Promotion mechanism of CeO2 addition on the low temperature SCR reaction over MnOx/TiO2: a new insight from the kinetic study

Abstract

CeO₂ addition showed a notable improvement on the low temperature selective catalytic reduction (SCR) performance of MnO_x/TiO₂. In this work, a new insight into the promotion mechanism was established from the steady state kinetic study. The kinetic rate constants of the SCR reaction through the Eley–Rideal mechanism, those of the SCR reaction through the Langmuir–Hinshelwood mechanism, and those of the non-selective catalytic reduction (NSCR) reaction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ were obtained according to the steady state kinetic analysis. After comparing the reaction kinetic constants of NO reduction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂, the mechanism of the addition of CeO₂ for NO reduction over MnO_x/TiO₂ was discovered according to the relationship between the reaction rate constants and the catalyst properties. Because the oxidation ability of MnO_x/TiO₂ increased, the rate constant of the SCR reaction over MnO_x/TiO₂ increased remarkably after CeO₂ addition, resulting in a notable promotion of N₂ formation. The oxidation of NH₃ to NH over MnO_x/TiO₂ required two Mn⁴⁺ cations on the adjacent sites. However, the probability of two Mn⁴⁺ cations occurring on the adjacent sites on MnO_x/TiO₂ obviously decreased after CeO₂ addition, although the oxidation ability of MnO_x/TiO₂ increased. Therefore, the rate of N₂O formation during NO reduction over MnO_x/TiO₂ did not vary notably after CeO₂ addition. As a result, both the SCR activity and N₂ selectivity of NO reduction over MnO_x/TiO₂ improved after CeO₂ addition.