Kinetic modeling of Fe‐BEA as NH3‐SCR catalyst—effect of phosphorous

The focus of this work is to investigate whether a previously developed microkinetic deactivation model for hydrothermally treated Fe‐BEA as NH₃‐SCR catalyst can be applied to describe chemical deactivation of Fe‐BEA due to phosphorous exposure. The model describes the experiments well for Fe‐BEA before and after phosphorous exposure by decreasing the site density, representing deactivation of sites due to formation of metaphosphates blocking the active iron sites, while the kinetic parameters are kept constant. Furthermore, the results show that the activity for low‐temperature selective catalytic reduction (SCR) is very sensitive to loss of active monomeric iron species due to phosphorous poisoning compared to high‐temperature SCR. Finally, the ammonia inhibition simulations show that exposure to phosphorous may affect the internal transport of ammonia between ammonia storage sites buffering the active iron sites, which results in a lower SCR performance during transient conditions. © 2014 American Institute of Chemical Engineers AIChE J, 61: 215–223, 2015     

A kinetic model of the hydrogen assisted selective catalytic reduction of NO with ammonia over Ag/Al2O3

A global kinetic model which describes H₂‐assisted NH₃‐SCR over an Ag/Al₂O₃ monolith catalyst has been developed. The intention is that the model can be applied for dosing NH₃ and H₂ to an Ag/Al₂O₃ catalyst in a real automotive application as well as contribute to an increased understanding of the reaction mechanism for NH₃‐SCR. Therefore, the model needs to be simple and accurately predict the conversion of NO_x. The reduction of NO is described by a global reaction, with a molar stoichiometry between NO, NH₃ and H₂ of 1:1:2. Further reactions included in the model are the oxidation of NH₃ to N₂ and NO, oxidation of H₂, and the adsorption and desorption of NH₃. The model was fitted to the results of an NH₃‐TPD experiment, an NH₃ oxidation experiment, and a series of H₂‐assisted NH₃‐SCR steady‐state experiments. The model predicts the conversion of NO_x well even during transient experiments. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4325–4333, 2013     

Monitoring the Ammonia Loading of Zeolite‐Based Ammonia SCR Catalysts by a Microwave Method

Exhaust gas aftertreatment systems, which reduce nitrogen oxide emissions of heavy‐duty diesel engines, commonly use a selective catalytic reduction (SCR) catalyst. Currently, emissions are controlled by evaluating NO_x or NH₃ in the gas phase downstream the catalyst and calculating the NH₃ loading via a chemical storage model. Here, a microwave‐cavity perturbation method is proposed in which electromagnetic waves are excited by probe feeds and the reflected signals are measured. At distinct resonance frequencies, the reflection coefficient shows a pronounced minimum. These resonance frequencies depend almost linearly on the NH₃ loading of a zeolite‐based SCR catalyst. Since the NH₃ loading‐dependent electrical properties of the catalyst material itself are measured, the amount of stored ammonia can be determined directly and in situ. The cross‐sensitivity towards water can be reduced almost completely by selecting an appropriate frequency range.     

Deactivation mechanism of activated carbon supported copper oxide SCR catalysts in C2H4 reductant

Current state‐of‐the‐art NH₃‐SCR technology based on vanadium catalysts suffers problems associated with NH₃ slip and poisoning of the catalyst and blockage of heat recovery steam generators (HRSG). If environmentally‐friendly catalysts capable of efficient operation at lower temperatures could be developed that used a reductant other than NH₃, the issues with current state‐of‐the‐art SCR could be significantly lessened. Hence, in this study, activated carbon (AC) supported copper oxide‐based catalysts for SCR while using C₂H₄ as a reductant was discussed. Reaction testing of catalysts demonstrated high initial NO conversion with steeply declining activity over 2 h of testing when C₂H₄ was used as the reductant; in comparison, with the same catalyst and NH₃ as the reductant, stable, long‐term NO conversion was achieved, but at a lower rate than the initial reactivity with C₂H₄. As a consequence, catalyst characterization studies were performed to assess deactivation mechanisms when C₂H₄ was the reductant. These studies included x‐ray diffraction, BET surface area and porosity, temperature programmed reduction, scanning electron microscopy, Raman spectroscopy and x‐ray photoelectron spectroscopy of both fresh and deactivated catalysts. The analytical results showed the surface area and porosity of the catalyst remained unchanged and the initially highly‐dispersed Cu species became agglomerated and more crystalline during reaction testing. Furthermore, carbon black was also detected on the catalyst surface after testing, presumably formed during the decomposition of C₂H₄. Both agglomeration of the active Cu species and blockage by carbon deposits would decrease the availability of active sites and lead to decreased catalytic activity.     

