Reaction Pathways in the Selective Catalytic Reduction Process with NO and NO2 at Low Temperatures

Effect of Cu loading on the performance and kinetics of Cu/SAPO-34 catalysts for selective catalytic reduction with NH3

A series of X%Cu/SAPO-34 (X = 1.0, 2.0, 4.0 and 6.0) catalysts were prepared by ultrasonic impregnation method for selective catalytic reduction (SCR) of NO_x with ammonia. The effect of different Cu loadings on the selective catalytic reduction of NO by molecular sieve catalysts was examined on a fixed-bed reactor. Catalyst physicochemical properties were characterized and analyzed using XRD, TEM, NH₃-TPD, H₂-TPR, and in situ DRIFTS. Catalysts were used in reaction kinetics studies from the perspective of transient and steady-state kinetics. Cu/SAPO-34 catalyst with 4% Cu loading had the best denitrification efficiency and wide activity window. Copper species were highly dispersed on the catalyst surface. Cu/SAPO-34 catalyst with 4% Cu loading had rich acidic sites and excellent redox performance. Cu/SAPO-34 catalysts with 4% Cu loading possess minimal activation energy and were lower than commercial catalysts. According to the results of in situ IR, transient and steady-state analysis, the Cu/SAPO-34 catalyst with 4% Cu loading in the NH₃-SCR reaction process was mainly E-R mechanism, and there was L-H mechanism.

Estimation of Entropy Generation in a SCR-DeNOx System with AdBlue Spray Dynamic Using Large Eddy Simulation

In this work, the entropy generation analysis is extended to the multi-phase fluid flow within a Large Eddy Simulation (LES) framework. The selected study case consists of a generic selective catalytic reduction (SCR) configuration in which the water/AdBlue is injected into a cross-flow of the internal combustion (IC) engine exhaust gas. The adopted numerical modules are first assessed by comparing with experimental data for film thickness in the case of AdBlue injection and then with H₂O mass fraction and temperature for water injection case. Subsequently, the impact of heat transfer, fluid flow, phase change, mixing and chemical reaction due to AdBlue injection on the entropy generation is assessed. Hence, the individual contributions of viscous and heat dissipation together with the species mixing, chemical reaction during the thermal decomposition of urea into NH₃ and dispersed phase are especially evaluated and analysed. In comparison to the shares of the viscous and mixing processes, the entropy generation is predominated by the heat, chemical and dispersed phase contributions. The influence of the operating parameters such as exhaust gas temperature, flow rate and AdBlue injection on entropy generation is discussed in details. Using a suitable measures, the irreversibility map and some necessary inferences are also provided.

Calcium poisoning mechanism on the selective catalytic reduction of NOx by ammonia over the γ-Fe2O3 (001) surface

γ-Fe₂O₃ has an excellent low-temperature selective catalytic reduction (SCR) deNO_x performance, but its resistance to alkaline earth metal calcium (Ca) is poor. In particular, the detailed mechanism of Ca poisoning on the γ-Fe₂O₃ catalyst at the atomic level is not clear. Hence, the density functional theory method was used in this research to investigate the influence mechanism of Ca poisoning on the NH₃-SCR over the γ-Fe₂O₃ catalyst surface. The findings reveal that NH₃, NO, and O₂ molecules can bind to the γ-Fe₂O₃ (001) surface to generate coordinated ammonia, monodentate nitroso, and adsorption oxygen species, respectively. The main active site is Fe₁-top. For the γ-Fe₂O₃ with Ca poisoning, the Ca atom has a high adsorption energy on the surface of γ-Fe₂O₃ (001), which covers the catalyst surface and reduces the active sites. The presence of Ca atom decreases the adsorption performance of NH₃, while slightly improving the NO and O₂ adsorption. In particular, the Ca atom restrains the NH₃ activation and NH₂ formation, which is detrimental to the NH₃-SCR process.

