Study on the effect of erbium doping on vanadium–manganese based NH3-SCR catalysts

Abstract

With the increasingly strict motor vehicle emission regulations, the demand for the performance of on-board SCR catalysts has been continuously rising. This work aims to develop composite metal oxide catalysts with a broad temperature window. A series of TiV_0.1Mn_0.01Er_mO_x (m = 0.005, 0.01, 0.015, 0.02) catalysts were prepared by the solution combustion method. Performance tests show that appropriate loading of Er not only improves the low-temperature performance but also enhances the high-temperature performance. However, loading more Er elements only improves the high-temperature performance, while the low-temperature performance decreases instead. The optimized TiV_0.1Mn_0.01Er_0.01O_x catalyst in this paper exhibits an active temperature window of 160–470 °C. BET characterization reveals that Er doping increases the specific surface area. The XRD pattern shows diffraction peaks of anatase TiO₂, indicating that the active components are distributed in an amorphous state on the support surface. NH₃-TPD curves show that the total acid amount of TiV_0.1Mn_0.01Er_0.01O_x is higher than that of the TiV_0.1Mn_0.01O_x catalyst, and the acid amount distribution across the entire temperature range is also higher than that of TiV_0.1Mn_0.01O_x, implying that more NH₃ is adsorbed on the surface for reactions. TEM images show that sintering occurs in some catalysts, but the surface pore structure remains abundant. In situ diffuse reflection was used to study the mechanism of the TiV_0.1Mn_0.01Er_0.01O_x catalyst, revealing that the catalyst surface is rich in Brønsted and Lewis acid sites. NH₃ is adsorbed to form NH₄⁺ and coordinated NH₃, with a higher content of Lewis sites playing a dominant role. NO forms monodentate, bidentate, and bridged nitrates on the catalyst surface, with similar contents of the three nitrates, all participating in the SCR reaction. This paper suggests that both the reaction mechanism of NH₃ adsorption and activation followed by reaction with gaseous NO, and the reaction mechanism involving NO oxidation to form nitrates are present. Therefore, the surface of TiV_0.1Mn_0.01Er_0.01O_x simultaneously exhibits the Eley–Rideal and Langmuir–Hinshelwood reaction mechanisms.