NO Adsorption and Reaction on Aged Pd–Rh Natural Gas Vehicle Catalysts: A Combined TAP and Steady-State Kinetic Approach

A combined temporal analysis of products (TAP) and steady-state kinetic study was achieved to characterize the surface reactivity of fresh and aged bimetallic Pd–Rh/Al₂O₃ Natural-Gas Vehicle catalysts. Single NO pulse TAP experiments were performed on a stabilized surface after exposure to successive NO pulses until to get a steady-state NO conversion. Outlet flow curves recorded during such experiments show fast reaction steps taking place on noble metal particles and a slow process during NO desorption ascribed to the involvement of spill-over effect of chemisorbed NO molecules from the metal to the support. This slow process attenuates on the aged sample likely due to an alteration of the metal/support interface induced by particle sintering at high temperature. Thermal aging also alters the surface composition of bimetallic Pd–Rh particles which leads to changes in the products distribution from NO dissociation. A similar selectivity behavior is observed from steady-state kinetic measurements during the NO/H₂ reaction. Interestingly, a weak partial pressure dependency of the selectivity reflects a surface Rh enrichment of Pd–Rh particles during aging.