Effect of Particle Size and Composition on CO-Tolerance at Pt–Ru/C Catalysts Analyzed by In Situ Attenuated Total Reflection FTIR Spectroscopy

The particle size dependence of activities for the hydrogen oxidation reaction (HOR) on monodisperse Pt2Ru3 (d = 2.6, 3.6, and 4.5 nm) supported on carbon black prepared by the nanocapsule method (Pt2Ru3/C) was investigated by in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy in 1% CO (H2-balance)-saturated 0.1 M HClO4. The highest CO-tolerant HOR activity at both 25 and 60 °C was observed for the smallest Pt2Ru3 particle (d = 2.6 nm), with a Pt-rich top surface and an Ru-rich core. At 25 °C, the suppression of the adsorption of CO on Ru sites was a dominant factor to maintain the electronic modification effect that weakens the bond strength of COad. The adsorption of CO on Ru at all catalysts was found to decrease markedly by increasing the temperature to 60 °C, probably because of a decrease in the bond strength of COad on Ru sites. At 60 °C, the coverage of bridged CO, θ­[COB], on Pt sites was found to be lowest on the smallest Pt2Ru3 particle. Because COB blocks two active sites for the HOR, the largest number of HOR active sites can be available at the smallest particle. The effect of chemical composition on the CO-tolerance was also analyzed on Pt2Ru3/C and PtRu/C with the identical particle size d = 2.6 nm. While both catalysts exhibited nearly comparable CO-tolerance at 25 °C, Pt2Ru3/C showed higher CO-tolerance at 60 °C, which is ascribed to the lower θ­[COB], probably because of an electronic effect of the Ru-rich core.