CATALYTIC DECOMPOSITION OF NITROGEN MONOXIDE OVER VALENCY-CONTROLLED LA2CUO4-BASED MIXED OXIDES

The direct decomposition of nitrogen monoxide (NO) to N2 and O2 was ca rried out over valency-controlled La2-xAx'Cu1-yBy'O4 (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 oxy gen TPD. It was confirmed by XPS that the surface and bulk composition s generally agreed and by redox titration that the average oxidation n umber (AON) of copper varied widely from 1.60 to 2.30. XPS showed that the valency of surface copper agreed with AON for AON less-than-or-eq ual-to 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-xSrxCUO 4 increased with x in parallel. The catalytic activity for NO decompos ition showed a maximum at x = 0.4-0.5, and a good correlation was foun d between the catalytic activity and AON for all of the catalysts test ed. The decomposition of dinitrogen oxide (N2O) was also Carried out f or La2-xSrxCuO4 for a comparison. Based on the much slower rate of dec omposition for NO than that for N2O and the significantly high rate of oxygen TPD, it was concluded that the oxygen desorption was not the r ate-determining step in the steady state. The relative activities of t he catalysts studied are all well explained by a mechanism in which th e active sites for NO decomposition over these catalysts are coordinat ively unsaturated Cu2+ ions on the surface that can be easily oxidized to Cu3+ upon NO adsorption.