Underpotential deposition oxidative

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Colletti LP, Teklay D, Stickney JL (1994) Thin-layer electrochemical studies of the oxidative underpotential deposition of sulfur and its application to the electrochemical atomic layer epitaxy deposition of CdS. J Electroanal Chem 369 145-152... 

Alois GD, CavaUini M, Innocent M, Foresti ML, Pezzatini G, GuideUi R (1997) In situ STM and electrochemical investigation of sulfur oxidative underpotential deposition on Ag(lll). J Phys Chem B 101 4774 780... 

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. 

Although not the subject of this article, double layer studies are briefly discussed in this paragraph in order to demonstrate that ex situ XPS studies indeed provide information about the state of the electrode exposed to an electrochemical environment at a defined potential. A crucial step in any ex situ experiment is the emersion of the electrode. Here the question arises whether the electrochemical double layer or part of it is preserved at the interface after emersion and transfer. Winograd et al. [10,11] first demonstrated that the electrode under UHV conditions still remembers the electrode potential applied at the time of emersion. These authors investigated oxide formation on Pt and the underpotential deposition of Cu and Ag on Pt by means of XPS and proved that the electrochemically formed oxide layer and... 

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. 

The catalytic properties of a Pt/Sn combination were observed on different kinds of electrode materials alloys [90], electro co-deposits of Pt and Sn [89, 90], underpotential deposited tin [42] or a mixture of tin oxide and platinum deposited on glass [95], All different materials present a marked influence on methanol electrooxidation. 

Melroy and co-workers88 recently reported on the EXAFS spectrum of Pb underpotentially deposited on silver (111). In this case, no Pb/Ag scattering was observed and this was ascribed to the large Debye-Waller factor for the lead as well as to the presence of an incommensurate layer. However, data analysis as well as comparison of the edge region of spectra for the underpotentially deposited lead, lead foil, lead acetate, and lead oxide indicated the presence of oxygen from either water or acetate (from electrolyte) as a backscatterer. 

Electrodes modified by underpotential deposition of metal were subjected as electrocatalysts to reduction of oxygen,oxidation of formic acid, and other processes in which polycrystalline metal substrates were used (see review in Ref. 151). Electrocatalysis of single-crystal electrodes modified by underpotential deposition was also investigated, as reviewed by Ad2iC. ... 

Since a well-defined condition of the underpotential deposition metal at a definite coverage is better for studying its modified surface at a definite coverage, an irreversibly adsorbed underpotential deposition metal is desirable Sb on Pt(lll) for CO oxidation,Bi and Te on Pt(lOO) for formic acid oxidation, " Sb + Bi on Pt(lOO) for formic acid oxidation, and others found in these references. 

A special case is when the electrochem-ically active components are attached to the metal or carbon (electrode) surface in the form of mono- or multilayers, for example, oxides, hydroxides, insoluble salts, metalloorganic compounds, transition-metal hexacyanides, clays, zeolites containing polyoxianions or cations, intercalative systems. The submonolayers of adatoms formed by underpotential deposition are neglected, since in this case, the peak potentials are determined by the substrate-adatom interactions (compound formation). From the ideal surface cyclic voltammetric responses, E° can also be calculated as... 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

Underpotential deposited layers have a strong effect on the electro-catalytic properties of electrodes for surface-sensitive reactions such as organic oxidations, hydrogen evolution, oxygen reduction, etc. A review on this subject has recently been published by Adzic [131a, b]. 

In the case of oxides or sulphides, poisoning has been observed to be very weak [99,168]. One reason is that these materials, especially oxides, are usually obtained as porous layers [169, 170]. But another, more exciting explanation is that metal deposition on non-metallic surfaces is more difficult than on metals, since the bond formed between the surface and deposited metal atoms is weaker [168], Therefore, no underpotential deposition takes place, which pushes the potential range of impurity deposition to more cathodic potentials. If deposition takes place, it occurs at a high overpotential, and the resulting discharged particles form clusters rather than monolayers, thus leaving most of the active surface uncovered. 

