Elementary Steps and Site Requirements for NOx Adsorption and Oxidation on Metal and Oxide Surfaces
- Author(s): Weiss, Brian M.
- Advisor(s): Iglesia, Enrique
- et al.
NO oxidation catalysts are used in conjunction with NOx adsorbents to remove toxic nitrogen oxides from combustion effluents that lack CO and residual hydrocarbons as reductants. Efficient NOx trapping strategies require detailed knowledge of the reaction mechanism and the structural requirements for NO oxidation and for NOx adsorption, which are investigated here by kinetic, isotopic, and spectroscopic methods.
NO oxidation rates on Pt, PdO, RhO2, and Co3O4 catalysts increase linearly with O2 and NO pressures and are inhibited by NO2, consistent with a kinetically-relevant NO oxidation step that requires the reaction of O2 with vacancies (*) on active sites nearly saturated with oxygen atoms (O*). Equilibrated reactions between catalyst surfaces and NO and NO2 molecules establish the concentration of oxygen vacancies during NO oxidation. Measurements of 16O2-18O2 exchange rates are used to confirm and to further investigate the O¬2 activation steps proposed for NO oxidation. NO oxidation rates on all catalysts are markedly higher on large metal and oxide clusters that bind oxygen more weakly than small clusters; consequently, oxygen vacancies become more abundant as cluster size increases. Both RhO2 and Co3O4 undergo one-electron reductions to intermediate oxidation states (Rh2O3 and CoO) when vacancies form during NO oxidation, which allows both Rh and Co oxides to effectively catalyze NO oxidation.
The pervasive NO2 inhibition of NO oxidation turnovers leads to synergistic effects between NOx adsorbents and NO oxidation catalysts. NO oxidation rates are much higher when NO2 adsorption sites are present to bind NO2 as it forms on catalyst sites, and NO oxidation rates depend critically on the rate of NO2 adsorption on oxide substrates.
NOx adsorption on BaO-containing solids leads to nitrites (NO2-) and nitrates (NO3-). The formation of nitrites on BaO/Al2O3 requires vicinal co-adsorption of NO and NO2 to displace carbonates that form during exposure to CO2 in combustion effluent. NO and NO2 bind rapidly as nitrite, but NOx uptakes as nitrite are limited in CO2-rich streams by thermodynamics that result in most binding sites being occupied with carbonate at high CO2/NOx ratios. Nitrates are more stable, and their formation is required to achieve large NOx uptakes from combustion effluent. The conversion of nitrites to nitrates, however, requires N2O4 molecules as oxidants that form slowly by non-activated homogeneous NO2 dimerization reactions, consistent with nitrate formation rates that increase with (NO2)2 and are independent of adsorbate coverage. Pt clusters present in close proximity to nitrite-saturated BaO domains provide a catalytic route for the formation of N2O4 that reacts subsequently with nitrites. These elementary steps lead to nitrate formation rates on BaO/Pt/Al2O3 that are inhibited by NO and are proportional to NO2 pressure and the coverage of unreacted nitrites. The results herein identify the physical limitations of oxidation catalysts and NOx adsorbents that are used for NOx trapping and provide new opportunities for optimizing NOx storage materials by exploiting previously unrecognized synergies between the catalyst and adsorbent components.