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

Infrared spectra and nitrate and nitrite formation rate data free of transport artifacts provide rigorous evidence for the identity of the adsorbed species and the elementary steps required for adsorption of NO, NO2, and CO2 on BaO/Al2O3 with and without Pt clusters that act as oxidation catalysts. NO/NO2 adsorption occurs via initial formation of nitrites and their subsequent oxidation to nitrates on samples presaturated with carbonates by exposure to CO2. Nitrites form much faster than nitrates at low NO2 pressures via displacement of carbonates and vicinal coadsorption of NO and NO2 molecules. As a result, the dynamics of nitrite formation and of their subsequent oxidation can be independently measured during exposure to NO/NO2 mixtures over a broad temperature range (453-673 K). Nitrites form rapidly upon exposure to NOx, but the presence of CO2 limits equilibrium NOx uptakes because of unfavorable thermodynamics, rendering nitrite formation an impractical strategy for NOx removal from CO2-rich combustion effluent streams. Nitrate thermodynamics is much more favorable, but the rate of nitrite oxidation to nitrates is limited by slow homogeneous NO2 dimerization to N2O4, which acts as the oxidant in nitrite conversion to nitrates on Pt-free Ba/Al2O3. This mechanism is consistent with nitrate formation rates that are second-order in NO2 pressure and essentially independent of NO pressure, sample temperature, and residual coverages of unreacted nitrites. Pt clusters present in close proximity to (but not atomic contact with) nitrite-saturated BaO domains provide a catalytic route for the formation of the N2O4 oxidant required to convert nitrites to stable nitrates and for the effective removal of NOx from CO2-containing streams. Nitrate formation rates on BaO/Pt/Al2O3 are proportional to NO2 pressures, inhibited by NO and proportional to the residual coverage of unreacted nitrites, consistent with rates limited by reactions between N2O4 molecules, in equilibrium with NO2 in steps mediated by Pt surfaces, and nitrites. These detailed kinetic and spectroscopic studies provide mechanistic details previously unavailable about the bifunctional character of NO2 reactions that form stable nitrates on BaO domains and about the essential role of oxidant formation on metal clusters in rendering such adsorption strategies practical for the removal of NOx from CO2-rich streams.