Studying the NOx-trap mechanism over a Pt-Rh/Ba/Al2O3 catalyst by operando FT-IR spectroscopy

Abstract

In situ FTIR spectroscopy coupled with mass spectrometry have been used to study the mechanism of nitrates formation and reduction over a common Pt-Rh/Ba/Al₂O₃ NO_x storage catalyst. Our device consists of a transmission reactor cell (having a very small dead volume) dedicated to the evolution of surface species, and of a mass spectrometer combined with a FTIR micro-cell for gas analysis, allowing time resolved analysis in stationary and transient conditions. After studying the nitration properties of the catalysts under a lean flow, we have investigated its activity and selectivity in nitrate reduction under different reducing agents (CO, H₂), using a rich mixture very close to the real composition too. The alternate exposure to rich and lean streams has allowed the identification of some steps in the reduction pathway, active sites, intermediate species and by-products for NO_x-trap reaction. In particular we have differentiated the role of the support and the noble metal in the mechanism, as well as of isocyanate adspecies and ammonia among the detected species.