Spectroscopic characterization of a molybdena/silica system photoreduced in a carbon monoxide atmosphere

Abstract

A supported MoO₃/SiO₂ system prepared via molybdate impregnation or via Mo(CO)₆ heterogenization was reduced at 823 K with H₂ and the CO adsorption on the reduced molybdena phase was investigated by i.r., u.v.–vis. and e.s.r. techniques.

The formation of Mo^V ions has been identified by e.s.r. spectroscopy and weak i.r. bands at 2203 and 2181 cm^–1 have been assigned to CO adsorbed on Mo^V and Mo^IV, respectively.

A photoreduction process, carried out in a CO atmosphere at room temperature, lead to further reduction of molybdena; in particular, two bands at 2128 and 2080 cm^–1 were very strong in Mo(CO)₆/SiO₂ samples and have been assigned to a pair of CO molecules linearly adsorbed on distorted tetrahedral Mo^II ions. This species is in equilibrium with a CO species bridged between two vicinal Mo^II ions (v_CO= 2043 cm^–1) and the equilibrium process can be also monitored using u.v.–visible reflectance spectroscopy. The possibility of obtaining a molybdena phase in which Mo ions have a valence lower than four is thus confirmed.