Abstract
Temperature-programmed reduction (TPR) and cyclic voltammetry have been used to investigate Pt and PtRu dispersed catalysts unsupported and supported on conducting carbon at various stages of the catalyst preparation. Differences in the TPR profiles and the corresponding uptake of H 2 by unsupported and supported Pt(NH 3) 2(NO 2) 2 dried from HNO 3 solution at 120 °C are attributed to an interaction via ligand exchange between the nitrato-substituted Pt complex and nucleophilic surface groups formed on the carbon support by the action of HNO 3. A similar interaction was observed for carbon-supported Ru salts prepared from RuNO(NO 3) x . Support interactions present after the drying stage led to an enhanced catalyst dispersion and to the elimination of the original interaction after activation in air at 300 °C. Bimetallic PtRu catalysts display reducibilities in TPR intermediate between those observed for the pure components in both unsupported and supported forms. The influence of the carbon support is absent after activation in air. At 300 °C, activation produces a high bimetallic dispersion (90 m 2 g −1) whereas activation at 400 °C leads to more perfect alloy formation and consequently to an increased intrinsic activity as an electrocatalyst for methanol oxidation at the expense of a reduced dispersion (54 m 2 g −1). Cathodic processes at 0.1–0.4 V (vs RHE) during the initial linear potential sweep, corresponding to oxide reduction, are observed for air-activated Pt and PtRu catalysts. Electrochemical reduction of oxide species in this way is analogous to the characteristic processes observed in TPR except that initial cathodic reduction extends ca.4 atomic layers into the oxide particles. Our conclusions concerning air-activated Pt catalysts, largely composed of PtO 2, are in essential agreement with similar studies of oxide layers formed electrochemically on smooth Pt electrodes.
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