Improvement of Propylene Epoxidation Caused by Silver Plasmon Excitation by UV-LED Irradiation on a Sodium-Modified Silver Catalyst Supported on Strontium Carbonate

Abstract

The effect that UV-LED irradiation exerted on a sodium-modified silver catalyst supported on strontium carbonate (Ag-Na/SrCO3) was examined during an epoxidation of propylene to propylene oxide. Based on our previous study, we used Ag(56)-Na(1)/SrCO3 in this study. The numbers in parentheses refer to the weight percentage of silver and sodium. Although this catalyst system did not contain typical photocatalysts such as titanium oxide or tungsten oxide, UV-LED irradiation of Ag(56)-Na(1)/SrCO3 resulted in an evident improvement in the selectivity and yield of propylene oxide. Such an advantageous effect of UV-LED irradiation could not be discussed based on the bandgap used in photocatalysts and, therefore, we proposed a mechanism based on the plasmon excitation of silver, which could be accomplished using the irradiation wavelength of UV-LED to produce electrons. Since the lifespan of these electrons is expected to be short, it is difficult to place them into direct contact with the gas phase of oxygen. Once the generated electrons move to SrCO3, however, the lifespan is improved, which could allow suitable contact with oxygen in the gas phase to form active oxygen. If the oxygen is active for epoxidation as hydrogen peroxide, this could explain the improvement in activity from UV-LED irradiation.