High Temperature SCR Over Cu-SSZ-13 and Cu-SSZ-13 + Fe-SSZ-13: Activity of Cu2+ and [CuOH]1+ Sites and the Apparent Promoting Effect of Adding Fe into Cu-SSZ-13 Catalyst

Cu-SSZ-13 catalysts were synthesized with Si: Al?=?4.5 and 25, to obtain materials with isolated Cu²⁺ and [CuOH]¹⁺ sites, respectively. The catalysts were tested for the selective catalytic reduction of NOx (SCR), NO oxidation and NH₃ oxidation. Cu²⁺ sites presented the highest NO rates and lowest NH₃ rates, as the temperature was increased from 300 °C to 650 °C, during SCR and NH₃ oxidation, respectively. None of the Cu-SSZ-13 catalysts presented activity for NO oxidation, consistent with the absence of copper oxide clusters. In addition, catalysts composed by mechanical mixtures of Cu-SSZ-13?+?Fe-SSZ-13 with Si: Al?=?4.5 and 25 were tested for SCR, NO oxidation and NH₃ oxidation, to study the effect of the presence of iron together with Cu-SSZ-13 for improving its SCR working temperature range. Higher reaction rates for NO oxidation and NH₃ oxidation over Cu-SSZ-13?+?Fe-SSZ-13 showed a more relevancy of side reactions that makes a combined effect of Fe-SSZ-13 and Cu-SSZ-13 not a real improvement in high temperature SCR.

Graphic Abstract

     

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

An overview is given of the selective catalytic reduction of NO_x by ammonia (NH₃‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH₃‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH₃‐SCR reaction over Fe‐ZSM‐5, with isolated species being the most active.     

Unifying redox kinetics for standard and fast NH3‐SCR over a V2O5‐WO3/TiO2 catalyst

A dynamic Mars–van Krevelen kinetic model that unifies Standard and Fast SCR reactions into a single redox approach is herein proposed for V‐based catalysts for NO_x removal from Diesel exhausts. Such a mechanistic model is consistent with the detailed catalytic chemistry proposed for the NH₃‐NO/NO₂ reacting system in which NO₂ disproportionates to form nitrites and nitrates, nitrates are reduced by NO to nitrites in a key redox step, and nitrites react with NH₃ to form N₂ via decomposition of unstable ammonium nitrite. Intrinsic kinetic parameters were estimated by global multiresponse nonlinear regression of 42 transient runs. The model accounts for stoichiometry, selectivity, and kinetics of the global SCR process, reproducing successfully both the steady‐state and transient behaviors of the SCR reacting system over the full range (0–1) of NO₂/NO_x feed ratios in the 175–425°C temperature range. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.     

Reduction of Nitrogen Oxides by Ammonia Over Iron‐Containing Catalysts

Selective catalytic reduction (SCR) of NO_x by NH₃ is an efficient technology for the removal of nitrogen oxides from diesel exhaust. It is a disadvantage that the active component in commercial catalysts, V₂O₅, is toxic and melts at ～650 °C. An alternative catalyst system, based on iron as the active component, was developed in this work. For this purpose, a number of supports were taken and impregnated with Fe₂O₃ and Fe₂O₃/WO₃ by an incipient wetness technique. The synthesized catalysts were tested in a diesel model exhaust using temperature programmed reaction. The screening accomplished to date, resulted in a 5.8 mol.‐% Fe₂O₃/Al₂O₃ catalyst that exhibited outstanding activity in the temperature range between 150–375 °C with selective formation of N₂. However, this catalyst was significantly deactivated by thermal aging at 600 °C. In contrast, the activity of the sample with 1.4 mol.‐% Fe₂O₃ and 0.8 mol.‐% WO_x showed very high thermal stability as well as selective N₂ production over the whole temperature regime, but it had lower SCR activity.     

