Researchers and environmentalists have been working on alternative solutions to the concerns listed above and explored various approaches. Another critical thing to be discussed here is the NO solubility problem in water. Some other toxic gases such as CO
2 and SO
2 are easily treated via the alkali absorption method
[1][2]. Since the solubility problem NO can be treated such as other gases, it required some combined or integrated system applied for NO reduction
[3]. This solubility problem indicated the NO
x removal would be easier if chemical modifications were used for its absorption and reduction. Many ways came to notice, but most practice has been undertaken by wet scrubbing using Fe-EDTA. There are several reasons for considering flue gas treatment and reduction techniques, which are discussed herein. The MnO
x/CNT
s catalysts were found to have unusual SCR activity at low temperatures when they were first synthesized. When using the optimal 1.2 percent MnO
x/CNT
s catalyst at 80–180 °C, the NO conversion ranged between 57.4 and 89.2 percent. This occurred from the use of amorphous MnO
x catalysts, which have a higher ratio of Mn
4+ to Mn
3+ and O
S to (O
S + O
L) than the crystalline MnO
x catalysts
[4]. By impregnation and in situ deposition methods, the same Ce/Mn molar ratio was achieved in the preparation of Ce (1.0) Mn/TiO
2 catalysts. In comparison to the impregnation-prepared Ce(1.0)Mn/TiO
2-IP catalyst, the in situ deposition-prepared Ce(1.0)Mn/TiO
2-SP catalyst demonstrated superior catalytic activity throughout a wide temperature range (150–300 °C) and at high-gas hourly-spaced velocities ranging from 10,500 to 27,000 h
−1. Furthermore, the Ce(1.0)Mn/TiO
2-SP catalyst produced by the in situ deposition approach has superior sulphur resistance to the Ce(1.0)Mn/TiO
2-IP catalyst
[5][6]. By using the citric acid–ethanol dispersion method, a variety of Gadolinium (Gd)-modified MnO
x/ZSM-5 catalysts were produced and assessed using a low-temperature NH
3-SCR reaction. Of them, the GdMn/Z-0.3 catalyst, which had a molar ratio of 0.3 for Gd to Mn, had the maximum catalytic activity, and it was capable of achieving a 100 percent NO conversion in the temperature range of 120–240 degrees Celsius. Furthermore, when tested in the presence of 100 ppm SO
2, GdMn/Z-0.3 demonstrated superior SO
2 resistance when compared to Mn/Z. It was demonstrated that such catalytic efficacy was primarily driven by surface chemisorbed oxygen species, a wide surface area, an abundance of Mn
4+ and, a proper acidity and reducibility, and the of the catalyst, among other factors
[7].