Preparation Of Efficient Catalysts For Methane Decomposition

Various Ni-based catalysts were prepared by impregnation of Ni(NO₃)₂·6H₂O solution, using Al₂O₃, MgO, TiO₂, CeO₂, Diamond, SiC as carriers of Ni. The XRD patterns of samples show that Ni²⁺ were reduced to Ni⁰ when the catalysts were calcined and reduced at certain conditions. Surface area of Al₂O₃ is 103.39m²/g and others are all less than 10m²/g, which were measured by BET method. The changes of surface area increasing were discovered except 40Ni/Al₂O₃ catalyst after carriers loading metal Ni. It can be seen from SEM images that active component relatively distributed more averagely on Al₂O₃ with higher surface area.Catalysts were studied in a fixed-bed quarts reactor. In order to research different effects of reaction condition on catalytic activity, the reactions of methane decomposition were carried out using various Ni-loading catalysts in conditions of different temperature and space velocity. The results showed that the order of the catalytic activity of different support materials at 600℃and space velocity of 12000ml·g^-1·h^-1 in 120min was as follows: 33Ni/O-Diamond<40Ni/SiC<40Ni/CeO₂<40Ni/TiO₂<41.2Ni/MgO<40Ni/Al₂O₃. The catalysts made with carries of high surface area attained high catalytic activity. The initial conversion of methane gradually increases along with the increasing of reaction temperature. Too high temperature resulted in catalysts deactivating rapidly. The catalyst of 40Ni/Al₂O₃ remained high catalytic activity at 650℃and methane conversion reached 57% in 120min. The methane conversion increased with the decreasing of space velocity. The low space velocity wasn't propitious to increasing hydrogen yield, although it can increase methane conversion. The catalysts deactivated rapidly if space velocity is too high. The methane conversion reached 52% and remained steadily in 120min over 40Ni/Al₂O₃ at space velocity of 48000ml·g^-1·h^-1.The catalytic life was studied over 40Ni/SiC, 40Ni/TiO₂, 40Ni/Al₂O₃. The results showed that the catalysts exhibited constant catalytic activity for above 7h at temperature of 600℃and space velocity of 12000ml·g^-1·h^-1. The catalytic activity of 40Ni/Al₂O₃ was higher than 40Ni/SiC and 40Ni/TiO₂ markedly and keeped steadily above 45% for 8h. The highest methane conversion reached 54% over 40Ni/Al₂O₃ at space velocity of 48000ml·g^-1·h^-1 and temperature of 650℃and remained steadily in 7h.It's well known that reaction conditions effect carbon form through methane decomposition. SEM investigation revealed that filamentous and encapsulateng shaped carbon were formed under different temperature over Ni/O-Diamond catalysts by contrast with 40Ni/Al₂O₃ catalysts which just formed filamentous carbon. It can be seen that the reaction temperature and space velocity effect the diameter of filamentous carbon. The form of deposited carbon depended on the balance between carbon producing rate and carbon pervasion and transfer rate in nickel.Reaction kinetics of catalytic decomposition of methane over 40Ni/Al₂O₃ has been studied. The kinetic model based on hypothetic mechanism was established according to adsorption and dehydrogenation of methane. It's proved that the first step of dehydrogenation is the rate-determining step over 40Ni/Al₂O₃ catalysts.