Aqueous colloidal metal oxides

Using the simple hydrolysis/condensation of metal salts under basic aqueous conditions in the presence of carbo- or sulfobetaines, respectively, colloidal Pt02 was obtained, the analytical data of which correspond to pure a-Pt02 and to commercial samples of Adams catalyst. Bi- and trimetallic colloidal metal oxides as precursors for fuel-cell catalysts, e.g., colloidal Pt/RuO and Pt/Ru/WO, have also been prepared this way [5If]. 

This chapter focuses mainly on the author s own contributions in the area of aqueous nanoscaled transition metal oxides. Specifically, the preparation in high concentration and application of aqueous colloids comprising metal oxide and mixed metal oxide nanopartides will be described. The chapter begins with a short overview of the author s earlier work on methods for size- and shape-selective preparation of transition metal (zerovalent) colloids, because this led to the development of simple and practical ways to prepare the corresponding aqueous transition metal oxide coUoids. 

The 3M Company manufactures a continuous polycrystalline alurnina—sihca—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal sihca and dimethylform amide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extmded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-p.m diameter aluminum borosihcate fibers, which are suitable for fabrication into textiles for use at temperatures up to 1427°C. 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Jambor, J.L. Dutrizac, J.E. (1998) Occurrence and constitution of natural and synthetic fer-rihydrite, a widespread iron oxyhydroxide. Chem. Rev. 98 2549-2585 James, R.O. ElealyT.W. (1972) Adsorption of hydrolyzable metal ions at the oxide-water interface. Ill A thermodynamic model of adsorption. J. Colloid Interface Sci. 40 65-81 James, R.O. Parks, G.A. (1982) Characterization of aqueous colloids by their electrical double layer and intrinsic surface chemical properties. Surface Colloid Sci. 12 119-126... 

Metal oxide, e.g. RUO2 [35,36] and WO3 [151], loaded Cd8 colloidal suspensions with and without Ft functionalization were investigated for water decomposition under visible light. WO3-Pt/Cd8, in an aqueous solution of methyl viologen (MV ) which serves as an electron relay, has been found most efficient to date in terms of water splitting, see reactions (7.4.1) to (7.4.8) [151] ... 

These metal and metal oxide catalysts must work as a kind of electron pool which brings about multi-electron process for H2 and 02 generation. Silver colloids were studied as electron pool for H2 formation under y-irradiation in the aqueous system composed of Ag° colloids, acetone, 2-propanol and SDS S9). The colloids (average diameter 140 A) of 2.5 x 10 4 M can store 1 coulomb/1, corresponding to the storage of 450 electrons/particle 60 ... 

P. Bamhard et al, Aqueous Slurry Explosives with Colloidal Hydrous Metal Oxide , USP 4058420 (1976) CA 88, 52662 (1978)... 

Usually there is a lot of effort required to make nanomaterials by electrochemical means. In aqueous solutions the electrodeposition of nanocrystalline metals requires pulsed electrodeposition and the addition of additives whose reaction mechanism hitherto has only been partly understood (see Chapter 8). A further shortcoming is that usually a compact bulk material is obtained instead of isolated particles. The chemical synthesis of metal or metal oxide nanoparticles in aqueous or organic solutions by colloidal chemistry, for example, also requires additives and often the desired product is only obtained under quite limited chemical conditions. Changing one parameter can lead to a different product. 

Finally we shall argue that present-day theories of the nonprimitive models of the electric double layer have considerable difficulty in treating properly ion adsorption in the Stern inner region at metal-aqueous electrolyte interfaces and we suggest that this region is a useful concept which should not be dismissed as unphysical. Indeed Stern-like inner region models continue to be used in colloid and electrochemical science, for example in theories of electrokinetics and aqueous-non-metallic (e.g., oxide) interfaces. 

Amberger 6 has used lanolin as protective colloid. The lanolin is impregnated with an aqueous solution of a platinous salt, and intimately mixed with the requisite quantity of alkali hydroxide to precipitate out the platinum as hydrated oxide. This is reduced to colloidal metallic platinum by hydrazine hydrate. Both the lanolin and the colloidal metal are dissolved by light petroleum or chloroform, and the whole of the metal with a portion of the lanolin may be reprecipitated on addition of alcohol—a reaction affording a means of increasing the concentration of the metal in the preparation. 

Surface functional groups in the suspended particles determine the type of process that will take place. In the case of inorganic colloidal particles (e.g., clays) the main functional groups are silanol (=Si — OH) and aluminol (=AI — OH), whereas in metal oxides or hydroxides the functional groups are (=M — OH). These groups may become protonated or deprotonated, depending on the pH of the aqueous medium, by sorption of H+ or OH- ions as follows ... 

Dispersion stability is critically important for all CMP slurries. For example, particle-particle aggregation may significantly increase the mean particle size of a slurry and result in surface defects such as scratches and delamination. Severe aggregation can also accelerate the settling of slurry, which can drastically alter the physical and chemical characteristics of a slurry. Therefore, a proper dispersion of metal oxide or abrasive particles in an aqueous environment is fundamentally important for CMP applications [58]. One of the most effective and commonly used methods for the stabilization of a colloidal dispersion involves the use of surfactant. 

An alternative way to achieve the photodissociation of water consists in the use of aqueous suspensions of powdered or colloidal semiconductors, in general loaded with noble-metal and/or noble-metal-oxide catalysts which act as short-circuited photoelectrolysis cells. Titanium dioxide was certainly (and is still being) the semiconductor most frequently employed in such systems. 

Crawford, R.J., Harding, LH., and Mainwaring, D.E., The zeta potential of iron and chromium hydrous oxides during adsorption and coprecipitation of aqueous heavy metals, J. Colloid Interf. Sci., 181, 561, 1996. 

Towle SN, Bargar JR, Brown GE Jr, Parks GA (1999b) Sorption of Co(II) on metal oxide surfaces II. Identification of Co(II)(aq) adsorption sites on (1-102) and (0001) surfaces of a-Al203 by grazing-incidence XAFS spectroscopy. J Colloid Interface Sci 217 312-321 Trainor TP (2001) X-ray Scattering and X-ray Absorption Spectroscopy Studies of the Structure and Reactivity of Aluminum Oxide Surfaces. Ph.D. Dissertation. Department of Geological Environmental Sciences, Stanford University, Stanford, CA, USA Trainor TP, Brown GE Jr, Parks GA (2000) Adsorption and precipitation of aqueous Zn(II) on alumina powders. J Colloid. Interface Sci 231 359-372... 

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