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     

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.     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

Simulation of NOx and soot abatement with Cu‐Cha and Fe‐ZSM5 catalysts

The embodiment of the NO_x selective catalytic reduction (SCR) functionality in a diesel particulate filter (DPF), so‐called SCR‐on‐Filter (SCRoF), is investigated through numerical modeling with SCR kinetics corresponding to Cu‐Chabazite and Fe‐ZSM5 catalysts. The results of the simulations of the SCR activity, performed in the absence and presence of soot, indicate that the presence of soot negligibly affects the NO_x conversion efficiency, given the slow dynamics of passive regeneration. Conversely, the reduction in cake thickness by soot passive oxidation is significantly different in the absence of SCR activity (uncatalyzed DPF) compared to that in its presence (SCRoF). In fact, in the SCRoF only 60–80% of the original soot consumption obtained in the absence of SCR reaction over 1 h can be achieved. Individual Cu‐Chabazite and Fe‐ZSM5 catalysts, as well as in‐series layers of the two catalysts, are investigated to devise the widest temperature window for SCRoF. © 2016 American Institute of Chemical Engineers AIChE J, 63: 238–248, 2017     

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.     

Selective Catalytic Reduction of NOx with Ammonia over Fe‐Mo/ZSM‐5 Catalysts

The Fe‐Mo/ZSM‐5 catalysts were prepared by an impregnation method. A comparison of the catalytic activity for the reduction of NO_x with ammonia over Fe‐Mo/ZSM‐5, Fe‐ZSM‐5 and Mo/ZSM‐5 catalysts was carried out. Also, the effects of the conditions used for calcination as well as of the Fe/Mo ratio on the catalytic performance of Fe‐Mo/ZSM‐5 catalysts were studied. It was found that Fe‐Mo/ZSM‐5 is more active than Fe‐ZSM‐5 and Mo/ZSM‐5 separately. Fe‐Mo/ZSM‐5 exhibited the best performance for SCR reaction with a Fe/Mo ratio of 1.5 and yielded the highest NO_x conversion of 96 % at a temperature of 430 °C. The results also showed that the performance of Fe‐Mo/ZSM‐5 is sensitive to the conditions used during calcination. The bulk phase and surface composition of the Fe‐Mo/ZSM‐5 catalysts were determined by XRD, BET, and XPS techniques, respectively. The results revealed that the surface Mo percentage is the largest when the Fe/Mo ratio is 1.5, which may be related to its higher activity for catalytic reduction of NO_x. XRD results indicate that the best catalytic performance of the Fe/Mo = 1.5 sample results from a strong interaction among Fe, Mo and HZSM‐5. In addition, it can be tentatively presumed that the surface nitrous species from the calcinations play an important role in SCR of NO_x over Fe‐Mo/ZSM‐5 catalysts.     

Fe‐Beta@CeO2 core‐shell catalyst with tunable shell thickness for selective catalytic reduction of NOx with NH3

A series of core‐shell structural deNO_x catalysts using small‐grain Beta supporting FeO_x nanoparticles as the core and tunable CeO₂ thin film thickness as sheaths were designed and controllably synthesized. Their catalytic performances were tested for selective catalytic reduction of NO_x with NH₃ (NH₃‐SCR). It was found that CeO₂ shell thickness plays an important role in influencing the acidity and redox properties of the catalysts. Fe‐Beta@CeO₂ core‐shell catalysts exhibit excellent resistance to H₂O and SO₂ and high NO_x conversion (above 90%) in the wide temperature range (225–565°C). The kinetics result indicates that the coating of CeO₂ shell significantly increases the pore diffusion resistance of Fe‐Beta@CeO₂ catalysts. Furthermore, in situ DRIFT results reveal that CeO₂ shell can promote the formation of NO₂ and cis‐ species. But too thick CeO₂ shell (～20 nm) would result in the formation of inactive nitrate species, and thereby lead to a decrease of high‐temperature activity of the catalysts. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4430–4441, 2017     

Effects of copper loading on NH<Subscript>3</Subscript>-SCR and NO oxidation over Cu impregnated CHA zeolite

