Mechanistic studies of the NO-CO reaction on Rh/Al2O3 under net-oxidizing conditions

The dynamic behavior of NO and CO adsorbates over a 0.2 wt% Rh/Al2O3 catalyst was studied under net-oxidizing conditions by in situ infrared spectroscopy combined with pulse transient techniques. The observed sequence of adsorbate and CO2 concentration profiles during the pulse reaction studies reveals that Rh+(CO)(2) is an active adsorbate which reacts with adsorbed oxygen to form CO2, The observed transient behaviors of Rh+(CO)(2), Rh-NO-, and CO2 during the pulse reactions can be explained by the proposed NO-CO reaction mechanism in which (i) Rh-NO-dissociates to form adsorbed nitrogen and oxygen and (ii) adsorbed oxygen further reacts with Rh+(CO)(2) to produce CO2. Addition of air to the NO-CO reactant stream produces a net-oxidizing environment for the NO-CO reaction. Adsorbed oxygen from air reacts with Rh-NO+ producing the nitrate species and providing more Rh sites for the NO-CO reaction at 473 K. At 573 K and above, adsorbed oxygen reacts with Rh+(CO)(2) producing CO2, blocks the reduced Rh-0 sites for Rh-NO-, oxidizes Rh-o/Rh+ to Rh2+ sites, and inhibits the NO conversion and N2O formation. Rh sites are readily reduced and return to their initial state and the catalyst returns to its initial activity when air is withdrawn from the reactant stream. Preservation of the reduced Rh sites for Rh-NO- is required to maintain the catalyst activity for the NO-CO reaction under net-oxidizing conditions. (C) 1998 Academic Press.