Catalysis, bimetallic surfaces

Bimetallic surfaces are well known for showing radically different chemistry from the individual components and the catalysis industry frequently makes use of these properties to tune catalysts, a recent example is the alloying of... 

This section reports a series of examples of application of the cluster model approach to problems in chemisorption and catalysis. The first examples concern rather simple surface science systems such as the interaction of CO on metallic and bimetallic surfaces. The mechanism of H2 dissociation on bimetallic PdCu catalysts is discussed to illustrate the cluster model approach to a simple catalytic system. Next, we show how the cluster model can be used to gain insight into the understanding of promotion in catalysis using the activation of CO2 promoted by alkali metals as a key example. The oxidation of methanol to formaldehyde and the catalytic coupling of prop)me to benzene on copper surfaces constitute examples of more complex catalytic reactions. 

Quantum chemical simulations based on density functional theory (DFT) are widely regarded as reaching the appropriate compromise between chemical accuracy and the need to study structurally complex extended materials in order to tackle problems associate with heterogeneous catalysis involving alloys. A review of DFT and heterogeneous catalysis can be found in the previous SPR and that review also listed several general reviews of applications and foundations of DFT. Experimental and theoretical studies of monolayer bimetallic surfaces were recently reviewed. In... 

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]. 

This chapter eoncentrates primarily on issues of heterogeneous catalysis. The thermodynamic and kinetic factors are outlined that are important in defining the surfaee chemistry of bimetallic surfaces. In addition, the various approaches will be introdueed that are utilised by surfaee scientists in an attempt to measure the composition of bimetallic surfaces under the influence of adsorbates. Furthermore, the ehapter will investigate the difficulties encountered when attempting to obtain accurate measurements on nanoseale bimetallie particles under environments typically encountered in a eatalytic reaction. By way of contrast, the relevance of much more aceurate measurements on well-defined surfaces under idealised ultrahigh vacuum (UHV) conditions will be questioned. 

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. 

Chemical reconstruction of alloy and/or bimetallic surfaces will be more complex, and it is obvious that the catalysis of alloy and/or bimetallic surface can not be explained by the traditional idea of ligand effect and ensemble effect for the sites. That is, chemical reconstruction of alloy surface will occur by selective segregation or selective reaction of metal atoms. As a result, a new surface will be prepared sometimes, which is responsible to the prominent catalytic activity and/or selectivity of alloy and/or bimetallic surfaces. If this is the case, the catalysis of alloy surface is entirely different form the idiomatic idea of ligand effect" and " ensemble effect". 

Consequently, the roles of Rh atoms in the three way catalyst are summarized as i) NO + H2 reaction is structure sensitive on Pt and Rh surfaces but is structure insensitive on the Pt deposited Rh as well as on Rh deposited Pt surfaces, that is, the bimetallic surfaces are activated during catalysis by the adsorption of NO and/or O2, and ii) the active surface composed of Rh-0 and Pt atoms may require about 0.3 monolayer of Rh on the Pt surface, iii) Voltammogram is a valuable method to diagnose the surface during catalysis or by adsorption. 

Surface Chemistry and Catalysis on Some Platinum-Bimetallic Catalysts... 

Since 1976 until present time Toshima-t5q)e nanocolloids always had a major impact on catalysis and electrocatalysis at nanoparticle surfaces [47,210-213,398-407]. The main advantages of these products lie in the efficient control of the inner structure and morphology especially of bimetallic and even multimetallic catalyst systems. 

Bimetallic nanoparticles (including monometallic ones) have attracted a great interest in scientific research and industrial applications, owing to their unique large sur-face-to-volume ratios and quantum-size effects [1,2,5,182]. Since industrial catalysts usually work on the surface of metals, the metal nanoparticles, which possess much larger surface area per unit volume or weight of metal than the bulk metal, have been considered as promising materials for catalysis. 

Markovic NM, Radmilovic V, Ross PN. 2003. Physical and electrochemical characterization of bimetallic nanoparticle electrocatalysts. In Wieckowski A, Savinova E, Vayenas C, eds. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Dekker, pp. 311-342. 

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