Adatom Modified Surfaces

The development of new electrocatalytic materials by deposition of foreign atoms on a host metal is currently a very active area of research in electrochemistry. In this regard, most of the studies have been focused on platinum as a substrate, due to the very high reactivity of this metal for fuel-cell reactions. Still, a surface modifier is necessary to improve the catalytic activity and stability of the electrode and to avoid side poisoning reactions. It has been shown that the modification of platiniun surfaces with submonolayer amounts of elements of the p-block of the periodic table leads, in many cases, to electrode materials with greatly improved catalytic properties.  

Nevertheless, the molecular-level understanding of the catalytic enhancement is, in general, not well established. In this regard, three main effects are nsnally invoked to explain the catalytic enhancement the electronic (ligand) effect, associated with chmig- 

Section V.l. The effect of selenium and lead deposition is illustrated in Fig. 22 because these cases provide representative examples for all studied adatoms. Results for the unmodified Pt(l 11) are also included in this fignre for the sake of comparison. 

On the other hand, the results obtained in the medium-high coverage region have a more straightforward explanation. In this region, two different behaviors are observed, represented by S and 

on the one hand, which displace the towards higher values, and Bi and Pb, on the other hand, which clearly decrease the pme 

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In the presence of Bi or Te, the C=0 bond is weakened, as concluded from the displacement of the CO stretching band to lower wavenumbers. There is also a change in the dependence of the band frequency on electrode potential, with the slope dv/dE increasing for the adatom-modified surfaces. These changes indicate that the adatom alters the electronic properties of the surface, increasing the amount of electronic backdonation and stabilizing the adsorbed CO molecule. No catalytic enhancement is expected from this effect. 

Attard GA, Hazzazi O, Wells PB, Chment V, Hertero E, Feliu JM. 2004. On the global and local values of the potential of zero total charge at well-defined platinum surfaces Stepped and adatom modified surfaces. J Electroanal Chem 568 329-342. 

At the same time, many lattice dynamics models have been constructed from force-constant models or ab-initio methods. Recently, the technique of molecular dynamics (MD) simulation has been widely used" " to study vibrations, surface melting, roughening and disordering. In particular, it has been demonstrated " " " that the presence of adatoms modifies drastically the vibrational properties of surfaces. Lately, the dynamical properties of Cu adatoms on Cu(lOO) " and Cu(lll) faces have been calculated using MD simulations and a many-body potential based on the tight-binding (TB) second-moment aproximation (SMA). " ... 

According to (7.8) and (7.12), the stoichiometry (m/n) can be extracted from the slope of the plots of adatom charge density versus hydrogen (or hydrogen plus anion) charge density. Some representative plots are shown in Fig. 7.3. The conclusions extracted from this kind of analysis are summarized in Tables 7.1 and 7.2 for Pt(l 11) and Pt(lOO) modified surfaces, respectively. The extension of this analysis... 

In some cases, the maximum coverage of the adatom and the stoichiometry of the reaction have been corroborated by independent techniques, normally involving the transfer of the modified surface to ex situ conditions. 

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

UPD of Pb on n-Se is induced by illumination [320]. The Pb adatoms modify the Se surface and form surface states in the band gap. In the dark, the Pb UPD occurs only on the Se surface where PbSe clusters have been deposited. The deposited Pb atoms interact irreversibly with surface Se atoms to form PbSe monolayer [320, 321]. The... 

For hydrogen as well as for oxygen evolution, where current densities are very high, modified electrodes are uncommon. Actually, adatom activated surfaces may fall into this category although they are not customarily included there, and in any case the use of the term becomes only a semantic question. To the knowledge of the present author, only very few investigations have been carried out with classically... 

In a separate series of experiments, the influence of sulfur on the decomposition of a mixture consisting of CO/C2H4/H2 over iron was investigated. Previous work [17] had shown that while iron did not catalyze the decomposition of ethylene, even in the presence of hydrogen, when a small fraction of CO was added to the reactant, a dramatic increase in the rate of decomposition of the olefin was observed. This behavior was rationalized according to a model in which the presence of coadsorbed CO resulted in what is believed to be reconstruction of the iron to form a surface, which favors dissociative chemisorption of ethylene. In the current study, we have extended this study to include the case where sulfur is preadsorbed on the metal surface in an attempt to determine how such adatoms modify the coadsorption characteristics of CO and C2H4 on iron. 

The presence of small amount of foreign atoms on the electrode surface changes greatly the reaction selectivity of CO2 reduction, as exemplified by metal electrodes alloyed with other metals or adatom modified electrode. Adatom modified electrodes were prepared by underpotential deposition (UPD) or overpotcntial deposition techniques." The coverage of the adatoms ranged between 1.1 and 1.6, mostly between 1.25 and 1.35. CO selectivity r in CO2 reduction, defined as F(CO)/[F(CO) + F(HCOO)], was... 

In this respect the special role of irreversible adsorption of foreign metal adatoms should be mentioned as this type of adsorption provides the possibihty of producing stable modified electrodes for electrocatalytic purposes (for instance Sn, Bi adatom modified electrodes). In some cases, the irreversibly adsorbed adatom undergoes well-defined surface redox processes that could be structure sensitive (for instance. As, Sb, and Bi adatoms on platinum substrates or two-dimensional Bi -i- As alloys supported on Pt(lll). ... 

Pt(lll) [116]. Bi causes a decrease of CO oxidation [117] on Pt(lOO), which agrees with its stabilization of this adsorbate deduced from in situ IR studies [118, 119], while As has an opposite effect [119]. Both adatoms modify the CO-stripping process with a small catalytic effect observed at very positive potentials (0.7 V). The catalysis has been ascribed to the adatom-mediated oxygen transfer with a possible electronic effect for As [119]. Similar behavior is observed with Sb, which decreases the adsorption of CO on Pt(lOO) and Pt(lll) surfaces by removing the twofold bridging geometry, as deduced from the relative vco band intensities [76]. At the saturation Sb coverage, only linear CO is observed. 

Adsorption of Gases on Surfaces Modified by Electronegative Adatoms... 

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