Adsorption of oxalate on rutile particles in aqueous solutions: a spectroscopic, electron-microscopic and theoretical study

Abstract

The adsorption of oxalic acid from the aqueous phase at the surface of rutile nanoparticles has been investigated by attenuated total-reflection Fourier-transformed infrared (ATR-FTIR) measurements. A combination of high resolution transmission electron microscopy (HRTEM) and Wulff-type construction was used to elucidate the typical morphology of the nanocrystals. It is estimated that (110)-type facets present more than 85% of the exposed surface in the powder. The aqueous system was also studied quantum-chemically using the semiempirical method MSINDO. Geometry optimizations have been performed, and the vibration spectra of the most stable surface complexes have been calculated. A sequence of model types has been applied in the quantum-chemical calculations in order to take into account the effect of interaction of water and oxalic acid on the adsorption mechanism and the vibration spectra. It was found that the presence of the aqueous phase significantly changes the stability of the oxalic acid surface complexes compared with the bare TiO(2) surface. The combination of experimental and theoretical information allowed identification of three species as the main contributors to the surface speciation. Two bidentate species were found with the C-C bond parallel to the TiO(2) surface, one monoprotonated and one deprotonated, and a third species being monodentate and monoprotonated.

Oxalic acid at the TiO 2 /water interface under UV(A) illumination: Surface reaction mechanisms

“…The same photocatalyst materials used previously for the respective experiments carried out under dark conditions [1,2] were employed in the present work. They consist of pure rutile (R15 provided by Millennium Inorganic Chemicals, now CRISTAL GLOBAL) and pure anatase (commercial product S230 from Kemira) nanoparticles with respective surface areas of 65 and 230 m 2 g À1 (BET adsorption) and average particle sizes of 15 nm (rutile) and 4-7 nm (anatase), respectively.…”

“…A TiO 2 (rutile or anatase) layer was deposited on the ZnSe ATR crystal (2.3 g m À2 and 1-3 lm thick) [1,2,22] by drying at room temperature an aliquot of 200 ll of a 5.75 g l À1 TiO 2 suspension together with 200 ll of water (see Refs. [1,2]).…”

“…A thorough study of the adsorption of oxalic acid at supported rutile [1] and anatase [2] nanoparticles showed the coexistence of different binding structures, resulting in different adsorption enthalpies. While at rutile (1 1 0) [1], the most stable and abundant structure is a bidentate complex, a monodentate structure is found to be the most favoured adsorbed oxalate species at the (1 0 0) anatase surface [2]. http://dx.doi.org/10.1016/j.jcat.2014.…”

“…Since, in particular, commercially produced TiO 2 nanocrystallites do not exhibit the equilibrium faces naturally occurring in macroscopic single crystals [9], photocatalytic reaction models assuming the well-known crystallographic facets may be unrealistic. Different exposed surfaces will definitively lead to different chemical interactions with a given adsorbate [1,2] due to a different coordination of the surface atoms, thus likely triggering different reaction mechanisms. A thorough study of the adsorption of oxalic acid at supported rutile [1] and anatase [2] nanoparticles showed the coexistence of different binding structures, resulting in different adsorption enthalpies.…”

“…Different exposed surfaces will definitively lead to different chemical interactions with a given adsorbate [1,2] due to a different coordination of the surface atoms, thus likely triggering different reaction mechanisms. A thorough study of the adsorption of oxalic acid at supported rutile [1] and anatase [2] nanoparticles showed the coexistence of different binding structures, resulting in different adsorption enthalpies. While at rutile (1 1 0) [1], the most stable and abundant structure is a bidentate complex, a monodentate structure is found to be the most favoured adsorbed oxalate species at the (1 0 0) anatase surface [2].…”