Chemisorption surface segregation

In this section, we extend the above formalism to that for an alloy surface within the CPA, which serves as the model for the pre-chemisorption substrate. The model discussed here is based on that of Ueba and Ichimura (1979a,b) and Parent et al (1980). For a comprehensive introduction to alloy surfaces see Turek et al (1996). A feature of surface-alloy models, which is different from bulk ones, is that the CP in layers near the surface is different from that in the bulk, due to the surface perturbation. Moreover, the alloy concentration in the surface layers may be quite different from that in the bulk, a feature known as surface segregation. (See Ducastelle et al 1990 and Modrak 1995 for recent reviews.) We assume that both of these surface effects are confined to the first surface layer only. 

However, surface segregation (cs / Cb) has a radical effect on AE, as can clearly be seen in Fig. 6.3(b). Cu/Ni alloys are known (Kelley and Ponec 1981, Ouannasser et al 1997) to have an enriched Cu concentration in the surface layer for all bulk concentrations. As a result, the alloy shows a more Cu-like behaviour than it would if it were non-segregated. In particular, AE has a value significantly closer to that for pure Cu than in the case where cs = Cb, and this occurs at all bulk concentrations c. The smallest change in AE occurs in Cu-rich alloys, which is understandable, because these alloys have mostly Cu in the surface layer anyway, so the effect of surface segregation is relatively small. Thus, surface segregation has a lesser effect in these alloys than in Ni-rich ones, which have mostly Ni in the bulk, but may have a Cu majority in the surface layer. Clearly, then, the concentration cs of the surface layer is the primary parameter in determining the chemisorption properties of the DBA. 

Vickerman and Ertl (1983) have studied H2 and CO chemisorption on model Cu-on-Ru systems, where the Cu is deposited on single-crystal (0001) Ru, monitoring the process using LEED/Auger methods. However, the applicability of these studies carried out on idealized systems to real catalyst systems has not been established. Significant variations in the electronic structure near the Eermi level of Cu are thought to occur when the Cu monolayer is deposited on Ru. This implies electron transfer from Ru to Cu. Chemical thermodynamics can be used to predict the nature of surface segregation in real bimetallic catalyst systems. 

Chemisorption-induced metal atom reorganization at low temperatures had been discovered in the 1960s by field ion microscopy in chemisorption studies of nitrogen and carbon monoxide on a tungsten single-crystal tip (55-58). Likewise, surfece reconstruction was concluded from research of chemisorption on metal and bimetal films (59-61). This surface reconstruction was observed particularly on open crystal faces the more closely packed faces are often corroded from their edges with the former faces. Surface reconstruction is of particular relevance to alloy surfaces, where it leads to chemisorption-induced segregation of one alloy component to the... 

The aim of this chapter is to review our understanding of the fundamental processes that yield improved electrocatalytic properties of bimetallic systems. Three classes of bimetallic systems will be discussed bulk alloys, surface alloys, and overlayer(s) of one metal deposited on the surface of another. First, we describe PtjM (M=Ni, Co, Fe, Cr, V, and Ti) bulk alloys, where a detailed and rather complete analysis of surface structure and composition has been determined by ex situ and in situ surface-sensitive probes. Central to our approach to establish chemisorption and electrocatalytic trends on well-characterized surfaces are concepts of surface segregation, relaxation, and reconstruction of near-surface atoms. For the discussion on surface alloys, the emphasis is on Pd-Au, a system that highlights the importance of surface segregation in controlling surface composition and surface activity. For exploring adsorption and catalytic properties of submonolayer and overlayer structures of one metal on the surface of another, we summarize the results for Pd thin metal films deposited on Pt single-crystal surfaces. For all three systems, we discuss electrocatalytic reactions related to the development of materials... 

They describe quite successfully the surface concentration ratio as a function of the bulk concentration ratio at a fixed temperature, which is mostly room temperature or zero temperature. However, they do not describe the temperature dependence of segregation. Experimentally strong variation of the surface concentration ratio and even cross-overs were observed in chemisorption-induced segregation as a function of temperature. 

We now have to think how the chemical interaction of the components of the alloy when at the surface affects their ability in chemisorption. Before we can look at this, however, we must address the problem of surface segregation. 

With cobalt as the catalyst, CO-chemisorption rules the nature of the active sites by controlling surface segregation and thus even construction of the tme FT-catalyst [9, 10,16]. 

The heat of surface segregation is the difference between the heat of chemisorption of I/2N2 and the heat of segregation... 

There is evidence of a promoting action of chromium on nickel catalysts for the reaction of hydrogenation of valeronitrile in our conditions. Introduction of chromium increased the initial specific activity and the selectivity. The promoting effect of chromium on activity could be correlated to the increase of the metallic surface. Another explanation could be that the Cr+ segregated at the surface of the catalyst may play the role of a Lewis acid center and may be responsible for a better chemisorption of valeronitrile on the catalysts, through nitrogen lone pair electrons or the n orbital of the CN bond. However, further examination of the results obtained (see Fig. 3)... 

At the higher metal level (2.0-4.5% Ni with up to 2% Sb) used to study artificially contaminated materials, XRD results have shown the formation of Ni-Sb alloys (NiSb x<0.08) whereas XPS data have indicated that a non-reducible antimony oxide, a well dispersed reducible Sb phase together with reducible Sb (that form an alloy with reducible Ni), were present. Selective chemisorption data for unsupported Ni-powders showed that one surface structure can effectively passivate 2-3 Ni atoms with respect to H2 chemisorption. XPS examination confirmed that Sb segregates at the surface of Ni particles where it can drastically affect the electron properties of neighboring Ni atoms thus reducing their activity. 

---