Preparation of mono- or zerovalent nickel by single or successive one-electron-transfer steps in the photoreduction of silica-supported nickel catalysts

Abstract

Ni/SiO/sub 2/ supported catalysts prepared by competitive cation exchange can be reduced by UV irradiation in hydrogen at 77 K to lead by a one-electron process to Ni/sup +/ species. These Ni/sup +/ ions which exhibit an EPR signal at g/sub 1/ = 2.68, g/sub 2/ = 2.32, and g/sub 3/ = 2.007 and a band at 838 nm in the UV-vis and near-IR reflectance spectra bind molecular hydrogen to form (similarly ordered Ni(H/sub 2/))/sup +/ pseudotetrahedral surface complexes. The photoreduction process in carbon monoxide occurs appreciably only at 25/sup 0/C and depends on the CO pressure. It is a one-electron-reduction process, and (Ni(CO)/sub n/)/sup +/ species (n = 2-4) are observed by EPR and IR for pressures above 10 Torr. For lower pressures, the formation of metal is observed by IR and ferromagnetic resonance via the reduction by two consecutive one-electron-transfer steps. The first step proceeds from the photoproduction of (Ni/sup +/-O/sup -/)* excited states. The second step is due to the release of electrons from a reservoir which has been tentatively identified to pseudo-carbonates characterized by IR bands at 1750-1850 cm/sup -1/ and by TPD peaks of CO at 803 K and of CO/sub 2/ at 513 andmore » 803 K. The formation of the latter species is the result of the quenching of the excited state by CO molecules which react with the O/sup -/ activated surface oxygens to lead to CO/sub 2//sup -/ which with O/sup 2 -/ ions generates CO/sub 3//sup 3 -/ radical ions.« less