Interactions of nitrogen oxides (NO x ) with (n)MnO x −(1 − n)CeO2 binary oxides were studied to use them for sorptive NO x removal at low temperatures (≃150 °C). The formation of MnO x −CeO2 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% O2, and He balance (W/F = 0.24 g·s/cm3). The cumulative NO x uptake was increased by decreasing the reaction temperature and/or by increasing O2 concentration, indicative of chemisorption via oxidation of NO/NO2. 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 NO2 by lattice oxygens bound to Mn and subsequent coordination to Ce in adjacent surface sites. XPS and O2-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.

No abstract

The reactivity of NO x sorbed onto MnO x -CeO 2 toward H 2 was studied in the presence of impregnated Pd catalyst by use of TPR, H 2 pulse reactions, in situ FT-IR, and steady-state NO-H 2 -O 2 reactions. The Pd-loaded catalyst after saturated NO x -sorption at 150 °C could be regenerated by micropulse injections of H 2 , which ensure the reduction of monodentate, bidentate, and ionic nitrates sorbed on MnO x -CeO 2 into N 2 . In the steady-state NO-H 2 -O 2 reaction, gaseous NO x was first sorbed onto MnO x -CeO 2 and subsequently reduced at the PdO/MnO x -CeO 2 boundary. Despite the nonselective character of Pd catalysts toward NO x -H 2 reaction, Pd/MnO x -CeO 2 attained 65% NO-conversion of a stream of 0.08 vol % NO, 2 vol % H 2 , and 6 vol % O 2 in He at a low temperature of 150 °C, compared to ∼30% for Pd/γ-Al 2 O 3 , the reaction on which was more suppressed by the competitive H 2 -O 2 reaction. The combination of NO x sorbability of MnO x -CeO 2 and H 2 activation of Pd catalysts was found to give rise to a synergistic effect, thus paving the way to development of NO x -sorbing catalysts for selective deNO x processes at low temperatures (<150 °C).

No abstract

The reactivity of NO x -adsorbing material, MnO x −CeO2, toward H2 has been studied in the presence of impregnated Pd catalysts by the use of micropulse reactions, in situ DRIFT spectroscopy, and chemisorption experiments. It was found that NO x adsorbed on Pd/MnO x −CeO2 can be largely removed by micropulse H2 injections at 150 °C, which ensure the conversion of nitrite adsorbates (NO2 -) into N2 and thus recover the adsorbability. The results of O2−H2 titration evidenced that the role played by Pd is to produce atomic hydrogen, which spills over onto MnO x −CeO2 and leads to the reduction of the surface. The amount of spilt-over hydrogen was strongly dependent on temperature and the composition of MnO x −CeO2; the equimolar oxide (0.5MnO x −0.5CeO2) exhibited ca. 270-fold larger hydrogen uptake compared to the number of surface Pd atom (n H/n Pd   s = 270) at 150 °C. The anion vacancies thus formed on MnO x −CeO2 could be refilled by oxygen spilt-over from Pd. When H2 was supplied after saturated by the oxidative NO adsorption at 150 °C, hydrogen spilt-over from Pd caused not only the reduction of nitrite species adsorbed on MnO x −CeO2, but also the reaction between the anion vacancy and gaseous NO. Consequently, two types of spillover-assisted mechanisms for NO−H2 reactions were proposed to explain the reason the reduction of adsorbed nitrite spreads out to the whole surface of MnO x −CeO2.