Simple oxides

The theory of catalysis on oxides has been developed by Hauffe [38], Wolkenstein [39], and others. Normally, the oxides applied in catalysis are semiconductors with chemical formulas AO, A02, A203, A304, etc., where A denotes the metallic element. 

The composition of these oxides normally departs from the precise stoichiometry, expressed in their chemical formulae. For example, in the case of a stoichiometric oxide, such as A05, where 8 = 0, we will have only thermal disorder, where the concentration of vacancies, and interstitials will be determined by the Schottky, Frenkel, and anti-Frenkel mechanisms [40-42] (these defects are explained in more detail in Chapter 5). In the case of the Schotky mechanism, the following equilibrium, described with the help of the Kroger-Vink notation, [43] develops [40] 

The Physical Chemistry of Materials Energy and Environmental Applications 

The catalytic activity of these simple oxides has been correlated [38,39] with its ability to chemisorb simple molecules such as CO and N20 via an electron transfer, which results in a change in the electron transport properties in the semiconductor solid oxide. In this sense, the catalytic activity of simple oxides has been correlated with the band gap, and the number of d-electrons for 3d metal oxides [22], 

The Metal(iv) Oxides and Related Oxide Phases.—Simple Oxides. By far the 

Attar-Bashi, C. Eabom, J. Vend, and D. R. M. Walton, J. Organometallic Chem., 1976,117, 

Sawyer, W. B. Ewert, and G. M. Brissey, Synth. React. Inorg. Metal-Org. Chem., 1976,6,337. 

Guiller, M. Coudurier, and J. B. Donnet, Bull Soc. chim. France, 1975, 1563. 

A simple oxide catalyst can be used in either the bulk state or supported on an inert oxide support material. The bulk oxides are usually prepared using a precipitation-calcination sequence similar to those described in Chapter 9 for the preparation of support oxides. In general, the simple semiconductor oxides are not very good catalysts for synthetic reactions. The insulator oxides, however, can be used as solid acids and bases for a number of reactions. Alumina has been used as an acid catalyst for the vapor phase rearrangement of cyclohexanone oxime to caprolactam (Eqn. 10.9). Modification of the y-alumina surface by the addition of 10-20% of B2O3 increased its activity for this reaction, giving caprolactam in 80% selectivity even after several hours of continuous operation.  

Basic oxides, particularly MgO and BaO, have been used to promote a number of base catalyzed reactions.2.3 To be effective the oxide must first be heated to remove the water, carbon dioxide and oxygen that may have adsorbed 

The only thermodynamically stable crystallographic modification of alumina (AI2O3) is a-Al203, or known as corundum. Comndum has a hexagonal crystal 

Optical and mechanical properties of transparent AI2O3 ceramics are highly dependent on their grain size and residual porosity. Various strategies have been employed to control the grain sizes and minimize the residual porosity. For this purpose, fine-grained transparent AI2O3 ceramics have recently attracted much 

Effects of the nature of the dopants, thermal pretreatment, and sintering temperature on SPS of transparent alumina have been systematically investigated [68]. A slurry of a-Al203 was doped with Mg, Zr, and La nitrates or chlorides, with concentrations of 150-500 weight ppm and then freeze-dried to produce nanosized doped powders ( 150 nm). The powders were sintered by SPS to yield transparent polycrystaUine alumina ceramics. Transparency of the nanosized AI2O3 ceramics was shown to depend mainly on the way the powder was prepared, as well as the nature of the dopants. RIT values at 640 nm of the samples doped with Zr02, MgO, and La203 were 40.1, 44.1, and 48.1 %, as compared to 30.5 % for pure alumina. 

A two-step pressing method was reported to be able to significantiy improve the optical properties of alumina ceramics with SPS at high heating rates [69]. In this case, commercial alumina powder could be consolidated at 1150 °C at a heating 

Besides the applications for lighting and domes, transparent nanostructured Y-AI2O3 ceramics have various other applications, such as a humidity sensor of Y-AI2O3 [75]. Nanostructured Y AlaOs ceramics were prepared from Al-Sec-Butoxide (C12H27AIO3) by using a sol-gel process. The sensors had a long term stability of up to two years. 

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Variable oxidation state is also exhibited in the oxides themselves among metals in this region of electronegativity. Thus lead, for example, forms the monoxide PbO (+2) and the dioxide PbO 2 ( + 4) (the compound Pbj04 is not a simple oxide but is sometimes called a compound oxide). Similarly, manganese gives the oxides MnO and Mn02-... 