On the Efficiency of NH3–SCR Catalysts for Heavy Duty Vehicles Running on Compressed Natural Gas in Synthetic Gas Bench Scale

This work aims to study the effectiveness of NH₃–SCR after-treatment systems, initially developed for a Diesel application, on Heavy duty natural gas engines working in lean conditions for exhaust gas pollutants abatement. Commercial oxidation and NH₃–SCR catalysts were investigated for respectively CH₄, CO oxidation and NO_X reduction. In this study, we showed that the NH₃–SCR coupled with an oxidation catalyst lead to significant conversion of CH₄, CO and NO_X, and can be used as after-treatment system for pollutants providing from CNG lean burn engines.     

Mechanistic Study of the NO + NH<Subscript>4</Subscript>NO<Subscript>3</Subscript> Reaction on H- and Fe/H-BEA Zeolites Using <Superscript>15</Superscript>N and <Superscript>18</Superscript>O Labeled Species

Reduction of NO by NH₃ over metal-promoted zeolites represents the principal reaction in the selective catalytic reduction (SCR) technology for NOx removal from Diesel engine exhausts. It has been established that addition of ammonium nitrate (AN) to the reaction mixture substantially enhances the rate of this reaction, decreasing the temperature necessary for an efficient deNOx process. Nevertheless, the nature of this effect has not been completely elucidated. To investigate the NO?+?AN reaction mechanism, we have used individual reactants labeled with either ¹⁵N or ¹⁸O (or both isotopes), thus obtaining an experimental background for proposing the route of the SCR accelerated by AN addition. For this study, we have used as the catalysts H-BEA and Fe/H-BEA zeolites with various Si/Al ratios and various amounts and states of the iron species.     

Effect of Preparation Procedure on the Catalytic Properties of Fe-ZSM-5 as SCR Catalyst

Fe-ZSM-5 catalysts, prepared by different methods, have been characterized by BET, ICP-AES, XRD, XPS, NH₃-TPD and NO-TPD and evaluated for NO_x reduction according to standard NH₃-SCR, NH₃ oxidation and NO oxidation, in absence and presence of water. The presence of water has a significant influence on both the SCR and oxidation reactions. The most active catalyst for NH₃-SCR is prepared by ion exchange using FeCl₂ as iron precursor. The XPS results indicate that Fe²⁺ ions are the main active sites for the SCR reactions, while Fe³⁺ ions are the primarily active sites for oxidation of ammonia.     

A Modeling Study of NH3 Slip Catalysts: Analysis of the SCR/PGM Interactions

We present a simulation study which comparatively analyzes the performances of four configurations of NH₃ slip catalysts (ASC) for automotive NH₃–SCR based Diesel aftertreatment systems. The comparison in terms of NH₃ conversion and N₂ selectivity over a wide range of operating conditions highlights the positive interaction of NH₃–SCR and PGM oxidation chemistries, which is best exploited by a dual-layer configuration with the SCR catalyst on top. This study also emphasizes the key role of modeling and simulation tools in the development of complex exhaust aftertreatment systems as well as the importance of a clear understanding of the catalytic chemistry involved in automotive deNOx converters.     

