In situ Raman spectroscopy study of silver particle size effects on unpromoted Ag/α-Al2O3 during ethylene epoxidation with molecular oxygen

In situ Raman spectroscopy and parallel fixed bed reactor studies were conducted under ethylene epoxidation conditions with O2 at 1 atm and 200 ℃ on unpromoted Ag/α-Al2O3 catalysts with different Ag particle sizes. It was found that for Ag particles of 20–50 nm, the weight normalized conversion rate decreased rapidly with increasing Ag particle size but remained almost constant above 50 nm. On the other hand, the apparent TOF increased with increasing Ag particle sizes in the 20–170 nm studied range, while ethylene oxide selectivity at zero residence time was nearly constant (55 ± 4%). Raman bands at 815 (all Ag sizes) and 880 (Ag sizes > 100 nm) cm−1 were identified and assigned to active molecular oxygen species. The 880 cm−1 species was assigned to a molecular oxygen complex structure stabilized by subsurface oxygen. The presence of the 880 cm−1 oxygen species likely explain the higher apparent TOF in larger Ag particles (>100 nm).