In Situ High Temperature SERS Study of Oxygen Adsorbed on Ag: Support and Electrochemical Promotion Effects

Abstract

AbstractIn situ surface enhanced Raman spectra (SERS) of adsorbed oxygen were obtained during oxygen adsorption and ethylene epoxidation on Ag catalyst surfaces at temperatures between 25 and 400 °C and atmospheric total pressure. Three types of Ag catalysts were investigated: (I) Ag supported on a‐Al2O3, (II) Ag films deposited on quartz and (III) Ag films deposited on the O2‐ conducting solid electrolyte Y2O3‐stabilized ZrO2 (YSZ). In the last case the effect of the application of external voltages (‐2 V to +2 V) on the SERS was also examined “in situ” in order to investigate the effect of Non‐Faradaic Electrochemical Modification of Catalytic Activity (NEMCA). Bands at 240, 345, 815, 870, 980 and 1630 cm−1 were observed for the oxygen‐Ag/YSZ system. The 815 and 980 cm−1 bands were also observed for the Ag/quartz and Ag/a‐Al2O3 samples. The intense band at 815 cm−1, assigned to the O‐O stretching vibration of a molecularly adsorbed oxygen species, exhibited an isotope shift of 20 cm−1 upon replacing 16O2 with 18O2. This band is present during ethylene epoxidation at temperatures up to 350 °C and its relative Raman intensity decreases with decreasing O2 to C2H4 ratio. The excitation profile of the 815 cm−1 band and the existence of the overtone at 1630 cm−1 indicate that preresonance Raman enhancement is also responsible for the high intensity spectra obtained. In the case of Ag/YSZ it was found that increasing catalyst potential strengthens the O‐O bond and increases the band intensity, while decreasing catalyst potential causes a decrease in bond strength and band intensity. These observations can be accounted for within the framework of previous NEMCA studies. However the change in relative band intensity can also be due to the effect of applied voltage and changing Fermi level and work function on the enhancing mechanism of SERS.