Fast SCR Research Articles

The Activity of CeVO4-Based Catalysts for Ammonia-SCR: Impact of Surface Cerium Enrichment

The ammonia-SCR catalytic activity of unsupported CeVO4 with an excess of CeO2 was investigated in standard and fast-SCR conditions. Solids were obtained from a hydrothermal synthesis route under a mild condition and then stabilized after aging in a wet atmosphere at 600 and 850 °C. Particular attention was paid to the role of excess CeO2 and the consequences of hydrothermal aging on physical–chemical properties and catalytic activity. The XRD patterns put into evidence the formation of the zircon-type structure of CeVO4 in agreement with a segregation of cubic face-centered structure of ceria (CeO2). Along with adding an excess of CeO2, high specific surface area (102 m2/g) for the 11wt% CeO2/CeVO4 solid was obtained. The presence of CeO2 nanoparticles in addition to CeVO4 nanoparticles have limited the decrease in the specific surface area after aging at 600 and 850 °C. The catalyst with 11wt% CeO2/CeVO4 exhibited the best catalytic performances in standard and fast SCR conditions after thermal aging at 600 °C.

Develop high efficient of NH3-SCR catalysts with wide temperature range by ball-milled method

To develop high efficient NH3-SCR catalysts with wide temperature range, a series of novel catalysts based on MnOx-CeO2/TiO2 and V2O5-WO3/TiO2 with various mixing-ratios were prepared via ball-milled method for selective catalytic reduction of NO by NH3. N2-Sorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and in-situ diffuse reflectance infrared transform spectroscopy (in-situ DRIFTS) were used to characterize the catalysts. The ball-milling catalysts exhibited over 90% NO conversion in 200–400 °C, good N2 selectivity and high poison resistance to SO2 and H2O in broad temperature window when compared to their original materials (e.g., the 5MnCe/Ti-5VW/Ti (BM-6 h). The good SCR performance of xMnCe/Ti-yVW/Ti (BM-6 h) indicated that the milled-mixing method was a promising way in developing wide temperature range SCR catalysts. The XPS results showed that the ratio of (V3++V4+)/Vn+, Mn4+/Mn3+ and Ce3+/Ce4+ on the surface of xMnCe/Ti-yVW/Ti (BM-6 h) samples were higher than that on the MnCe/Ti(raw/BM-6 h) and VW/Ti(raw/BM-6 h), and increased as the duration of ball-milled process extended. The possible routes for electron transfer between the V, Mn and Ce components on the ball-milled mixing catalysts were proposed. The DRIFTS results showed that the surface of 5MnCe/Ti-5VW/Ti (BM-6 h) was mainly covered by ionic NH4+ at low temperature due to abundant Brønsted acid sites. The ionic NH4+ reacted with NO2 nitro compounds and gaseous NO via “fast SCR” reaction route at low temperature. In addition, the coordinated NH3 dominated the SCR reaction by reacting with gaseous NO in “standard SCR” route at high temperature (i.e. ≥350 °C).

Hydrothermal aging alleviates the inhibition effects of NO2 on Cu-SSZ-13 for NH3-SCR

Fresh and hydrothermally aged Cu-SSZ-13 catalysts were tested for the standard and fast selective catalytic reduction of NOx with NH3 (NH3-SCR). Interestingly, NO2 exhibited distinctly different effects on these two catalysts. NOx conversion over fresh Cu-SSZ-13 was clearly inhibited by NO2, while hydrothermal aging could reverse this inhibition effect. It was found that the NOx conversion decreased with progressive hydrothermal aging under standard SCR (SSCR) conditions but increased under fast SCR (FSCR) conditions. The N2 adsorption/desorption isotherms indicated the formation of mesopores induced by hydrothermal aging, which is favorable for gas diffusion. Temperature programmed desorption (TPD) and in situ DRIFTS, combined with other auxiliary measurements, demonstrated that L-NH3 (NH3 adsorbed onto Lewis acid sites) is more active toward both NO+O2 and NO2 for the formation of N2, while B-NH3 (NH3 adsorbed onto Brønsted acid sites) is mainly responsible for the formation of NH4NO3. The amount and stability of NH4NO3 formed under FSCR conditions decreased with progressive hydrothermal aging due to the loss of Brønsted acid sites.

