Isolation of active adsorbates for the NO-CO reaction on Pd/Al2O3 by selective enhancement and selective poisoning

A novel experimental approach which couples in situ infrared (IR) with selective enhancement and poisoning has been developed to identify the active adsorbate for the NO-CO reaction on Pd/Al2O3. In situ IR studies of the steady-state NO-CO reaction show that NO adsorbs as Pd-N drop Odelta+, Pd-N=O, and Pd-N=(O delta-); CO adsorbs as Pd-C drop O and (Pd)(Pd)>C=O. Addition of H-2 to 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 O-2 to 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 CO2 and Pd-C drop O suggests that Pd-C drop O is the active adsorbate leading to CO2. This study demonstrates that combined in situ IR with careful selection of enhancing and poisoning reagents allows unambiguous identification of active adsorbates under reaction conditions. (C) 1998 Academic Press.