The synthesis of the TiO2@IrOx particles with 40 wt% iridium is based on three process steps which are elucidated below. For the reaction solution, 1.39 g of IrCl3 • H2O (53.7% iridium, 99.8% purity (metals basis), Thermo Fisher Scientific) were dissolved in 50 mL deionized (DI) water (18.2 MΩ cm) and the pH value was adjusted to 11 with 1 M KOH. In a second flask, 1.12 g of TiO2 (rutile, < 5 µm, > 99.9% (trace metals basis), Sigma Aldrich) were dispersed in 100 mL DI water and 1 mL of 1 M KOH (pH 11) by ultrasonication in an ultrasonic bath for 20 minutes. Both solutions were mixed, and 30 mL of isopropanol (> 99.8%, Sigma-Aldrich) were added as hole scavenger. The first reaction step was the photodeposition for which the reaction solution was exposed to UV-C radiation (254 nm wavelength) from a laboratory lamp (UVP Multiple Ray Lamp, 8 W, Analytik Jena) with a power density of 10 -12 mW cm -2 at the solution surface (measured with UVpro Radiometer, Nuvonic GmbH) for 24 hours. Secondly, the UV radiation was switched off and the reaction solution was heated at 100 °C under reflux for 1 h. After cooling down to room temperature, the solution was filtered, yielding a clear filtrate and a black powder indicating that all the iridium precursor was consumed. During filtering, the powder was washed with 1 L of DI water. After drying the powder at 70 °C for at least 4 hours and grinding it with an agate mortar for 10 minutes, the catalyst powder underwent an annealing step as the third part of the synthesis. The powder was heated to 350 °C at a rate of 10 K min -1 and held at the target temperature for 30 min under a continuous flow of synthetic air (100 mL min -1 ).