Formation of Hydrous Oxides

Hydrous oxide preparation is a routine procedure in gravimetric analysis, the transition metal ions usually being precipitated in hydrous form by addition of base. While oxide suspensions pre-oared in this manner may be used to examine double layer 

Cathodic electroprecipitation is a technique used commercially to prepare nickel hydroxide deposits in the battery field.25 In this case a nickel salt is present in solution at low pH (ca. 3.0) and hydrogen gas evolution around the cathode causes a local increase in pH, resulting in the precipitation of an adherent layer of nickel hydroxide at the metal surface. Similarly, anodic electroprecipitation is used commercially26 to produce layers of another highly active battery material, y-MnO2. 

The mechanism of hydrous oxide growth on repetitive cycling is now reasonably well understood, at least at a qualitative level. 

One of the most versatile and convenient techniques used in recent times to generate hydrous oxides in a form suitable for examining their redox behavior is that of potential cycling. 

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Studied. The phenomena observed were explained similarly to other noble metals by the assumption of the role of premonolayer oxidation and formation of hydrous oxides. 

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

The properties of the el ectrical doubl e 1 ayer (EDL) have been the subject of considerable research (1,3,5,8,10). Unlike reversible electrodes, where surface potential is controlled and charge develops in response to changes in electrode potential, mineral surfaces develop potential in response to the formation of surface charge (8). On the surface of hydrous oxides, for example, hydroxyl groups... 

The formation of Au-OHad or surface oxides on gold in alkaline electrolyte was in fact proposed to explain some of the electrocatalytic properties observed for a gold electrode (e.g., incipient hydrous oxide/adatom mediator model ). Our previous measurement of the interfacial mass change also indicated the formation of Au oxides (AU2O3, AuOHorAu(OH)3) on gold nanoparticle surfaces. A detailed delineation of the catalytic mechanism is part of our on-going work. 

Processes controlling metal ion attenuation in add mine drainage streams. Geochim. Cosmochim. Acta 47 1957-1973 Charlet, L. A. Manceau (1993) Structure, formation, and reactivity of hydrous oxide particles Insights from x-ray absorption spectroscopy. In Buffle, J. van Leeuwen, H.P. 

Temperature and air feed is carefully controlled in the process to suppress any formation of antimony tetroxide (86204). Antimony trioxide is separated from any arsenic trioxide (AS2O3) that may be present as an impurity by volatilization, as the latter is much more volatile than the former. It may be also prepared by alkaline hydrolysis of antimony trichloride and subsequent dehydration of hydrous oxide under controlled heating (rapid or vigorous heating may partially oxidize 8b(III) to Sb(V). 

Figure 2 The fate of phosphorus during soil formation can be viewed as the progressive dissolution of primary mineral phosphorus (dominantly apatite), some of which is lost from the system by leaching (decrease in Ptotai). and some of which is reincorporated into nonoccluded, occluded, and organic fractions within the soil. Nonoccluded phosphorus is defined as phosphate sorbed to surfaces of hydrous oxides of iron and aluminum, and calcium carbonate. Occluded phosphorus refers to phosphorus present within the mineral matrix of discrete mineral phases. The initial buildup in organic phosphorus results from organic matter return to soil from vegetation supported by the soil. The subsequent decline...

Charlet, L., and Manceau, A. (1993) Stracture, Formation and Reactivity of Hydrous Oxide Particles Insights from X-ray Absorption Spectroscopy. In Environmental Particles, Vol. 2, J. Buffle and H. P. Van Leeuwen, Editors. Lewis, Boca Raton, FL. 

After the water was collected in the reaction vessel, 288 ppm ferrous sulfate was added. Addition of a stoichiometric quantity (100 ppm) of potassium permanganate thus resulted in formation of hydrous ferric and manganese oxides. Scavenging eflBciencies for this precipitation reaction have been reported previously (5, 6). The resulting suspension was agitated for 10-20 min and allowed to settle for 12-24 hr, after which the supernatant hquid was siphoned. The precipitate was removed and... 

Amorphous aluminum oxide has recently been proved to extract lithium from brines and bitterns having lithium concentrations of 0.83 and 13.1 mg/1, respectively. The sorption may be explained by the formation of hydrous lithium aluminum oxide. The sorption capacity of amorphous hydrous aluminum oxide was found to be 4.0 mmol/g. For brines and bitterns the lithium concentration factors on the sorbent attained values of 370 and 130, respectively equilibrium was reached after 7 days. The desorption of lithium ions was carried out with boiling water yielding a maximum concentration factor of lithium in the eluate of 46 in reference to the initial lithium concentration of the brines. Lithium was separated from the eluates by solvent extraction with cyclohexane containing thenoyltrifluoracetone and trioctyl-phosphine oxide, subsequent back extraction with hydrochloric acid, and precipitation of lithium phosphate by addition of K3P04. The purity of the precipitate amounted to at least 95% I7 21). 

Cationic Surface Complexes. The first paper of the series (4), in i section entitled Preliminary Approach to the Interfacia] Coordination Chemistry of Hydrous Oxides, addressed the mechanism of cation adsorption v hvdroxylated mineral surfaces. Here the term coordination was reserved -r complex formation between cations and surface functional groups through b inding that can be either electrostatic or covalent, or a mixture of both. The dichotomous qualification is much in the spirit of the classic Stem (11) ricture of strong adsorption. 

Titanium dioxide can be improved by doping with metals. Titanium dioxide participates in photochemical processes. Its mechanism involves the formation of positive holes in the valence band and electron promotion to the conductive band irradiated by UV. Both electrons and holes react with the surrounding material. By doping Ti02 crystals with various metals, electron and hole recombination centers are formed. Also, the crystal is coated with a layer of hydrous oxides which decompose hydroxyl radicals. This is applied to various grades of Ti02 which gives them a unique performance in applications where UV durability is required. 

The production of catalyst particles of suitable configuration and hardness is an essential part of catalyst manufacture . Most heterogeneous catalysts are produced by processes that involve formation of solids from aqueous solutions. Precipitation is frequently employed in preparation of hydrous oxide catalysts. To avoid occluded or adsorbed impurities, ammonia or ammonium salts are often used as well as nitrates of the desired metal constituents. Calcination removes the nitrogen-containing components. Anions such as Cl or 864 or cations such as Na are avoided these often are poisons if they are present in the final catalyst. 

Gel formation is especially well suited to the preparation of hydrous oxide catalysts such as those of Si, Al, Fe. Also, an all-embracing gel can be used to produce the catalyst particle in a preferred size and spherical form by allowing droplets of the recently rapidly... 

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