Identifying active sites for fast NH3-SCR of NO/NO2 mixtures over Fe-ZSM-5 by operando EPR and UV–vis spectroscopy

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Sustainable product-based approach in the production of olefins using a dual functional ZSM-5 catalyst

Investigation of the current industrial processes, such as methanol to olefin (MTO) and hexane to olefin (HTO), in terms of green and sustainable chemistry approaches in order to design the process, catalyst and reactor from the beginning in such a way as to minimize environmental pollution is compulsory. Therefore, the synthesis of a group of multifunctional catalysts, which can be used simultaneously in both industrial processes to produce a variety of products, was studied. The effect of incorporation of different metals (Fe, Mn, Zn, Ga and Al) on the strengthening of each of the products was also studied. The investigation of reactor productivity (WHSV_HTO = 25) in HTO showed that the production efficiency of propylene in microchannels is higher than the ideal value for all samples, which is a significant improvement for sustainable approaches in future technologies. Considering the overall performances, Ga-ZM showed the best performance in both processes due to the high P/E ratio. The significant effect of Ga on the increasing of propylene was confirmed in MTO at 400 °C (P/E ≃ ∞), which indicated the dramatic effect of this metal in directing the reaction mechanism to an olefin-based cycle by converting almost all ethylene to propylene by methylation.

A review on identification, quantification, and transformation of active species in SCR by EPR spectroscopy

Electron paramagnetic resonance (EPR) is the only technique that provides direct detection of free radicals and samples that contain unpaired electrons. Thus, EPR had an important potential application in the field of selective catalytic reduction of nitrogen oxide (SCR). For the first time, this work reviewed recent developments of EPR in charactering SCR. First, qualitative analysis focused on recognizing Cu, Fe, V, Ti, Mn, and free-radical (oxygen vacancy and superoxide radical) species. Second, quantification of the active species was obtained by a double-integral and calibration method. Third, the active species evolved because of different thermal treatments and redox-thermal processes under reductants (NH₃ and NO). The coordination information of the active species in catalysts and their effects on SCR performances were concluded from mechanism viewpoints. Finally, potential perspectives were put forward for EPR developments in characterizing the SCR processes in the future. After all, EPR characterization will help to have a deep understanding of structure-activity relationship in one catalyst.

Low-temperature selective catalytic reduction of NO with NH3 over an FeO x /β-MnO2 composite

A series of Fe-modified β-MnO₂ (FeO _x /β-MnO₂) composite catalysts were prepared by an impregnation method with β-MnO₂ and ferro nitrate as raw materials. The structures and properties of the composites were systematically characterized and analyzed by X-ray diffraction, N₂ adsorption-desorption, high-resolution electron microscopy, temperature-programmed reduction of H₂, temperature-programmed desorption of NH₃, and FTIR infrared spectroscopy. The deNO _x activity, water resistance, and sulfur resistance of the composite catalysts were evaluated in a thermally fixed catalytic reaction system. The results indicated that the FeO _x /β-MnO₂ composite (Fe/Mn molar ratio of 0.3 and calcination temperature of 450 °C) had higher catalytic activity and a wider reaction temperature window compared with β-MnO₂. The water resistance and sulfur resistance of the catalyst were enhanced. It reached 100% NO conversion efficiency with an initial NO concentration of 500 ppm, a gas hourly space velocity of 45 000 h^-1, and a reaction temperature of 175-325 °C. The appropriate Fe/Mn molar ratio sample had a synergistic effect, affecting the morphology, redox properties, and acidic sites, and helped to improve the low-temperature NH₃-SCR activity of the composite catalyst.

