Huazhen Chang

Improvement of Activity and SO2 Tolerance of Sn-Modified MnOx–CeO2 Catalysts for NH3-SCR at Low Temperatures

The performances of fresh and sulfated MnOx-CeO₂ catalysts for selective catalytic reduction of NOx by NH₃ (NH₃-SCR) in a low-temperature range (T < 300 °C) were investigated. Characterization of these catalysts aimed at elucidating the role of additive and the effect of sulfation. The catalyst having a Sn:Mn:Ce = 1:4:5 molar ratio showed the widest SCR activity improvement with near 100% NOx conversion at 110-230 °C. Raman and X-ray photoelectron spectroscopy (XPS) indicated that Sn modification significantly increases the concentration of oxygen vacancies that may facilitate NO oxidation to NO₂. NH₃-TPD characterization showed that the low-temperature NH₃-SCR activity is well correlated with surface acidity for NH3 adsorption, which is also enhanced by Sn modification. Furthermore, as compared to MnOx-CeO₂, Sn-modified MnOx-CeO₂ showed remarkably improved tolerance to SO₂ sulfation and to the combined effect of SO₂ and H₂O. In the presence of SO₂ and H₂O, the Sn-modified MnOx-CeO₂ catalyst gave 62% and 94% NOx conversions as compared to 18% and 56% over MnOx-CeO₂ at temperatures of 110 and 220 °C, respectively. Sulfation of SnO₂-modified MnOx-CeO₂ may form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO₂ during NH₃-SCR at T > 200 °C.

Substitution of WO3 in V2O5/WO3–TiO2 by Fe2O3 for selective catalytic reduction of NO with NH3

To improve N2 selectivity and lower the cost, WO3 in V2O5/WO3–TiO2 was substituted by a low cost composition Fe2O3 for selective catalytic reduction (SCR) of NO with NH3. The SCR reaction over V2O5/Fe2O3–TiO2 mainly followed the Eley–Rideal mechanism (i.e. the reaction between activated ammonia species and gaseous NO). There were two active components on V2O5/WO3–TiO2 for the activation of adsorbed NH3 (i.e. V5+ and Fe3+). The acid sites on V2O5/Fe2O3–TiO2 mainly resulted from the support Fe2O3–TiO2, so the adsorbed NH3 preferred to be activated by Fe3+ rather than by V5+. V5+ on V2O5/Fe2O3–TiO2 could accelerate the regeneration of Fe3+ on Fe2O3–TiO2 due to the rapid electron transfer between V5+ and Fe2+ on the surface, so the activation of adsorbed NH3 by Fe3+ was promoted. As some NH3 adsorbed on V2O5/Fe2O3–TiO2 was not activated by Fe3+, the inactivated NH3 could then be activated by V5+ on the surface. As a result, 2% V2O5/Fe2O3–TiO2 showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 300–450 °C. Furthermore, the emission of 2% V2O5/Fe2O3–TiO2 to the fly ash can be prevented by an external magnetic field due to its inherent magnetization. Therefore, 2% V2O5/Fe2O3–TiO2 could be a promising low-cost catalyst in NO emission control.

Design Strategies for P-Containing Fuels Adaptable CeO2–MoO3 Catalysts for DeNOx: Significance of Phosphorus Resistance and N2 Selectivity

Phosphorus compounds from flue gas have a significant deactivation effect on selective catalytic reduction (SCR) DeNOx catalysts. In this work, the effects of phosphorus over three catalysts (CeO2, CeO2-MoO3, and V2O5-MoO3/TiO2) for NH3-SCR were studied, and characterizations were performed aiming at a better understanding of the behavior and poisoning mechanism of phosphorus over SCR catalysts. The CeO2-MoO3 catalyst showed much better catalytic behavior with respect to resistance to phosphorus and N2 selectivity compared with V2O5-MoO3/TiO2 catalyst. With addition of 1.3 wt % P, the SCR activity of V2O5-MoO3/TiO2 decreased dramatically at low temperature due to the impairment of redox property for NO oxidation; meanwhile, the activity over CeO2 and CeO2-MoO3 catalysts was improved. The superior NO oxidation activity contributes to the activity over P-poisoned CeO2 catalyst. The increased surface area and abundant acidity sites contribute to excellent activity over CeO2-MoO3 catalyst. As the content of P increased to 3.9 wt %, the redox cycle over CeO2 catalyst (2CeO2 ↔ Ce2O3 + O*) was destroyed as phosphate accumulated, leading to the decline of SCR activity; whereas, more than 80% NOx conversion and superior N2 selectivity were obtained over CeO2-MoO3 at T > 300 °C. The effect of phosphorus was correlated with the redox properties of SCR catalyst for NH3 and NO oxidation. A spillover effect that phosphate transfers from Ce to Mo in calcination was proposed.

