Catalysts For Reduction Of NOx Research Articles

Effects of alkali and alkaline earth metals on Cu-SSZ-39 catalyst for the selective catalytic reduction of NOx with NH3

Alkali/alkaline earth metals from urea solution and engine lubricating oil can induce the chemical deactivation of NH3-SCR catalysts. Cu-SSZ-39, with excellent catalytic performance and hydrothermal stability, is thought to be a good choice for the NOx abatement from diesel vehicles. In this work, the effects of alkali/alkaline earth metals on Cu-SSZ-39 were investigated for the first time. The deactivation effects of alkali/alkaline earth metals followed the order of K > Mg > Ca > Na. The surface area, isolated Cu2+ ions and acid sites greatly decreased after the introduction of alkali/alkaline earth metals. Na and K had greater influence on the zeolite surface acidity than Ca and Mg with the same loading amount (1 mmol per gram catalyst). The deterioration of the zeolite structure and the transformation of isolated Cu2+ ions to CuOx were observed after poisoning by each of the alkali/alkaline earth metals. The distribution of copper species is a very important factor determining the SCR performance of Cu-SSZ-39 after alkali/alkaline earth metals poisoning. This work provides a new insight into understanding deactivation mechanisms in Cu-based small-pore zeolites.

Two-dimensional MnFeCo layered double oxide as catalyst for enhanced selective catalytic reduction of NOx with NH3 at low temperature (25–150 °C)

Low temperature catalysts for selective catalytic reduction of NOx with NH3 plays an important role in flue gas treatments. Here, a two-dimensional MnFeCo layered double oxide were synthetized, characterized and employed as NH3 selective catalytic reduction catalyst, showing excellent low temperature de-NOx performance. MnFeCo layered double oxide exhibited a specific surface area of 92 m2/g with average pore size and volume of 2 nm and 1.5 cm3/g. At 100 °C, the MnFeCo layered double oxide performed an efficient NO conversion of 100 %. Even at 25 and 50 °C, NO conversions reached values of 49 and 86 %, respectively. Furthermore, the NO conversion reached an efficiency of 89 % in the presence of H2O vapor (5 %) and SO2 (100 ppm). Based on that, it can be said that this NH3 selective catalytic reduction catalyst shows great potential in NOx denitration of stationary industrial installations at a low temperature range.

Preparation of Quasi-MIL-101(Cr) Loaded Ceria Catalysts for the Selective Catalytic Reduction of NOx at Low Temperature

At present, the development of novel catalysts with high activity Selective Catalytic Reduction (SCR) reaction at the low temperature is still a challenge. In this work, the authors prepare CeO2/quasi-MIL-101 catalysts with various amounts of deposited ceria by a double-solvent method, which are characterized by X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and so on. The results show that the increase of Ce content has a great influence on the catalytic property of the catalyst. The introduction of Ce can promote the conversion between Cr3+ and Cr5+ and increase the proportion of lattice oxygen, which improves the activity of the catalyst. However, the catalyst will be peroxidized when the content of Ce is too high, resulting in the decline of the catalytic activity. This experiment indicates that CeO2/quasi-MIL-101 plays a significant role in the NH3-SCR process at the low temperature when the loading of Ce is 0.5%. This work has proved the potential of this kind of material in NH3-SCR process at the low temperature, providing help for subsequent studies.

Influence of CeO2 loading on structure and catalytic activity for NH3-SCR over TiO2-supported CeO2

A series of supported CeO2/TiO2 catalysts were prepared to explore the influence of CeO2 loading on these catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR). The catalysts were investigated in detail by means of XRD, Raman, H2-TPR, NH3-TPD, XPS, in situ DRIFTS, and NH3-SCR reaction. The activity of the catalyst is closely related to the content of CeO2. When the loading of CeO2 is near the dispersion capacity (1.16 mmol Ce4+/100 m2 TiO2), the catalytic activity is better. This may be because that the dispersed CeO2 is the active species and the catalyst has appropriate redox property, along with the larger amounts of surface Ce content and surface adsorbed oxygen species. Finally, a possible reaction mechanism via the Langmuir-Hinshelwood (L-H) mechanism is tentatively proposed to further understand the NH3-SCR reaction.

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh as novel monolithic catalysts for NOx reduction

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh were developed as novel monolithic catalysts for NOx reduction.