Catalytic Performance of One-Pot Synthesized Fe-MWW Layered Zeolites (MCM-22, MCM-36, and ITQ-2) in Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

The application of layered zeolites of MWW topology in environmental catalysis has attracted growing attention in recent years; however, only a few studies have explored their performance in selective catalytic reduction with ammonia (NH3-SCR). Thus, our work describes, for the first time, the one-pot synthesis of Fe-modified NH3-SCR catalysts supported on MCM-22, MCM-36, and ITQ-2. The calculated chemical composition of the materials was Si/Al of 30 and 5 wt.% of Fe. The reported results indicated a correlation between the arrangement of MWW layers and the form of iron in the zeolitic structure. We have observed that one-pot synthesis resulted in high dispersion of Fe3+ sites, which significantly enhanced low-temperature activity and prevented N2O generation during the reaction. All of the investigated samples exhibited almost 100% NO conversion at 250 °C. The most satisfactory activity was exhibited by Fe-modified MCM-36, since 50% of NO reduction was obtained at 150 °C for this catalyst. This effect can be explained by the abundance of isolated Fe3+ species, which are active in low-temperature NH3-SCR. Additionally, SiO2 pillars present in MCM-36 provided an additional surface for the deposition of the active phase.

Quantification of Redox Sites during Catalytic Propane Oxychlorination by Operando EPR Spectroscopy

Identification and quantification of redox-active centers at relevant conditions for catalysis is pivotal to understand reaction mechanisms and requires development of advanced operando methods. Herein, we demonstrate operando EPR spectroscopy as an important technique to quantify the oxidation state of representative CrPO₄ and EuOCl catalysts during propane oxychlorination, an attractive route for propylene production. In particular, we show that the space-time-yield of C₃ H₆ correlates with the amount of Cr²⁺ and Eu²⁺ ions generated over the catalysts during reaction. These results provide a powerful strategy to gather quantitative understanding of selective alkane oxidation, which could potentially be extrapolated to other functionalization approaches and operating conditions.

A plasma thermal slag-derived from hazardous waste has a born hydrothermal stability

Hydrothermal instability restricts performances of silica-based catalysts, which have wide applications in both industry and environment. For the first time, plasma-thermal slag was revealed to be a catalyst with a born hydrothermal stability in selective catalytic reduction of nitric oxide. The slag catalyst removed 98.5 % of NO with a high N₂ selectivity (> 95 %) at 200 °C. After a hydrothermal treatment at 900 °C, the activity of the slag only decreased to 84.0 %. According to characterizations of XRD, HTREM, XPS, and EPR, active metals existed in coordination states in the slag at first. Under hydrothermal conditions, these species transformed to short-range single crystals, which were hindered from sintering by surrounded Si-O bands. At the same time, in-situ DRIFT indicated that more Brønsted and Lewis acid sites were formed. Hence, enough active sties were reserved for effective catalytic reduction of nitric oxide. The main result of this work helps us to understand hydrothermal stability of a catalyst. What's more, the high-value-added utilization of plasma-thermal slag is in favor of the development of hazardous-waste treatment.

Polyphenylene as an Active Support for Ru-Catalyzed Hydrogenolysis of 5-Hydroxymethylfurfural

Selective transformation of biomass feedstocks to platform molecules is a key pursuit for sustainable chemical production. Compared to petrochemical processes, biomass transformation requires the defunctionalization of highly polar molecules at relatively low temperatures. As a result, catalysts based on functional organic polymers may play a prominent role. Targeting the hydrogenolysis of the platform chemical 5-hydroxymethylfurfural (5-HMF), here, we design a polyphenylene (PPhen) framework with purely sp²-hybridized carbons that can isolate 5-HMF via π-π stacking, preventing hemiacetal and humin formation. With good swellability, the PPhen framework here has successfully supported and dispersed seven types of metal particles via a newly developed swelling-impregnation method, including Ru, Pt, Au, Fe, Co, Ni, and Cu. Ru/PPhen is studied for 5-HMF hydrogenolysis, achieving a 92% yield of 2,5-dimethylfuran (DMF) under mild conditions, outperforming the state-of-the-art catalysts reported in the literature. In addition, PPhen helps perform a solventless reaction, achieving direct 5-HMF to DMF conversion in the absence of any liquid solvent or reagent. This approach in designing support-reactant/solvent/metal interactions will play an important role in surface catalysis.

