Property, active species and reaction mechanism of NO and NH3 over mesoporous Fe-Al-SBA-15 via microwave assisted synthesis for NH3-SCR

Abstract

A series of bimetallic Fe-Al-SBA-15 DeNOx catalysts were synthesized by microwave (MW) and conventional heating (CH) methods with various levels of iron and aluminum contents, all of which was used to identify and study the roles of aluminum and different iron species in SCR. Synergistic effect between iron and aluminum was achieved showing a satisfactory NO conversation of approximately 95% over Fe-Al-SBA-15-MW at temperature as low as 350°C. The catalysts were characterized by XRD, TEM, N2 desorption/adsorption, UV–vis DRS, 27Al MAS NMR, in situ DRIFTS and EPR. Through UV–vis DRS and 27Al MAS NMR, we found that MW method tended to produce more oligomeric FexOy clusters and tetrahedral framework aluminum than CH method. We also did some research about the reaction mechanism of NH3-SCR. Specifically, oligomeric FexOy clusters played an important role in NO oxidation. NH3 were mainly adsorpted to the tetrahedral framework aluminum and existed as NH4+ ions. Although the reaction between adsorbed NH3 and O2 occurred, the reaction rate was much slower than that between NH3 and NO+O2. Moreover, in situ EPR indicated that Fe3+ species, which did not react with pure NH3, could be easily reduced to Fe2+ by NH3+NO. When O2 was introduced along with NH3 and NO, the non-paramagnetic Fe2+ species would be reoxidezd to Fe3+, which meant that O2 participated in the reaction cycle and had a great function on reoxidization of Fe2+. Besides, the active complex NH4+(NO2) which attributed to g′∼3.3 in EPR might be the key intermediates in the SCR reaction.