Methanol oxidation catalysis and substructure of PtRu bimetallic nanoparticles

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Abstract

Catalytic material of PtRu nanoparticles supported on carbon (PtRu/C) for direct methanol fuel cells was synthesized by a polyol reduction method. Addition of phosphorus was effective for downsizing PtRu particles and improving their catalytic activity. The activity obtained was six times of that of a commercial catalysis. The samples were analyzed by techniques of X-ray absorption fine structure (XAFS) at Pt LIII-edge and Ru K-edge, transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF). These results indicated a core–shell structure consisting of a Pt-rich core and Ru-rich shell. By examining coordination numbers determined by XAFS analysis, we found a clear correlation between the catalytic activity and the Pt–Ru atomic pair frequency occurring on the particle surface, which supports the “bi-functional mechanism”.

Graphical abstract

PtRu nanoparticles catalyst supported on carbon was synthesized and analyzed. The results indicated a clear correlation between the catalytic activity and Pt–Ru atomic pair frequency occurring on the particle surface, which support the “bi-functional mechanism”.

Introduction

Fuel cells have been attracting much attention as a next-generation power source for various applications [1], [2], [3], [4]. Direct methanol fuel cells (DMFC) are one of the most promising power sources for movable and portable devices and electric vehicles. The DMFC uses methanol as its fuel, while other many types of fuel cell use hydrogen gas [5]. DMFC has a volumetric energy density about 10 times larger than the lithium ion batteries. However, the effective cell voltage of the DMFC is significantly lower compared with thermodynamic voltage because of a considerable overpotential occurring in methanol oxidation reactions at its anode. Platinum is intrinsically an excellent catalyst for methanol oxidation and is usually used in anodes in DMFC. Pt catalysts are, however, easily poisoned by CO formed on methanol oxidation. To overcome this problem, scientists have been looking for new Pt-based alloys with high tolerance for this CO poisoning [6], [7]. PtRu alloy was found as a catalyst with a high CO tolerance [8], [9], [10], and its improvement was described as a “bi-functional mechanism” by Watanabe et al. [11]. Generally, downsizing of catalysis particles leads to a higher specific surface then to reductions in weight and cost of fuel cells [12], [13], but no report was found regarding application of this mechanism to nanoparticles of PtRu alloy. For developing much more advanced catalytic material, it is necessary to check whether the “bi-functional mechanism” would be valid for the nanoparticles system. In this work, we synthesized PtRu alloy nanoparticles loaded on a porous carbon support by a polyol process technique. Phosphorus was added in the process to reduce particle size further [14], [15], [16]. The catalytic activity was measured by linear sweep voltammetry (LSV). Atomic substructure in the synthesized PtRu nanoparticles was examined by the X-ray absorption fine structure (XAFS) measurements and some other complementary techniques: X-ray diffraction measurements (XRD), transmission electron microscope observations (TEM) and X-ray fluorescence spectroscopy (XRF) analyses. Correlations between these data sets regarding catalysis and structure were studied. The present result will provide a new insight into the atomic substructure of the catalysis and will contribute to development of more advanced fuel cells.

Section snippets

Sample preparation

Five samples (Samples A–E) of PtRu nanoparticles supported on carbon (PtRu/C) catalysts were synthesized by a polyol reduction method with various compositions of initially loaded materials, as tabulated in Table 1. Only Sample A was synthesized without phosphorus, but the other four included it. Sample B was synthesized with less phosphorus than Samples C–E. Samples D and E were synthesized with less Ru than the others. The details of the synthesis procedure have been reported in references

Catalytic activity of PtRu/C catalyst

Fig. 1 shows the potential–current curves obtained with the present samples, which reflect anode polarization involved in methanol oxidation. The catalytic activities of our five PtRu/C catalysts were higher than that of the commercial one, TEC61E54, and they are in the order of Samples A–E over the whole potential range. We noticed that the catalytic activity increases with increasing phosphorus addition to the source materials. Sample E has the highest activity, six times higher than that of

Conclusion

Catalysts of 2-nm bimetallic PtRu nanoparticles supported on carbon were successfully synthesized with a polyol reduction method with phosphorus addition. Their catalytic activities for methanol oxidation were far higher than the commercial one; the highest one was six times greater. Coordination numbers and inter-atomic distances around Pt and Ru were probed by the EXAFS analysis, from which the catalytic activity was found to be correlated clearly with a pairing factor: PRu = NRu–Pt/(NRu–Pt + N

Acknowledgments

The X-ray absorption experiments were performed at the BL-19B station of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2005A0904-RI-p) and at the BL-7C station of Photon Factory with the approval of the High Energy Accelerator Research Organization (KEK) (Proposal No. 2005G032). We thank the staff of the SPring-8 and the Photon Factory for their assistance with the data collection using the synchrotron radiation.

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