Surface ordered alloys

However, the predse structure of the catalyst and the precise role of CeC>2 in the present case and of Bi is not completely clear. In general terms, several explanations for the rate and selectivity enhancements by the promoter are possible [80] (a) geometric blocking of a fradion of sites and generation of specific surface ensembles, viz. formation of an ordered alloy (b) neighboring atom participation (Fig. 11.4), although the partial oxidized state of the promoter (Bix+) of the model is not confirmed by surface studies (LEED, XPS, EXAFS) (c) occurrence of bi-fundional catalysis, assuming that O or OH radicals formed on the promoter participate in the oxidation. 

For the metals Co, Ni and Pd and perhaps others it appears to be a good approximation to assume, in spite of the hybridization, that part of the Fermi surface is s-like with mrff me, and part d-hke with meff me. The current is then carried by the former, and the resistance is due to phonon-induced s-d transitions. This model was first put forward by Mott (1935) and developed by many other authors (e.g. Coles and Taylor 1962) for reviews see Mott (1964) and Dugdale and Guenault (1966). Applications of the model have also been made to ordered alloys of the type Al6Mn, Al7Cr by Griiner et al (1974), where the width A of the d-band is the same as it would be for an isolated transitional-metal atom in the matrix, but most of the Fermi surface is assumed to be (s-p)-like. The behaviour of the disordered Pd-Ag alloy series is particularly interesting. The 4d-bands of the two constituents are well separated, as shown particularly by... 

A completely novel approach to technical electrolysis for anodic oxygen evolution from alkaline solution is the use of amorphous metals, i.e. chilled melts of nickel/cobalt mixtures whose crystallization is prevented by the addition of refractory metals like Ti, Zr, B, Mo, Hf, and P (46-51). For this type of material, enhanced catalytic activity in heterogeneous catalysis of gas-phase reactions has been observed (51). These amorphous metals are shown to be more corrosion resistant than the respective crystallized alloys, and the oxides being formed at their surfaces often exhibit a higher catalytic activity than those formed on ordered alloys, as shown by Kreysa (52-54). 

The simplest surface specific feature of an ordered phase is the fact that there usually are different truncations of the bulk ordered alloy by the same surface orientation. In this case the problem is to find the stable truncation which, as we will show in this section, is usually directly related to the surface segregation energy of the deposited element to the corresponding surface of the substrate. 

Another important result presented in this figure is the behavior of the surface energies of partially ordered p(2xl)-NiAl alloys in the surface layer. Such partially ordered alloys have the same ordered p(2xl) structure, but the excess of Ni atoms form partial antisite defects on the A1 sublattice. One can... 

Fig. 14. First-principles results for the surface energy of random and ordered surface alloys on Cu(lll). Cu2Pd and CusPd are Vs Vs nd (2x2) ordered alloys correspondingly. The dotted line is the stability line which is the surface energy of a disordered alloy with the maximal possible value of the SRO parameter for a given 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]. 

Compared to SRO effects on surface segregation in solid solutions, the role of LRO should be naturally more prominent and common. Its elucidation requires calculations that take into account various factors contributing to the net segregation characteristics in ordered alloys including the temperature dependence the crystal bulk structure and surface orientation, effective bulk and surface interatomic interactions (NN, non-NN) in relation to segregation driving forces, deviation from exact stoichiometry, possible surface relaxation and reconstruction, atomic vibrations, etc. This section attempts to quantify some of these factors and present several possible scenarios of segregation/order interplay. 

Another class of ideally bulk-truncated equiatomic surfaces of alloys (e.g., Ll2(100) and DO3(100) with AB3 bulk stoichiometry), exhibits more diverse segregation/order interplay compared to the previous class ( non-segregated , Cs=cij, equiatomic termination). Because the segregation vs. 

In addition to the segregation/order factor, and depending on its magnitude, the crystal structure and surface orientation can strongly affect the surface composition in ordered alloys. For example, unlike the case of the equiatomic bulk truncated composition of Ll2(100), LRO tends to maintain the Ll2(lll) surface with nominal bulk concentration (0.25). Therefore, the two ordered surfaces are expected to exhibit quite different segregation characteristics for the same r value (Fig. 10). Moreover, SRO causes pronounced changes of surface sublattice and average compositions associated with a considerable reduction of the order-disorder transition temperature (especially in fee alloys). 

These energetic parameters were used in FCEM calculations assuming segregation at the three outmost layers only. As shown in Fig. 15, the segregation tendency prevails only in the B32 ordered alloys and the surface concentration decreases with temperature (entropy-driven monotonous desegregation). This behavior is associated with the distinctly high segregation/order factor (sec. 3.1). On the other hand, ordered bulk truncation with surface concentration very... 

---