NH3-SCR Activity Research Articles

Ce and Zr Modified WO3-TiO2 Catalysts for Selective Catalytic Reduction of NOx by NH3

A series of Ce and/or Zr modified WO3-TiO2 catalysts were synthesized by the impregnation method, which were employed for NH3-SCR reaction. The T50 contour lines of NOx were used to quickly optimize catalyst composition, the Ce20Zr12.5WTi catalyst was considered to be the optimization result, and also exhibited excellent NH3-SCR activity and thermal stability with broad operation temperature window, which is a very promising catalyst for NOx abatement from diesel engine exhaust. The excellent catalytic performance is associated with the formation of Ce-Zr solid solution. The introduction of Zr to CeWTi catalyst facilitated the redox of Ce4+/Ce3+ and the formation of more acid sites, more Ce3+ ions, more oxygen vacancies, larger quantities of surface adsorbed oxygen species and NH3, which were beneficial for the excellent selective catalytic reduction (SCR) performance.

Effect of calcination temperature on low-temperature NH3-SCR activity and the resistance of SO2 with or without H2O over Fe–Mn–Zr catalyst

Abstract A novel Fe–Mn–Zr was prepared by co-precipitation method and calcined at different temperature. The NO removal capacity, N2 selectivity and SO2 resistance with or without H2O over Fe–Mn–Zr were investigated in a fixed-bed microreactor. The results showed that the catalysts calcined at 400–600 °C exhibit 100% NO removal efficiency in the range of 170–360 °C. FMZ-500 shows a better tolerance to both 100 and 200 ppm SO2 than other samples and excellent resistance to H2O and can achieve over 93% NO removal efficiency in the presence of SO2 and H2O. FMZ-500 catalyst can completely recover NO removal capacity when SO2 and H2O shut off. When Fe–Mn–Zr is calcined at 700 and 800 °C, NO removal efficiency at 170 °C are 90% for FMZ-700 and 35% for FMZ-800 and their resistances to SO2 are very poor. When Fe–Mn–Zr is calcined at 400, 500 and 600 °C, MnO2, Mn2O3 and Fe2O3 are homogeneously dispersed and exist mainly in an amorphous state as well as similar reduction properties. When the calcination temperature increases to 800 °C, the crystallinity of oxides significantly increases and reduction temperatures shift from 318 and 413 °C to 420 and 478 °C. High temperature calcination causes the decrease of surface area and Lewisacid sites, corresponding to low NH3 adsorption.

The effects of tungsten and hydrothermal aging in promoting NH3-SCR activity on V2O5/WO3-TiO2 catalysts

The fresh catalysts of V2O5-TiO2 (VTi) and V2O5/WO3-TiO2 (VWTi) were prepared via an impregnation method. The hydrothermal aged samples were obtained at 750 °C for 24 h under airflow with 10 vol% water vapor. Compared to the fresh catalysts, the catalytic performance of the aged catalysts (VTi-A and VWTi-A) has been significantly improved. Particularly, VWTi-A displayed the best activity over a wide temperature range (NOx conversion: >90% @ 250–550 °C). Structural characterizations indicated that the surface area of the aged catalysts was dramatically decreased, and the morphologies of catalysts also changed. The X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), and Raman results demonstrated that hydrothermal aging greatly influenced the migration and agglomeration of vanadium species. As a result, numerous active polymeric VOx species were emerged on the surface of catalysts. More importantly, XPS results revealed that more low valence vanadium species (V4+ and V3+) was produced on the VWTi-A catalysts than that of VTi-A after hydrothermal aging. These low valence vanadium species resulted in considerably higher SCR activity than the high valence vanadium species (V5+). Consequently, both polymeric surface VOx species and low valence vanadium species determined the enhanced low-temperature SCR behavior of the aged catalysts.

