Special Issue "Selective Catalytic Reduction of NOx"

A special issue of Catalysts (ISSN 2073-4344). This special issue belongs to the section "Metal Catalysis".

Deadline for manuscript submissions: closed (28 February 2018)

Special Issue Editor

Guest Editor
Prof. Dr. Oliver Kröcher

Paul Scherrer Institut, CH-5232 Villigen, Switzerland
Website | E-Mail
Interests: emission control catalysts, selective catalytic reduction, diesel oxidation catalysts, methane oxidation, bioenergy, catalytic processes for gaseous and liquid biofuels, operando spectroscopy

Special Issue Information

Dear Colleagues,

The recent diesel scandal made the public again aware of the fact that NOx emissions from diesel engines are a major threat to human health and by no means easy to avoid. The most efficient process to reduce NOx emissions from lean exhaust gases is selective catalytic reduction (SCR) with ammonia, which has undergone tremendous development over the past decades. Originally only applied in stationary power plants and industrial installations, SCR systems are now installed also in millions of mobile diesel engines, ranging from off-road machineries, heavy-duty and light-duty trucks and passenger cars, to locomotives and ships. These applications are particularly challenging due to the varying operation conditions of mobile diesel engines with respect to exhaust gas temperature, exhaust gas flow, NOx inlet concentrations, ambient temperature and available installation space. As a matter of fact, many problems are still encountered with all the different SCR applications and much research is being conducted to overcome them.

Submissions to this special issue on “Selective Catalytic Reduction of NOx” are welcome in the form of original research papers or short reviews that reflect the state of research in the SCR field on the following topics: Selective catalytic reduction of NOx (SCR) for diesel vehicles/stationary power plants/industrial installations, SCR catalyst research and development (V-based catalysts, Fe-zeolites, Cu-zeolites), catalyst deactivation, SCR reaction mechanisms, SCR kinetics and modelling, structure-function relationships in SCR catalysts and dosage/decomposition of reducing agents for SCR.

Prof. Dr. Oliver Kröcher
Guest Editor

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Keywords

  • Selective catalytic reduction (SCR) with ammonia/urea
  • SCR in diesel vehicles, stationary power plants and industrial installations
  • SCR catalyst research and development on V-based systems, Fe-zeolites and Cu-zeolites
  • Catalyst deactivation
  • SCR reaction mechanisms
  • SCR kinetics and modelling
  • Structure-function relationships in SCR catalysts
  • Control, dosage and decomposition of reducing agents for SCR

Published Papers (17 papers)

