Selective Reduction Of NO Research Articles

Porous washcoat structure in CeO2 modified Cu‐SSZ‐13 monolith catalyst for NH3‐SCR with improved catalytic performance

AbstractA series of CeO2 modified Cu‐SSZ‐13 monolith catalysts were prepared by embedding CeO2 into the washcoat of Cu‐SSZ‐13 monolith catalyst through solvent combustion method. These CexCu‐SSZ‐13 catalysts were studied in the selective catalytic reduction (SCR) of NO with NH3, among which the Ce2Cu‐SSZ‐13 catalyst exhibited the best low‐temperature activity, hydrothermal stability, and sulfur resistance. The physicochemical properties of the catalysts were characterized using multiple methods. Results showed that the acidity, redox capacity, and ammonia adsorption capacity significantly enhanced after CeO2 modification, thus leading to the high performance of Ce2Cu‐SSZ‐13 catalyst. Furthermore, the introduction of CeO2 induced the fast SCR reaction by promoting the oxidation of NO to NO2. Analog calculation suggested that the porous structure generated via solvent combustion in the washcoat effectively increased the diffusion rate of reaction. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT) analysis showed that Brønsted acid sites were the main active center and the reaction followed Eley–Rideal mechanism.

Catalytic Performance of Bimetallic Systems (Cu-Fe, Cu-Mn, Fe-Mn) Based on Spherical MCM-41 Modified by Template Ion-Exchange in NH3-SCR Process

Mesoporous silica of MCM-41 type with spherical morphology was modified with copper, iron, or manganese as well as pairs of these metals by template ion-exchange (TIE) method. The obtained samples were characterized with respect to their structure (XRD), morphology (SEM-EDS), textural parameters (low-temperature N2 sorption), surface acidity (NH3-TPD), transition metal loadings (ICP-OES), their deposited forms (UV-vis DRS) and reducibility (H2-TPR). The catalytic performance of monometallic and bimetallic samples in the selective catalytic reduction of NO with ammonia (NH3-SCR) was tested. The best catalytic results presented a bimetallic copper-manganese sample, which was significantly more active than the mechanical mixture of monometallic copper and manganese catalysts. The synergistic cooperation of manganese and copper species is possibly related to charge relocation between them, resulting in activation of the catalyst in oxidation of NO to NO2, which is necessary for the fast NH3-SCR reaction.

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH3. However, their activity is unexpectedly inhibited in the presence of NO2 at low temperatures. This is strikingly distinct from the NO2-accelerated NOx conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as CuII species (fw-Cu2+ and NH3-solvated CuII with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these CuII sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the CuII sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.

Effect of Ammonia on the Intermediates and Chemical Pathway of Selective Catalytic Reduction of NO by Ethanol over a Pt/Al2O3 Catalyst

Effect of Ammonia on the Intermediates and Chemical Pathway of Selective Catalytic Reduction of NO by Ethanol over a Pt/Al₂O₃ Catalyst

Effects of sulfate on denitration performance of rare earth tailings for selective catalytic reduction of NO with NH3

This paper explores the effects of concentration of acid activation on the properties of rare earth tailings catalyst. The results show that the denitration activity of rare earth tailings catalyst is enhanced by acid activation. The best denitration activity of catalyst, activated by sulfuric acid of 8 mol/L, is 86%.

