The ammonia storage and ammonia species reactivity within Cu-ZSM-5 with different copper electronic states

Abstract

Three Cu-ZSM-5 catalysts having different copper species: 1) the isolated Cu2+ ions, 2) the structures of Cu2+ ions with extra-lattice oxygen (ELO), and 3) the CuO-like nanoparticles, were comparatively studied for their ammonia storage capacity under NH3-Ar, NH3-O2-Ar, and NH3-NO-O2-Ar (SCR-like) feeds. EPR, DR UV–vis, DRIFT techniques and TPD modes were used to evaluate the effect of Cu-state in the O2-activated catalysts on their ammonia storage capacity and conversion of the adsorbed ammonia under Ar, O2-Ar, or NO-O2-Ar feeds. Several types of NH3-species were generated in the NH3-saturated and SCR-treated catalysts. It was shown that NH3-species within the NH3-saturated catalysts, which contained the Cu-structures with ELO or CuO-like nanoparticles, were more reactive compared with the catalyst with isolated Cu2+ ions. For the first two samples, the ammonia sorption capacity determined from the sorption-desorption dynamics at given temperature (25, 200, 300 °C) was higher than amount of the TPD(Ar)-desorbed ammonia under Ar. It is due to selective oxidation of some amount from the coordinated NH3-species, for which ELO of Cu-structures is consumed, the process is accompanied by reduction of Cu2+ to Cu+. The total quantity of the TPD-desorbed ammonia as well as the amount of ammonia coordinated in [Cu(NH3)n]2+ complex decreased with increasing sorption temperature and under O2-Ar or NO-O2-Ar feeds used for TPD mode. Regardless of Cu-states in the catalysts, the molar ratio of the ammonia storage capacity to copper loading was shown by the sorption-desorption dynamics to be ca. 2 at 200 °C, versus 4 typical of the catalyst saturated at 25 °C. These values are consistent with DR UV–vis data, showing a.b. at 14100-14500 cm−1 and 15700−15800 cm−1, respectively. The NH3-TPD(Ar) mode with the NH3-saturated catalysts showed the molar ratio of the TPD-desorbed ammonia to copper loading (ν(NH3)/ν(Cu) equal to 2 for the sample with isolated Cu2+ions; but for two others, the ν(NH3)/ν(Cu) was < 1. Finally, this work is trying to manifest the role of isolated Cu2+ ions and Cu-structures with ELO for SCR-activity.