Simultaneous improvement of ammonia mediated NOx SCR and soot oxidation for enhanced SCR-on-Filter application

The integration of NOx reduction and catalytic soot oxidation was investigated for the SCRoF (Selective Catalytic Reduction on Filter) applications. By physically mixing a commercial SCR catalyst (either Fe-ZSM-5 and Cu-ZSM-5) with a soot oxidation catalyst (K/CeO2-PrO2), it was possible to lower the soot oxidation temperature by more than 150 degrees and, by optimizing the catalysts mass ratio in the mixture, NOx conversion simultaneously increased, because NO oxidation induced a fast SCR reaction pathway, unlike during standard SCR. Such an improvement in NOx conversion was more pronounced with the Fe-ZSM-5 than with the Cu-ZSM-5 zeolite, as the latter was more sensitive to the NO2/NOx ratio. In order to make the soot oxidation catalyst inactive towards ammonia oxidation, poisoning of the surface acid sites with 3.0 wt.% K2CO3 (corresponding to only 1.0 wt.% K) was performed. In the soot oxidation and SCR catalysts physical mixture, the soot was oxidized mainly by O2 and the contribution of NO2 to oxidation was negligible, as NO2 itself was a key reactant in the (kinetically much faster) SCR reaction.

Promoting effect of Ce and Mn addition on Cu-SSZ-39 zeolites for NH3-SCR reaction: Activity, hydrothermal stability, and mechanism study

Abstract In this work, Ce and Mn were added to Cu-SSZ-39 zeolites by ion-exchange process, and their activity performance for selective catalytic reduction of NOx with ammonia (NH3-SCR), especially hydrothermal stability (hydrothermal aging at 750, 800, 850 °C) was investigated systematically. It is found that the addition of Ce and Mn, Mn more evidently, both can greatly improve the activity of Cu-SSZ-39 catalyst. Moreover, MnCu-SSZ-39 catalyst have shown superior hydrothermal stability compared with Cu-SSZ-39 catalyst, and outstandingly its activity is not only not decreased significantly like Cu-SSZ-39 but improved obviously after hydrothermal aging at as high as 850 °C. The high and improved activity of Ce and Mn added Cu-SSZ-39 is ascribed to more active CuO crystallites formation in the inner channel and higher acidity strength. The superior hydrothermal stability of MnCu-SSZ-39 may be due to its well-preserved structure and acid density even after hydrothermal aging at as high as 850 °C. Moreover, during the hydrothermal aging process, not only Cu oxides aggregation is inhibited, but also more Cu2+ ion species are induced to ion exchange sites in MnCu-SSZ-39, which may account for its excellent hydrothermal stability (even higher than the activity of fresh samples). The mechanism analysis has shown NH3-SCR reaction over Cu-SSZ-39 and CeCu-SSZ-39 samples follow the “Langmuir–Hinshelwood” (L-H) mechanism, while on MnCu-SSZ-39, it follows both “L–H” and “Eley–Rideal” (E–R) mechanisms, which may also be another reason for its excellent activity and hydrothermal stability. Moreover, “Fast SCR” mechanisms also occur and contribute over hydrothermal aged A-MnCu-SSZ-39 sample, in addition to “L-H” and “E-R” mechanisms. Thus, the easily prepared MnCu-SSZ-39 catalyst has shown the excellent activity and super hydrothermal stability, with 90% NO conversion at 245 °C and broader temperature window of 245 °C to 550 °C, even after hydrothermal aging at 850 °C, therefore indicate great potential for future application in NOx purifying from diesel engine exhaust.

