Metal adatom

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

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. 

Adzic RR. 1984. Electrocatalytic properties of surfaces modified by foreign metal adatoms. In Gerisher H, Tobias CW, eds. Advances in Electrochemistry and Electrochemical Engineering. Volume 13. New York Wiley-Interscience. pp. 159-260. 

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. 

Adzic RR, Tripkovic AV, Markovic NM. 1983. Structural effects in electrocatalysis oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms. J Electroanal Chem 150 79-88. 

The use of conventional electrochemical methods to study the effect of metal adatoms on the electrochemical oxidation of an organic adsorbate may be in some cases of limited value. Very often, in the potential region of interest the current due to the oxidation of an organic residue is masked by faradaic or capacitive responses of the cocatalyst itself. The use of on-line mass spectroscopy overcomes this problem by allowing the observation of the mass signal-potential response for the C02 produced during the oxidation of the adsorbed organic residue. 

This effect, called the third body effect by Conway and co-workers [101], is however controversial [102], The main argument against this theory is the fact that there is a specificity of catalytic behavior for each kind of metal adatom. Even adatoms producing similar geometrical blocking effects, present different catalytic properties. So, for instance, tin and lead [97] occupy two Pt atoms, but tin produces... 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

Electrochemical reduction of carbon monoxide in dry nonaqueous media at moderate to low pressures leads to the formation of the 1,3-cyclobutanedione dianion (squarate) at current efficiencies, up to about 45% depending on the cathode material [1,2]. In aqueous solution, electroreduction can lead to the formation of methane and other hydrocarbon products. The role of the metal/adatom in determining the extent of CO and hence hydrocarbon formation during the reduction of carbon dioxide is related to the ability of the electrode material to favor CO formation (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and stabilize HCCO [3, 4]. 

At low adsorbate coverages the surface structure of the deposited metal is determined by the substrate periodicity. Thus, under these conditions the adsorbate-substrate interaction is predominant. At higher coverages the adsorbate may continue to follow the substrate periodicity or form coincidence structures with new periodicities that are unrelated to the substrate periodicity. The ordering geometry of large-radius metallic adatoms (especially K, Rb and Cs) shows relatively little dependence on the substrate lattice they tend to form hexagonal close-packed layers on any metal... 

Fig. 4.27 Activation energies of surface diffusion of 5-d transition metal adatoms on the W (110) surface. Squares are data obtained with a few adatoms on a plane by Bassett and circles are data with one adatom on a plane by Tsong Kellogg.

In the experiment discussed above, no directional dependence of the pair interaction is attempted. Pair interactions are simply assumed to be isotropic on the W (110) surface. The pair interaction, in general, should depend both on the direction of the adatom-adatom pair bond and on the bond length. Thus pair energies should therefore be measured for each possible pair bond. A preliminary study in this direction has been reported by the same authors for Si-Si interaction on the W (110) surface.94 Si-Si interaction is of particular interest since (1) Si atoms interact with one another in solid state by forming covalent bonds rather than metallic bonds it would be interesting to see how the interaction of Si adatom pairs on a metal surface is different from that of metal adatom pairs (2) semiconductor-metal interfaces are technologically important... 

Underpotential deposition of metal adatoms at foreign metal electrodes shows a strong effect on the kinetics of inner sphere redox reactions such as the reduction of Cr(OH2)sCl2+ [130] due to electrostatic and specific interactions. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

The interaction of CO with alloy impurities is significantly different if the impurity is a metal adatom on the surface, or a substitutional impurity in the surface. The adsorption properties of CO on Au/Ni(lll) surfaces has been studied with Au in the form of adatoms and substitutional impurities. In the case of adatoms, the adsorption energy of CO in sites that include a nearest neighbor of the Au adatom is reduced by up to 1.2 eV. At larger distances from the adatom the adsorption energies are nearly unchanged. The Au adatom was found to bind CO much more strongly than a flat Au(lll) surface. In contrast, the substitutional Au impurity bonds CO more weakly than a flat Au(lll) surface. 

So-called underpotential deposited species arise when an electrochemical reaction produces first, on a suitable substrate adsorbent metal, a two-dimensional array or in some cases two-dimensional domain structures (cf. Ref 100) at potentials lower than that for the thermodynamically reversible process of bulk crystal or gas formation of the same element. The latter often requires an overpotential for initial nucleation of the bulk phase. The thermodynamic condition for underpotential deposition is that the Gibbs energy for two-dimensional adatom chemisorption on an appropriate substrate must be more negative than that for the corresponding three-dimensional bulk-phase formation. Underpotential electrochemisorption processes commonly involve deposition of adatoms of metals, adatoms of H, and adspecies of OH and O. 

The electrochemistry and surface chemistry of such UPD species has been the subject of several previous reviews [6, 7, 99, 100) and many original papers Ref 99 reviews, in thorough detail, electrocatalysis induced or modified by UPD metal adatoms which really change the intrinsic catalytic nature of the substrate metal surfaces. It is surprising, however, that very little work has been done until recently (cf Refs. 75, 101-106) on the adsorbed species that are the kinetically involved intermediates in overall Faradaic reactions proceeding continuously at appreciable net rates (or equivalent current densities), for example, in the reactions of H2, O2, and CI2 evolution and other processes such as O2 reduction (more work, relatively, has been done on that reaction) or H2 oxidation proceeding at appreciable overpotentials. Such intermediates are conveniently referred to as OPD species. 

Electrodes may also be modified by deposition of metal adatoms at potentials several hundred millivolts positive to the reversible potential for metal deposition. A submonolayer of adatoms may lower overpotentials for electron transfer processes or improve the selectivity of an electrocatalytic surface. Underpotential deposition and electrocatalysis have been discussed in a review [182]. 

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