In Situ X-ray Absorption Spectroscopy Study of the Deactivation Mechanism of a Ni-SrTiO₃ Photocatalyst Slurry Active in Water Splitting

We used in situ X-ray absorption spectroscopy (XAS) to investigate the composition-performance correlation of Ni-SrTiO₃ photocatalysts active for water splitting. After preparation and exposure to ambient conditions, the Ni particles on SrTiO₃ consist of Ni(0) and Ni(II) phases, with a 4:1 at % ratio, in a metal/oxide core/shell configuration, as confirmed by XPS and TEM-EDX. In situ XAS experiments using an aqueous slurry of the Ni-SrTiO₃ photocatalyst and simultaneous continuous exposure to 365 nm light with a power density of 100 mW cm^-2 and the X-rays do not reveal significant changes in oxidation state of the Ni particles. Contrarily, when the X-rays are discontinuously applied, UV excitation leads to oxidation of a significant fraction of Ni(0) to Ni(II), specifically to NiO and Ni(OH)₂ phases, along with cocatalyst restructuring. Ni dissolution or oxidation to higher valence states (e.g., Ni(III)) was not observed. The UV light-induced oxidation of Ni(0) causes the hydrogen evolution rate to drop to similar rates as observed for pristine SrTiO₃, suggesting that Ni(0) is the active phase for H₂ generation. Our results underscore the importance of assessing the effects of (continuous) X-ray exposure to (photo)catalyst-containing aqueous slurries during in situ XAS experiments, which can significantly influence the observation of compositional and structural changes in the (photo)catalysts. We ascribe this to X-ray induced water photolysis and formation of free electrons, which in this study quench SrTiO₃ photoholes and prevent Ni oxidation.