The effect of Ni content on a highly active Ni-Al_2O_3 catalyst prepared by the homogeneous precipitation method

摘要

Highly active Ni-Al_2O_3 catalysts were prepared by the homogeneous precipitation method with a variety of high nickel contents ranging from 30 to 70 wt.%. The effects of nickel content on the physicochemical properties and catalytic activities of the Ni-Al_2O_3 catalysts were investigated. XRD measurements showed that the catalyst with 30 Ni wt.% only had a diffraction peak corresponding to NiAl_2O_3) whereas the catalysts of 50, 60 and 70 Ni wt.% had diffraction peaks corresponding to NiO and NiAl_2O_3. Hydrogen chemisorption results showed that the nickel surface area increased with increasing nickel content in the order: 30 < 40 < 50 < 60 < 70 Ni wt.%. Specifically, the nickel surface area increased steadily from 11 to 22 m~2/g with increasing the nickel content from 30 to 50 wt.%, after which it stayed nearly constant at 22 m2/g despite the increase in nickel content from 50 to 70 wt.%. TEM images of the reduced Al_2O_3 catalysts revealed that the average sizes of the Ni particles were 12,13 and 16 nm for the catalysts with 30, 50 and 70 Ni wt.%, respectively, suggesting that a higher nickel content yielded a larger Ni particle. The catalytic performance of methane steam reforming showed that the catalytic reaction rate increased steadily with increasing nickel content from 30 to 50 wt.%, after which it stayed nearly constant despite that the nickel content increased to over 50 wt.%. As a result, about 50 wt.% of nickel was found to be a reasonable nickel content to obtain the maximum catalytic activity.