Increased SCR performance of Cu-CHA due to ammonium nitrate buffer: Experiments with oscillating NO/NO2 ratios and application to real driving cycles

Currently, the literature on ammonium nitrate formation across SCR catalysts mainly sees ammonium nitrate formation as an undesired problem because it can inhibit the catalytic activity. This paper shows that, besides the inhibition effects, ammonium nitrate formation on the catalyst can also positively influence the catalytic activity because it acts as an NO2 buffer that stores NO2 under NO2-excess conditions and releases NO2 under NO2-deficient conditions.To demonstrate the working principle of the ammonium nitrate buffer, in a first step the ammonium nitrate loading is determined as a function of the NO2/NOx ratio in the feed, using a titration scheme recently proposed in the literature. Then, step tests are performed where at otherwise constant feed conditions, the NO2/NOx ratio is switched between 75 % NO2/NOx and 25 % NO2/NOx. During the slow frequency switching (3 min per phase), an increased NOx conversion is observed after both the transition to NO2-rich and NO2-lean. This can be explained by a build-up of ammonium nitrate during NO2-rich and the consumption of ammonium nitrate during NO2-lean conditions owing to the enhanced SCR reaction. When the frequency of the NO2/NOx switches is increased to 30 s, the ammonium nitrate storage loading can no longer follow the fluctuations and stays at an intermediate level. During this scenario, the catalyst shows a constant high conversion close to the Fast SCR reaction.Finally, it is demonstrated that the ammonium nitrate buffer contributes to the overall NOx conversion during standard driving cycles. In particular, the NO2 emissions are predicted significantly better by a model that accounts for the ammonium nitrate storage equilibrium.

Effects of Ce over TiO2 supported MnOx-based Catalyst for NOx Reduction by Ammonia

Ce modified MnOx-based catalysts have attracted much attention due to its high activity for selective catalytic reduction of NOx by NH3 (NH3-SCR) at low-temperatures. However, the most important role of Ce on the NH3-SCR performance of MnOx-based catalysts has not been confirmed. Herein, the typical Ce-Mn/TiO2 catalyst was synthesized through incipient-wetness impregnation method, the positive role of Ce on Ce-Mn/TiO2 catalyst in the NH3-SCR process was revealed by combining different activity tests (including NO oxidation and NH3 oxidation) and characterizations (including XRD, XPS and He-TPDMS experiments). It was found that the introduction of Ce can promote the dispersion of MnOx on TiO2 support. Meanwhile, the doping of Ce in MnOx can also increase the content of Mn4+ species. The Mn4+ species plays a crucial role in NO oxidation reaction, which can trigger the “Fast SCR” reaction and promote the conversion of NOx. This work provides insight into the catalyst design for NH3-SCR process at low-temperature.

The effect of non-selective oxidation on the Mn2Co1Ox catalysts for NH3-SCR: Positive and non-positive

In order to explore the generation of NO2 and N2O on the Mn-Co catalysts in the NH3-SCR process, Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts, prepared by two different complexing agents, i.e., polyethylene glycol (PEG) and urea as double complexing agents, and ammonia (PA), were further investigated by NO + O2 and NH3 + O2 experiments, besides NH3-SCR test. Catalytic activity and physicochemical properties of the catalysts were compared and analyzed through NH3-TPD, O2-TPD, H2-TPR, XRD, Raman and XPS. The NH3-SCR results show that the Mn2Co1Ox (PEG) exhibits higher NO conversion than Mn2Co1Ox (PA) due to larger amount of lattice oxygen accompanied with more Con+, and surface adsorbed oxygen species, as well as the improvement of surface acidity, proved by O2-TPD, XPS and NH3-TPD, respectively. Furthermore, the separate oxidation experiments show that Mn2Co1Ox (PEG) effectively exhibits the oxidation of NH3 to N2O and also the oxidation of NO to NO2 by O2 respectively, which can influence the catalytic performance. The XRD, XPS and Raman characterization results indicate that Mn2Co1Ox (PEG) contains more Co3O4 than Mn2Co1Ox (PA), that can produce non-selective pure oxidation reaction by the NO + O2 and NH3 + O2 experiments of Co3O4 phase. Information from in situ infrared experiments is that the strongly adsorbed NH3 species on the Mn2Co1Ox (PEG) is susceptible to deep dehydrogenation and react with NOx to produce N2O in the presence of O2. Meanwhile, the oxidation site (Co3O4) can also oxidize NO to more NO2, thereby improving the “fast SCR” reaction. However, if the proportion of oxidation sites is high, NO and NH3 can be oxidized to more N2O will be produced by the reaction between NO and NH3.

