A Highly Effective Catalyst of Sm-MnOx for the NH3-SCR of NOx at Low Temperature: Promotional Role of Sm and Its Catalytic Performance

Sm-Mn mixed oxide catalysts prepared by the coprecipitation method were developed, and their catalytic activities were tested for the selective catalytic reduction (SCR) of NO with ammonia at low temperature. The results showed that the amount of Sm markedly influenced the activity of the MnOx catalyst for SCR, that the activity of the Sm-Mn mixed oxide catalyst exhibited a volcano-type tendency with an increase in the Sm content, and that the appropriate mole ratio of Sm to Mn in the catalyst was 0.1. In addition, the presence of Sm in the MnOx catalyst can obviously enhance both water and sulfur dioxide resistances. The effect of Sm on the physiochemical properties of the Sm-MnOx catalyst were investigated by XRD, low-temperature N2 adsorption, XPS, and FE-SEM techniques. The results showed that the presence of Sm in the Sm-MnOx catalyst can restrain the crystallization of MnOx and increase its surface area and the relative content of both Mn4+ and surface oxygen (OS) on the surface of the Sm-MnOx catalyst. NH3-TPD, NO-TPD, and in situ DRIFT techniques were used to investigate the absorption of NH3 and NO on the Sm-MnOx catalyst and their surface reactions. The results revealed that the presence of Sm in the Sm0.1-MnOx catalyst can increase the absorption amount of NH3 and NO on the catalyst and does not vary the SCR reaction mechanism over the MnOx catalyst: that is, the coexistence of Eley–Rideal and Langmuir–Hinshelwood mechanisms (bidentate nitrate is the active intermediate), in which the Eley–Rideal mechanism is predominant.