Stoichiometric number oxygen reduction

The reduction of NO by H2 on La203 and Sr-promoted La203 was examined by Huang et al. [59]. Both N2 and N2O were observed as products. The sequence of elementary steps proposed for the catalytic cycle was the following, which invoked one type of site (S) to adsorb and activate H2 while the second type ( ) interacted with the oxygen-containing reaction intermediates. Stoichiometric numbers for an elementary step are included when needed ... 

Unlike a geometrical factor, the value of the factor

with composition in a predictable way. To illustrate this, suppose that stoichiometric MO2 is heated in a vacuum so that it loses oxygen. Initially, all cations are in the M4+ state, and we expect the material to be an insulator. Removal of O2- to the gas phase as oxygen causes electrons to be left in the crystal, which will be localized on cation sites to produce some M3+ cations. The oxide now has a few M3+ cations in the M4+ matrix, and thermal energy should allow electrons to hop from M3+ to M4+. Thus, the oxide should be an n-type semiconductor. The conductivity increases until

reduction continues, eventually almost all the ions will be in the M3+ state and only a few M4+ cations will remain. In this condition it is convenient to imagine holes hopping from site to site and the material will be a p-typc semiconductor. Eventually at x = 1.5, all cations will be in the M3+ state and M2C>3 is an insulator (Fig. 7.3). 

Oxygen-transfer reactions have been shown to occur from cobalt(III)-nitro complexes to alkenes coordinated to palladium.472 Thus ethylene and propene have been oxidized stoichiometrically in quantitative yields to acetaldehyde and acetone respectively, with the concomitant reduction of the nitro- to the nitrosyl-cobalt analog. A catalytic transformation with turnover numbers of 4-12 can be achieved at 70 °C in diglyme. The mechanism shown in Scheme 11 has been suggested. 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

The use of sulphur or sulphur compounds, even in the presence of a metal compound (most often vanadium), as a catalyst for the reductive carbonylation of nitroarenes have also attracted some attention [222-235]. Sulphur compounds are isoelectronic to their selenium analogues and some similarities in the reactivity of COS and H2S with respect to SeCO and H2Se is to be expected. Sulphur has the very important advantage of having a much lower toxicity than selenium, but the reactivity of its compounds is also much lower. As a consequence, several of the processes reported employ a stoichiometric amount of sulphur or, in the catalytic processes, the catalytic efficiency is extremely poor. Catalytic ratios around 2-4 are conunon and this number is never higher than 10. This implies than a very large amount of sulphur is always necessary and needs to be separated from the products and recycled. The form in which sulphur is present at the end of the process has never been reported. If oxygenated products are formed at least in part, as it is likely, their elimination may be costly or environmentally problematic. 

The intrinsic behavior of oxides under ion irradiation with the usual sputtering conditions (e.g., 2-5 keV Ar ) has been addressed recently by studying stoichiometric single-crystal targets or well-defined bulk-phase materials [73-76]. The extent of the reduction, as established by the oxygen/metal ratio from XPS measurements, was confirmed by that calculated from the initial and reduced state(s) of the metallic components developed during ion bombardment [75]. Reliable chemical shift data for reduced components are available for a number of multivalent oxide-forming metals [77, 78], and can be used in peak synthesis. 

The important question as to whether the chemical steps involved in the transient oxygen storage and release processes previously described (Sections 3.3.1 and 3.3.2) should be considered as equilibrium controlled processes (reversible elementary steps) has recently been addressed. It was found that reduction of ceria by CO or Hg to form COg or HgO, respectively, is very important, and if not included in proposed reaction schemes, a number of effects experimentally observed under the dynamic conditions of OSC measurement cannot be understood. For example (i) the observed OSC is lower in the presence of HgO and COg in the CO/He and Og/He alternate pulse or step-change gas compositions (ii) the observed OSC depends on the CO/Hg concentration (iii) when the catalyst is exposed to a rich-lean step, the outlet gas composition remains rich for some time after the inlet has switched to lean (iv) when a previously oxidized catalyst is subjected to a short rich pulse followed by stoichiometric operation, the catalyst emits a delayed pulse of CO/ Hg during the stoichiometric phase (v) when a catalyst is operated with an oscillating air/fuel ratio at slightly rich conditions, the... 

---