An Unexplored O 2 ‐Involved Pathway for the Decarboxylation of Saturated Carboxylic Acids by TiO 2 Photocatalysis: An Isotopic Probe Study

The aerobic decarboxylation of saturated carboxylic acids (from C 2 to C 5 ) in water by TiO 2 photocatalysis was systematically investigated in this work. It was found that the split of C 1 C 2 bond of the acids to release CO 2 proceeds sequentially (that is, a C 5 acid sequentially forms C 4 products, then C 3 and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic‐labeled H 2 18 O. As much as ≈42 % of oxygen atoms of the produced acetic acids were from dioxygen ( 16 O 2 ). 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 H 2 16 O was replaced by H 2 18 O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H 2 16 O under an atmosphere of 18 O 2 . 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 H 2 O through hole/OH radical oxidation. In the presence of conduction band electrons, O 2 can independently accomplish such C 1 C 2 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 O 2 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 O 2 ‐dependent decarboxylation competes with hole/OH‐radical‐promoted decarboxylation and depends on TiO 2 surface defects at which Ti 4c sites are available for the simultaneous coordination of substrates and O 2 .