Scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy (TEM) have been used to determine the dimensions of a series of palladium clusters stabilized by tetraalkylammonium salts. Electrochemically prepared colloids were used in which the average diameter of the inner metal core was varied between 2 and 4 nanometers, and the size of the ammonium ions was adjusted in the series (+)N(n-C(4)H(9))(4) < (+)N(n-C(8)H(17))(4) < (+)N(n-C(18)H(37))(4). The difference between the mean diameter determined by STM and that measured by TEM allows the determination of the thickness of the protective surfactant layer. On the basis of these studies, a model of the geometric properties of ammonium-stabilized palladium clusters has been proposed. Suggestions for the mechanism of the STM imaging process are also made.

Nanostructured metal clusters and colloids are of interest as catalysts for organic and inorganic reactions. as electrocatalysts in fuel cells. and as components for materials with special electronic, optical, or magnetic properties."' They are usually prepared by chemical reduction of metal salts. In order to prevent undesired agglomeration with formation of either large metal particles in the micrometer range or metal powders, stabilizers such as special ligmds,r31 polymers,[41 or tetraalkylaminonium salts[" generally need to be used. Nevertheless, high yields, simple isolation and purification. and control of the cluster size is only seldom possible.[61 Recently we described an electrochemical process by which metal foils are anodically dissolved, and the metal salts formed as intermediates are cathodically reduced with quantitative formation of metal clusters stabilized by tetraalkylammonium salts.'7. Besides the simple isolation and the high purity of the clusters, another advantage is the control of particle size (1 -10 nm) by adjusting the current density, that is, the overpotential. Tetraalkylammonium salts serve simultaneously as supporting electrolyte and stabilizer for the cluster. Though this method can be employed for relatively easily oxidizable metals (for example, Pd, Ni, Cu, Au, etc.),['. ' I for anodically less readily soluble metals like Pt, Rh. Ru, and M o it functions poorly or not at all. This is unfortunate, because precisely these metals are of interest as potential catalysts.We now report a solution to this problem based on the use of metal salts."] Initially a mixture of PtClz and (NRJBr in acetonitrile/THF (1 :3) was electrolyzed in a simple electrolysis cell["] consisting of a Pt anode and a Pt cathode. Unfortunately. only a small amount of Pt-containing material was formed, which was difficult to isolate. Because the oxidation process at the anode (for example, Br-+ Br') possibly caused problems, in a renewed attempt we used PtClz as the metal source and (NRJOAc as the supporting electrolyte and stabilizer. in the hope that the Kolbe electrolysis would ensure the chemical "disposal" at the anode (Scheme 1).

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Structural investigations using EXAFS spectroscopy and standard analytical methods were performed on electrochemically synthesized tetrabutylammonium bromide stabilized Pd/Pt bimetallic clusters with different Pd/Pt ratios. The results of these investigations allow the determination of a detailed model concerning the structure of these 3.5 nm sized bimetallic clusters. The cores of the clusters are enriched in platinum metal, while the outer shell is enriched in palladium metal. The nanostructured particles are partially oxidized. The amounts of oxide in the outer shells are larger than in the cores of the clusters. Also, a small amount of the metal is present as metal bromide. The ammonium salt stabilizer forms a protective layer around the cluster.

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