Adsorption and thermal decomposition of Mo(CO)6 on TiO2(110)

Abstract

The adsorption, reaction and thermal desorption of Mo(CO)₆ on TiO₂(110) has been investigated by TPD, Fourier-transform reflection–absorption IR spectroscopy (FT-RAIRS) and XPS. The p-polarised FT-RAIRS spectrum of the first layer adsorbed at 150 K is characterised by a pair of transmission and absorption bands in the range 2000–2023 and 1964 cm^–1, respectively. A second pair of transmission and absorption bands are observed at 2035–2045 and 1954–1950 cm^–1 for the multilayer species produced at higher coverages. Each pair of transmission/absorption bands is assigned to the coupling of the normal/tangential component of the p-polarised radiation to the vibrationally coupled modes of an isotropic physisorbed layer. The coupling of dipoles parallel to the surface results in a normal mode red shifted from the singleton frequency, and the coupling of dipoles perpendicular to the surface results in a blue shift. The small difference between the mono- and multi-layer peak positions is probably a result of differences in this vibrational coupling rather than any significant interaction of Mo(CO)₆ with the surface. This is reflected in the TPD which indicates the desorption of the multilayer and monolayer at similar temperatures around 200 K. FT-RAIRS indicates that thermal desorption may be accompanied by some rearrangement of the physisorbed layer, and the formation of weakly bound subcarbonyl species, Mo(CO)₅, characterised by a single transmission band at 1950 cm^–1. XPS confirms the removal of CO from Mo(CO)₆ to produce 0.2 ML (monolayer) of an unstable and weakly bound subcarbonyl species which decarbonylates to produce small Mo particles above 220 K. Exposure of the TiO₂(110) surface at 400 K to Mo(CO)₆ results in dissociative adsorption to produce small Mo particles and adsorbed carbon. There is also no evidence that the reaction of Mo(CO)₆ with the prehydroxylated TiO₂(110) surface results in the formation of any stable subcarbonyl species.