Formic acid oxidation adatoms

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. 

Femandez-Vega A, Feliu JM, Aldaz A, Clavilier J. 1991. Heterogeneous electrocatalysis on well-deflned platinum surfaces modifled by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt(lll)-As system. J Electroanal Chem 305 229-240. 

Figure 7.14 Current density for formic acid oxidation as a function of the fraction of Pt surface atoms blocked by adatoms on two different electrodes (a) Bi/Pt(lll) (b) Sb/Pt(100). (Reprinted with permission from Leiva et al. [1997].)...

The good coincidence between the model and the experimental data depicted in Fig. 7.14 supports the idea that this model captures the essential aspects of the effect of different adatoms on the electrocatalysis of formic acid oxidation. 

Similarly to formic acid oxidation, modification of the electrode surface by adatoms or other species could result in a significant enhancement of the oxidation rate of methanol. 

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

Ad-atom electrocatalysis in the case of formic acid oxidation can be divided into two terms (1) inhibition of surface poisoning by CO formation (indirect pathway), and (2) true enhancement of the rate of oxidation (direct pathway). For most adatoms, these eftects are mixed. An example for the mixed case is the Sb ad-atom adsorption on Pt(lOO) [96]. The reaction takes place on single Pt sites and randomly... 

The use of adatoms of foreign metals obtained by imderpotential deposition on the platinum surface is another convenient method for investigating the effect of a promoter on the electrocatalytic properties of platinum. However, the effect of adatoms in this case has been shown to be not as effective for electrooxidation of methanol as for the oxidation of other organic molecules such as formic acid adatoms of tin, however, showed a positive effect on the rate of methanol oxidation. ... 

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. 

Other model reactants are simple organic molecules, for example, formic acid [381, 382]. Pt(lll) exerts lower catalytic influence on HCOOH oxidation than do Pt(lOO) and Pt(llO) faces. However, in the presence of Pb adatoms on Pt(lll) a strong catalytic influence has been observed [383]. The poisonous species production in HCOOH oxidation is then inhibited. Electrochemical reduction of CO2 to glycolate/glyoxylate and oxalic acid has been studied [384]. Other products such as formic acid accompanied by CO and methane have also been detected [385]. In the latter case, the efficiency of the competing process of hydrogen evolution has been suppressed to less than 3.5%. 

In acidic medium, the electrocatalytic oxidation of glyoxal on platinum in the potential range 1 to 1.5 V/RHE leads mainly to formic acid (60%) and CO (40%). With lead adatoms, it becomes possible to oxidize glyoxal between 0.4 and 1.0 V/RHE leading mainly to CO2 formation (46%), while the selectivity towards, glyoxylic acid is sensibly increased (28%). At pH=7 and 1,9 V/RHE, the main oxidation product is formic acid (99%). Otherwise, in acidic medium the oxidation is more selective towards glyoxylic acid (70%), when the applied potential is in the range of 1.80 to 2.13 V/RHE. 

Considering these precedents, it is no wonder that during the last 2 years most of the papers devoted to the study of oxidation of formic acid are dealing with the problem of how the catalytic activity can be enhanced or the poison formation can be minimized by adatoms or other species (acetonitrile, nitromethane). °... 

It has been observed that the rate of electrocatalytic oxidation of formic acid at R electrodes can be enhanced significantly by the incorporation of adatoms, such as Pb, Bi, Tl, and Cd. The most significant enhancement has been seen with Pb adatoms, which were shown to be accompanied by a dramatic decrease in the poisoning phenomena on R. ° A similar type of behavior has been observed with systems based on Pt/Cdg, Pt/TI and Pt/Biads, although to a lesser extent. One of the most plausible explanations given rationalizing the improved characteristics of electro-oxidation in the presence of adatoms was proposed by Shibata and Motoo ° who invoke a... 

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. 

Campbell SA, Parsons R (1992) Effect of Bi and Sn adatoms rai formic acid and methanol oxidation at well defined platinum surfaces. J Chem Soc Faraday Trans 88 833-841... 

In other cases, a simpler simulation based on statistical models is preformed. This kind of simulation was performed to determine the effect of the different adatoms in the oxidation of formic acid on... 

In the first case, the activity of the surface is directly proportional to the number of pair ad-atom-surface sites, whereas in the latter case, the activity is proportional to the number of unoccupied surface sites. Since oxidation of formic acid takes place though a parallel-path mechanism, the effects of different levels of poisoning are also considered. For the cases in which the surface is completely covered by poison, both types of ad-atoms (the catalytically effective ad-atoms and the third-body adatoms) produce similar qualitative effects, that is, both types increases the current for the oxidation of formic acid [65]. Of course, the catalytic enhancement is higher in the case of the ad-atoms that modifies the electronic properties of the surface, since the global effect will be the combination of the electronic enhancement and the third-body effect (any ad-atom always acts as a third body). This is the case, for instance, for the Pt(lOO) surfaces modified with ad-atoms [65-67]. For the surfaces with low poisoning, that is, the Pt(lll)... 

Minevski LV, Adzic RR. Oxidation of formic acid at a high surface area supported platinum modified by foreign metal adatoms. J Appl Electrochem 1988 18 240-4. 

---