Metal oxide adsorbed oxygen

Metal oxides, 31 78-79, 89, 102, 123, 157-158, 191, 32 199-121 see also Amorphous metal oxides Sulfate-supported metal oxides specific oxides adsorbed oxygen on, 27 196-198 binary, surface acidity, 27 136-138 catalytic etching, 41 390-396 coordination number, 27 136 electrocatalysts, 40 127-128 Fe3(CO)i2 reaction with, 38 311-314 Lewis acid-treated, 37 169-170 multiply-valent metals, electrocatalytic oxidations, 40 154-157 superacids by, 37 201-204 surface acidity, methods for determining, 27 121... 

The composition of the surface may also depend on gas pressure, for example, a surface may change from that of a metal with adsorbed oxygen to a surface metal oxide (JJ-JP) or to a metastable (subsurface) oxide that cannot be identified in UHV or by other analysis (60,61). It is apparent that such pressure effects have a strong impact on the catalytic properties and that measurements under elevated pressure are desirable. 

Figure 11 Schematic representation of the reaction pattern for the OER (a) formation of adsorbed hydroxyl radicals ( OH) by water discharge (b) oxygen evolution by electrochemical oxidation of ( OH) species (c) formation of a metal oxide (d) oxygen evolution by chemical decomposition of the oxide. (From Ref. 10. Copyright 1999, The Electrochemical Society, Inc. Reprinted with permission.)...

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... 

Cobalt(Il) dicobalt(Ill) tetroxide [1308-06-17, Co O, is a black cubic crystalline material containing about 72% cobalt. It is prepared by oxidation of cobalt metal at temperatures below 900°C or by pyrolysis in air of cobalt salts, usually the nitrate or chloride. The mixed valence oxide is insoluble in water and organic solvents and only partially soluble in mineral acids. Complete solubiUty can be effected by dissolution in acids under reducing conditions. It is used in enamels, semiconductors, and grinding wheels. Both oxides adsorb molecular oxygen at room temperatures. 

As the system passes from the active to the passive state the initial interaction depends on the composition of the aqueous phaseAn initial chemisorbed state on Fe, Cr and Ni has been postulated in which the adsorbed oxygen is abstracted from the water molecules. This has features in common with the metal/gaseous oxygen interaction mentioned previously. With increase in anodic potential a distinct phase oxide or other film substance emerges at thicknesses of 1-4 nm. Increase in the anodic potential may lead to the sequence... 

The oxygen vacancies then diffuse to the gas interface where they are annihilated by reaction with adsorbed oxygen. The important point, however, is that metal is consumed and oxide formed in the same reaction zone. The oxide drift has thus only to accommodate the net volume difference between the metal and its equivalent amount of oxide. In theory this net volume change could represent an increase or a decrease in the volume of the system, but in practice all metal oxides in which anionic diffusion predominates have a lower metal density than that of the original metal. There is thus a net expansion and the oxide drift is away from the metal. 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

These reactions demonstrate the Brflnsted base role of adsorbed oxygen perviously found on Ag(llO) and show further that more active transition metals which themselves activate C-H bonds catalytically oxidize via a two-step mechanism in which the surface intermediates are scavenged by adsorbed oxygen. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

In the anodic polarization of metals, surface layers of adsorbed oxygen are almost always formed by reactions of the type of (10.18) occurring in parallel with anodic dissolution, and sometimes, phase layers (films) of tfie metal s oxides or salts are also formed. Oxygen-containing layers often simply are produced upon contact of the metal with the solution (without anodic polarization) or with air (the air-oxidized surface state). 

The first step of oxide-layer formation is oxygen adsorption (chemisorption). In the case of platinum, the process stops at this stage, and depending on the conditions, an incomplete or complete monolayer of adsorbed oxygen is present on the platinum surface. In the case of other metals, layer formation continues. When its thickness 5 has attained two to three atomic diameters, the layer is converted to an individual surface phase that is crystalline (more seldom, amorphous) and has properties analogous to those of the corresponding bulk oxides. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

In above sections the main attention has been paid to adsorption-caused change in electrophysical characteristics of semiconductor adsorbent caused by surface charging effects. However, as it was mentioned in section 1.6, the change in electrophysical characteristics of such adsorbents can be caused by other mechanisms, e.g. by direct interaction of absorbate with the surface defects provided (as in the case of oxide adsorbents) by superstoichiometric atoms of metals and oxygen... 

The release of N2 occurs within function 3. It involves the dissociation of NO (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to a supported homogeneous catalytic process involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. 

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