Irregular influence of alkali metals on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia

SCR activity of Cu-SAPO-34 catalyst was reduced by alkali metal ions. The alkali metals ions (Li+, Na+ and K+) have shown irregular influences on Cu-SAPO-34. The order of poisoning strengths under 400 °C was found to be: Na+ > K+ > Li+, which is not consistent with the basicities of their corresponding metals. Experimental results and calculations showed that the alkali metal ions readily replace H+ and Cu2+/Cu+ ions. These exchanges result in the loss of Brønsted acid sites and migration of isolated Cu2+ ions in Cu-SAPO-34, which decrease the NH3-SCR activity. Both the basicity and ion diameter will affect the exchanging behavior of an alkali ion. Na+ and Li+ ions will influence both H+ and Cu2+/Cu+ ions but K+ ions only preferably replace the H+. We hypothesize that K+ cannot enter into a small ring (6-membered ring) to replace a Cu2+/Cu+ ion because of its large ion diameter. The displaced Cu2+/Cu+ ions will transfer to adjacent unbonded Al site to form a CuAlO2 species.

Activity enhancement of WO3 modified FeTiOx catalysts for the selective catalytic reduction of NOx by NH3

A series of WO3 modified iron-titanate mixed metal oxides catalysts (the molar ratio of Fe/Ti = 4:1) were synthesized by conventional impregnation method and tested for selective catalytic reduction of NOx by NH3. The addition of WO3 species could enhance the catalytic performance of FeTiOx and 6 W/FeTiOx showed optimal activity with above 80 % NO conversion under a wide operating temperature window of 200–400 °C. It also exhibited excellent stability in the presence of H2O and SO2 at 250 °C, even as low as 225 °C, which provided a suitable deNOx candidate for stationary sources. The characterization methods including XRD, BET, XPS, H2-TPR, NH3-TPD, NO-TPD and in situ DRIFTs were used to investigate the structure and surface properties of catalysts and their influence on the catalytic activity. The results demonstrated that the impregnation of WO3 species led to the strong electronic inductive effects between Fe3+and W6+, Ti4+ and W6+ in the forms of Fe(3+n)+-O-W(6−n)+ and Ti(4+n)+-O-W(6−n)+ structures, increasing the ratio of chemical adsorbed oxygen species, surface OH-, which were stimulative to low-temperature NH3-SCR reaction. The increased amount of surface Brønsted acid sites mainly came from W-OH species. Besides, the reaction pathway over 6 W/FeTiOx mainly followed Eley-Rideal mechanism at 225 °C through the reaction between gaseous NO and adsorbed NH3 species.

Enhancement of the Hydrothermal Stability of WO3/Ce0.68Zr0.32O2 Catalyst by Silica Modification for NH3-SCR

Hydrothermal stability of catalysts for selective catalytic reduction of NOx with NH3 (NH3-SCR) is essential to develop a promising candidate catalyst for removing NOx emitted from diesel engines. ...

Hierarchical three-dimensionally ordered macroporous Fe-V binary metal oxide catalyst for low temperature selective catalytic reduction of NOx from marine diesel engine exhaust

A series of hierarchical three-dimensionally ordered macroporous Fe10-xVx binary metal oxides (3DOM-Fe10-xVx) catalysts were designed and synthesized for selective catalytic reduction of NOx from marine diesel engine exhaust. The physical-chemical performance of the 3DOM-Fe10-xVx catalysts were subjected by various techniques and found that the 3DOM-Fe10-xVx with the Fe to V atomic ratio of 9.0/1.0 exhibited a wider active temperature window, better soot resistance performance compared with the conventional Fe9.0V1.0 catalyst due to its special 3DOM structure. Even when simultaneously exposed to the soot and high concentration of sulfur dioxide reaction feeds, the 3DOM-Fe9.0V1.0 catalyst also displayed superior catalytic activity. Interestingly, we found that the soot had a reduction activity for NOx at high reaction temperature, which can effectively extend its active temperature window. The 3DOM-Fe10-xVx catalysts prepared in this work could be potentially a good candidate for the cargo ships deNOx catalysts to satisfy the more stringent exhaust emission rules.

