Reduction molecules

However both groups agree on the stoichiometries, which are all of the type 2 Cr(n) 1 reductant molecule, and on the rate laws, which are generally... 

The thermal stability of NOx adsorbed species and their reactivity in the presence of gaseous reductant molecules was addressed by thermal decomposition in He (TPD) or by heating in flowing H2/He mixtures [temperature-programmed surface reaction (TPSR)], respectively. In these cases, after NOx adsorption and He purge at the adsorption temperature (300 100oC), the samples were cooled to RT under flowing He. Then the samples were heated at 15°C/min up to 500-600°C in He (TPD) or in He + H2 (2000 ppm) (H2-TPSR). 

The common idea on the mechanisms governing the reduction of NO adsorbed species over LNT catalysts is that the regeneration process includes at first the release of NO, from the catalyst surface (i.e. from the alkali- or alkali-earth metal compound), followed by the reduction of the released NO to N2 or other products [11]. The reduction of the released NO in a rich environment is thought to occur according to the TWC chemistry and mechanisms in particular, it was suggested that NO is decomposed on reduced Pt sites [38], or that a direct reaction occurs between released NO species and the HC reductant molecules on the precious metal sites [39],... 

A platinum (0.05 wt%)-impregnated ETS-lO(Cs) sample showed spectra similar to that of ETS- 10(Cs) (g... < gxx. gyy). except that the Ti3+ signal intensity increased by a factor of about 2.4 compared with that of the ETS-lO(Cs) sample. Although the reduction in Ti3+ intensity by Cs is attributed to greater stabilization of Ti4+ ions by the more basic and larger Cs atoms, the increase in the intensity induced by platinum is attributed to better activation of the reductant molecules (H2) by platinum and the consequently greater reduction of Ti4+ to Ti3+. In other words, both cesium and platinum influence the reducibility... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Reductive dissolution of transition metal oxides by organic reductants can be described as occurring in the following sequence of steps (i) diffusion of reductant molecules to the oxide surface,... 

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

Observations of triply and doubly bridging hydroxo groups in otherwise low-valent carbonyl clusters is particularly exciting in view of the potentially important role of such ligands in catalytic reductions. Molecules such as Ru4(CO)i<>(C = CHPrl)(OH)(PPh2) are worthy of detailed study. 

Several of the key issues are reflected in the debate over the appropriate use of pe to describe redox conditions in natural waters (129-131). The parameter is defined in terms of the activity of solvated electrons in solution (i.e., pe = - log e ), but the species e aq does not exist under environmental conditions to any significant degree. The related concept of pe (132), referring to the activity of electrons in the electrode material, may have a more realistic physical basis with respect to electrode potentials, but it does not provide an improved basis for describing redox transformations in solution. The fundamental problem is that the mechanisms of oxidation and reduction under environmental conditions do not involve electron transfer from solution (or from electrode materials, except in a few remediation applications). Instead, these mechanisms involve reactions with specific oxidant or reductant molecules, and it is these species that define the half-reactions on which estimates of environmental redox reactions should be based. 

The reductive dissolution of metal oxides such as Mn(III/IV) oxides by organic reductants occurs by the following sequential steps (Stone, 1986) (1) diffusion of reductant molecules to the oxide surface, (2) surface chemical reaction, and (3) diffusion of reaction products from the oxide surface. Steps (1) and (3), which are transport steps, are influenced by both the interfacial concentration gradient and the electrical potential gradient due to the net charge of the oxide surface. 

It should also be pointed out that the rate of each of the reaction steps (precursor complex formation, electron transfer, and breakdown of successor complex) is affected by the chemical characteristics of the metal oxide surface sites and the nature of the reductant molecules. These aspects are discussed in detail in an excellent review by Stone (1986), and the reader is encouraged to refer to this article. 

Quantitative information on the number of sites may be obtained from a volumetric or gravimetric study of the adsorption of oxidizing or reducting molecules. Adsorption calorimetry can be used to determine the energetics of the site distribution, as in the case of acidic and basic sites. 

It is quite clear that conversion between the fully oxidized and fully reduced forms of cytochrome oxidase is a four electron process126). In accord with the four electron accepting role of dioxygen, the titration of reduced oxidase with 02 (obtained in situ from /r-peroxo-bis (pentamine cobalt(II) tetranitrate) indicated complete oxidation of one reductant molecule by each molecule of 02127). There is no evidence for the liberation of intermediate oxidation states of oxygen such as peroxide or superoxide. The presence of two hemes and two coppers has lead to the assumption (but not without some strong exception, vide infra) that all four metal ions are involved in the red-ox process. 

Focusing on tlie intrinsic effectiveness of ammonia-type reductants, we have been investigating the application of a urea solution for tlie NO reduction in diesel exhaust. Since urea decomposes into two reductant molecules (ammonia and/or cyanic acid) above 300 °C in tlie presence of water, it can serve as a safe reductant reservoir. A liigh performance of urea solutions in NO reduction has already been reported in detail elsewhere [16,22]. The otlier disadvantages remained to be solved are (iii) the ammonia slip, (iv) unfavorable ammonia oxidation to N2O and NO, and... 

Fig, 11 (A) shows the SCR of NOx over Ag/ANOi and Ag-Pd AITN catalysts by Cdlr.at various temperatures. Both NOx conversions increased with increase in the reaction temperature and reached a maximum at 4.37 X for Ag-Pd/ANO and al 470 XI for Ag/ANO.. The NOx conversions then decreased with further increase of reaction temperature. The highest rate of NOx conversion was 82% over Pd-Ag/ANO, which was higher than the 73% rate over Ag/ANO . Obviously, a trace amount of Pd added into Ag/AI 03 can enhance the NOx conversion in the presence of excess oxygen and water vapor. This effect of adding metals is considered to be favorable for activating reductant molecules, for example, the scission of a C-C bond and partial oxidation. Fig. 11 (B) show-s the conversions of into COx over Pd-Ag/ANOjt and Ag AUO v Similar to NOx conversion, the curve of CM I, conversion for Ag/AI>Oi shifted to lower temperatures after the addition of Pd. This result suggests that Ag-Pd/ANOt can activate Cdl, to react with NO O. ... 

NADPH The reduced form of nicotinamide adenine dinucleotide phosphate, which is produced in photosynthesis and serves as the primary reductant molecule in plant cells. 

Table 1 shows that the B-reducer dramatically induces the decrease of Fdc and the increase of Bs with its addition increasii from 0 to 100 mL/L. It is due to the fact that despite the absence of salt bridge (leading to a lower efficiency of plating current) to prevent Fe2+ from oxidization in the solution, the B-reducer can protect the as-synthesized CoFe film effectively fi-om oxidization since B-reducer, acting as a reductant molecule, can reduce Fe3+ (produced during plating) back into Fe2+ ... 

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