Characterization of Silica-Supported NiI−H2 Complexes by Electron Paramagnetic Resonance and Their Titration Using CO Adsorption Measurements

The Ni I species generated by UV-irradiation in the presence of hydrogen at 77 K of Ni/SiO 2 samples where nickel was deposited on silica using four different methods are studied by electron paramagnetic resonance. Four different signals are observed and tentatively assigned to silica-supported Ni I (H 2 ) n complexes resulting from the reduction of two different three-coordinated Ni II precursors (Ni II 3c ). The total amount of Ni I obtained by photoreduction was measured using a new titration method based on CO chemisorption. The principles of this method are described. The reduction degree is related to the amount of Ni II 3c precursors. The highest reduction degree ([Ni I /Ni total ] = 40%) is obtained for samples where Ni 2+ was introduced by cationic exchange of the silica support with [Ni(en) 3 ] 2+ (en = ethylenediamine). The results are consistent with concomitant electron paramagnetic resonance measurements.