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Research

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Open AccessArticle Ammonia Concentration Distribution Measurements on Selective Catalytic Reduction Catalysts
Catalysts 2018, 8(6), 231; https://doi.org/10.3390/catal8060231
Received: 18 April 2018 / Revised: 16 May 2018 / Accepted: 22 May 2018 / Published: 1 June 2018
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Abstract
This work presents the methodology and accurate evaluation of ammonia concentration distribution measurements at the selective catalytic reduction (SCR) catalyst outlet cross-section. The uniformity of ammonia concentration is a crucial factor influencing overall SCR effectiveness, and it contributes to the necessity of employing
[...] Read more.
This work presents the methodology and accurate evaluation of ammonia concentration distribution measurements at the selective catalytic reduction (SCR) catalyst outlet cross-section. The uniformity of ammonia concentration is a crucial factor influencing overall SCR effectiveness, and it contributes to the necessity of employing a reliable test method. The aftertreatment system design (mainly its geometrical features) can be evaluated in detail. The ammonia concentration is measured at the SCR catalyst outlet at grid points covering from the center to the outer edges of the catalyst. Its execution requires the introduction of a probe hovering over the back face of the SCR. To obtain the expected accuracy, it is necessary to measure a sufficient number of points in a reasonable timeframe. In order to achieve that, a fully automatic sampling device was developed. Sample results are presented showing the capabilities of the created test stand and its importance for the design development and validation stages of SCR-based engine aftertreatment. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessFeature PaperArticle Nb-Modified Ce/Ti Oxide Catalyst for the Selective Catalytic Reduction of NO with NH3 at Low Temperature
Catalysts 2018, 8(5), 175; https://doi.org/10.3390/catal8050175
Received: 9 March 2018 / Revised: 18 April 2018 / Accepted: 23 April 2018 / Published: 26 April 2018
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Abstract
Recently, great attention has been paid to Ceria-based materials for selective catalytic reduction (SCR) with NH3 owing to their unique redox, oxygen storage, and acid-base properties. Two series of bimetallic catalysts issued from Titania modified by Ce and Nb were prepared by
[...] Read more.
Recently, great attention has been paid to Ceria-based materials for selective catalytic reduction (SCR) with NH3 owing to their unique redox, oxygen storage, and acid-base properties. Two series of bimetallic catalysts issued from Titania modified by Ce and Nb were prepared by the one-step sol-gel method (SG) and by the sol-gel route followed by impregnation (WI). The resulting core-shell and bulk catalysts were tested in NH3-SCR of NOx. The impregnated Nb5/Ce40/Ti100 (WI) catalyst displayed 95% NOx conversion at 200 °C (GHSV = 60,000 mL·g−1·h−1, 1000 ppm NOx, 1000 ppm NH3, 5% O2/He) without forming N2O. The catalysts were characterized by various methods including ICP-OES, N2-physisorption, XRD, Raman, NH3-TPD, DRIFTS, XPS, and H2-TPR. The results showed that the introduction of Nb decreases the surface area and strengthens the surface acidity. This behavior can be explained by the strong interaction between Ceria and Titania which generates Ce-O-Ti units, as well as a high concentration of amorphous or highly dispersed Niobia. This should be the reason for the excellent performance of the catalyst prepared by the sol-gel method followed by impregnation. Furthermore, Nb5/Ce40/Ti100 (WI) has the largest NH3 adsorption capacity, which is helpful to promote the NH3-SCR reaction. The long-term stability and the effect of H2O on the catalysts were also evaluated. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Mobility of NH3-Solvated CuII Ions in Cu-SSZ-13 and Cu-ZSM-5 NH3-SCR Catalysts: A Comparative Impedance Spectroscopy Study
Catalysts 2018, 8(4), 162; https://doi.org/10.3390/catal8040162
Received: 14 March 2018 / Revised: 8 April 2018 / Accepted: 9 April 2018 / Published: 18 April 2018
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Abstract
The mobility of NH3-solvated Cu ions within the zeolite framework has been recently identified as a key factor for the kinetics of the selective catalytic reduction of NOx with NH3 (NH3-SCR) over Cu-zeolite catalysts at low temperatures.
[...] Read more.
