Bulk-Metal Deposition

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

In some cases, the electrode material is the limiting factor of the electrochemical stability window. In a metal salt solution, underpotential deposition (UPD) may occur. In some examples, such as gold or platinum electrodes in the presence of lithium ions, the UPD appears at potentials that are substantially higher than the bulk metal deposition [4-6], In addition, some metals may possess catalytic activity for specific reduction or oxidation processes [7-12], Many nonactive metals (distinguished from the noble metals), including Ni, Cu, and Ag, which are commonly used as electrode materials, may dissolve at certain potentials that are much lower than the oxidation potentials of the solvent or the salt. In addition, some electrode materials may be catalytic to certain oxidation or reduction processes of the solution components, and thus we can see differences in the stability limits of nonaqueous systems depending on the type of electrode used. 

Underpotential deposition Underpotential deposition (upd) occurs when monolayers (or submonolayers) of a metal ad-atom are deposited on a foreign metal substrate at potentials positive of the reversible Nernst potential for bulk deposition [16]. Monolayers will only form when a low work function metal is deposited onto the surface of a higher work function substrate. In this case, the metal ad-atom-substrate bond is greater than the ad-atom-ad-atom bond formed in bulk metal deposition. Upd phenomena have been the subject of extensive work using SPMs and of particular interest is the role of coadsorbed anions on this process, as a function of electrode potential. 

Foreign metal ad-atoms can be deposited on an electrode electrochemically in the so-called underpotential region, that is, the potential region positive to the reversible Nernst potential for the bulk metal deposition. 

Saturate coverages with ad-atoms are obtained at potentials close to the potential for the bulk metal deposition. The maximum surface concentration is about Fmax ... 

The-preparatlon of Pd+Cu catalysts has been studied by consecutive reduction of Cu onto Pd or Pd/C. In general, bulk Cu deposition is kinetically more favourable, although adsorbed Cu (submonolayer) is more stable. Various methods for the elimination of bulk metal deposition, are discussed. Hydrogenation in formic acid or ionadsorption followed by hydrogenation are found to be suitable methods for practical application. Pd/C catalysts modified by bulk or adsorbed Cu, have different selectivities in the partial reduction of 4-... 

From catalytic aspects latter structure seems to be more favourable. In this paper, through the example of Pd+Cu catalysts, we wish to present methods by which bulk metal deposition can be avoided, and bimetallic catalysts with well-designed uniform surface structure can be attained. The surface structure of bimetallic catalysts has been studied by an electrochemical polarization (EP) method and catalytic properties were analyzed by selective reduction of 4-chloronitrobenzene (CNB). 

The electrode potential of an adsorbed metal/metal ion system can be calculated from the work function differencies of base and adsorbed metals (ref.11), or derived from polarization curves. On the basis of the above equations, by appropriate choice of the metal ion concentration and pH values, the potential conditions given in Fig. 3 can be attained. In this way, in aqueous acidic medium, a number of metals with standard electrode potentials negative to hydrogen (Table 1.) are capable of adsorption on Pd without bulk metal deposition, e.g. Pb or Cd (ref.15). 

In the case of Cu, however, which is a metal with an electrode potential positive to hydrogen, bulk metal deposition in aqueous medium cannot be avoided. By changing the pH value, Cu concentration, temperature, and H2 pressure, no realistic values preventing bulk metal deposition can be attained. 

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