Study of the nitric oxide reduction of SCR-NH3 on γFe2O3 catalyst surface with quantum chemistry

The surface reaction process of γFe2O3 catalyst for NOx removal with NH3 was studied by the first principles and density functional theory. The results show that the NH3 molecule is easily adsorbed on the surface of the catalyst, then dehydrogenated to produce NH2, NH and other products, yet most of the products are NH2 since the activation energy barrier from NH2 to NH is high under anaerobic condition. The next main step is that NH2 reacts with gaseous NO to generate NH2NO intermediate products, and then it decomposes to N2 and H2O. The NH3 dehydrogenation is the rate determining step with the activation energy barrier of 88.040 kJ/mol. However, the NH fragment will be formed when oxygen exists since the activation energy barrier from NH2 to NH reduces under aerobic condition. After its formation, the NH fragment will combine with NO to yield NHNO and decomposes to N2 and hydroxyl. In addition, NO2 can be produced on the surface by the absorbed NO reacting with an active O atom, and the NO2 can carry out fast SCR process. Otherwise, the N2O is not easily formed on the surface of γFe2O3 catalyst which means that the catalyst has good selectivity.

Assessment of the Single-Site Kinetic Model for NH3-SCR on Cu-Chabazite for the Prediction of NOx Emissions in Dynamometer Tests

A transient single-site kinetic model consisting of NH3-SCR (selective catalytic reduction) related reactions (NH3 adsorption/desorption, NH3 oxidation, NO oxidation, standard SCR, fast SCR, NO2-SCR, NH4NO3, and N2O formation) on a commercial multi-site Cu-Chabazite washcoated monolith was developed using laboratory scale synthetic gas bench (SGB) data and its potential towards predicting the downstream NOx concentrations was assessed using SGB and dynamometer tests. Kinetic model described the transient and steady-state data obtained for the model development in the SGB well. Single-site nature of the model enabled the prediction of the reactivity of the lower temperature active Cu site much better than the activity associated with higher temperature Cu sites. Validation experiments in the SGB at 185 °C which consisted of varying NH3 and NO2/NOx ratios were very well predicted. The kinetic model was also successful in predicting the instantaneous NOx emissions and cumulative NOx and N2O amount released in real-size SCR reactors during World Harmonic Transient Cycle (WHTC) in the 220–290 °C range. Model was in slight disagreement with measured cumulative NO but was in good in agreement with NO2 for the World Harmonic Stationary Cycle (WHSC) tests in dynamometer and 235–339 °C range, respectively. The developed model was found sufficient for both design and calibration of aftertreatment systems (ATS).

Effect of the NH4NO3 Addition on the Low-T NH3-SCR Performances of Individual and Combined Fe- and Cu-Zeolite Catalysts

We have measured NOx conversions and N2O productions over Fe-BEA and Cu-SAPO catalysts and over their sequential arrangements under Enhanced SCR conditions, resulting from the addition of an aqueous solution of ammonium nitrate (AN) to the typical Standard SCR feed stream, and we have compared them to those observed under Standard and Fast SCR conditions. The expected strong enhancement of the poor low temperature activity of the Fe-BEA catalyst was confirmed: both NH3 and NOx conversions and N2O formations similar to those of the Fast SCR reaction were achieved when cofeeding ammonium nitrate. On the other hand, the Cu-SAPO efficiency was drastically decreased by the addition of AN at low temperatures, possibly due to trapping of the ammonium nitrate salt within the SAPO zeolite, characterized by smaller pores than those of the BEA zeolite. The Cu-SAPO performances were recovered only at T > 250 °C with a huge release of N2O due to the thermal decomposition of AN. The combined system with the Fe-zeolite sample placed upstream of the Cu-zeolite also exhibited outstanding low temperature deNOx performances, with even lower N2O production than over the Fe-zeolite only at the same Enhanced SCR (E-SCR) conditions.

