| AVS 54th International Symposium | |
| Surface Science | Monday Sessions |
| Session SS1-MoM |
| Session: | Catalysis on Clusters and Nanoparticles |
| Presenter: | J. Nakamura, University of Tsukuba, Japan |
| Authors: | J. Nakamura, University of Tsukuba, Japan K. Izumi, University of Tsukuba, Japan Y. Iwasaki, University of Tsukuba, Japan T. Kondo, University of Tsukuba, Japan |
| Correspondent: | Click to Email |
Reduction in Pt usage is one of the key requirements for the commercialization of polymer electrolyte fuel cells (PEFCs) for use in everyday life, because of its high price and limited availability, and the difficulty of finding suitable substitutes. We have reported that use of carbon nanotube (CNT) as an electrode material results in high performance with low loading of Pt. That is, the electric power of 12 wt% Pt/CNT was found to be higher than that of 29wt% Pt/carbon black. Moreover, we have observed that RuPt/CNT catalysts show higher tolerance for CO poisoning compared to RuPt/carbon black(CB) catalysts. We have thus found superior performance of CNT as a support material. The CNT used is thicker tubes with diameter of 50-80 nm compared to single wall CNT (0.7-1.5 nm). The diameter of catalyst particles ranges from 2 to 8 nm. The use of thick CNTs in PEFC is very promising because the price is going down recently. Why does CNT show good performance as a support material? There are several physical and chemical reasons. Here, we emphasize the chemical aspect of the interface between catalysts and carbon electrode. That is, difference between CNT and generally used carbon black (CB) is the surface of carbon, at which catalyst particles are attached and electron flows. The surface of thick CNT can be regarded as basal plane of graphite, while the surface of CB is composed of edges of graphene sheets. We thus use a model system of metal catalysts/HOPG (Highly oriented pyrolytic graphite). STM observation shows that Pt particles are attached like two dimension islands on HOPG instead of spherical particles. XPS measurements show that Pt 4f core level is shifted to higher energy compared to bulk Pt. The 2-d Pt particles on HOPG show very high catalytic activity for H2-D2 exchange reaction at high pressures (24 Torr). However, the catalytic activity is reduced by a factor of 30-50 upon heating above 320 K in the presence of hydrogen although no decrease in the amount of Pt is observed by the heating. This is consistent with lower peak temperature shift in TPD of H2 from Pt on HOPG. The change in the catalytic properties of 2-d Pt particles can be ascribed to electronic modification due to a lattice strain of 2-d Pt islands on HOPG. This explains the support effect of carbon supported fuel cell catalysts, that is, Pt/CNT described above.