Low-temperature deNOx Research Articles

Kinetic modelling of the NOx reduction by H2 on Pt/WO3/ZrO2 catalyst in excess of O2

This paper addresses the elementary kinetic mean field modelling of the NOx reduction by H2 on Pt/WO3/ZrO2 under oxygen-rich conditions. Pt/WO3/ZrO2 was recently shown to reveal substantial low-temperature deNOx activity with enhanced N2 selectivity referred to traditional Pt catalysts. The model was developed based on a postulated reaction mechanism as well as kinetic examinations and implied a network of 48 reaction steps described by Arrhenius-based rate expressions. Kinetic parameters were taken from literature and by fitting calculations, while pre-exponential factors of adsorption were estimated from kinetic gas theory. For validation, experiments were simulated and thermodynamic consistency was proven by checking the Gibbs free enthalpy of the catalytic surface reactions. As a result of the kinetic model, the formation of OH surface species was identified as the rate determining step of H2 oxidation, while the reduction of NO predominately occurs by dissociation of chemisorbed NO.

Economical way to synthesize SSZ-13 with abundant ion-exchanged Cu+ for an extraordinary performance in selective catalytic reduction (SCR) of NOx by ammonia.

In this study, an economical way for SSZ-13 preparation with the essentially cheap choline chloride as template has been attempted. The as-synthesized SSZ-13 zeolite after ion exchange by copper nitrate solution exhibited a superior SCR performance (over 95% NOx conversion across a broad range from 150 to 400 °C) to the traditional zeolite-based catalysts of Cu-Beta and Cu-ZSM-5. Furthermore, the opportune size of pore opening (∼3.8 Å) made Cu-SSZ-13 exhibiting the best selectivity to N2 as well as satisfactory tolerance toward SO2 and C3H6 poisonings. The characterization (XRD, XPS, XRF, and H2-TPR) of samples confirmed that Cu-SSZ-13 possessed the most abundant Cu cations among three investigated Cu-zeolites; furthermore, either on the surface or in the bulk the ratio of Cu(+)/Cu(2+) ions for Cu-SSZ-13 is also the highest. New finding was announced that CHA-type topology is in favor of the formation of copper cations, especially generating much more Cu(+) ions than the others, rather than CuO. The activity test of Cu(CuCl)-ZSM-5 (prepared by a solid-state ion-exchange method) clearly indicated that Cu(+) ions could make a major contribution to the low-temperature deNOx activity. The activity of protonic zeolites (H-SSZ-13, H-Beta, H-ZSM-5) revealed the topology effect on SCR performances.

Influencing factors on low-temperature deNOx performance of Mn–La–Ce–Ni–Ox/PPS catalytic filters applied for cement kiln

Effects of catalyst loading amount, reaction temperature, O2 concentration, NH3/NO molar ratio and SO2 on low-temperature catalytic performance of the Mn–La–Ce–Ni–Ox (Mn/La/Ce/Ni=2.5:2.5:1:1 in mol)/PPS for NH3-SCR of NO were mainly studied. Results showed that the filter with catalyst loading amount of 250g/m2 obtained more than 95% NO removal efficiency at 200°C under filtration velocity of 1m/min. The NO removal efficiency was still above 85% by injecting 300ppm SO2 at 200°C. Anyway, the Mn–La–Ce–Ni–Ox/PPS catalytic filter is promising to achieve the expectation of simultaneous removing particulate and NOx from low-sulfur flue gas in cement kiln.

Effect of Fe doping on low temperature deNOx activity of high-performance vanadia anatase nanoparticles

VFe/TiO2 catalysts have been prepared using a solgel based method. Fe was introduced using 3 different methods. The resulting substances were characterized with N2-physisorption, NH3-TPD, H2-TPR, XPS and XRD. Extrudates of the VFe/TiO2 powders were prepared using sepiolite (20wt.%) as binder. The activities of the composite materials in the selective catalytic reduction (SCR) of NO with NH3 were measured at temperatures of up to 200°C and in the presence of 20vol.% H2O. Presence of Fe can increase the surface area, enhance the redox properties, increase the number of surface acid sites, increase the share of surface adsorbed oxygen and does not induce the formation of crystalline V2O5. However, the measurements performed here show that Fe doping does not cause activity enhancement of the V/TiO2.

Bifunctional Ag-based catalyst for NOxreduction with E-diesel fuel

The excellent low-temperature deNOx performance of Ag/AlF3/Al2O3 by simulated E-diesel fuel has been achieved, mainly due to the multiple roles of AlF3 impregnated onto Al2O3. It increases the formation of the highly dispersed Ag ions and the catalyst surface acidity as well as the formation of the reaction intermediates such as cyanides and isocyanates. A plausible process for the formation of the surface composition of Ag/AlF3/Al2O3 has been proposed by using the results from XPS and FTIR.

