Colloids zerovalent transition-metal

Upon the addition of CO or H2 in the presence of appropriate stabilizers, the controlled chemical decomposition of zerovalent transition metal complexes yields isolable products in multigram amounts [49]. The growth of metallic Ru particles from Ru(COT) (COD) (COT = cyclooctatetraene, COD = cycloocta-1,5-diene) with low-pressure dihydrogen was first reported by Ciardelli et al. [49a]. This material was, however, not well characterized, and the colloidal aspect of the ill-defined material seems to have been neglected in this work. Bradley and Chaudret [49b-l] have demonstrated the use of low-valent transition metal olefin complexes as a very clean source for the preparation of nanostructured mono- and bimetallic colloids. 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

In materials chemistry, nanoparticles of noble metals are an original family of compounds. Well-defined in terms of their size, structure and composition, zero-valent transition-metal colloids provide considerable current interest in a variety of applications. Here, the main interest is their application in catalysis. Zerovalent nanocatalysts can be generated in various media (aqueous, organic, or mixture) from two strategic approaches according to the nature of the precursor, namely (i) mild chemical reduction of transition-metal salt solutions and (ii) metal atom... 

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... 

THF as Colloidal Stabilizer for Early Transition Metal Colloids Solvents such as THE or propylenecarbonate can act as powerful colloidal stabilizers. Small sized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THE after [BEtsH J-reduction of the pre-formed THF-adducts (Eq. (2.4)) [82-84, 92]. 

During the last 2-3 decades a vast amount of knowledge regarding the preparation and characterization of nanostructured transition metal colloids in the zerovalent form has accumulated [1, 2]. The emphasis has been largely on the development of methods for the control of size and, more recently, even of shape [3]. In the majority of cases application in catalysis was not pursued systematically, i.e., generally only a simple model reaction such as the hydrogenation of cyclohexene was studied. 

In contrast to nanoscaled transition metal colloids in the zerovalent form, less is known regarding the analogous metal oxides, i.e., colloidal MO (M=metal n= 1, 2, 3, etc.). Of course, such species in solid bulk form or as immobilizates on surfaces or in solid carriers have been known for a long time in heterogeneous catalysis [4], in semiconductor technology and other areas [3, 5]. More recently special methods have been developed for their preparation in constrained environments [6], in microemulsions [7], or as one- or two-dimensional nanostructures [5d, 8]. 

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 next question to be asked concerns the composition of the particles. For the case of monometallic particles this could in principal be a trivial question. If the particle is known to contain a single element, the only question which then arises concerns the oxidation states of the metal, and this can be determined by X-ray photoelectron spectroscopy. For example, the colloidal metals described in Section 6.2.2.1 and prepared by Dye and coworkers by alkalide and electride reduction of salts of gold, copper, platinum, nickel, and molybdenum (as well as several main group metals and metalloids) were analyzed by XPS [73] which showed the presence of only zerovalent metal. Oxidized metal was detected only for nickel and molybdenum (among the transition metals) and this only after exposure to an oxidizing solvent such as methanol. These results show that if a sufficiently powerful reducing agent is used in the colloid synthesis, the surface of the particles can be kept in a reduced metallic state. 

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