1. Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, 1-1 Gakuen-cho, Sakai, Osaka 593, Japan
2. Laboratoire de Reactivite de Surface, Universite P. et M. Curie, URA 1106-CNRS, 4 Place Jussieu, Tour 54, 75252 Paris Cedex 05, France