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization

With the looming goal of carbon neutrality and increasingly stringent environmental protection policies, gas purification in coal-fired power plants is becoming more and more intense. To achieve the NOx emission standard when coal-fired power plants are operating at full load, wide-temperature denitrification catalysts that can operate for a long time in the range of 260–420°C are worthy of study. This review focuses on the research progress and deactivation mechanism of selective catalytic reduction (SCR) denitration catalysts applied to a wide temperature range. With the increasing application of SCR catalysts, it also means that a large amount of spent catalysts is generated every year due to deactivation. Therefore, it is necessary to recycle the wide temperature SCR denitration catalyst. The challenges faced by wide-temperature SCR denitration catalysts are summarized by comparing their regeneration processes. Finally, its future development is prospected.

Effects of Flue Gas Impurities on the Performance of Rare Earth Denitration Catalysts

Selective catalytic reduction (SCR) is still the most widely used process for controlling NOx gas pollution. Specifically, commercial vanadium-based catalysts have problems such as narrow operating temperature range and environmental pollution. Researchers have developed a series of cerium-based catalysts with good oxygen storage performance and excellent redox performance of CeO2. However, the anti-poisoning performance of the catalyst is the key to its application. There are many kinds of impurities in the flue gas, which has a huge impact on the catalyst. The deposition of substances, the reduction of active sites, the reduction of specific surface area, and the reduction of chemically adsorbed oxygen will affect the denitration activity of the catalyst to varying degrees, and the poisoning mechanism of different impurities on the catalyst is also different. Therefore, this review divides the impurities contained in flue gas into different types such as alkali metals, alkaline earth metals, heavy metals, and non-metals, and summarizes the effects and deactivation mechanisms of various types of impurities on the activity of rare earth catalysts. Finally, we hope that this work can provide a valuable reference for the development and application of NH3-SCR catalysts for rare earth denitration in the field of NOx control.

Ceria–tungsten–tin oxide catalysts with superior regeneration capacity after sulfur poisoning for NH3-SCR process

A combined study on the anti-sintering ability, SO2-poisoning mechanism and thermal regeneration property of CeWSnOx catalysts for NH3-SCR reaction.

Reaction Pathways of Standard and Fast Selective Catalytic Reduction over Cu-SSZ-39

Cu-SSZ-39 exhibits excellent hydrothermal stability and is expected to be used for NO_x purification in diesel vehicles. In this work, the selective catalytic reduction (SCR) activities in the presence or absence of NO₂ were tested over Cu-SSZ-39 catalysts with different Cu contents. The results showed that the NO_x conversion of Cu-SSZ-39 was improved by NO₂ when NO₂/NO_x = 0.5, especially for the catalysts with low Cu loadings. The kinetic studies showed two kinetic regimes for fast SCR from 150 to 220 °C due to a change in the rate-controlling mechanism. The activity test and diffuse reflectance infrared Fourier transform spectra demonstrated that the reduction of NO mainly occurred on the Cu species in the absence of feed NO₂, and when NO₂/NO = 1, NO could react with NH₄NO₃ on the Brønsted acid sites in addition to undergoing reduction on Cu species. Thus, NO₂ can promote the SCR reaction over Cu-SSZ-39 by facilitating the formation of surface nitrate species.

Improvement of NOx uptake/release over Pd/Beta by propylene: shielding effect of intermediates on adsorbed NOx species

Passive NOx adsorbers (PNAs) are capable of trapping NOx at low temperature and releasing the trapped NOx into the gas circuit at higher temperatures, where downstream NOx reduction catalysts are activated. Hydrocarbons have a significant effect on the performance of PNAs, nonetheless research in this area has been overlooked. Here the chemistry of NOx adsorption and desorption in the presence of C3H6 was studied. For different pore-size zeolites (BEA, MFI and CHA), the addition of C3H6 always increased the NOx adsorption capacity at a low temperature and raised the NOx desorption temperature. Spectroscopic and computational investigations were performed using the model Pd/Beta to unravel the relevant mechanism. Fourier transform infrared (FTIR) spectra indicated that more Pd+ was formed in the presence of C3H6, which contributed to higher NOx storage capacity. An intermediate Pd-NC3H6O was probed and its evolution procedure was modeled by density functional theory (DFT) calculations. The results showed that a shielding effect of Pd-NC3H6O on Pd+-NO improved the NOx desorption temperature. This investigation has important implications for how short-chain olefins and even more complex gas mixtures affect the NOx adsorption and desorption performance of Pd/zeolite.