Type 1, simple oxides Type 11, mixed oxides... 

Chemisorption of alkanethiols as well as of di- -alkyl disulfides on clean gold gives indistinguishable monolayers (251) probably forming the Au(l) thiolate species. A simple oxidative addition of the S—S bond to the gold surface is possibly the mechanism in the formation of SAMs from disulfides ... 

Butyric acid, the simple oxidation product of -butyraldehyde, is used chiedy in the production of cellulose acetate butyrate [9004-36-8]. Sheets of cellulose acetate butyrate are used for thermoformed sign faces, bUster packaging, goggles, and face shields. 

Isobutyric acid, the simple oxidation product of isobutyraldehyde, is employed in the esterification of TMPD to form the mono- and diesters of TMPD. Some isobutyric acid is also used in the production of isobutyronittile, an organo-phosphate pesticide precursor. 

Catalysis by Metal Oxides and Zeolites. Metal oxides are common catalyst supports and catalysts. Some metal oxides alone are industrial catalysts an example is the y-Al202 used for ethanol dehydration to give ethylene. But these simple oxides are the exception mixed metal oxides are more... 

Cobalt(II) chloride hexahydrate [7791-13-1], C0CI2 6H20 is a deep red monoclinic crystalline material that deflquesces. It is prepared by reaction of hydrochloric acid with the metal, simple oxide, mixed valence oxides, carbonate, or hydroxide. A high purity cobalt chloride has also been prepared electrolyticaHy (4). The chloride is very soluble in water and alcohols. The dehydration of the hexahydrate occurs stepwise ... 

Cobalt(II) nitrate hexahydrate [10026-22-9], Co(N02)2 6H20, is a dark reddish to reddish brown, monoclinic crystalline material containing about 20% cobalt. It has a high solubiUty in water and solutions containing 14 or 15% cobalt are commonly used in commerce. Cobalt nitrate can be prepared by dissolution of the simple oxide or carbonate in nitric acid, but more often it is produced by direct oxidation of the metal with nitric acid. Dissolution of cobalt(III) and mixed valence oxides in nitric acid occurs in the presence of formic acid (5). The ttihydrate forms at 55°C from a melt of the hexahydrate. The nitrate is used in electronics as an additive in nickel—ca dmium batteries (qv), in ceramics (qv), and in the production of vitamin B 2 [68-19-9] (see Vitamins, VITAMIN B22)-... 

Although our simple oxide film model explains most of the experimental observations we have mentioned, it does not explain the linear laws. How, for example, can a material lose weight linearly when it oxidises as is sometimes observed (see Fig. 21.2) Well, some oxides (e.g. M0O3, WO3) are very volatile. During oxidation of Mo and W at high temperature, the oxides evaporate as soon as they are formed, and offer no barrier at all to oxidation. Oxidation, therefore, proceeds at a rate that is independent of time, and the material loses weight because the oxide is lost. This behaviour explains the catastrophically rapid section loss of Mo and W shown in Table 21.2. 

In 1931 Ing pointed out that formula (II) and (III) do not contain methyl or potential methyl groups in j ositions 6 and 8 which they occupy in cytisoline. Further, a partially reduced quinoline ought to oxidise easily to a benzenecarboxylic acid and so far the only simple oxidation, products recorded from cytisine were ammonia, oxalic acid and isovaleric acid. Distillation of cytisine with zinc dust or soda-lime yields pyrrole and pyridine, but no quinoline. On these grounds Ing suggested that cytisine should be formulated without a quinoline nucleus, and that the reactions which indicate the presence of an aromatic nucleus in the alkaloid can be accounted for by an a-pyridone ring. This a-pyridone nucleus can... 

All of these ehimnddon reacdons contain fi-carbonyl groups in the nltro compounds Of course, masked carbonyl groups are also frequently employed for such fi-elimination of HNO, as shown in Eq 7131, Eq 7 133, and Eq 7 133In these cases, the sulfinylmethyl or hydroxymethyl group is converted into the carbonyl group by the Pummerer rearrangement or by simple oxidation... 

In the gas-cooled reactor, reaction.between the coolant and the moderator results in formation of a proportion of carbon monoxide in the atmosphere. This gas can be carburising to nickel-base alloys but the results of tests in which CO2 was allowed to react with graphite in the furnace indicate that the attack on high-nickel alloys is slight, even at moderately high temperatures and is still mainly due to simple oxidation. 