Site-Specific Iron Substitution in STA-28, a Large Pore Aluminophosphate Zeotype Prepared by Using 1,10-Phenanthrolines as Framework-Bound Templates

An AlPO4 zeotype has been prepared using the aromatic diamine 1,10-phenanthroline and some of its methylated analogues as templates. In each case the two template N atoms bind to a specific framework Al site to expand its coordination to the unusual octahedral AlO4 N2 environment. Furthermore, using this framework-bound template, Fe atoms can be included selectively at this site in the framework by direct synthesis, as confirmed by annular dark field scanning transmission electron microscopy and Rietveld refinement. Calcination removes the organic molecules to give large pore framework solids, with BET surface areas up to 540 m2  g-1 and two perpendicular sets of channels that intersect to give pore space connected by 12-ring openings along all crystallographic directions.

Selective catalytic reduction of NOx by NH3 over CeVO4-CeO2 nanocomposite

In this study, it was found that the CeVO₄-CeO₂ nanocomposite possessed remarkably selective catalytic reduction (SCR) performance and wider active temperature scope. And, the promotion principle was explored based on BET, XRD, XPS, H₂-temperature-programmed reduction, NH₃-temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform (DRIFT) techniques. The characterization outcomes manifested that the CeVO₄-CeO₂ nanocomposite could inhibit its crystallinity and enhance the concentrations of chemisorbed oxygen species and Ce³⁺, which was advantageous to the SCR process. Moreover, the in situ DRIFT technique manifested that the NH₃-SCR reaction over Ce_0.75V_0.25O_y was enhanced effectively through the mechanism of L-H.

Multivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV-Vis/ATR-IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas-Liquid Phase Reactions

Operando EPR, XANES/EXAFS, UV‐Vis and ATR‐IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6‐tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least‐squares fitting (MCR‐ALS) of simultaneously recorded XAS and UV‐Vis data sets, it was found that an initially formed (bpy)(NMI)Cu^I‐ complex (bpy=2,2′‐bipyridine, NMI=N‐methylimidazole ) is converted to two different Cu^II species, a mononuclear (bpy)(NMI)(CH₃CN)Cu^II‐OOH species detectable by EPR and ESI‐MS, and an EPR‐silent dinuclear (CH₃CN)(bpy)(NMI)Cu^II(μ‐OH)₂⋅Cu^II (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)Cu^II‐TEMPO monomers that are also EPR‐silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non‐reducible Cu^II leads to slight deactivation at extended reaction times.

High-Efficiency Catalytic Conversion of NOx by the Synergy of Nanocatalyst and Plasma: Effect of Mn-Based Bimetallic Active Species

Three typical Mn-based bimetallic nanocatalysts of Mn−Fe/TiO2, Mn−Co/TiO2, Mn−Ce/TiO2 were synthesized via the hydrothermal method to reveal the synergistic effects of dielectric barrier discharge (DBD) plasma and bimetallic nanocatalysts on NOx catalytic conversion. The plasma-catalyst hybrid catalysis was investigated compared with the catalytic effects of plasma alone and nanocatalyst alone. During the catalytic process of catalyst alone, the catalytic activities of all tested catalysts were lower than 20% at ambient temperature. While in the plasma-catalyst hybrid catalytic process, NOx conversion significantly improved with discharge energy enlarging. The maximum NOx conversion of about 99.5% achieved over Mn−Ce/TiO2 under discharge energy of 15 W·h/m3 at ambient temperature. The reaction temperature had an inhibiting effect on plasma-catalyst hybrid catalysis. Among these three Mn-based bimetallic nanocatalysts, Mn−Ce/TiO2 displayed the optimal catalytic property with higher catalytic activity and superior selectivity in the plasma-catalyst hybrid catalytic process. Furthermore, the physicochemical properties of these three typical Mn-based bimetallic nanocatalysts were analyzed by N2 adsorption, Transmission Electron Microscope (TEM), X-ray diffraction (XRD), H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The multiple characterizations demonstrated that the plasma-catalyst hybrid catalytic performance was highly dependent on the phase compositions. Mn−Ce/TiO2 nanocatalyst presented the optimal structure characteristic among all tested samples, with the largest surface area, the minished particle sizes, the reduced crystallinity, and the increased active components distributions. In the meantime, the ratios of Mn4+/(Mn2+ + Mn3+ + Mn4+) in the Mn−Ce/TiO2 sample was the highest, which was beneficial to plasma-catalyst hybrid catalysis. Generally, it was verified that the plasma-catalyst hybrid catalytic process with the Mn-based bimetallic nanocatalysts was an effective approach for high-efficiency catalytic conversion of NOx, especially at ambient temperature.

