Reactions Involving Metal Oxides

Dehydrogenation of hydrocarbons by cationic metal oxide clusters was studied in several experimental gas-phase and theoretical works. Rozanska and Sauer [132] used DFT methods to establish the mechanism of the oxidative dehydrogenation of hydrocarbons by VjOy (the smallest monocationic polynuclear closed-shell V species). Schwarz et al. [133] studied the gas-phase dehydrogenation of alkanes with Ni202 (see reaction below). 

In a theoretical work, we compared cationic Ni202H2 [134] with Ni202H2 [135], which are the (potential) products of the reaction of Ni202 and Ni202 with H2 or with alkanes. They also could be regarded as products of the NiO/NiO + H2O reaction. The analysis of these systems completes the comparison of neutral and cationic nickel oxides. The lowest lying isomer of Ni202H2 has a rhombic 

A system composed of Ni202 and H2 with a large distance between the subsystems has been considered to get estimates for the reaction energy. For the reactions Ni202 + H2 - Ni2(OH)2° , estimates of about 2eV for the reaction of neutral Ni202 and 3.5 eV for the reaction of the cation have been obtained. These values very clearly support the assumption of a higher reactivity of the cation, inferred already from the electronic structure. 

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Many of these reactions involving Mn and Fe have been shown to be linked with microbially mediated (e.g., chemolithotrophic) processes, in particular, bacterial reduction of Fe and Mn oxides are also capable of oxidizing sulfides to SO42-, and NH3 to NO3- under anaerobic conditions. Other important reactions involving metal oxides include the oxidation of NH3 to N2 via Mn oxides in the presence of oxygen. 

The ligand coupling mechanism has also been suggested to occur during a number of oxidation reactions involving metal oxides, such as for example ... 

Thermochemical calculations of reactions involving metal oxides (Reactions (25) to (27)) were not carried out because of unknown oxidation states and thermodynamic properties of many metal oxides. However, according to Jones (1987), the heats of reaction of Reactions (25) and (26) are small. 

EXAMPLE 2 - at what size must an AI2O3 monocrystal be crushed to increase its energy from 500 kJmole (500 kJ.mole is a typical value of the energies brought into play in chemical reactions involving metallic oxides) If Ysv l2 3 = 1 J and P0AI2O3 = 4.1Q3 kg m, the answer is at a size less than that of the ciystal cell ... 

Superimposed on this simple equiUbrium are complex reactions involving the oxides and hydrides of the respective metals. At about 400°C, the metal phase resulting from the reaction of sodium and potassium hydroxide contains an unidentified reaction product that precipitates at about 300°C (15). 

This chapter presents detailed and thorough studies of chemical synthesis in three quite different chemical systems zinc ferrite, intermetallic, and metal oxide. In addition to different reaction types (oxide-oxide, metal-metal, and metal oxide), the systems have quite different heats of reaction. The oxide-oxide system has no heat of reaction, while the intermetallic has a significant, but modest, heat of reaction. The metal oxide system has a very large heat of reaction. The various observations appear to be consistent with the proposed conceptual models involving configuration, activation, mixing, and heating required to describe the mechanisms of shock-induced solid state chemistry. 

Half reactions involving the oxidation of a metal in aqueous solutions... 

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 order to illustrate this principle, let the effect of temperature on the equilibrium constant of an exothermic reaction, involving the oxidation of a metal to its oxides, be considered. Upon increasing the temperature of this reaction some of the metal oxides will dissociate into the metal and oxygen and thereby reduce the amount of heat released. This qualitative conclusion based on Le Chatelier s principle can be substantiated quantitatively from the Varft Hoff isochore. 

If the electrode is covered with a film, then anodic oxidation of the metal does not involve facetting. The surface of the metal either becomes more rough (if the film is discontinuous) or becomes very lustrous (continuous films). Films, especially continuous films, retard the electrode reaction of metal oxidation. The metal is said to be in its passive state. 

This type of reaction involves both oxidation and addition of groups, so it is known as an oxad reaction. Alkyl derivatives of PC13 can be prepared by the reactions with Grignard reagents and metal alkyls illustrated in the following equations ... 