Effects of different introduction methods of Ce4+ and Zr4+ on denitration performance and anti-K poisoning performance of V2O5-WO3/TiO2 catalyst

The purpose of this work is to explore the effects of the introduction methods of Ce4+ and Zr4+ on the physicochemical properties, activity, and K tolerance of V2O5-WO3/TiO2 catalyst for the selective catalytic reduction of NOx by NH3. Four different methods, namely pre-impregnation, post-impregnation, co-impregnation, and co-precipitation, were used to synthesize a series of V2O5-WO3-TiO2-CeO2-ZrO2 catalysts. The catalysts were characterized by XRD, BET, NH3-TPD, XPS, and H2-TPR techniques. Moreover, the activity and anti-K poisoning performance were tested by an NH3-SCR model reaction. The results show that the introduction of Ce4+ and Zr4+ can improve the catalytic performance of V2O5-WO3/TiO2 catalyst, but the impregnation method cannot enhance the anti-K poisoning performance. Ce4+ and Zr4+ introduced by co-precipitation method can effectively improve the tolerance of K, which is mainly due to the incorporation of Ce4+ and Zr4+ into TiO2 lattice to form a uniform TiO2-CeO2-ZrO2 solid solution, resulting in the optimal surface acidity and redox performance, and reducing the decreases caused by K-poisoning. Furthermore, based on the best introduction method, we further optimized the molar ratio of Ce4+/Zr4+. It is found that the catalyst exhibits the best anti-K poisoning performance when the molar ratio of Ce4+/Zr4+ is 2:1.

Facile fabrication of nanosheet-assembled MnCoOx hollow flower-like microspheres as highly effective catalysts for the low-temperature selective catalytic reduction of NOx by NH3.

A series of MnCoOx flower-like hollow microspheres with various molecular proportions of reactant were prepared through simple solvothermal method for the ammonia selective catalytic reduction (SCR) at low temperatures. The as-prepared samples have been applied by various characterization techniques to explore the formation process of the morphology and physicochemical properties. The Mn(1)Co(1)Ox presented the optimal intrinsic catalytic performance (95% NOx conversion at 75 °C), favorable thermal stability, and strong SO2 resistance. The excellent properties mainly related to its higher specific surface area and abundant active sites originated from hollow microsphere special structure consists of abundant nanosheets, robust redox properties beneficial for the strong interaction between the manganese and cobalt, larger number of acidic sites and stronger acid strength, etc., which collaboratively dominate its catalytic properties of NH3-SCR at low temperatures.

Reduction of NO with new vanadium-carbon xerogel composites. Effect of the oxidation state of vanadium species

Vanadium-carbon composites were successfully prepared by sol-gel methods, and their behaviour as NOx-reduction catalysts was tested. Two different synthesis approaches were employed: the distribution of VOx (i) along the carbon matrix and (ii) on the carbon spheres surface. This allows to modify the carbon-VOx interaction and thus, evaluate the role of vanadium oxidation state in the NOx abatement. The distribution of VOx along the carbon matrix favours the dispersion and reducibility of VOx nanoparticles and improves the carbon-VOx contact stabilizing vanadium low oxidation states (V2+), however part of the VOx particles were embedded on the carbon matrix becoming inactive. The distribution of those species on the carbon spheres surface improves the accessibility to the VOx active sites but at the expense of the decrease of the carbon-VOx interaction and VOx dispersion and reducibility. The oxidation state of the active phase is related with the global synthesis procedure and that reducibility depends on the interactions between phases. Catalytic results evidence that the NO reduction is strongly influenced by the oxidation state of vanadium. Commercial V2O5 and carbon-V2O5 physical mixture are poorly reactive whereas a stronger activity improvement activity was observed for C-VOx composites reaching conversion higher than 91% at 450 °C.

Promotional effects of Fe on manganese oxide octahedral molecular sieves for alkali-resistant catalytic reduction of NOx: XAFS and in situ DRIFTs study

Currently, selective catalytic reduction (SCR) of NOx with NH3 in the presence of alkali metal ions by using vanadium-free catalysts is still a big challenge for the removal of NOx for stationary sources. In this work, improved reduction of NOx with NH3 in the presence of alkali metal ions over novel Fe-doped manganese oxide octahedral molecular sieve (OMS-2) catalysts has been demonstrated and the promotional effects of Fe have been clarified. The Fe-doped OMS-2 (Fe-OMS-2) catalysts exhibited excellent NH3-SCR activity, improved N2 selectivity and enhanced alkali resistance. The X-ray absorption fine structure spectroscopy (XAFS) results indicated that the trapped K+ ions could make the catalyst structure more stable. In situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTs) results revealed that the Fe doping could enhance adsorption of NH3 species and adsorb various types of NOx species namely monodentate nitrate, bridged nitrate, bidentate nitrate and chelating nitrite. These formed intermediates on the Fe-OMS-2 catalysts were more reactive and thus more effectively participated in the SCR reactions. Superior alkali resistance of the Fe-OMS-2 catalysts was due to improved redox species, more acid sites and stronger adsorption of NOx species. The present investigations in this work may lead to an alternative development of high-performance non-vanadium catalysts for alkali-resistant NOx reduction.

