Underpotential deposition monolayers

Another intriguing approach to electrocatalysis involves the use of underpotential-deposited monolayers and submonolayers of foreign metal adatoms on metal substrates. Such layers afford unique electronic and morphological surface properties, not usually achievable with pure metal or alloys. Underpotential-deposited layers have been found to have high catalytic activity for such reactions as H2 generation, 02 reduction, and certain electro-organic reactions. 

The mass of an underpotentially deposited monolayer of lead on gold was measured in situ by using a quartz crystal (110). Crystals were operated at their third harmonic, and a frequency change of 46 Hz corresponding to a mass gain of 2.8 x 10 g/cm was noted. 

The additivity principle was well obeyed on adding the voltammograms of the two redox couples involved even though the initially reduced platinum surface had become covered by a small number of underpotential-deposited mercury monolayers. With an initially anodized platinum disk the catalytic rates were much smaller, although the decrease was less if the Hg(I) solution had been added to the reaction vessel before the Ce(lV) solution. The reason was partial reduction by Hg(l) of the ox-ide/hydroxide layer, so partly converting the surface to the reduced state on which catalysis was greater. 

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. 

Most recently, we have been able to obtain the in situ surface EXAFS spectrum of a half-monolayer of underpotentially deposited copper on a bulk Pt(lll) single crystal pretreated with iodine. The spectrum shown in Fig. 23 is a bit noisy (due to limited number of scans) but at least five well-defined oscillations can be observed. Preliminary data analysis indicates that the copper adatoms sit on threefold hollow sites with copper neighbors at 2.80 0.03 A. This distance is very close to the Pt—Pt distance in the (111) direction and indicates the presence of a commensurate... 

Figure 23. In situ X-ray absorption spectrum for half a monolayer of copper underpotentially deposited on a bulk Pt (111) electrode pretreated with iodine.

For the case of surface truncation rods, the technique is based on the detection of diffraction peaks between Bragg peaks. Although this requires careful alignment and some a priori knowledge of the structure, monolayer sensitivity can be achieved. In fact, Samant et a/.138 have recently performed an in situ surface diffraction study of lead underpotentially deposited on silver employing this technique along with grazing incidence diffraction. It is clear that this technique will also find widespread use in the near future. 

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],... 

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]. 

As an example [13] we consider the underpotential deposition of thallium on silver (Fig. 15.13). At potentials above the onset of the upd of thallium the SHG signal decreases, at first slowly, then more rapidly. The adsorption of thallium causes a strong rise in a(o ), because the region in which the electronic density decays to zero becomes more extended with an angle of incidence of 45° this shows up as a drastic increase in the signal. A similar behavior is seen in other systems, and often even fractions of a monolayer can be detected. 

Underpotential deposition is described as less than monolayer metal deposition on a foreign metal substrate, which occurs at more positive potentials than the equilibrium potential of a metal ion deposed on its own metal, expressed by the Nemst equation. Kolb reviewed state-of-the-art Underpotential deposition up to 1978. As Underpotential deposition is a process indicative of less than a monolayer metal on a substrate, it is expected to be quite sensitive to the surface stmcture of the substrate crystal a well-defined single-crystal electrode preparation is a prerequisite to the study of Underpotential deposition. In the case of Au and Ag single-crystal electrodes, Hamelin and co-workers extensively studied the necessary crystal surface structure, as reviewed in Ref. 2. 

The process of monolayer deposition of metal ions in underpotential deposition is strongly affected by anion-specific adsorption, and the two processes at the electrode interface must be elucidated if one is to understand underpotential deposition phenomena in a unified way. 

In situ STM images were observed in the cases of Zn underpotential deposition on Au(lll) with and without r in phosphate solutions/ Underpotential deposition of T1 on Au(lOO) and Au(ll 1) was investigated by X-ray scattering and STM in perchloric acid solution. Both measurements gave c(p x 2) monolayer structures before bulk deposi-... 

EQCM frequency of 20 Hz, which corresponds to a one-third monolayer of sulfate species adsorption/desorption. However, the electricity from the above cyclic voltammogram current is calculated to be about 1 x 10 C ctn i.e., 6 x 10 molecules cin" which is about one-tenth of a monolayer. This may indicate that sulfate adsorption on Au(lll) is associated with a partial charge transfer process. In Fig. 25b, an increase in EQCM frequency was observed as for (a), and a decrease in the frequency was observed at the Cu underpotential deposition region. The frequency change due to Cu underpotential deposition is determined to be 35 Hz,... 

Ex situ LEED and XPS studies were conducted to demonstrate the effects of CUand Br" these anions form densely packed incommensurate structures on Cu underpotential deposition at the full monolayer. ... 

One way to view UPD is as formation of a surface compound. In other words, deposition of the first atomic layer of an element on a second element involves a larger deposition driving force than subsequent layers, as it benefits from the AG of compound formation. For deposits formed at underpotential, once the substrate is covered the deposition stops because the reaction is surface limited. No more of the substrate element is available to react, unless it can quickly diffuse to the surface through or around the initially deposited monolayer (an example would be amalgam formation at a mercury electrode surface). Subsequent deposition is then only observed when the bulk deposition potential has been exceeded. 

For example, the voltammogram in Fig. 1 depicts Ag UPD on an I-coated Pt(lll) electrode [26], Three features can be attributed to the UPD of Ag, each of which results in the formation of a new structure on the surface, as indicated by the FEED patterns diagrammed in the circles. It was concluded in that work that UPD involved more than a single monolayer of Ag. Ag depositing at underpotential reacted with the Pt substrate as well as with the adsorbed I atom layer. It is also interesting to note that Ag underpotentially deposited in Fig. 1 reacted with the adsorbed atomic layer of I atoms to form a monolayer of the I-VII compound Agl on the Pt surface. 

Ft partial and full monolayers have been deposited onto alternative metal particle cores. Brankovic, Wang, and Adzic found Ft spontaneously deposited onto hydrogen-reduced Ru particles at a surface coverage of 0.11 and 0.50. An alternative method of depositing Ft onto Fd, Fd alloy, and Au particles has been developed by Sasaki et al. and Zhang et al. This involves the deposition of a Cu overlayer by underpotential deposition followed by galvanic displacement by Ft. Complete monolayers of Ft have been claimed and the technique has been extended to preparation of mixed monolayers of Ft and other precious metals. ... 

SAMs controlling electrochemistry, whereas the reverse holds for the other one, both topics are inseparably intertwined as exemplified by the underpotential deposition of metal on SAM-modified electrodes where patterned SAMs allow localization of metal UPD, which in turn affects the monolayer. 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

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