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- froin yttria-stabilized zirconia (YSZ) was studied at 25-400 degrees 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 +/- 03 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 O-2. 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. (C) 2015 Elsevier Inc. All rights reserved.