The mobility of NH3-solvated Cu ions within the zeolite framework has been recently identified as a key factor for the kinetics of the selective catalytic reduction of NOx with NH3 (NH3-SCR) over Cu-zeolite catalysts at low temperatures. Here, we utilize in situ impedance spectroscopy to explore the mobility of NH3-solvated CuII ions, i.e., CuII(NH3)n, in Cu-SSZ-13 and Cu-ZSM-5 zeolites with varied Cu ion exchange levels, and observed that both the zeolite framework (CHA or MFI) and the Cu exchange level influence the high-frequency dielectric relaxation processes that are associated with the short-range (local) motion of CuII(NH3)n. Our results suggest that the local motion of CuII(NH3)n species is favored within the CHA framework due to the unique cage structure, and thereby contribute to the overall ion conductivity at high frequencies, which, on the contrary, is not observed for ZSM-5, where NH3-solvated Cu2+ ions do not experience a comparable constrained space for local motion. This study sheds new light on the mobility of Cu active sites under NH3-SCR related reaction conditions and may contribute to an advanced understanding of the underlying mechanism. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle DRIFT Study on Promotion Effect of the Keggin Structure over V2O5-MoO3/TiO2 Catalysts for Low Temperature NH3-SCR Reaction
Catalysts 2018, 8(4), 143; https://doi.org/10.3390/catal8040143
Received: 10 March 2018 / Revised: 30 March 2018 / Accepted: 30 March 2018 / Published: 3 April 2018
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Abstract
Heteropoly acids (HPAs) with the Keggin structure have been widely used in NOx removal. Two kinds of catalysts (those with and without the Keggin structure) are prepared for studying the effect of the Keggin structure on the NH3-SCR reaction. A
[...] Read more.
Heteropoly acids (HPAs) with the Keggin structure have been widely used in NOx removal. Two kinds of catalysts (those with and without the Keggin structure) are prepared for studying the effect of the Keggin structure on the NH3-SCR reaction. A series of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analyses are conducted to investigate the surface-adsorbed species on the catalysts during the SCR reaction. The mechanism for enhancing low-temperature activity of the catalysts is proposed. Furthermore, the effect of NH4+ in the Keggin structure is also investigated. Results indicate that both the Langmuir–Hinshelwood (L-H) and Eley–Rideal (E-R) mechanisms occurred in the NH3-SCR reaction over the catalyst with the Keggin structure (Cat-A); in addition, when more acid sites are provided, NOx species activity is improved and more NH4+ ions participate in reaction over Cat-A, thus promoting SCR activity. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle The Role of Fe2O3 Species in Depressing the Formation of N2O in the Selective Reduction of NO by NH3 over V2O5/TiO2-Based Catalysts
Catalysts 2018, 8(4), 134; https://doi.org/10.3390/catal8040134
Received: 28 February 2018 / Revised: 23 March 2018 / Accepted: 27 March 2018 / Published: 30 March 2018
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Abstract
Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of
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Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of N2O formation in the selective catalytic reduction of NO by NH3 (NH3-SCR). The promoted VWT and c-VWT catalysts all gave a significantly decreased N2O production at temperatures >400 °C compared to the unpromoted samples. However, such a promotion led to the loss in high temperature NO conversion, mainly due to the oxidation of NH3 to N-containing gases, particularly NO. Characterization of the unpromoted and promoted catalysts using X-ray diffraction (XRD), NH3 adsorption-desorption, and Raman spectroscopy techniques could explain the reason why the promotion showed much lower N2O formation levels at high temperatures. The addition of Fe2O3 to c-VWT resulted in redispersion of the V2O5 species, although this was not visible for 2.73% Fe2O3/VWT. The iron oxides exist as a highly-dispersed noncrystalline α-Fe2O3 in the promoted catalysts. These Raman spectra had a new Raman signal that could be tentatively assigned to Fe2O3-induced tetrahedrally coordinated polymeric vanadates and/or surface V-O-Fe species with significant electronic interactions between the both metal oxides. Calculations of the monolayer coverage of each metal oxide and the surface total coverage are reasonably consistent with Raman measurements. The proposed vanadia-based surface polymeric entities may play a key role for the substantial reduction of N2O formed at high temperatures by NH3 species adsorbed strongly on the promoted catalysts. This reaction is a main pathway to greatly suppress the extent of N2O formation in NH3-SCR reaction over the promoted catalysts. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Investigation of Iron Vanadates for Simultaneous Carbon Soot Abatement and NH3-SCR
