Revealing insights into the unexpected effects of ZnO on selective catalytic reduction (SCR) activity of the CeOx-WO3 catalyst

Abstract

This study delved into the intriguing impact of zinc oxide (ZnO) on the selective catalytic reduction (SCR) performance of a CeOx-WO3 catalyst with a Ce/W molar ratio of 3:2 (denoted as CW). The introduction of ZnO, at low concentrations of 1 wt% or 3 wt%, resulted in a diminution of the catalyst’s surface acid sites, which hampered the adsorption of NH3 and negated the benefits of improved NH3 activation due to the enhanced oxidative capacity. Consequently, the proceeding of the SCR reactions was significantly impeded, with a marked decrease in NOx conversion to approximately 42 % at 200 °C, which was nearly half that of the virgin CW catalyst. In contrast, increasing ZnO loading to 5 wt% facilitated the formation of ZnWO4, which introduced additional strong acid sites and bolstered NH3 adsorption at moderate and high temperatures. This, in conjunction with the heightened reactivity of surface Ce4+, resulted in a resurgence of deNOx performance, with a NOx conversion increase of ∼ 13 % at 250 °C compared to the catalyst with 3 wt% ZnO. Conversely, escalating ZnO loading to 7 wt% disrupted the formation of ZnWO4 and the excess ZnO neutralized the catalyst’s acid sites, thereby hindering NH3 adsorption and reducing NOx conversion. These insights were instrumental for the prospective application of the CeOx-WO3 catalysts under heavy metal-rich conditions.