Alloying surface structure sensitivity

R.C. Yeates and G.A. Somorjai. Surface Structure Sensitivity of Alloy Catalysis Catalytic Conversion of /i-Hexane over Au-Pt(lll) and Au-Pt(lOO) Alloy Cry.stal Surfaces. / Catal. 103 208 (1987). 

For iron, cobalt, nickel, and their alloys, the most sensitive technique for characterizing the particle surface is the measurement of magnetic properties. Thus, we synthesized cobalt nanoparticles of 1.6 nm (ca. 150 atoms), 2 nm (ca. 300 atoms) and 4 nm (a few thousand atoms) mean size. The structure of the particles is hep in the latter case and polytetrahedral in the first two cases. The 4 nm particles display a saturation magnetization equal to that of bulk... 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Is it known that the rate of hydrogenolysis reactions are extremely sensitive to effects of alloying, surface contamination, poisoning, etc. Consequently, in all cases where supported metals are used there must be concern as to whether apparent particle size effects are due to structure sensitivity or to some minor contamination effect. In the few cases where clean single crystal surfaces have been used there is evidence of a structure effect.338 However, the maximum change in activity between different crystal faces seems to be about a factor of 10. For Ni single crystals the (100) surface is more active than the (111) surface. A similar conclusion has been reached for oriented Ni powder samples.339... 

Many of the Cu 100 based surface alloys discussed in this chapter are relatively well characterised in terms of their layerwise compositional profile, geometric structure and thermal stability. However, it is clear that the majority of structural studies performed to date have made the (often necessary) assumption that a single homogeneous structural phase with a somewhat idealised compositional profile is present. In many cases, particularly for adsorbates which exhibit considerable bulk solubility in copper this may be a oversimplification. Future work to investigate the sensitivity of quantitative probes of surface structure and composition such as LEED, ion scattering spectroscopies and photo-electron diffraction to structural heterogeneity will be invaluable. 

The chemisorption of acetylene on the Sn/Pt(100) surface alloys revealed similar chemistry and provided additional information on the structure sensitivity of these reactions [53, 54]. While 15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3V2xV2)R45°-Sn/Pt(100) alloy, no such benzene desorption occurred from related surfaces, as shown in Fig. 2.6. 

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... 

The electro-oxidation of H2/CO mixtures is a very complicated reaction with many variables, e.g., temperature, CO concentration, and pH. From a practical standpoint, the structure sensitivity is not so interesting, since at any realistic level of CO, e.g., >10 ppm CO, and temperature the pure-Pt surface is highly poisoned by adsorbed CO, and the electrode polarization is impractically large. It is, however, still of fundamental importance to know the structure sensitivity of the reaction on pure Pt in order to understand the properties of Pt-based alloy catalysts that are not so highly poisoned by the CO, i.e., so-called CO-tolerant catalysts. For our purposes here, we discuss only one characteristic measure of the structure sensitivity, shown in Figure 14. For a wider range of results, we refer the interested reader to [54] and references therein. Figure 14 shows the current for H2 oxidation at 50-mV... 

The dissociative adsorption of the oxygen molecule is unlikely and the first electron transfer to the oxygen molecule [Eq. (63)] is considered to be the rate determining step. The formation of H2O2 and various forms of platinum oxides on the surface reduces the theoretical potential obtainable by ORR. In fact, potential losses in PEMFC arise predominantly because of the sluggishness of the ORR. The structural sensitivity of ORR has been discussed over the Pt-based electrodes. Pt/C is commonly used for the cathode material, and Pt-based alloys, such as PtCr and PtNi, have also been investigated. 

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