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

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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.

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