Distinct NO2 Effects on Cu-SSZ-13 and Cu-SSZ-39 in the Selective Catalytic Reduction of NOx with NH3

Cu-SSZ-13 and Cu-SSZ-39, with similar structures, are both highly active and hydrothermally stable in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR), attracting great attention for applications on diesel vehicles. In this study, it was interestingly found that NO₂ has distinct effects on the NO_x conversion over Cu-SSZ-13 and Cu-SSZ-39, with an inhibiting effect for Cu-SSZ-13 but a promoting effect for Cu-SSZ-39. The distinct NO₂ effects were found to be associated with the differences in the reactivity of surface NH₄NO₃, a key intermediate for NH₃-SCR, on these two Cu-based small-pore zeolites. Cu-SSZ-13 has excellent standard SCR activity, but the reactivity of surface NH₄NO₃ with NO is relatively low, which would induce the accumulation of NH₄NO₃ on the surface and thus inhibit NO_x conversion. Surface Brønsted acid sites play key roles in the reduction of surface NH₄NO₃ by NO, and Cu-SSZ-39 showed much higher surface acidity than Cu-SSZ-13. Compared with Cu-SSZ-13, the intrinsic standard SCR activity of Cu-SSZ-39 was lower but NH₄NO₃ could be reduced by NO rapidly on Cu-SSZ-39, even faster than the reduction of NO by the adsorbed NH₃ on Cu active sites; thus, NO_x conversion was promoted by NO₂ on Cu-SSZ-39. This work provides an improved understanding of fast SCR on Cu-based small-pore zeolites.

Getting insight into the effect of CuO on red mud for the selective catalytic reduction of NO by NH3

A series of copper-modified red mud catalysts (CuO/PRM) with different copper oxide contents were synthesized by wet impregnation method and investigated for selective catalytic reduction of NO by NH₃ (NH₃-SCR). The catalytic results demonstrated that the red mud catalyst with 7 wt% CuO content exhibited the excellent catalytic performance as well as resistance to water and sulfur poisoning. The red mud support and copper-containing catalysts were characterized by XRF, XRD, N₂ adsorption-desorption, HRTEM, EDS mapping, XPS, H₂-TPR, NH₃-TPD and in situ DRIFT. The obtained results revealed that well dispersed copper oxide originating from 1 to 7 wt% CuO contents was more facile for the redox equilibrium of Cu²⁺ + Fe²⁺ ↔ Cu⁺ + Fe³⁺ shifting to right to form Cu⁺ and surface oxygen species than crystalline CuO generating from high CuO loading (9 wt% CuO), which was beneficial to the enhancement of reducibility and the formation of Lewis acid sites on the catalyst surface. All these factors made significant contributions to the improvement of NH₃-SCR activities for CuO/PRM catalysts. Moreover, in situ DRIFT analysis combined with DFT calculated results confirmed that the finely dispersed copper species not only enhanced the NH₃ activation but also promoted the NO_x desorption, which synergistically facilitated the NH₃-SCR process via the Eley-Rideal mechanism.

Density functional theory (DFT) studies of vanadium-titanium based selective catalytic reduction (SCR) catalysts

Based on density functional theory (DFT) and basic structure models, the chemical reactions on the surface of vanadium-titanium based selective catalytic reduction (SCR) denitrification catalysts were summarized. Reasonable structural models (non-periodic and periodic structural models) are the basis of density functional calculations. A periodic structure model was more appropriate to represent the catalyst surface, and its theoretical calculation results were more comparable with the experimental results than a non-periodic model. It is generally believed that the SCR mechanism where NH₃ and NO react to produce N₂ and H₂O follows an Eley-Rideal type mechanism. NH₂NO was found to be an important intermediate in the SCR reaction, with multiple production routes. Simultaneously, the effects of H₂O, SO₂ and metal on SCR catalysts were also summarized.

