Optimal morphologies of Ag particles on α-Al2O3 for ethylene epoxidation

Abstract

In industry, silver-based catalysts are the mainstream choice for converting ethylene to ethylene oxide (EO), and the morphology of Ag particles plays an important role in their catalytic performance; however, related studies are scarce. In this work, evidence from SEM shown that we successfully synthesized three different morphologies of Ag particles induced by different solvent, i.e., flower-like Ag (Fl-Ag), spheroidal Ag (Sp-Ag) and sheet-like Ag (Sh-Ag), dissolve them respectively into the mixed solution of silver nitrate/oxalic acid to form a suspension, then load onto α-Al₂O₃. Through were calculated from the XRD peak area the Ag(100) surface occupancy has the order of Fl-Ag (30 %) > Sh-Ag (20 %) > Sp-Ag (15 %), and the selectivity of EO has positive relation with similar order of Fl-Ag/Al₂O₃ (85 %) > Sh-Ag/ Al₂O₃ (72 %) > Sp-Ag/Al₂O₃ (70 %). The selectivity of Fl-Ag/Al₂O₃ for EO at 210 °C was 85 %. It is shown that Ag(100) surface has better O₂ interaction and activated than that of Ag(111) surface, and (100) has a lower activation energy than the side reaction (0.9 eV vs 1.0 eV), facilitates the conversion of ethylene to ethylene oxide. This study provides a new method to design and optimize silver catalysts.