Study on the ZnO-poisoning behaviors on CeO_x–MoO₃ catalysts for the selective catalytic reduction of NO_x with NH₃

In this study, the impacts of zinc oxide (ZnO) loading on the selective catalytic reduction (SCR) performance of the cerium oxide-molybdenum oxide (CeO_x–MoO₃) catalyst were systematically investigated. With the introduction of ZnO, the catalyst’s nitrogen oxides (NO_x) reduction activity presented a downward trend. At 300 °C, the NO_x conversion efficiency of the sample with 4 wt% ZnO loaded dropped below 70%, ~ 22% lower than that of the fresh sample. This decrease was attributed to the neutralization of the catalysts’ acid sites by the added ZnO, which inhibited the adsorption of ammonia as the reductant. Furthermore, the introduction of ZnO resulted in a decrease in the relative content of Ce³⁺, which interfered with the formation of oxygen vacancies and the activation of oxygen. This weakened oxidative ability slowed down the activation of the reactants. Even though the specific surface area of the CeO_x–MoO₃ catalyst was enhanced due to ZnO, this promotional effect was insufficient to completely overcome the aforementioned negative impacts. Consequently, the efficiency of the NO_x elimination process over the ZnO-loaded catalysts was adversely affected. Despite the addition of ZnO, the Eley–Rideal reaction pathway continued to dominate in the deNO_x process.