In Situ/Operando Spectroscopic Studies on the NH3–SCR Mechanism over Fe–Zeolites

Reduction/oxidation half-cycles of the selective catalytic reduction of NO with NH3 (NH3–SCR) over Fe-exchanged mordenite (MOR) zeolites at 300 °C were investigated by in situ/operando spectroscopy (infrared, UV–vis, and Fe K-edge X-ray absorption near edge structure) and density functional theory (DFT) calculation. The reduction of Fe3+ into Fe2+ and the simultaneous formation of N2 and H2O in the reduction half-cycle (under NO + NH3) were demonstrated by different spectroscopic results. In the subsequent oxidation half-cycle (under O2 or NO + O2), Fe2+ was reoxidized into Fe3+. The reduction half-cycle comprises several elementary steps. Reduction of Fe3+–OH by NO producing Fe2+ and NO+ species was observed at low temperatures (<100 °C), while N2 formation due to the reduction of NO+ was observed under subsequent NH3 exposure at 100 °C. Under transient conditions, NH3 on Brønsted acid sites (B–NH3) reacted with NO to generate N2 when the coverage of B–NH3 was low, indicating that B–NH3 is not a spectator but a reservoir of NH3. Transition state calculation theoretically suggested that the formation of nitrous acid (HONO) intermediates from [Fe3+(OH–)2]+ at a Al site and gaseous NO was a facile process (Ea = 29.2 kJ/mol). Combining the experimental observation and DFT calculation, the mechanism of the reduction half-cycle over Fe–zeolites was proposed; [Fe3+(OH–)2]+ is reduced by NO to produce a HONO intermediate, which then reacts with NH3 on Brønsted acid sites to yield H2O and N2 via NO+ species. Based on the mechanistic insights above, Fe–zeolites (MOR and β) with different Fe loadings and Si/Al ratios were tested for NH3–SCR reaction. Consequently, 2.7 wt % Fe-loaded zeolites with a relatively large number of Brønsted acid sites (Al-rich β with a Si/Al ratio of 5) showed the highest NOx conversion in a low-temperature region.

Effect of O2 on Selective Catalytic Reduction of NO by C3H6 over Fe Catalysts Supported on Porous Clay Heterostructures (Fe-PCH)

K10 montmorillonite was used as the raw clay to prepare porous clay heterostructures (PCH) by organic template intercalation and tetraethylorthosilicate (TEOS) was used as silicone source. Fe was load on the PCH support by impregnation method to prepare Fe-PCH catalysts. The catalytic activity tests were carried out in a fixed bed micro-reactor in the atmosphere of 0.1% C3 H6 , 0.1%NO, 0~10%O2 in N2 balance. The total flow rate was 100ml/min and corresponding gas hourly space velocity (GHSV) was 15000h-1. The mircro-reaction pathways were studied by Diffuse Reflection Infrared Fourier Transform Spectrometer (DRIFTS). The influence of O2 on the SCR-C3 H6 reactivity over Fe-PCH was analyzed. The results showed that FePCH catalyst exhibited the best catalytic performance without oxygen, which reached the maximum NO conversion of 100% at 4000 C. When the concentration of O2 is below a critical value, [O2 ]crit, the catalytic activity gradually decreased with the increasing concentration of O2 . When the concentration of O2 is above [O2 ]crit, the NO conversion was only 20%~30%. Fe-PCH catalysts were characterized by transmission electron microscope (TEM), N2 adsorption/desorption and X-ray photoelectron spectroscopy (XPS) techniques. The results showed that Fe-PCH had supermicroporous and mesoporous structures. The active components on Fe-PCH catalyst were mainly α-Fe2 O3 nanorods, exposing (024) and (104) planes. The in situ DRIFTS technique was used to detect the reactive intermediates over the surface of Fe-PCH catalyst, which could help analyze the mechanism of the influence of oxygen on catalytic activity. The results presented that oxygen could inhibit the formation of isocyanate species over Fe-PCH catalyst during C3 H6 -SCR, which might be the reason for the decrease of NO conversion. Finally, a reasonable reaction path of C3 H6 -SCR over Fe-PCH catalyst was proposed.

Promotional role of Nb modification on CuCeOx catalyst for low temperature selective catalytic reduction of NO with NH3: A mechanism investigation

CuCeOx and CuCeNbOx-k with different Nb doping ratios were obtained by the citric acid method and applied in NH3-SCR process. The activity test results elucidated that the addition of Nb effectively ameliorated the low-temperature SCR performance. Among them, CuCeNbOx-0.4 catalyst had the finest SCR activity (more than 90% in 185–370 °C), highest N2 selectivity, excellent SO2&H2O resistance and thermal stability. Numerous characterization analysis demonstrated that CuCeNbOx-0.4 had the largest specific surface area, the strongest surface acidity and the most excellent redox properties. The conclusions of in situ DRIFT manifested the Nb modification generated more activated species, enhanced the reactivity of ad-NH3 and ad-NOx species, and the SCR reaction on CuCeNbOx-0.4 mainly followed the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Furthermore, the Nb doping also formed the dual redox cycle (Cu2+ + Nb4+ ↔ Cu+ + Nb5+, Cu2+ + Ce3+ ↔ Cu+ + Ce4+), which facilitated the low-temperature SCR reaction.

