A Review on Selective Catalytic Reduction of NOx by NH3 over Mn–Based Catalysts at Low Temperatures: Catalysts, Mechanisms, Kinetics and DFT Calculations

Abstract

It is a major challenge to develop the low-temperature catalysts (LTC, <250 • C) with excellent efficiency and stability for selective catalytic reduction (SCR) of NO x by NH 3 from stationary sources. Mn-based LTC have been widely investigated due to its various valence states and excellent redox performance, while the poisoning by H 2 O or/and SO 2 is one of the severe weaknesses. This paper reviews the latest research progress on Mn-based catalysts that are expected to break through the resistance, such as modified MnO x -CeO 2 , multi-metal oxides with special crystal or/and shape structures, modified TiO 2 supporter, and novel carbon supporter (ACF, CNTs, GE), etc. The SCR mechanisms and promoting effects of redox cycle are described in detail. The reaction kinetics will be a benefit for the quantitative study of Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms. This paper also introduces the applications of quantum-chemical calculation using density functional theory to analyze the physic-chemical properties, explicates the reaction and poisoning mechanisms, and directs the design of functional catalysts on molecule levels. The intensive study of H 2 O/SO 2 inhibition effects is by means of the combination analysis of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT), and the amplification of tolerance mechanisms will be helpful to design an excellent SCR catalyst.

NOx Emissions from Euro 5 and Euro 6 Heavy-Duty Diesel Vehicles under Real Driving Conditions

“…The trucks and large buses with GVWs of 40 tons or higher, which were often driven under high-speed conditions, showed no significant differences compared to the 7-15-ton trucks. SCR systems applied to Euro 6 HDV can maintain low levels of NOx emissions regardless of GVW due to the improved low temperature performance of SCR catalysts and the insulation of exhaust lines [31,32]. The current vehicle emission factors were calculated as a function of the average vehicle speed by driving in various modes separately from the certification test mode in the chassis dynamometer.…”

Enhancement of SCR performance of monolithic Mn–Ce/Al2O3/cordierite catalysts by using modified deposition precipitation method

“…MnO x –CeO x catalysts have attracted intensive interest because of their outstanding activities at low temperature as a result of multiple valence states of Mn cations and the oxygen storage and redox properties of Ce . Most of the MnO x –CeO x low‐temperature SCR catalysts reported in the literatures are in powder or granule form; however, in gas‐phase reactions, honeycomb monolith catalysts have some favorable characteristics, for example, the pressure drop is two orders of magnitude lower, higher resistance to attrition and higher dust tolerance . Cordierite is preferentially used as the basis material of the monolithic catalyst because of its high mechanical stability and low thermal expansion coefficient .…”

NOx Removal Performance Optimization of NiMnTi Mixed Oxide Catalysts by Tuning the Redox Capability

“…[5,6] With the in-depth research about the catalysts in the NH 3 -SCR field, the superiority of transition metal oxide catalysts are gradually acknowledged. [7][8][9] Notably, the selection of metal elements and the preparation methods of mixed metal oxides catalysts have an impact on the activity, selectivity, sulfur resistance and stability. [10,11] Hence, exploiting catalysts with remarkable performance and broad activity temperature region remains to be a rising consensus by selecting the proper metal element and preparation approach.…”