Colloidal mixed-metal oxides

Sadakane, M., Asanuma, T., Kubo, J. et al. (2005) Facile procedure to prepare three-dimensionally ordered macroporous (3DOM) perovskite-type mixed metal oxides by colloidal crystal templating method, Chem. Mater. 17, 3546. 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

Macroporous VPO Phases. - The macroscale templating of bulk mixed metal oxide phases in the presence of colloidal sphere arrays typically consists of three steps shown in Figure 18. First, the interstitial voids of the monodisperse sphere arrays are filled with metal oxide precursors. In the second step, the precursors condense and form a solid framework around the spheres. Finally, the spheres are removed by either calcination or solvent extraction leading to the formation of 3D ordered macroporous structures [137]. 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

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 chemistry described here also opens the door for a new approach to the production of the analogous sulfides, selenides and teUurides [3, 5] Initial experiments showed that it is possible to transform aqueous colloidal Pt02 into colloidal forms of PtS and PtS2 by treatment with Na2S [58]. Mixed metal oxides/sulfides are intermediates, but so far no attempts have been made to isolate and characterize such unusual aqueous colloids. 

If mixed-metal alkoxide precursors can be prepared and their reactivity can be controlled, ordered macroporous mixed metal oxides can be produced by a sol-gel method using a colloidal crystal template. Alkoxides of Ti and Zr with other metals are easily prepared by mixing titanium alkoxide and zirconium alkoxide with alkoxides of other metals or metal salts, and the formed mixed metal alkoxides can infiltrate the voids of templates. Therefore, production of mixed metal oxides with Ti and Zr is straightforward. 

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. 

Methods for preparing bimetallic colloids containing gold have been described in Section 3.2.3. Their composition may be easily tuned since solutions of the mixed chloride precursors are normally used, and are reduced in a variety of ways after addition of a stabiliser. As already mentioned in Section 4.3.6, for gas-phase catalytic reactions, after depositing the colloid on an oxide support the stabilisers must be removed, but for liquid-phase reactions they may be retained providing access to the metal is still possible. 

Since then, other colloidal oxide systems have been investigated in order to prepare ceramic mesoporous membranes designed for ultrafiltration. The preparation of an electronically conductive membrane from a Ru02 Ti02 mixed oxides sol and the application to an electro-ultrafiltration process [25,26], as well as the preparation of titania and zirconia ultrafiltration membranes [27], have been described following a colloidal process in which a partial destabilization of a metal oxide colloidal suspension is used to produce top layers with different pore size and pore volume in the mesoporous range. In agreement... 

Colic, M., Fisher, M.L., and Fuerstenau, D.W, Electrophoretic behavior and viscosities of metal oxides in mixed surfactant systems. Colloid Polym. Sci., 276, 72, 1998. 

The binder system should have a molar ratio of silica to alkali metal oxide which ranges from 3.5 to 10, preferably 3.5 to 7. This ratio is significant because the ratios of soluble potassium, lithium or sodium silicates commercially available as solutions lie within a relatively narrow range. Most of sodium silicates are within the range of Si02/Na20 of about 2 1 to 3.75 1. Thus, overall ratios of binder compositions obtained by admixing colloidal silica, such as ratios of 4 1, 5 1, 7 1 are mainly an indication of what proportions of colloidal silica and soluble silicates were mixed since the amount of amorphous silica in the soluble silicate at ratios of 2 1 to 3.75 1 are small. 

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