The hydrofluoride method can be used successfully both for the preparation of complex fluoride compounds and of complex oxides. The main advantage is that the synthesis is performed at relatively lower temperatures. In addition, the complex oxide material is formed through its respective fluoride compound and the product obtained is therefore more consistent. For instance, Co4Nb209 can be prepared using the hydrofluoride method at 900-1100°C, whereas the regular synthesis, based on the interaction of simple oxides, requires extended treatment at about 1400°C. 

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

In a very recent study, it has been demonstrated116 that zinc 5,15-bis(3,5-di-tert-butylphenyl)-porphyrin (13) without any activating halogen atoms at the chromophore can be directly linked in a very simple oxidative coupling reaction with silver(I) hexafluorophosphate to a mixture of porphyrin dimers, trimers and tetramers. The separation of the product mixture was achieved by gel-permeation chromatography based on the molecular weights of the oligomers. The dimer when re-exposed to the same reaction conditions yielded 25% of the tetramer.116... 

The conversion of the dehydrotrimer 135 into the corresponding bis-cuprate followed by coupling with dibromide 131 (Cadiot-Chodkiewicz conditions) gave the expanded [5]pericycline 122 in 53% isolated yield (Scheme 28) [4]. The more versatile approach by simple oxidative cyclooligomerization of dehydrooligomers of type 135 under high dilution conditions as shown in Scheme 28 provided the acetylene-expanded [3]- 82, [5]- 122 and [6]pericyclines 163 in reasonable to excellent yields [4,7]. 

Silver does not form a simple oxide by direct oxidation in air, but the metal does form a black tarnish with oxygen... 

Westheimer ° has reviewed other inductions of the chromic acid oxidation of iodide, indicating how these reactions afford insight into the mechanism of the simple oxidation. 

Since all the oxidations follow the same basic kinetics and involve an oxidising metal ion in trace quantities, the details are not reiterated here. The simple oxidation of Ag(I) by persulphate is discussed on p. 475. 

Reasonable NO conversion can be achieved using n-decane as reductant. In the absence of sulfur dioxide, the catalytic activity is roughly related to the r ucibility of the Cu phase of Cu ions in zeolites the reaction temperature needed to reach 20% NO conversion parallels that of the TPR peak (Table 7). This relation also practically holds for Cu on simple oxides, therefore a redox mechanism in which reduction of Cu + cations is the slow step could account for the results. 

We distinguish electrodes consisting of simple oxides, from those consisting of complex oxide systems. The latter include cations of different metals or cations of a given metal in different valence states. An example for the latter type is cobalt cobaltite C03O4 (a spinel structure) containing Co and Co ions. 

Simple Oxides of Base Metals Electrodes of lead dioxide, Pb02, which in contrast to other base-metal oxides are stable in sulfuric acid are an example for a simple oxide system. In a number of cases, this electrode serves as the anode in the electrosynthesis of organic compounds in acid media. 

Nickel oxide anodes are another example for a relatively simple oxide electrocatalyst used rather widely in the oxidation of organic substances (alcohols, amines, etc.) in alkaline solutions at relatively low anodic potentials (about +0.6 V RHE). These processes, which occur at an oxidized nickel surface, are rather highly selective. As an example, we mention the industrial oxidation of diacetone-L-sorbose to the corresponding acid in vitamin C synthesis. This reaction occurs at nickel oxide electrodes with chemical yields close to 100%. 

Although the band model explains well various electronic properties of metal oxides, there are also systems where it fails, presumably because of neglecting electronic correlations within the solid. Therefore, J. B. Good-enough presented alternative criteria derived from the crystal structure, symmetry of orbitals and type of chemical bonding between metal and oxygen. This semiempirical model elucidates and predicts electrical properties of simple oxides and also of more complicated oxidic materials, such as bronzes, spinels, perowskites, etc. 

The chemical compositions of materials are usually expressed in terms of simple oxides calculated from elemental analysis determined by x-ray fluorescence. For spent foundry sand, the chemical parameters include bulk oxides mass composition, loss on ignition, and total oxygen demand. Table 4.6 lists the general chemical properties of spend foundry sand. It is shown that spent foundry sand consists primarily of silica dioxide. 

Site-binding constants have been determined for only a limited range of simple oxides with only one type of surface site. Multiple-surface site minerals occurring in the deep-well environment such as silicates, aluminosilicates, and complex oxides (such as manganese oxide) will require much more complex TLMs. 

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