Catalysts 2018, 8(4), 130; https://doi.org/10.3390/catal8040130
Received: 7 March 2018 / Revised: 22 March 2018 / Accepted: 22 March 2018 / Published: 26 March 2018
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Abstract
FeVO4 and Fe0.5Er0.5VO4 were prepared and loaded over standard Selective Catalytic Reduction (SCR) supports based on TiO2-WO3-SiO2 (TWS) and redox active supports like CeO2 and CeZrO2 with the aim of
[...] Read more.
FeVO4 and Fe0.5Er0.5VO4 were prepared and loaded over standard Selective Catalytic Reduction (SCR) supports based on TiO2-WO3-SiO2 (TWS) and redox active supports like CeO2 and CeZrO2 with the aim of finding a suitable formulation for simultaneous soot abatement and NH3-SCR and to understand the level of interaction between the two reactions. A suitable bi-functional material was identified in the composition FeVO4/CeZrO2 where an SCR active component is added over a redox active support, to increase carbon oxidation properties. The influence of the presence of ammonia in soot oxidation and the effect of the presence of soot on SCR reaction have been addressed. It is found that the addition of NO and NO/NH3 mixtures decreases at different levels the oxidation temperature of carbon soot, while the presence of carbon adversely affects the NH3-SCR reaction by increasing the oxidation of NH3 to NO, thus lowering the NO removal efficiency. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Enhanced Low Temperature NO Reduction Performance via MnOx-Fe2O3/Vermiculite Monolithic Honeycomb Catalysts
Catalysts 2018, 8(3), 100; https://doi.org/10.3390/catal8030100
Received: 31 January 2018 / Revised: 22 February 2018 / Accepted: 27 February 2018 / Published: 28 February 2018
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Abstract
Selective catalytic reduction of NOx by ammonia (NH3-SCR) was the most efficient and economic technology for De-NOx applications. Therefore, a series of MnOx/vermiculite (VMT) and MnOx-Fe2O3/VMT catalysts were prepared by an
[...] Read more.
Selective catalytic reduction of NOx by ammonia (NH3-SCR) was the most efficient and economic technology for De-NOx applications. Therefore, a series of MnOx/vermiculite (VMT) and MnOx-Fe2O3/VMT catalysts were prepared by an impregnation method for the selective catalytic reduction (SCR) of nitrogen oxides (NOx). The MnOx-Fe2O3/VMT catalysts provided an excellent NO conversion of 96.5% at 200 °C with a gas hourly space velocity (GHSV) of 30,000 h−1 and an NO concentration of 500 ppm. X-ray photoelectron spectroscopy results indicated that the Mn and Fe oxides of the MnOx-Fe2O3/VMT catalyst were mainly composed of MnO2 and Fe2O3. However, the MnO2 and Fe2O3 components were well dispersed because no discernible MnO2 and Fe2O3 phases were observed in X-ray powder diffraction spectra. Corresponding MnOx-Fe2O3/VMT monolithic honeycomb catalysts (MHCs) were prepared by an extrusion method, and the MHCs achieved excellent SCR activity at low temperature, with an NO conversion greater than 98.6% at 150 °C and a GHSV of 4000 h−1. In particular, the MnOx-Fe2O3/VMT MHCs provided a good SCR activity at room temperature (20 °C), with an NO conversion of 62.2% (GHSV = 1000 h−1). In addition, the NO reduction performance of the MnOx-Fe2O3/VMT MHCs also demonstrated an excellent SO2 resistance. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Mn-Ce-V-WOx/TiO2 SCR Catalysts: Catalytic Activity, Stability and Interaction among Catalytic Oxides
Catalysts 2018, 8(2), 76; https://doi.org/10.3390/catal8020076
Received: 28 January 2018 / Revised: 4 February 2018 / Accepted: 6 February 2018 / Published: 12 February 2018
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Abstract
A series of Mn-Ce-V-WOx/TiO2 composite oxide catalysts with different molar ratios (active components/TiO2 = 0.1, 0.2, 0.3, 0.6) have been prepared by wet impregnation method and tested in selective catalytic reduction (SCR) of NO by NH3 in a
[...] Read more.
