Nature of Catalytic Active Sites Present on the Surface of Advanced Bulk Tantalum Mixed Oxide Photocatalysts

Abstract

The most active photocatalyst system for water splitting under ultraviolet (UV) irradiation (270 nm) is the promoted 0.2% NiO/NaTaO3:2% La photocatalyst with an optimized photonic efficiency of 56%, but fundamental issues about the nature of the surface catalytic active sites and their involvement in the photocatalytic process still need to be clarified. This is the first study to apply cutting-edge surface spectroscopic analyses to determine the surface nature of tantalum mixed oxide photocatalysts. Surface analysis with high- resolution X-ray photoelectron spectroscopy (1−3 nm) and high-sensitivity low-energy ion scattering spectroscopy (0.3 nm) indicates that the NiO and La2O3 promoters are concentrated in the surface region of the bulk NaTaO3 phase. The NiO is concentrated on the NaTaO3 outermost surface layers, while La2O3 is distributed throughout the NaTaO3 surface region (1−3 nm). Raman and UV−vis spectroscopy revealed that the bulk molecular and electronic structures, respectively, of NaTaO3 were not modified by the addition of the La2O3 and NiO promoters, with La2O3 resulting in a slightly more ordered structure. Photoluminescence spectroscopy reveals that the addition of La2O3 and NiO produces a greater number of electron traps resulting in the suppression of the recombination of excited electrons and holes. In contrast to earlier reports, La2O3 is only a textural promoter (increasing the BET surface area by ∼7-fold by stabilizing smaller NaTaO3 particles) and causes an ∼3-fold decrease in the specific photocatalytic TORs (micromoles of H2 per square meter per hour) rate because surface La2O3 blocks exposed catalytic active NaTaO3 sites. The NiO promoter was found to be a potent electronic promoter that enhances the NaTaO3 surface-normalized TORs by a factor of ∼10−50 and turnover frequency by a factor of ∼10. The level of NiO promotion is the same in the absence and presence of La2O3, demonstrating that there is no promotional synergistic interaction between the NiO and La2O3 promoters. This study demonstrates the important contributions of the photocatalyst surface properties to the fundamental molecular/electronic structure−photoactivity relationships of promoted NaTaO3 photocatalysts that were previously not appreciated in the literature.