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Systematic fixed-bed reactor studies were performed under propylene epoxidation conditions with O2 (1 atm, 250 °C) to examine the impact of potassium and oxidative/reductive pretreatments on Cu/SiO2 catalysts. At similar propylene conversion and copper particle sizes (2–3 nm), hydrogen pretreatment of K-promoted Cu/SiO2 significantly increased propylene oxide (PO) selectivity, making it the primary product (∼40 %). Characterization of the spent catalysts using XPS and H2-TPR revealed a unique Cu-K interaction following H2 pretreatment and suggested a potential role for subsurface oxygen in the reaction. Activity tests with CuO and Cu2O, both with and without K, showed that K deactivates CuO while enhancing PO/acrolein ratio for Cu2O. Operando Modulation Excitation-Phase Sensitive Detection- Diffuse Reflectance Visible-Near Infrared Spectroscopy (ME-PSD-DRVis-NIR) revealed charge transfer correlations. These correlations suggested that the combined K and H2 increases the population of mixed-valent Cum+ (1 < m < 2) (Cu1+ and Cu2+ ensemble) sites and modulates the strength of oxygen adsorption and activation at these sites, thereby improving PO selectivity and formation rate. These results demonstrate that controlling the copper oxidation state through promoters and pretreatments enhances PO selectivity in direct propylene epoxidation with O2, providing valuable insights for improving catalytic activity and identifying crucial oxygen species.