Study on Performance of Transition Metal-Doped Catalysts for DeNOx at Low-Temperature

Mn-Ce based catalysts doped with transition metal were synthesized via a sol-gel method for low-temperature Selective catalytic reduction DeNOx. NOx conversion of these catalysts was evaluated under 100 OC-300 OC. H2-Temperature Programmed Reduction was used to investigate the reduction capability effect on NOx conversion. On this basis, sulfur resistance of catalysts was analyzed and Fourier Transform Infrared (FI-IR) spectra were used to discuss sulfur poisoning mechanism of catalysts. The results indicated that NOx conversion of catalysts was Mn-Ce-W0.03 Mn-Ce-Y0.03 > Mn-Ce-Zr0.03 > Mn-Ce >Mn-Ce-La0.03 > Mn-Ce-Pr0.03.W, Zr, Y elements was doped to improve catalytic activity at low-temperature and enhance anti-sulfur ability of catalysts. Especially, W element can restrain sulfate formation and reduce the channel blocking of the catalyst and Mn-Ce-W0.03 catalyst displays optimal performance of resistance to sulfur poisoning.

SCR catalyst coated on low-cost monolith support for flue gas denitration of industrial furnaces

NH3–SCR honeycomb catalyst was prepared by coating powdery active metal components on the surface of blank monolith supports and tested at the temperatures of 250–350°C, and superficial gas velocities of 5.3–6.3m/s. The tests were performed with a simulated gas mixture containing NO, SO2, O2, H2O and N2 or a real flue gas produced on line. The results showed that the coated honeycomb catalyst has an extraordinary high activity for low-temperature deNOx in the presence of 5vol.% steam and 1000ppm SO2. Its activity increased with increasing the coating thickness and reached at 110μm thickness as comparable as that of the 100% active components-molded catalyst, suggesting the deNOx reaction proceeded mainly in the surface layers of the honeycomb cell wall and hence coated honeycomb catalyst should be more cost-efficient than 100% active components-molded one. In a real coal-burned flue gas stream the catalyst maintained essentially its low temperature activity and stability, further verifying its high industrial applicability. Then, the effects of NO concentration and NH3/NO ratio on NO conversion were investigated to seek a suitable kinetic model for prediction of the catalyst’s deNOx rate and reactor design for pilot plant research. The data obtained showed that a simple power-law kinetic equation with first order in NO and zero order in ammonia can be applied to the coated honeycomb catalyst, and was used for determination of the kinetic parameters. Finally, simulation calculation was performed under arbitrary conditions to provide valuable information for design of pilot-plant scale reactors.

Research Progress of Low-Temperature SCR DeNOx Catalysts

Low-temperature SCR DeNOx process is considered to be a potential technique to meet the stringent environment regulations. It can avoid the problem such as blocking and eroding often generated in medium temperature, polluting and poisoning caused by alkali elements and so on. In this paper, a review of DeNOx catalysts about noble metals, molecular sieve, carbon-based and metal oxides catalyst was presented. The affecting factors of the low-temperature activation were comprehensively expounded, such as precursor introduction method, preparation method, supporter pretreatment and catalyst structure. Then mechanism of SO2 poisoning and effect of active compositions were analyzed. The dual effect of SO2 on the activity of low temperature catalysts mainly results from the formation of certain sulfur-containing species on catalyst surface. Water vapor decreased the number of available active sites attributed to competitive adsorption among H2O and other reactants. Moreover, the co-existence of H2O and SO2 resulted in the decrease of activity obviously. So the way to improve resistance of SO2 and H2O poisoning was summed up. Finally, future research and development directions in the developing low-temperature SCR for removal of NOx technology were proposed.