In situ regeneration of sulfated Cu/SAPO-34 catalyst for the selective catalytic reduction of NOx with NH3

The regeneration mechanism of sulfated Cu/SAPO-34 catalysts in reducing atmospheres (5% H2/Ar and NH3/Ar at 600 °C) was investigated by using a series of characterization methods. Hydrogen regeneration improved the catalysts’ NH3-SCR activity from 150 to 550 °C remarkably, while ammonia regeneration exhibited relatively less obvious promotion effect. The TPR of hydrogen and EPR spectra data show that the regeneration process affects the amount and the coordination of the active isolated Cu2+ species significantly. The desulfation increases the amount of isolated copper ions and improves the catalytic activity of SCR.

Speciation of Cu Cations in Cu-CHA Catalysts for NH3-SCR: Effects of SiO2/AlO3 Ratio and Cu-Loading Investigated by Transient Response Methods

We develop an experimental protocol to quantify the speciation of Cu ions (ZCuIIOH and Z2CuII) in Cu-CHA catalysts for the NH3 Selective Catalytic Reduction of NOx (NH3-SCR). Toward this end, we performed four transient tests, namely H2-TPR, NO + NH3 TPR, NO2 adsorption + TPD, and NH3 adsorption + TPD, over two sets of Cu-CHA research catalysts characterized by different Cu contents (0–2.1% w/w) and SiO2/Al2O3 (SAR) ratios (10–25). Preliminary H2-TPR tests on the samples with the extreme SAR and Cu loading values were used to identify the variability range of the fractions of ZCuIIOH and Z2CuII species in these catalysts. The ZCuIIOH fraction was found to vary between 0.55 (at Cu/Al = 0.11) and 0.79 (at Cu/Al = 0.29). NO+NH3 TPR runs demonstrated that the NO + NH3 mixture is a much stronger reducing agent than H2: full reduction of all the Cu was obtained already at lower temperature, and differences in the reducibility of ZCuIIOH and Z2CuII were strongly attenuated. Both the integral NO consumption and t...

Environmentally-friendly tourmaline modified CeMnFeOx catalysts for low-temperature selective catalytic reduction of NOx with NH3

Abstract The tourmaline/manganese-iron-cerium-oxide composites prepared by the hydrothermal method was investigated for NOx removal in simulated flue gas. The microstructure of the catalyst has been characterized by X-ray diffractometer (XRD), temperature-programmed reductions (TPR) and X-ray photoelectron spectra (XPS). The incorporation of tourmaline could remarkably enhance the activity of tourmaline/Mn-Fe-Ce catalyst at low temperature. H2-TPR results indicated that H2 adsorption amount decreased after the incorporation of tourmaline. The XPS results revealed that the incorporation of tourmaline could enhanced the contents of high valence Oα (Oα/O) in the composite catalysts, which is favorable for oxidation process of elemental manganese. Overall, the catalytic performance of tourmaline/Mn-Fe-Ce was closely related to the addition of tourmaline. When the addition of tourmaline was 2%, the composite catalyst showed the best catalytic performance: The NOx conversion of the composite catalyst is 100% at 170–230 °C. Moreover, the denitrification mechanism of composites was studied. The addition of tourmaline improved the catalytic process, and stable bidentate nitrate species was activated at low temperature during the denitrification reaction. As a mineral material, tourmaline has excellent far-infrared properties and spontaneous polarization properties, which can significantly improve the denitration performance of the catalyst. The use of mineral materials as raw materials to prepare denitration catalysts provides new ideas for future research.

