Determination of the Platinum and Ruthenium Surface Areas in Platinum−Ruthenium Electrocatalysts by Underpotential Deposition of Copper. 2. Effect of Surface Composition on Activity

Abstract

Submonolayer amounts of Ru have been deposited on a polycrystalline Pt bead electrode from a solution containing 6.68 × 10 -4 mol dm -3 RuNO(NO 3 ) 3 using an impinging jet apparatus. Deposition of Ru on a platinum surface almost completely covered with H ads due to exchange of the adsorbed hydrogen with Ru 3+ produces a surface coverage of 0.12 ( 0.02. The reduction of Ru occurs via two separate pathways: in one, metallic Ru is deposited on the surface; in the other, the complex is reduced to an intermediate oxidation state and remains soluble. The latter process, which occurs at potentials less than about 0.25 V has not been previously reported and may explain some of the discrepancies seen in the literature. For characterization of the resultant Pt(Ru) surface we compare the approach developed by Motoo and Watanabe (J. Electroanal. Chem. 1975, 60, 267-273) 1 and Frelink et al. (Langmuir 1996, 12, 3702) 2 with that utilizing copper upd. The latter approach is applicable for Ru coverage from 0 up to at least 0.80, whereas the former methods are only applicable up to a coverage of ca. 0.30. On addition of Ru to a clean Pt surface a small initial drop in total surface area is seen, but the area then remains quite constant with increasing Ru coverage. In comparison, the charge measured from stripping of a saturated CO layer adsorbed at 0.3 V(RHE) shows an increase with Ru coverage. The most active surface for the oxidation of an adsorbed CO layer as measured by the potential at which half of the CO has been oxidized is found to contain a Ru surface coverage of 0.2. In comparison, for the oxidation of methanol at 25 °C, it is found that there is a broad maximum in catalytic activity for Ru surface coverage in the range 0.25-0.5. The long term poisoning rate of the electrodes is highly dependent upon Ru coverage, showing a minimum over the range 0.4-0.6.

Determination of the real surface area of powdered materials in cavity microelectrodes by electrochemical impedance spectroscopy

“…The electrode was polarized at E = 0.33 V for 100 s then the potential was scanned at 10 mV s −1 between 0.33 and 0.6 V. The background current was substracted from the copper stripping charge and the real surface area was determined assuming the monolayer charge of 420 C cm −2 [16,17].…”

“…For Pt-based electrocatalysts, the measurement of H UPD charge includes contributions from bisulfate adsorption, vary for different crystallographic planes [8] and the total charge is sensitive to sur-UPD is an alternative method to evaluate PtRu surface area [2,16,17] although several problems were also raised [2].…”

Activity of selenium modified ruthenium-electrodes and determination of the real surface area

“…The same is true for pseudo-capacitive charges such as hydrogen and oxygen adsorption, which are often used for the determination of the surface area of Pt and Au. For pure Ru and PtRu alloys, a method based on upd of Cu was described by Kucernak and coworkers [10,11]. We have shown [12] that this method cannot be simply transferred on RuSe x catalysts, because the amount of Cu, which can be deposited in the upd-range, decreases with the surface concentration of Se.…”

Catalysis for Direct Methanol Fuel Cells

“…Therefore, the activation-controlled (COOH) 2 oxidation current is proportional to the Pt surface area. Another useful method is the underpotential deposition of Cu [106]. Cu undergoes underpotential deposition onto Pt and Ru metal surfaces and can subsequently be stripped from the surface.…”