Interaction between Nickel Oxide and Support Promotes Selective Catalytic Reduction of NOx with C3 H6.

Selective catalytic reduction of nitrogen oxides (NOx ) with C3 H6 (C3 H6 -SCR) was investigated over NiO catalysts supported on different metal-oxides. A NiAlOx mixed oxide phase was formed over NiO/γ-Al2 O3 catalyst, inducing an immediate interaction between NiOx and AlOx species. Such interaction resulted in a charge transfer from Ni to Al site and the formation of Ni species in high oxidation state. In comparison to other NiO-loaded catalysts, NiO/γ-Al2 O3 catalyst exhibited the highest NOx conversion at temperature higher than 450 °C, but a poor C3 H6 oxidation activity due to the decreased nucleophilicity for surface oxygen species. By temperature-programed NO oxidation, it is indicated that nitrate species were rapidly formed and stably maintained at high temperature over NiO/γ-Al2 O3 catalyst. In situ transient reactions further verified the Langmuir-Hinshelwood mechanism for C3 H6 -SCR, where both gaseous NO and C3 H6 were adsorbed and activated on catalyst surface and reacted to generate N2 . Due to the strong metal-support interaction over NiO/γ-Al2 O3 catalyst, both nitrate and Cx Hy Oz intermediates were well preserved to attain high C3 H6 -SCR activity.

Novel Methods for Assessing the SO2 Poisoning Effect and Thermal Regeneration Possibility of MOx-WO3/TiO2 (M = Fe, Mn, Cu, and V) Catalysts for NH3-SCR.

In this study, the sulfur resistance and thermal regeneration of a series of MOx-WO3/TiO2 (denoted as MW/Ti, M = Fe, Mn, Cu, V) catalysts were investigated. After in situ sulfur poisoning, the selective catalytic reduction (SCR) activity of the poisoned catalysts was inhibited at low temperatures but was promoted at high temperatures. After thermal regeneration, the FeW/Ti catalyst was more thoroughly regenerated among nonvanadium-based catalysts. To investigate the impacts of sulfur poisoning, characterizations including X-ray diffraction, thermogravimetric analysis, H2 temperature-programmed reduction, and SO2 temperature-programmed desorption were applied. It was discovered that different sulfur-containing species blocked the adsorption of NH3/NO to a distinct extent over all of the catalysts, thus affecting the catalytic activity. The effect depends on which are dominant (NO or NH3) during the reaction at different temperatures. The difference in regeneration depends on the formation of sulfate species. The ratio of Mx(SO4)y to NH4HSO4 generated on the catalysts was adopted to assess the possibility of regeneration. The ratios were 0.5, 1.4, 1.5, and 1.7 for VW/Ti, FeW/Ti, CuW/Ti, and MnW/Ti catalysts, respectively. The lower the ratio was, the easier the catalyst could be regenerated. Meanwhile, the sulfate species could be decomposed more easily on the poisoned FeW/Ti catalyst. FeW/Ti is an excellent candidate for low- and medium-temperature NH3-SCR among nonvanadium-based catalysts.

Low-temperature selective catalytic reduction of N2O by CO over Fe-ZSM-5 catalysts in the presence of O2.

Nitrous oxide (N2O) is an important ozone-depletion substance and greenhouse gas. Selective catalytic reduction (SCR) of N2O by CO is considered an effective method for N2O elimination. However, O2 exhibited a significant inhibition effect on the catalytic performance of N2O reduction by CO. A series of iron-based catalysts were prepared to investigate the effect of O2 in SCR of N2O by CO. The Fe-Z-pH2 (Fe-ZSM-5 ion-exchanged under pH of 2) catalyst manifested superior activity at low temperature and excellent O2 resistance in N2O reduction process. The characterization results from UV-vis DR spectra and XPS indicated that α-sites are the main active sites in Fe-Z-pH2, and they were inert to O2 but highly active to N2O. It could be concluded that the competition effect between N2O and O2 was very important over different catalysts. O2 is more prevalent over α-Fe2O3 catalyst, while N2O dominates over Fe-Z-pH2 catalyst. Moreover, in the presence of O2, Fe-Z-pH2 exhibited better performance for N2O removal than non-noble mixed oxide catalysts, which might broaden the application of low-temperature SCR of N2O by CO.

