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Chapter 1
POLYOXOMETALATE-PROTECTED METAL NANOPARTICLES: SYNTHESIS, STRUCTURE AND CATALYSIS
YIFENG WANG
School of Chemistry and Chemical Engineering, Shandong University,
Jinan 250199, PR China
IRA A. WEINSTOCK
Department of Chemistry, Ben Gurion University,
POB 653, Beer Sheva, 84105 Israel
1.Introduction
Metal-oxygen cluster–anions (polyoxometalates, or POMs)
1,2 constitute a large and rapidly growing class of discrete molecular structures with applications ranging from catalysis
3,4 to functional materials.
5 POMs are inexpensive, minimally or non-toxic, negatively-charged clusters com-prised of early-transition metals, usually in their d
0 or d
1 electronic configurations (e.g., V(V), Mo(VI) and Mo(V), or W(VI)), bridged by oxygen atoms (formally O
2-, or occasionally HO
-, ions). Representative POMs range from small isopolyanions, such as
(<1 nm in diameter), slightly larger Keggin or Wells-Dawson heteropolyanions (e.g.,
and
), and partially reduced “wheel-like” oxomolybdate nanoclusters
6,7 that contain up to 176 Mo atoms (4.1 nm in diameter
8). These and related polyoxoanions, and derivatives prepared by incorporation of main-group, transition-metal,
9,10 or f-block
11–14 cations, are used as molecular models of magnetic oxides,
15 and in applications from catalysis
3,16–20and electron transport in fuel cells,
21,22 to the design of functional-nanocomposite materials.
23These and thousands of other POMs can be prepared in quantity via either traditional (serial and kinetically controlled) or thermodynamic (one-pot) syntheses in water. As a class, POMs possess extensive and reversible redox chemistries,
24–26 which is central to their use in many applications.
25,27–30Their reduction potentials, acidities, and other key properties relevant to catalysis and materials science, can be extensively yet readily altered by the elemental composition of the POM cluster itself. In addition, because many redox-active POMs are well defined and stable in solution, they are deployed as physico-chemical “probes” of electron-transfer processes.
31–34As molecular anions, it is not surprising that polyoxometalates can stabilize colloidal metal nanoparticles in solution. And, given the ease with which POMs can be converted from water-soluble to organic-solvent soluble forms (as a function of their counter cations), they have been used with considerable success to stabilize metal nanoparticles in both media. While individual bonds between the oxide ligands of polyoxometalates and metal(0)-nanoparticle surfaces appear to be considerably weaker than those, for example, between alkanethiol ligands and gold(0) surfaces, data from numerous publications demonstrate that POMs can be far more effective than simple organic or inorganic ions such as citrate or phosphate.
Studies of POMs on planar metal and graphite surfaces, by Anson,35 Nadjo and Keita,36–38 Klemperer,39 Gewirth40–42 and Barteau,43,44 and others,45 demon-strated the formation of POM monolayers with well-defined packing geometries. During that time, Finke46–49 demonstrated that POMs stabilized Ir(0) and Rh(0) nanaoparticles in organic solvents through a combination of the electrostatic mechanisms common to anion-stabilized colloids and the steric mechanisms typical of, for example, alkanethiol-protected gold(0) nanoparticles. The combined electrostatic and steric mechanism was proposed in response to data indicating direct bonding between POMs and the metal(0) nanoparticle surface. These direct interactions are entirely consistent with the formation of highly stable POM monolayers on planar surfaces.
More recently, Weinstock used cryogenic transmission electron microscopy ( cryo-TEM) to image POM-stabilized Ag(0) and Au(0) nanoparticles in water.50,51Well-defined POM monolayers were observed covering the nanoparticles. The close distances between POMs on the metal(0) surfaces suggested that POM counter-cations must be extensively incorporated as critical structural components of the POM monolayer. This role is analogous to that of counter-cations in POM monolayers on planar surfaces, and between oxomolybdate macro-ions in single-walled POM vesicles.52 Hence, the stability of POM-protected metal(0) nanoparticles likely derives from several phenomena, namely, electrostatic forces typical of anion-stabilized colloids, direct bonding between POMs and the metal(0) surfaces, and the formation of numerous ionic and hydrogen bonds between POMs bound closely to one another on the metal(0) surface. This unique class of metal-nanoparticle structures is the subject of this chapter.
The chapter is divided into three sections, which concern the synthesis, structure and reactivity of POM-stabilized metal(0) nanoparticles. Unlike most nanoparticle-protecting ligands, POMs are redox active, and their reduced forms can reduce metal cations to colloidal metal(0), which are then stabilized by the oxidized POM anions. These methods are described, along with reductions of metal cations to metal(0) nanoparticles by traditional reducing agents, but in the presence of POM ligands. The second section, on the structures of POM-stabilized metal(0) nanoparticles, includes important observations from many groups, along with a detailed summary of new findings obtained with the aid of cryo-TEM imaging. Unlike alkanethiol-Au(0) nanoparticles, the POM-protected metal(0) particles are soluble analogs for the metal(0)/metal-oxide interfaces in heterogeneous metal-oxide supported metal(0) catalysts. Because of this, and given the redox behavior of the POMs themselves, POM-protected metal(0) nanoparticles have been investigated for use as catalysts for selective oxidations and other reactions. Results from those studies, reviewed in the last section of this chapter, indicate that POM-protected metal(0) nanoparticles can serve as active catalysts with unique selectivities, and in some cases, unprecedented rates and stabilities. Recent information concerning the detailed structures of POM-protected metal(0) nanoparticles provide new options for establishing structure–reactivity relationships for this versatile clas...