Effect of iron loading on the performance and structure of Fe/ZSM-5 catalyst for the selective catalytic reduction of NO with NH3

A series of Fe/ZSM-5 catalysts with different Fe contents were prepared by impregnation method. The catalysts were characterized by TEM, XRD, H₂ temperature-programed reduction (H₂-TPR), temperature-programed desorption of ammonia (NH₃-TPD), and in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and the catalytic activity test was also carried out on selective catalytic reduction (SCR) denitration device. Results showed that the single metal iron-supported ZSM-5 catalyst has high deNO_x activity in the medium-high temperature range, and the optimal loading of Fe active component is 10 wt%; the deNO_x efficiency over 80% at the range of 350-450 °C and 431 °C reaches the maximum of 96.91%. Iron species can be finely dispersed on the surface of the carrier as amorphous oxides, and the crystalline structure of zeolite is retained. The significant redox performance, highly dispersed nanoparticles, and rich Lewis acid sites on the surface of catalyst are favorable for the SCR denitration reaction. Fe/ZSM-5 10 wt% catalyst has rich Lewis acid sites and less B acid sites and Lewis acidic sites play an important role during the reaction. Only Eley-Rideal (E-R) mechanism existed during the NH₃-SCR reaction process, and there is no denitration reaction being accomplished by L-H mechanism at 150 °C.

Structure–Activity Relationship Study of Mn/Fe Ratio Effects on Mn−Fe−Ce−Ox/γ-Al2O3 Nanocatalyst for NO Oxidation and Fast SCR Reaction

A series of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts were synthesized with different Mn/Fe ratios for the catalytic oxidation of NO into NO2 and the catalytic elimination of NOx via fast selective catalytic reduction (SCR) reaction. The effects of Mn/Fe ratio on the physicochemical properties of the samples were analyzed by means of various techniques including N2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD) and NO-TPD, meanwhile, their catalytic performance was also evaluated and compared. Multiple characterizations revealed that the catalytic performance was highly dependent on the phase composition. The Mn15Fe15−Ce/Al sample with the Mn/Fe molar ratio of 1.0 presented the optimal structure characteristic among all tested samples, with the largest surface area, increased active components distributions, the reduced crystallinity and diminished particle sizes. In the meantime, the ratios of Mn4+/Mnn+, Fe2+/Fen+ and Ce3+/Cen+ in Mn15Fe15−Ce/Al samples were improved, which could enhance the redox capacity and increase the quantity of chemisorbed oxygen and oxygen vacancy, thus facilitating NO oxidation into NO2 and eventually promoting the fast SCR reaction. In accord with the structure results, the Mn15Fe15−Ce/Al sample exhibited the highest NO oxidation rate of 64.2% at 350 °C and the broadest temperature window of 75–350 °C with the NOx conversion >90%. Based on the structure–activity relationship discussion, the catalytic mechanism over the Mn−Fe−Ce ternary components supported by γ-Al2O3 were proposed. Overall, it was believed that the optimization of Mn/Fe ratio in Mn−Fe−Ce/Al nanocatalyst was an extremely effective method to improve the structure–activity relationships for NO pre-oxidation and the fast SCR reaction.