[CO-SCR Performance and Mechanism over Co3O4 Catalysts].

Nitrogen oxide (NOx) is an important precursor for many air pollution problems such as fine particulate matter and ground-level ozone. Because air pollution levels increase daily, it is important to control NOx emissions from industrial boiler flue gas. A series of different Co3O4 catalysts was prepared in this study by different methods. The effects of the preparation methods on selective catalytic reduction of NO by CO (CO-SCR) were investigated. The catalysts were characterized by BET, XRD, HR-TEM, and Raman. The results show that the Co3O4-S catalyst, prepared by solid grinding with cobalt sulfate as the precursor, had better CO-SCR activity and H2O resistance and that Co3O4-C, prepared by solid grinding with cobalt acetate as the precursor, showed excellent H2O resistance. The NO oxidation results showed that better NO oxidation activity over the catalysts is an important reason for the improved CO-SCR activity. The Raman results indicate that more Co2+ ions appeared on the surface of Co3O4-S, which benefited the formation of oxygen vacancies. The H2-TPR results showed better redox property of the Co3O4-S catalyst. The HR-TEM results shoes that the (111) and (220) crystal planes were exposed mainly on Co3O4-S and Co3O4-O and that more (220) crystal planes are conducive to improved reaction.

Cu/SAPO-34 prepared by a facile ball milling method for enhanced catalytic performance in the selective catalytic reduction of NOx with NH3.

Cu/SAPO-34 catalysts were prepared by different solid-state ion exchange methods, i.e., mechanical mixing (Cu/SAPO-34-M) and ball milling (Cu/SAPO-34-B), and were used for selective catalytic reduction of NOx with NH3 (NH3-SCR) reaction. Compared with Cu/SAPO-34-M, Cu/SAPO-34-B exhibited more excellent NH3-SCR catalytic activity. Various characterization methods, including XRD, SEM, N2 adsorption-desorption, UV-vis, H2-TPR, EPR, NO + O2-TPD, NH3-TPD, and in situ DRIFTS, were used to elucidate their different catalytic performances. The characterization results suggested that ball milling could be beneficial for increasing the amount of isolated Cu2+ ions in the Cu/SAPO-34 catalyst. In comparison with Cu/SAPO-34-M, Cu/SAPO-34-B had more isolated Cu2+ ions, which mainly contributed to the NOx adsorption and Lewis acid sites. Furthermore, ball milling could improve the redox property of the Cu/SAPO-34 catalyst. The in situ DRIFTS results verified that NH3 adsorbed on Lewis acid sites were more active than those adsorbed on Brønsted acid sites. Therefore, it was believed that ball milling was a suitable method to prepare more efficient Cu/SAPO-34 catalysts for NOx removal from diesel exhaust.

Heterogeneous activation of persulfate by Co3O4-CeO2 catalyst for diclofenac removal.

A series of Co3O4-CeO2 mixed metal oxides were synthesized by co-precipitation method and successfully used to activate persulfate for diclofenac removal. The effects of Co:Ce mole ratio, calcination temperature and calcination time on the catalytic activities were investigated. Results showed that the activity of Co3O4-CeO2 catalysts increased with Co:Ce mole ratio from 1:9 to 7:3, and decreased with the calcination temperature from 300 to 800 °C. 90% diclofenac was removed with Co7Ce3-300-1 catalyst (Co:Ce = 7:3, calcinated at 300 °C for 1 h) after 15 min. Moreover, short calcination time and low temperature resulted in smaller crystallite size, more structural defects, more active crystal surfaces and larger surface area of the catalyst, which led to higher removal efficiency of diclofenac. The high ratios of Co2+/Co3+, Ce3+/Ce4+ and Oads/Olatt were very important to enhance the catalytic activity. Finally, a potential reaction mechanism was proposed based on characterization of the fresh and spent catalysts.

New Insight into SO2 Poisoning and Regeneration of CeO2-WO3/TiO2 and V2O5-WO3/TiO2 Catalysts for Low-Temperature NH3-SCR.

