چكيده به لاتين
Thermocatalytic decomposition of methane (TDM) is an efficient technology to produce hydrogen with high purity and without COx components. The noble and transition metals were used in this process. The most common catalyst which used in this process is the nickel-based components. This metal has suitable activity but relatively short life-time. This study is focused on the developing catalytic systems over the nickel-based catalysts that have suitable activity and stability in the TDM process. The first part of this research was related to the synthesize of the various spinel powders containing MgAl2O4, CaAl2O4, BaAl2O4, SrAl2O4, FeAl2O4, and ZnAl¬2O4 prepared by the mechanochemical method, and the synthesized catalysts were employed as catalyst support for the nickel-based with 40 wt.% of NiO in the TDM process. The results demonstrated that the FeAl2O4-supported nickel catalyst presented the highest stability among the studied catalysts. The decline in the initial CH4 conversion was about 21% at 575 ℃ during the 300 min time on stream. However, other samples were completely deactivated during the stability test, which was related to the deposition and growth of the carbon nanofiber that covers the available catalyst active sites. The second part of this study is dedicated to the study of various NiO contents (20, 30, 40, 50, and 60 wt.%) on the textural characteristics and catalytic efficiency of the FeAl2O4 supported nickel catalysts in the TDM process. The obtained results demonstrated that the SBET of the synthesized catalysts reduced from 62 to 26 m2.g-1 by increasing the nickel loading from 20 to 60 wt.%, which is ascribed to the blockage of FeAl2O4 support pores. Furthermore, the activity results showed that the catalytic activity improved by increasing the nickel content from 20 to 50 wt.% due to the rise of active site concentration. However, the methane conversion was reached to 40% at 600 ℃ over the NiO(50)/FeAl2O4 catalyst. The more increment of nickel content decreased the catalytic efficiency due to the decline in active phase dispersion. At third part of this research, the influence of various metal oxides (CuO, Cr2O3, Co3O4, ZnO, and MnO2) promoted NiO/FeAl2O4 catalysts were studied on the textural characteristics and catalytic efficiency. The controlled preparation method caused the sample doped with MnO2 to show the highest specific surface area (47 m2.g-1) and the lowest particle size (10.6 nm) among the promoted samples. The activity test revealed that the MnO2(10)-NiO(50)/FeAl2O4 catalyst was the most active and stable catalyst in the CH4 decomposition reaction. At 700 ℃, the CH4 conversion and H2 yield were 62.3 and 66 % over this catalyst. Also, the decline in its initial activity was only 3 % during the 300 min reaction time at 575 ℃. Also, the results indicated that the catalytic efficiency and rate of carbon production increased by raising the MnO2 content from 5 to 10 wt.%. However, the high amount of the deposited carbon and decline in metal dispersion led to the catalytic activity being reduced when the MnO2 loading was increased from 10 to 15 wt.%. The last part of this research is centralized on designing and optimizing the preparation conditions of the NiO(50)/FeAl2O4 catalyst in the TDM. The mesoporous FeAl2O4 powders were synthesized by various methods, and the supported nickel catalysts were employed in the TDM. The results showed that the catalyst supported on FeAl2O4 synthesized by modified co-precipitation (MCOP) method possessed the best efficiency, and CH4 conversion and H2 yield were 74 and 82 % at 675 ℃, respectively. Taguchi L9 statistical experimental design was applied to investigate and optimize the preparation parameters, including surfactant type (CTAB, PVP, P123), pH (9, 10, 11), aging temperature (40, 60, and 80 ℃), and aging time (0, 15, 30 h). The obtained outcomes revealed that the optimized sample displayed a mesoporous nanocrystalline structure with an appropriate BET area (73 m2.g-1) and pore volume (0.17 cm3.g-1), and its maximum methane conversion was 80.8 % at 700 ℃. The results indicated that the Taguchi procedure was impressive in the optimization of the synthesis parameters, and the error value between the predicted value and the actual value was lower than 4%. The study of the presence of the plasma reactor is the last topic that is discussed in this dissertation. The results showed that the catalytic efficiency was significantly improved by introducing an electric field to the reactor system.
