Bulk alloys

Most of the present implementations of the CPA on the ab-initio level, both for bulk and surface cases, assume a lattice occupied by atoms with equal radii of Wigner-Seitz (or muffin-tin) spheres. The effect of charge transfer which can seriously influence the alloy energetics is often neglected. Several methods were proposed to account for charge transfer effects in bulk alloys, e.g., the so-called correlated CPA , or the screened-impurity model . The application of these methods to alloy surfaces seems to be rather complicated. 

Provided the mole fraction of A does not fall below N, then the oxide AO will be formed exclusively. The important criterion is the ratio of the oxidation parabolic rate constant to that of the diffusion coefficient of For A1 in Fe, the parabolic rate constant is very low, whilst the diffusion coefficient is relatively high, whereas the diffusion coefficient of Cr is much lower. Hence, the bulk alloy composition of A1 in iron required for the exclusive formation of AI2O3 at any given temperature is lower than the Cr concentration required for the exclusive formation of CrjOj. 

Figure 7. SNIFTIR spectra of the adsorbed intermediates involved in the oxidation of 0.1 M CHjOH in 0.5 M HCIO4 on a smooth RO.ORuO.l bulk alloy (p-po-larized light modulation potential Ail = 0.3 V averaging of 128 interferograms). Electrode potential (mV/RHE) (1) 370, (2) 470, (3) 570, (4) 670, (5) 770.

These conclusions from the infrared reflectance spectra recorded with Pt and Pt-Ru bulk alloys were confirmed in electrocatalysis studies on small bimetallic particles dispersed on high surface area carbon powders.Concerning the structure of bimetallic Pt-Ru particles, in situ Extended X-Ray Absorption Fine Structure (EXAFS>XANES experiments showed that the particle is a true alloy. For practical application, it is very important to determine the optimum composition of the R-Ru alloys. Even if there are still some discrepancies, several recent studies have concluded that an optimum composition about 15 to 20 at.% in ruthenium gives the best results for the oxidation of methanol. This composition is different from that for the oxidation of dissolved CO (about 50 at.% Ru), confirming a different spatial distribution of the adsorbed species. 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

The XRD and TEM showed that the bimetallic nanoparticles with Ag-core/Rh-shell structure spontaneously form by the physical mixture of Ag and Rh nanoparticles. Luo et al. [168] carried out structure characterization of carbon-supported Au/Pt catalysts with different bimetallic compositions by XRD and direct current plasma-atomic emission spectroscopy. The bimetallic nanoparticles were alloy. Au-core/Pd-shell structure of bimetallic nanoparticles, prepared by co-reduction of Au(III) and Pd(II) precursors in toluene, were well supported by XRD data [119]. Pt/Cu bimetallic nanoparticles can be prepared by the co-reduction of H2PtClg and CuCl2 with hydrazine in w/o microemulsions of water/CTAB/ isooctane/n-butanol [112]. XRD results showed that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (111) plane of the PtCu3 bulk alloy. 

Gasteiger HA, Ross PN, Cairns EJ. 1993. LEIS and AES on sputtered and annealed polycrystalline Pt-Ru bulk alloys. Surf Sci 293 67-80. 

WeinertM, Watson RE. 1995. Core-level shifts in bulk alloys and surface adlayers. Phys Rev B 51 17168-17180. 

Gasteiger HA, Markovic N, Ross PN, Caims EJ. 1993. Methanol electrooxidation on well-characterized platinum-mthenium bulk alloys. J Phys Chem 97 12020-12029. 

Kabbabi A, Faure R, Durand R, Beden B, Hahn F, Leger JM, Lamy C. 1998. In situ FTIRS study of the electrocatalytic oxidation of carbon monoxide and methanol at platinum-ruthenium bulk alloy electrodes. J Electroanal Chem 444 41-53. 

PtRu nanoparticle electrocatalyst with bulk alloy properties prepared through a sono-chemical method Langmuir 22 10446-10450. 

Ge Q, Desai S, Neurock M, Kourtakis K. 2001. CO adsorption on Pt-Ru surface alloys and on the surface of Pt-Ru bulk alloy. J Phys Chem B 106 9533-9536. 

Figure 14.12 CO bulk electro-oxidation at PtRu alloys, (a, b) PcRui j /Ru(0001) (x = 0.07, 0.25, 0.47) surface alloys measured in a flow cell with a CO-saturated electrolyte, (c) Freshly sputtered Pto.sRuo.s bulk alloy in a rotating disk electrode setup (data from Gasteiger et al. [1995]), compared with a Pto.53Ruo,47/Ru((X)01) surface alloy.

Basnayake R, Li Z, Katar S, Zhou W, Rivera H, Smotkin ES, Casadonte DJ, Korzeniewski C Jr (2006) PtRu nanoparticle electrocatalyst with bulk alloy properties prepared through a sonochemical method. Langmuir 22 10446-10450... 

An empirical treatment developed by Kolb et al. [81, 82] relating UPD behavior to the difference in work function between the substrate and depositing species has been used to explain anomalous co-deposition behavior observed in Ni-Fe and Ni-Zn alloys [83]. Although the relationship appears to hold for pure underpotential deposition limited to a monolayer, it does not satisfactorily predict bulk alloy behavior. For example, based on work function data alone, one would expect Zn-Al and Sb-Al alloys to be formed by underpotential alloy deposition. Recent reports in the literature, however, indicate that alloying in these systems does not occur [46, 84]. 

The semiconductors that have been the subject of numerous investigations in bulk, alloyed, or nanocrystalline form include Si, Ge, doped diamond, SiC, (B, Al, Ga, In)(N, P, As, Sb), and (Zn, Cd, Hg, Pb)(0, S, Se, Te). Nature has been exceptionally benign in providing NMR-active isotopes at natural abundances exceeding 4% for all of the preceding elements except in the cases of 13C, 33S, and 170, and enrichment with isotopic-labels has become more common. 

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