Reaction Mechanism of Low-Temperature Selective Catalytic Reduction of NO x over Fe-Mn Oxides Supported on Fly-Ash-Derived SBA-15 Molecular Sieves: Structure–Activity Relationships and in Situ DRIFT Analysis

Fly ash emissions caused by coal combustion have been increasing for many years, causing serious environmental pollution. Coal combustion also causes large amounts of NO x to be emitted to the atmosphere, and this has caused environmental problems such as acid rain, which cannot be ignored. The denitrification catalyst V2O5/WO3–TiO2 gives a good denitrification efficiency at a high temperature, but the catalyst has poor efficiency and is difficult to use at low temperatures (100–300 °C). Therefore, we introduce a new method based on the use of fly ash to control NO x output. We used a two-step alkali hydrothermal method to prepare SBA-15 mesoporous molecular sieves from fly ash obtained from a thermal power plant in Inner Mongolia (China). A series of bimetallic iron and manganese oxides were supported on the fly-ash-derived SBA-15 catalyst, and excellent NO conversion was found for selective catalytic reduction (SCR) of NO with NH3 at low temperatures. The catalysts were characterized by XRD, XPS, NH3-, O2-, and CO2-TPD, H2-TPR, BET analysis, SEM, TEM, and DRIFT spectroscopy. The denitration activity and denitration mechanism over the catalysts are discussed. The mechanisms of NO reduction and N2O formation over Mn/SBA-15 and Fe-Mn/SBA-15 were investigated through in situ DRIFT studies and a transient reaction study. The strong oxidation, low acidity, and high basicity of the Fe-Mn/SBA-15 catalyst contributed to a large amount of nitrate being produced during the catalysis. The nitrate decomposed to produce N2O, resulting in a decrease in N2 selectivity. The denitration mechanism of the Fe-Mn/SBA-15 catalyst in the SCR reaction followed Langmuir–Hinshelwood, Eley–Rideal, and Mars–van Krevelen mechanisms.