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.