Synergistic effect between the redox property and acidity on enhancing the low temperature NH3-SCR activity for NOx removal over the Co0.2CexMn0.8-xTi10 (x = 0–0.40) oxides catalysts

A series of Co0.2CexMn0.8-xTi10 (x = 0, 0.05, 0.15, 0.25, 0.35 and 0.40) oxides catalysts were prepared by the sol-gel method and used for NH3-SCR. These catalysts were characterized by means of XRD, SEM-EDS, H2-TPR, NO/NH3 oxidation, NH3 (NO and SO2)-TPD, XPS and in situ DRIFTS. It was found that the Co0.2Ce0.35Mn0.45Ti10 catalyst showed excellent NH3-SCR activity and a broaden temperature window (180–390 °C), accompanied with good resistance to SO2 and H2O. It could be concluded that the redox ability of Co0.2Ce0.35Mn0.45Ti10 could be reduced by Co and Ce doping, which resulted in high NH3-SCR activity at high temperature and good resistance of SO2. The addition of Co and Ce can supply more acid sites and NOx adsorption sites over Co0.2Ce0.35Mn0.45Ti10. Thus, more surface Brønsted acid and Lewis acid sites, NOx adsorption sites and modest redox ability of Co0.2Ce0.35Mn0.45Ti10 play key roles in the special NH3-SCR activity due to the interactions among Ce, Co, Mn and Ti oxides. Furthermore, the results of in situ DRIFTS study reveal the NH3-SCR reactions over Co0.2Ce0.35Mn0.45Ti10 and Co0.2Mn0.8Ti10 catalysts are mainly controlled by E-R mechanism (>210 °C) and the L-H mechanism (<210 °C), respectively.

Transition metals (Co, Zr, Ti) modified iron-samarium oxide as efficient catalysts for selective catalytic reduction of NOx at low-temperature

A series of iron-samarium mixed oxide catalysts modified by zirconium, cobalt and titanium were prepared by co-precipitation method, and the effects of modified transition metal species and calcination temperature in preparation process on NH3-SCR activity were investigated. The catalysts were characterized by TG-DSC, SEM, EDS-mapping, BET, XRD, XPS, PY-IR, NH3-TPD, O2-TPD and in situ DRIFTS. We also investigated the effects of gas hourly space velocity (GHSV), KCl, water vapor and SO2 on NH3-SCR activity over the optimal catalyst. The results indicated that the addition of transition metal could alter the activity of iron-samarium mixed oxide catalyst. In all the samples tested, titanium was illustrated to be the most suitable assistant that could increase surface acid species and acid amount, meanwhile broaden the reaction temperature window and improved the N2 selectivity. Moreover, Ti0.15Sm0.075Fe0.775Ox-350 sample possessed best NO conversion rate and excellent resistance to water vapor and SO2. However, The NH3-SCR activity of K/Ti0.15Sm0.075Fe0.775Ox-350 was much lower than that of the fresh Ti0.15Sm0.075Fe0.775Ox-350. Furthermore, in situ DRIFT study verified the NH3-SCR reaction of Ti0.15Sm0.075Fe0.775Ox-350 involved both E–-R mechanism and L–-H mechanism.

Water promotion mechanism on the NH3-SCR over Fe-BEA catalyst

The water promotion mechanism in the Fe-BEA catalyst on the NH3-SCR activity was investigated by in situ DRIFT studies. While a significant amount (mol/g) of Lewis acid sites was formed on the surface of the Fe-BEA without water addition, supplementary hydroxyl species caused Lewis acid sites to be transformed into Brønsted acid sites, thus resulting in more NO3−/NO2− and NH4+ species adsorbed on the surface which increased the SCR activity under the presence of water.

