Metal , underpotential deposition

In view of the overwhelming success of PS in surface science, it is not surprising that XPS has been used rather early for the study of electrochemically modified electrode surfaces. Winograd et al. [10-12] were the first to use this spectroscopy for the study of oxide formation on Pt electrodes and also for the investigation of metal underpotential deposition (UPD) on Pt. Although a standard surface analytical tool, XPS has not found a corresponding consideration in electrochemistry. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Another important area in which X-ray surface scattering is applied is the underpotential deposition of metals. Underpotential deposition is the phenomenon by which one metal deposits on another at a potential positive of its normal reduction potential. For example, Pb deposits at underpotentials on Ag. This is due to the fact that the Gibbs energy for formation of a Pb-Ag bond is less than that for formation of a Pb-Pb bond. Other metals which undergo underpotential deposition on Ag, Au, and Pt are T1 and Bi. On the basis of the electrochemistry observed in formation of the metal monolayers, there is good reason to expect that they are well ordered. Tl, Pb, and Bi all form an incommensurate monolayers on Au(lll). On Au(lOO), Tl and Bi form an incommensurate monolayer with a c(2 X 2) surface structure [14]. On the other hand, underpotential deposition of Pb on Ag(lll) leads to an incommensurate monolayer [13]. These studies demonstrate clearly that the nature of the monolayer formed depends on both the nature and structure of the substrate metal. 

CHANGES IN THE SURFACE STRUCTURE OF PLATINUM-TYPE METAL INDUCED BY METAL UNDERPOTENTIAL DEPOSITION AND ANION ADSORBATES... 

Summary. The potential of in-situ scanning probe techniques for the local investigation of surface properties and reactions at "nonideal" electrodes is presented in a typical example in the field of metal underpotential deposition, the essential role of the step dislocations for the local progress of adsorbate formation and also for the longterm adsorbate stability is shown and discussed for the adsorption of Pb and TI monolayers at stepped Ag(l 11) electrodes. 

Recently, the structure of the solid/liquid interface has been studied with a wide range of in-situ structural techniques. In particular, scanned probe microscopes [1-5] and synchrotron-based methods [6-9] have yielded a wealth of structural information. The ultimate goal of this work is an understanding of the structure and reactivity of the electrode surface at the atomic level. One of the most extensively studied processes is metal underpotential deposition (UPD) [10], which involves the formation of one or more metal monolayers at a potential positive of the reversible Nemst potential for bulk deposition. 

Huckaby D A and Blum L 1991 A model for sequential first-order phase transitions occuring in the underpotential deposition of metals J. Eiectroanai. Chem. 315 255-61... 

Zinc and tin The electrodeposition of Zn [52] has been investigated in acidic chloroaluminate liquids on gold, platinum, tungsten, and glassy carbon. On glassy carbon only three-dimensional bulk deposition was observed, due to the metal s underpotential deposition behavior. At higher overvoltages, codeposition with A1... 

Copper electrodeposition on Au(111) Copper is an interesting metal and has been widely investigated in electrodeposition studies from aqueous solutions. There are numerous publications in the literature on this topic. Furthermore, technical processes to produce Cu interconnects on microchips have been established in aqueous solutions. In general, the quality of the deposits is strongly influenced by the bath composition. On the nanometer scale, one finds different superstmctures in the underpotential deposition regime if different counter-ions are used in the solutions. A co-adsorption between the metal atoms and the anions has been reported. In the underpotential regime, before the bulk deposition begins, one Cu mono-layer forms on Au(lll) [66]. 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

The energetic aspects of underpotential deposition can be investigated by a slow (i.e., a few millivolts per second) potential scan starting at a potential so high that no adsorption takes place. As the potential is lowered, one or more current peaks axe observed, which are caused by the adsorption of the metal ions (see Fig. 4.9). According to the usual convention, the adsorption current is negative (i.e., cathodic). Different peaks may correspond to different adsorption sites, or to different structures of the adsorbate layer. If the potential is scanned further past the equilibrium potential bulk deposition is observed. 

In order to investigate the dependence of a fast reaction on the nature of the metal, Iwasita et al. [3] measured the kinetics of the [Ru(NH,3)6]2+/3+ couple on six different metals. Since this reaction is very fast, with rate constants of the order of 1 cm s-1, a turbulent pipe flow method (see Chapter 14) was used to achieve rapid mass transport. The results are summarized in Table 8.1 within the experimental accuracy both the rate constants and the transfer coefficients are independent of the nature of the metal. This remains true if the electrode surfaces axe modified by metal atoms deposited at underpotential [4]. It should be noted that the metals investigated have quite different chemical characteristics Pt, and Pd are transition metals Au, Ag, Cu are sd metals Hg and the adsorbates T1 and Pb are sp metals. The rate constant on mercury involved a greater error than the others... 

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