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Figure 64. AFM images of Ge/GeO2 sample obtained before (a) and after (b) indentation experiments and the variation of the hardness with the contact depth. Reprinted with permission from [24], S. Mathur et al., Mafer. Sci. Forum 386-388, 341 (2002). © 2002, Trans Tech Publications.

for example, thermolysis of BaSn2(OBu')6 (formed by the Lewis acid-base reaction of Ba(OBu')2 and Sn(OBu')2 [325]) produces a two-phase material (Sn/BaSnO3) where metallic tin particles are dispersed in a barium stannate matrix [604]. Veith et al. have extended this approach to those metal centers for which a meta-stable oxidation state is not easily accessible [605] by incorporating reducing lig-ands into the precursor. Thus, hydride-modified aluminum ferf-butoxide, [H2Al(OBu')]2, disproportionates upon pyrol-ysis (450-550 °C) to yield an Al/Al2O3 composite [605]. A similar observation was made with the Ga compound [H2Ga(OBu')]2 (Table 4), which produced a Ga/Ga2O3 composite [302].

Silica-supported metal particles constitute a large class of important heterogeneous catalysts [532, 606]. The efficiency of such catalytic systems depends on the size and shape of the metal particles and on the nature of particle-support interactions. For this reason, considerable attention has been paid to the development of synthetic strategies to tailor metal/silica (M/SiO2) composite materials.

Lukehart et al. have incorporated complexes of various metals (Ge, Ag, Cu, Os, Pd, Pt, Re, and Ru) in silica xero-gel matrices (Table 4) to obtain molecularly doped xerogels [294, 303].

Scheme 30.

H+ or base catalyst

SO xerogel*x"M"

Subsequent thermal treatment of the doped xerogels afforded metal nanoclusters highly dispersed in the bulk of the xerogel matrix [294, 303], which is chemically controlled because of the covalent bonding of the metal complex in the matrix (Scheme 30). When a dinuclear complex such as ds-Pt(PPh3)2-(Ph)(SnPh2Cl), with a bimetallic core (Pt:Sn = 1:1) is used, a nanocomposite containing nanocrystals of PtSn (niggliite) intermetallic dispersed in silica matrix is formed (Fig. 65). The selective formation of single-phase PtSn particles demonstrates that control over the composition of the nanomaterial can be achieved through proper choice of the core structure of the molecular precursor [607].

Tilley et al. have used tris(alkoxy)siloxy derivatives of different metals such as Cu, Al, Ti, Zr, Hf, V, and Cr to obtain metal/silica and metal oxide/silica nanocomposites [103, 249, 299, 301, 311, 313] and have investigated their catalytic behavior. The molecular structure of the copper(I) derivative, [Cu(OSi(OBu')3]4, is shown in Figure 66.

The precursors of the general formula M[OSi(OBu')3]„ are well-defined oxygen-rich compounds that can be ther-molyzed to amorphous, homogeneous metal oxide/silica materials at low temperatures, via the elimination of isobutene and water. For example, the group 4 complexes, M[OSi(OBu')3]4 (M = Ti, Zr, Hf), convert readily to high-surface-area MO2 • 4SiO2 materials at temperatures as low as 135 °C (Eq. (24)).

HO, MeOH

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