Real time surface infrared spectroscopy in the study of surface reaction kinetics

With recent improvements in both Fourier Transform and dispersive infrared spectrometers, surface sensitivities have been achieved which are sufficient for time-resolved surface infrared Spectroscopy. This time-resolved capability, along with high spectral resolution (better than 10 cm^-1 ), and the ability to probe through high ambient pressures, has enabled infrared spectroscopy to be successfully applied to the study of surface reaction kinetics. For example, time-resolved infrared absorption features in the 1200 cm^-1 to 2300 cm^-1 region were used in an investigation of oscillatory oxidation of carbon monoxide over a platinum foil catalyst under ambient pressures up to 600 Torr. Adsorbed carbon monoxide and gas-phase carbon dioxide were measured simultaneously, with time resolution of 600 msec per spectrum. In addition, broadband reflectance measurements were used to infer adsorbed oxygen levels from the same real time spectra. These measurements indicated a slow, periodic variation in the number of active sites on the catalyst, in phase with the variations in reaction rate. Parallel studies (Auger and IR) of a Pt foil in UHV suggested that periodic variation in the surface carbon level drives the activity and reaction rate oscillations. The time resolution and high pressure capability of surface infrared Spectroscopy were critical for achieving in-situ measurements on all relevant time scales. Other recents applications of surface infrared spectroscopy, including improvements in time resolution (down to tens of milliseconds per spectrum) will also be discussed.