Oxide electrodes have been observed to be almost immune from poisoning effects due to traces of metallic impurities in solution [99]. This is undoubtedly due primarily to the extended surface area. It can be anticipated that the calcination temperature must have a sizable effect. But in addition, a different mechanism of electrodeposition must be operative. Chemisorption on wet oxides is usually weak because metal cations are covered by OH groups [479]. As a consequence, underpotential deposition of metals is not observed on Ru02, although metal electrodeposition does takes place. However, electrodeposited metals give rise to clusters or islands and not to a monomolecular layer like on Pt. Therefore, the oxide active surface remains largely uncovered even if metallic impurities are deposited [168]. Thus, the weak tendency of oxides to adsorb ions, and its dependence on the pH of the solution is linked to their favorable behavior observed as cathodes in the presence of metallic impurities. 

Pitner and Hussey studied the electrochemistry of tin in acidic and basic AICI3/I-ethyl-3-methyl-imidazolium chloride-based ionic liquids by using voltammetry and chronoamperometry at 40 °C [15]. They reported that the Sn(II) reduction process is uncomplicated at a platinum substrate, where in the atidic ionic liquid the reduction wave was observed at +0.46 V on the Pt electrode and the oxidation at +0.56 V. When they used a gold electrode instead of platinum, they observed an underpotential deposition of a tin monolayer and an additional underpotential deposition process that was attributed to the formation of tin-gold alloy at the surface. The deposition of tin on glassy carbon was controlled by nudeation. 

With increasing amounts ofbenzoic acid (0 to 0.38 wt.%) we observe a decreasing peak current for the aluminum deposition (see Figure 8.7), the aluminum oxidation peak disappears and the underpotential deposition (UPD) of aluminum is also strongly diminished. These experiments lead to the conclusion that the additive molecules block the active growth sites and therefore the peak current of aluminum deposition increases with decreasing additive concentration. A further consequence... 

Lead and mercury are deposited as micron-sized clusters, predominantly at intercrystallite boundaries [105] so does lithium from the polyethylene oxide solid electrolyte. What is more, Li intercalates into the sp2-carbon [22, 138], Thus, observations on the Li intercalation and deintercalation enable one to detect non-diamond carbon on the diamond film surface. Copper is difficult to plate on diamond [139], There is indirect evidence that Cu electrodeposition, whose early stages proceed as underpotential deposition, also involves the intercrystallite boundaries [140], We note that diamond electrodes seem to be an appropriate tool for use in the well-known electroanalytical method of detection of traces of metal ions in solutions by their cathodic accumulation followed by anodic stripping. The same holds for anodic deposition, e.g. of, Pb as PbCh with subsequent cathodic reduction [141, 142], Figure 30 shows the voltammograms of anodic dissolution of Cd and Pb cathodically predeposited from their salt mixtures on diamond and glassy carbon electrodes. We see that the dissolution peaks are clearly resolved. The detection limit for Zn, Cd, and Pb is as low as a few ppb [143]. 

Types of electrode/solution interface studied include oxide films on metals, monolayer deposits obtained by underpotential deposition, adsorption, and spectroelectrochemistry in thin-layer cells. 

Noble metal electrodes include metals whose redox couple M/Mz+ is not involved in direct electrochemical reactions in all nonaqueous systems of interest. Typical examples that are the most important practically are gold and platinum. It should be emphasized, however, that there are some electrochemical reactions which are specific to these metals, such as underpotential deposition of lithium (which depends on the host metal) [45], Metal oxide/hydroxide formation can occur, but, in any event, these are surface reactions on a small scale (submonolayer -> a few monolayers at the most [6]). 

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. 

The Cat, or its product of electrode oxidation or reduction Cat, is immobilized at the electrode surface and decreases the overpotential for oxidation or reduction of the S, without being involved in the chemical redox reaction with the S. Typical example is the catalytic effect of underpotential deposited layer of lead on a platinum electrode, on anodic oxidation of methanol [v]. 

The high mass sensitivity of ETSM sensors renders them particularly suited for the analysis of monolayer and submonolayer films. In fact, the earliest applications of the ETSM involved studying the electrochemical deposition of monolayers, including the formation of metal oxides [207], electrosorption of halides [208], and the underpotential deposition of metal atoms [209-213]. In some cases, the electrovalency (i.e., the ratio of moles of electrons transferred at the electrode to moles of adsorbate deposited) was found to vary with adsorbing species the adsorption of iodide onto gold, for example, occurs with complete charge transfer from the halide to the electrode, whereas the adsorption of bro-... 

An interesting aspect of UPD (underpotential deposition), which may be of great practical importance, is its effect on electrocatalysis. Studies of the oxidation of organic molecules on platinum have shown a significant catalytic effect, caused by a UPD layer of lead, as shown in... 

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