Ni/SiO2 Materials Prepared by Deposition−Precipitation:  Influence of the Reduction Conditions and Mechanism of Formation of Metal Particles

The influence of the parameters of temperature-programmed reduction (TPR) on the size of the metal particles and on the extent of nickel reduction was investigated for a Ni/SiO2 sample (24 wt % Ni) prepared by deposition−precipitation whose supported phase is a 1:1 nickel phyllosilicate. Three reduction parameters were investigated: the gas flow rate, the heating rate, and the final reduction temperature. The size of the metal particles and the extent of nickel reduction were determined by transmission electron microscopy, hydrogen chemisorption, chemical titration by sulfuric acid, and magnetic measurements. The results show that TPR up to 900 °C leads to fully reduced nickel particles whose average size varies between 35 and 65 Å, depending on the reduction parameters. It has been possible to get smaller particles of about 20 Å by isothermal reduction at 450 °C, but nickel was not fully reduced. Experiments on the sintering of the nickel oxide particles before reduction and of the nickel metal particles showed that it was not possible to obtain metal particles larger than 70 Å, probably owing to a strong metal oxide− and metal−support interaction, respectively. A mechanism of formation of metal particles for samples prepared by deposition−precipitation is proposed.