Design Strategies for CeO2–MoO3 Catalysts for DeNOx and Hg0 Oxidation in the Presence of HCl: The Significance of the Surface Acid–Base Properties

A series of CeMoOx catalysts with different surface Ce/Mo ratios was synthesized by a coprecipitation method via changing precipitation pH value. The surface basicity on selective catalytic reduction (SCR) catalysts (CeMoOx and VMo/Ti) was characterized and correlated to the durability and activity of catalyst for simultaneous elimination of NOx and Hg(0). The pH value in the preparation process affected the surface concentrations of Ce and Mo, the Brunauer-Emmett-Teller (BET) specific surface area, and the acid-base properties over the CeMoOx catalysts. The O 1s X-ray photoelectron spectroscopy (XPS) spectra and CO2-temperature programmed desorption (TPD) suggested that the surface basicity increased as the pH value increased. The existence of strong basic sites contributed to the deactivation effect of HCl over the VMo/Ti and CeMoOx catalysts prepared at pH = 12. For the CeMoOx catalysts prepared at pH = 9 and 6, the appearance of surface molybdena species replaced the surface -OH, and the existence of appropriate medium-strength basic sites contributed to their resistance to HCl poisoning in the SCR reaction. Moreover, these sites facilitated the adsorption and activation of HCl and enhanced Hg(0) oxidation. On the other hand, the inhibitory effect of NH3 on Hg(0) oxidation was correlated with the competitive adsorption of NH3 and Hg(0) on acidic surface sites. Therefore, acidic surface sites may play an important role in Hg(0) adsorption. The characterization and balance of basicity and acidity of an SCR catalyst is believed to be helpful in preventing deactivation by acid gas in the SCR reaction and simultaneous Hg(0) oxidation.

A novel magnetic Fe–Ti–V spinel catalyst for the selective catalytic reduction of NO with NH3 in a broad temperature range

Fe–Ti–V spinel showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 250–400 °C, and an external magnetic field can effectively prevent the emission of a vanadium based catalyst to the environment due to its magnetization.

Comparison of preparation methods for ceria catalyst and the effect of surface and bulk sulfates on its activity toward NH3-SCR.

A series of CeO2 catalysts prepared with sulfate (S) and nitrate (N) precursors by hydrothermal (H) and precipitation (P) methods were investigated in selective catalytic reduction of NOx by NH3 (NH3-SCR). The catalytic activity of CeO2 was significantly affected by the preparation methods and the precursor type. CeO2-SH, which was prepared by hydrothermal method with cerium (IV) sulfate as a precursor, showed excellent SCR activity and high N2 selectivity in the temperature range of 230-450 °C. Based on the results obtained by temperature-programmed reduction (H2-TPR), transmission infrared spectra (IR) and thermal gravimetric analysis (TGA), the excellent performance of CeO2-SH was correlated with the surface sulfate species formed in the hydrothermal reaction. These results indicated that sulfate species bind with Ce(4+) on the CeO2-SH catalyst, and the specific sulfate species, such as Ce(SO4)2 or CeOSO4, were formed. The adsorption of NH3 was promoted by these sulfate species, and the probability of immediate oxidation of NH3 to N2O on Ce(4+) was reduced. Accordingly, the selective oxidation of NH3 was enhanced, which contributed to the high N2 selectivity in the SCR reaction. However, the location of sulfate on the CeO2-SP catalyst was different. Plenty of sulfate species were likely deposited on CeO2-SP surface, covering the active sites for NO oxidation, which resulted in poor SCR activity in the test temperature range. Moreover, the resistance to alkali metals, such as Na and K, was improved over the CeO2-SH catalyst.

