The Role of Adsorbed Formate and Oxygen in the Oxidation of Methanol at a Polycrystalline Pt Electrode in 0.1 M KOH: An In Situ Fourier Transform Infrared Study

Abstract

The oxidation of methanol at a polycrystalline Pt anode was investigated with in situ external reflectance Fourier transform infrared (FTIR) spectroscopy in both nitrogen- and oxygen-saturated 0.1 M KOH. The data strongly suggest that adsorbed formate is either the, or at least an important, intermediate in the direct pathway, with attack on this intermediate (rather than its formation) being the rate-determining step. The HCOOads is formed via an Eley−Rideal-type reaction between solution methanol and OHads. Linearly adsorbed CO (COL) appears to be a spectator rather than a participant in the methanol oxidation reaction, at least up to relatively high potentials, i.e., 0.1 V. The primary products of the oxidation are formate up to −0.5 V, then bicarbonate as well as formate and finally only CO2 at potentials greater than or equal to −0.3 V. Clearly, the pH in the thin layer drops below 10.25 at potentials greater than −0.5 V and below 6.37 in the latter potential region, these being pK a,2 and pK a,1, respectively, of carbonic acid. This indicates the disadvantage of the external reflectance approach, where the thin layer causes very restricted diffusion, compared to those techniques such as surface-enhanced infrared spectroscopy (SEIRAS) that employ attenuated total reflectance. However, the external reflectance approach has a major advantage over SEIRAS of detecting solution-borne intermediates and products, which are inaccessible to the latter technique because of its lack of sensitivity beyond the near-electrode region. At potentials greater than −0.1 V, the CO2 solution band remained constant as excess CO2 was forced into bubbles, pushing electrolyte out of the thin layer. In oxygen-saturated solution, no COL was observed. The O2-reduction products clearly modified the Pt surface and altered the characteristics of the Pt oxide film formed at higher potentials, which, among other effects, removed the nucleation sites for CO2 bubble formation. Because of the bubble formation in N2-saturated solution, it was difficult to judge whether more CO2 was formed in O2-saturated KOH, but the amounts of solution formate and bicarbonate formed in the latter were significantly higher than those in N2-saturated solution.

An in Situ FTIR Study of Ethanol Oxidation at Polycrystalline Platinum in 0.1 M KOH at 25 and 50 °C

“…In-situ FTIR data at potentials above transition The gain of CO 2 (band at 2340 cm -1 ) clearly shows that the pH, at least in some regions of the thin layer, has fallen below the pK a of carbonic acid, 6.4 [25][31] [32]. However, similar water loss features were observed in our previous studies on methanol oxidation in KOH [31] and attributed to the loss of highly hydrogen-bonded bulk water [45] from the thin layer due to CO 2 gas bubble formation, and this does not seem an unreasonable explanation, particularly given the fact that the other loss features (see table 1) and gain features are observed at both temperatures, suggesting a single mechanism. The formation of acetic acid clearly shows that the pH in the thin layer at these potentials is ≤ 4.7, the pK a of acetic acid 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 [21].…”

“…As described in our previous papers[25][31][32], at higher potentials during our in-situ FTIR experiments in alkaline solution there is a substantial change in pH in the electrolyte immediately above the electrode and trapped in the thin layer. The swing in pH at higher potentials was ascribed to a combination of[31][32]: (a) relatively slow diffusion of OHions across the electrode surface over a timescale of tens of minutes for an electrode of the radius that we use; and (b) the exhaustion of reactant in the thin electrolyte layer, which leads to further changes in the ambient conditions at the electrode surface. The swing in pH at higher potentials was ascribed to a combination of[31][32]: (a) relatively slow diffusion of OHions across the electrode surface over a timescale of tens of minutes for an electrode of the radius that we use; and (b) the exhaustion of reactant in the thin electrolyte layer, which leads to further changes in the ambient conditions at the electrode surface.…”

“…[43]; this value varies very little between 14 ºC and 45 ºC[44]. The substantial drop in pH in thin-layer FTIR spectroscopic experiments has been modelled in terms of the slow diffusion of OHions across the electrode surface coupled with the exhaustion of reactant[31][32].…”

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

“…In alkaline medium the reaction routes and product distribution alter due to the change in the work function of the electrode material: the main products on a polycrystalline Pt electrode [ 64 ] are formate (formic acid in alkaline medium dissociates rapidly to formate ion − HCOO ) and bicarbonate ( − 3 HCO ) in the low potential region (> 0.2 V) as well as CO 2 above a potential of 0.7 V. On Pt(111) electrode [65] the main products are formate and CO 2 . According to DFT calculation the electrocatalytic activity of Pd on alcohol oxidation is strongly depended on pH [66] and at high pH solutions Pd could be an alternative for Pt.…”

Computational screening for selective catalysts: Cleaving the C C bond during ethanol electro-oxidation reaction

“…9 Christensen et al have shown that the interfacial pH drops at higher potentials due to the high consumption of OH -which is not completely counterbalanced by the OH -diffusion from the bulk-phase. [10][11][12] This phenomenon leads to a transition from alkaline to acidic conditions at the interphase. The transition potential varies with the diffusion rate of OH -which is dependent on the temperature and mass flow-rate.…”