Experimental assessment of the bifunctional NH3-SCR pathway and the structural and acid-base properties of WO3 dispersed on CeO2 catalysts

Abstract

A bifunctional pathway for selective catalytic reduction (SCR) with NH3 was established using a mixture of WO3/ZrO2 possessing strong acidity and CeO2 possessing moderate redox activity. The physical mixture (tight contact) of WO3/ZrO2 (4.2 W/nm2) and CeO2 provided enhanced SCR reactivity but did not improve the redox property assessed by NO oxidation. No improvement of the SCR conversion, however, was observed for mixtures of individual pellets (loose contact), suggesting the requirement of submicrometer-level proximity of the acidic sites to the redox centers. WO3/CeO2 (5.3 W/nm2) with intermediate acidic strength had higher SCR and NO oxidation rates than the aforementioned physical mixture of WO3/ZrO2 and CeO2. The weakly basic sites (OH species) on CeO2 were replaced stoichiometrically with strongly acidic sites on the WO3 domains along with an increase in the W density (0–24.4 W/nm2), and monoclinic WO3 crystallites then formed at approximately 10 W/nm2. The dispersed WO3 was in a distorted octahedral environment in all WO3/CeO2 samples. The intrinsic SCR reactivity was controlled by only the W surface density; the SCR rate (per surface area) increased within the polytungstate submonolayer region (<5–10 W/nm2) and then nearly reached a plateau. The NO oxidation rate also increased with an increase of the W density in a similar trend to the SCR rate, which is indicative of the formation of redox-active centers, rendering the synergetic enhancement of SCR reactivity. These findings and conclusions reached here provide useful guidance for new bifunctional strategies to achieve practical SCR performance.