CHARACTERIZATION OF SILICA-SUPPORTED NI-I-H-2 COMPLEXES BY ELECTRON-PARAMAGNETIC-RESONANCE AND THEIR TITRATION USING CO ADSORPTION MEASUREMENTS

The Ni-I species generated by UV-irradiation in the presence of hydrog en at 77 K of Ni/SiO2 samples where nickel was deposited on silica usi ng four different methods are studied by electron paramagnetic resonan ce. Four different signals are observed and tentatively assigned to si lica-supported Ni-I(H-2)(n) complexes resulting from the reduction of two different three-coordinated Ni-II precursors (Ni-3c(II)). The tota l amount of Ni-I obtained by photoreduction was measured using a new t itration method based on CO chemisorption. The principles of this meth od are described. The reduction degree is related to the amount of Ni- 3c(II) precursors. The highest reduction degree ([Ni-I/Ni-total] = 40% ) is obtained for samples where Ni2+ was introduced by cationic exchan ge of the silica support with [Ni(en)(3)](2+) (en = ethylenediamine). The results are consistent with concomitant electron paramagnetic reso nance measurements.