Role of hydrogen-spillover in H2-NO reaction over Pd-supported NOx-adsorbing material, MnOx-CeO2

The reactivity of NOx-adsorbing material, MnOx-CeO2, toward H-2 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 NOx adsorbed on Pd/MnOx-CeO2 can be largely removed by micropulse H-2 injections at 150 degreesC, which ensure the conversion of nitrite adsorbates (NO2-) into N-2 and thus recover the adsorbability. The results of O-2-H-2 titration evidenced that the role played by Pd is to produce atomic hydrogen, which spills over onto MnOx-CeO2 and leads to the reduction of the surface. The amount of spilt-over hydrogen was strongly dependent on temperature and the composition of MnOx-CeO2; the equimolar oxide (0.5MnO(x)0.5CeO(2)) exhibited ca. 270-fold larger hydrogen uptake compared to the number of surface Pd atom (n(H)/n(Pd) (s) = 270) at 150 degreesC. The anion vacancies thus formed on MnOx-CeO2 could be refilled by oxygen spilt-over from Pd. When H-2 was supplied after saturated by the oxidative NO adsorption at 150 degreesC, hydrogen spilt-over from Pd caused not only the reduction of nitrite species adsorbed on MnOx-CeO2, but also the reaction between the anion vacancy and gaseous NO. Consequently, two types of spillover-assisted mechanisms for NO-H-2 reactions were proposed to explain the reason the reduction of adsorbed nitrite spreads out to the whole surface of MnOx-CeO2.