Underpotential work function

Fig. 31. Work function change, referred to the clean Pt electrode surface, as a function of electrode potential for Ag underpotential deposition onto Pt. The work function of bulk Cu would correspond... 

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. 

An empirical treatment developed by Kolb et al. [81, 82] relating UPD behavior to the difference in work function between the substrate and depositing species has been used to explain anomalous co-deposition behavior observed in Ni-Fe and Ni-Zn alloys [83]. Although the relationship appears to hold for pure underpotential deposition limited to a monolayer, it does not satisfactorily predict bulk alloy behavior. For example, based on work function data alone, one would expect Zn-Al and Sb-Al alloys to be formed by underpotential alloy deposition. Recent reports in the literature, however, indicate that alloying in these systems does not occur [46, 84]. 

Willis, underpotential deposition. 1313 Wojtowicz, 1381 Work function of the metal, 887 and chemical potential 835 definition, 835 in electrochemistry, 835 and surface potential, 835 underpotential deposition and, 1316 Working electrode, potential, 1061 Wright, 1495... 

Fig. 7.145. Underpotential shift AEp, in different S(poly)/Mez+ systems as a function of the difference of electron work functions of polycrystalline S and Me, AO = Os - OMe. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, p. 50, copyright 1996 John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.)...

Kolb [109] found a correlation for a large number of upd systems of the underpotential at 0upd = 0.2 from voltametric curves and the differences between work functions of the substrate and the corresponding metal... 

Swathirajan and Bruckenstein [113] have recently discussed the thermodynamics and kinetics of underpotential deposition phenomena on polycrystalline electrodes. Since work functions and surface structural features vary in different crystal faces, upd phenomena on single crystals have shown distinctive differences [109]. 

Underpotential deposition of metals is a commonly observed phenomenon, which has been found to occur for dozens of metal couples in both aqueous and nonaqueous media. It is characterized by the distance between the peak potential observed during a slow cathodic sweep, and the reversible potential for deposition of the metal in the same solution. This potential difference has been related to the difference in the electronic work function of the two metals concerned, although this correlation does not always liold strictly. Much research has been devoted to the study of underpotential deposition of metals on single... 

An attempt was made by Gerischer, Kolb, and Przasnyski [3.122, 3.229, 3.230] to empirically correlate the difference of work functions of 3D S and 3D Me phases, A0= 0s- 0Me, with a characteristic underpotential shift, AEp, corresponding to the most positive Meads desorption peak in an anodic stripping curve (cf. Figs. 3.2-3.6). A linear relation... 

Quite different adsorption characteristics are formed in the case of the underpotential deposition of metal ions [49]. As described in section 10.2, underpotential deposition (UPD) is the process by which a metal ion adsorbs on a different metal substrate at a potential more positive than that at which it is electrodeposited on itself. UPD occurs when there is a significant difference between the work functions of the depositing metal phase M and the substrate metal S. Consider as an example the electrodeposition of Pb on Au. The work function of polycrystalline Au is approximately 300 mV greater than that of polycrystalline Pb (see table 8.2). This also means that the PZC of the Au electrode is positive of that for Pb. As a result, Pb adsorbs on Au more readily than it does on Pb. The adsorption process is accompanied by significant charge transfer. In fact, an estimate of pad for Pb " " on Au is close to zero, indicating that the cation is essentially discharged. 

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. 

Kolb et al. (23) observed that the potential of stripping of the bulk deposit is shifted towards lower po. n ials with respect to the potential of desorption of the UPD layer this difference was defined as the underpotential shift, AEp. The underpotential shift was correlated to the difference in work functions, O, between the substrate, S, and the UPD species, M, and expressed by the following equation ... 

Kolb DM, Frzasnyski M, Gerischer H (1974) Underpotential deposition of metals and work function differences. J Electroanal Chem 54(l) 25-38... 

The isotherm has a Langmuir basis and implies additivity of the various UPD adatom-substrate and adatom-adatom interactions. The AE q term represents the underpotential of the most positive stripping peak of the UPD adlayer where the UPD ML coverage approaches zero. The term is the Temkin parameter describing the UPD layer-substrate interactions such as the electrode work function change with the UPD adlayer coverage. The term is the Frumkin parameter representing the lateral adatom interactions within the UPD adlayer. 

One of the most important applications of EC-STM has been for investigations of metal electrodeposition, due to the great importance of the latter in materials science fields. Both underpotential deposition (UPD), jjj which a monolayer of metal adatoms is generated at a lower work function metal substrate,and bulk deposition - have been investigated using EC-STM for numerous metal/substrate systems. EC-STM studies of UPD on Au surfaces include observations of the deposition of Ag, 2 Cu, Cd, Pb, and Hg on Au(lll). The strength of EC-STM as a... 

The values of AE jpd observed ejcperimenially have been related to the difference in the electron work function of the two metals concerned. Much research has been devoted to the study of underpotential deposition on single-crystal metal substrates. 

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