Infrared investigation of CO adsorption on thermally reduced silica-supported molybdenum catalysts

Abstract

Molybdenum – silica catalysts thermally reduced in hydrogen have been studied by i.r. spectroscopy using CO as the probe molecule. ¹²CO and ¹³CO adsorption at different temperatures (beam temperature and 77 K) were studied as functions of (a) preparation (grafting and impregnation), (b) molybdenum loading, (c) time of reduction treatment, (d) CO pressure and (e) time of CO contact. Infrared spectra show that similar surface species, in differing proportions, are present in all samples. Monocarbonyl species are formed on Moⁿ⁺ cations probably with 3 ⩽n⩽ 5. Two types of metal carbonyl species are also formed: a —OC—Mo(CO)₅ species chemisorbed on Moⁿ⁺ cations and a ‘physisorbed’ Mo(CO)₆ species weakly held on the silica support. Reduction to metal is favoured when the Mo dispersion decreases and when the time of reduction increases.