In situ DRIFTS study of the mechanism of low temperature selective catalytic reduction over manganese-iron oxides

Abstract

To investigate the mechanism of selective catalytic reduction (SCR) of NO x with NH 3 , Ti 0.9 Mn 0.05 Fe 0.05 O 2-δ catalyst was prepared by self-propagating high-temperature synthesis (SHS) method and evaluated at 25–450 °C. The catalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The possible SCR mechanism over Ti 0.9 Mn 0.05 Fe 0.05 O 2-δ was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Ti 0.9 Mn 0.05 Fe 0.05 O 2-δ showed both high SCR activity and N 2 selectivity over a broad temperature window of 100–350 °C. The XRD and TEM results indicated that the active components of Mn and Fe were in a highly dispersed state and in an amorphous form on TiO 2 . The DRIFTS results revealed that Brönsted acid sites were the active centers for NO removal and monodentate nitrates were the key intermediate in the SCR reaction. At 150 °C, both Langmuir-Hinshelwood and Eley-Rideal mechanisms are involved in the SCR reaction, while the former one mechanism dominates the catalytic activity of Ti 0.9 Mn 0.05 Fe 0.05 O 2-δ . Additionally, the presence of O 2 significantly affects NO oxidation and coordinated NH 3 activation. Selective catalytic reduction reactions over Ti 0.9 Mn 0.05 Fe 0.05 O 2-δ catalysts possibly proceed according to both Langmuir-Hinshelwood and Eley-Rideal mechanisms, but NO more likely reacts as an adsorbed species.