Low temperature selective catalytic reduction of nitric oxide with urea over activated carbon supported metal oxide catalysts

Selective catalytic reduction of nitrogen oxides (SCR) with loaded urea is a method for removing NO under oxygen-rich and low-temperature conditions, which can solve the inhibitory effect of oxygen on the catalyst and the slip of ammonia. In present study, a series of activated carbon (wo-AC, co-AC, cs-AC and nu-AC) supported metal (Mn, Fe, Co, Cu and Zn) oxide catalysts with loading urea were prepared by ultrasonic assisted impregnation. The catalysts were used for NO removal at 50-120°C and characterized by XRD, SEM, GFAAS, EDS, XPS, BET and FTIR techniques. The effects of activated carbon type, loaded active element, metal oxides loading, temperature fluctuation on catalytic activity and the catalytic stability were also studied in this paper. The results indicated that nutshell-based activated carbon was more suitable as a carrier than other activated carbons, and urea-10Mn/nu-AC catalyst yielded a higher NO conversion than other catalysts. Besides, for used activated carbons, the larger specific surface area, more micropores distribution and the larger number of hydroxyl group and cyano terminal group are beneficial to the catalytic process. Moreover, the downward trend of NO conversion with increasing temperature suggested the adsorption of reactant gases played a crucial role in the catalytic process of urea-SCR.

Preparation of Mesoporous Mn-Ce-Ti-O Aerogels by a One-Pot Sol-Gel Method for Selective Catalytic Reduction of NO with NH3

Novel Mn-Ce-Ti-O composite aerogels with large mesopore size were prepared via a one-pot sol-gel method by using propylene oxide as a network gel inducer and ethyl acetoacetate as a complexing agent. The effect of calcination temperature (400, 500, 600, and 700 °C) on the NH3-selective catalytic reduction (SCR) performance of the obtained Mn-Ce-Ti-O composite aerogels was investigated. The results show that the Mn-Ce-Ti-O catalyst calcined at 600 °C exhibits the highest NH3-SCR activity and lowest apparent activation energy due to its most abundant Lewis acid sites and best reducibility. The NO conversion of the MCTO-600 catalyst maintains 100% at 200 °C in the presence of 100 ppm SO2, showing the superior resistance to SO2 poisoning as compared with the MnOx-CeO2-TiO2 catalysts reported the literature. This should be mainly attributed to its large mesopore sizes with an average pore size of 32 nm and abundant Lewis acid sites. The former fact facilitates the decomposition of NH4HSO4, and the latter fact reduces vapor pressure of NH3. The NH3-SCR process on the MCTO-600 catalyst follows both the Eley-Rideal (E-R) mechanism and the Langmuir-Hinshelwood (L-H) mechanism.

Quantitative determination of the Cu species, acid sites and NH3-SCR mechanism on Cu-SSZ-13 and H-SSZ-13 at low temperatures

The NH₃-SCR mechanism and the number of acid sites and various Cu species on Cu-SSZ-13 and H-SSZ-13 were quantitatively determined.

Morphology-Oriented ZrO2-Supported Vanadium Oxide for the NH3-SCR Process: Importance of Structural and Textural Properties

ZrO₂ supports, with diverse morphologies (hollow sphere, star, rod, mesoporous), were produced using hydrothermal and evaporation-induced self-assembly methods. Zirconia-supported vanadium oxide catalysts were prepared by wet impregnation and used for the low-temperature selective catalytic reduction (SCR) of NO with ammonia. Characterization of catalysts includes N₂ physisorption, elementary analysis, X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by H₂, and temperature-programmed desorption of NH₃. Significant differences in terms of activity are measured. 3 wt % V₂O₅ supported on mesoporous ZrO₂ (V/MZ) presents excellent N₂ yields (>90%, in the 200-400 °C interval), with a wide operating temperature window (NO conversion > 95%, in the 225-425 °C interval), and less interesting performances were obtained when vanadium oxide is supported over stars, hollow spheres, and rods. Surface characterization showed a content of tetravalent vanadium ion, when supported, decreasing in the order of mesoporous > hollow sphere > star > rod. This order is in perfect agreement with the order of performance of the catalyst in the NH₃-SCR reaction. The impact of tetravalent ion's presence on the surface is confirmed by diffuse reflectance infrared Fourier transform spectroscopy analysis, Brønsted acid sites generated on the surface, and the V⁴⁺-OH species involved in the reaction. The production of more important nitrite species over the tetragonal supported vanadium oxide catalyst could be another reason for the excellent NH₃-SCR performance displayed by the V/MZ catalyst. When supported over monoclinic zirconia, like vanadium oxide over star-type morphology, the adsorbed NH₃ species (NH₄⁺ and coordinated NH₃) reacted with NO _x adsorption species (nitrate) to form ammonium nitrate. Ammonium nitrate can be decomposed to N₂ and N₂O (or NO₂). Thus, NO conversion curves and N₂ yield curves over tetragonal zirconia (MZ) at lower temperature were ahead of those over V/star ZrO₂ because of the higher V⁴⁺ surface content and more active B acid sites associated with an easy formation of the nitrito intermediate.