New Insights into the Role of WO3 in Improved Activity and Ammonium Bisulfate Resistance for NO Reduction with NH3 over V–W/Ce/Ti Catalyst

The W-modified V–W/Ce/Ti-x% catalysts are prepared to investigate their selective catalytic reduction (SCR) activity. The V–W/Ce/Ti-5% with 5% WO3 loading shows superior NH3-SCR activity and higher ammonium bisulfate (ABS) resistance at 280 °C. The 5% WO3 doping can induce the isolated tetrahedrally coordinated vanadium species to transform into the oligomeric ones and strongly combine with neighboring V and Ce via synergetic interaction (V4+ + Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+), which favors the electron conduction among these active sites and significantly enhances the NOx conversion rate. Moreover, WO3 doping suppresses the formation of ABS and metal sulfate species over active sites by lowering the alkalinity of V–W/Ce/Ti-5%. The synergetic interaction through electron transfer can lower the thermal stability of ABS species and cause the NH3-SCR reaction between NO + O2 and ABS to completely prohibit the formation of ABS because of the faster de-NOx conversion rate with 5% WO3 loading.

Rare earth salt of 12-tungstophosphoric acid supported on iron oxide as a catalyst for selective catalytic reduction of NOx

A series of rare earth Keggin-type tungstophosphorate doped iron oxide catalysts were prepared by physical mixing method. The effects of type and the content of rare earth element over NH3-SCR activity were investigated and the catalysts were characterized by FT-IR, XRD, SEM, EDS, XPS, BET, TGA, ICP-AES, H2-TPR and NH3-TPD. It is indicated that the addition of rare earth Keggin-type tungstophosphorate could increase the NH3-SCR activity of the iron oxide catalysts, while the effect of rare earth species on SCR activity over FeyOx is not obvious. Effects of O2 concentration, SO2 and SO2 + H2O on NH3-SCR activity over NdPW/FeyOx(25) and the control experiments were also investigated, the results of which illustrates that O2 exerts a significant promotion effect on the SCR reaction and the impact of SO2 (470 ppm) on catalytic activity is almost negligible. NdPW/FeyOx(25) presents appealing properties of hydrothermal stability and SO2-tolerance, and exhibits a great potential in practical applications.

Promotional effect of H2O2 modification on the cerium-tungsten-titanium mixed oxide catalyst for selective catalytic reduction of NO with NH3

Several cerium-tungsten-titanium mixed oxide catalysts were prepared through the hydrothermal precipitation method, and the influence of H2O2 modification on the NH3-SCR activity of Ce20W10Ti100Oz was investigated. The samples were characterized by XRD, N2 adsorption-desorption, XPS and NH3-TPD to reveal the effect of H2O2 modification on its structure properties and redox properties. In addition, the NH3-SCR mechanisms at 200 °C for Ce20W10Ti100Oz catalyst modified by 2 ml H2O2(1 mol/L) was also studied by in situ DRIFTS. The NH3-SCR activity of Ce20W10Ti100Oz was improved by H2O2 modification, and the suitable volume of H2O2 (1 mol/L) is 2 ml. The introduction of H2O2 refined the pore size of Ce20W10Ti100Oz and enlarged its BET surface area. In addition, H2O2 modification improved the molar ratio of Ce3+/Ce4+ on the surface of catalyst and made large amount of active oxygen belonging to surface hydroxyl formed on its surface with the total concentration of the active absorbed oxygen (Oα+O'α) enhanced. They all contributed to enhance the NOx conversion over Ce20W10Ti100Oz catalyst. The intensity of acid sites for Ce20W10Ti100Oz was also improved by H2O2 modification, especially the Brønsted acid sites. Ionic NH4+ adsorbed on the Brønsted acid sites was mainly the active adsorbed NH3 species in the NH3-SCR reaction at 200 °C, and the reaction between active adsorbed NH3 species and gaseous NOx mainly occurred on Ce20W10Ti100Oz-2 at 200 °C following the E-R mechanism.

