The electrocatalytic oxidations of water-soluble alcohols and diols and some other small organic molecules
(MeOH, EtOH, 2-fluoroethanol, 2,2-difluoroethanol, 2,2,2-trifluoroethanol, CH2(OH)2, ethylene glycol, n-PrOH,
i-PrOH, 1,4-butanediol, 1,3-propanediol, 1,2-propanediol, t-BuOH, neopentyl alcohol, benzyl alcohol, 2-Me-1-PrOH, formic acid, acetaldehyde, acetic acid, propionaldehyde, propionic acid, acetone, glycolaldehyde,
glyoxal, glycolic acid, glyoxylic acid, oxalic acid) have been compared in aqueous 0.5 M H2SO4 at C and Au
electrodes modified with an electrodeposited Pt−Sn catalyst at low (−0.1 to 0.5 V vs SCE) potentials. The
Pt−Sn catalyst is electrochemically deposited and forms as a smooth deposit on Au electrodes. At a catalyst
loading of ∼0.5 mg/cm2 the electrodeposition of the Pt−Sn catalyst results in formation of adherent ∼0.8
μm size particles on C electrodes. In general, for organic molecules containing only C, H, and O with two
or more carbon atoms, the presence of H atoms on both the α- and β-carbon results in a relatively negative
potential for onset of catalytic current (usually between −0.1 and −0.2 V vs SCE) at Pt−Sn compared to Pt
alone. Of the alcohols and diols studied, formaldehyde (which exists in aqueous solutions as the hydrated
form, CH2(OH)2) shows the highest electrocatalytic currents at the Pt−Sn catalyst. The ultimate product, via
HCOOH, is CO2. EtOH is oxidized only to acetic acid on Pt−Sn electrodes. The EtOH and n-PrOH oxidation
yields, determined by exhaustive electrolysis, are 4 e- per molecule, while i-PrOH yields only 2 e- per molecule.
We confirmed the generation of acetic acid, propionic acid, and acetone as the final products for the oxidations
of EtOH, n-PrOH, and i-PrOH, respectively, by 13C NMR and/or GCMS. The electron yield of the oxidation
of ethylene glycol at Pt−Sn surfaces is only 4 e- per molecule instead of the value of 8 e- per molecule
expected for the oxidation of ethylene glycol to oxalic acid. Glycolic acid (CHOCOOH) is the oxidation
product by GCMS. This substance is not electrocatalytically oxidized on Pt−Sn at potentials negative of
∼ +0.4 V vs SCE in comparison to the −0.1 V vs SCE onset for ethylene glycol oxidation. MeOH, the
molecule with the highest electron yield on Pt−Sn (6 e- per molecule), unfortunately shows the most positive
potential onset for oxidation among the group of alcohols compared in this study, while the two intermediates
along the path of oxidation to CO2, formaldehyde, and formic acid are oxidized on Pt−Sn at very negative
potentials compared to MeOH. Thus, the conversion of MeOH to formaldehyde is the efficiency-determining
step in the oxidation of MeOH to CO2.