Effect of Temperature on Oxygen Reduction Reaction Kinetics for Pd Core-Pt Shell Catalyst with Different Core Size

The oxygen reduction reaction (ORR) activity and the coverage of oxide species over a Pd core-Pt shell catalyst on carbon support (Pt/Pd/C) with various Pd core sizes (2.3, 4.3, and 8.0 nm) were investigated in the temperature range from 25 to 60 degrees C and compared with a Pt/C catalyst (TEC10V30E, Tanaka Kikinzoku Kogyo). The apparent rate constant (k(app)) of Pt/Pd/C increased with increasing core size at 25 degrees C. However, k(app) of Pt/Pd/C started to decrease at 50-60 degrees C, while that of Pt/C behaved according to the general Arrhenius equation. Eventually, the 2.3 nm core showed the highest k(app) at 60 degrees C, and the 8.0 nm core was almost the same as that of Pt/C. According to the electrochemical measurements, the coverage of oxide species on Pt/Pd/C was quite smaller than that of Pt/C. However, it increased dramatically with increasing temperature from 25 to 60 degrees C. Among the Pt/Pd/C, the 8.0 nm core showed the most obvious oxide coverage increase at 60 degrees C, which was almost identical to that of Pt/C. In contrast, the 2.3 nm core showed the lowest oxide coverage at 60 degrees C, which was expected to be the cause of the largest k(app). Operando X-ray absorption spectroscopy indicated that the Pt-Pt bond length in Pt/Pd/C was shorter than that in the Pt/C at 25 degrees C due to compressive surface strain from the Pd core, which is the reason why Pt/Pd/C has higher activity than Pt/C. On the other hand, as Pd has a thermal expansion coefficient higher than that of Pt, Pt/Pd/C showed a Pt-Pt bond length larger than that of Pt/C at 60 degrees C. A longer Pt-Pt bond length extension was observed at 60 degrees C in the 8.0 nm core compared to that in the other catalysts.

Automated summary

The study revealed that the size of Pd core in Pt/Pd/C catalyst significantly influences the ORR activity and oxide species coverage, with the activity of Pt/Pd/C diminishing with increasing temperature due to the compressive surface strain from the Pd core.