Studies On The Performance And Mechanism Of The Novel Catalysts Loaded With Manganese And Iron For Low Temperatrue NH₃ -SCR

To address the more and more stringent emission regulations of NOx on the diesel engines in recent years, the selective catalytic reduction with ammonia (NH3-SCR), attributing to its well-developed technology in the application for the removal of NOx in the flue gas, has been considered to be one of the most promising technical routes. However, it is accepted that the traditional vanadia-based commercial catalysts, which is the core of the SCR technology, exist a significant problem: the narrow active reaction temperature window (300-400℃). Therefore, the researches on the non vanadia-based catalysts have been carrying out to tackle the difficulty.In order to solve the problems above, the low temperature NH3-SCR catalysts were investigated in this study. The detailed research consists of four sections as followed:(1) To improve traditional catalyst preparation methods, the novel self-propagating high-temperature synthesis (SHS) method was applied to synthesize a series of low temperature catalysts loaded with manganese and iron.(2) The effects of different active components on the catalytic performance were investigated on the test bench using simulated engine exhaust. It was found that Ti0.9Mn0.1O2-δand Ti0.9Fe0.1O2-δhad excellent activity and selectivity at low temperature respectively, while the dual metal oxides catalyst-Ti0.9Mn0.05Fe0.05O2-δperformed very well both in activity and selectivity: the DeNOx efficiency sustained 100% in the temperature window of 120-320℃and N2 selectivity kept more than 70% all the time. The results of the characterization of the catalysts by BET, XRD, SEM, FTIR, TEM showed that Ti0.9Mn0.05Fe0.05O2-δ, on the surface of which the active components had outstanding dispersion, possessed the very good specific surface area, BJH desorption pore volume and pore size distributions.(3) The studies on the influence of ignition temperature for combustion synthesis had also been carried out. The results indicated that the NOx removal efficiency and N2 selectivity decreased with the increasing ignition temperature. Through the characterization of the catalysts, it was pointed out that the decline of the catalytic performance was caused by the worse coordinating effect among the metal oxides and the degeneration of the surface physical structure, such as the specific surface area, pore volume, pore properties, crystal phase, etc.(4) The reaction process and mechanism were investigated by in situ DRIFTS. The results revealed that NO could strongly adsorb on the catalyst surface to generate nitrate species, while ammonia could suppress this process by adsorption competition. Br?nsted acid sites and Lewis acid sites both participated in this SCR reaction, however, the former was more active. Eley-Rideal and Langmuir-Hinshelwood mechanism took effect in this reaction parallelly, while the latter was the main pathway. In addition, the monodentate nitrate is the key nitrate species in the SCR reaction.