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) o n 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-prec ipitation whose supported phase is a 1:1 nickel phyllosilicate. Three reduc tion parameters were investigated: the gas flow rate, the heating rate, and the final reduction temperature. The size of the metal particles and the e xtent of nickel reduction were determined by transmission electron microsco py, hydrogen chemisorption, chemical titration by sulfuric acid, and magnet ic measurements. The results show that TPR up to 900 degreesC leads to full y reduced nickel particles whose average size varies between 35 and 65 Angs trom, depending on the reduction parameters. It has been possible to get sm aller particles of about 20 Angstrom by isothermal reduction at 450 degrees C, but nickel was not fully reduced. Experiments on the sintering of the ni ckel oxide particles before reduction and of the nickel metal particles sho wed that it was not possible to obtain metal particles larger than 70 Angst rom, probably owing to a strong metal oxide- and metal-support interaction, respectively. A mechanism of formation of metal particles for samples prep ared by deposition-precipitation is proposed.