Electrocatalytic Oxidation of Methanol and Ethanol: A Comparison of Platinum-Tin and Platinum-Ruthenium Catalyst Particles in a Conducting Polyaniline Matrix

A method is described for preparing polyaniline/Pt-Ru assemblies that are effective catalysts for electrochemical oxidation of MeOH or EtOH in aqueous H₂SO₄. The Pt-Ru particles have been deposited into polyaniline by electrochemical deposition from aqueous H₂SO₄ containing Pt(IV) and Ru(III) from addition of K₂PtCl_e and K₂RuCl₅· xH₂O, respectively. The activity of the polyaniline/Pt-Ru assemblies for MeOH or EtOH oxidation is higher than that of polyaniline-coated electrodes modified with Pt alone. For the polyaniline/Pt-Ru system, high catalytic activity is observed for Pt-Ru particles that are codeposited from solutions containing from ∼ 0.5 to 2 mM Ru(III) when a 3 mM Pt(IV) solution is used. Characterization of the electrodes by SEM shows that the codeposition procedure yields roughly spherical catalyst particles 300–400 nm in diameter dispersed throughout the polyaniline. XPS analysis shows Ru to be present in two different oxidation states, most likely Ru(0) and Ru(IV). The Pt-Ru catalyst is compared to a Pt-Sn catalyst which can also be dispersed in a polyaniline matrix. For electrodes prepared from 3 mM Pt(IV) with an appropriate amount of Sn(IV) or Ru(III), the Pt-Ru catalyst is better for MeOH oxidation than the Pt-Sn catalyst, because it is more reproducible. Both catalysts show a significant (and similar) temperature dependence for MeOH or EtOH oxidation. For EtOH oxidation the Pt-Sn catalyst is superior to the Pt-Ru catalyst, and the activity of Pt-Sn at low potentials, where polyaniline is nonconducting, is limited by the resistivity of the polyaniline film. For EtOH oxidation the initial product is acetaldehyde which, unfortunately, degrades the conductivity of polyaniline. Oxidation of MeOH occurs at more positive potentials and does yield CO₂, though formaldehyde, an intermediate product, in aqueous acid also degrades the conductivity of polyaniline.