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In Situ and Operando Raman Spectroscopy Study of Structure Sensitivity in Unpromoted Ag/α-Al2O3 During Ethylene Epoxidation with Molecular Oxygen
Alzahrani, Hashim
Alzahrani, Hashim
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Abstract
This work investigates the structure sensitivity in gas-phase ethylene epoxidation with molecular oxygen to ethylene oxide (EO) on unpromoted Ag/α-Al2O3 by operando surface-enhanced Raman spectroscopy (SERS) to elucidate the effect of silver particle size on apparent turnover frequencies (TOF) and EO selectivity. The research gaps were identified by critically reviewing the published work for the past century. Catalysts were prepared by wet-incipient impregnation of a silver oxalate solution on α-Al2O3 (8 m2/g), leading to silver particle sizes in the 20-170 nm range by varying silver loading and temperature treatment in various atmospheres. XRD and SEM characterization of catalysts’ morphology confirmed the polycrystallinity of silver particles' bulk structure and demonstrated a linear proportional increase in the number of crystallites with average particle size. Oxygen available for the reaction was assessed by O2-TPD and ethylene TPSR and was ~50 and ~15% for silver particles 100 nm, respectively, indicating higher amounts of dissolved oxygen in large silver particles. Operando SERS experiments were conducted at 1 atm and 200 ℃. The weight normalized conversion rate decreased rapidly for silver particles between 20-50 nm, above which it plateaued. For the apparent TOF, an opposite relationship was observed, constant between 20-50 nm, followed by a rapid increase above 50 nm. EO selectivity was initially independent of silver particle size between 20-50 nm, but then increased up to particle sizes of 100 nm, remaining constant up to ~170 nm. Raman spectra showed two significant bands, one at 815 cm-1 for all catalysts and the second at 880 cm-1 only present in catalysts with silver particles 100 nm. They were assigned to diatomic oxygen complexes stabilized by atomically chemisorbed and subsurface oxygen, respectively. Oxygen species characterized by 880 cm-1 are likely responsible for higher TOF and EO selectivity for particles 100 nm. These results emphasize the need for subsurface oxygen species provided by large particles to enhance activity and EO selectivity.
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2022-05-31
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University of Kansas
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906210_1.pdf
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Chemical engineering, Catalysis, Epoxidation, Ethylene, Particle size, Raman, Structure Sensitivity