Study On Mechanism Of Methane Catalytic Cracking Reaction Of Copper-Doped Catalysts

In a fusion reactor,deuterated and tritiated methane has been generated as a result of the interaction between the plasma and the first wall during the D-T fusion reaction。Under the consideration of ALARA and economic point, the deuterium and tritium existed in the gas was need to be recovered and purified. Therefore, it’s very important to directly recover deuterium and tritium from the deuterated and tritiated methane by means of methane decomposition.Ni-SiO2 and Ni-Cu-SiO2 catalyst were prepared by Sol-gel method and used SiO2 to be supports, and investigated the effect of Cu contents on the structure of Ni-based catalysts, reaction temperature, catalytic activity and by-generated carbon structures. The XRD and SEM characterization results showed that the doping of the Cu contents can enhance the dispersion of the Ni metal particles, the TPR characterization results proved that the introduction of Cu decreased the reduction temperature of NiO particles. In the studying of the catalytic performance over Ni-based catalyst, it found that the stability of the catalyst could be improved by doping of Cu contents in the high reaction temperature, but increased of the Cu contents could decrease the catalytic ability. Moreover, from the results of TEM, Ni-Cu metal particles were located at the tip of generated carbon filament, and the introduction of Cu contents could prevent the spherical carbon species formed. In addition, octopus-like carbon structures were generated by Ni-Cu-SiO2 catalyst.Using the characteristic of Ni-Cu particles followed tip-growth mode to form carbon filaments, the continuous temperature reaction mode was designed. Compared to the traditional mode (constant temperature reaction mode), the Continuous temperature reaction mode could enhance the dispersion of the Ni-Cu particles, so it could remarkable improve the catalytic performance of Ni-Cu-SiO2 catalyst at 750℃。During the constant temperature reaction mode at 750℃ the methane conversion increased from less than 14%(constant temperature reaction mode) to more than 35%, and the lifetime increased from less than 20min to 300min. Moreover, the relationship between reaction temperature and the structures of catalysts, catalytic performance, generated carbon structures was established, and analyzed the effect of structure changed.The step-wise rising temperature reaction mode, the stage methane flow control reaction mode and the stage temperature control reaction mode were designed to test the catalytic performance of 65%Ni-10%Cu-25%SiO2catalysts and generate by-products carbon structures, and combined with TEM images, TEM-EDX data, XRD data,Raman spectra and TGA-DTA curves, andit was found that the Ni-Cu particles were in the quasi-liquid state around 680℃. The quasi-liquid state caused the structure of Ni-Cu particle became instable, and the particles started to suffer the fragmentation process and phase separation. Moreover, higher temperature could make contribution to the high degree of graphitization, and the temperature also caused some of these carbon structures migrated to the surface of the Ni-Cu particles. Although fragmentation process, phase separation, carbon structures migrated and graphitization together make a significant contribution to catalyst deactivation at higher temperatures, the high degree of graphitization might play the most important role in the deactivation of Ni-Cu catalysts at higher temperature. From the above mentioned results, the detailed deactivation mechanism was explained.For the deactivated 65%Ni-10%Cu-25%SiO2catalysts, it could be covered activity by increasing reaction temperature. When the reactiom temperature was heated up to 900℃, the methane conversion increased to 15%. In the studying of the structures and catalytic performance of three carbon structure species (CFs, SWCNTs and CBs), the results showed the disordered carbon structure played the role of catalyst in methane decomposition at higher temperature and caused the methane conversion increased. The BET surface area characterization results shows the surface area, the pore size distribution and micropores remarkable decreased, which proved the methane decomposition reaction occurred in the micropores, and the generated carbon blocked the pore mouth might be the key factor to cause catalyst lose activity.Finally, the activity and lifetime of deactivated Ni-SiO2 and Ni-Cu-SiO2 catalysts after regeneration with air was also investigated. With the regeneration times increased, the activity of Ni-SiO2 catalysts decreased and while the lifetime didn’t change obviously. The lifetime and catalytic activity of Ni-Cu-SiO2 both decreased after regeneration with air which was different from that of Ni-SiO2 and the lifetime during methane decomposition after regeneration with air increased with the increase of the Cu content. The XRD and BET characterization results found the weaker interactionbetween Ni-Cu particles and SiO2 supports caused the metal particles sintered into big clusters during the regeneration processing, the catalysts with big particle size would form spherical carbon structure and reduce the activity.