Electrocatalytic Oxidation of Small Carbohydrate Fuels at Pt−Sn Modified Electrodes

Abstract

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.