Promotion effect and mechanism of the addition of Mo on the enhanced low temperature SCR of NO_x by NH₃ over MnO_x/Γ-Al₂O₃ catalysts

A series of Mn/γ-Al₂O₃ and Mn_xMo_y/γ-Al₂O₃ catalysts were prepared by using Incipient Wetness Impregnation (IWI) method. The catalytic performance tests showed that the Mn₃Mo_1.25/γ-Al₂O₃ demonstrated a higher SCR performance (NO conversion of around 96%) at a broad low temperature range (150–300 °C). The characterization showed that the addition of Mo to the Mn/γ-Al₂O₃ catalysts promoted the dispersion of MnO_x on the surface of γ-Al₂O₃. The adsorption of NO could form two different species, i.e., nitrites and nitrates, on the surface of the catalyst. The presence of nitrites is beneficial to low temperature SCR. It is found that the existence of Mo in the catalyst favours the formation of Mn³⁺, which plays a critical role in the adsorption of NH₃ and therefore improves NH₃ adsorption capacity of the MnO_x/γ-Al₂O₃ catalysts. The low temperature SCR of the Mn₃Mo_1.25/γ-Al₂O₃ catalyst was found to mainly follow L-H mechanism, but E-R mechanism also plays a role to some extent. Moreover, it is found that the addition of Mo not only mitigates the deactivation of the catalysts, but also broadens the effective temperature range of the SCR catalysts.