Possible deNO_x management under net oxidizing conditions: a molecular beam study of ¹⁵NO + CO + O₂ reaction on Pd(111) surfaces

Abstract

Isothermal kinetic measurements of ¹⁵NO reduction with CO on Pd(111) surfaces were carried out under net-oxidizing conditions with ¹⁵NO + CO + O₂, using a molecular beam instrument (MBI). Transient state (TS) and steady state (SS) kinetic details of the above reaction were obtained for a wide range of temperature and beam compositions, especially with O₂-rich compositions. Increasing O₂ content, generally, suppresses ¹⁵NO reduction in the SS; nonetheless, irrespective of O₂ content, ¹⁵N₂ was produced in TS, and to a significant extent under SS conditions too. Sustainable N2 production between 450 and 600 K and with low to moderate amount of oxygen was observed, and the extent of NO decomposition was also quantified. The ratio of ¹⁵N₂:¹⁵N₂O was generally found to be around 8:1 under most of the reaction conditions. Maxima in the SS reaction rates of all products were observed between 500 and 600 K. Compared to other elementary reaction steps, a slow decay observed with N + N → N₂ step under SS beam oscillation conditions demonstrates its contribution to the rate limiting nature of the overall reaction. Fast beam switching experiments have been performed alternately between O₂-lean and -rich conditions, thus highlighting the effectiveness of ¹⁵NO reduction in TS, irrespective of the beam composition. Possibly in a future technology initiative, this aspect could be exploited to manage more ¹⁵NO reduction on Pd-based catalysts.