The location of nickel oxide and nickel in silica-supported catalysts: Two forms of ?NiO? and the assignment of temperature-programmed reduction profiles

The preparation of nickel catalysts supported on a range of silicas results in the formation of two distinct types of “NiO” which reduce at very different temperatures under temperature-programmed reduction (TPR) conditions. Examination of the effects of pore structure and experiments designed to concentrate the oxides in either the smaller (~9 nm) or the larger pores (15–30 nm) show that the more reducible oxide is located mainly in the small pores and the less reducible oxide in the large pores. The more reducible oxide resembles bulk NiO and has negligible interaction with the silica. The less reducible oxide is either in the form of crystallites so small as to make nucleation of the reduction to metal difficult or as surface nickel silicates or hydroxysilicates. Reoxidation of the reduced catalyst followed by TPR shows that the nickel oxide and nickel crystallites are immobilized in the pores of the silica at temperatures up to 600 °C.