A series of Mn-Ce-V-WOx/TiO2 composite oxide catalysts with different molar ratios (active components/TiO2 = 0.1, 0.2, 0.3, 0.6) have been prepared by wet impregnation method and tested in selective catalytic reduction (SCR) of NO by NH3 in a wide temperature range. These catalysts were also characterized by X-ray diffraction (XRD), Transmission Electron Microscope (TEM), in situ Fourier Transform infrared spectroscopy (in situ FTIR), H2-Temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The results show the catalyst with a molar ratio of active components/TiO2 = 0.2 exhibits highest NO conversion value between 150 °C to 400 °C and good resistance to H2O and SO2 at 250 °C with a gas hourly space velocity (GHSV) value of 40,000 h−1. Different oxides are well dispersed and interact with each other. NH3 and NO are strongly adsorbed on the catalyst surface and the adsorption of the reactant gas leads to a redox cycle with the valence state change among the surface oxides. The adsorption of SO2 on Mn4+ and Ce4+ results in good H2O and SO2 resistance of the catalyst, but the effect of Mn and Ce are more than superior water and sulfur resistance. The diversity of valence states of the four active components and their high oxidation-reduction performance are the main reasons for the high NO conversion in this system. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Surface Species and Metal Oxidation State during H2-Assisted NH3-SCR of NOx over Alumina-Supported Silver and Indium
Catalysts 2018, 8(1), 38; https://doi.org/10.3390/catal8010038
Received: 1 January 2018 / Revised: 15 January 2018 / Accepted: 17 January 2018 / Published: 19 January 2018
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Abstract
Alumina-supported silver and indium catalysts are investigated for the hydrogen-assisted selective catalytic reduction (SCR) of NOx with ammonia. Particularly, we focus on the active phase of the catalyst and the formation of surface species, as a function of the gas environment. Diffuse
[...] Read more.
Alumina-supported silver and indium catalysts are investigated for the hydrogen-assisted selective catalytic reduction (SCR) of NOx with ammonia. Particularly, we focus on the active phase of the catalyst and the formation of surface species, as a function of the gas environment. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy was used to follow the oxidation state of the silver and indium phases, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to elucidate the formation of surface species during SCR conditions. In addition, the NOx reduction efficiency of the materials was evaluated using H2-assisted NH3-SCR. The DRIFTS results show that the Ag/Al2O3 sample forms NO-containing surface species during SCR conditions to a higher extent compared to the In/Al2O3 sample. The silver sample also appears to be more reduced by H2 than the indium sample, as revealed by UV-vis spectroscopic experiments. Addition of H2, however, may promote the formation of highly dispersed In2O3 clusters, which previously have been suggested to be important for the SCR reaction. The affinity to adsorb NH3 is confirmed by both temperature programmed desorption (NH3-TPD) and in situ DRIFTS to be higher for the In/Al2O3 sample compared to Ag/Al2O3. The strong adsorption of NH3 may inhibit (self-poison) the NH3 activation, thereby hindering further reaction over this catalyst, which is also shown by the lower SCR activity compared to Ag/Al2O3. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Preparation and Performance of Modified Red Mud-Based Catalysts for Selective Catalytic Reduction of NOx with NH3
Catalysts 2018, 8(1), 35; https://doi.org/10.3390/catal8010035
Received: 16 December 2017 / Revised: 13 January 2018 / Accepted: 17 January 2018 / Published: 19 January 2018
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Abstract
Bayer red mud was selected, and the NH3-SCR activity was tested in a fixed bed in which the typical flue gas atmosphere was simulated. Combined with XRF, XRD, BET, SEM, TG and NH3-Temperature Programmed Desorption (TPD) characterization, the denitration
[...] Read more.
Bayer red mud was selected, and the NH3-SCR activity was tested in a fixed bed in which the typical flue gas atmosphere was simulated. Combined with XRF, XRD, BET, SEM, TG and NH3-Temperature Programmed Desorption (TPD) characterization, the denitration characteristics of Ce-doped red mud catalysts were studied on the basis of alkali-removed red mud. The results showed that typical red mud was a feasible material for denitration catalyst. Acid washing and calcining comprised the best treatment process for raw red mud, which reduced the content of alkaline substances, cleared the catalyst pore and optimized the particle morphology with dispersion. In the temperature range of 300–400 °C, the denitrification efficiency of calcined acid washing of red mud