Study on the NO2 production pathways and the role of NO2 in fast selective catalytic reduction DeNOx at low-temperature over MnOx/TiO2 catalyst

Selective catalytic reduction of NO by NH3 (SCR) at low temperature usually considered to be the combination of NO oxidation and “Fast SCR” process. However, the NO oxidation mechanism and the role of NO2 over “Fast SCR” process are controversial. Focusing on these problems, the in-deep studies were performed over the typical MnOx/TiO2 catalysts. Combining in-situ DRIFT, TPD-MS and DFT calculation results, an integrated NO oxidation mechanism was proposed based on an “Nitro-assisted oxygen activation” concept where the activation and transformation of O2 is linked with the transformation of nitro species. The OO bond of O2 can be weakened by generating a variation of bidentate nitrates, which can promote the break of OO bond and the formation of NO2. The formation of the variation of bidentate nitrates was the rate-determining step over NO oxidation. Additionally, we further reveal that the crucial role of NO2 over “Fast SCR” process lies in participating the hydrogen abstraction from coordinated NH3 and promoting the production of NH2 active species which can direct react with NO to generate N2 and H2O. The removability of NO2 can coupling the MnO2 (NO oxidation active component) and TiO2 support (“Fast SCR” active component) together. Our findings provide insight into the design and improvement of catalysts or reaction pathways over the low-temperature SCR process.

Experimental Study on SO2-to-SO3 Conversion Over Fe-Modified Mn/ZSM-5 Catalysts During the Catalytic Reduction of NOx

Mn–Fe/ZSM-5 catalysts with different Fe loading amounts were prepared using wet impregnation and the effects of these catalysts on SO3 formation during the catalytic reduction of NOx were assessed. Variations in the catalysts’ physicochemical properties with different Fe contents were also investigated using BET, XRD, SEM–EDS, FT-IR, H2-TPR and XPS techniques. The 9Mn9Fe catalyst gave the highest SO2-to-SO3 and NOx conversions, and adding Fe also improved the catalyst’s SO2 resistance. The presence of NO2 caused the formation of nitrate ions on the catalyst surface during fast SCR. These nitrate ions re-oxidized the active sites more rapidly than O2 at low temperatures, thereby promoting the redox reaction. BET, XRD and SEM–EDS showed that optimal Fe loading amount reduced MnOx crystallinity while improving surface dispersion. Adding excessive Fe caused the active compositions to accumulate on the catalyst surface. FT-IR, H2-TPR and XPS demonstrated that optimal Fe loading amount increased the quantity of Mn4+=O bonds while raising the concentrations of Mn4+ ions and lattice oxygen, thus enhancing SO2-to-SO3-conversion. Therefore, although suitable amounts of Fe and NO2 promoted NOx conversion, these species also significantly increased SO2-to-SO3 conversion.

Manganese oxide catalysts supported on zinc oxide nanorod arrays: A new composite for selective catalytic reduction of NOx with NH3 at low temperature

Abstract In this study, a new composite of low-temperature selective catalytic reduction (LT-SCR) catalyst is synthesized by impregnating MnOx onto ZnO nanorod arrays integrated with cordierite ceramic substrate (MnOx-ZnO/CC), which exhibits improved activities at 100–250 °C compared to its analogue that fabricated from TiO2 nanorod arrays (MnOx-TiO2/CC). By optimizing the synthesis conditions, the MnOx-ZnO/CC catalyst obtains a maximum NOx conversion efficiency of 96.8% at 200 °C. The introduction of ZnO nanorod arrays not only provides a high surface area, but also results in the formation of amorphous MnOx. A higher content of Mn4+, proper ratio of surface-adsorbed oxygen (Oα), and more Lewis acid sites are found on the surface of the MnOx-ZnO/CC catalyst than that on the MnOx-TiO2/CC counterpart. Additional information about the reaction mechanism is systematically investigated via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), the results of which demonstrate that the LT-SCR reactions over the MnOx-ZnO/CC catalyst generally occur between NOx and NH3 species in the adsorption status, following the Langmuir-Hinshelwood pathway. The enhanced activities can be attributed to the generation of abundant NH4NO3 originating from NO2 in the “Fast SCR” route, because the synergistic effect between MnOx and ZnO facilitates the oxidation of NO.

Cu Loading Dependence of Fast NH3-SCR on Cu/SSZ-13

The fast NH3-SCR kinetics are studied on Cu/SSZ-13 with different Cu loadings, under reaction conditions (temperature and space velocity) where both steady-state and differential NOx conversions are readily achieved. Cu loading is found to greatly influence low-temperature NOx turnover rates, however not to substantially affect apparent activation energies. The high reaction apparent activation energies (~ 160 kJ/mol) suggest that the rate-limiting kinetics for fast SCR involve the participation of NH4NO3, an intermediate that is of considerable stability in small-pore zeolites. The energetically demanding NH4NO3 + NO reactions make low-temperature fast SCR much slower than standard SCR over Cu-exchanged small-pore zeolite catalysts, in drastic contrast to vanadium and medium/large pore zeolite-based SCR catalysts.

