Abstract
The selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 has been studied by operando DRIFT spectroscopy on four Cu-faujasite catalysts, Cu(0.12)Na-FAU, Cu(0.14)H-FAU, Cu(0.16)Ba-FAU, and Cu(0.38)Na-FAU. From steady-state SCR activity at 523, 623, and 723 K with NO/NH3/O2/He (0.2/0.2/3.0/96.6, v/v), the feed was switched to NO/O2/He (0.2/3.0/96.8), and after stabilization to NO/NH3/He (0.2/0.2/99.6). The aim was to simulate the SCR catalytic cycle, which obeys a redox couple Cu+/Cu2+. The changes of the Cu oxidation state were identified by following an IR band near 900 cm-1, assigned to an internal vibration of [Cu-O-Cu]2+, or to a modification of adjacent T-O-T framework vibrations induced by [Cu-O-Cu]2+. At the steady state of the SCR at 523 K, there is no band visible near 900 cm-1 providing evidence that [Cu-O-Cu]2+ is reduced as Cu+. Moreover, the redox cycle with NO+O2 and NO+NH3 shows that the oxidation of Cu+ to [Cu-O-Cu]2+ by NO+O2 takes much longer than the reduction of [Cu-O-Cu]2+ by NO+NH3 at this temperature. These experiments demonstrate that the oxidation is the ratedetermining step when the SCR is carried out at low temperature. At 723 K, DRIFT examinations of the SCR at steady state shows that the catalysts are mainly composed of Cu oxo species, for example, [Cu-O-Cu]2+ with the band near 900 cm-1. On the other hand, the redox cycle with NO+O2 and NO+NH3 indicates that the oxidation became much faster than reduction. Therefore, and in contrast with the reaction at 523 K, the reduction of Cu2+ to Cu+ is the rate-determining step in the SCR at 723 K. The study of the redox properties of the various Cu-FAU catalysts by NO+O2 and NO+NH3 demonstrates that the high activity at low temperature of Cu(0.38)Na-FAU and Cu(0.16)Ba-FAU can be mainly described in terms of a fine-tuning of Cu redox properties