In this study, the poisoning effects of SO2 on the V2O5-WO3/TiO2 (1%VWTi) and CeO2-WO3/TiO2 (5%CeWTi) selective catalytic reduction (SCR) catalysts were investigated in the presence of steam, and also the regeneration of deactivated catalysts was studied. After pretreating the catalysts in a flow of NH3 + SO2 + H2O + O2 at 200 °C for 24 h, it was observed that the low-temperature SCR (LT-SCR) activity decreased significantly over the 1%VWTi and 5%CeWTi catalysts. For 1%VWTi, NH4HSO4 (ABS) was the main product detected after the poisoning process. Both of NH4HSO4 and cerium sulfate species were formed on the poisoned 5%CeWTi catalyst, indicating that SO2 reacted with Ce3+/Ce4+, even in the presence of high concentration of NH3. The decrease of BET specific surface area, NO x adsorption capacity, the ratio of chemisorbed oxygen, and reducibility were responsible for the irreversible deactivation of the poisoned 5%CeWTi catalyst. Meanwhile, the LT-SCR activity could be recovered over the poisoned 1%VWTi after regeneration at 400 °C, but not for the 5%CeWTi catalyst. For industrial application, it is suggested that the regeneration process can be utilized for 1%VWTi catalysts after a period of time after NH4HSO4 accumulated on the catalysts.

Excellent Activity and Selectivity of One-Pot Synthesized Cu-SSZ-13 Catalyst in the Selective Catalytic Oxidation of Ammonia to Nitrogen.

One-pot synthesized Cu-SSZ-13 catalyst treated with dilute HNO3 achieved superior activity and selectivity in the selective catalytic oxidation (SCO) of NH3 to nitrogen, in comparison with other zeolite-based catalysts and most of metal-oxide catalysts. Furthermore, the catalyst showed the similar or even higher catalytic activity than the partial noble-metal catalysts, and meanwhile its N2 selectivity was superior to most noble-metal catalysts. The characterization results demonstrated that more Cu2+ ions existing in Cu-SSZ-13 catalyst were advantageous to its NH3-SCO activity. The in situ DRIFTS results indicated that the reactivity of NH3 species adsorbed on Lewis and Brønsted acid sites over Cu-SSZ-13-O-H catalyst depended on the reaction temperature. The results of this study suggest the one-pot synthesized Cu-SSZ-13 is a promising NH3-SCO catalyst for practical application, either mobile or stationary pollution sources.

Diclofenac degradation in water by FeCeOx catalyzed H2O2: Influencing factors, mechanism and pathways.

The degradation of diclofenac in a like Fenton system, FeCeOx-H2O2, was studied in details. The influencing factors, reaction kinetics, reaction mechanism and degradation pathways of diclofenac were investigated. The optimum conditions were at a solution pH of 5.0, H2O2 concentration of 3.0mmol/L, diclofenac initial concentration of 0.07mmol/L, FeCeOx dosage of 0.5g/L, and 84% degradation of diclofenac was achieved within 40min. The kinetics of FeCeOx catalyzed H2O2 process involved adsorption-dominating and degradation-dominating stages and fitted pseudo-second order model and pseudo-first order model, respectively. Singlet oxygen 1O2 was the primary intermediate oxidative species in the degradation process; superoxide radical anion O2- also participated in the reaction. The surface cerium and iron sites and the oxygen vacancies in the FeCeOx catalyst were proposed to play an important role in H2O2 decomposition and active species generation. The detected intermediates were identified as hydroxylated derivatives (m/z of 310, 326 and 298), quinone imine compounds (m/z of 308, 278 and 264) and hydroxyl phenylamine (m/z of 178). The majority intermediates were hydroxylated derivatives and the minority was hydroxyl phenylamine. The degradation pathways were proposed to involve hydroxylation, decarboxylation, dehydrogenation and CN bond cleavage.

NiFe(C2O4)x as a heterogeneous Fenton catalyst for removal of methyl orange.

