REDUCTION OF MODEL STEAM REFORMING CATALYSTS - NIO/ALPHA-AL2O3

The effect of NiO loading (5-21 wt.-%) on the reduction of NiO/alpha-Al2O3 catalysts, prepared by multiple impregnation of nickel nitrate solution followed by calcination at 650-degrees-C, has been characterized using temperature-programmed reduction, isothermal hydrogen consumption, magnetization, X-ray diffraction, and electron microscopy. X-ray diffraction analysis of fresh catalysts indicated normal NiO crystallites about 30 nm in size. Studies from 270-degrees-C to 350-degrees-C show hydrogen consumption at lower temperatures is faster than the subsequent growth of nucleated clusters of nickel atoms into crystallites, with the rates of the two processes approaching each other at higher temperatures. As NiO loading increases, chemical reduction becomes more difficult but nickel crystallite growth is not affected. This decreased reducibility is believed due to Al3+ ion incorporation into NiO surface layers during impregnation. Isothermal hydrogen consumption from 270-degrees-C to 450-degrees-C follows a shrinking core model with NiO crystallites decreasing progressively in size. Magnetic measurements show crystallite growth is dependent on diffusion-controlled nucleation. Decreasing hydrogen flow has a profound effect on chemical reduction and nucleation but not on growth. Similar results are found with added water vapor. X-ray diffraction and transmission electron microscope measurements reveal 23 nm nickel crystallites with some evidence for Al-Ni alloy formation. A mechanism is proposed in which adsorbed water inhibits chemical reduction and nucleation, and foreign ions such as Al3+ increase this effect.