The kinetic modeling of photocatalytic reactions is a powerful tool for process optimization. We applied a holistic kinetic model for the gas-phase photocatalytic oxidation of HCl to Cl2 to identify suitable operation conditions and further optimization potential. We used a flat-plate photoreactor with UV LEDs and iodometric titration as online analytics and performed a comprehensive parameter variation. High O2 and moderate HCl partial pressures resulted in the highest reaction rates, indicating a favorable reactant ratio of 4:1. An Arrhenius dependence of the reaction rate with an apparent activation energy of 25.7 kJ mol–1 identifies a suitable reaction temperature of ∼120 °C. This temperature combines high reaction rates with high apparent quantum yields up to 8.4%, showing a logarithmic dependence of reaction rates on light intensity. The well-fitting kinetic model predicts that improving the intrinsic activity of the photocatalyst is the key for further enhancing the efficiency of photocatalytic HCl recycling.

We studied the photocatalytic aerobic oxidation of HCl over TiO2 for producing Cl2. Steady‐state Cl2 production rates were determined with a photocatalytic fixed‐bed gas‐phase reactor equipped with UV light‐emitting diodes (LEDs) using iodometric titration as online analytics. We found stable Cl2 production rates of up to 16 mmol h−1 m−2 for commercial anatase TiO2 Hombikat UV100. The rate increased linearly with temperature from 21 to 140 °C, indicating the acceleration of the limiting desorption rate of the coupled product water. Comparing different TiO2 polymorphs revealed that anatase possesses higher activity than rutile. The adsorption of HCl was monitored in situ by IR spectroscopy. The IR spectra indicated that HCl chemisorption chlorinates the surface of TiO2 under the reaction conditions, suggesting it to be the first step of the reaction mechanism. High stability opens up the opportunity of developing a promising photocatalytic process of HCl recycling at lower temperatures suitable for reaching full conversion.

SrTiO3 was prepared by a polymeric precursor method and applied in the photocatalytic aerobic oxidation of HCl in a flat‐plate reactor equipped with a UV LED array (368 nm). Reaction rates up to 240 mmol h−1 m−2 and apparent quantum yields up to 33 % using an illuminated area of 60 cm−2 were achieved with highly crystalline SrTiO3 calcined at 750 °C, outperforming commercially available SrTiO3 by a factor of almost 2. A gradual catalyst deactivation was observed, which was due to the formation of crystalline SrCl2×2 H2O on the surface confirmed by X‐ray diffraction, electron microscopy and X‐ray photoelectron spectroscopy (XPS). Increasing the HCl partial pressure accelerated both Cl2 formation and catalyst deactivation. XP spectra revealed an intrinsic surface segregation of Sr and the presence of several Sr‐ or O‐containing surface species. High Cl2 yields up to 42 % obtained with an illuminated area of 120 cm−2 encourage further research on a photocatalytic Deacon process for improved HCl recycling.

NaNbO 3 enriched with oxygen vacancies by Ni doping was successfully synthesized via a polymerized complex method and applied as a photocatalyst in the oxidation of cinnamyl alcohol (CA) to cinnamaldehyde in air. Reaction rates as high as 45 μmol h −1 were achieved under visible light with a high apparent quantum efficiency of 67.2% and excellent chemoselectivity larger than 99%. UV−vis, electron paramagnetic resonance, and attenuated total reflectance infrared spectroscopy results indicate that the CA molecules preferentially adsorb at the oxygen vacancies, thus enabling electron transfer between coordinatively bound CA and NaNbO 3 under visible light, inducing CA oxidation. The photocatalytic aerobic oxidation of CA is assumed to proceed via the one-photon pathway with H 2 O 2 as the coupled product. The photodeposited Pt nanoparticles on the surface not only enhanced the oxidation rate but also improved the selectivity to cinnamaldehyde substantially because of the fast decomposition of formed H 2 O 2 , in this way avoiding its consecutive oxidation by H 2 O 2 . The oxygen vacancies on the surface generated by Ni doping are identified to play a decisive role in the chemisorption of cinnamyl alcohol and the interface charge transfer.