Hydrous gold oxide

The mechanism of oxidation of D-glucose in alkaline media at gold electrodes has been investigated. Experiments using cyclic voltammetry at a rotating disc electrode indicated that the mass-transport-limited reaction proceeds an enediol intermediate hydrogen-bonded to catalytic hydrous gold oxide. The enediol is... 

Carbonate ion, like OH", precipitates hydrous gold oxides. 

Hydrous oxides are of major interest in many areas of technology, e.g., corrosion and passivation of metals, formation of decorative, protective, and insulating films, aqueous battery systems, catalysis and electrocatalysis, electrochromic display systems, pH monitoring devices, soil science, colloid chemistry, and various branches of material science. Detailed accounts of some of the nonnoble hydrous metal oxide systems, especially aluminum,1 have appeared recently. In the case of the noble metals such as platinum or gold most of the electrochemical work to date has been concerned with compact monolayer, and submonolayer, oxide growth. 

At gold oxide, an insulating oxide is formed up to 17 = 2.0 V. Then, the valence band reaches the Fermi level, and oxygen evolution and corrosion starts. A conducting hydrous layer is formed at a constant rate [108]. The electrode capacity remains constant, since the potential drop across the inner oxide is electronically limited, and the outer hydrous layer is ionically conducting. ETR can take place at the inner AU2O3/Au(OH)3-interface. 

Until comparatively recently, the only reported stable inorganic hydrosols were primarily sols of elements such as gold, sulphur, selenium, etc. and compoimds such as silica, lead iodate, silver halides, etc. A considerable amount of attention is now being paid, however, to the preparation of mono-dispersed hydrous metal oxides, which are chemically considerably more complex than other crystalline or stoichiometrically well-defined materials and are of interest as potential catalysts. Examples include the hydrous oxides of chromium and aluminium (spheres) and copper and iron (polyhedra) with particle sizes < 1 pm. One manufacturing procedure consists of ageing aqueous... 

The gold oxides, hydrous or anhydrous, dissolve inHX, e.g. ... 

The action of alkali on aqueous Au " solutions produces a precipitate, probably of AU2O3.JCH2O, which on dehydration yields brown AU2O3. This is the only confirmed oxide of gold. It decomposes if heated above about 160°C and, when hydrous, is weakly acidic, dissolving in cone alkali and probably forming salts of the [Au(OH)4] ion. 

The tendency of gold(III) chlorocomplexes to adsorb on iron (hydrous) oxides that tend to form via in situ precipitation in acidic pressure leaching operations needs to be controlled to avoid undesirable metal losses. 

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. 

Studies of 2.65 Ga gold systems in the Yilgarn Craton, Western Australia, have emphasised the interaction of intrinsically reduced (CH4 H2 N2 HCI) and oxidized (CO2 SO2) anhydrous fluids as well as hydrous fluids in deposit formation. Carbonate veins have between -4 and -6 %o, typical of mantle values and primary sulfate have of 8 to 12 %o. 

The generation of hydrous films on platinum at low potentials on the anodic sweep seems quite feasible from a thermodynamic viewpoint—especially in base. The inhibition here is evidently related to the need for six hydroxide ions to have access to coordination sites at the same platinum atom—an improbable condition for a metal atom in a regular surface site. Evidence will be presented later indicating that such hydrous oxide formation can, to a very limited extent, precede monolayer formation in the case of gold— the atoms of the latter involved in formation of the hydrous material are presumably at low lattice coordination sites on the surface. There is some evidence from recent single-crystal studies (see Section XIV) for this type of behavior in the case of platinum—it is obviously difficult to detect with polycrystalline substrates as with only a small fraction of a monolayer involved optical techniques would need to be extremely sensitive and electrochemical procedures are hampered by the fact that the redox behavior of the hydrous material coincides with that for adsorbed hydrogen. 

Thick hydrous oxide films on gold can readily be produced, as in the case of platinum, by either dc or potential cycling techniques. 

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