Simultaneous removal of COD and NH3‐N in secondary effluent of high‐salinity industrial waste‐water by electrochemical oxidation

BACKGROUND: Electrochemical oxidation has been applied successfully in industrial waste‐water treatment. The simultaneous removal of COD_Cr and NH₃‐N, as well as the corresponding mechanisms and reaction zone, were examined in this study. The reaction kinetics and the significant factors that affect removal performance were also studied. RESULTS: The COD_Cr removal efficiency without chlorides in waste‐water was only 11.8% after 120 min of treatment, which was much lower than the efficiency with chlorides, and agitation did not improve the performance. When the current density was increased from 2.5 to 10 mA cm^?2, the removal efficiency was improved. The removal efficiencies of COD_Cr and NH₃‐N were less at initial pH = 11 than at pH = 3 and 8.7 (without adjustment). The COD_Cr and NH₃‐N removal efficiencies were decreased by about 30% and 50%, respectively, when the electrode distance was increased from 4 to 12 cm. Instantaneous current efficiency decreased with increase in current density. CONCLUSIONS: The degradation of pollutants occurred mainly at the boundary layer between the electrode and the bulk solution. The indirect oxidation by active chlorine generated from the chlorides was proven to be the primary mechanism of electrochemical oxidation treatment. The removal of COD_Cr in this study followed a pseudo‐first‐order kinetic model. Copyright © 2011 Society of Chemical Industry     

A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5

Kinetic modeling, in combination with flow reactor experiments, was used in this study for simulating NH₃ selective catalytic reduction (SCR) of NO_x over Cu-ZSM-5. First the mass-transfer in the wash-coat was examined experimentally, by using two monoliths: one with 11 wt.% wash-coat and the other sample with 23 wt.% wash-coat. When the ratio between the total flow rate and the wash-coat amount was kept constant similar results for NO_x conversion and NH₃ slip were obtained, indicating no significant mass-transfer limitations in the wash-coat layer. A broad range of experimental conditions was used when developing the model: ammonia temperature programmed desorption (TPD), NH₃ oxidation, NO oxidation, and NH₃ SCR experiments with different NO-to-NO₂ ratios. 5% water was used in all experiments, since water affects the amount of ammonia stored and also the activity of the catalyst. The kinetic model contains seven reaction steps including these for: ammonia adsorption and desorption, NH₃ oxidation, NO oxidation, standard SCR (NO + O₂ + NH₃), rapid SCR (NO + NO₂ + NH₃), NO₂ SCR (NO₂ + NH₃) and N₂O formation. The model describes all experiments well. The kinetic parameters and 95% linearized confidence regions are given in the paper. The model was validated with six experiments not included in the kinetic parameter estimation. The ammonia concentration was varied from 200 up to 800 ppm using NO only as a NO_x source in the first experiment and 50% NO and 50% NO₂ in the second experiment. The model was also validated with transient experiments at 175 and 350 °C where the NO and NH₃ concentrations were varied stepwise with a duration of 2 min for each step. In addition, two short transient experiments were simulated where the NO₂ and NO levels as well as NO₂-to-NO_x ratio were varied. The model could describe all validation experiments very well.     

Selective catalytic reduction of NOx, by propene over copper-exchanged pillared clays

Copper-exchanged pillared clays were examined as an SCR catalyst for NO_x, removal by propene. Both micropores and mesopores were simultaneously developed by pillaring a bentonite with TiO₂. Therefore, TiO₂-pillared clay has about 8 to 9 times higher surface area and 3 times higher pore volume than the parent unpillared bentonite. The presence of water in the feed gas stream caused a small and reversible inhibition effect on NO removal activity of Cu/Ti-PILC. The water tolerance of Cu/Ti-PILC was higher than copper-exchanged zeolites such as CuHM and Cu/ZSM-5 due to its high hydrophobicity as confirmed by H₂O-TPD experiment. Copper-exchanged PILC was confirmed to be an active catalyst for NO_x, removal by propene. The addition of copper to TiO₂-pillared clay greatly enhanced the catalytic activity for NO removal. Cupric ions on Ti-PILC were active reaction sites for the present reaction system. The state of copper species on the surface of Ti-PILC varied with the content of copper and TiO₂. The catalyst having more easily reducible cupric ions showed maximum NO conversion at relatively lower reaction temperatures. It indicates that the redox behavior of cupric ions is directly related to NO removal mechanism. The redox property of cupric ions depended on the copper content and dehydration temperature of PILC.     

