Meiqing Shen

NH3-SCR over Cu/SAPO-34 catalysts with various acid contents and low Cu loading

Four Cu/SAPO-34 catalysts are used to research the effect of their acid content on their NH3-SCR activity, by changing the proportion of Si in the catalyst. Though the catalysts exhibit similar Cu loading performed by an ion-exchange method, they lead to different NO conversions in the temperature range of 120–600 °C. The NH3-TPD results show that varying the proportion of Si can adjust the acid content of SAPO-34. In the low temperature range, kinetic tests prove that the various acid contents did not affect the apparent activation energy (Ea) of the Selective Catalytic Reduction by NH3 (NH3-SCR) over Cu/SAPO-34. But it is found that the acid densities are related to the NO conversion at low temperature. In the high temperature range, increasing the acid content of the Cu/SAPO-34 catalysts inhibits the observed NH3 conversions in the NH3 oxidation results. As NH3 oxidation and NH3-SCR are competing reactions and the former can decrease NO conversion, this inhibition is therefore beneficial to NH3-SCR. Finally, the influence of the acid content of the catalysts on the NH3 oxidation is also investigated by kinetic tests.

The effect of synthesis methods on Cu species and active sites over Cu/SAPO-34 for NH3-SCR reaction

The changed status of copper species and active Cu sites of NH3-SCR over the Cu/SAPO-34 zeolites synthesized by three different methods has been investigated in this work. The combination of the EPR, H2-TPR and SEM results demonstrate that different synthesis methods do not affect the types of Cu species, but affect the distribution. The dominant Cu species is CuO-clusters for the precipitation sample, and it is isolated Cu2+ for the ion-exchange sample. While, for the one-pot catalyst, both isolated Cu2+ and surface CuO species are present in comparable amount. Additionally, hydrothermal treatment can also change the distribution of Cu species, it prompts the migration of Cu species from the external surface CuO into the ion exchange locations and forms more isolated Cu2+. The kinetic results show all the Cu/SAPO-34 catalysts display the same apparent activation energy and TOF values, indicating that the three synthetic methods do not change the reaction pathway of NH3-SCR and the active Cu sites (isolated Cu2+ in the vicinity of six-membered rings).

Catalytic performance and hydrothermal durability of CeO2–V2O5–ZrO2/WO3–TiO2 based NH3-SCR catalysts

Ceria modified V2O5–ZrO2/WO3–TiO2 catalysts with different Ce loading (0, 5, 10 wt%) have been evaluated in the NH3-SCR process before and after hydrothermal aging (750 °C, 10 wt% H2O/air for 12 h). Compared with only Zr containing catalysts, addition of Ce greatly enhances the low temperature activity of the fresh catalyst, but it deactivates obviously after aging. The catalysts were characterized by XRD, XPS, H2-TPR and DRIFTS. The results suggest that not only the enrichment of Ce3+ and the increased redox properties, but also the more active adsorbed nitrates on CeO2 modified catalysts benefit the SCR reaction. The catalyst deactivation after aging is mainly due to the sintering and segregation of CeO2 on the catalysts surface, suggesting the poor hydrothermal stability of the Ce component. However, additional provided NO2 will compensate for the activity loss due to hydrothermal aging and significantly improve the low temperature SCR activity, suggesting a high NO2 sensitivity of the Ce component. Moreover, it was found that a ceria, zirconia containing catalyst exhibits superior SCR activities, with both the fresh and aged 1%V2O5–10%CeO2–10%ZrO2/WO3–TiO2 catalysts showing high NOx conversions (>95%) and selectivity to N2 (>98%) in a wide temperature range of 200–500 °C, at a space velocity of 120 000 h−1 in a simulated exhaust containing 500 ppm NOx (NO2/NO = 1) and 5% H2O.

