Efficient Sm modified Mn/TiO2 catalysts for selective catalytic reduction of NO with NH3 at low temperature

Abstract

A series of Sm modified Mn/TiO2 catalysts were proposed for selective catalytic reduction of NO with NH3 and their catalytic activity was evaluated at low temperature. The effect of Sm on the catalytic activity of Mn-Sm/TiO2 was comprehensively studied using XRD, low-temperature N2 adsorption, XPS, NH3-TPD and H2-TPR. The results showed that the 20Mn-10Sm/TiO2 catalyst, prepared by ultrasonic impregnation, exhibited NOx conversion of higher than 80 % at the temperature range of 110–250 °C with a gas hourly space velocity of 80,000 h−1. The weak Lewis acid site and redox ability may be crucial properties for catalytic performance as a result of the increase of Mn4+ and surface chemisorption oxygen as well as weak acid sites due to the enhanced interaction among Mn, Sm and Ti after addition of Sm. The in situ DRIFTS experiments were conducted to investigate the adsorption of NH3 and NO with their surface reaction. The results revealed that the SCR reactions on the surface of Mn/TiO2 catalyst follow both Langmuir-Hinshelwood and Eley-Rideal mechanism and bridging nitrate is the important intermediate. While, that on surface of 20Mn-10Sm/TiO2 catalyst comply with only Eley-Rideal mechanism, as the introduction of Sm changes the reaction path by inhibiting the adsorption of NO in the presence of NH3. Besides, SCR reaction rate over 20Mn-10Sm/TiO2 catalyst is fast as a result of large amount of -NH2 species formed on the surface of 20Mn-10Sm/TiO2 catalyst for the breaking of NH bonds become easier over the Sm modified catalyst, which is beneficial for SCR activity of 20Mn-10Sm/TiO2 catalyst at low temperature. The TG-DSC-MS results revealed that the addition of Sm reduce the sulfation of Mn/TiO2 catalyst.