MnOx−CeO₂ Binary Oxides for Catalytic NOx Sorption at Low Temperatures. Sorptive Removal of NOx

Interactions of nitrogen oxides (NO x ) with ( n )MnO x −(1 − n )CeO 2 binary oxides were studied to use them for sorptive NO x removal at low temperatures (≃150 °C). The formation of MnO x −CeO 2 solid solutions with the fluorite-type structure at n ≃ 0.5 was found to be quite effective in accelerating NO x sorption from flowing mixtures of 0.08% NO, 2% O 2, and He balance (W/F = 0.24 g·s/cm 3 ). The cumulative NO x uptake was increased by decreasing the reaction temperature and/or by increasing O 2 concentration, indicative of chemisorption via oxidation of NO/NO 2 . In situ FT-IR diffuse reflectance spectrometry demonstrated that adsorption of NO x as bidentate, monodentate, and ionic nitrates is responsible for the large uptake. These adsorbates are produced by oxidative adsorption, which is caused by NO oxidation to NO 2 by lattice oxygens bound to Mn and subsequent coordination to Ce in adjacent surface sites. XPS and O 2 -TPD studies suggested that the active site for NO oxidation should be related to the redox of Mn and accompanying reversible sorption/desorption of lattice oxygens.