Promotive effect of H₂O on low-temperature NO reduction by CO over Pd/La_0.9Ba_0.1AlO_3-δ

For future removal of NO_x by catalysts, low-temperature NO reduction is desired. Results confirmed that a drastic improvement of catalytic activity by H₂O on NO–CO–O₂ reaction over Pd/La_0.9Ba_0.1AlO_3-δ catalyst at the low temperature of 473 K or below. In a humidified condition, NO reaction with CO on Pd/La_0.9Ba_0.1AlO_3-δ proceeded without being affected by competitive adsorption of NO and CO, whereas that on Pd/Al₂O₃ was inhibited by strong adsorption of CO on a Pd surface. From in-situ DRIFTS measurements, results showed that nitrite species on the support react with CO adsorbed onto Pd at the periphery of Pd particles and that carbonate species accumulated on Pd/La_0.9Ba_0.1AlO_3-δ are removed rapidly in a humidified condition. Although NO reduction proceeds dominantly on the Pd surface in a dry condition, supplied steam promotes desorption of the surface carbonate to advance the reaction of nitrite with CO for de-NO_x in a humidified condition. This mechanism occurs specifically on Pd/La_0.9Ba_0.1AlO_3-δ by virtue of the lattice oxygen and oxygen vacancy on La_0.9Ba_0.1AlO_3-δ.