Palladium 2, EXAFS analysis

Extended X-ray absorption fine stmcture (EXAFS) analysis has been employed recently for the investigation of the role of colloids in catalysis. For Heck reactions with Herrmarm s paUadacycle or with [Pd(OAc)2]/P(tert-Bu)3, no indication of formation of coUoidal palladium was found [44]. By contrast, Koningsberger and co-workers found that in aUyHc amination with allylpalladium complexes with bidentate phosphine Hgands, from the early stages of the reaction onward, deactivation occurs by formation of di- and trinuclear clusters, which aggregate to larger clusters and finally to palladium black [45]. 

The most straightforward case of electrostatic adsorption occurs with the most stable metal complexes such as platinum or palladium tetraammines, [(NH3)4Pt]+ and [(NH3)4Pd]+ or PTA and PdTAs, respectively. Over the wide pH and concentration ranges that can be employed during syntheses from the lab to the industrial scale, the relatively strongly bound ammine ligands of these coordination complexes remain intact. This has been most directly confirmed in the case of Pt by extended X-ray absorption fine structure (EXAFS) analysis of PTA solutions and adsorbed species [23], and for PdTAs less directly by XPS [24]. 

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]... 

Toshima [162, 164, 165] has studied a series of PVP protected colloidal bimetallic PtPd catalysts in olefin hydrogenation catalysis. The highest activity for the hydrogenation of 1,3-cyclooctadiene to cyclooctene was observed at a Pd Pt radio of 4 1. On the basis of EXAFS analysis [165] it was concluded that the distribution of the two metals in the particles is nonuniform, and that at the most active composition the colloid particles had a platinum core coated with palladium. A less active 1 1 Pd/Pt catalyst, with the same particle size, apparently had a more uniform distribution of metals, with both platinum and p ladium atoms at the surface. 

Grunwaldt et al. (2003b) reported XAFS measurements recorded during palladium-catalyzed alcohol oxidation in supercritical CO2. A commercial shell-impregnated catalyst consisting of 0.5 wt% Pd on alumina was used for benzyl alcohol oxidation (to benzaldehyde) in supercritical CO2 with pure O2 as oxidant. The conditions were 353 K and 150 bar. The results are summarized in Table 8. The authors reported only Pd XANES data, not EXAFS data, and thus the analysis is limited to information about the average oxidation state of the palladium. 

Extended X-Ray Absorption Fine Structure (EXAFS) Spectroscopic Analysis of 6-Mercaptopurine Riboside Complexes of Platinum(II) and Palladium(II)... 

Contrary to expectation, the reaction product 8, a black amorphous powder soluble in water and acetic acid, was found to have composition and properties quite different from those expected for the platinum analog of the Pdsgi giant clus-ter.157] According to elemental analysis, substance 8 has the reproducible elemental composition Pt8phen3(0Ac)4(0H)4(H20)6, which is quite different from the expected minimum formula Pt9(phen)o.96(OAc)2.89 for the Pt analog of palladium giant cluster 6. On the basis of the elemental composition, the formal oxidation state of platinum in substance 8 is close to (+1). This value was confirmed by the EXAFS data. ... 

Structural parameter derivation from curve-fitting analysis of EXAFS spectra of the supported PdQ2 catalyst and the attached Pd catalyst, respectively, established the Pd-Q distances in the catalysts to be almost the same, but with the latter having a higher palladium coordination number close to four (Table 5.6-2). This confirmed the quite uniform preparation of the attached catalyst on the support with mainly [PdCU] " species present. 

Initially, the as-synthesized catalyst was characterized by EXAFS and XANES (region a), and then the reaction mixture comprising benzyl alcohol/cyclohexane (solvent) saturated with 02/mixture was introduced to the catalyst at 50 °C (region b). Palladium was found to be in a partially oxidized state and there was no catalytic activity for this material under this condition as evidenced from the IR spectrum of the effluent from the reactor. Then, the catalyst material was exposed to H2-saturated cyclohexane to reduce the palladium to metallic state (region C). After about 30 min of the exposure, aU the palladium atoms are found to be in the metallic state. This has been confirmed by both XANES and EXAFS data, as shown in Figure 12.10. Analysis of the EXAFS data showed evidence for the formation of palladium hydride. This leads to a more elongated EXAFS function and to a shift... 

In further experiments, preformed TBAX-stabilized Pd-colloids were shown to interact with iodobenzene in a stoichiometric reaction with formation of Ph-PdX species, possibly in an ionic form as indicated by UVA is and NMR analysis. De Vries made the claim, (based on TEM, MS, EXAFS, filtration studies and Finke inhibition tests) that in standard Mizoroki-Heck reactions at high temperatures (>120 °C), the palladium catalyst is rapidly reduced to Pd(0) which tends to form soluble colloids. The atylating agent continuously attacks the palladium atoms at the rim of the nanoparticles leading to the formation of soluble anionic complexes, completion of a Mizoroki-Heck cycle in solution. 

Elaborate synthetic approaches have been developed that enable significant control over the size and shape of palladium nanostructures. In order to understand the properties of the materials formed based on the preparation method, several characterization techniques have been used. These include electron microscopy, scanning probe microscopy (SPM), nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, infrared (IR) spectroscopy, electrochemistry, X-ray diffraction (XRD), thermogravimetric analysis (TGA), electron diffraction, photoelectron spectroscopy, dynamic light scattering (DLS), extended X-ray absorption fine structure (EXAFS), BET surface area analysis andX-ray reflectivity (XRR). In the following section we will describe the information provided by each of these characterization techniques. 

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