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

CeO2 addition showed a notable improvement on the low temperature selective catalytic reduction (SCR) performance of MnOx/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 MnOx/TiO2 and MnOx-CeO2/TiO2 were obtained according to the steady state kinetic analysis. After comparing the reaction kinetic constants of NO reduction over MnOx/TiO2 and MnOx-CeO2/TiO2, the mechanism of the addition of CeO2 for NO reduction over MnOx/TiO2 was discovered according to the relationship between the reaction rate constants and the catalyst properties. Because the oxidation ability of MnOx/TiO2 increased, the rate constant of the SCR reaction over MnOx/TiO2 increased remarkably after CeO2 addition, resulting in a notable promotion of N-2 formation. The oxidation of NH3 to NH over MnOx/TiO2 required two Mn4+ cations on the adjacent sites. However, the probability of two Mn4+ cations occurring on the adjacent sites on MnOx/TiO2 obviously decreased after CeO2 addition, although the oxidation ability of MnOx/TiO2 increased. Therefore, the rate of N2O formation during NO reduction over MnOx/TiO2 did not vary notably after CeO2 addition. As a result, both the SCR activity and N-2 selectivity of NO reduction over MnOx/TiO2 improved after CeO2 addition.