Oxide sites

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

Marazano and co-workers have also applied the reactions of tryptamine with various Zincke salts, including 115 (Scheme 8.4.39), in the synthesis of pyridinium salts such as 116. This type of product is useful for further conversion to dihydropyridine or 2-pyridone derivatives. For example, in a different study, Zincke-derived chiral pyridinium salts could be oxidized site-selectively with potassium ferricyanide under basic conditions as a means of chiral 2-pyridone synthesis (117 —> 118, Scheme 8.4.40). 

Almost simultaneously, Lindahl and co-workers proposed that Cluster C is the CO oxidation site based on EPR and ENDOR studies of the cyanide adduct of the enzyme (134). That proposal was based on the premise that CO and cyanide compete for the same binding site. Additionally, Xia and Lindahl have shown that, by mild SDS treatment, they can partially dissociate CODH/ACS, which is a tetra-meric enzyme with an subunit composition, into an isolated a subunit and an form (135). The form has the same level of CO oxidation activity as the native protein indicating that the a subunit is not involved in CO oxidation and that the /8 subunit must contain the clusters required for CO oxidation (135). In addition, CO2 alters the g values of the Credi form of the enzyme (136). 

Spontaneous redox reactions can aiso occur by indirect eiectron transfer. In an indirect electron transfer, species invoived in the redox chemistry are not aiiowed to come into direct contact with one another. Instead, the oxidation occurs at one end of a wire and transfers eiectrons to the wire. Reduction occurs at the other end of the wire and removes eiectrons from the wire. The wire conducts eiectrons between the oxidation site and the reduction site. 

Step 1 represents adsorption of ammonia and step 2 its activation. The irreversible step 3 is obviously not elementary in nature, but unfortunately much information on the level of elementary steps is not available. Step 4 describes water formation and step 5 is the reoxidation of the site. Step 6 describes the blocking of sites by adsorption of water. The model thus relies on partially oxidized sites and vacancies on an oxide, similarly to the hydrodesulfurization reaction described in Chapter 9. The reactions are summarized in the cyclic scheme of Fig. 10.15. 

The catalytic activity for F/Cl exchange depends on the amount of reversibly oxidized sites with a linear relationship between the activity and the number of such sites [3,4]. 

The activity of the FePeCli6-S/tert-butyl hydroperoxide (TBHP) catalytic system was studied under mild reaction conditions for the synthesis of three a,p-unsaturated ketones 2-cyclohexen-l-one, carvone and veibenone by allylic oxidation of cyclohexene, hmonene, and a-pinene, respectively. Substrate conversions were higher than 80% and ketone yields decreased in the following order cyclohexen-1-one (47%), verbenone (22%), and carvone (12%). The large amount of oxidized sites of monoterpenes, especially limonene, may be the reason for the lower ketone yield obtained with this substrate. Additional tests snggested that molecular oxygen can act as co-oxidant and alcohol oxidation is an intermediate step in ketone formation. 

The role of the Pt-Ba interaction in the mechanism of adsorption of NO species was also studied by a kinetic model reported in the literature [16]. The model, which consists of 10 elementary reversible steps, is based on the oxidation of NO to N02 over Pt and on the storage of N02 over Ba, and it was used to simulate the data collected over both the physical mixture and the ternary Pt-Ba/y-Al203 1/20/100 w/w sample. A spillover reaction between Pt and Ba oxide sites has also been included in the model to account for the observed lower thermal stability of Ba-nitrates in the presence of Pt [16]. Essentially, the model assumes that the adsorption of NO proceeds through the nitrate route and does not consider the nitrite route. 

In summary, the total oxidation of propylene to C02 occurred at a higher rate than partial oxidation to propylene oxide and acetone total and partial oxidations occurred in parallel pathways. The existence of the parallel reaction pathways over Rh/Al203 suggest that the selective poisoning of total oxidation sites could be a promising approach to obtain high selectivity toward PO under high propylene conversion. 

A few relatively recent published examples of the use of NMR spectroscopy for studying polymer degradation/oxidation processes will now be discussed briefly. At the early stages of degradation, the technique can be used to provide chemical identification and quantification of oxidised species for polyolefins, oxidation sites can be identified by the chemical shifts of -CH2- groups a and ji to carbons bonded to oxygen [85]. Spin-spin relaxation times may be determined by a pulse sequence known as the Hahn spin-echo pulse sequence. 

The above is formally equivalent to the picture of a coordinatively unsaturated surface (CUS) put forward by Burwell et al. (8) in their discussion of chromia. The acid-base formalism does have the advantage of drawing attention to the analogy of acid and base catalyzed reactions. If a hydrocarbon undergoes reaction at these sites via loss of a proton to the oxide site, the reaction should be analogous to a base catalyzed reaction if it undergoes reaction via the loss of a hydride to the zinc site or addition of a proton from the oxide site, the reaction should be analogous to an acid catalyzed reaction. This view, which we find useful, is implicit in the discussion that follows. 

Raman spectroscopy has been successfully applied to the investigation of oxidic catalysts. According to Wachs, the number of Raman publications rose to about 80-100 per year at the end of the nineties, with typically two thirds of the papers devoted to oxides [41]. Raman spectroscopy provides insight into the structure of oxides, their crystallinity, the coordination of metal oxide sites, and even the spatial distribution of phases through a sample when the technique is used in microprobe mode. As the frequencies of metal-oxygen vibrations found in a lattice are typically between a few hundred and 1000 cm 1 and are thus difficult to investigate in infrared, Raman spectroscopy is clearly the indicated technique for this purpose. 

Surface site densities used in the computation of the oxide site concentrations presented in this paper were determined by either rapid tritium exchange or acquired from published values (18). Reported total site densities for hydrous metal oxides show relatively little variation generally they range by less than a factor of 3. Since [M], [SOM], [H] and x are known or can be determined from experimental data, uncertainties in estimates of the total site concentration are directly translated into uncertainties in the calculated partitioning coefficient. 

Powders give statistically mixed phases and, possibly, spatially unseparated reduction and oxidation sites, as well as poor space charge layers for carrier separation. This leads to high rates of bulk and surface recombination, as well as solution species back reactions. Light scattering losses add a further decrease in... 

Tetranuclear Manganese Complexes Modelling the Photosynthetic Water Oxidation Site... 

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