NOx Interactions with Dispersed BaO: Adsorption Kinetics, Chemisorbed Species, and Effects of Oxidation Catalyst Sites

BaO adsorbents, together with transition metal catalysts, can be used to trap smog-causing NO from combustion effluent streams that lack CO or residual hydrocarbons as reductants. 3 Transition metals catalyze NO oxidation to NO2, a molecule that readily adsorbs on many porous solids, but which also inhibits NO oxidation by titrating active sites on catalytic surfaces with oxygen atoms. 8 As a result, NO2 adsorption rates on BaO or BaCO3 strongly influence the effectiveness of the catalysts required to convert NO to NO2 as part of lean de-NOx removal strategies. Infrared and X-ray photoelectron spectroscopic studies suggest that NO2 initially adsorbs on BaO-containing solids as nitrites with either sequential 16 or concurrent 17 formation of nitrates. Nitrate formation can involve peroxides (BaO2) 5,18,19 or direct interactions of NO2 5,8 23 or NO/O2 15,16,20 with BaO surfaces or with preformed nitrites. Nitrite and nitrate formation on BaO may also be mediated by intermediates formed on proximate metal clusters. 22 The elementary steps for NOx adsorption processes and the transport pathways between Pt and BaO sites remain unclear and controversial, at least in part, because previous studies have been conducted at conditions leading to complete NO2 conversion, which preclude unequivocal interpretations of adsorption and reaction rates. Here, we report infrared spectra and kinetic data under conditions of strict kinetic control to probe the nature of the