High performance liquid chromatography (hplc)

The mixtures
were prepared using methanol (HPLC, gradient grade, ≥99.8%
(GC), <0.01% water, VWR International), ethanol (Puriss., absolute,
≥99.8% (GC), Sigma-Aldrich), formic acid (Suprapur, 98–100%
(ACS), Merck KGaA), D₂O (99.9 atom % D, Sigma-Aldrich),
and Milli-Q water (18.2 MΩ cm). All substances were used without
further purification. To avoid the overlap between the C=O
stretching mode (∼1720 cm^–1)^{26 (link)} and the H–O–H bending mode (∼1650–1750
cm^–1),^{28 (link)} we measure the
H–O–D bending region for the water–formic acid
system. We prepared samples using H₂O:D₂O mixtures
and formic acid. We used 1:1 H₂O:D₂O mixtures
for samples with formic acid mole fractions of 0, 0.02, 0.08, 0.14,
and 0.32. 1:3 H₂O:D₂O mixtures and pure D₂O were used for formic acid mole fractions of 0.53 and 0.66,
respectively. The determination of x_HOD is given in the Supporting Information. The aqueous binary mixtures were contained in a trough with a diameter
of 5 cm for the SFG measurements.

Phenol (99.5%, Roth, Karlsruhe, Germany) was used as the pollutant in the photocatalytic degradation tests. Synthetic air (20.5 ± 0.5 vol% O₂, Air Liquide, Berlin, Germany) and hydrogen peroxide (H₂O₂, 30 wt%, Fisher Chemical, Schwerte, Germany) were used as the oxygen sources. For the immobilization of the photocatalysts onto the steel plates, a silica binder prepared from tetraethylorthosilicate (TEOS, 98%, Sigma-Aldrich, Schnelldorf, Germany), hydrochloric acid (HCl, 37%, Roth, Karlsruhe, Germany), 1-propanol (99.5%, Roth, Karlsruhe, Germany), 2-propanol (HPLC, VWR Chemicals, Dresden, Germany), and Levasil (Obermeier, Bad Berleburg, Germany) was used. As eluents in the HPLC analysis, acetonitrile (ACN, HPLC, VWR Chemicals, Dresden, Germany) and ultrapure water (Synergy UV system, Burlington, MA, USA) were used. As commercial titanium dioxide (TiO₂) photocatalysts, P25 (99.5%, Evonik, Essen, Germany), P90 (100%, Evonik, Essen, Germany), PC105 (100%, Millenium/Cristal ACTivTM, Thann, France), and PC500 (100%, Millenium/Cristal ACTivTM, Thann, France) were investigated. All photocatalysts have been extensively characterized in our previous publications and the main characteristics are summarized in Table 1.

Acetone, dichloromethane (DCM), hexane, 2-propanol (iPrOH), toluene, ethyl-acetate (EtOAc) (reagent grade, Molar Chemicals, Hungary) were purified by distillation. Acetonitrile (MeCN), tetrahydrofuran (THF), methanol (MeOH), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), pyridine (HPLC grade, VWR, Germany), cyclohexane, 1,4-dioxane (reagent grade, Reanal, Hungary), chloroform, 1,5-diaminoanthraquinone (Sigma-Aldrich, Germany) were used without further purification.
NMR: ¹H and ¹³C-NMR spectra were recorded in CDCl₃ and DMSO-d₆ at 25 °C on a Bruker Avance DRX-400 and a Bruker AM 360 spectrometer at 400 and 360 MHz, respectively, with tetramethylsilane as the internal standard.
UV–vis: The UV–vis spectra were recorded on an Agilent Cary 60 spectrophotometer (Agilent, Santa Clara, CA, USA) in a quartz cuvette of 1.00 cm optical length. A 3.00 cm³ solution was prepared from the sample.