Adatoms oxidation

On the other hand, in certain cases (e.g., Ad/Pt(l 11) in HCIO4 solutions), the adatom oxidation and anion adsorption on the free sites (OH adsorption in HCIO4) overlap. Then, another refinement to this analysis would be to use (7.7) and (7.10) to calculate qp according to... 

Applying the common equations for the thermodynamics of reversible cells, it is possible to extract energetic parameters for the adatom redox reaction. This approach requires the measurement of voltammograms at different temperatures. If we consider that the adatom oxidation reaction involves the formation of the hydroxide, we can write the following equation for the overall cell reaction ... 

It is worth mentioning that, in some cases, the adatom oxidation reaction may involve the formation of oxide species. In particular, the oxidation of electronegative adatoms, such as Te and Se, involves the interchange of four electrons, giving rise to Te(IV) and Se(IV) species. The formation of tetravalent hydroxide species seems very unlikely, both for steric reasons and because of the strong polarizing character of the cation, and hence the following equation for the overall cell reaction can be proposed ... 

At low CO coverages, the adatom oxidation peak can be distinguished from the CO oxidation peak. Lateral interactions between CO and the adatom stabilize the elemental Bi state, increasing the potential of the adatom redox peak. For As, a displacement of the redox peak to lower potentials is observed, indicating an stabilization of the As(III) state on the CO-As mixed adlayer. 

Spasojevic MD, Adzic RR, Despic AR. 1980. Electrocatalysis on surfaces modified by foreign metal adatoms Oxidation of formaldehyde on platinum. J Electroanal Chem 109 261-269. 

The experiments using Sn adatoms are Intended to test for a correlation between the activity of these species as promoters for CO oxidation kinetics and their influence on the CO vibrational spectrum. Watanabe et. al. have proposed an "adatom oxidation" model for the catalytic activity of these adatoms (23). They propose that the function of the Sn adatoms is to catalyze the generation of adsorbed 0 or OH species at a lower potential than would be required on unpromoted Pt (23). The latter species then react with neighboring adsorbed CO molecules to accomplish the overall oxidation reaction. One implication of this proposed mechanism is that the adsorbed adatom is expected to have little, if any, direct interaction with the adsorbed CO reactant partner. Vibrational spectroscopy can be used to test for such an interaction. 

The bottom spectrum was obtained by cycling the electrode in CO-free SnCl /HjSO solution to ensure formation of a partial Sn adlayer and then replacing the cell contents with CO-saturated solution. The v(C0) band is still observed, which shows that the Sn adatoms do not saturate the surface even in the absence of competitive CO adsorption. The intensity and frequency of the v(C0) band have both decreased, which confirms that the CO adlayer is only partially complete. There is no evidence for a change in v(C0) beyond that expected for the coverage dependence expected in acid solution. This shows that there is no strong interaction between adsorbed CO molecules and neighboring Sn adatoms, in support of the assumptions used in the adatom oxidation model discussed above. 

Regarding surface spectroelectrochemistry, the intense interest in the electrochemistry of single crystal surfaces, adatoms, oxide layers, and monolayers in the last decade has seen not only the enhancement of traditional methods (e.g., infrared (IR) and Raman microspectroscopy) but also the development of relatively new surface specific methods such as second-harmonic generation, sum-frequency generation, surface plasmon resonance (SPR), and surface-enhanced resonance spectroscopy (SERS). The increased access to synchrotron radiation has led to X-ray absorption and X-ray diffraction methods becoming more feasible as in situ techniques for thin films or species generated close to the electrode. In this article, for... 

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

The UPD and anodic oxidation of Pb monolayers on tellurium was investigated also in acidic aqueous solutions of Pb(II) cations and various concentrations of halides (iodide, bromide, and chloride) [103]. The Te substrate was a 0.5 xm film electrodeposited in a previous step on polycrystalline Au from an acidic Te02 solution. Particular information on the time-frequency-potential variance of the electrochemical process was obtained by potentiodynamic electrochemical impedance spectroscopy (PDEIS), as it was difficult to apply stationary techniques for accurate characterization, due to a tendency to chemical interaction between the Pb adatoms and the substrate on a time scale of minutes. The impedance... 

The use of adatoms of foreign metals obtained by imderpotential deposition on the platinum surface is another convenient method for investigating the effect of a promoter on the electrocatalytic properties of platinum. However, the effect of adatoms in this case has been shown to be not as effective for electrooxidation of methanol as for the oxidation of other organic molecules such as formic acid adatoms of tin, however, showed a positive effect on the rate of methanol oxidation. ... 

