Probing Surface Sites of TiO₂: Reactions with [HRe(CO)₅] and [CH₃Re(CO)₅]

Abstract

Two carbonyl complexes of rhenium, [HRe(CO)(5)] and [CH(3)Re(CO)(5)], were used to probe surface sites of TiO(2) (anatase). These complexes were adsorbed from the gas phase onto anatase powder that had been treated in flowing O(2) or under vacuum to vary the density of surface OH sites. Infrared (IR) spectra demonstrate the variation in the number of sites, including Ti(+3)-OH and Ti(+4)-OH. IR and extended X-ray absorption fine structure (EXAFS) spectra show that chemisorption of the rhenium complexes led to their decarbonylation, with formation of surface-bound rhenium tricarbonyls, when [HRe(CO)(5)] was adsorbed, or rhenium tetracarbonyls, when [CH(3)Re(CO)(5)] was adsorbed. These reactions were accompanied by the formation of water and surface carbonates and removal of terminal hydroxyl groups associated with Ti(+3) and Ti(+4) ions on the anatase. Data characterizing the samples after adsorption of [HRe(CO)(5)] or [CH(3)Re(CO)(5)] determined a ranking of the reactivity of the surface OH sites, with the Ti(+3)-OH groups being the more reactive towards the rhenium complexes but the less likely to be dehydroxylated. The two rhenium pentacarbonyl probes provided complementary information, suggesting that the carbonate species originate from carbonyl ligands initially bonded to the rhenium and from hydroxyl groups of the titania surface, with the reaction leading to the formation of water and bridging hydroxyl groups on the titania. The results illustrate the value of using a family of organometallic complexes as probes of oxide surface sites.

Effect of carbon supports on RhRe bifunctional catalysts for selective hydrogenolysis of tetrahydropyran-2-methanol

“…As standard foils were unable to fit in the beamline, this value had to be assumed based on previous studies. 28 This value should be close to the actual S 0 2 under our conditions, but a small error in absolute (not relative) coordination numbers may have been introduced. Standard procedures using Artemis software were employed to fit the XAS data to obtain coordination numbers.…”

Cytotoxicity, cellular localization and photophysical properties of Re(I) tricarbonyl complexes bound to cysteine and its derivatives

“…The Debye-Waller parameter (σ 2 ) values for the Re-C and Re-(N/O) scattering paths in the EXAFS fitting model of 1 seem reasonable (0.0021 ± 0.001 Å 2 and 0.0055 ± 0.002 Å 2 , respectively; Table 1), while that of the longer Re-C-O (n leg = 3) path is relatively small (0.0022 ± 0.002 Å 2 ). In the EXAFS model fitting of Re(CO) 3 bound to a TiO 2 surface, a similar σ 2 value was reported for the corresponding multiple scattering path [53]. By replacing H 2 O with a cysteinate ligand in 2, and introducing a Re-S path in its EXAFS model fitting, an unreasonably small σ 2 value was obtained for the Re-N path (Table 1), which is probably due to the interference between their EXAFS oscillations (see Figure S1).…”

In situ IR study of surface hydroxyl species of dehydrated TiO2: towards understanding pivotal surface processes of TiO2 photocatalytic oxidation of toluene

“…14 Previous studies reported that the treatment of TiO 2 under vacuum led to the disappearance of bridged -OH, accompanied by creation of Ti 3+ surface-defect sites. 23 Based on these results, we assigned the 3734 and 3715 cm À1 bands to terminal hydroxyls and the other four to bridged -OH. Bands in the 3600-3750 cm À1 range have been assigned to isolated hydroxyl groups, while bands in the 3600-2500 cm À1 range are due to the adsorption of un-dissociated water molecules, -OH oscillators involved in H-bonds.…”