Third-body effect

A different mechanism seems to operate in the case of poison formation from methanol [Herrero et al., 1993]. In this case, modification of the Pt(lll) surface by Bi deposition only causes a linear decrease in the amount of poison formed, indicating the existence of a mere third-body effect. Complete inhibition of the poisoning reaction is achieved for > 0.23, i.e., before the surface is completely covered. This suggests the existence of ensemble requirements for this reaction, which need enough free contiguous Pt sites to take place. 

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... 

In the following section, we will reexamine some earlier recombination measurements to see if third-body effects may have played a role. The rare gas dimer ions provide a good starting point. 

Babcock, L.M. and Streit, G.E. (1984) third-body effects in termolecular reactions Halide ion addition to boron trifluoride and boron trichloride./. Phys. Chem., 88, 5025. 

S20g2-, 348, 349 third-body, effect on CO+O, 121 thorium ions, and H2O2 + I-, 407 —, catalysis of H202 + S203 , 362 time-of-flight mass spectrometer, and CI2+O, 19... 

The ignition limit is a feature of systems containing approximately 0.5% H2 or more. If we begin the discussion by imagining a pure H2 -I- O2 system in which the H2 is then systematically replaced by CO, we find initially that the (second) limit moves to lower ambient temperatures and higher pressures. This feature can be modelled quantitatively simply by including the relative third body effect of CO, which is less than 1 (i.e., CO is a less... 

The difference may arise from the radical character of the monomers versus the closed-shell electronic structure of the dimer Nj O4. In the former, because of the high electron afhnity, a harpooning process is expected, while in the dimers the interaction is short range with covalent character. It is important to realize that for collision energies of more than 10.5 kcal the cross section for the N2O4 reaction exceeds that of the monomeric process. It is difficult to conclude on the third-body effect in this reaction because of the differences in the electronic structure between the two species NO2 and N2O4. Usually one would like to investigate the cluster s effect where only small perturbations exist. 

The inhibition can occur through either an electronic or a third-body effect. The presence of small amounts of Bi or As is able to modify the electronic properties of Pt(lll) in such a way that the spontaneous dissociation of formic acid is almost completely inhibited. It has also been estimated that the modification of the electronic properties by Bi adatoms that leads to the inhibition of formic acid dissociation extends over a distance of approximately seven Pt atoms. In contrast to this, the modification of electronic properties has no effect on the poison formation from methanol since the inhibition observed can be ascribed to a simple third-body mechanism. 

The mechanism of the catalytic action of metal adlayers in the oxidation of organic molecules has been interpreted by using several models including a third-body effect, an electronic effect, and the bifunctional catalyst mechanism. 

Lead Pronounced effects of Pb (Fig. 3) were interpreted in terms of a third-body effect by Adzic and coworkers [35]. Pb adatoms suppress adsorption of hydrogen and strongly bound intermediates, in particular, those interacting with two or three surface sites. The same model was assumed for Bi and Tl. Adsorbed H was considered to take part in the formation of strongly bound intermediates [18,19]. Current responses to potential sweeps into the H adsorption region [18,19, 65] and crystal quartz microbalance measurements [66]... 

Bi-Pt pairs were assumed to be responsible for the catalytic effect, without poison formation. Figure 7 shows a comparison between the experiment and the calculated curve. The agreement is surprisingly good considering the crude assumptions that adatoms cover one surface site and that they are randomly distributed. Bi is known to inhibit three H adsorption sites on Pt(lll) [75]. Nevertheless, the simulation data seem to corroborate the electronic effect for the Bi-Pt system. The data for Pt(lll) and Pt(lOO) revealed the role of substrate in determining the effect of Bi in suppressing poison adsorption (the electronic effect for Pt(l 11), but a third body for Pt(lOO)). In addition, on the same surface (Pt(lOO)), Bi can suppress poison formation by a third-body effect and can catalyze... 

Antimony The effect of Sb adlayers on Pt(lOO) [60] were ascribed to the suppression of poison formation, and the maximum oxidation current is obtained at high coverage, = 0.9(0.3), when no poison is detected at the surface. Substantial currents are observed for this surface. The maximum activity, however, does not surpass the intrinsic activity of Pt(lOO). A third-body effect was found operative for this system. Kizhakevariam and Weaver [76] found that Sb inhibits the adsorption of CO on Pt (100) and Pt (111) by decreasing the twofold binding geometries deduced from the relative vco band intensities. The optimum catalysis for... 

The effects of antimony, tin, and lead additions to the palladium black catalyst was analyzed [110]. Accordingly, each adatom strongly promotes formic acid oxidatirai in an electrochemical cell and reduces the amount of CO poison that develops on the catalyst surface after 1 h of oxidation. The authors attributed this effect to the third body effect (steric effect) but did not discard an electronic effect regarding that a decrease in the CO binding energy on palladium due to the presence of the adatoms, using XPS technique, was observed. 

Several authors [111, 112] suggest two catalytic effects third-body effect and electronic effect. However, the third-body effect cannot account solely for the observed catalytic effects. Thus, an electronic interaction between the updPb and the Pt substrate is likely as well. Other aspect concerning the lead deposits is the microstructural change during electrochemical experiments. Electrodeposited layers, near stoichiometric PtPb, yields a smooth compact surface that suffers microstructure changes after cyclic electrochemical experiments [113], The dealloyed structure... 

Fig. 3.1 Catalyst-mediated formic acid electrooxidation mechanisms (a) ensemble/ third-body effect and (b) bifunctional mechanism. The catalyst atoms (open circle) commonly Ft or Pd and (filled circle) secondary metal atom...

Nanoparticle-faceted terraces, comers, and edges presence (a) Third-body effect... 

In a relatively simple model by Leiva et al., the impact of adatom coverage in terms of a nearest-neighbor electronic effect versus a third-body effect was developed and compared to experimental results for both bismuth (Bi)- and antimony (Sb)-modified Pt single-crystal surfaces, shown in Fig. 3.7 [56]. One of... 

Separate modeling efforts have evolved to include solvatirai, electronic alterations in the d-band center, and the third-body effect. The findings suggest a combination of a positive electronic shift in the d-band center and third-body effect promotes the dehydration pathway. There exists a need for a cohesive hybrid model that also includes nanopaiticle attributes. 

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. 

Fig. 4.2 Plot of direct formic acid fuel cell performance at 0.6 V for Pt/C anodes as a function of Pb and Sb adatom coverages. The experimental data is compared to the two formic acid electrooxidation models proposed by Leiva (solid line) electronic enhancement and (dashed line) third-body effect [29]...

Generally, the ad-atoms cause positive catalytic effects with significant enhancement of the electrocatalytic activity of platinum in several cases. Several reviews have been published on this subject [26-28, 67]. Very recently Ross [68] and Jarvi and Stuve [29] have discussed the more recent advances in our understanding of the fundamentals of Ci electrocatalysis by ad-atoms. The different types of enhancement (third-body effect, bifunctional mechanism, poison destabilization, and electronic modification) are well documented. The new information obtained from the in situ spectroscopic studies about the nature of poisons and the dependence of their coverages on potential, as well as the use of single-crystal electrodes with defined surface structure and specific reactivity, enables a deeper insight in the electrocatalysis by ad-atoms. As a general rule, one can say that, except for methanol, the more susceptible... 

---