Enhancement of the oxygen reduction reaction (ORR) was examined with Pd₃Pb ordered intermetallic nanoparticles (NPs) supported on titania (Pd₃Pb/TiO₂). The Pd₃Pb/TiO₂ catalyst was synthesized by conventional wet chemical method with Pd and Pb ion precursors, a reducing agent and TiO₂ powder under ambient temperature. Powder X-ray diffraction (pXRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy measurements indicated the formation of the ordered intermetallic phase of Pd₃Pb in the NP forms on the TiO₂ surfaces. Electrochemical measurements showed that the Pd₃Pb/TiO₂ catalyst can remarkably enhance the ORR in alkaline environment due to the unique surface of Pd₃Pb NP and the strong interaction between Pd₃Pb and TiO₂ when compared with TiO₂-supported Pd and Pt NPs. The onset potential of Pd₃Pb/TiO₂ was shifted towards to higher potential by 110-150 mV when compared to Pd/TiO₂ and Pt/TiO₂.

The catalyst-coated GC electrodes were used to study the ORR activities and kinetics at the Pd/TiO₂ and Pd₃Pb/TiO₂ electrodes. Figure 1A shows the ORR polarization curves of Pd/TiO₂ (a) Pd₃Pb/TiO₂ (b) and Pt/TiO₂ (c) catalysts obtained in O₂-saturated 0.1M KOH(aq) solution at a rotation speed of 2000 rpm. The polarization curves show a well-defined diffusion limiting current region from -0.8 V to -0.4 V. The onset potential of the Pd/TiO₂ (a) for the ORR is -0.25 V. On the other hand, the Pd₃Pb/TiO₂ (b) shows the onset potential at -0.14 V, which is more positive than that of Pd/TiO₂, indicating that the Pd₃Pb NPs feature a significant enhancement in the electrocatalytic ORR in an alkaline environment when compared with Pd/TiO₂. Further, the electrocatalytic ORR activity of Pd₃Pb/TiO₂ was also compared with Pt/TiO₂ (c) catalyst. The Pt/TiO₂ catalyst showed an onset potential at -0.22 V (Fig. 1A (c)). In order to evaluate the kinetic parameter of Pd₃Pb/TiO₂ during the course of ORR, the polarization curves were recorded at different disk rotation rate in rotating disk electrode (RDE) experiments. Fig. 1B depicts the dependence of ORR curves on the disk rotation rate for the Pd₃Pb/TiO₂-coated GC. The number of electrons involved in ORR can be determined from the slope of the Koutecky-Levich (K-L) plot. The diffusion coefficient of O₂ (D_O2 = 1.9 x 10^-5 cm² s^-1) , the kinetic viscosity of the solution (0.01 cm² s^-1) and the bulk concentration of O₂ (C_O2 =1.2 x 10^-6 mol cm^-3) were used to estimate the number of electrons.  From the straight lines shown in the K-L plots (Fig.1C), the number of electron transferred in ORR was calculated to be 3.65-3.76 at around -0.08-0.28 V, indicating that the ORR is relatively dominated by a four-electron reduction of O₂ in the potential region from -0.08 to -0.28 V. A chronoamperometric study was conducted to investigate the durability of the catalyst for the ORR. The current-time curves on Pd₃Pb/TiO₂ (a) and Pd/TiO₂ (b) catalysts in O₂ saturated 0.1 M KOH(aq) solution are shown in Fig. 1D. It is clear that for ORR on Pd₃Pb/TiO₂ (a) at constant applied potential of -0.1 V, the current densities of the catalysts change quickly in the initial stage and after reaching to 5 mAcm^-2, the current density decrease slowly. After 90 min, the current density decreased to -3.9 mAcm^-2 (to 78%). The above results clearly reveal that the Pd₃Pb/TiO₂ is more stable than that of Pd/TiO₂ in terms of catalytic activities of oxygen.

Fig. 1 (A) ORR polarization curves of (a) Pd/TiO₂, (b) Pd₃Pb/TiO₂ and (c) Pt/TiO₂ catalysts in O₂-saturated 0.1 M KOH solution at a scan rate of 10 mV s^-1 and electrode rotation rate of 2000 rpm. (B) the dependence of ORR current density of the Pd₃Pb/TiO₂-coated GC electrode in a 0.1 M KOH solution on the electrode rotation rate. (C) K-L plots obtained from the ORR curves for the Pd₃Pb/TiO₂ at different potentials. (D) Chronoamperometry curves of (a) Pd₃Pb/TiO₂ and (b) Pd₃Pb/CB catalysts in O₂-saturated 0.1 M KOH at electrode potential of -0.1 V.