Oxidation by water vapor

Tungsten reacts with oxygen at high temperatures. The finely-divided powder is pyrophoric. But the bulk metal begins to oxidize at about 400°C. The metal oxidizes rapidly when heated in air or oxygen at red heat. Two simple oxides are known, a blue monoclinic dioxide, WO2, and a lemon yellow trioxide, WO3. The trioxide, WO3, is the most stable oxide and the ultimate product of heating the metal in oxygen. Many other oxides also are known, but they are of nonstoichiometric compositions and are unstable. The metal also is oxidized by water vapor at red heat. 

Norbert Berkowitz Surely it is possible that the primary alteration of the cell walls is caused by hydrolysis and oxidation by water vapor and air, or they may undergo these reactions inorganically after actual immersion in water. It is of course possible that breakdown products resulting from these reactions were used or altered by microorganisms. 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

Bulk tungsten does not react with water but will be oxidized by water vapor at elevated temperature, e.g., at 600 °C. 

Figure 1. Effect of argon carrier flow role upon the deposit oxidation by water vapor at 1056°C. Key to water vapor partial pressure O, 58 mm Hg and , 362 mm Hg.

Figure 9. Effect of hydrogen upon deposit oxidation by water vapor at 1056°C.

Syngas can be produced in the endothermic process of methane oxidation by water vapor, often referred to as the steam reforming of methane ... 

Note that vmlike the equations for Fe, the majority of the equations for Co do not contain a water partial pressure factor. This is not surprising since Co is much more resistant to oxidation than is Fe. Nevertheless, it has been reported that vmder FT conditions, Co crystals smaller than 5 iixa are oxidized by water vapor (56). It is possible that the smaller the Co crystals, the lower the Ph2o/ H2 ratio at which they will oxidize and so become inactive. If this process is reversible, then it seems feasible that the effect of the water vapor can be incorporated into the kinetic equation. If this is so, then equations (9) and (10) could well be more applicable at high conversions (i.e., vmder commercial conditions). 

Holstein, W. and Machiels, C. (1996). Inhibitionof Methanol Oxidation by Water Vapor—Effect on Measured Kinetics and Relevance to the Mechanism, J. Catal., 162, pp. 118—124. 

Holstein, W.L. and Machiels, C.J. Inhibition of methanol oxidation by water vapor-effect on measnred kinetics and relevance to the mechanism. J. Catal. 1996,162, 118-124. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

The corrosion behavior of plutonium metal has been summarized (60,61). a-Plutonium oxidizes very slowly in dry air, typically <10 mm/yr. The rate is accelerated by water vapor. Thus, a bright metal surface tarnishes rapidly in normal environments and a powdery surface soon forms. Eventually green PUO2 [12059-95-9] covers the surface. Plutonium is similar to uranium with respect to corrosion characteristics. The stabilization of 5-Pu confers substantial corrosion resistance to Pu in the same way that stabilization of y-U yields a more corrosion-resistant metal. The reaction of Pu metal with Hquid water produces both oxides and oxide-hydrides (62). The reaction with water vapor above 100°C also produces oxides and hydride (63). 

Silicon carbide has very high thermal conductivity and can withstand thermal shock cycling without damage. It also is an electrical conductor and is used for electrical heating elements. Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential usehilness as technical ceramics in aerospace appHcation, eg, the carbides (qv) of B, Nb, Hf, Ta, Zr, Ti, V, Mo, and Cr. Ba, Be, Ca, and Sr carbides are hydrolyzed by water vapor. 

BeryUium chloride [7787-47-5], BeCl2, is prepared by heating a mixture of beryUium oxide and carbon in chloride at 600—800°C. At pressures of 2.7—6.7 Pa (0.02—0.05 mm Hg) beryllium chloride sublimes at 350—380°C. It is easily hydrolyzed by water vapor or in aqueous solutions. BeryUium chloride hydrate [14871-75-1] has been obtained by concentrating a saturated aqueous solution of the chloride in a stream of hydrogen chloride. ChloroberyUate compounds have not been isolated from aqueous solutions, but they have been isolated from anhydrous fused salt mixtures. 

In some Orsat apparatus models, water vapor may also be determined hydrogen may also be determined by its oxidation to water vapor. 

The catalytic activity for NO oxidation [reaction(l)] was strongly inhibited by water vapor, because this reaction occurs on Lewis acid sites of zeolite as... 

When the temperature of a carbonate reservoir that is saturated with high-viscosity oil and water increases to 200° C or more, chemical reactions occur in the formation, resulting in the formation of considerable amounts of CO2. The generation of CO2 during thermal stimulation of a carbonate reservoir results from the dealkylation of aromatic hydrocarbons in the presence of water vapor, catalytic conversion of hydrocarbons by water vapor, and oxidation of organic materials. Clay material and metals of variable valence (e.g., nickel, cobalt, iron) in the carbonate rock can serve as the catalyst. An optimal amount of CO2 exists for which maximal oil recovery is achieved [1538]. The performance of a steamflooding process can be improved by the addition of CO2 or methane [1216]. 

The second class of reactions is similar to conventional combustion or burning reactions that yield mostly carbon oxides and water vapor, as summarized by... 

A two-step mechanism and resulting rate law can be developed as follows. Reactive carbon sites, C (total number Nc,), are assumed to exist on the surface of the solid. These can be oxidized reversibly by water vapor ... 

Manganese In its pure form a fairly good catalyst it can be promoted by certain oxides and metals. However, it is very easily oxidized and rendered inactive by water vapor. As the pure metal, 0.8%, promoted, up to 2.5%. 

Complementary work on the interaction of oxides with water vapor and the properties of transition metal ions in mineral structures was valuable in pointing the way to some of the variables found significant. These projects are supported, respectively, by the National Science Foundation (GP172) and the Advanced Research Projects Agency, through the Stanford Center for Materials Research. 

Pure oxygen or air are generally used as the oxidant in SOFCs. The most frequently considered fuels are H2, CO, coal and light alkanes C H2 +2, mainly methane. The moderate oxidation of alkanes or coal by water vapor or carbon dioxide provides H2-C0-C02-H20 mixtures with various compositions. Then the thermodynamic cell EMF is a function of fuel gas composition in the close vicinity of the anode and can be calculated considering the following equilibria ... 

IV sites. A protonated pyridine has never been observed, and the Lewis acid sites on titanium oxides cannot be converted into Br0nsted sites by water vapor adsorption (217). Although Jones and Hockey (216) suggest that the chemistry of surface hydroxyl on rutile corresponds more closely to that of the OH" ion rather than that of the hydroxyl group, no surface reactions similar to that observed with alumina [Eq. (14)] have since been reported. 

Poisoning by water vapor is a reversible effect and can be overcome by redrying the catalyst. Alkali poisoning, on the other hand, is permanent and may involve the formation of a salt, such as a manganite or cobaltite in the surface layer. In such cases, the manganese or cobalt atom is more completely coordinated and the reactivity of the surface considerably lessened thereby. Carbon monoxide is therefore oxidized only stoichiometrically by poisoned Mn02. 

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