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 (<i>d</i> = 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 (<i>d</i> = 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 CO_ad. 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 CO_ad on Ru sites. At 60 °C, the coverage of bridged CO, θ­[CO_B], on Pt sites was found to be lowest on the smallest Pt₂Ru₃ particle. Because CO_B 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 <i>d</i> = 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[CO_B], probably because of an electronic effect of the Ru-rich core