Deposition of Ru Adatoms on Pt Using Organometallic Chemistry: Catalysts for Electrooxidation of MeOH and Adsorbed Carbon Monoxide

Abstract

Ru adatoms were deposited on black Pt gauze by hydrogenating Ru(COD)(η3-C3H5)2 (1, COD is 1,5-cyclooctadiene) over the gauze at low temperatures in hexanes solution under 1 atm of dihydrogen gas. A series of Pt−Ruad surfaces were prepared by interrupting the hydrogenation after deposition of 0.05, 0.10, 0.30, 0.32, 0.44, 0.70, and 3.5 equiv of Ru adatoms. The ratio of currents in the “double layer” to those in the “hydride” region in the potentiodynamic responses of these surfaces (0.5 M H2SO4, 25 °C, sweep range 0.025−0.70 V, 5 mV/s) increased as the equivalents of Ru adatoms increased. Stripping voltammetry of adsorbed monolayers of carbon monoxide from these surfaces (0.5 M H2SO4, 25 °C, sweep range 0.025−0.70 V, 5 mV/s) showed a drop by 120 mV in the CO stripping peak potential upon deposition of 0.05 equiv of Ru adatoms on Pt. Deposition of more Ru adatoms did not cause significant further drops in the CO stripping peak potential. The surfaces were evaluated as catalysts for the electrooxidation of MeOH. The measured currents were normalized to the specific surface areas of the gauzes measured before deposition of Ru adatoms by hydrogenation of 1. The order of activity of MeOH-poisoned Pt−Ruad surfaces toward the potentiodynamic oxidation of MeOH (0.5 M H2SO4, 25 °C, sweep range 0.025−0.60 V, 5 mV/s) was 0.05 > 0.10 > 0.30 ∼ 0.44 > 0.7 > 0 (Pt) equiv of Ru adatoms. The activity of Pt increased relative to the other surfaces as the potential increased, becoming more active than 0.30, 0.70, and 0.44 equiv Pt−Ruad at the upper limit of the sweeps. Surfaces with low equivalents of Ru adatoms (from 0.05 to 0.10 equiv) were the most active toward the potentiostatic oxidation of MeOH (E = 0.4 V, 0.5 M H2SO4, 0.5 M MeOH, 25 °C) with between 50 and 28 times higher turnover numbers than black Pt. The activation energies for oxidation of MeOH over 0.05 and 0.10 equiv of Pt−Ruad were 37 and 45 kJ/mol, respectively, at 0.4 V, 0.5 M H2SO4, and 0.5 M MeOH.