Single-phase SO2-resistant to poisoning Co/Mn-MOF-74 catalysts for NH3-SCR

Single-phase Co/Mn-MOF-74 solid materials were synthesized successfully via the hydrothermal method. The effect of Co/Mn atom ratio on the structure of Co/Mn-MOF-74 solids and their catalytic activity for low-temperature NH3-SCR of NOx were investigated. Very high NO conversion values (>96.0%) at 180–240 °C were obtained using the Mn0.66Co0.34-MOF-74 chemical composition (optimal content of Co). Furthermore, Mn0.66Co0.34-MOF-74 showed outstanding resistance to SO2 poisoning and the incorporation of Co was shown to cause the weakening of the SO2 adsorption strength on the catalyst surface, thus to enhance catalyst's SO2 resistance to poisoning. These features were probed by in situ DRIFTS and catalytic performance studies. The present work suggests that Mn0.66Co0.34-MOF-74 could be likely considered as a prospective industrial NH3-SCR NOx control catalyst.

Mo doping as an effective strategy to boost low temperature NH3-SCR performance of CeO2/TiO2 catalysts

A novel CeO2/TiO2-MoO3 (Ce/TM) catalyst was synthesized by using of Mo as a bulk dopant to boost the NH3-SCR performance of CeO2/TiO2 catalyst. It displayed 100% NO conversion at 200–350 °C under 60,000 mlg−1 h−1 and high tolerance to H2O and SO2 at 250 °C. Characterization results manifested that the doping of Mo not only resulted in more Brønsted acid and Lewis acid sites formed on the Ce/TM surface, but also increased the specific surface area and redox ability of the catalyst, all of which account for the enhanced NH3-SCR activity.

The promoting effects of amorphous CePO4 species on phosphorus-doped CeO2/TiO2 catalysts for selective catalytic reduction of NOx by NH3

A series of phosphorus-doped CeO2/TiO2 catalysts were prepared by impregnation method and the effects of P on NH3-SCR performance over CeO2/TiO2 were investigated. The P-doped catalysts showed much better deNOx performance than Ce10/Ti catalyst. It was found that the optimal phosphorus loading was 3 wt.% (calculated as P2O5). The Ce10P3/Ti catalyst also exhibited excellent resistance to H2O. The samples were characterized by BET, XRD, XPS, NH3-TPD, H2-TPR and in-situ DRIFT studies. It was found that the ratio of surface Ce3+ and surface adsorbed oxygen species were promoted by phosphorus doping, thus the transfer of surface oxygen and the regeneration of active sites could be accelerated accordingly. Moreover, it was confirmed that the amorphous CePO4 appeared with excellent redox property after the doping of phosphorus. The amorphous CePO4 species could promote NH3-SCR activity by activating NH4+ species on the Brønsted acid sites of phosphate radical, forming NH2 intermediates to react with gaseous NO via an Eley–Rideal mechanism.

Synthesis of Cu0.5Mg1.5Mn0.5Al0.5Ox mixed oxide from layered double hydroxide precursor as highly efficient catalyst for low-temperature selective catalytic reduction of NOx with NH3

We report a novel NH3-SCR catalyst Cu0.5Mg1.5Mn0.5Al0.5Ox synthesized from layered double hydroxides with superior activity in a wide temperature range and improved SO2 and H2O resistance comparing to conventional doped Mn/γ-Al2O3. This catalyst results in a high NOx removal efficiency of 87.0–96.6% in the low temperature range of 100–250 °C, much better than Mn/γ-Al2O3 (35.0–67.2%). Besides, it exhibits significant resistance to SO2 and H2O due to the existence of Cu and Mg. The promoting effects of Cu and Mg are thoroughly investigated using various physico-chemical techniques. The superior NH3-SCR activity of Cu0.5Mg1.5Mn0.5Al0.5Ox catalyst can be associated with its high specific surface area, high reducibility of MnO2 and CuO species, abundance of acid sites, and the well dispersion of MnO2 and CuO species. The interactions between SO2 and NH3, and the degradation mechanism caused by SO2 were investigated using in-situ DRIFT analysis.