Improved alkali-tolerance of FeOx-WO3 catalyst for NO removal via in situ reserving FeOx active species

The improvement of alkali tolerance of de-NOx catalysts remains an immense challenge for selective catalytic reduction (SCR) of NO by NH3. In this work, NOx removal efficiency was nearly retained in the presence of potassium over FeOx-WO3 (FW) catalysts via protecting iron active species using different template agents. Typically, FW using urea (FW-(urea)) as template agent showed superior de-NOx performance after being poisoned by potassium (K/FW-(urea)). Urea could promote the facile binding of potassium with WO3, which acted as alkali-poisoning sites so that iron active species were reserved to a great extent. Additionally, potassium species tended to be aggregated over K/FW-(urea) and thus a lot of iron active sites could be exposed. Besides, K/FW-(urea) catalyst has higher surface acidity, superior redox ability, stronger interaction of WO3 with Fe2O3, and more surface adsorbed oxygen compare with other parallel poisoned samples. This is the reason for the remarkable ability of potassium resistance over K/FW-(urea). The strategy of template regulation could offer a novel route for maintaining considerable NH3-SCR performance in the presence of alkali.

Design of robust Co-doped Mn3O4 spinel catalysts for selective catalytic reduction of NO with NH3 at low temperatures

Developing an efficient catalyst with high intrinsic active sites for selective catalytic reduction of NOx with ammonia (NH3-SCR) at ultra-low temperature flue gas is challenging but desirable. Herein, a Co-doped Mn3O4 catalyst (Co0.04Mn2.96O4) was created by a confinement-pyrolysis-oxidation strategy using bimetallic Co–Mn–MOF–74 as the precursor, which exhibited excellent SCR performance with 96% NO conversion at 130 °C under a GHSV of 100,000 h−1. Benefited from the uniform microstructure of catalyst and combined with characterization and theoretical calculations, an efficient Cotet–O–Mnoct active site is revealed for NH3-SCR, and the formation of oxygen vacancies on Cotet–O–Mnoct site plays a crucial role in the adsorption and activation of reactants, which facilitating the low–temperature NH3-SCR activity. This work provides new insights into the design and synthesis of highly active NH3-SCR catalysts with high intrinsic active sites used at low temperatures.

Interaction between Nickel Oxide and Support Promotes Selective Catalytic Reduction of NOx with C3 H6.

Selective catalytic reduction of nitrogen oxides (NOx ) with C3 H6 (C3 H6 -SCR) was investigated over NiO catalysts supported on different metal-oxides. A NiAlOx mixed oxide phase was formed over NiO/γ-Al2 O3 catalyst, inducing an immediate interaction between NiOx and AlOx species. Such interaction resulted in a charge transfer from Ni to Al site and the formation of Ni species in high oxidation state. In comparison to other NiO-loaded catalysts, NiO/γ-Al2 O3 catalyst exhibited the highest NOx conversion at temperature higher than 450 °C, but a poor C3 H6 oxidation activity due to the decreased nucleophilicity for surface oxygen species. By temperature-programed NO oxidation, it is indicated that nitrate species were rapidly formed and stably maintained at high temperature over NiO/γ-Al2 O3 catalyst. In situ transient reactions further verified the Langmuir-Hinshelwood mechanism for C3 H6 -SCR, where both gaseous NO and C3 H6 were adsorbed and activated on catalyst surface and reacted to generate N2 . Due to the strong metal-support interaction over NiO/γ-Al2 O3 catalyst, both nitrate and Cx Hy Oz intermediates were well preserved to attain high C3 H6 -SCR activity.