Chabazite Synthesis and Its Exchange with Ti,Zn, Cu,Ag and Au for Efficient Photocatalytic Degradation of Methylene Blue Dye

A chabazite-type zeolite was prepared by the hydrothermal method. Before ion exchange, the chabazite was activated with ammonium chloride (NH₄Cl). The ion exchange process was carried out at a controlled temperature and constant stirring to obtain ion-exchanged chabazites of Ti⁴⁺ chabazite (TiCHA), Zn²⁺ chabazite (ZnCHA), Cu²⁺ chabazite (CuCHA), Ag⁺ chabazite (AgCHA) and Au³⁺ chabazite (AuCHA). Modified chabazite samples were characterized by X-ray diffraction (XRD), scanning electron microscope equipped with energy-dispersive spectroscopy (SEM-EDS), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), N₂ adsorption methods and UV–visible diffuse reflectance spectroscopy (DRS). XRD results revealed that the chabazite structure did not undergo any modification during the exchange treatments. The photocatalytic activity of chabazite samples was evaluated by the degradation of methylene blue (MB) in the presence of H₂O₂ under ultraviolet (UV) light illumination. The photodegradation results showed a higher degradation efficiency of modified chabazites, compared to the synthesized chabazite. CuCHA showed an efficiency of 98.92% in MB degradation, with a constant of k = 0.0266 min⁻¹ following a first-order kinetic mechanism. Then, it was demonstrated that the modified chabazites could be used for the photodegradation of dyes.     

OPTIMIZATION OF LEACHING OF COPPER FROM OXIDIZED COPPER ORE IN NH3-(NH4)2SO4 MEDIUM

Studies were carried out for selective leaching of Cu with simultaneous avoidance of iron dissolution during leaching of oxidized copper ore in an aqueous NH₃-(NH₄)₂SO₄ system. The effects of leaching parameters, such as ammonia concentration, ammonium sulphate concentration, leaching time, and solid-to-liquid ratio, were investigated on leaching of copper. A 2ⁿ factorial experimental design method in the dissolution experiments was used. In addition, the “Steepest Ascent” method was also applied to determine the optimum leaching conditions. It was observed that the most effective parameters on the leaching of copper were ammonia concentration and leaching time. Only 0.17% of iron in ore was dissolved in ammonia and ammonium sulphate medium. The optimum conditions established for maximum copper recovery were: ammonia concentration 2.824 mol L^?1, ammonium sulphate concentration 0.236 mol L^?1, solid-to-liquid ratio 0.167 g mL^?1, leaching time 2 h. Fixed parameters chosen in the experiments were: room temperature, average particle size 2.8 mm, stirring speed 500 rpm. Under the optimum conditions established for maximum copper recovery, the percentage of leached copper was 98.87.     

Low‐Temperature Selective Catalytic Reduction of NO on an Iron Ore Catalyst in a Magnetically Fluidized Bed

Low‐temperature selective catalytic reduction of NO from simulated flue gas by NH₃ on admixtures of iron ore and iron catalyst in a magnetically fluidized bed was experimentally studied. It was found that iron ore exerts a prominent catalytic effect on NO removal resulting in high removal efficiency without magnetic fields. Since iron ore is insensitive to magnetic fields due to its antiferromagnetic property, NO removal efficiency with iron ore as catalyst is only slightly influenced by magnetic fields. The NO removal efficiency, however, can be greatly improved using admixtures of iron ore and iron as catalyst in a magnetically fluidized bed. At defined magnetic field intensity and the same temperature, the removal efficiency reaches higher values compared with that without magnetic fields.     

Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO2 by NH3 over Fe/ZSM-5

The reaction kinetics of selective catalytic reduction (SCR) by NH₃ on NO (standard SCR) and on NO + NO₂ (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N₂ production rate was transiently promoted in the absence of gaseous NH₃; this enhancement can be attributed to the negative reaction order of NH₃ (between −0.21 and −0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NO_ad and O_ad to form NO₂. In the fast SCR, however, the promotion behavior in the absence of gaseous NH₃ was not observed and the apparent NH₃ order changed from positive to negative with NH₃ concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO₂ and NH₃ was occurring due to strong NO₂ adsorption; this must be the main reason for the absence of the promotion effect.