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Pollution from Ammonia (NH3)
Ammonia (NH3) is a toxic compound, originating from gas and wastewater sources in agriculture and industry. Therefore, converting NH3 into non-toxic N2 has become one of the top tasks for environmental treatment catalyst research. In this article, we will review the harmful effects as well as important sources of NH3 emissions.
Sources of NH3, NOx and N2O in agricultural soil
Nitrogen oxide (NOx) and sulfur oxide (SOx) have long been considered the main agents of environmental acidification. In the air, they are easily converted into nitric acid and sulfuric acid. However, NH3 emissions are also indirect agents of environmental acidification. The reaction between NH3 and acids in the air such as H2SO4 and HNO3 forms gas layers containing ammonium sulfate ((NH4)2SO4) and ammonium nitrate (NH4NO3). These salts are then oxidized by bacteria in the soil, producing HNO3 again. In addition, ammonia is also toxic to human health. At low concentrations, NH3 creates a stinging sensation, and at high concentrations, it can even cause blindness. Its odor can cause severe allergies to people exposed to it. Therefore, ammonia is considered a long-term cause of bronchitis. In addition, in industry, gas leaks NH3 will corrode equipment, thereby clogging production processes. In 1999, a study published in the journal Environmental Science demonstrated the role of ammonia in the formation of urban photochemical smog.
NH3 is emitted in large quantities from agricultural and livestock activities. The Netherlands is one of the countries most affected by the acidification of agricultural land from NH3 pollution. More than half of the forest land is affected, most of the swamps are acidified, the nitrate concentration in the soil exceeds the allowable level and continues to increase. Recently, it has also been found that the northeastern part of Carolina is suffering from negative impacts from NH3 pollution in the soil. In industry, NH3 is often used as a NOx reducing agent by selective catalytic reduction processes, according to the following reaction:
NH3 + NO + ¼ O2 → N2 + H2O.
This reaction is only effective when using excess ammonia, so it creates a very high risk of NH3 leakage. In addition, NH3 also leaks from many industrial production sources such as soda production, nitric acid, metallurgy industry...
Therefore, NH3 treatment, NH3 oxidation to N2 has become a big challenge in the industry today, despite the fact that NH3 treatment processes have been studied a lot over the past 30 years. NH3 treatment can be divided into two main groups: NH3 oxidation in the liquid phase and NH3 oxidation in the gas phase. In water, NH3 can be completely oxidized by the chlorinating agent (Cl2). However, after NH3 treatment, the process of removing chlorine in water is necessary. Two other methods used to oxidize NH3 solution are catalytic wet oxidation and electrochemical method.
In the gas phase, the method commonly used in the past was to dissolve NH3 gas into H2SO4 solution to collect ammonium sulfate. Ammonium sulfate will then be used in the fertilizer industry. However, the demand for ammonium sulfate in industry is also decreasing. Therefore, another method to treat gas-phase NH3 that is receiving much attention is ammonia oxidation at high temperature in the presence of metal catalysts (Pt, Pd, Cu, Ag, Ni, Fe...) or metal oxides (Co3O4, Cr2O3, CuO, Fe2O3...). The following articles will introduce this method in more detail.
A new technique for environmental NH3 removal involves the use of catalytic reactions that selectively oxidize NH3 to N2 and H2O. This method can be applied to treat ammonia sources at low and high concentrations, as well as in both the gas and liquid phases.
The oxidation reaction of NH3 with O2 in air can theoretically take place in the following ways:
2 NH3 + 3/2 O2 → N2 + 3 H2O + 151 kcal
2 NH3 + 2 O2 → N2O + 3 H2O + 132 kcal
2 NH3 + 5/2 O2 → 2 NO + 3 H2O + 108 kcal
Both NO and N2O are toxic gases to the environment and living organisms. N2O is one of the main agents causing the greenhouse effect while NO is easily oxidized by O2 to NO2, the source of acid rain. Exposure to N2O for a long time can cause suffocation, vitamin B12 deficiency, or paralysis of the whole body. NO is even more dangerous, causing blood vessel blockage, epithelial cancer, colitis, etc. Therefore, the role of catalysts used in the treatment processes of NH3 contaminated sources is not only to increase the speed of the NH3 oxidation reaction, but also to increase the selectivity of the reaction, promote the formation of N2 (a gas that is not toxic to the environment) and minimize the content of NOx gases (the main agent causing acid rain, environmental pollution) in the product. Controlling the product composition makes the NH3 oxidation reaction with catalysts one of the most interesting and important heterogeneous catalytic processes.
The search for these catalysts began in the early 20th century. Noble metals and transition metal oxides were the first to be studied for the NH3 decomposition reaction because they often exhibit effective oxidation catalytic activity for many oxidation reactions on other substances. A large number of research results have been published for many catalytic materials, from metals to metal oxides. For example, in the presence of Platium or cobalt oxide, at temperatures ranging from 750 to 900°C, NH3 will be oxidized by O2 mainly to NOx. On the contrary, at low temperatures below 300°C, the products of NH3 oxidation with catalysts such as metals such as Pt, Pd, Cu, Ag, Au or metal oxides such as Co3O4, MnO2, CuO, CaO, Fe2O3… are N2 and N2O. Increasing the temperature and O2/NH3 content will increase the NH3 conversion activity but will reduce the selectivity for N2 [2]. Another study also showed that, for oxide catalysts, at low temperatures, the lower the activity of the catalyst (ZnO, TiO2, MoO3), the higher the selectivity for N2 as the final product.
Comparison table of the activities of different catalysts for the oxidation reaction of NH3 (gas) at high concentrations with O2 as a catalyst [1]
([NH3] = 11400 ppm in He, catalyst mass: 0,2 g)
The oxidation of NH3 by O2 on Pt metal catalysts has attracted a lot of research in the world. Many reaction mechanisms have been proposed before 1960, however, they are only based on assumptions due to lack of experimental evidence. Recently, Mieher and Ho analyzed the oxidation of NH3 on Pt (111) using TPD (Temperature programmed desorption), EELS (electron energy loss spectroscopy), LEED (low energy electron diffraction) techniques. These techniques allow to identify intermediate compounds during the reaction process. Thanks to that, the two authors concluded that the reaction process will be through the simple cleavage of NH3 by oxygen atoms, followed by the combination of N atoms with O to form NO, or with N atoms themselves to form N2 [3].
NH3 + O → NH2 + OH
NH2 + O → NH + OH
NH + O → N + OH
N + O → NO
N + N → N2
A recent study by Van den Broek and van Santen showed that NH and OH are the main particles adsorbed on the surface of Pt after the oxidation of NH3 in air. The reaction between NH and OH is the rate-determining step of the entire reaction. N2O will be formed when NO reacts with N adsorbed on the catalyst surface [4]. By theoretical calculations on the Pt6 cluster, Fahmi and van Santen also found that NH3 will only dissociate when oxygen is present on the catalyst surface. The reaction mechanism is also similar to what Mieher and Ho found by experimental analysis methods [5]. These studies have laid the foundation for further studies on the NH3 oxidation mechanism on other catalysts such as Cu, Ag and transition metal oxides.
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Source: diendankienthuc.net
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