Bimetallic catalysts single-crystal surfaces

For a further discussion of the structure and properties of bimetallic systems, see Sections 2.6 and 3.2.3 for the preparation of bimetallic catalysts, see Section 4.6 and for the mechanisms by which they work in oxidations, see Section 8.2.2. Most textbooks of physical chemistry have sections on adsorption and catalysis, but they frequently focus on studies made under ultra-high vacuum conditions with single crystal surfaces. While this work produces beautiful pictures, it has limited relevance to the more mundane world of practical catalysis. Other introductory treatments of about the level of this chapter, or slightly more advanced, are available,5,7,11 as are deeper discussions of the kinetics of catalysed reactions.12 14 Industrial processes using catalysts have also been described in detail.15,16... 

Bimetallic alloy surfaces are also of great importance. Most metallic materials used commercially, and in particular metal-based, heterogenous catalysts, have multicomponent alloy phases. Despite their obvious importance, alloy single-crystal surfaces have not been studied so extensively in the past. A first step in understanding the chemistry of these surfaces is a thorough characterization of the structure of such surfaces. These attempts are part of efforts to overcome the material gap between commercial catalysts and surface-science studies. 

While attempting to use platinum in fuel cells, it has been demonstrated that its surface exhibits important electrocatalytic activities toward the oxidation of organic compounds. However, this effect can sometimes be enhanced by the use of bimetallic surfaces [1-10]. The physical mixture and the electronic interaction of the alloy components lead to a modification in the interaction between the adsorbate and the substrate in an electrocatalytic reaction. As a consequence of the structural changes at the single crystal surfaces during the electrochemical activation (examined with in situ STM) [11], it has been demonstrated that most of the catalysts are constituted by randomly oriented islands [12-14]. 

For single crystal surfaces, a reaction is deemed insensitive if its rate is about the same on all low Miller index planes, but since these differ from small metal particles in not having atoms of very low co-ordination number, the term face sensitivity should be used in this case. Two further approaches to the general problem have been tried (1) systematic variation of particle in supported metal catalysts, and (2) alteration of the composition of the surface of bimetallic catalysts, either supported or unsupported (Section 5.7). These lead respectively to particle size sensitivity and ensemble size sensitivity, but the three types are not necessarily exactly the same. 

Many types of bimetallic catalysts have been used to investigate electrocatalytic performance, including single-crystal surfaces [6], sputtered particles [5,7], carbon-supported catalysts fabricated by impregnation reduction, and colloidal nanocrystals (NCs). Bimetallic single crystals are indispensable when investigating facet-specific electrocatalytic properties sputtered bimetallic particles can be used as model catalysts with a clean surface to study the effect of composition on... 

Johnson et al.67 studied CO hydrogenation on bimetallic catalysts consisting of cobalt overlayers on W (100) and (110) single crystals at 200°C, 1 bar at a H2/ CO ratio of 2. AES spectra showed the postreaction Co/W surfaces to have high coverages of both carbon and oxygen, with carbon line shapes characteristic of bulk carbidic carbon.67 The catalytic activity apparently could not be correlated with surface carbon level.67... 

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. 

The studies reviewed here are part of a continuing effort (4-10) to identify those properties of bimetallic systems which can be related to their superior catalytic properties. A pivotal question to be addressed of bimetallic systems (and of surface impurities in general) is the relative importance of ensemble (steric or local) versus electronic (nonlocal or extended) effects in the modification of catalytic properties. In gathering information to address this question it has been advantageous to simplify the problem by utilizing models of a bimetallic catalyst such as the deposition of metals on single- crystal substrates in the clean environment familiar to surface science. 

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

Abstract Thermally stable, ordered surface alloys of Sn and Pt that isolate threefold Pt, twofold Pt, and single-Pt atom sites can be produced by controlled deposition of Sn onto Pt single crystals and annealing. The strnctnre was established by characterization with several techniques, including ALISS, XPD, LEED, and STM. Chemisorption and catalysis studies of these well-defined, bimetallic surfaces also define the overall chemical reactivity of Pt-Sn alloys, clarify the role of a second-metal component in altering chemistry and catalysis on Pt alloys, and develop general principles that describe the reactivity and selectivity of bimetallic alloy catalysts. 

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

Three types of work have been undertaken using (1) single crystal and polycrystalline alloys and surface alloys, (2) intermetallic compounds, and (3) conventional supported bimetallic catalysts. 

To mimic the structural and electrOTiic envirmunent of the ft layers surrounding a particle core with smaller lattice parameters in the dealloyed Pt-Cu catalysts, we extended and applied these ideas to bimetallic single-crystal model surfaces that consist of ft overlayers with various thicknesses grown oti a Cu(lll) substrate. 

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