Reaction intermediates of ethanol electro-oxidation on platinum investigated by SFG spectroscopy

Abstract

Although electrochemical oxidation of simple organic molecules on metal catalysts is the basic ingredient of fuel cells, which have great technological potential as a renewable source of electrical energy, the detailed reaction mechanisms are in most cases not completely understood. Here, we investigate the ethanol-platinum interface in acidic aqueous solution using infrared-visible sum frequency generation (SFG) spectroscopy and theoretical calculations of vibrational spectra in order to identify the intermediates present during the electro-oxidation of ethanol. The complex vibrational spectrum in the fingerprint region imply on the coexistence of several adsorbates. Based on spectra in ultra-high-vacuum (UHV) and electrochemical environment from the literature and our density functional theory (DFT) calculations of vibrational spectra, new adsorbed intermediates, never before observed with conventional infrared (IR) spectroscopy, are proposed here: g 2-acetaldehyde, g 2-acetyl, ethylidyne, monodentate acetate, methoxy, tertiary methanol derivative, COH residue, g 2-formaldehyde, mono and bidentate formate, CH 3 and CH 2 residues. In addition, we present new evidences for an ethoxy intermediate, a secondary ethanol derivative and an acetyl species, and we confirm the presence of previously observed adsorbates: a tertiary ethanol derivative, bidentate acetate, and CO ad. These results indicate that the platinum surface is much more reactive, and the reaction mechanism for ethanol electro-oxidation is considerably more complex than previously considered. This might be also true for many other molecule-catalyst systems.

How Reactive are Metal Surfaces in Solution? A Comparison of the Electrochemical Adsorption of Organic Molecules on Pt At Low Potentials

“…We compare these results with that of ethanol at the same experimental conditions. The main goals of the present investigation were to confirm the spectral assignments proposed previously [15] and to gain insights on the detailed reaction pathways for the electrooxidation of ethanol. Specifically, since acetaldehyde and acetic acid are products of the ethanol electrooxidation on Pt in acidic medium, investigating the adsorption of these molecules on Pt (at low potentials) could help clarifying the assignment of the different bands in the SFG spectrum for the ethanol solution/Pt interface.…”

“…1e, where most of the resonances disappear, except for a narrow peak at 1,408 cm . Additional evidences for the absence of interference effects in the thin liquid film are presented in detail elsewhere [15], together with a discussion on the possible origin of the peaks observed in Fig. 1e.…”

“…Our spectra were not corrected for these effects. Details of our SFG spectrometer and data acquisition procedure can be found elsewhere [15].…”

“…SFG spectroscopy can be used as a powerful tool to obtain vibrational spectra of adsorbed species without contribution from the bulk [14]. Very recently, we reported an extensive study about adsorbed intermediates of ethanol electrooxidation on Pt in acidic medium investigated by SFG spectroscopy [15]. Here, we study by SFG spectroscopy the adsorption of acetaldehyde, acetic acid, and methanol on Pt in acidic medium in the fingerprint region at low potential (0.05 V vs. reversible hydrogen electrode (RHE)).…”

Trimetallic catalyst based on PtRu modified by irreversible adsorption of Sb for direct ethanol fuel cells

“…However, during this process some poisoning intermediates, such as CO, can be formed and adsorb strongly on the catalysts surfaces blocking the active sites from further oxidation reactions [2][3][4], thus decreasing the overall performance of the catalyst and the cell.…”