catalyst (ARM) was more than 70%. The doping of Ce significantly enhanced NH3 adsorption from weak, medium and strong acid sites, reduced the crystallinity of α-Fe2O3 in ARM, optimized the specific surface area and broadened the active temperature window, which increased the NOx conversion rate by an average of nearly 20% points from 250–350 °C. The denitration efficiency of Ce0.3/ARM at 300 °C was as high as 88%. The optimum conditions for the denitration reaction of the Ce0.3/ARM catalyst were controlled as follows: Gas Hourly Space Velocity (GHSV) of 30,000 h−1, O2 volume fraction of 3.5–4% and the NH3/NO molar ratio ([NH3/NO]) of 1.0. The presence of SO2 in the feed had an irreversible negative effect on the activity of the Ce0.3/ARM catalyst. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Influence of the Sodium Impregnation Solvent on the Deactivation of Cu/FER-Exchanged Zeolites Dedicated to the SCR of NOx with NH3
Catalysts 2018, 8(1), 3; https://doi.org/10.3390/catal8010003
Received: 24 November 2017 / Revised: 18 December 2017 / Accepted: 19 December 2017 / Published: 23 December 2017
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Abstract
The effect of the sodium addition mode was investigated on model Cu/FER selective catalytic reduction (SCR) catalysts with two copper loadings (2.8 wt. % and 6.1 wt. %) in order to compare samples with or without over-exchanged copper. Na was added by wet-impregnation
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The effect of the sodium addition mode was investigated on model Cu/FER selective catalytic reduction (SCR) catalysts with two copper loadings (2.8 wt. % and 6.1 wt. %) in order to compare samples with or without over-exchanged copper. Na was added by wet-impregnation using two solvents: water or ethanol. Catalysts were evaluated in Standard and Fast-SCR conditions, as well as in NO and NH3 oxidation. They were characterized by H2-TPR, NO and NH3 adsorption monitored by FT-IR. As expected, whatever the copper loading, ammonia adsorption capacity was decreased by Na additions. Interestingly, characterizations also showed that Na impregnation in water favors the migration of the Cu-exchanged species, leading to the formation of CuO extra-framework compounds. Consequently, for both copper loadings, Na impregnation in water led to a stronger catalyst deactivation than impregnation in ethanol. Finally, the NOx conversion at low temperature (250 °C) appeared mainly affected by the loss in NH3 adsorption capacity whereas the deNOx deactivation at high temperature (500 °C) was rather governed by the decrease in the exchanged copper ratio, which also induced a partial inhibition of NO and NH3 oxidation behaviors. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Consideration of the Role of Plasma in a Plasma-Coupled Selective Catalytic Reduction of Nitrogen Oxides with a Hydrocarbon Reducing Agent
Catalysts 2017, 7(11), 325; https://doi.org/10.3390/catal7110325
Received: 13 October 2017 / Revised: 26 October 2017 / Accepted: 28 October 2017 / Published: 31 October 2017
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Abstract
The purpose of this study is to explain how plasma improves the performance of selective catalytic reduction (SCR) of nitrogen oxides (NOx) with a hydrocarbon reducing agent. In the plasma-coupled SCR process, NOx reduction was performed with n-heptane as a
[...] Read more.
The purpose of this study is to explain how plasma improves the performance of selective catalytic reduction (SCR) of nitrogen oxides (NOx) with a hydrocarbon reducing agent. In the plasma-coupled SCR process, NOx reduction was performed with n-heptane as a reducing agent over Ag/γ-Al2O3 as a catalyst. We found that the plasma decomposes n-heptane into several oxygen-containing products such as acetaldehyde, propionaldehyde and butyraldehyde, which are more reactive than the parent molecule n-heptane in the SCR process. Separate sets of experiments using acetaldehyde, propionaldehyde and butyraldehyde, one by one, as a reductant in the absence of plasma, have clearly shown that the presence of these partially oxidized compounds greatly enhanced the NOx conversion. The higher the discharge voltage, the more the amounts of such partially oxidized products. The oxidative species produced by the plasma easily converted NO into NO2, but the increase of the NO2 fraction was found to decrease the NOx conversion. Consequently, it can be concluded that the main role of plasma in the SCR process is to produce partially oxidized compounds (aldehydes), having better reducing power. The catalyst-alone NOx removal efficiency with n-heptane at 250 °C was measured to be less than 8%, but it increased to 99% in the presence of acetaldehyde at the same temperature. The NOx removal efficiency with the aldehyde reducing agent was higher as the number of carbons in the aldehyde was more; for example, the NOx removal efficiencies at 200 °C with butyraldehyde, propionaldehyde and acetaldehyde were measured to be 83.5%, 58.0% and 61.5%, respectively, which were far above the value (3%) obtained with n-heptane. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Concept of Vaporized Urea Dosing in Selective Catalytic Reduction