Combination of Photocatalysis and HC/SCR for Improved Activity and Durability of DeNOx Catalysts

A photocatalytic HC/SCR system has been developed and its high deNOx performance (54.0-98.6% NOx conversion) at low temperatures (150-250 °C) demonstrated by using a representative diesel fuel hydrocarbon (dodecane) as the reductant over a hybrid SCR system of a photocatalytic reactor (PCR) and a dual-bed HC/SCR reactor. The PCR generates highly active oxidants such as O3 and NO2 from O2 and NO in the feed stream, followed by the subsequent formation of highly efficient reductants such as oxygenated hydrocarbon (OHC), NH3, and organo-nitrogen compounds. These reductants are the key components for enhancing the low temperature deNOx performance of the dual-bed HC/SCR system containing Ag/Al2O3 and CuCoY in the front and rear bed of the reactor, respectively. The OHCs are particularly effective for both NOx reduction and NH3 formation over the Ag/Al2O3 catalyst, while NH3 and organo-nitrogen compounds are effective for NOx reduction over the CuCoY catalyst. The hybrid HC/SCR system assisted by photocatalysis has shown an overall deNOx performance comparable to that of the NH3/SCR, demonstrating its potential as a promising alternative to the current urea/SCR and LNT technologies. Superior durability of HC/SCR catalysts against coking by HCs has also been demonstrated by a PCR-assisted regeneration scheme for deactivating catalysts.

Integrated DeNO x –DeSoot Catalytic Systems with Improved Low-Temperature Performance

Performance of the combined catalysts [CeO2–ZrO2 + FeBETA] and [Mn/CeO2–ZrO2 + FeBETA], prepared by mechanical mixing of the component powders, was studied in soot oxidation and NH3-SCR. [CeO2–ZrO2 + FeBETA] catalyst (with component volumetric ratio of 3/1) demonstrated efficient soot oxidation at 400–450 °C and DeNOx performance similar to that of FeBETA catalyst, despite the fact that the amount of zeolite was reduced by four times indicating a strong synergistic effect between the components. Low-temperature DeNOx performance of the [CeO2–ZrO2 + FeBETA] system can be additionally enhanced by doping the CeO2–ZrO2 with Mn. It was found that NOx conversion over [Mn/CeO2–ZrO2 + FeBETA] at 150–250 °C was steadily improved by increasing the Mn loading, and [5.4 % Mn/CeO2–ZrO2 + FeBETA] attained NOx conversion above 90 % at Treact ~190 °C. Comparative studies of soot oxidation and NH3-SCR over combined catalysts and individual components indicated that the soot oxidation activity is assigned to the soot oxidation over ceria–zirconia component, while enhanced low-temperature SCR performance is a result of a “bi-functional” SCR mechanism comprising NO oxidation to NO2 over CeO2–ZrO2 component followed by fast-SCR over FeBETA.

Novel MnWOx catalyst with remarkable performance for low temperature NH3-SCR of NOx

A novel W promoted MnOx catalyst (MnWOx) was used for the selective catalytic reduction (SCR) of NOx with NH3 at low temperatures, with high deNOx efficiency from 60 to 250 °C under relatively high space velocity. The MnWOx catalyst showed a unique core–shell structure with Mn3O4 covered by Mn5O8 while Mn4+ species at the outer surface served as a real active phase for NH3-SCR. The W doping resulted in the smaller particle size of MnOx active phase, increased the surface acidity and facilitated the NO/NH3 oxidation, thus enhancing low temperature deNOx efficiency by promoting both Langmuir–Hinshelwood and Eley–Rideal reaction pathways. This novel catalyst is promising to be used in the deNOx process for flue gas after dust removal and desulfurization.

Design of meso-TiO2@MnOx–CeOx/CNTs with a core–shell structure as DeNOx catalysts: promotion of activity, stability and SO2-tolerance

Developing low-temperature deNOx catalysts with high catalytic activity, SO2-tolerance and stability is highly desirable but remains challenging. Herein, by coating the mesoporous TiO2 layers on carbon nanotubes (CNTs)-supported MnOx and CeOx nanoparticles (NPs), we obtained a core-shell structural deNOx catalyst with high catalytic activity, good SO2-tolerance and enhanced stability. Transmission electron microscopy, X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and NH3 temperature-programmed desorption have been used to elucidate the structure and surface properties of the obtained catalysts. Both the specific surface area and chemisorbed oxygen species are enhanced by the coating of meso-TiO2 sheaths. The meso-TiO2 sheaths not only enhance the acid strength but also raise acid amounts. Moreover, there is a strong interaction among the manganese oxide, cerium oxide and meso-TiO2 sheaths. Based on these favorable properties, the meso-TiO2 coated catalyst exhibits a higher activity and more extensive operating-temperature window, compared to the uncoated catalyst. In addition, the meso-TiO2 sheaths can serve as an effective barrier to prevent the aggregation of metal oxide NPs during stability testing. As a result, the meso-TiO2 overcoated catalyst exhibits a much better stability than the uncoated one. More importantly, the meso-TiO2 sheaths can not only prevent the generation of ammonium sulfate species from blocking the active sites but also inhibit the formation of manganese sulfate, resulting in a higher SO2-tolerance. These results indicate that the design of a core-shell structure is effective to promote the performance of deNOx catalysts.

