Study On Nano Nickel-Based Catalysts For Methane Decomposition Reaction

In this paper, high active Ni-based catalysts for methane decomposition were designed and prepared according to the growth mechanism of carbon filaments. In order to adjust the crystalline size and metal-support interaction, nano-scale Ni/Al2O3 and Ni/SiO2 catalysts were prepared by co-precipitation method starting from Feitknecht structured precursor and reverse impregnation method starting from nickel oxide accordingly. Small amount of Cu was introduced as additive to further adjust the chemical state of Ni particles. The methane conversion activity was tested for both catalysts in an in-situ thermal balance and a fixed-bed tubular reactor. The effect of reaction conditions to the catalyst activity and the formation mechanism of carbon filaments were also discussed.The calcination processes of Ni/Al2O3 precursor by co-precipitation and Ni(OH)2 by reverse impregnation were studied by TG/DTA. XRD and TPR were also used to investigate the structure and reduction property of Ni/Al2O3 and Ni/SiO2 catalysts in oxide state. SEM, TEM and SAD images were taken to show the particle morphology and crystallization degree of Ni/SiO2 catalysts prepared by reverse impregnation. The results showed that, a Feitknecht structured precursor was obtained for Ni/Al2O3 catalyst by co-precipitation. After calcination, a mixed metal oxide with NiO as the framework was obtained. A strong interaction between NiO and Al2O3 leads to a bad crystallization of NiO lattice, which makes the reduction temperature of nickel goes much higher than in other composites. In Ni/SiO2 catalysts prepared by reverse impregnation, the NiO particle was high crystallized. SiO2 as a texture promoter parts the NiO particles by a weak metal-additive interaction, as can be seen from TPR and XRD profiles. Cu addition changed the chemical state of NiO by lowering its reduction temperature and increasing degree of crystallization.The catalytically active temperature range and the highest carbon deposition rate for each catalyst were obtained by an in-situ thermal balance by temperature programmed reaction. The activity and stability under constant temperature of each catalyst were also tested in a fixed-bed tubular reactor. The results indicated that high calcination temperature of Ni(OH)2 will lead to the sintering of NiO, thus lowering the activity of catalyst. Copper addition made the catalyst more carbon capacious and more stable at high temperature. The 75Ni-8Cu/SiO2 and 75Ni-15Cu/SiO2 were both more active than co-precipitated Ni-Cu/Al2O3 catalysts, with the former has the highest rate of carbon deposition. At 700℃, the methane conversion reaches about 50% over 75Ni-8Cu/SiO2 catalyst but with a rapid deactivation. The initial methane conversion over 75Ni-15Cu/SiO2 catalyst is about 35%, but the catalyst life is rather long and the overall carbon deposition amount reaches 384 gC/gNi.SEM and TEM were used to characterize the morphology of carbon filament produced on Ni/SiO2 serial catalyst. It was shown that Ni particles transform from pear shape to quasi hexahedron shape after Cu addition. The growth mode of carbon filaments also shifts from tip-growth model to octopus growth mode, with the arrangement of the graphite sheet from fishbone to parallel.