ADSORPTION AND THERMAL-DECOMPOSITION OF MO(CO)(6) ON TIO2(110)

The adsorption, reaction and thermal desorption of Mo(CO)(6) on TiO2(1 10) has been investigated by TPD, Fourier-transform reflection-absorpt ion IR spectroscopy (FT-RAIRS) and XPS. The p-polarised FT-RAIRS spect rum 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 spec ies 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 s urface results in a normal mode red shifted from the singleton frequen cy, and the coupling of dipoles perpendicular to the surface results i n a blue shift. The small difference between the mono- and multi-layer peak positions is probably a result of differences in this vibrationa l coupling rather than any significant interaction of Mo(CO)(6) with t he surface. This is reflected in the TPD which indicates the desorptio n of the multilayer and monolayer at similar temperatures around 200 K . FT-RAIRS indicates that thermal desorption may be accompanied by som e rearrangement of the physisorbed layer, and the formation of weakly bound subcarbonyl species, Mo(CO)(5), characterised by a single transm ission 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 subc arbonyl species which decarbonylates to produce small Mo particles abo ve 220 K. Exposure of the TiO2(110) surface at 400 K to Mo(CO)(6) resu lts in dissociative adsorption to produce small Mo particles and adsor bed carbon. There is also no evidence that the reaction of Mo(CO)(6) w ith the prehydroxylated TiO2(110) surface results in the formation of any stable subcarbonyl species.