Large Scale Synthesis of Pd Core/Pt Shell Structured Catalyst and Their Electrochemical Properties

Abstract

Introduction PEFC represents one of the most effective ways for hydrogen utilization because of its high conversion efficiency, low pollution, light weight and high power density. However, usage amount of the expensive Pt should be decreased for the worldwide commercialization of the PEFC. Core/shell structured catalyst is a promising candidate for the decrease of the Pt usage. Furthermore, it has been reported that ORR activity of the Pt monolayer (PtML) shell is enhanced when it is formed on the Pd core [1, 2]. Conventionally, the PtML shell is formed on the Pd core surface by a Cu under potential deposition (Cu-UPD)/Pt replacement process [1]. Recently, we have successfully developed a modified Cu-UPD/Pt replacement process in which precise potential control is needless and suitable for large scale synthesis of the carbon supported Pd core/Pt shell structured catalyst (Pt/Pd/C) [3, 4]. In this study, we firstly prepared carbon supported Pd core (Pd/C) in a large scale, and the Pt/Pd/C catalyst was synthesized with the modified Cu-UPD/Pt replacement process. Electrochemical properties of the Pt/Pd/C catalyst were compared with those of the catalyst synthesized with a small scale. Experiment Carbon supported Pd core (Pd/C) was prepared by impregnation/thermal reduction method. 60 g of Ketjen black EC 300J and Pd precursor were well mixed in water. The mixture was dried and thermally reduced in N2 gas atmosphere, yielding 85 g Pd/C core. 40 g Pd/C core was ultrasonically dispersed in 50 mM H2SO4 containing 20 mM CuSO4 and stirred with coexistence of a Cu sheet under Ar atmosphere. After pre-determined stirring time, the Cu sheet was removed and K2PtCl4 was added to replace the Cu shell with the Pt shell, forming the Pt/Pd/C catalyst. The Pd/C core and the Pt/Pd/C catalyst were characterized with ICP, XRD, CO adsorption, TEM and CV. ORR activity of the Pt/Pd/C catalyst was evaluated with RDE technique in O2 saturated 0.1M HClO4 at 298 K. Results and Discussion A problem in the large scale preparation of the Pd/C core (85 g/batch) is an agglomeration of the Pd core particles during the thermal reduction. DSC profile of the Pd precursor impregnated on the carbon support indicated that a large exothermic reaction occur when Pd precursor is reduced. Figure 1 shows dependence of the Pd core size on the thermal reduction temperature. The Pd core size could be controlled in 4.5 nm, which is same core size obtained in a small preparation scale (5 g/batch), optimizing the thermal reduction temperature. In the Pt shell formation process, it is essential to deaerate the residual O2, since the O2 oxidizes the Cu shell formed on the Pd core surface in the modified Cu-UPD process. In order to demonstrate the O2 removal, ORR current of a polycrystalline Pt electrode was measured at 0.4 V vs. RHE in O2 saturated 50 mM H2SO4 by changing deaeration time with N2 gas bubbling in small scale for the basis of O2 deaeration (Fig. 2). The ORR current was not detected with the N2 gas bubbling longer than 20 min. Based on this result, the N2 gas bubbling was conducted in large scale, by which Pt shell close to the PtML was formed on the Pd core surface. Table 1 summarizes electrochemical properties of the Pt/Pd/C catalysts produced by the large and the small scale syntheses. All the properties of the Pt/Pd/C catalyst synthesized by the large scale are equivalent to those of the catalyst synthesized by the small scale. Further optimizations in the large scale synthesis of the Pt/Pd/C catalyst will be presented at the meeting. Acknowledgement This work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan.