Alloy bimetallic alloys

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

Au (or Ag) content. Decanethiol-protected AuPt alloy bimetallic nanoparticles of ca. 2.5 nm in particle size were similarly prepared [58]. The preparations of PdPt [59] and AuPd [60] bimetallic nanoparticles in water-in-oil (w/o) microemulsions can be realized in two-phase reaction system, in which a surfactant molecule itself works as a protecting agent in these cases. 

In summary the simultaneous reduction method usually provides alloyed bimetallic nanoparticles or mixtures of two kinds of monometallic nanoparticles. The bimetallic nanoparticles with core/shell structure also form in the simultaneous reduction when the reduction is carried out under mild conditions. In these cases, however, there is difference in redox potentials between the two kinds of metals. Usually the metal with higher redox potential is first reduced to form core part of the bimetallic nanoparticles, and then the metal with lower redox potential is reduced to form shell part on the core, as shown in Figure 2. The coordination ability may play a role in some extent to form a core/shell structure. Therefore, the simultaneous reduction method cannot provide bimetallic nanoparticles with so-called inverted core/ shell structure in which the metal of the core has lower redox potential. 

We investigated on structure of CuPd (2 1) bimetallic nanoparticles by XRD [71]. Since the XRD peaks of the PVP-protected CuPd nanoparticles appeared between the corresponding diffraction lines of Cu and Pd nanoparticles, as shown in Figru e 11, the bimetallic alloy phase was clearly formd to be formed in CuPd (2 1) bimetallic nanoparticles. We also characterized Ag-core/Rh-shell bimetallic nanoparticles, which formed during simple physical mixing of the corresponding monometallic ones, by XRD coupled with TEM [148]. 

PtRu nanoparticles can be prepared by w/o reverse micro-emulsions of water/Triton X-lOO/propanol-2/cyclo-hexane [105]. The bimetallic nanoparticles were characterized by XPS and other techniques. The XPS analysis revealed the presence of Pt and Ru metal as well as some oxide of ruthenium. Hills et al. [169] studied preparation of Pt/Ru bimetallic nanoparticles via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal. XPS and other analytical data indicated that the preparation method provided fully alloyed bimetallic nanoparticles instead of core/shell structure. AgAu and AuCu bimetallic nanoparticles of various compositions with diameters ca. 3 nm, prepared in chloroform, exhibited characteristic XPS spectra of alloy structures [84]. 

By XPS spectra, Endo et al. [96] confirmed that formation of binary structure prevented Pd atoms from oxidation in the AuPd and PtPd bimetallic nanoparticles which exhibited higher catal5hic activity than monometallic ones. Wang et al. [112]. characterized PtCu bimetallic alloy nanoparticles Ijy XPS. XPS revealed that both elements in the nanoparticles are in zero-valence and possess the characteristic metallic binding energy. 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Once we have developed our basic model and shown how it may be used to estab-hsh trends in electrochemical reactivity, we will take the further step of applying it to the identification of new bimetallic electrocatalysts. We will introduce simple procedures to rapidly screen bimetallic alloys for promising electrocatalytic properties, and we will demonstrate the importance of including estimates of the alloys stabihty in the screening procedure. Finally, we will give examples of successful apphcation of this method to specific problems in the area of electrocatalyst development. 

Stamenkovic VR, Mun BS, Arenz M, Mayrhofer KJJ, Lucas CA, Wang GF, Ross PN, Markovic NM. 2007b. Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces. Nature Mater 6 241-247. 

Stamenkovic V, Arenz M, Lucas C, Gallagher M, Ross PN, Markovic NM. 2003. Surface chemistry on bimetallic alloy surfaces adsorption of anions and oxidation of CO on Pt3Sn(lll). J Am Chem Soc 125 2736-2745. 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Alcohol oxidation by enzymes, 610-613 Alloy/bimetallic catalysts, 6-7, 70-71, 245-266, 317-337 Anderson-Newns Hamiltonian, 33-34 Anion adsorption effects, 143, 174-175, 208-239, 254, 281-283, 336, 525, 535-536... 

Abstract A convenient method to synthesize metal nanoparticles with unique properties is highly desirable for many applications. The sonochemical reduction of metal ions has been found to be useful for synthesizing nanoparticles of desired size range. In addition, bimetallic alloys or particles with core-shell morphology can also be synthesized depending upon the experimental conditions used during the sonochemical preparation process. The photocatalytic efficiency of semiconductor particles can be improved by simultaneous reduction and loading of metal nanoparticles on the surface of semiconductor particles. The current review focuses on the recent developments in the sonochemical synthesis of monometallic and bimetallic metal nanoparticles and metal-loaded semiconductor nanoparticles. 

Brayner, R., Coradin, T., Vaulay, M.-J., Mangeney, C., Livage, J. and Fievet, F. (2005) Preparation and characterization of metal (Au)- and bimetallic alloys (AuNi)-gelatin nanocomposites. Colloids and Surface Science A, 256, 191-197. 

In some other cases, the intermetal electron transfer does not occur even during hour-long irradiations. The initial simultaneous reduction reactions of and M are followed by mixed coalescence and association of atoms and clusters with ions, homolog or not. Besides the dimerization of atoms of the same metal into M2 and M 2, coalescence of both types of atoms occurs twice more frequently. Subsequently, mixed coalescence and reduction reactions progressively build bimetallic alloyed clusters according to the statistics of encounters, therefore to the relative initial ion abundance [53]. 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

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