Electrocatalytic oxidation of small carbohydrate fuels at Pt-Sn modified electrodes

The electrocatalytic oxidations of water-soluble alcohols and diols and som e other small organic molecules (MeOH, EtOH, 2-fluoroethanol, 2,2-difluoroe thanol, 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 alcoh ol, benzyl alcohol, 2-Me-1-PrOH, formic acid, acetaldehyde, acetic acid, pr opionaldehyde, propionic acid, acetone, glycolaldehyde, glyoxal, glycolic a cid, 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 electrochemic ally deposited and forms as a smooth deposit on Au electrodes. At a catalys t loading of similar to 0.5 mg/cm(2) the electrodeposition of the Pt-Sn cat alyst results in formation of adherent similar to 0.8 mu m size particles o n 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 alpha - and beta-carbon results in a relatively negative potential for onset of c atalytic 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 e lectrocatalytic currents at the Pt-Sn catalyst. The ultimate product, via H COOH, is CO2. EtOH is oxidized only to acetic acid on Pt-Sn electrodes. The EtOH and n-PrOH oxidation yields, determined by exhaustive electrolysis, a re 4 e(-) per molecule, while i-PrOH yields only 2 e(-) per molecule. We co nfirmed the generation of acetic acid, propionic acid, and acetone as the f inal products for the oxidations of EtOH, n-PrOH, and i-PrOH, respectively, by C-13 NMR and/or GCMS. The electron yield of the oxidation of ethylene g lycol 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 oxali c acid. Glycolic acid (CHOCOOH) is the oxidation product by GCMS. This subs tance is not electrocatalytically oxidized on Pt-Sn at potentials negative of similar to +0.4 V vs SCE in comparison to the -0.1 V vs SCE onset for et hylene glycol oxidation. MeOH, the molecule with the highest electron yield on Pt-Sn (6 e(-) per molecule), unfortunately shows the most positive pote ntial onset for oxidation among the group of alcohols compared in this stud y, while the two intermediates along the path of oxidation to CO2, formalde hyde, and formic acid are oxidized on Pt-Sn at very negative potentials com pared to MeOH. Thus, the conversion of MeOH to formaldehyde is the efficien cy-determining step in the oxidation of MeOH to CO2.