Investigation of La promotion mechanism on Ni/SBA-15 catalysts in CH_4 reforming with CO_2

摘要

La-promoted Ni/SBA-15 catalysts (molar ratio of La_2O_3/Ni - 0.10, 0.25) were prepared by the wetness impregnation method for the reforming of CH_4 with CO_2. The La species are highly dispersed on the catalyst and cause smaller nickel particles in comparison with the Ni/SBA-15 catalyst. The presence of La in Ni/SBA-15 evidently improves the catalytic activity and stability. For the reaction mechanism study, a special infrared cell under rapid heat dissipating was designed to trap the transient intermediates (e.g., CH_xO~*) on the catalysts. For La-Ni/SBA-15 with La_2O_3/Ni - 0.10, the adsorbed Ni(CO)_4 and bridge-bonded CO (Ni_2-CO) form on the nickel surface with large particle size, while for the one with La_2O_3/Ni = 0.25, CH_xO~* (x =2, 3) surface species occur on small sized nickel particle. The results indicate that highly dispersed La effectively promotes CO_2 dissociation into oxygen atom which facilitates the formation of the different C1 intermediates from the adsorbed CH_4 on the neighboring nickel species depending on the varied La_2O_3/Ni ratio.