What News in the Surface Chemistry of Bulk and Supported Vanadia Based SCR-Catalysts: Improvements in their Resistance to Poisoning and Thermal Sintering

This paper reports a short review on the impact of poisoning effects and thermal aging on the reactivity of surface vanadate species on benchmark V₂ O₅ -WO₃ /TiO₂ Selective-Catalytic-Reduction catalysts. A renewed interest of this SCR technology is related to its diversification notably as after-treatment systems to treat the exhaust from Diesel engines with higher running temperature especially when SCR catalysts are coated on Diesel Particulates Filters. Particular attention is also paid to poisoning effects in conjunction with the progressive replacement of fossil fuels by bio-fuels containing alkaline contaminants which drastically deactivate the catalyst through neutralization of strong acid sites. Most of the investigations show that better insight into the mechanisms of poisoning and aging processes is needed especially to understand the relative sensibility of various vanadate species. This could provide original guidelines to catalyst preparation and or the developments of more stable bulk systems as exemplified.

Investigating the Low Temperature Formation of CuII -(N,O) Species on Cu-CHA Zeolites for the Selective Catalytic Reduction of NOx

In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of Cu^II -(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic reduction of NO_x with NH₃ (NH₃ -SCR). In particular, we investigated the reaction of NO and O₂ at low temperature (200 and 50 °C) on a series of Cu-CHA zeolites with different composition (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidation part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200 °C. At lower temperature a mixture of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO⁺ , whose amount was found to be proportional to the zeolite Brønsted site concentration. Nitrates were found to mainly form with Cu^II ions stabilized by one negative framework charge (Z), Z-[Cu(OH]^I or Z-[Cu(O₂ ]^I , without involvement of Z₂ -Cu^II ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH₃ . Finally, water was found to replace preformed chelating copper nitrates and deplete NO⁺ (though with different kinetics) at both temperatures, while favouring the presence of monodentate ones.

Quantum chemistry study on the oxidation of NO catalyzed by ZSM5 supported Mn/Co–Al/Ce

The detailed mechanism of NO oxidation catalyzed by ZSM5 supported Mn/Co–Al/Ce is investigated and revealed by Quantum Chemistry Calculation. A three-step catalytic mechanism for NO oxidation is proposed and studied. Theoretical results show that, the activate energies of reactions catalyzed by ZSM-5 supported Mn/Co (71.1[Formula: see text]kJ/mol/80.6[Formula: see text]kJ/mol) are much lower than that obtained from the direct NO oxidation. This indicates that the ZSM-5 supported Mn/Co has an obvious catalytic effect. When the active center Si is replaced by Al and Ce, the activation energies are further decreased to about 40[Formula: see text]kJ/mol. This indicates that the doping of Al and Ce can obviously improve the catalytic effect. The theoretical study illustrates that the catalysts for NO oxidation not only relate to the supported transition metal such as Co and Mn, but also highly relate to the activity centers such as Al and Ce.

One-pot construction of Fe/ZSM-5 zeolites for the selective catalytic reduction of nitrogen oxides by ammonia

A simple one-step hydrothermal approach is developed to construct Fe/ZSM-5 zeolites for the selective catalytic reduction of nitrogen oxides by ammonia.

The enhanced resistance to P species of an Mn–Ti catalyst for selective catalytic reduction of NOx with NH3 by the modification with Mo

The modification of Mn–Ti catalyst by Mo could enhance its resistance to P species.

Challenges and breakthroughs in post-combustion catalysis: how to match future stringent regulations

This short overview briefly summarizes the prominent evolutions and scientific breakthroughs in the development of end-of-pipe technologies with respect to the standard regulations of atmospheric pollutant emissions from automotive exhaust.