Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation—: A DEMS study

Abstract

The activity of unsupported high surface area PtRu catalysts of different Pt:Ru ratio towards preadsorbed CO and the methanol oxidation reaction (MOR) in 0.5 M sulfuric acid solution has been studied at room temperature using differential electrochemical mass spectrometry (DEMS). Adsorbed CO monolayer stripping DEMS experiments show that (i) the contribution of double-layer charging increases with the Ru content, reaching up to 50% of the total stripping charge at approximately 40 at% Ru; (ii) the onset of CO ad oxidation for Ru containing catalysts starts at approximately 0.3 V RHE, about 0.15 V more negative compared with Pt; and (iii) both the onset potential and the peak potential for CO ad-stripping depends on the Ru content, reaching the most negative values at medium Ru contents, for 20–60 at% Ru. CO ad-stripping was furthermore used to determine the active surface area of the PtRu catalysts. Based on the electron yield of 1.9 electrons per CO 2 product molecule CO ad can be identified as the stable adsorbed product of methanol dehydrogenation on all PtRu catalysts. Potentiodynamic methanol oxidation experiments show a clear effect of the chemical composition of the PtRu catalysts. The onset of CO 2 formation occurs most negative, at slightly below 0.3 V RHE, for PtRu catalysts containing about 40–60 at% Ru. In the technologically interesting potential regime of 0.4–0.5 V RHE PtRu catalysts containing small or medium amounts of Ru (15, 42, 46 at%) are most active, while at more positive potentials more Pt rich catalysts containing approximately 15 at% Ru are most active. These activities refer to the inherent chemical activity obtained by normalizing the oxidation current to the active surface area determined by CO ad-stripping. Without normalization, the Pt rich catalyst (15 at% Ru) would appear as the most active one, underlining the necessity to properly account for variations in the active surface area. For all compositions methylformate formation starts at potentials around 0.5 V RHE, about 0.2 V positive of the onset of CO 2 formation, indicating that at low anodic potentials complete oxidation of methanol to CO 2 is preferred. The high current efficiency of both the PtRu and the Pt catalysts for the MOR, with electron yields of slightly above six electrons per CO 2 product molecule, is attributed to readsorption and complete oxidation of partially oxidized reaction intermediates, which is more facile for electrodes with high catalyst loadings, as used here, than for electrodes with lower loadings or smooth electrodes.