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 N2 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 N2 selectivity of NO reduction over MnOx/TiO2 improved after CeO2 addition.

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