An efficient redox reaction between organic substrates in solution and photoinduced h⁺_vb/e⁻_cb on the surface of photocatalysts requires the substrates or solvent to be adsorbed onto the surface, and is consequentially marked by a normal kinetic solvent isotope effect (KSIE≥1). Reported herein is a universal inverse KSIE (0.6–0.8 at 298 K) for the reductive dehalogenation of aromatic halides which cannot adsorb onto TiO₂ in a [D₀]methanol/[D₄]methanol solution. Combined with in situ ATR-FTIR spectroscopy investigations, a previously unknown pathway for the transformation of these aromatic halides in TiO₂ photocatalysis was identified: a proton adduct intermediate, induced by released H⁺/D⁺ from solvent oxidation, accompanies a change in hybridization from sp² to sp³ at a carbon atom of the aromatic halides. The protonation event leads these aromatic halides to adsorb onto the TiO₂ surface and an ET reaction to form dehalogenated products follows.

An efficient redox reaction between organic substrates in solution and photoinduced h⁺_vb/e⁻_cb on the surface of photocatalysts requires the substrates or solvent to be adsorbed onto the surface, and is consequentially marked by a normal kinetic solvent isotope effect (KSIE≥1). Reported herein is a universal inverse KSIE (0.6–0.8 at 298 K) for the reductive dehalogenation of aromatic halides which cannot adsorb onto TiO₂ in a [D₀]methanol/[D₄]methanol solution. Combined with in situ ATR-FTIR spectroscopy investigations, a previously unknown pathway for the transformation of these aromatic halides in TiO₂ photocatalysis was identified: a proton adduct intermediate, induced by released H⁺/D⁺ from solvent oxidation, accompanies a change in hybridization from sp² to sp³ at a carbon atom of the aromatic halides. The protonation event leads these aromatic halides to adsorb onto the TiO₂ surface and an ET reaction to form dehalogenated products follows.

Invited for the cover of this issue is the group of Professor Jincai Zhao at the Institute of Chemistry, CAS. The image shows that dicarboxylic acids with an even number of carbon atoms were degraded more slowly than those with an odd number of carbon atoms. This unusual fluctuation of the reactivity of dicarboxylic acids against the numbers of carbon atoms is very closely related to the different pre-coordination mode of the acids with the photocatalyst surface sites. Read the full text of the article at 10.1002/chem.201303236.

The degradation behaviours of five straight-chain dicarboxylic acids (from ethanedioic acid to hexanedioic acid) were compared in aqueous TiO₂-based photocatalysis. When all other conditions were identical, the degradation rates were found to fluctuate regularly with the parity of the number of carbon atoms. Dicarboxylic acids with an even number of carbon atoms (e-DAs) always degraded more slowly than those acids with an odd number of carbon atoms (o-DAs). This unusual fluctuation in the reactivity for the degradation of dicarboxylic acids by TiO₂-based photocatalysis is very closely related to the different pre-coordination modes of the acids with the photocatalyst. Attenuated total reflection FTIR (ATR-FTIR) of e-DAs labelled with ¹³C showed that both carboxyl groups of the acid coordinate to TiO₂ through bidentate chelating forms. In contrast, only one carboxyl group of the o-DAs coordinated to TiO₂ in a bidentate chelating manner, whereas the other formed a monodentate binding linkage. The bidentate chelating form with bilateral symmetric coordination did not favour degradation. Isotope-labelling experiments were performed with ¹⁸O₂ to observe the different ways in which incorporated oxygen entered the initial decarboxylated products of e- and o-DAs. For the degradation of butanedioic acid, (45.9±0.5) % of the oxygen in the formed propanedioic acid came from H₂O, whereas for pentanedioic acid, (97.4±0.2) % of the oxygen in the formed butanedioic acid came from H₂O. Our results demonstrate that in TiO₂-based photocatalysis, the reactivity of active species, such as ^.OH/h_vb⁺, is far from non-selective and that the attacks of these active species on organic substrates are significantly affected by the coordination patterns of the substrates on the TiO₂ surface.

The aerobic decarboxylation of saturated carboxylic acids (from C₂ to C₅) in water by TiO₂ photocatalysis was systematically investigated in this work. It was found that the split of C¹C² bond of the acids to release CO₂ proceeds sequentially (that is, a C₅ acid sequentially forms C₄ products, then C₃ and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic-labeled H₂¹⁸O. As much as ≈42 % of oxygen atoms of the produced acetic acids were from dioxygen (¹⁶O₂). Through diffuse reflectance FTIR measurements (DRIFTS), we confirmed that an intermediate pyruvic acid was generated prior to the cut-off of the initial carboxyl group; this intermediate was evidenced by the appearance of an absorption peak at 1772 cm⁻¹ (attributed to CO stretch of α-keto group of pyruvic acid) and the shift of this peak to 1726 cm⁻¹ when H₂¹⁶O was replaced by H₂¹⁸O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H₂¹⁶O under an atmosphere of ¹⁸O₂. With the α-keto oxygen of pyruvic acid preserved in the carboxyl group of acetic acid, ≈24 % new oxygen atoms of the produced acetic acid were from molecular oxygen at near 100 % conversion of pyruvic acid. The other ≈76 % oxygen atoms were provided by H₂O through hole/OH radical oxidation. In the presence of conduction band electrons, O₂ can independently accomplish such C¹C² bond cleavage of pyruvic acid to generate acetic acid with ≈100 % selectivity, as confirmed by an electrochemical experiment carried out in the dark. More importantly, the ratio of O₂ participation in decarboxylation increased along with the increase of pyruvic acid conversion, indicating the differences between non-substituted acids and α-keto acids. This also suggests that the O₂-dependent decarboxylation competes with hole/OH-radical-promoted decarboxylation and depends on TiO₂ surface defects at which Ti_4c sites are available for the simultaneous coordination of substrates and O₂.