Catalysts 2017, 7(10), 307; https://doi.org/10.3390/catal7100307
Received: 7 September 2017 / Revised: 28 September 2017 / Accepted: 13 October 2017 / Published: 19 October 2017
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Abstract
This work tried to identify the influence of dosing vaporized urea solution in a selective catalytic reduction (SCR) system. In the SCR method, optimising the urea evaporation and mixing properties can significantly improve the NOx conversion efficiency in the catalyst. It can
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This work tried to identify the influence of dosing vaporized urea solution in a selective catalytic reduction (SCR) system. In the SCR method, optimising the urea evaporation and mixing properties can significantly improve the NOx conversion efficiency in the catalyst. It can also exert a positive effect on the uniformity of NH3 concentration distribution across the catalyst face. The concept of an electrically evaporated urea-dosing system was investigated and it was found that urea pre-heating prior to introduction into the exhaust gas is favourable for enhancing NOx removal under steady-state and transient engine operation. In the urea evaporating system the heating chamber was of a cylindrical tube shape and the urea vapour was introduced into the exhaust by means of a Venturi orifice. The concept urea dosing was only a custom-made solution, but proved to be superior to the regular dosing system operating in the liquid phase. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessArticle Experimental Research of an Active Solution for Modeling In Situ Activating Selective Catalytic Reduction Catalyst
Catalysts 2017, 7(9), 258; https://doi.org/10.3390/catal7090258
Received: 21 July 2017 / Revised: 29 August 2017 / Accepted: 29 August 2017 / Published: 31 August 2017
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Abstract
The effect of active solutions suitable for the in situ activation of selective catalytic reduction (SCR) catalysts was experimentally investigated using a designed in situ activation modeling device. To gain further insight, scanning electron microscopy (SEM), specific surface area analysis (BET), Fourier transform
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The effect of active solutions suitable for the in situ activation of selective catalytic reduction (SCR) catalysts was experimentally investigated using a designed in situ activation modeling device. To gain further insight, scanning electron microscopy (SEM), specific surface area analysis (BET), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) analyses were used to investigate the effects of different reaction conditions on the characteristics of the deactivated catalysts. The activation effect of loading V2O5, WO3 and MoO3 on the surface of the deactivated catalysts was analyzed and the correlation to the denitrification activity was determined. The results demonstrate that the prepared activating solution of 1 wt % vanadium (V), 9 wt % tungsten (W), and 6 wt % molybdenum (Mo) has a beneficial effect on the deactivation of the catalyst. The activated catalyst resulted in a higher NO removal rate when compared to the deactivated catalyst. Furthermore, the NO removal rate of the activated catalyst reached a maximum of 32%. The activity of the SCR catalyst is closely linked to the concentration of the active ingredients. When added in optimum amounts, the active ingredients helped to restore the catalytic activity. In particular, the addition of active ingredients, the availability of labile surface oxygen, and the presence of small pores improved the denitrification efficiency. Based on these results, active solutions can effectively solve the problem of denitrification catalyst deactivation. These findings are a reference for the in-situ activation of the selective catalytic reduction of nitrogen oxides (SCR-DeNOx) catalyst. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Review