Chemical poison and regeneration of SCR catalysts for NOx removal from stationary sources

Selective catalytic reduction (SCR) of NOx with NH3 is an effective technique to remove NOx from stationary sources, such as coal-fired power plant and industrial boilers. Some of elements in the fly ash deactivate the catalyst due to strong chemisorptions on the active sites. The poisons may act by simply blocking active sites or alter the adsorption behaviors of reactants and products by an electronic interaction. This review is mainly focused on the chemical poisoning on V2O5-based catalysts, environmental-benign catalysts and low temperature catalysts. Several common poisons including alkali/alkaline earth metals, SO2 and heavy metals etc. are referred and their poisoning mechanisms on catalysts are discussed. The regeneration methods of poisoned catalysts and the development of poison-resistance catalysts are also compared and analyzed. Finally, future research directions in developing poisoning resistance catalysts and facile efficient regeneration methods for SCR catalysts are proposed.

Reaction pathway investigation on the selective catalytic reduction of NO with NH3 over Cu/SSZ-13 at low temperatures.

The mechanism of the selective catalytic reduction of NO with NH3 was studied using Cu/SSZ-13. The adspecies of NO and NH3 as well as the active intermediates were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy and temperature-programmed surface reaction. The results revealed that three reactions were possible between adsorbed NH3 and NOx. NO2(-) could be generated by direct formation or NO3(-) reduction via NO. In a standard selective catalytic reduction (SCR) reaction, NO3(-) was hard to form, because NO2(-) was consumed by ammonia before it could be further oxidized to nitrates. Additionally, adsorbed NH3 on the Lewis acid site was more active than NH4(+). Thus, SCR mainly followed the reaction between Lewis acid site-adsorbed NH3 and directly formed NO2(-). Higher Cu loading could favor the formation of active Cu-NH3, Cu-NO2(-), and Cu-NO3(-), improving the SCR activity at low temperature.

The outstanding performance of LDH-derived mixed oxide Mn/CoAlOx for Hg0 oxidation

Layered double hydroxide (LDH)-derived Mn/CoAlOx catalysts prepared by coprecipitation showed excellent performance in the oxidation of elemental mercury (Hg0). The Hg0 conversion on 1Mn/CoAlOx was above 95% at 200–350 °C under a space velocity (GHSV) of 480000 h−1. Mn dopants dramatically improved both the adsorption capacity and oxidation ability of Hg0 due to the enhancement of surface oxygen species and surface-exposed transition metal cations. Correlations between the Mn dopants, the adsorption of catalysts for Hg0 oxidation and the redox behaviors of the catalysts were established. The results revealed that two chemical reaction equilibria may have existed between adsorbed Hg0 and O2 on the surface of Mn/CoAlOx. HCl promoted the chemical reaction equilibria to the positive direction. 1Mn/CoAlOx catalysts had a certain ability to resist water and sulfur poisoning. These LDH-derived Mn/CoAlOx catalysts were first reported for Hg0 oxidation in flue gas, which could be used for practical applications.

Identification of active sites and reaction mechanism on low-temperature SCR activity over Cu-SSZ-13 catalysts prepared by different methods

Cu-SSZ-13 catalysts with similar Si/Al and Cu/Al ratios were prepared by aqueous solution ion-exchange (Cu-SSZ-13-I) and one-pot synthesis (Cu-SSZ-13-O) methods. NH3-SCR tests, XRD, BET, SEM, HRTEM, EPR, H2-TPR, NH3-TPD, NO + O2-TPD, in situ DRIFTS, and kinetic tests were performed for the catalytic measurement, bulk characterization and mechanism estimation. The NH3-SCR results indicated that Cu-SSZ-13-O showed higher DeNOx activity than Cu-SSZ-13-I in the absence or presence of H2O across the entire temperature range. The results of EPR, H2-TPR and NO + O2-TPD showed that more Cu2+ ions existed in Cu-SSZ-13-O, which mainly accounts for the Lewis acid sites and the majority of the NOx adsorption or activation. The DRIFTS results showed that NH3 on Lewis acid sites was more active than that on Bronsted acid sites in the NH3-SCR reaction. Furthermore, the DRIFTS results also indicated that monodentate nitrates are the most active nitrate species. Compared with Cu-SSZ-13-I, Cu-SSZ-13-O showed stronger Lewis acid site strengths and had more abundant monodentate nitrate species. Therefore, the NH3-SCR reaction proceeded more easily over Cu-SSZ-13-O in comparison with Cu-SSZ-13-I. This could be the key factor that explains the more excellent low-temperature SCR activity of Cu-SSZ-13-O. In addition, at low reaction temperatures, Cu-SSZ-13-O was less affected by pore (i.e., intracrystalline) diffusion limitations.