A Spectroscopic and Electrochemical Approach to the Study of the Interactions and Photoinduced Electron Transfer between Catechol and Anatase Nanoparticles in Aqueous Solution

Abstract

We have combined in situ photoelectrochemical and spectroscopic techniques (Attenuated Total Reflection Infrared, ATR-IR, and Resonance Raman Spectroscopy) for the study of the charge-transfer complex formed upon adsorption of catechol on anatase nanoparticles in contact with aqueous acidic solutions. Vibrational spectroscopies reveal the existence of at least two adsorbate configurations: catecholate in a chelate configuration and molecularly adsorbed catechol, with apparent values of -12.3 and -10.5 kJ mol(-1), respectively. These values are significantly less negative than the values reported for anatase colloidal dispersions. The adsorption of both catechol species on the nanoparticulate anatase thin films follows the Freundlich isotherm. As revealed by resonance Raman spectroscopy, only the adsorbed chelating catecholate forms the charge-transfer complex. The electron transfer from the adsorbate to the anatase nanoparticles has been evidenced by the development of a negative photopotential upon 514.5 or 632.8 nm laser illumination of an anatase nanostructured thin film electrode in contact with a catechol solution. The time evolution of the Raman spectra shows an increasing fluorescence indicating that, upon electron injection, catechol polymerization occurs on the TiO2 surfaces. This conclusion is confirmed by in situ ATR-IR measurements, which show a progressive broadening of the catecholate bands together with the appearance of new signals. This study illustrates the benefits of combining electrochemical, infrared, and Raman techniques for the elucidation of processes occurring at the semiconductor/solution interface. Finally, evidence is given on the different adsorption and reactivity behavior found for suspensions and nanoporous thin films under equivalent experimental conditions.

Effect of phosphate on heterogeneous Fenton oxidation of catechol by nano-Fe 3 O 4 : Inhibitor or stabilizer?

“…By analysis of the second derivative spectra, besides the bands at 1480 and 1260 cm analysis. The strong band at 1261 cm −1 was assigned to ν(CO) of adsorbed catechol species, and the band at 1275 cm −1 was assigned to the combination of an ether C-O-C stretch between aromatic rings and the O-H bending vibration (Dubey et al, 1998;Lana-Villarreal et al, 2005), suggesting the formation of polymeric catechol species during the oxidation. The IR results were consistent with the results from previous work in which the formation of carboxylates, alcohols and dimers of catecholate species, such as 2-furanglycolic acid, 1,2-benzendicarboxylic acid and formic acid, oxalic acid, maleic acid and succinic acid, was identified for the oxidation of catechol by Fe 3 O 4 -H 2 O 2 , by GC-MS (Gas Chromatography-Mass Spectrometer), ESI-Q-TOF-MS and ion chromatography analysis (He et al, 2014).…”

Heterogeneous Fenton oxidation of catechol and 4-chlorocatechol catalyzed by nano-Fe3O4: Role of the interface

“…The band at 1480 cm À1 could be assigned to the m(CC) modes of the aromatic ring [34,35], and the other band at around 1260 cm À1 was assigned to m(CO) modes of phenyl-O groups [11,35]. Other peaks were also observed, such as C-H bending at 1213 and 1102 cm À1 for catechol, and at 1216, 1115 and 1070 cm À1 for 4-chlorocatechol, and the OH deformation and bending band at 1195 cm À1 (catechol) and 1405 cm À1 (4-chlorocatechol).…”

“…So ethers or dimers containing C-O and O-H might be generated and adsorbed on the nano-Fe 3 [34,35,39]. Above all, the characteristic peaks of adsorbed catechol or 4-chlorocatechol still existed during the oxidation, indicating that the adsorbed target organics were not mainly attacked by ROS.…”

“…Changes of the spectral features might result from the different micro-environment of the adsorbed organics upon addition of H 2 O 2 and also from the generation of some new ph-O compounds, such as 3,4-dihydroxybenzoic acid, according to the GC-MS analysis. During the oxidation, m a (COO) band at 1533 (catechol) and 1532 cm À1 (4-chlorocatechol) [34] and m s (COO) band at 1398 (catechol) [34] and 1389 cm À1 (4-chlorocatechol) [35,37] arose and increased in intensity, indicating the adsorption of generated carboxylic compounds. The shoulder at 1277 cm À1 and 1195 cm…”

“…, arising in the oxidation of catechol, were assigned to the overlapping peaks of asymmetric vibrations of C-O-C linkage and C-OH vibrations [35,38,39], and symmetric vibrations of C-O-C linkage [35,38].…”