Intermetallics

Heterometallic nanoparticle catalysts such as Pt-Re, Ir-Sn, Pt-Ru, Ag-Ru, Cu-Ru, and Pd-Ru occupy an important position in heterogeneous catalysis [531]. Published reports

[190, 532-534] on such catalyst systems reveal that enhanced catalytic performance apparently arises from the synergy between the component elements at the nanoscale, which is absent in solid solutions of the two bulk metals. Furthermore, the thin films of intermetallic combinations of a group 13 element and a transition metal are thermodynamically stable metal/semiconductor interfaces that can act as ohmic contacts or as Schottky barriers [535]. Although molecular beam epitaxy is used to deposit such materials, a large number of reports have appeared on the deposition of binary metal alloys by the gas-phase decomposition of single-source organometallic sources, which demonstrate the possibilities of controlling the stoichiometry of intermetallics.

Thomas and Johnson and co-workers have obtained discrete nanoparticles of the Pd-Ru bimetallic catalyst by gentle thermolysis of the mixed-metal Pd-Ru carbonylate cluster [Pd6Ru6(CO)24]2- (Fig. 35) [190]. HR-TEM revealed that the Pd-Ru bimetallic nanoparticles were of uniform size (ca. 1.7 nm diameter). A uniform distribution of nanoparticles within the pores of mesoporous silica offers a highly active catalyst for hydrogenation of alkenes and unsaturated aromatic systems [190]. The easy formation of Pd-Ru nanoparticles is due to the ability of the anionic molecular precursor to shed its cloak of carbonyl groups under mild thermal treatment (180 °C). A comparison of the catalytic performance with monometallic Ru and Pd clusters reveals that the bimetallic catalysts are far superior in performance to their monometallic analogs, suggesting a possible synergism between the two bimetallic nanoparticles [190].

Kaesz and co-workers have deposited stoichiometric films of intermetallics containing a group 14 element and a transition metal [536-538]. Precursor complexes with metal-metal bonds (e.g., (CO)4-CoGaCl2(THF)) [537] as well as those in which the metal centers are connected by bridging ligands such as [(py)(Et)Co(dmg-GaEt2)2] [538], [Ni(dmg-GaEt2)2] [538], and Pt[(dmg)(GaMe2)2] [536] have been used to deposit CoGa, NiGa, and PtGa2, respectively. Since the Co-Ga phase diagram mainly comprises the S-CoGa phase, which extends on either side of the 1:1 atomic ratio, the CoGa films were consistently monophasic. On the other hand, the phase diagrams in the Ga-Pt and Ga-Ni systems comprise several phases, and as a result a small amount of Pt2Ga3 is formed with the major and expected PtGa2 phase

Figure 35. Structure of the Pd6Ru6 core of the [Pd6Ru6(CO)24]2- cluster. Reprinted with permission from [190], R. Raja et al., Chem. Commun. 1571 (1999). © 1999, Royal Society of Chemistry.
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