Bimetallic systems surface composition

The latter report demonstrated the unique ability of this technique to resolve surface structure as well as surface composition at the electrified solid-liquid interfaces. In particular, STM has become an important tool for ex situ and in situ characterization of surfaces at the atomic level, in spite its significant limitations regarding surface composition characterization for bimetallic systems, such as the lack of contrast for different elements and the scanned surface area being too small to be representative for the entire surface. To avoid these limitations, STM has been mostly used as a complementary tool in surface characterization. 

In our early work with bimetallic systems, we noticed that, depending on the preparation procedure in UHV, different surface compositions could be produced over the same bulk material owing to the phenomenon of surface segregation [Stamenkovic et al., 2002]. It was essential, then, to establish a methodology for transferring a well-defined bimetallic system into an electrochemical environment for further electrochemical characterization. 

Information on the chemical state of iridium on going from the molecular precursors, and its adsorption on the surface of the support can be obtained by Ir Mossbauer spectroscopy. It allows to estimate the composition of the Ir-containing alloys that are possibly formed during the activation treatment of supported bimetallic systems. The main results obtained in the application of Ir Mossbauer spectroscopy to characterize two Ir-containing bimetallic supported nanoparticles, i.e., Pt-Ir on amorphous silica and Fe-Ir on magnesia are presented and discussed... 

In addition to modification of surfaces by non-metals, the catalytic properties of metals can also be altered greatly by the addition of a second transition metaP ". Interest in bimetallic catalysts has arisen steadily over the years because of the commercial success of these systems. This success results from an enhanced ability to control the catalytic activity and selectivity by tailoring the catalyst composition . A long-standing question regarding such bimetallic systems is the nature of the properties of the mixed-metal system which give rise to its enhanced catalytic performance relative to either of its individual metal components. These enhanced properties (improved stability, selectivity and/or activity) can be accounted for by one or more of several possibilities. First, the addition of one metal to a second may lead to an electronic modification of either or both of the metal constituents. This... 

Surface-to-subsurface migrations in bimetallic NPs can completely change the surface composition and drastically alter catalytic performance. Despite the importance of atomic mobility in bimetallic systems, little is known about the dynamic processes of NPs and clusters due, in large part, to the lack of suitable experimental techniques. 

Among ordered bimetallic systems, the Pt-Sn one can be considered at present as the most in-depth studied not only for its surface structural properties, but also for its reactivity and catalytic properties. A comparable detailed knowledge exists only for a few other cases, among platinum alloys we can cite the Ni-Pt and Co-Pt systems, examined for their catalytic properties and the Pt-Ti system studied for their electrocatalytic properties [5]. Sparse data relative to the surface properties of several other Pt alloys exist (e.g. FeaPt and CuaPt -[3] and PtaMn [51]. All these data available pertain to fee phases either random substitutional or ordered compounds. Data exist also for other cubic ordered alloys which are isostructural with the PtaSn compound, e.g. NiaAl [52, 53] and AuaPd [28] and finally the Au-Cu system, which has been object of interest as the prototypical LI2 or Pm3m ordered system in the CuaAu composition [54, 55]. 

The chemistry of bimetallic surfaces is controlled largely by the composition of the outermost layer of the surface and to a lesser extent by the layers immediately below the surface. It has been predicted for many years that the composition of the bulk of a bimetallic system will differ strongly from that of the surface and a vast number of publications exists where researchers have attempted to measure or calculate the composition of bimetallic surfaces. 

The ability to be able to ignore the presence of the adsorbate atoms in calculating the surface composition is a powerful aspect of the MEIS technique. However, it should be stated that this technique has so far only been used for single crystal substrates. No useful information has yet been reported for more catalytically relevant systems such as supported bimetallic clusters. 

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... 

Preliminary results indicate that the binding energy of acetate and CO increases if gold is substituted for palladium at sites which are located one layer beneath the surface. The binding energy, however, decreases if the gold is actually substituted into the surface. More work on the spatial and compositional effects, however, is required to better understand Pd/Au and other bimetallic systems. 

Surface Composition. - Bimetallic catalyst systems have received much interest because variation in alloy composition offers a ready method of altering the metallic properties of the catalyst. For a range of Fe-Ni catalysts, Matsuyama et al have attempted to answer a question fundamental to such systems, namely, how does the surface composition compare with that of the bulk Powdered Fe-Ni catalysts were prepared from a solution of Fe(N03)2 and Ni(N03)2- The mixture also contained radioactive [63-Ni] which emits 3 radiation with an f max of 67 keV and a penetration of about 200 layers of heavy metal. It was possible to measure the amount of Ni which existed in the surface layers of the alloy, since 3-cmission from the underlayers of the metal was weakened by self-absorption. 

This finding, together with the XRD data for the nanocrystal core properties [58], demonstrated that both the core and the surface of the bimetallic nanopartides exhibit bimetallic alloy properties. The detection of both Au-atop and Pt-atop CO bands on the surface of the alloy nanoparticles of a wide range of bimetallic composition can be correlated with the electronic effect as a result of the d-band shift of Pt in the bimetallic nanocrystals. There exists a stronger electron donation to the CO band by a Pt-atop site surrounded by Au atoms in the bimetallic alloy surface than that from the monometallic Pt surface as a consequence of the upshift in d-band center of Pt atoms surrounded by Au atoms, which explains the preference of Pt-atop CO over the Au-atop CO adsorption. The observed decrease of the Pt-atop CO band frequency with increasing Au concentration is in agreement with the d-band theory for the bimetallic system [172]. 

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