Investigation On Methane Catalytic Decomposition Reaction On Ni-based And Fe-based Catalysts

Methane catalytic decomposition can produce COx-free hydrogen and carbon nanomaterials simultaneously, which is prior to the conventional hydrogen production processes. In this work, both Ni-based and Fe-based catalysts are investigated for production of COx-free hydrogen and various types of nanocarbon through methane catalytic decomposition. The catalysts with high activity and stability are obtained and the approach to improve the stability of catalyst at high temperature is discussed. Carbon nanomaterials with different morphologies are successfully synthesized and the methanism is discussed.NiAl and NiCuAl catalyst derived from Feitknecht compound was used for the methane catalytic decomposition in a fluidized bed. The effect of reaction temperature, reduced temperature, the metal loading, and the gas velocity is investigated and these reaction conditions are optimized. The activation energies of methane catalytic decomposition on Ni/Al2O3 and NiCu/Al2O3 catalysts are measured in the chemical reation kinetic limted temperature regin, and the values are both 73±2 kJ/mol for the two catalysts. Thus it is suggested that the doping of Cu didn't change the state of activity sites because the each activity site is composed of several Ni atoms. Cu in the catalyst particles tends to be rich in the surface and will reduce the overall activity sites, but it has effect on reducing the opportunities of deactivation caused by encapsulation.The measurement of ppm level CO concentration is achieved by the gas chromatogram equipped with methanation reactor. The results show that the CO concentration can be 1800-2500 ppm in the trail gas when no gas purity devices are used. This value is decreased to 700-1040 ppm when a drier is used, while it drop to 250 ppm when both drier and deoxidization systems are used. The experiment results also show that the CO concentration can be smaller than 10 ppm when using pure methane as feed. For all the reaction process, the CO concentration increase with the increasement of temperature, and decrease with the time on stream.Two different reaction schemes were tested for methane catalytic reaction on NiCu/Al2O3 catalysts. The constant temperature reaction (CTR) is that reactions are conducted in a constant temperature, while the pre-induced reaction (PIR) is that reaction is first induced in low temperature and then reacted in a higher temperature. The results show that the PIR can improve the stability of catalyst dramatically. The HRTEM and EDS characterization results show that the composition of metal particles obtained in PIR is similar to particles obtained in low temperature CTR, in which the Ni/Cu is about 3:1, while for the particles obtained in high temperature CTR, this value is lower than 1:1. It is also observed that the structure of carbon nanofibers formed in PIR have the both characters of the carbon nanofibers formed in the low temperature CTR and high temperature CTR. It is suggested that in the induced period of reaction, the metal particles undergo a reconstruction process, and the particles rich in Cu or Ni are formed. When the reaction is conducted in low temperature CTR, those particles rich in Cu are with very low activity and the carbon nanofibers are difficult to grow on this types of particles. When the reaction is conducted in high temperature CTR, the particles rich in Ni are deactivated quickly and also not suitable to catalyze the growth of carbon nanofibers. When the PIR scheme is conducted, the induction process occurs in low temperature, and the state of metal particles can be reserved where the metal particles rich in Ni will be also active in high temperature.The doping effect of Mo, Cr and W on the Fe-based catalysts is inverstigated. The activity tests and the characterization results show that the doping of inactive Cr and W reduce the activity sites and decrease the methane conversion. The activity of catalysts is promoted by doping Mo, and the methane conversion increase from 14% to 40%. The XRD and TPR characterization showed that the doping of Mo can change the Fe redox cycle and enhance the dispersion of Fe. When the Mo/(Mo+Fe) value equals to 10%, the catalyst has the best performance. For the reaction conducted in fixed bed reactor, the methane conversion can stabilize at 44% for 180 min at 973 K, while the conversion can be 78% for 40 min at 1023 K.Catalysts prepared by citric acid sol-gel method and impregnation method are employed to produce nanocarbon with different morphologies by methane catalytic decomposition. Various catalysts are tested and the structure of nanocarbons is characterized by HRTEM and Raman. For the catalysts prepared by citric acid sol-gel method, the catalyst with a composition of Fe:Mo:Al=9:1:120 (mol ratio) can produce single wall carbon nanotubes with high purity. It is also found that Fe-Mo catalysts have better performance than Fe catalyst, which is due to the doping effect of Mo. For the catalysts prepared by impregnation, different types of nanocarbon, including multi-walled carbon nanobubes, nanosized carbon onion-like fullerene and nanosized graphite particle was obtained on different catalysts. It is also found that the Mo/MgO catalyst have good activity and stability in high temperature and is effective for producing highly graphitized multiple-walled carbon nanotubes with narrow diameter distribution. The XRD results showed that the Mo2C can be the activity species during the reaction. Based on the results of HRTEM, the mechanism of single wall carbon nanotubes growth and other form of nanocarbon is discussed.