Size reduction of PtRu catalyst particle deposited on carbon support by addition of non-metallic elements

doi:10.1016/j.cattod.2005.10.023

Catalysis Today

Volume 111, Issues 3–4, 15 February 2006, Pages 182-187

https://doi.org/10.1016/j.cattod.2005.10.023 Get rights and content

Abstract

Addition of non-metallic elements such as N, P and S was examined to reduce the catalyst particle size of PtRu deposited on a carbon support and to improve the catalytic performance. It was found that the addition of N, P and S reduced the size of PtRu catalyst particle and that P was the most effective additive on the size reduction. A well dispersed PtRu catalyst particle 2 nm in size was obtained by the addition of P. The maximum power density was observed to be 64 mW/cm² in a passive state of direct methanol fuel cell (DMFC) at room temperature by using the PtRuP anode catalyst. The PtRuP catalyst retained the size independently with surface area of the carbon support. This made it possible to use a carbon support with low surface area (lower porosity), resulting in better accessibility by molecules on the noble metal particles which were deposited prior to the external surface in compared to the micropore walls. The maximum power density was improved from 44 to 64 mW/cm² by using a less porous carbon support (140 m²/g).

Introduction

For advances in fuel cells, catalytic activity and utilization efficiency of catalyst should be simultaneously improved. Reduction of the particle size of catalytic component is effective to improve its mass diffusion rate [1], [2]. Generally, carbon supports with high specific surface areas are used to obtain fine Pt and PtRu catalyst particles [1], [2], [3], [4]. However, micropores exist in the carbon supports and the fine catalyst particles may be buried in the micropores. The catalyst particles buried in the micropores may lose their opportunities to contact with solid polymer electrolyte and fuel, resulting in the loss of catalytic property. The fraction of buried catalyst should increase with higher specific surface area, which will deteriorate efficiency of the catalyst. Pt is an expensive material and its deposits in the world are very limited, but it is still the best catalyst for the fuel cell application. To realize the fuel cell application, improvement of the catalytic activity and minimization of the Pt requirement will be essential. Therefore, a new technology that can simultaneously improve the catalytic activity and utilization efficiency of the catalyst is strongly demanded.

Watanabe et al. synthesized 1.7 nm of well dispersed Pt catalyst particle by using a carbon support with specific surface area of 1250 m²/g [2]. They stated that the area of wall of the micropores smaller than 2 nm in diameter was included in the Brunauer–Emmett–Teller (BET) surface area and the area mainly consisted of micropore walls on high surface area carbon supports, but the micropores should be inaccessible by molecules for both catalytic reaction and diffusion of oxygen. Efficiency of the Pt catalyst was electrochemically measured to be around 0.4 in their studies [5], [6]. Therefore, it seems difficult to improve both activity and efficiency by enhancing the specific surface area of carbon supports resulting in the reduction of size of catalyst particle.

It has been reported that addition of a non-metallic element, P, drastically reduced the size of Fe single crystal in a magnetic alumite film [7]. In this report, a new method is proposed to reduce the size of PtRu catalyst particle. It is shown that the addition P reduces the size of PtRu catalyst particle and that reduced size is retained independently with the specific surface area of the carbon support. It is demonstrated that the PtRuP catalyst is a promising candidate as both the catalytic activity and efficiency of catalyst are simultaneously improved.

Section snippets

Experimental

A PtRu catalyst was synthesized by a polyol process [8], [9], [10], [11] using ethylene glycol as a reductant. 1.69 mmol of platinum (II) acetylacetonate (Pt(acac)₂), 1.69 mmol of ruthenium (III) acetylacetonate (Ru(acac)₃) and 0.5 g of carbon support were mixed in ethylene glycol and the solution was refluxed for 4 h at 473 K with mechanical stirring under nitrogen atmosphere. After refluxing, ethylene glycol was evaporated and the obtained PtRu catalyst was washed with ion exchanged water and then

Results and discussion

At first in this study, a conventional PtRu catalyst was synthesized by a polyol process. TEM observation revealed that the size of PtRu catalyst particles was distributed from 2 to 10 nm and that its dispersion was heterogeneous. Maximum power density of DMFC using the PtRu anode catalyst was measured to be 38 mW/cm², which was far smaller than our target of 100 mW/cm². It has been reported that addition of P drastically reduced the size of acicular Fe single crystal in a magnetic alumite film [7]

Acknowledgements

The authors are indebted to S. Suzuki, Dr. M. Sugimasa, Y. Arishima and Professor O. Kitakami of Tohoku University for their fruitful discussion. We give our thanks to M. Watanabe for TEM observation. We also wish to thank Dr. N. Ota and T. Taniguchi for their encouragement throughout this work.

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Size reduction of PtRu catalyst particle deposited on carbon support by addition of non-metallic elements

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Acknowledgements

J. Electroanal. Chem.

Chem. Lett.

J. Electrochem. Soc.

J. Electrochem. Soc.

J. Electroanal. Chem.

J. Electroanal. Chem.

Jpn. J. Appl. Phys.

Chem. Lett.