1. Department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Korea
2. Exhaust Emission Engineering Team, Power Train Research and Development Center, Hyundai-Kia Motors Company, Jangduk-dong, Hwaseong 445-706, Korea
3. General Motors Research and Development Planning Center, Warren, Michigan 48090-9055, United States