Hao Dang

The mechanism of the effect of H2O on the low temperature selective catalytic reduction of NO with NH3 over Mn–Fe spinel

H2O showed a notable inhibition on the low temperature selective catalytic reduction (SCR) reaction over Mn based catalysts. However, the mechanism of H2O effect was not clear. In this work, the mechanism of H2O effect on the low temperature SCR reaction over Mn–Fe spinel was studied using the transient reaction study and the steady-state kinetic analysis. According to the steady-state kinetic analysis, the reaction kinetic rate constants of NO reduction over Mn–Fe spinel (including the rate constants of N2 formation through the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism, and the rate constants of N2O formation) in the presence of H2O and in the absence of H2O were compared. According to the transient reaction study, the effect of H2O on the elementary reactions of NO reduction over Mn–Fe spinel through both the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism was investigated. The results indicated that the effect of H2O on the low temperature SCR reaction over Mn–Fe spinel was not only attributed to the competition adsorption of H2O with NH3 and NOx, but also related to the decrease in the oxidation ability and the inhibition of the interface reaction.

The centralized control of elemental mercury emission from the flue gas by a magnetic rengenerable Fe–Ti–Mn spinel

A magnetic Fe-Ti-Mn spinel was developed to adsorb gaseous Hg(0) in our previous study. However, it is currently extremely restricted in the control of Hg(0) emission from the flue gas for at least three reasons: sorbent recovery, sorbent regeneration and the interference of the chemical composition in the flue gas. Therefore, the effect of SO2 and H2O on the adsorption of gaseous Hg(0) on the Fe-Ti-Mn spinel and the regeneration of spent Fe-Ti-Mn spinel were investigated in this study. Meanwhile, the procedure of the centralized control of Hg(0) emission from the flue gas by the magnetic Fe-Ti-Mn spinel has been analyzed for industrial application. The spent Fe-Ti-Mn spinel can be regenerated by water washing followed by the thermal treatment at 450 °C with no obvious decrease of its ability for Hg(0) capture. Meanwhile, gaseous Hg(0) in the flue gas can be remarkably concentrated during the regeneration, facilitating its safe disposal. Initial pilot test demonstrated that gaseous Hg(0) in the real flue gas can be concentrated at least 100 times by the Fe-Ti-Mn spinel. Therefore, Fe-Ti-Mn spinel was a novel magnetic regenerable sorbent, which can be used for the centralized control of Hg(0) emission from the flue gas.

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.

Elemental Mercury Oxidation over Fe–Ti–Mn Spinel: Performance, Mechanism, and Reaction Kinetics

The design of a high-performance catalyst for Hg0 oxidation and predicting the extent of Hg0 oxidation are both extremely limited due to the uncertainties of the reaction mechanism and the reaction kinetics. In this work, Fe-Ti-Mn spinel was developed as a high-performance catalyst for Hg0 oxidation, and the reaction mechanism and the reaction kinetics of Hg0 oxidation over Fe-Ti-Mn spinel were studied. The reaction orders of Hg0 oxidation over Fe-Ti-Mn spinel with respect to gaseous Hg0 concentration and gaseous HCl concentration were approximately 1 and 0, respectively. Therefore, Hg0 oxidation over Fe-Ti-Mn spinel mainly followed the Eley-Rideal mechanism (i.e., the reaction of gaseous Hg0 with adsorbed HCl), and the rate of Hg0 oxidation mainly depended on Cl• concentration on the surface. As H2O, SO2, and NO not only inhibited Cl• formation on the surface but also interfered with the interface reaction between gaseous Hg0 and Cl• on the surface, Hg0 oxidation over Fe-Ti-Mn spinel was obviously inhibited in the presence of H2O, SO2, and NO. Furthermore, the extent of Hg0 oxidation over Fe-Ti-Mn spinel can be predicted according to the kinetic parameter kE-R, and the predicted result was consistent with the experimental result.