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

The effect of NiO loading (5-21 wt.-%) on the reduction of NiO/alpha-A l2O3 catalysts, prepared by multiple impregnation of nickel nitrate so lution followed by calcination at 650-degrees-C, has been characterize d using temperature-programmed reduction, isothermal hydrogen consumpt ion, magnetization, X-ray diffraction, and electron microscopy. X-ray diffraction analysis of fresh catalysts indicated normal NiO crystalli tes about 30 nm in size. Studies from 270-degrees-C to 350-degrees-C s how hydrogen consumption at lower temperatures is faster than the subs equent 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 mor e difficult but nickel crystallite growth is not affected. This decrea sed reducibility is believed due to Al3+ ion incorporation into NiO su rface layers during impregnation. Isothermal hydrogen consumption from 270-degrees-C to 450-degrees-C follows a shrinking core model with Ni O crystallites decreasing progressively in size. Magnetic measurements show crystallite growth is dependent on diffusion-controlled nucleati on. Decreasing hydrogen flow has a profound effect on chemical reducti on and nucleation but not on growth. Similar results are found with ad ded water vapor. X-ray diffraction and transmission electron microscop e measurements reveal 23 nm nickel crystallites with some evidence for Al-Ni alloy formation. A mechanism is proposed in which adsorbed wate r inhibits chemical reduction and nucleation, and foreign ions such as Al3+ increase this effect.