چكيده به لاتين
Methane catalytic combustion (MCC) is an efficient and advanced technology to eliminate or reduce the pollutant emissions such as unburned hydrocarbons, NOx, COx, soot, etc. However, various catalysts were used in this process. In this study, the MnOx was selected as the primary catalyst due to the superior redox features, high oxygen storage capacity, and diverse oxidation states in the crystalline network. The first part of this research was related to the study of the MnOx, NiO, CoOx, Fe2O3, and Cr2O3 prepared by the mechanochemical method, and the synthesized catalysts were employed in the MCC process. The results showed that the MnOx catalyst possessed the best performance among the calcined catalysts, of which 90% of CH4 conversion was obtained at 400 ℃. Due to the sensitivity of the oxidation reaction to the catalyst structure, the second part of this study is dedicated to the study of various MnOx morphologies, including wire, rod, tube, and flower, which prepared by the hydrothermal and solution methods. The results revealed that the wire-like MnOx catalyst had the best performance in the MCC process owning to the better physicochemical features such as higher specific surface area (53 m2.g-1) of this catalyst. The influence of the different synthesis factors, including the hydrothermal temperature, pH value, aging time, and calcination temperature, were investigated. The results indicated that the sample was synthesized under the Thydro. = 240 °C, pH =3, aging time = 24 h, and Tcal. = 400 °C displayed the highest efficiency at low temperatures (T50% = 305 ℃, T90% = 403. The effect of alkaline earth metal additions (MgO, CaO, BaO, and SrO) to the MnOx catalyst using the mechanochemical preparation method was investigated. The results presented that the catalyst containing 10 wt.% of BaO has a great potential to improve the combustion efficiency, and the light-off temperature over this catalyst has been reduced by 50 ℃ compared to the pure MnOx. Therefore, the effect of different BaO contents (x = 5, 10, 15, 20) was investigated on the textural properties and catalytic efficiency. However, the BaO(10)-MnOx catalyst showed a higher reaction rate, and the 10 and 90% of methane conversion temperatures were about 305 and 427 ℃, respectively, which is related to the higher ability to supply oxygen through the components during the MCC process. The BaO(10)-MnOx was synthesized with four methods: mechanochemical, precipitation, combustion synthesis, and sol-gel (pechini). The results indicated that due to the structural properties such as surface properties and degree of reduction, the catalyst prepared by the mechanochemical method had shown better performance than others. The influence of oxygen storage capacity was studied with using CeO2, ZrO2, La2O3, and Y2O3 to promote the BaO(10)-MnOx efficiency. The results showed that the addition of promoters into the BaO-MnOx catalyst resulted in the increase in oxygen mobility, which could be confirmed with O2-TPD analysis. The addition of CeO2 has a more positive effect on the catalytic activity due to the higher surface area (39 m2.g-1) and oxygen storage capacity. Furthermore, the effect of various CeO2 contents (x = 1, 3, 5, 7) was also considered. The results confirmed that increasing the CeO2 contents up to 3 wt.% improved the combustion efficiency, and the 90% of CH4 conversion was obtained at about 350 ℃ over this catalyst. The influence of the addition of 0.5 wt.% of Pd, Pt, Rh, and Ru on the structural properties and catalytic efficiency of the BaO(10)-MnOx in the MCC process was investigated. However, the results demonstrated that the Rh/BM catalyst could dramatically change the light-off temperature and CH4 conversion. The influence of the Rh loadings (x = 0.25, 0.5, 0.75, 1) was another factor that was examined in the following, and the results show better performance of catalysts containing 1% weight of rhodium oxide in the MCC process. The T50% = 279℃ and T90% = 350℃, and the light-off temperature of this catalyst was about 50℃ lower than that obtained for the BaO(10)-MnOx. The mentioned catalyst was also utilized in the CO oxidation process, and the results presented that the complete conversion of CO to CO2 had occurred at about 100 ℃. The study of the reactor type is the last topic that is discussed in this dissertation. In this regard, we used two different systems, including a fixed bed reactor and a plasma combined fixed bed reactor on the A-MnOx (A: NiO, CuO, CoOx, and Fe2O3) catalysts. The results showed that applying an electric field can significantly improve the CH4 conversion efficiency, especially at low temperatures. However, the CH4 was converted to CO2 and H2O at ambient temperature under an electric field, which is related to the production of active radicals and energetic electrons. The effect of various contents of CuO (x = 5, 10, 15, and 20) as the sample with the highest efficiency was evaluated, and the results indicated that the catalyst containing 10 wt.% of CuO showed the best performance (69% at T = 100 ℃) in hybrid system.