Design of a hierarchical Fe-ZSM-5@CeO2 catalyst and the enhanced performances for the selective catalytic reduction of NO with NH3

Selective catalytic reduction of NOx by NH3 (NH3-SCR) is an effective technique for the abatement of NOx. Development of SCR catalyst is the core of the technique. Herein, a hierarchical Fe-ZSM-5@CeO2 catalyst with CeO2 nanoparticles decorating is originally fabricated via dopamine polymerization for NH3-SCR. Fe-ZSM-5@CeO2 possess a NOx conversion over 90% and N2 selectivity over 98% in the temperature range of 250–400 °C, showing an advanced catalytic activity compared with Fe-ZSM-5/CeO2. The synergistic effect of Fe-ZSM-5 and CeO2 over Fe-ZSM-5@CeO2 facilitates the formation of redox cycle between Ce3+/Ce4+ redox-couple and high rate active oxygen species (Oβ and Oγ), leading to the enhanced NH3-SCR performance. The feature of hierarchical structure with CeO2 outside provides a pathway to convert NO to NO2, facilitating the fast SCR. Meanwhile, the formed NO2 can activate Fe2+/Fe3+ redox-couple, offering an extra redox cycle which can further contribute to fast SCR. The outer CeO2 layer can also improve the water resistance of Fe-ZSM-5 as the catalytic activity of Fe-ZSM-5@CeO2 was impervious when exposed to 10% H2O. To be concluded, the hierarchical Fe-ZSM-5@CeO2 is proved to be a promising candidate for NH3-SCR due to its uniform distribution of active sites and the synergistic effect of Fe-ZSM-5 and CeO2 species.

Dual Promotional Effects of TiO2-Decorated Acid-Treated MnOx Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NOx

Alkali metals generated during waste incineration in power stations are not conducive to the control of nitrogen oxide (NO x) emission. Hence, improved selective catalytic reduction of NO x with ammonia (NH3-SCR) in the presence of alkali metals is a major issue for practical NO x removal. In this work, we developed a novel TiO2-decorated acid-treated MnO x octahedral molecular sieve (OMS-5(H)@TiO2) catalyst with improved alkali-resistant NO x reduction at low temperature, and the dual promotional effects of OMS-5(H)@TiO2 catalysts were clarified. It was found that the special structure of the acid-treated MnO x octahedral molecular sieve (OMS-5(H)) was responsible for the trapping of alkali metals and high deNO x activity at low temperature. Subsequently, the decoration by TiO2 further improved the redox properties by accelerating the high ratio of Mn4+ and Oα on the surface of the highly active (OMS-5(H)@TiO2) catalyst. Moreover, a thorough mechanism study via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) demonstrated that the acid treatment led to remarkable increment of acid sites, which enabled the catalyst to resist alkali metals in the form of ion exchange. Meanwhile, the decoration of TiO2 further increased the strength of the Lewis acid sites, assisting more active intermediate species to effectively take part in the deNO x reaction. Besides, a "fast SCR" process was observed to certify that the decoration of TiO2 promoted the improvement of low-temperature activity in the presence of alkali metals. The dual effects combining OMS-5(H) with TiO2 decoration in terms of alkali metal resistance and high catalytic activity at low temperature proved that the high-performance deNO x catalyst was successfully developed in this work. The work paves a way for the development of superior low-temperature SCR catalysts with improved NO x reduction efficiency in the presence of alkali metals.

