UV Raman studies of adsorbed oxygen and NOx species on Pt/γ-alumina catalysts

Raman spectra at 244 nm are obtained in situ from several Pt/γ-Al2O3 catalysts under gas flows containing O2, CO, NO2, and H2 at temperatures from 20 to 600°C. With O2 or NO2, a broad peak at 571 cm−1 is observed which we attribute to the Pt–O stretch mode of atomic oxygen. This marks the first direct spectroscopic observation of this fundamental species on a real catalyst. The peak can be easily removed with CO or H2, even at room temperature, and the process is completely reversible. With NO2 exposure, several peaks associated with surface NOx species also appear, the most prominent of which is a nitrate line at 1048 cm−1. Under reaction conditions, at 350°C with a stoichiometric CO/O2/N2 gas stream, the steady-state O coverage on the Pt particles is about half of the maximum coverage achieved with only an O2 flow. This observation paves the way for future in situ studies of catalytic mechanisms addressing the role of atomic O as an intermediate.