Effect of NiO content in mesoporous NiO-Al₂O₃ catalysts for high pressure partial oxidation of methane to syngas

The effect of NiO content in mesoporous NiO-Al₂O₃ catalysts on stability of the catalytic activity was investigated for partial oxidation of methane to syngas under the reaction conditions at 650 °C and 1 MPa over the contact time range from 0.1 to 5 ms. Mesoporous/porous NiO-Al ₂O₃ catalysts with a high BET surface area were prepared with NiO content of 3.7-75 wt%. From XRD patterns and TPR profiles observed in these catalysts before H₂ pretreatment, the presence of strong interactions between NiO and Al₂O₃ was indicated, which gave relatively small NiO particles in the catalysts. As results of activity tests conducted by changing the contact time from 5 ms to lower until the catalyst lost its oxidation activity, the order of the catalyst tolerance against oxidation of nickel active species on them was: NiO(75)-Al ₂O₃ > NiO(40)-Al₂O₃ > NiO(90)-Al₂O₃ > NiO(11)-Al₂O₃. As results of long runs and TG-DTA analysis, NiO(40)-Al₂O₃ showed the stable activity without any carbon deposition after 24 h reaction. This is because the Ni particle size and dispersion are 5.9-7.1 nm and 10.8%, respectively, which are considered to be suitable to prevent both carbon deposition and oxidation of nickel species during reaction, while NiO(75)-Al₂O₃ with Ni particle size of 11.7-15.9 nm gave high carbon deposition after a long run. The Ni particle size of 10 nm was a borderline for carbon deposition under reaction conditions at 1 MPa.