Effects of calcium substitute in LaMnO3 perovskites for NO catalytic oxidation

Abstract La1–xCaxMnO3 (x=0–0.3) perovskite-type oxides were synthesized by citrate sol-gel method. The physical and chemical properties were characterized by X-ray diffraction (XRD), Brumauer-Emmett-Teller method (BET), X-ray photoelectron spectroscopy (XPS), NO+O2-TPD (temperature-programmed desorption), activated oxygen evaluation and H2-TPR (temperature-programmed reduction) technologies. The results showed that NO catalytic oxidation activity was significantly improved by Ca substitution, especially for lower temperature activity. The La0.9Ca0.1MnO3 sample showed the maximum conversion of 82% at 300 °C. The monodentate nitrates played a crucial role for the formation of NO2. The reducibility of Mn4+ ions and reactivity of activated oxygen were favorable for the catalytic performances of NO oxidation.

Effect of iron doping into CeO2-ZrO2 on the properties and catalytic behaviour of Pd-only three-way catalyst for automotive emission control

Ce(0.67)Zr(0.33)O(2) doped with iron oxide was prepared and the corresponding Pd-only three-way catalysts were examined and characterized. Pd/CZFe(1%) catalyst exhibits the best catalytic performance for CO, HC, NO and NO(2) elimination and the widest operation window. The doping of iron oxide with 1% loading suggests the formation of more homogeneous Ce-Zr-Fe-O ternary solid solution, which seems to facilitate the reduction of Ce(4+)→Ce(3+) or the formation of oxygen vacancy and to promote the interaction between Ce-Zr and Fe. Moreover, the Ce redox behaviour for surface reduction suggests depending not only on the formation of homogeneous Ce-Zr-Fe-O but also on the surface property of the sample. The increase in the concentration of oxygen vacancies under all atmospheres for CZFe(1%) sample also results in the enhancement of oxygen storage complete capacity.

Preparation of FexCe1-xOy solid solution and its application in Pd-only three-way catalysts

Abstract FeOx-CeO2 mixed oxides with increasing Fe/(Ce+Fe) atomic ratio (1–20 mol%) were prepared by sol-gel method and characterized by X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) and Hydrogen temperature-programmed reduction (H2-TPR) techniques. The dynamic oxygen storage capacity (DOSC) was investigated by mass spectrometry with CO/O2 transient pulses. The powder XRD data following Rietveld refinement revealed that the solubility limit of iron oxides in the CeO2 was 5 mol% based on Fe/(Ce+Fe). The lattice parameters experienced a decrease followed by an increase due to the influence of the maximum solubility limit of iron oxides in the CeO2. TPR analysis revealed that Fe introduction into ceria strongly modified the textual and structural properties, which influenced the oxygen handling properties. DOSC results revealed that Ce-based materials containing Fe oxides with multiple valences contribute to the majority of DOSC. The kinetic analysis indicated that the calculated apparent kinetic parameters obey the compensation effect. The three-way catalytic performance for Pd-only catalysts based on the Fe doping support exhibited the redundant iron species separated out of the CeO2 and interacted with the ceria and Pd species on the surface, which seriously influenced the catalytic properties, especially after hydrothermal aging treatment.

Redox behaviors and structural characteristics of Mn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox

Mn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox samples synthesized by sol-gel method were tested for redox properties through the dynamic oxygen storage measurement and characterized using X-ray diffraction, BET, electron paramagnetic resonance, and X-ray photoelectron spectroscopy. The results showed that redox performances of ceria-based materials could be enhanced by synergetic effects between Mn-O and Ce-O. Fresh and aged samples were characterized with the fluorite-type cubic structure similar to CeO2, and furthermore, the thermal stability of Mn0.1Ce0.9Ox materials was improved by the introduction of some Zr atoms. From XPS, it could be concluded that Mn2+/Mn3+ redox couples existed on the surface of Mn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox samples. Electron paramagnetic resonance researches revealed that there were three types of Mn2+ species: isolated Mn2+ substituting for Ce4+ ions in the lattice with a cubic symmetry, ones in defect with a noncubic symmetry, and at the surface of samples.

