Isolation of Active Adsorbates for the NO–CO Reaction on Pd/Al2O3by Selective Enhancement and Selective Poisoning

Abstract

A novel experimental approach which couplesin situinfrared (IR) with selective enhancement and poisoning has been developed to identify the active adsorbate for the NO–CO reaction on Pd/Al2O3.In situIR studies of the steady-state NO–CO reaction show that NO adsorbs as Pd–N≡Oδ+, Pd–N=O, and Pd–N//Oδ−; CO adsorbs as Pd–C≡O andPdPd>C=O. Addition of H2to the NO–CO flow as a reaction enhancer which reacts with adsorbed nitrogen and oxygen results in depletion of Pd–N=O, while addition of O2to the NO–CO flow as a reaction poison leads to accumulation of Pd–N=O on the Pd surface. These observations suggest that Pd–N=O is the active adsorbate involved in NO dissociation. The observed variation in the concentrations of CO2and Pd–C≡O suggests that Pd–C≡O is the active adsorbate leading to CO2. This study demonstrates that combinedin situIR with careful selection of enhancing and poisoning reagents allows unambiguous identification of active adsorbates under reaction conditions.