Catalytic and modulation excitation visible-NIR spectroscopic insights into the consequences of K promotion and pretreatment on Cu/SiO2 for direct propylene epoxidation

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 Cu^m+ (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.