Structure–Reactivity Insights from Design of Experiments: Facet Orientation of SrTiO ₃ Determines the Impact of Humidity and Light Intensity on the Photocatalytic Gas-Phase Oxidation of Acetone

Optimizing photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) typically relies on one-variable-at-a-time (OVAT) methods, which overlook crucial interactions and may not result in optimized process conditions. In this study, we apply design of experiments (DoE) to systematically evaluate the effects of relative humidity (RH) and light intensity (LI) on photomineralization of acetone over two SrTiO3 (STO) photocatalysts: cubic (c-STO) and {110}-truncated (trd-STO). Using a three level full-factorial design (RH = 2, 31, or 54%; LI = 0.6, 1.2, or 2.4 mW·cm–2), we reveal statistically significant interactions between RH, LI, and photocatalyst morphology. trd-STO outperformed cubic-STO, exhibiting high reaction rates in a broader optimum range, greater stability, and superior light efficiency under conditions of high humidity. Notably, while a high relative humidity of 54% severely inhibited the acetone conversion using cubic-STO, trd-STO maintained activity. By polynomial regression models and isoresponse surface plots, we determined the optimal operating conditions and highlight that variable interactions are strongly crystal facet-dependent. Our results demonstrate that DoE is a powerful tool for identifying “sweet spots” in VOC photomineralization and to reveal how surface facet engineering impacts photocatalytic behavior under realistic environmental conditions.