A commercial V2O5-WO3/TiO2 catalyst used at an NH3-SCR deNOx process in an oil-fired power plant: cause of an increase in deNOxing and NH3 oxidation performances at low temperatures

The extent of both NOx removal and NH3 oxidation in the selective reduction of NOx by NH3 over a sample of a commercial V2O5-WO3/TiO2 catalyst that had been used for 20,000 h at an NH3-SCR deNOx process in a domestic heavy oil-fired power plant has been determined using an on-line IR-based system coupled with a modified White cell and the catalyst has been characterized to ascertain the reason why it possessed visibly different behaviors in the deNOx reaction, compared to a fresh sample. The on-site-used catalyst gave not only much higher deNOx activity at all temperatures less than 400 °C than that indicated for a fresh catalyst but also a greatly increased NH3 oxidation reaction. SEM-EDX measurements with samples of the used and fresh catalysts represented the presence of a huge amount of sulfur in the former sample, very consistent with sulfur amounts determined by a C/S analyzer; however, this did not create the measured enhancement in the low-temperature deNOx and NH3 oxidation performances. ICP measurements indicated a significant increase, by 1% as a basis of V2O5, in an amount of vanadium species after the on-site use and such vanadium species came from heavy oils used to generate electricity at the power plant and were in the form of polycrystalline V2O5 nanoparticles as acquired by XRD measurements. All these results propose that the increased V2O5 content might be associated with the noticeable difference in SCR activity and NH3 oxidation between both of the samples.

DeNOx performance of Ag/Al2O3 catalyst using simulated diesel fuel–ethanol mixture as reductant

The NOx reduction activity of Ag/Al2O3 catalyst has been investigated systematically, using a mixture of simulated diesel fuel (SD) and ethanol (E) as the reductant with special interest in the low temperature deNOx performance below 350°C. The Ag loading and catalyst operating conditions such as C1/NOx and ethanol/simulated diesel fuel (E/SD) ratios have been optimized in order to achieve the maximum deNOx performance of the Ag/Al2O3 catalyst. Ammonia has been identified as the most abundant reaction intermediate over the Ag/Al2O3 catalyst under the optimized operating conditions. To further enhance the deNOx performance by utilizing the NH3 formed over the Ag/Al2O3 catalyst, a dual-bed reactor system has been employed with CuZSM5 catalyst in the rear bed consecutively following the front bed containing the Ag/Al2O3. The NOx-to-N2 conversion in this dual-bed system increased up to 85% from 275 to 450°C, mainly due to the NH3 oxidation to N2 by the CuZSM5 in the rear bed. Physicochemical characterization of the Ag/Al2O3 catalysts by UV–vis, TEM and EELS has indicated that both ionic and metallic Ag formed on the catalyst surface play important roles for the high deNOx performance of the present catalytic system; the ionic Ag including Ag+ and Agδ+ is the active reaction site for the reduction of NOx to N2, while the metallic Ag is responsible for the partial oxidation of hydrocarbons (HCs) which promotes the initiation of the HC/SCR process in the low temperature range. The optimum ionic/metallic (I/M) ratio of Ag species on Al2O3 support surface appears to be in the range of 1.4–1.7, depending upon the catalyst temperature range of interest.

A Metal-Monolithic Anodic Alumina-Supported Ag Catalyst for Selective Catalytic Reduction of NOx with Propene in the Presence of Hydrogen

The effect of H2 addition on the selective catalytic reduction of NOx with propene (C3H6-SCR of NOx) has been investigated using a metal-monolithic anodic alumina-supported Ag catalyst. The low temperature de-NOx activity of the Ag/Al2O3 catalyst is markedly enhanced by the addition of a small amount of H2, which is associated with the improved propene activation at low temperatures. The addition of H2 greatly promotes the nitrate decomposition at low temperatures, which is believed to alleviate nitrate poisoning and initiate the SCR reaction at low temperatures, because of the formation of free silver sites. Furthermore, it is suggested that a small amount of H2 addition will further reduce the freed Ag+ species into Agnδ+ and eventually lead to the improved de-NOx activity, because Agnδ+ is more active in propene activation than Ag+ at low temperatures. However, excess H2 and high Ag loading mainly promoted the nonselective combustion of propene with oxygen, rather than the selective oxidation of propene with NOx.

