1. Contribution from the Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898, Debye Institute, Laboratory of Inorganic Chemistry and Catalysis, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands, and Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616