Redox reactions involving nitric oxide have important implications beyond their fundamental chemistry as demonstrated by the controversy in the biomedical literature regarding conditions under which generation of NO leads to the amelioration or the exacerbation of oxidative stress in mammalian systems (95). Oxidative stress is defined as a disturbance in the balance between production of reactive oxygen species (pro-oxidants) and antioxidant defenses (96). Reactive oxygen species include free radicals and peroxides as well as other reactants such as oxidative enzymes with metal ion sites in high oxidation states. The... 

The Belousov-Zhabotinsky (BZ) reaction involves the oxidation of an organic species such as malonic acid (MA) by an acidified aqueous bromate solution in the presence of a metal ion catalyst such as the Ce(m)/Ce(IV) couple. At excess [MA] the stoichiometiy of the net reaction is... 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

Reaction between metal oxide vapor and solid carbon. A novel method of preparation of ultra-high surface area carbides55-59 involves the reaction of solid carbon with vaporized metal oxide precursors like Mo03 or WO2. The synthesis uses high specific surface area activated carbons and the final product appears to retain a memory of the porous structure of the starting material. The carbon acts like a skeleton around which the carbides are formed, and catalytically active samples with surface areas between 100 and 400 m2g 1 are generated. Materials prepared by this method are described by Ledoux et al. (chapter 20). 

Evidence is now accumulating to show that reactions involving metals might be the common denominator underlying AD and PD. In these disorders, an abnormal reaction between a protein and a redox-active metal ion (copper or iron) promotes the formation of ROS. It is especially intriguing how the antioxidant Cu/Zn-SOD activity can convert into a pro-oxidant activity, a theme echoed in the recent proposal that Ap and PrP, the proteins respectively involved in AD and prion diseases, possess similar redox properties [Bush, 2002],... 

Another common type of reaction involving sulfur compounds is the oxidation of thiols or thiolates to disulfides. This process is found to be very sensitive to the presence of metal ions. The metal can act as the primary oxidant, or dioxygen may be involved in a reaction with a co-ordinated thiol (Fig. 9-10). Very often, oxidation reactions involving metal ions and thiols are catalytic in the metal. 

The use of centrifugation to separate the liquid from solid phases in traditional batch or tube techniques has several disadvantages. Centrifugation could create electrokinetic effects close to soil constituent surfaces that would alter the ion distribution (van Olphen, 1977). Additionally, unless filtration is used, centrifugation may require up to 5 min to separate the solid from the liquid phases. Many reactions on soil constituents are complete by this time or less (Harter and Lehmann, 1983 Jardine and Sparks, 1984 Sparks, 1985). For example, many ion exchange reactions on organic matter and clay minerals are complete after a few minutes, or even seconds (Sparks, 1986). Moreover, some reactions involving metal adsorption on oxides are too rapid to be observed with any batch or, for that matter, flow technique. For these reactions, one must employ one of the rapid kinetic techniques discussed in Chapter 4. 

In recent years, there has been a great deal of interest in the mechanisms of electron transfer processes.52-60 It is now recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. 

During the Gupta Dynasty (320—480 A.D.) the production of iron in India achieved a remarkable degree of sophistication as attested by the Dhar Pillar, a seven-tonne, one-piece iron column made in the fourth century A.D. This implies that the production of metallic iron from the ores was a well-established process, and the people involved at that time were aware of the reverse reaction involving the oxidation of iron to produce the oxide (the familiar rusting of iron). 

Turn now to complex formation and a half-reaction involving two oxidation states of a metal in solution (charges omitted for simplicity) ... 

The forward reaction involves formal oxidation of the metal, accompanied by an increase in coordination number it is an OA. The reverse reaction is an example of RE, which involves a decrease in both oxidation number and coordination number. 

DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species, but differ in their identification of the nature of the adsorbed reactive ammonia (protonated ammonia vs. molecularly coordinated ammonia), of the active sites (Br0nsted vs. Lewis sites) and of the associated reaction intermediates [16,17]. Concerning the mechanism of SO2 oxidation over DeNOxing catalysts, few systematic studies have been reported up to now. Svachula et al. [18] have proposed a redox reaction mechanism based on the assumption of surface vanadyl sulfates as the active sites, in line with the consolidated picture of active sites in commercial sulfuric acid catalysts [19]. Such a mechanism can explain the observed effects of operating conditions, feed composition, and catalyst design parameters on the SO2 SO3 reaction over metal-oxide-based SCR catalysts. 

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