This paper studies a heterogeneous Fenton catalyst NiFe(C2O4)x, which showed better catalytic activity than Ni(C2O4)x and better re-usability than Fe(C2O4)x. The methyl orange removal efficiency was 98% in heterogeneous Fenton system using NiFe(C2O4)x. The prepared NiFe(C2O4)x had a laminated shape and the size was in the range of 2-4 µm, and Ni was doped into catalyst's structure successfully. The NiFe(C2O4)x had a synergistic effect of catalyst of 24.7 for methyl orange removal, and the dope of Ni significantly reduced the leaching of Fe by 77%. The reaction factors and kinetics were investigated. Under the optimal conditions, 0.4 g/L of catalyst dose and 10 mmol/L of hydrogen peroxide concentration, 98% of methyl orange was removed within 20 min. Analysis showed that hydroxyl radicals and superoxide radicals participated in the reaction. With NiFe(C2O4)x catalyst, the suitable pH range for heterogeneous Fenton system was wide from 3 to 10. The catalyst showed good efficiency after five times re-use. NiFe(C2O4)x provided great potential in treatment of refractory wastewater with excellent property.

Preparation of a magnetic N-Fe/AC catalyst for aqueous pharmaceutical treatment in heterogeneous sonication system.

High efficiency and facile separation are desirable for catalysts used in water treatment. In this study, a magnetic catalyst (nitrogen doped iron/activated carbon) was prepared and used for pharmaceutical wastewater treatment. The catalyst was characterized using BET, SEM, XRD, VSM and XPS. Results showed that iron and nitrogen were successfully loaded and doped, magnetic Fe2N was formed, large amount of active surface oxygen and Fe(II) were detected, and the catalyst could be easily separated from water. Diclofenac was then degraded using the catalyst in ultrasound system. The catalyst showed high catalytic activity with 95% diclofenac removal. Analysis showed that ·OH attack of diclofenac was a main pathway, and then ·OH generation mechanism was clarified. The effects of catalyst dosage, sonication time, ultrasonic density, initial pH, and inorganic anions on diclofenac degradation were studied. Sulfate anion enhanced the degradation of diclofenac. Mechanism in the catalytic ultrasonic process was analyzed and reactions were clarified. Large quantity of oxidants was generated on the catalyst surface, including ·OH, O2-, O- and HO2·, which degraded diclofenac efficiently. In the solution and interior of cavitation bubbles, ·OH and "hot spot" effects contributed to the degradation of diclofenac. Reuse of the catalyst was further investigated to enhance its economy, and the catalyst maintained activity after seven uses.

Surface Tuning of La0.5Sr0.5CoO3 Perovskite Catalysts by Acetic Acid for NOx Storage and Reduction.

Selective dissolution of perovskite A site (A of ABO3 structure) was performed on the La1 - xSrxCoO3 catalysts for the NOx storage and reduction (NSR) reaction. The surface area of the catalysts were enhanced using dilute HNO3 impregnation to dissolve Sr. Inactive SrCO3 was removed effectively within 6 h, and the catalyst preserved the perovskite framework after 24 h of treatment. The tuned catalysts exhibited higher NSR performance (both NOx storage and NO-to-NO2 oxidation) under lean-burn and fuel-rich cycles at 250 °C. Large amounts of NOx adsorption were due to the increase of nitrate/nitrite species bonding to the A site and the growth of newly formed monodentate nitrate species. Nitrate species were stored stably on the partial exposed Sr(2+) cations. These exposed Sr(2+) cations played an important role on the NOx reduction by C3H6. High NO-to-NO2 oxidation ability was due to the generation of oxygen defects and Co(2+)-Co(3+) redox couples, which resulted from B-site exsolution induced by A-site dissolution. Hence, our method is facile to modify the surface structures of perovskite catalysts and provides a new strategy to obtain highly active catalysts for the NSR reaction.

Design Strategies for CeO2-MoO3 Catalysts for DeNOx and Hg(0) 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.

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.

A novel mechanism for poisoning of metal oxide SCR catalysts: base-acid explanation correlated with redox properties.

A novel mechanism is proposed for the poisoning effect of acid gases and N2O formation on SCR catalysts involving base-acid properties correlated with redox ability of M-O or M-OH (M = Ce or V) of metal oxides and the strength of their basicity responsible for resistance to HCl and SO2 at medium and low temperatures.

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.

Design strategies for P-containing fuels adaptable CeO2-MoO3 catalysts for DeNO(x): 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.

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.