Shangchao Xiong

Competition of selective catalytic reduction and non selective catalytic reduction over MnOx/TiO2 for NO removal: the relationship between gaseous NO concentration and N2O selectivity

In this work, a novel phenomenon was discovered that N2O selectivity of NO reduction over MnOx/TiO2 was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N2O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley–Rideal mechanism (the reaction of over-activated NH3 with gaseous NO) and the Langmuir–Hinshelwood mechanism (the reaction of adsorbed NO3− with adsorbed NH3 on the adjacent sites) could contribute to the formation of N2O. Kinetic study demonstrated that N2O selectivity would be independent of gaseous NO concentration if NO reduction over MnOx/TiO2 mainly followed the Langmuir–Hinshelwood mechanism. If NO reduction over MnOx/TiO2 mainly followed the Eley–Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more –NH2 was used to reduce gaseous NO to form N2 and the further oxidization of –NH2 to –NH was restrained, resulting in an obvious decrease of N2O selectivity. The Eley–Rideal mechanism played an important role in NO reduction over MnOx/TiO2, especially at higher temperatures. Therefore, N2O selectivity of the low temperature SCR reaction over MnOx/TiO2 decreased especially at higher temperatures after the increase of gaseous NO concentration.

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.

Effect of CeO2 for a high-efficiency CeO2/WO3–TiO2 catalyst on N2O formation in NH3-SCR: a kinetic study

In this study, we investigated the effects of CeO2 for a high-efficiency CeO2/WO3–TiO2 catalyst on N2O formation in NH3-SCR reaction using a kinetic method. The results demonstrated that CeO2 is very effective for enhancing the SCR (4NH3 + 4NO + O2 → 4N2 + 6H2O) reaction rate and thus suppressing the NSCR reaction (4NH3 + 4NO + 3O2 → 4N2O + 6H2O) due to the competition between these two reactions. Therefore, N2O formation under SCR reaction conditions could be remarkably inhibited with the addition of CeO2 to the catalyst. On the other hand, CeO2 can also enhance the C–O reaction (4NH3 + 5O2 → 4NO + 6H2O) over the catalyst, so N2O formation under NH3 oxidation conditions was promoted due to the lack of SCR reaction. In addition, we also found another interesting phenomenon wherein N2O formation over WO3–TiO2 (without CeO2) decreased gradually at high temperature from 350 to 450 °C, while N2O formation over CeO2/WO3–TiO2 kept increasing with the reaction temperature, which was also associated with the promotional effect of CeO2 on SCR reaction and the competition between SCR and NSCR reactions.

Role of WO3 in NO Reduction with NH3 over V2O5-WO3/TiO2: A New Insight from the Kinetic Study

WO3 role in NO reduction over V2O5-WO3/TiO2 was studied by comparing the kinetic parameters of the selective catalytic reduction (SCR) reaction, the non selective catalytic reduction (NSCR) reaction and the C–O reaction. The results show that the SCR reaction over V2O5/TiO2 was promoted after WO3 incorporation, while the NSCR reaction and the C–O reaction were both restrained.Graphical Abstract