FIGURE 16.1 Charging curves recorded when an adsorbed layer of oxygen adatoms or a phase oxide layer are formed (a) galvanostatic (b) potentiostatic. 

Adatoms produce a strong change in catalytic properties of the metal on which they are adsorbed. These catalytic effects are highly specific. They depend both on the nature of the metal and on the nature of the adatoms they also depend on the nature of the electrochemical reaction. For instance, tin adatoms on platinum strongly (by more than two orders of magnitude) enhance the rate of anodic methanol oxidation. 

Despite the fact that in many cases, metal electrodes with adatoms are catalyti-cally highly active, they have found rather limited practical nse in electrochemical devices. This is dne to the low stability of these electrodes The adatoms readily nndergo oxidation and desorption from the surface, whereupon the catalytic activity is no longer boosted. In some cases, attempts have been made to extend the existence of the active condition by adding the corresponding ions to the working electrolyte of the electrochemical device so as to secure permanent renewal of the adatom layer. 

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

Another important difference in the poison formation reaction is observed when studying this reaction on Pt(lll) electrodes covered with different adatoms. On Pt(lll) electrodes covered with bismuth, the formation of CO ceased at relatively high coverages only when isolated Pt sites were found on the surface [Herrero et al., 1993]. For formic acid, the formation takes place only at defects thus, small bismuth coverages are able to stop poison formation [Herrero et al., 1993 Macia et al., 1999]. Thus, an ideal Pt(lll) electrode would form CO from methanol but not from formic acid. This important difference indicates that the mechanism proposed in (6.17) is not vahd. It should be noted that the most difhcult step in the oxidation mechanism of methanol is probably the addition of the oxygen atom required to yield CO2. In the case of formic acid, this step is not necessary, since the molecule has already two oxygen atoms. For that reason, the adatoms that enhance formic acid oxidation, such as bismuth or palladium, do not show any catalytic effect for methanol oxidation. 

The FTIR studies revealed that the formation of CO2 is only detected when the CO starts to be oxidized (Fig. 6.18). Therefore, it was proposed that the mechanism has only one path, with CO as the C02-forming intermediate [Chang et al., 1992 Vielstich and Xia, 1995]. This has two important and practical consequences. First, methanol oxidation will be catalyzed by the same adatoms that catalyze CO oxidation, mainly ruthenium. Second, since the steric requirements for CO formation from methanol are quite high, the catalytic activity of small (<4nm) nanoparticles diminishes [Park et al., 2002]. 

Femandez-Vega A, Feliu JM, Aldaz A, Clavilier J. 1991. Heterogeneous electrocatalysis on well-deflned platinum surfaces modifled by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt(lll)-As system. J Electroanal Chem 305 229-240. 

I,eiva E, Iwasita T, Hertero E, FeUu JM. 1997. Effect of adatoms in the electrocatalysis of HCOOH oxidation. A theoretical model. Langmuir 13 6287 6293. 

Wang H, Baltruschat H. 2007. DEMS study on methanol oxidation at poly- and monocrystalline platinum electrodes The effect of anion, temperature, surface structure, Ru adatom, and potential. J Phys Chem C 111 7038-7048. 

The process for this irreversible adsorption has not been investigated in detail. The mechanism by which the metal is deposited has not been unambiguously elucidated, and several possibilities have been proposed. One possibility is the formation of local cells, with the ion of the adatom being reduced and either hydrogen [Szabo and Nagy, 1978] or platinum [Clavilier et al., 1988] being oxidized ... 

In most cases, the adatom deposition is reductive, as sketched in (7.1), because the adatom precursor in the solution is in an oxidized state. However, oxidative adsorption... 

The Ge adatoms do not remain adsorbed after oxidation, and the characteristic voltammetric profile of the blank is recovered after about 2-3 cycles up to 1.2 V. The Ge reduction process does not take place in a well-defined peak. 

The S oxidation process involves desorption of S adatoms, and the formation of H2SO4 species has been tentatively proposed. The maximum coverage was determined from AES and LEED measurements. 

At near-saturation coverages, the oxidation peak potential is displaced to approximately 1.03 V. There is a second oxidation process at 1.1 V that involves desorption of Te adatoms, which has been ascribed to the displacement of the Te(IV) oxide by surface Pt oxide. 

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