Catalytic Decomposition of Nitrogen Monoxide over Valency-Controlled La2CuO4-Based Mixed Oxides

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Abstract
The direct decomposition of nitrogen monoxide (NO) to N2 and O2 was carried out over valency-controlled La2−xA′xCu1−yB′yO4 (A′ = Sr, Ba, Ca, Ce; B′ = Zr, Al) mixed oxide catalysts with a K2NiF4-type structure. The catalysts were characterized by XRD, XPS, redox titration, and oxygen TPD. It was confirmed by XPS that the surface and bulk compositions generally agreed and by redox titration that the average oxidation number (AON) of copper varied widely from 1.60 to 2.30. XPS showed that the valency of surface copper agreed with AON for AON ≤ 2, while it was in the Cu2+ state for AON > 2.0. The amounts of oxygen desorbed in TPD and the oxygen nonstoichiometry of La2−xSrxCuO4 increased with x in parallel. The catalytic activity for NO decomposition showed a maximum at x = 0.4—0.5, and a good correlation was found between the catalytic activity and AON for all of the catalysts tested. The decomposition of dinitrogen oxide (N2O) was also carried out for La2−xSrxCuO4 for a comparison. Based on the much slower rate of decomposition for NO than that for N2O and the significantly high rate of oxygen TPD, it was concluded that the oxygen desorption was not the rate-determining step in the steady state. The relative activities of the catalysts studied are all well explained by a mechanism in which the active sites for NO decomposition over these catalysts are coordinatively unsaturated Cu 2+ ions on the surface that can be easily oxidized to Cu3+ upon NO adsorption.