Cu/CHA catalysts with various Cu loadings (0.5 wt%–6.0 wt%) were synthesized via incipient wetness impregnation. The catalysts were applied to the selective catalytic reduction (SCR) of NO with NH₃ and NO oxidation reaction. XRD and N₂ adsorption-desorption data showed that CHA structure was maintained with the incorporation of Cu, while specific surface areas decreased with increasing Cu loading. At intermediate Cu loading, 4 wt%, the highest NH₃-SCR activity was observed with ～98% N₂ selectivity from 150 °C to 300 °C. Small amounts of water, 2%, slightly increased NO conversion in addition to the remarkable N₂O and NO₂ reduction at high temperature. Water effects are attributed to the improved Cu ion reducibility and mobility. NO oxidation results provided no relation between NO₂ formation and SCR activity. Physicochemical properties, NO conversion, N₂ selectivity, and activation energy data showed that impregnated samples’ molecular structure and catalytic activity are comparable to the conventional ion-exchanged (IE) samples’ ones.     

Mesoporous Fe-containing ZSM-5 zeolite single crystal catalysts for selective catalytic reduction of nitric oxide by ammonia

Mesoporous and conventional Fe-containing ZSM-5 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in NO selective catalytic reduction (SCR) with NH₃. It was found that mesoporous Fe-ZSM-5 catalysts exhibit higher SCR activities than comparable conventional catalysts. Furthermore, conventional Fe-ZSM-5 catalysts have maximum activity at ~2.5 wt% Fe while for the mesoporous system, optimal NO conversion is obtained for the catalysts with ~6 wt % Fe.     

Potential and Limitations of Natural Chabazite for Selective Catalytic Reduction of NOx with NH3

The potential of the natural chabazite for the selective catalytic reduction (SCR) of NOx with NH₃ is evaluated in the present work. Activity tests were performed under technically relevant reaction and temperature conditions for the fresh and hydrothermally aged catalysts. The natural chabazite before and after alkaline removal as well as after iron and copper addition were compared. The structural as well as surface and bulk properties were elucidated by a variety of complementary characterization techniques, i.e. XRD, XPS, EPR, BET, NH₃‐TPD, ex situ and in situ XAS. The results indicate that an important facet for using the natural chabazite for the standard and fast SCR reactions is the removal of alkaline metals, which at the same time also leads to a partial change of the structure and the size of the iron‐containing particles. The performance and especially the hydrothermal stability can be further improved by copper addition.     

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     

Screening of Fe–Cu-Zeolites Prepared by Different Methodology for Application in NSR–SCR Combined DeNOx Systems

The NO_x NH₃-SCR performance of several Cu and Fe catalysts supported on BETA and ZSM-5 zeolites has been studied in single SCR and double NSR–SCR configuration, and the activity related to the nature and reducibility of metal species on the catalyst surface. Intermediate ammonia formed in NSR improved greatly NO_x conversion at the exit of the double NSR–SCR configuration, which was practically totally converted to N₂.     

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.     

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.     

The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13

Abstract  

The effect of Cu loading on the selective catalytic reduction of NO_x by NH₃ was examined over a series of Cu ion-exchanged (20–80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80–95%) were obtained over all catalyst samples between 250 and 500 °C, and at the gas hourly space velocity of 200,000 h⁻¹. Both NO reduction and NH₃ oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures (>500 °C) due to excess NH₃ oxidation. The optimum Cu ion exchange level appears to be ~40–60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600 °C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the “fast” SCR (i.e., NO/NO₂ = 1), only a slight improvement in NO_x reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N₂ was observed; only a very small amount of N₂O was produced even in the presence of NO₂. The apparent activation energies for NO oxidation and NO SCR were estimated to be ~58 and ~41 kJ/mol, respectively.     

