Non-reducible oxides

Even if, except at defect sites[58, 59], the hydrogen does not easily dissociate on the surface oxides[l, 60], distributions of hydrogen atoms are involved in surface reactions (for instance the decomposition of the formic acid) and have motivated both experimental and theoretical works. The decomposition is made at high temperature on activated MgO powders[61, 62], On a non reducible oxides, the heterolytic... 

While the rates for the gold-catalyzed oxidations of CO are only slightly influenced by the presence of H2 over the pure titania supports (Table 1), the activities of the gold catalysts prepared from Ti02-Si02 supports for the oxidation of CO significantly increase when H2 is present in the feed (Table 2). This behavior is characteristic of non-reducible oxides such as alumina [1]. This clearly is an effect of the silica matrix in which the gold nanoparticles are dispersed, that is emphasized by the low conversion levels reached. 

The above classification suggests that under properly chosen condition the subject of this chapter, i.e. metal ion-metal nanocluster ensemble sites (MIMNES) can be formed in most of the above types of catalysts. For instance, from bimetallic clusters of type (i) and (ii) MIMNES can be formed under conditions of mild oxidation. In catalysts type (iii) MIMNES should exist both under oxidative and reductive environment. In catalysts type (iv) any metal-support interaction with the involvement of non-reducible oxide can also be considered as MIMNES. The only requirement for the formation of MIMNES is the atomic closeness of the two types of sites. 

In the following sections, we elaborate in more detail on how the structural and electronic properties of the oxide control its catalytic behavior by following a few example sterns. We describe the applications to oxidation chemistry over reducible and non-reducible oxides, and acid chemistry via specific example catalytic systems. 

In addition to the wide range of metal oxide catalysts that can cany out oxidation via redox catalysis, there are a host of other materials that can carry out oxidation over non-reducible metal oxides. The oxidation mechanisms over non-reducible metal oxides are quite different and typically involve the production of free radical intermediates. The mechanisms tend to contain both heterogeneous and homogeneous activation and functionality. The oxide is used to activate a free radical process that can then proceed in the gas phase or at the surface. Li-substituted MgO and the rare earth metal oxides are two classes of materials that are considered non-reducible oxidation catalysts. Here we wiU specifically focus on the activation of alkanes over non-reducible metal oxides. 

On non-reducible oxides, H2S can exchange with surface hydroxyl groups. 

Active gold catalysts are advantageous in that water usually enhances the catalytic activity [39]. Reducible or semiconductive metal oxide supports do not need moisture for room temperature catalytic activity, while non-reducible metal oxides such as AI2O3 and Si02 do [39] (Figure 10). 

We consider problems related to electrophysical properties of sintered polycrystalline oxides as well as their adsorption changes. We also analyze the difference in adsorption induced changes of electrophysical characteristics of stoichiometric and non-stoichiometric partially reduced oxide adsorbents. 

According to Summers and Chang from NASA s Ames Research Center, Moffett Field (1993), the oxidation of Fe2+ to Fe3+ provided a possibility for the reduction of nitrites and nitrates to ammonia. This reaction would have been of great importance, as NH3 is required in many syntheses of biogenesis precursors. The authors assume that nitrogen was converted to NO in a non-reducing atmosphere, and thence to nitrous and nitric acids. These substances entered the primeval oceans in the form of acid rain , and here underwent reduction to NH3 with the help of Fe2+, thus raising the pH of the oceans to 7.3. Temperatures above 298 K favoured this reaction, which can be written as ... 

In formula XII, A = reducing terminal unit (negligible). B = non-reducing terminal unit B consumes two molecules of oxidant to give one molecule of formic acid. C = point of branching. Each non-terminal unit (in the chains BC and elsewhere in the interior of the molecule) consumes one molecule of oxidant but produces no formic acid.]... 

Starch source D. P.a from osmotic measurements Unit-chain length from methylation Unit-chain length from periodate oxidation No. of non-reducing terminal groups/ molecule References... 

An important question is how this system can work with sugar alcohols and non-reducing sugars. The oxidation is catalysed by the electrode surface, which means that the response is dependent on the electrode potential of the catalytic state rather than the redox potential. 

Enthalpies of oxidation of stoichiometric and even non-stoichiometric oxides may similarly be obtained by heating reduced oxides in air in an adiabatic calorimeter to a temperature at which the oxidation proceeds sufficiently fast [56] ... 

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