Pd-doped or Pd impregnated 30% La0.7Sr0.3CoO3/Al2O3 catalysts for NOx storage and reduction

Here we report the effects of the incorporation of palladium into 30% La0.7Sr0.3CoO3/Al2O3 catalyst by different methods on the NOx storage and reduction behaviour. Several catalysts were prepared incorporating four palladium nominal loadings (0.75, 1.5, 2.25 and 3.0%) by doping the perovskite formulation (30% La0.7Sr0.3Co1-yPdyO3/Al2O3) or by wetness impregnation over alumina-supported perovskite (y% Pd-30% La0.7Sr0.3CoO3/Al2O3). The results of X-Ray diffraction, N2 adsorption-desorption at −196 °C, electron microscopy, temperature programmed techniques, Raman and X-ray photoelectron spectroscopies demonstrated that the wetness impregnation method induces the formation of small PdO particles homogenously distributed over the surface in strong interaction with the perovskite. Meanwhile, doping method leads to the formation of a mix between intraframework palladium species and surface agglomerated PdOx particles with weaker interaction with the perovskite phase. Therefore, palladium accessibility and the synergetic effects with the perovskite are lower for Pd-doped samples. The NO-to-NO2 conversion is similar irrespective of the palladium content and the incorporation method. This confirms the perovskite phase as the main active site for NO oxidation. However, NOx adsorption during lean conditions and NOx reduction to N2 during rich periods are significantly promoted after the incorporation of palladium, especially by impregnation method. This enhancement is assigned to better NOx adsorption sites regeneration and to a promotion of NOx reduction rate, respectively. Thus, the best De-NOx performance of Pd-impregnated catalysts is derived from the higher efficient use of the palladium active sites. Among the developed formulations, the 1.5% Pd-30% LSCO/Al2O3 sample shows the best balance between NOx removal efficiency and minimum palladium content. This sample shows a NO removal efficiency and nitrogen yield as high as 86.2% and 69.5%, respectively. Based on these results, the developed formulation is revealed as a promising alternative to the NSR model catalyst (1.5% Pt-15% BaO/Al2O3) for NOx removal in the automotive application.

Enhanced hydrothermal stability of a Cu-SSZ-13 catalyst for the selective reduction of NOx by NH3 synthesized with SAPO-34 micro-crystallite as seed

The distribution of framework Si and Al in the Cu-SSZ-13 catalyst, for NH3 selective catalytic reduction of NOx, is manipulated via a new strategy, using SAPO-34 as a seed in hydrothermal synthesis instead of the conventional SSZ-13 micro-crystallite. The SAPO-34 seed has a strong impact on the hydrothermal stability of the catalyst, moderating the dealumination as well as the agglomeration of the active Cu2+ ions in the Cu-SSZ-13 with hydrothermal treatment at 800 °C.

Investigation of an irreversible NOx storage degradation Mode on a Pd/BEA passive NOx adsorber

Passive NOx adsorbers (PNAs) are a proposed solution for cold start NOx emissions, via low temperature NOx trapping and subsequent release at a temperature where downstream NOx reduction catalysts are active. Pd/zeolites have been reported to have significant NOx storage capacities. However, their real exhaust compatibility has not been completely evaluated. In this work, Pd/BEA was used to study the mechanism of an observed PNA degradation. NO adsorption, NO and CO co-adsorption and temperature programmed desorption/oxidation were performed on a series of differently aged Pd/BEA catalysts to evaluate the NOx storage capacity and desorption temperature. An irreversible NOx storage degradation mode caused by low temperature CO exposure was observed. H2 temperature programmed reduction results indicate this irreversible degradation can be attributed to the loss of Pd2+. Additionally, STEM images revealed Pd particle migration/agglomeration after hydrothermal aging and CO/H2 exposure. Larger particles, formed during CO exposure, lead to the irreversible degradation.

Fe-modified Ce-MnOx/ACFN catalysts for selective catalytic reduction of NOx by NH3 at low-middle temperature.

A series of MnOx/ACFN, Ce-MnOx/ACFN, and Fe-Ce-MnOx/ACFN catalysts on selective catalytic reduction (SCR) of NOx with NH3 at low-middle temperature had been successfully prepared through ultrasonic impregnation method, and the catalysts were characterized by SEM, XRD, BET, H2-TPR, NH3-TPD, XPS, and FT-IR spectroscopy, respectively. The results demonstrated that the 15 wt% Fe(1)-Ce(3)-MnOx(7)/ACFN catalyst achieved 90% NOx conversion (100~300 °C), good water resistance, and stability (175 °C). The excellent catalytic performance of the Fe(1)-Ce(3)-MnOx(7)/ACFN catalyst was mainly attributed to the interaction among Mn, Ce, and Fe. The doping of Fe promoted the dispersion of Ce and Mn and the formation of more Mn4+ and chemisorbed oxygen on the surface of a catalyst. This work laid a foundation for the successful application of active carbon fiber in the field of industrial denitrification, especially in the aspect of denitrification moving bed. Graphic abstract.