Characterization of Ni/SiO2 catalysts prepared by successive deposition and reduction of Ni2 ions

Different Ni/SiO2 specimens have been synthesized by impregnation (sample Ni-i-Si) and grafting Ni2+ ions from nickel amminocomplex solution on (i) SiO2 (sample Ni-1-Si) and (ii) reduced Ni-1-Si (sample Ni-2-Si). They have been characterized by TPR, IR spectroscopy, DR UV-VIS spectroscopy, XPS, XRD, TEM, and FMR. Due to the formation of surface silicates, the Ni2+ ions from Ni-1-Si exhibits low acidity and forms carbonyls only at low temperatures. They are characterized by a higher reduction temperature (TPR peak at 956 K) than the Ni2+ ions from the Ni-i-Si sample (two TPR peaks at 652 and 710 K). Highly dispersed metal particles are formed after reduction of the Ni-l-Si sample (ca. 2 nm in diameter), the CO adsorption reveals the lack of dense crystal planes. The reduced Ni-1-Si sample is characterized by a lower dispersion of nickel (main diameter of the metal particles of 16 mm). Exposure of the reduced Ni-1-Si sample to oxygen causes oxidation of all of the surface situated metal nickel to Ni2+. These Ni2+ ions are reduced much more easily (TPR peak at 489 K) than those initially deposited. Part of the silica surface is regenerated during the reduction of nickel and is not blocked again after oxidation, which allows subsequent deposition of Ni2+ ions on the reduced catalysts. As a result, the nickel concentration increases. Reduced Ni-2-Si sample is also characterized by small nickel particles (ca. 2-3 nm in diameter). The results evidence that subsequent deposition-reduction of Ni2+ ions on silica can be used to prepare highly dispersed nickel catalysts With different nickel concentration. (C) 1999 Academic Press.