Early-Stage Deactivation of Platinum-Loaded TiO 2 Using In Situ Photodeposition during Photocatalytic Hydrogen Evolution Greta M. Haselmann and Dominik Eder* Technische Universitä t Wien, Institut fü r Materialchemie, Getreidemarkt 9, 1060, Vienna, Austria * S Supporting Information ABSTRACT: This work unravels a sudden deactivation of Pt/TiO 2 (P25) during the initial stages of photocatalytic H 2 evolution from aqueous solution that, until now, has gone unnoticed, using a unique combination of in situ photodeposition of Pt with an on-line gas detector system. Utilizing a set of techniques, including high- resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS-UV-vis), X-ray powder diffraction (XRD), Raman spectroscopy, and physisorption, we were able to attribute this deactivation to a shift in mechanism, accompanied by an increase in CO concentration. Key to this phenomenon is the ratio of Pt atoms to oxygen vacancies, which were created through ultrasonic pretreatment and in situ UV irradiation in the bulk and surface, respectively. We also observed a potential additional contribution to the deactivation by encapsulation of the Pt nanoparticles, indicating that strong metal-support interaction (SMSI) may indeed happen in aqueous and ambient conditions. Furthermore, we encourage implementing the concept of a “dynamic” catalyst to photochemistry that opens up a new approach toward understanding the complex mechanisms and kinetics in heterogeneous photocatalysis. KEYWORDS: photocatalysis, TiO 2 , deactivation, mechanism, defects, SMSI, photodeposition, hydrogen ■ INTRODUCTION Heterogeneous photocatalysis covers a range of cutting-edge applications that address important socioeconomic areas such as energy, 1,2 environment, 3,4 hygiene and disinfection, 5,6 and recently also gained considerable impact on the development of novel green processes. 7 The comparability of photocatalytic activities generally constitutes a major challenge, because they are dependent on a variety of parameters, including reaction temperature, light intensity, and amount and type of sacrificial agent, as well as specific reactor setups. Very often, this problem is addressed by using a common benchmark system as reference and, less consistently, by comparing quantum efficiency values, which are less system-sensitive than actual activity values. This renders it crucial to have a good benchmark system that ensures stable and reliable activity levels. TiO 2 in particular of type P25so far has been the most widely studied material in photocatalysis. P25 is a commercial product that is commonly synthesized by flame-spray pyrolysis and has demonstrated high photocatalytic activities, making it a highly popular reference material. P25 is a mixed-phase compound that consists of anatase, rutile, and amorphous phases. The phase composition is typically characterized by an anatase:rutile ratio of 80:20, while the presence of an amorphous phase is very often neglected in the literature. Ohtani et al. determined the ratio of anatase, rutile, and amorphous phase in a sample by selective dissolution to be 78:14:8, while simultaneously noting the inconsistency in composition between different samples. 8 Still, when composited with platinum nanoparticles as co- catalysts, TiO 2 remains one of the most active photocatalysts for both oxidation (e.g., dye degradation, water purification) and reduction (e.g., hydrogen evolution) reactions. Therefore, Pt/TiO 2 has evolved as the most common reference photo- catalyst. Here, we report on a detailed investigation of the initial stages of photocatalytic hydrogen formation using Pt-loaded P25 in which we observed an unexpected sudden deactivation, which differs distinctly from previously reported passivation of photocatalysts. We were able to observe this effect due to the unique combination of in situ photodeposition of Pt with an on-line gas detector system. Employing a CO detector, in addition to H 2 and CO 2 , allowed us to pinpoint this deactivation to a shift in mechanism that is accompanied by an increased formation of CO. Deactivation of Pt/P25 has drastic consequences and may even be the reason for the large deviations within reported literature results, i.e., activities may have been obtained from already deactivated samples and thus underestimate the actual potential of TiO 2 . Received: March 16, 2017 Revised: June 1, 2017 Published: June 6, 2017 Research Article pubs.acs.org/acscatalysis © XXXX American Chemical Society 4668 DOI: 10.1021/acscatal.7b00845 ACS Catal. 2017, 7, 4668-4675