Low Temperature-Based Abatement Technique: History
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Nitrogen and sulpher oxides (NOx, SOx) have become a global issue in recent years due to the fastest industrialization and urbanization. Numerous techniques are used to treat the harmful exhaust emissions, including dry, traditional wet and hybrid wet-scrubbing techniques. However, several difficulties, including high-energy requirement, limited scrubbing-liquid regeneration, formation of secondary pollutants and low efficiency, limit their industrial utilization. Regardless, the hybrid wet-scrubbing technology is gaining popularity due to low-costs, less-energy consumption and high-efficiency removal of air pollutants. 

  • removal
  • wet-scrubbing
  • catalytic systems

1. Metal Ligand Absorption

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 CO2 and SO2 are easily treated via the alkali absorption method [46,47]. Since the solubility problem NO can be treated such as other gases, it required some combined or integrated system applied for NO reduction [48]. This solubility problem indicated the NOx 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 MnOx/CNTs catalysts were found to have unusual SCR activity at low temperatures when they were first synthesized. When using the optimal 1.2 percent MnOx/CNTs catalyst at 80–180 °C, the NO conversion ranged between 57.4 and 89.2 percent. This occurred from the use of amorphous MnOx catalysts, which have a higher ratio of Mn4+ to Mn3+ and OS to (OS + OL) than the crystalline MnOx catalysts [49]. By impregnation and in situ deposition methods, the same Ce/Mn molar ratio was achieved in the preparation of Ce (1.0) Mn/TiO2 catalysts. In comparison to the impregnation-prepared Ce(1.0)Mn/TiO2-IP catalyst, the in situ deposition-prepared Ce(1.0)Mn/TiO2-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/TiO2-SP catalyst produced by the in situ deposition approach has superior sulphur resistance to the Ce(1.0)Mn/TiO2-IP catalyst [50,51]. By using the citric acid–ethanol dispersion method, a variety of Gadolinium (Gd)-modified MnOx/ZSM-5 catalysts were produced and assessed using a low-temperature NH3-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 SO2, GdMn/Z-0.3 demonstrated superior SO2 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 Mn4+ and, a proper acidity and reducibility, and the of the catalyst, among other factors [52].

2. Metal Ligand Stability

Among transition metal chelating complexes, Fe-EDTA is the most favorable and stable chelate against the long-range of pH. Comparison for the metal chelate system are also shown in Figure 3. On top of that, ferrous (Fe(II)) has the great affinity towards the NO compared to other d-block elements. By taking advantage of this Fe-EDTA stability and greatest affinity, it has been used for NOx removal from flue gas. Another advantage of this process (wet scrubbing) is the simultaneous removal of NOx and SO2 under ambient conditions. Moreover, the NOx interaction with Fe(II) is direct chemically binding and forms a very stable bond among other transition metals [12,53].
Figure 3. (A) Relative stability of Fe-EDTA with other metals, (B) relative NO gas and transition metal interactions.

3. Principle of Gas Absorption

Basically, this concept is composed of two steps, (i) Fe-EDTA solution absorbs NOx molecule via making metal-nitrosyl-complex and then (ii) this NOx reduced into N2O, N2 and N-S compounds by utilizing the SO2, which is a compulsory part of flue gas. This SO2 transformed into SO32− by alkali absorption and reduced NOx by converting itself into SO42− ions. During the reduction of NOx, the scrubber (Fe-EDTA) also regenerated irrespective the valence form of iron, Fe(II) or Fe(III). As Fe(III) is more stable due to more stability than Fe(II), it cannot be restored without the help of the electron donor externally. The number of electron donors used to reduce Fe(III) back to Fe(II) depends upon the source, condition and more important the NOx reduction process.

This entry is adapted from the peer-reviewed paper 10.3390/nano11123301

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