Effects of MnO 2 crystal structure and surface property on the NH 3-SCR reaction at low temperature

Abstract

Two manganese oxides with the same nanorod-shaped morphology but different crystal structures, tunnel and layer structures, were synthesized and investigated for selective catalytic reduction of NOx with NH3 (NH 3-SCR) at low temperature. Tunneled α-MnO 2 had much higher catalytic activity than layered δ-MnO 2 under the same reaction conditions. Experiment results revealed that the surface area was not the main factor to affect the NH 3-SCR activities over the MnO 2 nanorods and that the activities were structure sensitive. Structure analysis and temperature-programmed desorption experiments of NH 3 (NH 3-TPD) suggested that the exposed (110) plane of α-MnO 2 had many Mn cations in coordinatively unsaturated environment, while all of the Mn cations on the exposed (001) plane of δ-MnO 2 were in coordinatively saturated environment. Thus, α-MnO 2 possessed many more Lewis acid sites. Furthermore, α-MnO2 has weaker Mn-O bonds and an efficient tunnel structure, which are favorable characteristics for NH3 adsorption. Moreover, X-ray photoelectron spectroscopy (XPS) and thermal gravimetric (TG) analysis indicated that α-MnO 2 obtained a higher capability for NH 3 and NOx activation than δ-MnO 2. The crystal structure and surface properties of α-MnO 2 are more suitable to the adsorption of NH 3 and activation of NH 3 and NOx, which accounts for the higher catalytic activity of the α-MnO2 nanorods. © Editorial office of Acta Physico-Chimica Sinica.