On the local sensitivity of different IR techniques: Ba species relevant in NOx storage-reduction

Abstract

IR spectroscopy is widely used to elucidate reaction mechanisms in NO_x storage and reduction (NSR). Observed band positions and assignments of vibrational modes, however, differ remarkably among the various investigations. We report an IR study of barium species relevant in NSR, aiming to clarify the source of the reported discrepancies and different surface and bulk sensitivity of various IR measurement configurations. Four IR techniques, namely, transmission IR spectroscopy (TIRS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), attenuated total reflection infrared spectroscopy (ATR-IRS), polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), all suitable for in situ studies of the reaction system, were used. Depending on the IR technique certain bands undergo a clear band shift or disappearance, evidently showing different surface and bulk sensitivity. In spectra of barium nitrate recorded by the more bulk sensitive IR techniques, i.e. TIRS, ATR-IRS, and PM-IRRAS, fewer bands appeared than in the more surface sensitive DRIFTS spectra. This work constitutes a collection of IR spectra of reference barium compounds for the clarification of species present in the NSR catalyst system. The band position or the presence of certain bands assigned to the same chemical species may deviate if the spectra were measured by different IR techniques, especially if the compared IR techniques differ in surface/bulk sensitivity. This implies that the band assignment valid for spectra measured by DRIFTS can be transferred to TIRS, ATR-IRS, and PM-IRRAS only with precautions.