A kinetic study of the NO to NO2 oxidation mechanism over Fe-FER: A combined analysis of operando surface and gas phase data

Abstract Diesel engines when compared with gasoline engines produce higher amount of NOx due the operating conditions. Among the NOx produced NO represents the great majority while, under air excess conditions, the removal of NOx is necessarily catalytic and still challenging. In the ammonia based SCR, it is often admitted that, when dealing with the original formulation developed for stationary sources (V/TiO2), the fast SCR proceeds in presence of an equimolar mixture of NO and NO2. Most of the catalytic formulations developed up to now (except Cu based zeolites) thus suffer the same rate determining step consisting in the NO to NO2 oxidation. Aiming at improving the catalytic efficiency through the catalyst composition, it is fundamental to understand the reaction mechanism at its elementary steps in order to properly design the active sites. The choice regarding the distinct possible hypothetical mechanisms was thus here guided by the nature of the active sites. It is indeed worth knowing, for a better tuning of the catalyst formulation, whether isolated, oligomeric or dual iron sites are involved in the catalytic loop. The approach in this work consists in a systematic study of the influence of the inlet NO concentration under O2 excess on both the initial reaction rate (NO to NO2 conversion level kept below 10%) and the NO coverage level onto iron measured by operando FTIR. Isothermal experiments were performed over an aged FeFER catalyst at three distinct temperatures. The whole set of data was then processed and compared to the expected evolution derived from five possible mechanisms. The best mechanism determined in the frame of this study involves isolated iron sites onto which NO and O2 co-adsorb and the corresponding rate determining step consists in the dissociation of the so-formed Fe_NOO2 intermediate species. The associated activation energy and reaction enthalpy values are then evaluated.

Investigation of the robust hydrothermal stability of Cu/LTA for NH3-SCR reaction

Recently copper ion-exchanged LTA zeolites were proved to be robust for NH3-SCR reaction. In this study, Cu/LTA catalysts with Si/Al = 15 and Cu/Al = 0.4 were synthesized via incipient wetness impregnation (IWI) method, following degreening/hydrothermal aging at different temperatures (750, 800, 850, 900 °C), and used to catalyze standard SCR, fast SCR and NH3/NO oxidation reactions. Catalysts were characterized with surface area/pore volume, powder X-Ray diffraction (XRD), nuclear magnetic resonance (NMR), H2-temperature programmed reduction (H2-TPR) and in situ Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS). Through the BET surface areas, XRD and NMR results, it can be found that the framework structure stability of Cu/LTA catalysts during hydrothermal aging was outstanding, even after harsh aging at 900 °C. Moreover, various Cu species, including Z-Cu2+, Z-[Cu(OH)]+ and CuOx clusters, were quantified for Cu/LTA catalysts hydrothermally aged under various temperatures with H2-TPR and in situ DRIFTS. An imperative finding in this study is the exceptional hydrothermal stability of [Cu(OH)]+ and the gradual conversions of both Cu2+ and CuOx clusters to [Cu(OH)]+ with increasing aging temperature. It is worth noting that this phenomenon is exactly the opposite of Cu/SSZ-13. As it is known from the literature (Song et al., 2017), the formation of CuOx not only decreases the selectivity of NOx conversion, but also can cause deterioration of zeolite structure, since the ion-exchanged copper stabilizes the zeolite. This may also explain why the hydrothermal stability of Cu/LTA samples is outstanding.

Nature of active sites in Cu-LTA NH3-SCR catalysts: A comparative study with Cu-SSZ-13

Here, a comparative study is made on four pairs of Cu-LTA and Cu-SSZ-13 zeolites with the same bulk Si/Al ratio (16) but different Cu/Al ratios (0.14–0.65), in an attempt to understand the similarities and differences in the nature of their catalytically active sites for the NH3-SCR reaction. Two different types of copper ions, i.e., Cu2+ and [Cu(OH)]+, can exist in these two catalyst systems. However, their location and distribution were characterized to be quite different. Unlike the case in Cu-SSZ-13, both Cu2+ and [Cu(OH)]+ ions in Cu-LTA catalysts with Cu/Al ≤ 0.50 are predominantly located near the single 6-ring sites in the LTA structure, and copper species when over-exchanged reside within the 8-ring windows. It was also found that both Cu-SSZ-13 and Cu-LTA have nitrate and ammonium nitrate as reaction intermediates, but the relative amounts of these intermediates are always smaller in the latter catalyst system regardless of the Cu/Al ratio. This is primarily due to the preferential location of copper ions in single 6-rings that are spatially more confined than the 8-ring window sites. Consequently, Cu-LTA shows a better deNOx activity under the fast SCR conditions than Cu-SSZ-13.