NOx-sorbing metal oxides, MnOx-CeO2.: Oxidative NO adsorption and NOx-H2 reaction

(n)MnOx-(1- n)CeO2 binary oxides have been studied for the sorptive NO removal and subsequent reduction of NOx sorbed to N-2 at low temperatures (less than or equal to 150 degreesC). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO3-) and/or nitrite (NO2-) species on the surface depending on temperature, 02 concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NOx toward H-2 was examined for MnOx-CeO2 impregnated with Pd, which is known as a nonselective catalyst toward NO-H-2 reaction in the presence of excess oxygen. The Pd/MnOx-CeO2 catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H-2 at 150 degreesC. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnOx-CeO2 surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H-2-O-2 combustion, H-2-NO reaction was suppressed to a certain extent in the presence of O-2. Nevertheless, Pd/MnOx-CeO2 attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H-2, and 6% O-2 in He at as low as 150 degreesC, compared to ca. 30% conversion for Pd/gamma-Al2O3 at the same temperature. The combination of NOx-sorbing materials and H-2-activation catalysts is expected to pave the way to development of novel NOx-sorbing catalysts for selective dcNO(x) at very low temperatures.