Alloy surface ordering

X-ray scattering studies at a renewed pc-Ag/electrolyte interface366,823 provide evidence for assuming that fast relaxation and diffu-sional processes are probable at a renewed Sn + Pb alloy surface. Investigations by secondary-ion mass spectroscopy (SIMS) of the Pb concentration profile in a thin Sn + Pb alloy surface layer show that the concentration penetration depth in the solid phase is on the order of 0.2 pm, which leads to an estimate of a surface diffusion coefficient for Pb atoms in the Sn + Pb alloy surface layer on the order of 10"13 to lCT12 cm2 s i 820 ( p,emicai analysis by electron spectroscopy for chemical analysis (ESCA) and Auger ofjust-renewed Sn + Pb alloy surfaces in a vacuum confirms that enrichment with Pb of the surface layer is probable.810... 

W(1 0 0) surface [164]. A similar very small dependence of So on Ts was also observed elsewhere [165]. This lead to the suggestion [164, 168] that the precursor responsible for dissociation may not be fully accommodated at Ts, but had sufficient lifetime at the surface to undergo dissociation if it encountered a defect (or step) site. It also leads to the series of experiments in which H2 and N2 dissociation was investigated on the W(1 0 0)-c(2 x 2)Cu alloy surface in order to establish the effect of changing the activation barrier to direct dissociation in the surface unit cell, and concomitant effects on the indirect dissociation channel. 

G. M. Schwab, J. Phys, Colloid Chem., 64, 581 (1950), and earlier papers with coworkers, has shown that the reaction HCOOH —>112 + CO2 is nearly zero-order on a number of metal and alloy surfaces. 

The primaiy emphasis in this review article is to showcase the use of LEISS to examine the outermost layers of Pt-Co alloys in order to correlate interfacial composition with electrocatalytic reactivity towards oxygen reduction. In some instances, it is desirable to compare the properties of the outermost layer with those of the (near-surface) bulk an example is when it becomes imperative to explain the unique stability the alloyed Co under anodic-oxidation potentials. In such cases. X-ray photoelectron spectroscopy and temperature-programmed desorption may be employed since both methods are also able to generate information on the electronic (binding-energy shift measurements by XPS) and thermochemical (adsorption enthalpy determinations by TPD) properties at the sub-surface. However, an in-depth discourse on these and related aspects was not intended to be part of this review article. 

Ng et al. (144) examined the NO-CO, CO-O2, and CO-N2O reactions on a Pto,io-Rho.9o (IH) surface. The NO-CO activity of this alloy surface is similar to that of Rh(lll) at temperatures from 573 to 648 K in that the two surfaces are represented by the same activation energy, reaction orders, and selectivity. The turnover frequencies are slightly lower than those for Rh(lll) when compared on a per surface atom basis however, the rates per surface Rh atom are virtually unchanged. The authors suggested that the primary effect of Pt is to dilute the Rh surface concentration. 

The BFS method has been applied to a variety of problems, ranging from the determination of bulk properties of solid solution fee and bee alloys and the defeet strueture in ordered bee alloys [28] to more speeifie applieations ineluding detailed studies of the strueture and eomposition of alloy surfaees [29], ternary [30] and quaternary alloy surfaees and bulk alloys [31,32], and even the determination of the phase strueture of a 5-element alloy [33]. Previous appheations have foeused on fundamental features in monatomie [26] and alloy surfaces [29] surface energies, reconstructions, surface structure and surface segregation in binary and higher order alloys [34,35] and multilayer relaxations [36,37]. While most of the work deals with metallic systems, the lack of restrictions on the type of system that can be studied translated into the extension of BFS to the study of semiconductors [38]. 

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