectroscopy of hydrogen on transition metal surfaces Pd(IOO) and Pd(111), 578-588, 1986, with permission from Elsevier Science.

Electrons can be transferred in various ways. Single electrons may directly reduce transition metals such as Fe (ferric ion) to Fe (ferrous ion) or Cu (cupric ion) to Cu (cuprous ion). Hydrogen atoms and hydride ions (H ) may serve... 

Typically, as a chiral catalyst for the catalytic asymmetric hydrogenation, a transition metal complex bearing a chiral bulky ligand is used, " so that such a substrate molecule can only approach in a particular orientation, allowing the external reductant to reduce the substrate in a predictable fashion. Alternatively, the asymmetric hydrogenation of C=0 and C=N bonds may be accomplished through the use of catalytic amounts of an organocatalyst. This topic is discussed in chapter 32 of this book. 

Hydrogen peroxide, in combination with reducing agents (transition metals), also is used in those appHcations where its high water- and low od-solubiHty is not a problem or is easily overcome. 

Activation methods can be divided into two groups. Activation by addition of selected metals (a few wt%), mainly transition metals, e.g., fine powders of Fe, Ni, Co, Cr, Pt, Pd, etc. ", or chlorides of these metals when these are reducible to the metal by hydrogen during presintering. The mechanism of activation is not understood (surface tension, surface diffusion, etc.) but is related to the electronic structure of the metal additive. Activation by carbon is also effective. Alternatively, activation utilizes powders in a specially activated state, e.g., very fine (submicronic) powders. ... 

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