The promoting mechanism of SO42− on CeO2 for selective catalytic reduction of NO by NH3: A DFT study

The promoting mechanism of SO42− on CeO2 for selective catalytic reduction of NO by NH3: A DFT study

Performance of Cu-Natural Chabazite During Selective Catalytic Reduction of NO with NH3-Effect of H2O Vapor and Metal Content

Performance of Cu-Natural Chabazite During Selective Catalytic Reduction of NO with NH3-Effect of H2O Vapor and Metal Content

The effect of CNTs on V-Ce/TiO2 for low-temperature selective catalytic reduction of NO

The effect of CNTs on V-Ce/TiO2 for low-temperature selective catalytic reduction of NO

Influence of Framework n(Si)/n(Al) Ratio on the Nature of Cu Species in Cu‐ZSM‐5 for NH3‐SCR‐DeNOx

AbstractNanosized Cu‐containing ZSM‐5 catalysts with different n(Si)/n(Al) ratio of 18.9–50.5 were prepared by ion‐exchange. The physico‐chemical characterization clearly shows that the molar ratio of framework T atoms influences the nature and distribution of copper species. According to DR UV‐Vis, TPR‐H2, EPR, or FT‐IR spectroscopy analyses, the amount of aggregated copper species increases with increasing the framework n(Si)/n(Al) ratio. Thus, the activity of the Cu‐containing ZSM‐5 with n(Si)/n(Al) ratio of 47.0—50.5 in the selective catalytic NO reduction with NH3 (NH3‐SCR‐DeNOx) significantly decreases compared to the other materials (n(Si)/n(Al) ratio of 18.9—19.6). The reaction mechanism has been discussed in light of the results of 2D COS (two‐dimensional correlation spectroscopy) analysis of IR spectra and catalytic properties of the zeolites. The results make evident that enhanced activity of Cu‐containing ZSM‐5 in NH3‐SCR‐DeNOx is correlated with the formation of different NOx− under the experimental conditions.

Unveiling Secondary-Ion-Promoted Catalytic Properties of Cu-SSZ-13 Zeolites for Selective Catalytic Reduction of NOx.

The incorporation of secondary metal ions into Cu-exchanged SSZ-13 zeolites could improve their catalytic properties in selective catalytic reduction of NOx with ammonia (NH3-SCR), but their essential roles remain unclear at the molecular level. Herein, a series of Cu-Sm-SSZ-13 zeolites have been prepared by ion-exchanging Sm ions followed by Cu ions, which exhibit superior NH3-SCR performance. The NO conversion of Cu-Sm-SSZ-13 is nearly 10% higher than that of conventional Cu-SSZ-13 (175-250 °C) after hydrothermal ageing, showing an enhanced low-temperature activity. The Sm ions are found to occupy the six-membered rings (6MRs) of SSZ-13 by X-ray diffraction Rietveld refinement and aberration-corrected scanning transmission electron microscopy. The Sm ions at 6MRs can facilitate the formation of more active [ZCu2+(OH)]+ ions at 8MRs, as revealed by temperature-programmed reduction of hydrogen. X-ray photoelectron spectroscopy and density functional theory (DFT) calculations indicate that there exists electron transfer from Sm3+ to [ZCu2+(OH)]+ ions, which promotes the activity of [ZCu2+(OH)]+ ions by decreasing the activation energy of the formation of intermediates (NH4NO2 and H2NNO). Meanwhile, the electrostatic interaction between Sm3+ and [ZCu2+(OH)]+ results in a high-reaction energy barrier for transforming [ZCu2+(OH)]+ ions into inactive CuOx species, thus enhancing the stability of [ZCu2+(OH)]+ ions. The influence of the ion-exchanging sequence of Sm and Cu ions into SSZ-13 is further investigated by combining both experiments and theoretical calculations. This work provides a mechanistic insight of secondary ions in regulating the distribution, activity, and stability of Cu active sites, which is helpful for the design of high-performance Cu-SSZ-13 catalysts for the NH3-SCR reaction.

Promoting H2O/SO2 resistance of Ce-Mn/TiO2 nanostructures by Sb5+/Sb3+ addition for Selective catalytic reduction of NO with NH3

