Adatoms reaction enhancement mechanisms

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... 

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 electrocatalytic oxidation of organic molecules has been studied very extensively using various supports and adatoms. There are several proposed mechanisms in the literature to interpret the role of adatoms. It is assumed, for instance, that adatoms block the poison formation not leaving enough space for these reactions. According to another hypothesis, adatoms act as redox intermediates. Independently from the explanation, it is a fact that very often a significant enhancement of the catalytic activity can be observed and these results could provide very useful information for liquid-phase catalytic reactions.60... 

Particles created by photodissodation may also move across the surface with enhanced reactivity. The formation of CO2 upon irradiation of a CO + O2 adlayer on Pt(l 11) was indeed the first hint for a "hot" adatom mechanism of this reaction, as discussed in Chapter 3 [70]. Subsequent detailed studies of this system [71] showed that the CO2 molecules formed in this way at 25K substrate temperature are strongly peaked along the surface normal and exhibit translational energies up to 1.35 eV. 

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