New insight into the promotion effect of Cu doped V2O5/WO3–TiO2 for low temperature NH3-SCR performance

The promotion effect of Cu on the V/WTi catalyst for the selective catalytic reduction of NOx by NH3 was investigated in the temperature range of 150–400 °C. The Cu addition shows a superior NH3-SCR performance in comparison with the V/WTi sample. The catalysts were characterized by XRD, Raman, EPR, H2-TPR, XPS, and in situ DRIFTS techniques. The obtained results reveal that the Cu oxides in close proximity to V oxides on the surface facilitate the formation of double redox couples of V5+/V4+ and Cu2+/Cu+, which may play a critical role in the superior NH3-SCR performance. The electronic interactions caused by the redox cycle of Cu2+ + V4+ ↔ V5+ + Cu+ could significantly improve the redox properties of the vanadium species, which is beneficial for the activation of NH3 species bound to the vanadium species. Moreover, the redox cycle of Cu2+ + V4+ ↔ V5+ + Cu+ induces the formation of high-activity nitrate species adsorbed on Cu species. The kinetic analysis reveals that the Cu doping induces the decrease of the activation energy (Ea) of NH3-SCR.

Deactivation mechanism of SO2 on Cu/SAPO-34 NH3-SCR catalysts: structure and active Cu2+

The deactivation mechanism of Cu/SAPO-34 ammonia selective catalytic reduction catalysts (NH3-SCR) by SO2 poisoning has been systematically investigated using a range of analytical techniques in order to study the influence on both the zeolitic framework and the active Cu2+ ions. The different sulfate samples were obtained by SO2 poisoning over Cu/SAPO-34 NH3-SCR catalysts as a function of time and concentration in the feed. The obtained results reveal that the SO2 poisoning could seriously decrease NO conversion during the whole temperature range (100–500 °C). The XRF results shows that there is almost no sulfur existing on the SAPO-34 support. The ex situ DRIFTS and BET results reveal that SO2 poisoning has a less-pronounced effect on its framework structure. The TPR and EPR results demonstrate that SO2 poisoning has a significant influence on the coordination environment and the content of the active isolated Cu2+ species. The kinetic results demonstrate that the SO2 poisoning does not influence the apparent activation energy (Ea) of the NH3-SCR reaction over Cu/SAPO-34 catalysts. The decline of the NH3-SCR activity is due to the reduction of the number of isolated Cu2+ ions.

The migration of Cu species over Cu–SAPO-34 and its effect on NH3 oxidation at high temperature

The influence of Cu species migration, through hydrothermal treatment, on the NH3 oxidation mechanism and its inhibition on NH3-SCR activity over Cu–SAPO-34 catalysts were studied. XRD, SEM, EPR, H2-TPR, CO-DRIFTS were conducted to estimate the Cu species distribution. The results revealed that the CuO species initially existed on the external surface of the fresh impregnated Cu/SAPO-34 catalyst, and converted to isolated Cu2+ and nanosized CuO in the cavity of the SAPO-34 support during hydrothermal treatment. The NH3-SCR activity over the hydrothermally treated Cu/SAPO-34 catalysts improved due to the increase of the isolated Cu2+ amount, while the NH3 oxidation activity declined. Furthermore, the NO amount generated in the NH3 oxidation reaction over the hydrothermally treated samples was much less than the fresh one, which was also caused by the migration of Cu species. In addition, the kinetic NH3 oxidation was performed at temperatures from 340 °C to 440 °C, and the results indicated that CuO species were the active sites for the NH3 oxidation. In conclusion, the Cu species migration caused the decrease of the NH3 oxidation activity and the great variation in the generated NO amount. Furthermore, it was proposed that the NH3 oxidation mechanism over the Cu/SAPO-34 catalysts contained two steps: firstly, molecular NH3 reacted with O2 at CuO sites; secondly, the generated NO was reduced by NH3 to N2 at isolated Cu2+ sites.