A series of xSb-4Ce-10Mn/TiO2 (x = 0, 2, 3, 4, 5, 6) catalysts were prepared by a reverse co-precipitation method. The catalysts were characterized by XRD, TEM, Raman, XPS, etc. The effect of the Sb5+/Sb3+ loading on the NO conversion and H2O/SO2 resistance was studied. The xSb-4Ce-10Mn/TiO2 displayed the highest low-temperature catalytic activity, best N2 selectivity and best H2O/SO2 resistance at × = 4. The Sb5+/Sb3+ addition slightly decreased the catalytic activity, but helped to improve the H2O/SO2 resistance. Meanwhile, the Sb5+/Sb3+ addition decreased the Mn4+ and Ti4+ content, but increased the Ce3+ and surface adsorbed oxygen content of the catalyst. In the presence of SO2, the Brønsted acid sites of the 4Sb-4Ce-10Mn/TiO2 increased. Results showed the Sb5+/Sb3+ addition enlarged the surface area, made (NH4)2SO4 easy to decompose and inhibited the MnSO4 formation, which contributed to the improvement of the H2O/SO2 resistance. Transient in situ DRIFT spectra showed the reaction of the NH3-SCR over the catalysts belongs to the Langmuir-Hinshelwood (L-H) mechanism.

Effects of the preparation method of Cu⁃SAPO⁃34 catalysts in de-NOx SCR by propylene

Cu-SAPO-34 catalysts prepared by incipient wet impregnation (IWI), liquid ion-exchange (LIE) and one-pot (OP) hydrothermal methods were applied in selective catalytic reduction of NO with propylene (C3H6-SCR). Under diesel engine exhaust conditions containing 10% O2 and 5% H2O, Cu-SAPO-34(OP) obtained ~95% NOx conversion and ~95% N2 yield at 250–300 °C. Compared the IWI and LIE samples, the Cu-SAPO-34(OP) sample had abundant Lewis acidic sites, the largest specific surface area, and the most isolated Cu2+ species, thus it exhibited the best de-NOx performance. In contrast, more inactive CuO species formed on the surface of the LIE and IWI samples. The isolated Cu2+ ions in the 8MR rings have high reducibility, and are supposed to be the main active sites of Cu-SAPO-34 catalysts in the C3H6-SCR reaction. In situ DRIFTS suggests that isolated copper ions are favourable for the adsorption and activation of NOx and C3H6, thus enhancing the formation of isocyanate (-NCO) and -CN species, which act as key intermediates and further produce N2. This study can be helpful to provide some references for the design of HC-SCR catalyst and application of diesel NOx abatement.

Alkali poisoning of Fe-Cu-ZSM-5 catalyst for the selective catalytic reduction of NO with NH3

Fe (2 wt%)-Cu (1.5 wt%)-ZSM-5 SCR catalyst contacted 1.5 wt% of Na and 1.8 wt% of K in order to simulate poisoning by species more specifically contained in exhaust gases from exhaust gases of diesel engines and power plants. Poisoning agents do not cause loss of surface area nor pore occlusion. XRD and SEM results showed that alkali metals introduction did not deteriorate the crystallinity and morphology of zeolite crystals. However, a significant loss of surface acidity was observed upon alkali-poisoned catalysts causing a dramatic deactivation of the NH3-SCR of NO reaction. Na-doped catalyst showed higher low-temperature SCR activity, while potassium has a stronger deactivation effect on Fe-Cu-ZSM-5 than sodium beyond 400 °C.

Understanding the Water Effect for Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-13 Catalysts

The catalytic activity of commercial Cu-SSZ-13 monolith catalysts was measured in the presence and absence of water, which demonstrated an inhibiting role below 300 °C and a promoting role above 300 °C. Furthermore, temperature-programed desorption illustrated that hydrolysis of Z2Cu would take place to generate ZCuIIOH and [Cu(H2O)6]2+ species, which inhibited ammonia adsorption over Lewis acid sites but promoted nitrate formation and gaseous NO2 desorption. The temperature-programed surface reaction and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were further performed to elucidate the influence of water on reaction mechanisms. Basically, the NH3-SCR reaction mainly took place at Cu centers acting as Lewis acid sites of Cu-SSZ-13 catalysts in the low-temperature regime. Water inhibited the reaction by occupying and blocking Lewis acid sites to prevent [Cu(NH3)2]2+ formation and NO oxidation from occurring, limiting the formation of NH4NO3 intermediates. In the high-temperature regime, gaseous NO2 could directly interact with NH4+ molecules adsorbed onto Brønsted acid sites. The addition of water could facilitate the production of gaseous NO2 decomposed by surface nitrates and further promote the NH3-SCR reaction rate at high temperatures.