Effect of Ce doping on the resistance of Na over V2O5-WO3/TiO2 SCR catalysts

Enhanced performance of Ce doping on the catalytic activity of V2O5-WO3/TiO2 catalysts and its resistance properties to Na poisoning have been investigated in the research. Different Na species (NaCl and Na2O) were deposited on CeO2-V2O5-WO3/TiO2 catalysts by impregnation method. The results presented that CeO2-V2O5-WO3/TiO2 catalysts exhibited a higher NH3-SCR activity and better Na resistance than the V2O5-WO3/TiO2 catalyst, and its poisoning resistance to Na2O is more superior than to NaCl, caused by more acid sites over the 10%CeO2-V2O5-WO3/TiO2 (Ce10VWTi) catalyst, which indicated that the poisoning resistance of Na2O on Ce10VWTi catalysts is higher than that of NaCl. Combined with the characterization results, the dopant of Ce could enhance the surface chemisorbed oxygen on the Na poisoning V2O5-WO3/TiO2 catalysts, facilitate the redox cycle, and increase the intensity of acid sites due to the newly formed Brønsted acid sites stemmed from Ce3+−NH4+, thereby promoting catalytic activity and Na poisoning resistance.

Aerosol-assisted deposition of Mn-Fe oxide catalyst on TiO2 for superior selective catalytic reduction of NO with NH3 at low temperatures

A series of Me-Fe/TiO2 catalysts was prepared via an aerosol-assisted deposition method, and used for the selective catalytic reduction of NO with NH3 (NH3-SCR) for the first time. Experimental results show that the spray-deposition Mn-Fe/TiO2, denoted Mn-Fe/TiO2(SD), demonstrates high NH3-SCR activity (>95% conversion) with high N2 selectivity (>90%) at temperatures as low as 80–150 °C at a space velocity of 50,000 h−1. Also, the benefits of the spray deposition for preparing Mn-Fe/TiO2 are highlighted by comparison with co-precipitation and wet impregnation. This efficient and reliable technique offers attractive opportunities for large-scale manufacture of promising materials with superior deNOx catalytic performances.

Enhancement of the NH3-SCR catalytic activity of MnTiOx catalyst by the introduction of Sb

Sb was used as the additive to enhance the NH3-SCR performance of MnTiOx catalyst. Experimental results revealed that MnSbTiOx-0.2 demonstrated outstanding NH3-SCR activity in a wide temperature window (100–400 ℃) and high resistance to SO2. Characterization results indicated the introduction of Sb into MnTiOx catalyst could improve the dispersity of active species and generate more surface acid sites, Mn4+ and surface adsorbed oxygen species. In situ DRIFT spectra proved that the presence of Sb in MnSbTiOx-0.2 catalyst could form new Brønsted acid sites, along with the increased surface Lewis acidity. The improved adsorption of NH3 and NOx species over MnSbTiOx-0.2 catalyst had a stimulating effect on the NH3-SCR reaction over it through Langmuir−Hinshelwood (L-H) route. Moreover, Sb species in MnSbTiOx-0.2 could improve the reactivities of ad-NH3 and ad-NOx species in SCR reaction. All these factors contributed to the excellent performance of MnSbTiOx-0.2 catalyst.

The smart surface modification of Fe2O3 by WOx for significantly promoting the selective catalytic reduction of NOx with NH3

The deposition of WOx species using conventional impregnation method is a very simple and efficient way to adjust the surface acidity and redox ability of hematite Fe2O3 simultaneously, significantly improving its catalytic activity and N2 selectivity for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The deposited WOx species on Fe2O3 surface was in highly unsaturated coordination state acting as an effective surface modifier. Through the electronic inductive effect in WOFe bonds, the oxidation ability of surface Fe(3+δ)+ species was well increased, yet the over-oxidation of NH3 on bulk Fe2O3 was significantly suppressed, which was quite beneficial to the improvement of NH3-SCR activity and N2 selectivity simultaneously. Besides, the deposited WOx species could also supply abundant surface reactive Lewis and Brønsted acid sites for NH3 adsorption and activation during the SCR reaction. The NH3-SCR process on this WOx/Fe2O3 catalyst mainly followed an Eley-Rideal (E-R) reaction pathway between gaseous NO and active NH3 adsorbed species, which was the main reason for its superior SO2 resistance. This opens a new route for the preparation of highly efficient, selective, cost effective and SO2 resistant NH3-SCR catalysts for the deNOx process on diesel engines.