Selective catalytic reduction by urea in a fluidized‐bed reactor

The selective catalytic reduction (SCR) of NO_x by urea as a reducing agent was carried out over fresh and sulfated CuO/γ‐Al₂O₃ catalysts in a fluidized‐bed reactor. The optimum temperature ranges for NO reduction on the fresh and sulfated CuO/γ‐Al₂O₃ catalysts were 300–350 °C and 400–450 °C, respectively. NO reduction with the sulfated CuO/γ‐Al₂O₃ catalyst was somewhat higher than that with the fresh CuO/γ‐Al₂O₃ catalyst. N₂O formation increased with increasing reaction temperature. Ammonia (NH₃) slip increased with increasing gas velocity and decreased with increasing reaction temperature. Copyright © 2003 Society of Chemical Industry     

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

     

Bifunctional mesoporous Cu–Al–MCM-41 materials for the simultaneous catalytic abatement of NOx and VOCs

This study explored the possibility of using waste organic solvent as the source of volatile organic compound (VOC) and it served as a reducing agent of selective catalytic reduction (SCR) deNO_x process, in which the VOC itself can be catalytically oxidized on the mesoporous Cu and/or Al substituted MCM-41 catalysts. The synthesized Cu–Al–MCM-41 catalysts were extensively characterized by powder low-angle X-ray diffraction (XRD), N₂ adsorption–desorption measurements, transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), ²⁷Al magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma–mass spectrometer (ICP–MS) analysis. The XRD, TEM and N₂ adsorption–desorption studies clearly demonstrated the presence of a well ordered long range hexagonal array with uniform mesostructures. The Cu–Al–MCM-41 materials showed a better long-term-stability than that of copper ion-exchanged H–ZSM-5 (Cu–ZSM-5) zeolite. The Cu–Al–MCM-41 material was found to be an efficient catalyst than that of Cu–MCM-41 without aluminum for the simultaneous catalytic abatement of NO_x and VOCs, which was attributed to the presence of well dispersed and isolated Cu²⁺ ions on the Cu–Al–MCM-41 catalyst as observed by UV–Vis DRS and EPR spectroscopic studies. And the presence of aluminum (Al³⁺ ions) within the framework of Cu–Al–MCM-41 stabilized the isolated Cu²⁺ ions thus it led to higher and stabilized activity in terms of NO_x reduction.     

Insights into Supported Copper(II)‐Catalyzed Azide‐Alkyne Cycloaddition in Water

Cross‐linked polymeric ionic liquid material‐supported copper (Cu‐CPSIL), imidazolium‐loaded Merrifield resin‐supported copper (Cu‐PSIL) and silica dispersed CuO (CuO/SiO₂), were prepared and proved to be efficient catalysts for the one‐pot synthesis of 1,4‐disubsituted‐1,2,3‐triazoles by the reaction of alkyl halides with sodium azide and terminal alkynes in water at room temperature. Moreover, these supported copper catalysts were recovered quantitatively from the reaction mixture by simple filtration and reused for five consecutive recycles without significant loss of catalytic activity. Among the three immobilized copper catalysts, Cu‐CPSIL exhibited excellent catalytic activity for the reaction of aliphatic bromides, sodium azide and terminal alkynes. The differences in the catalytic performances of the catalysts could be ascribed to the copper dispersion and the interaction between copper and the supports. In addition, water was used as the reaction media and the proton provider, the latter was found to be very important for the reaction. The XPS results suggested that the supported Cu(II) catalysts were reduced to catalytic Cu(I) species via alkynes homocoupling reaction. By means of IR and ESI‐MS studies, a possible mechanism of cycloaddition based on the reduction of Cu(II) to Cu(I) species was proposed.     

TPD‐TG‐MS Investigations of the Catalytic Conversion of Glycerol over MOx‐Al2O3‐PO4 Catalysts

The catalytic performance of bifunctional catalysts, MO_x‐Al₂O₃‐PO₄, that contain acidic centers and different transition metal oxide components were evaluated in the gas‐phase dehydration of glycerol using the TPD‐TG‐MS technique and a continuous flow reactor experiment. The initial catalytic activity and selectivity to acrolein and acetol significantly depends on the acidity and the type of transition metal oxide. The higher the total acidity, the higher the acrolein selectivity in the order W > Mo > Cu > V~ Fe ~Cr > Ce. On the other hand, Mn‐, Cr‐, and Fe‐containing catalysts favor the formation of products of oxidative C‐C cleavage. TPD‐TG‐MS investigations of catalysts loaded with glycerol are useful tools for fast‐screening of initial activities of catalysts in the gas‐phase dehydration of glycerol.