PdCu bimetallic

PdCo PdPt/C) [163,164] and PdsPt/C [165] were found to demonstrate better alcohol tolerance and comparable and even better electrocatalytic performance than commercial Pt/C. A superior ORR activity and methanol tolerance as compared to commercial Pt/C and Pd/C catalysts have also been reported with a novel nanotubular mesoporous PdCu bimetallic nanocomposite comprising interconnected alloy nanoparticles with sizes around 3nm. The improved performance is related to the special structural configuration of the catalyst [166]. 

This section reports a series of examples of application of the cluster model approach to problems in chemisorption and catalysis. The first examples concern rather simple surface science systems such as the interaction of CO on metallic and bimetallic surfaces. The mechanism of H2 dissociation on bimetallic PdCu catalysts is discussed to illustrate the cluster model approach to a simple catalytic system. Next, we show how the cluster model can be used to gain insight into the understanding of promotion in catalysis using the activation of CO2 promoted by alkali metals as a key example. The oxidation of methanol to formaldehyde and the catalytic coupling of prop)me to benzene on copper surfaces constitute examples of more complex catalytic reactions. 

The activation treatment of the precursors has been carried out by decomposition under flowing Ar up to 670 K (0.5 K/mn). Indeed, previous works on Pd and PdCu supported on silica have shown that such an Ar treatment leads to metallic Pd and Pd-Cu bimetallic phases, well dispersed on the support (8). To remove the carbonaceous residues, all the samples have been calcined under O2 at 720 K and reduced under H2 at 870 K. Their compositions have been controlled by chemical analysis... 

Bimetallic particles of PdCu and PdCur were first prepared by simultaneous condensation of both metals on NaCl (100) single crystals cleaved in situ under UHV conditions.The same kind of particles have been produced by decomposition of Pd(acac)2 and Cu(acac)2 in solution, on previously cleaned MgO micro-cubes. It has been shown " that small Pd particles (<6 nm) prepared from solution in this manner had the same characteristics (shape, structure, and structure of the interface) as particles condensed under UHV conditions. 

Pd and bimetallic PdCu and PdCu3 particles have been prepared, both under UHV conditions and by decomposition of organometallic compounds, on MgO and ZnO micro-crystals, single crystals or thin epitaxial layers. The shapes and structures of the particles and the structures of the interfaces, and the dependence of these on particle size and annealing conditions, were determined by HRTEM and WBDF imaging. 

For bimetallic PdCu particles crystallized on MgO, an ordered structure (type CsCl) is always obtained after annealing. The particles are epitaxially oriented (001) on the substrate, with perfect coincidence between the lattices. The correspondence between experimental and simulated HRTEM images, and numerical analysis, are indicative of a layer of Pd at the interface between the alloy and MgO, with the atomic columns aligned with those of MgO. 

EXAFS analysis of colloidal metals has begun to shed light on this complex state of affairs. In the few examples reported so far for bimetallic colloids, a nonuniform distribution of metals has been observed. Ibshima [164, 165] has studied a series of bimetallic PtPd catalysts, and concluded that the distribution of the two metals in the particles is nonuniform on the basis of differences in their respective mean coordination numbers. In our laboratory, colloidal 4.0 nm PdCu/PVP has been analyzed by this method and the distribution of the metals in the alloy particles shown to be nonuniform. However the component which is enriched at the surface appears to be palladium, in contrast to the segregation of copper to the surface of bulk copper-palladium alloys. [203] In addition, the ability of surface deposited copper to dissolve in a preformed palladium particle has been clearly demonstrated on the basis of the analysis of the coordination sphere of the copper. [204]... 

Adsorbed carbon monoxide can serve as a useful infrared probe of surface composition in bimetallic colloids if both metals bind CO. This is exemplified in the infrared spectrum of CO on a PVP stabilized colloidal alloy CutgPdjT, [38] Carbon monoxide adsorbs readily onto these PdCu particles (co. 45 A) in dichloro-methane at 25 °C, as shown by the infrared absorption spectrum in Figure 6-28. By comparing this to the IR spectrum of CO on a pure palladium colloid of similar size [34] in Figure 6-27d, it can be clearly seen that CO occupies both palladium and copper sites. Whereas the bands at 2046 cm" and 1936 cm" are in the frequency ranges found for linear and bridged CO on the pure palladium particles, the new band at 2089 cm corresponds to CO on surface copper atoms, thus demonstrating that both metals are present at the surfaces of the particles. 

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