Low‐Temperature De‐NOx by Selective Catalytic Reduction Based on Iron‐Based Catalysts

AbstractLow‐temperature De‐NOx technology is a new topic in the area of flue gas treatment. This paper presents experimental studies on selective catalytic reduction (SCR) of NOx from synthetic flue gas over iron‐based catalysts with ammonia in a fluidized reactor. The iron‐based catalysts used in the experiments are particles of Fe3O4 and γ‐Fe2O3, and they are analyzed by X‐ray diffraction (XRD) and Mössbauer spectroscopy, before and after reaction. It is observed that the efficiency of De‐NOx with γ‐Fe2O3 catalyst is high at temperatures from 200–290 °C and the maximum efficiency is seen to exceed 90 % at 250 °C. The effects of magnetic fields on SCR De‐NOx using γ‐Fe2O3 particles as catalysts are also studied by applying uniform and coaxial magnetic fields to the fluidized bed. The results suggest that SCR De‐NOx on γ‐Fe2O3 catalyst is fit for operation below 200 °C in the fluidized reactor in conjunction with the effects of magnetic fields.

High deNOx performance of Mn/TiO 2 catalyst by NH 3

The deNOx performance of the s-Mn/TiO 2 catalyst prepared by sol–gel method has been significantly enhanced, particularly in the low temperature region. Mn is basically incorporated into the matrix of Ti and becomes a structural component of s-MnO 2/TiO 2 mixed oxide, whereas by impregnation method it simply exists on the surface of i-Mn/TiO 2. The well-dispersed Mn over s-Mn/TiO 2 catalyst is the primary cause for the high deNOx performance compared to that over i-Mn/TiO 2. The addition of Fe onto the Mn/TiO 2 catalyst improves the NOx removal activity by NH 3 in a wide operating temperature window, including the low temperature region less than 250 °C, regardless of the catalyst preparation methods. Including NO 2 (NO 2/NO = 1) in the feed gas stream further enhances the low temperature deNOx activity of Mn/Fe/TiO 2 catalyst.

TPD of NO2− and NO3− from Na-Y: The relative stabilities of nitrates and nitrites in low temperature DeNOx catalysis

Temperature programmed depletion (TPD) was performed for both surface nitrates (H+–NO3−) and surface nitrites (H+–NO2−) on Na-Y. The depletion curves are each adequately described with a pre-exponential of 1013 and activation energies that vary as a function of coverage from ∼100 to 200kJmol−1. Multiple adsorption sites could lead to the observed variation in activation energies. NO2− is more kinetically stable than NO3− on Na-Y. The effect that the method of preparation of NO3− and NO2− could have on the relative stabilities of the nitrates and nitrites is discussed. The trends for the relative stabilities of different nitrates and nitrites on Na-Y and BaNa-Y are examined as a function of the cationic adsorption site and the counter-ion (e.g., H+ and NO+). The possible roles of the stabilities of nitrates and nitrites in low temperature DeNOx catalysis are considered.

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250°C and could potentially deactivate low temperature NOx reduction catalysts by blocking active sites. It is shown that NO reduces neat NH4NO3 above its 170°C melting point, while acidic solids catalyze this reaction even at temperatures below 100°C. NO2, a product of the reduction, can dimerize and then dissociate in molten NH4NO3 to NO++NO3−, and may be stabilized within the melt as either an adduct or as HNO2 formed from the hydrolysis of NO+ or N2O4. The other product of reduction, NH4NO2, readily decomposes at ≤100°C to N2 and H2O, the desired end products of DeNOx catalysis. A mechanism for the acid catalyzed reduction of NH4NO3 by NO is proposed, with HNO3 as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNOx systems could help mitigate catalyst deactivation at low operating temperatures (<150°C).

Efficient decomposition of NO by ammonia radical-injection method using an intermittent dielectric barrier discharge

Although many NO decomposition systems have been developed using plasmas such as dielectric barrier discharges (DBDs), corona discharges, surface discharges, glow discharges, and microwave discharges, the present system is unique on the viewpoint of the use of an intermittent one-cycle sinusoidal power source to generate DBD plasma. There are several features of the system: (1) easy control of the electric power consumed in the DBD plasma, and (2) DBD-plasma generation used only for the production of ammonia radicals. The system employs a radical injection system, where the radicals are produced in a separate discharge chamber, called radical injector, from NO flow field. This enables an efficient production of ammonia radicals being appropriate for DeNOx. It is shown from the temperature dependence of NO removal (DeNOx) characteristics that the present system is a low-temperature DeNOx system compared to a conventional thermal DeNOx system, and NO decomposition is performed over a wide range of gas temperature containing NO. Surveying parametric characteristics of DeNOx, the energy efficiency is improved by a factor of 30% compared to the previously obtained result.