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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.
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Hydrogen activation by rhodium under the cover of a copper oxide thin film
The activation of reactants by catalytically active metal sites at metal-oxide interfaces is important for understanding the effect of metal-support interactions on nanoparticle catalysts and for tuning activity and selectivity. Using a combined experimental and theoretical approach, we studied the activation of H2 and the effect of CO poisoning on isolated Rh atoms completely or partially covered by a copper oxide (Cu2O) thin film. Temperature programmed desorption (TPD) experiments conducted in ultra-high vacuum (UHV) show that neither a partially nor a fully oxidized Cu2O layer grown on a Rh/Cu(111) single-atom alloy can activate hydrogen in UHV. However, in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments performed at elevated H2 pressures reveal that Rh significantly accelerates the reduction of these Cu2O thin films by hydrogen. Remarkably, the fastest reduction rate is observed for the fully oxidized sample with all Rh sites covered by Cu2O. Both TPD and AP-XPS data demonstrate that these covered Rh sites are inaccessible to CO, indicating that Rh under Cu2O is active for H2 dissociation but cannot be poisoned by CO. In contrast, an incomplete oxide film leaves some of the Rh sites exposed and accessible to CO, and hence prone to CO poisoning. Density functional theory calculations demonstrate that unlike many reactions in which hydrogen activation is rate limiting, the rate-determining step in the dissociation of H2 on thin-film Cu2O with Rh underneath is the adsorption of H2 on the buried Rh site, and once adsorbed, the dissociation of H2 is barrierless. These calculations also explain why H2 can only be activated at higher pressures. Together, these results highlight how different the reactivity of atomically dispersed Rh in Cu can be depending on its accessibility through the oxide layer, providing a way to engineer Rh sites that are active for hydrogen activation but resilient to CO poisoning.
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- Award ID(s):
- 1653561
- PAR ID:
- 10595356
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Catalysis
- Volume:
- 440
- Issue:
- C
- ISSN:
- 0021-9517
- Page Range / eLocation ID:
- 115779
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jcat.2024.115779