Fe and/or Mn oxides supported on fly ash-derived SBA-15 for low-temperature NH3-SCR

Fe and/or Mn/SBA-15 catalysts were prepared by the impregnation method using low-cost fly ash-derived SBA-15 molecular sieve as support and tested towards NH3-SCR. Powder XRD, N2 adsorption, XPS, H2-TPR and SEM techniques were used for catalyst characterization and investigation of relationships with the catalytic activity. The Fe-Mn/SBA-15 catalyst exhibited significantly higher NH3-SCR activities than Mn/SBA-15 or Fe/SBA-15 in the 150–250 °C range owing to some developed synergistic effects between Mn and Fe. In particular, the addition of Mn improved the dispersion of Fe2O3 on the catalyst surface, while the addition of Fe promoted the formation of single β-MnO2 phase. The high dispersion of Fe2O3, the appropriate Mn4+/Mn3+ ratio, the concentration of adsorbed oxygen and the low-T enhanced redox activity of the Fe-Mn/SBA-15 improved its NH3-SCR activity. This work opens new avenues of recycling fly ash formed in coal-fired power plants (reducing environmental pollution) and developing low-cost SCR catalysts for NOx pollution control.

Cu-ZSM-5 zeolite supported on SiC monolith with enhanced catalytic activity for NH3-SCR

Here, we developed a new kind of hetero-structural catalyst through the homogeneous growth of Cu-ZSM-5 zeolite onto silicon carbide (SiC) monolith by using a one-pot hydrothermal method combined with an ion-exchange process in a rotary oven. The new catalyst formulation was tested towards NH3-SCR, where the catalytic activity and stability of Cu-ZSM-5/SiC at T > 350 °C appear significantly higher than those obtained with Cu-ZSM-5 alone. In particular, the NO consumption rate of the former is 7.5 times larger on the average than that of the latter. This remarkable enhancement of catalytic activity is ascribed to the very good thermal conductivity of the SiC monolithic support. This research work could provide the means for the future design of more active and stable monolithic Cu-ZSM5-based catalytic systems with even lower Cu loading than the ones used in present de-NOx applications.

Different copper species as active sites for NH3-SCR reaction over Cu-SAPO-34 catalyst and reaction pathways: A periodic DFT study

Cu-SAPO-34 SCR catalyst has drawn much attention because of its high NH3-SCR activity and N2 selectivity. Different copper species as active sites for SCR reaction in Cu-SAPO-34 catalyst were studied by DFT calculations, primarily focusing on copper sites in six-membered rings (6 MR) and eight-membered rings (8 MR). The relative energies of various Cu species were calculated and organized as phase diagrams to investigate the stabilities of Cu species. The diagrams show that Cu2+ can exist in 6 MR, and [Cu-O-Cu]2+ tends to form in 8 MR.The DFT results show that the adsorption energies for NH3 on Cu+ and Cu2+ in 8 MR are the highest and but NH3 can hardly dissociate in 8 MR, which indicate isolated Cu ions located in 8 MR will hinder the NH3-SCR reaction. However, the additional Cu ion and O atom in 8 MR effectively reduces the dissociation energy barrier. These results indicate that isolated Cu2+ is the active site in 6 MR, and [Cu-O-Cu]2+ is the active site in 8 MR. Besides, the NH3 dissociation on Cu2+ and [Cu-O-Cu]2+ are the key steps for the reactions in 6 MR and 8 MR.