Breaking the CC Bond in the Ethanol Oxidation Reaction on Platinum Electrodes: Effect of Steps and Ruthenium Adatoms

Abstract

Ethanol is a possible candidate to use in fuel cells and, for that reason, the study of its oxidation has deserved much attention during the last decade.[1] When compared to methanol, it presents some advantages: it can be obtained directly from biomass distillation, it has a higher energy density since 12 electrons are exchanged in the complete oxidation to CO 2 and it is less toxic. The activity of platinum electrodes, which is the best pure metal for the oxidation of both methanol and ethanol, is similar, since the current densities measured at a given potential are comparable. [1,2] However, the electrocatalysis of ethanol oxidation presents a major problem due to CÀC bond breaking. The general scheme for the oxidation of ethanol on platinum electrodes is the following. The first step is the oxidation of ethanol to acetaldehyde transferring two electrons [Reaction (1), Scheme 1]. Acetaldehyde can, in turn, be oxidized to acetic acid, transferring two additional electrons. Acetic acid can be considered as a final product since its oxidation is very difficult. In fact, acetic acid is stable on platinum electrodes at potentials where oxygen evolution takes place. Additionally, it can be considered as a poison for the oxidation, since acetate adsorbs strongly on the surface and hinders further ethanol oxidation.[2] Since only four electrons per ethanol molecule are transferred in the formation of acetic acid, the energy density of this path is only one third of the value that could be obtained for the complete oxidation of ethanol through Reactions (3)-(5) (Scheme 1), which represents the route in which 12 e À per molecule are transferred.Experimentally, it is found that the main product in the oxidation of ethanol is acetic acid and thus the effective energy density of the process is much smaller than the theoretical value. Therefore, the crucial step in the ethanol oxidation reaction is the breaking of the CÀC bond and the transformation of the fragments into CO 2 . Herein we try to gain insight into this process by using surfaces with a well-defined geometry. Platinum surfaces with terraces with (111) symmetry separated by monoatomic (110) steps provide a unique and excellent environment to study this reaction. It has been shown that on the (111) terraces ethanol oxidation process occurs exclusively through the route that exchanges 4 e À , that is, the final product is only acetic acid.[2] On the other hand, (110) step sites are very active for the CÀC bond breaking step, since the amounts of CO and CO 2 produced increases with the step density. [3,4] Thus, the CÀC bond breaking is localized only in certain sites on the surface. On the steps it is also possible to deposit adatoms and to study their effect on the reaction, determining the role of the adatom in the oxidation mechanism. This is an ideal situation because the adatom is deposited close to the active site and therefore a small quantity of them can modify significantly the catalytic activity of the site. Thus, its effect on the reaction can be easily...

On the electrochemical properties of platinum stepped surfaces vicinal to the (100) pole. A computational study

“…The large difference in the reactivity of the steps in the (111) terrace with respect to the terrace sites explain why the M a n u s c r i p t 17 reactivity of the real Pt(111) electrode is, for some reactions, governed by the number of defects, which behave in a similar way to step sites. Thus, it has been proposed that some reactions on a real Pt(111) electrode occurs only on the defects, such as CO oxidation [62], CO formation from formic acid [19,20] or the cleavage of the C-C bond in ethanol oxidation [15,16]. Since those reactions does not occur on the ideal Pt(111) plane, the number of defects clearly affects the reactivity, and significant differences can be observed between Pt(111) electrodes with different quality.…”

“…For the surfaces vicinal to the (111) stereographic pole, that is, those containing terraces with (111) symmetry, significant effects in reactivity of the surface have been found. For those surfaces, the step sites are the active sites on these surfaces for the oxidation of adsorbed CO [14], the cleavage of the C-C bond in the ethanol oxidation reaction [15][16][17][18], or the formation of CO in the formic acid oxidation reaction [19][20][21], whereas the (111) terrace sites have a negligible activity for those processes. On the other hand, for the surfaces vicinal to the (100) pole, the reactivity of the (100) domains is much less affected by the presence of the steps.…”

The breaking of the CC bond in ethylene glycol oxidation at the Pt(111) electrode and its vicinal surfaces

“…Platinum single crystal working electrodes preparation, reagents and experimental procedures, including FTIR set-up, is the same as described in previous papers [1][2][3][4][5][6]. Pt(111) and its vicinal surfaces Pt(S)[n(111)x(100)], with Miller indices Pt(n+1, n-1, n-1) and Pt(S)[n(111)x(111)] or Pt(S)[(n-1)(111)x(110)], with Miller indices Pt(n, n, n-2), where n is the number of terrace rows, were used.…”

“…This single crystal contains the most compact arrangement of surface atoms and is expected to be an important fraction of surface sites in platinum nanoparticles. The inclusion of steps in the structure leads to a better performance, because C-C breaking takes place at steps [2,3]. In contrast, EG seems to effectively dissociate at platinum, in such a way that CO was considered to be the only relevant residue from spontaneous interaction of the fuel with the different basal planes [4].…”

The correlation of electrochemical and fuel cell results for alcohol oxidation in acidic and alkaline media

“…According to these authors on Pt(111) terraces ethanol oxidation occurs via acetaldehyde to acetic acid and only a small CO 2 fragment is observed. In the case of Pt (110) Pt (111) Pt (110) Pt (110) Pt (100) In addition, it is also possible to modify SCE surfaces with adatoms of another metal [101][102][103][104] and these electrodes are of special interest because they resemble bimetallic nanoparticles that will be discussed in Section 5.4. For instance addition of Ru atoms on Pt(332) surface revealed that Ru has only an active role in the oxidation of CO ads to CO 2 , but has no role in enhancement of -CH 3 transformation to CO nor C-C bond cleavage [101].…”