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Open AccessFeature PaperReview Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts
Catalysts 2018, 8(4), 140; https://doi.org/10.3390/catal8040140
Received: 2 March 2018 / Revised: 27 March 2018 / Accepted: 28 March 2018 / Published: 31 March 2018
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Abstract
Cu/SSZ-13 Selective Catalytic Reduction (SCR) catalysts have been extensively studied for the past five-plus years. New and exciting fundamental and applied science has appeared in the literature quite frequently over this time. In this short review, a few topics specifically focused on a
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Cu/SSZ-13 Selective Catalytic Reduction (SCR) catalysts have been extensively studied for the past five-plus years. New and exciting fundamental and applied science has appeared in the literature quite frequently over this time. In this short review, a few topics specifically focused on a molecular-level understanding of this catalyst are summarized: (1) The nature of the active sites and, in particular, their transformations under varying reaction conditions that include dehydration, the presence of the various SCR reactants and hydrothermal aging; (2) Discussions of standard and fast SCR reaction mechanisms. Considerable progress has been made, especially in the last couple of years, on standard SCR mechanisms. In contrast, mechanisms for fast SCR are much less understood. Possible reaction paths are hypothesized for this latter case to stimulate further investigations; (3) Discussions of rational catalyst design based on new knowledge obtained regarding catalyst stability, overall catalytic performance and mechanistic catalytic chemistry. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessFeature PaperReview Strategies of Coping with Deactivation of NH3-SCR Catalysts Due to Biomass Firing
Catalysts 2018, 8(4), 135; https://doi.org/10.3390/catal8040135
Received: 28 February 2018 / Revised: 22 March 2018 / Accepted: 22 March 2018 / Published: 30 March 2018
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Abstract
Firing of biomass can lead to rapid deactivation of the vanadia-based NH3-SCR catalyst, which reduces NOx to harmless N2. The deactivation is mostly due to the high potassium content in biomasses, which results in submicron aerosols containing mostly
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Firing of biomass can lead to rapid deactivation of the vanadia-based NH3-SCR catalyst, which reduces NOx to harmless N2. The deactivation is mostly due to the high potassium content in biomasses, which results in submicron aerosols containing mostly KCl and K2SO4. The main mode of deactivation is neutralization of the catalyst’s acid sites. Four ways of dealing with high potassium contents were identified: (1) potassium removal by adsorption, (2) tail-end placement of the SCR unit, (3) coating SCR monoliths with a protective layer, and (4) intrinsically potassium tolerant catalysts. Addition of alumino silicates, often in the form of coal fly ash, is an industrially proven method of removing K aerosols from flue gases. Tail-end placement of the SCR unit was also reported to result in acceptable catalyst stability; however, flue-gas reheating after the flue gas desulfurization is, at present, unavoidable due to the lack of sulfur and water tolerant low temperature catalysts. Coating the shaped catalysts with thin layers of, e.g., MgO or sepiolite reduces the K uptake by hindering the diffusion of K+ into the catalyst pore system. Intrinsically potassium tolerant catalysts typically contain a high number of acid sites. This can be achieved by, e.g., using zeolites as support, replacing WO3 with heteropoly acids, and by preparing highly loaded, high surface area, very active V2O5/TiO2 catalyst using a special sol-gel method. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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Open AccessReview Sulfur and Water Resistance of Mn-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review
Catalysts 2018, 8(1), 11; https://doi.org/10.3390/catal8010011
Received: 6 December 2017 / Revised: 26 December 2017 / Accepted: 3 January 2018 / Published: 7 January 2018
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Abstract
Selective catalytic reduction (SCR) with NH3 is the most efficient and economic flue gas denitrification technology developed to date. Due to its high low-temperature catalytic activity, Mn-based catalysts present a great prospect for application in SCR de-NOx at low temperatures. However,
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Selective catalytic reduction (SCR) with NH3 is the most efficient and economic flue gas denitrification technology developed to date. Due to its high low-temperature catalytic activity, Mn-based catalysts present a great prospect for application in SCR de-NOx at low temperatures. However, overcoming the poor resistance of Mn-based catalysts to H2O and SO2 poison is still a challenge. This paper reviews the recent progress on the H2O and SO2 resistance of Mn-based catalysts for the low-temperature SCR of NOx. Firstly, the poison mechanisms of H2O and SO2 are introduced in detail, respectively. Secondly, Mn-based catalysts are divided into three categories—single MnOx catalysts, Mn-based multi-metal oxide catalysts, and Mn-based supported catalysts—to review the research progress of Mn-based catalysts for H2O and SO2 resistance. Thirdly, several strategies to reduce the poisonous effects of H2O and SO2, such as metal modification, proper support, the combination of metal modification and support, the rational design of structure and morphology, are summarized. Finally, perspectives and future directions of Mn-based catalysts for the low-temperature SCR of NOx are proposed. Full article
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx)
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