The mechanistic cause of enhanced C3H8-SCR activity of Ag/Al2O3 by H-2 addition was investigated by kinetic and in situ infrared (IR) studies. Kinetic studies found that hydrogen addition results in decreased activation energy for NO reduction (from 224 to 61 kJ mol(-1)). decreased reaction orders with respect to O-2 (from 1.38 to 0.01) and C3H8 (from 1.91 to 0.76), and increased order with respect to NO (from -2.53 to 0.49). The essential role of H-2 is in the reductive activation of molecular oxygen into reactive oxygen species involved in the oxidative activation of C3H8. Addition of hydrogen can also retard nitrate poisoning by reducing nitrates and thus decreasing nitrate coverage. N-2 formation via the unsteady-state reaction of nitrates with hydrogen and NH3 formation via the NO + H-2 reaction also occur, demonstrating an additional effect of hydrogen on the promotion of NO reduction under the unsteady-state and unselective conditions. (c) 2006 Elsevier Inc. All rights reserved.