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Adatoms additive

A common method for improving formic acid electrooxidation activity is through the incorporation of foreign adatoms in sub- or monolayer coverages onto metal electrocatalyst surfaces (substrates). Adatoms are usually deposited onto the metal surface either by under potential deposition (UPD) or by irreversible adsorption [17]. The two dominant reaction enhancement mechanisms for the direct dehydrogenation pathway, as described in Sect. 3.3 of the previous chapter for formic acid electrooxidation, are the third-body and electronic effects. The type of enhancement mechanism due to adatom addition is dependent on the substrate/adatom... [Pg.71]

To illustrate the primary effects of adatom addition, single-crystal electrodes are discussed here. Feliu and Herrero have extensively studied formic acid electrooxidation on Pt single-crystal substrates modified with an array of various adatoms. They have established a connection between the electronegativity of the adatoms in relation to Pt and the type of active enhancement mechanism incurred as a function of adatom coverage [42]. Their results support inhibition of the indirect pathway on Pt(lll) terraces and they have demonstrated that COads formation occurs at step and defect sites. For Pt(l 11) substrates decorated with electropositive adatoms, such as Bi, Pb, Sb, and Te, the electronic enhancement is extended to the second or third Pt atom shell from the adatom. While for electronegative adatoms, in respect to Pt, the third-body effect dominates with increased coverages, such as S and Se. [Pg.72]

Many surfaces have additional defects other than steps, however, some of which are illustrated in figure A1.7.1(b). For example, steps are usually not flat, i.e. they do not lie along a single low-mdex direction, but instead have kinks. Terraces are also not always perfectly flat, and often contain defects such as adatoms or vacancies. An adatom, is an isolated atom adsorbed on top of a terrace, while a vacancy is an atom or group of atoms missing from an otiierwise perfect terrace. In addition, a group of atoms called an island may fonn on a terrace, as illustrated. [Pg.287]

The above features indicate that for the parallel to the surface directions the weakest coupling between the Au adatom and the surface atoms occurs for the (111) face, while the strongest for the (100) surface. However, the opposite happens for the normal to the surface direction. In addition, the phonon modes of Au adatom are found in lower frequencies than those of Cu adatom. [Pg.154]

Electronegative adatoms cause significant changes in the metal surface electronic stmcture, manifest as changes in the surface work function. In general electronegative additives increase the work function of the metal substrate. Typical examples are shown in Figures 2.9 and 2.10 for the adsorption of Cl and coadsorption of Cl and O on the work function of... [Pg.31]

The effect induced by different electronegative additives is more pronounced in the case where the additive adatoms occupy the most coordinated sites forming ordered structures (e.g Cl addition onNi(lOO)). In this case (Fig. 2.28) one modifier adatom affects 3-4 CO adsorption sites and complete disappearance of the CO p2-peak is observed above modifier coverages of -0.25 or less. The lack of ordering and the tendency of the modifier to form amorphous islands (e.g. P on Ni(100)) diminishes the effect. Thus in the case of P on Ni(100) the disappearance of the CO p2-peak is observed at P coverages exceeding 0.6. [Pg.59]

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. [Pg.70]

Addition of very small amounts of Bu4Sn can completely transform the performance of these catalysts by poisoning the hydrogenation sites. For example, when a Ni°/Si02 catalyst is used, the best result corresponds to a 2-carene yield of 30%, with at least 30% of the carenes transformed into by-products. Addition of 0.04 mole of Bu4Sn/Nis results in an increase of the yield of 2-carene, up to 37%, and a decrease of the amount of by-products to less than 10%. In this case, tin is present as adatoms on the most hydrogenating sites (very hkely those situated on the faces rather than on corners and edges). [Pg.202]

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. [Pg.186]

Completely different behavior is observed with S and Se, as shown in Fig. 7.8. With these adatoms, deposition on the terrace starts from the very beginning and no selectivity towards the step is observed. Additionally, deposition of the adatom changes the hydrogen adsorption energy on the (110) step sites, as reflected by the progressive shift of the peak at 0.12 V towards higher potential values. [Pg.225]

The mechanism of the cocatalytic effect is still a matter of investigation. For most of the systems of interest in electrocatalysis, data for characterization of the surface by means of spectroscopic UHV methods are still missing. Also measurements of changes in the electronic properties of the metal in the presence of adatoms in addition to more intensive application of in situ and on-line methods are desirable for a systematic search of new catalytic materials. [Pg.160]

The modulation of the charge of the adsorbed atom by the vibrations of heavy particles leads to a number of additional effects. In particular, it changes the electron and vibrational wave functions and the electrostatic energy of the adatom. These effects may also influence the transition probability and its dependence on the electrode potential. [Pg.141]

The relative importance of the two mechanisms - the non-local electromagnetic (EM) theory and the local charge transfer (CT) theory - remains a source of considerable discussion. It is generally considered that large-scale rough surfaces, e.g. gratings, islands, metallic spheres etc., favour the EM theory. In contrast, the CT mechanism requires chemisorption of the adsorbate at special atomic scale (e.g. adatom) sites on the metal surface, resulting in a metal/adsorbate CT complex. In addition, considerably enhanced Raman spectra have been obtained from surfaces prepared in such a way as to deliberately exclude one or the other mechanism. [Pg.118]

The general theory of 4.3 can now be applied, with some modification, due to the fact that the substrate electronic structure consists of discrete states arising from the metal film, in addition to the delocalized band states of the semiconductor. The adatom GF (5.56) can be written as (cf. (4.70))... [Pg.85]


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