National Research Council of Canada. Energy, Mining and Environment
The effect of Pt average particle size (1.9-6.7 nm) on the catalytic activity of 909 ppm carbon monoxide and ethylene oxidation by O2- from yttria-stabilized zirconia (YSZ) was studied at 25-400 °C. The results show that CO and C2H4 oxidation in the absence of oxygen in the gas feed is strongly size dependent in analogy to their catalytic oxidation in oxygen-rich environment. Without oxygen, the local nano-galvanic cell mechanism is proposed, where CO and C2H4 electrooxidation at the three-phase boundary (tpb) is accompanied by partial surface electroreduction of zirconia. The smallest Pt nanoparticles (NPs) (1.9 ± 0.4 nm) have higher TOF, higher intrinsic rates and lower activation energies compared with the larger Pt NPs (4.4 ± 0.3 nm), for example 30.3 vs. 64.8 kJ mol-1 for CO and 12.5 vs. 38.4 kJ mol-1 for C2H4 oxidation, respectively. In the presence of oxygen, the catalytic oxidation occurs at significantly lower temperatures than without O2. In this case, the oxidation reaction takes place via a sacrificial promoter-type mechanism, proposed earlier for catalyst nanoparticles supported on YSZ, where thermally induced O2- backspillover migrates from YSZ to the gas-exposed surface of Pt NPs in parallel with the electrooxidation reaction at tpb.