Numerical Investigation of SCR Mixer Design Optimization for Improved Performance

The continuous increase in the number of stringent exhaust emission legislations of marine Diesel engines had led to a decrease in NOx emissions at the required level. Selective catalyst reduction (SCR) is the most prominent and mature technology used to reduce NOx emissions. However, to obtain maximum NOx removal with minimum ammonia slip remains a challenge. Therefore, new mixers are designed in order to obtain the maximum SCR efficiency. This paper reports performance parameters such as uniformity of velocity, ammonia uniformity distribution, and temperature distribution. Also, a numerical model is developed to investigate the interaction of urea droplet with exhaust gas and its effects by using line (LM) and swirl (SM) type mixers alone and in combination (LSM). The urea droplet residence time and its interaction in straight pipe are also investigated. Model calculations proved the improvement in velocity uniformity, distribution of ammonia uniformity, and temperature distribution for LSM. Prominent enhancement in the evaporation rate was also achieved by using LSM, which may be due to the breaking of urea droplets into droplets of smaller diameter. Therefore, the SCR system accomplished higher urea conversion efficiency by using LSM. Lastly, the ISO 8178 standard engine test cycle E3 was used to verify the simulation results. It has been observed that the average weighted value of NOx emission obtained at SCR outlet using LSM was 2.44 g/kWh, which strongly meets International Maritime Organization (IMO) Tier III NOx (3.4 g/kWh) emission regulations.

Highly selective catalytic reduction of NOx by MnOx-CeO2-Al2O3 catalysts prepared by self-propagating high-temperature synthesis

We first present preparation of MnO_x-CeO₂-Al₂O₃ catalysts with varying Mn contents through a self-propagating high-temperature synthesis (SHS) method, and studied the application of these catalysts to the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Using the catalyst with 18 wt.% Mn (18MnCe1Al2), 100% NO conversion was achieved at 200°C and a gas hourly space velocity of 15384hr^-1, and the high-efficiency SCR temperature window, where NO conversion is greater than 90%, was widened to a temperature range of 150-300°C. 18MnCe1Al2 showed great resistance to SO₂ (100 ppm) and H₂O (5%) at 200°C. The catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy, Fourier transform infrared spectroscopy, and H₂ temperature programmed reduction. The characterization results showed that the surface atomic concentration of Mn increased with increasing Mn content, which led to synergism between Mn and Ce and improved the activity in the SCR reaction. 18MnCe1Al2 has an extensive pore structure, with a BET surface area of approximately 135.4m²/g, a pore volume of approximately 0.16cm³/g, and an average pore diameter of approximately 4.6 nm. The SCR reaction on 18MnCe1Al2 mainly followed the Eley-Rideal mechanism. The performances of the MnO_x-CeO₂-Al₂O₃ catalysts were good, and because of the simplicity of the preparation process, the SHS method is applicable to their industrial-scale manufacture.

Promotional Effect of Cerium and/or Zirconium Doping on Cu/ZSM-5 Catalysts for Selective Catalytic Reduction of NO by NH3

The cerium and/or zirconium-doped Cu/ZSM-5 catalysts (CuCexZr1−xOy/ZSM-5) were prepared by ion exchange and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR). Activities of the catalysts obtained on the selective catalytic reduction (SCR) of nitric oxide (NO) by ammonia were measured using temperature programmed reactions. Among all the catalysts tested, the CuCe0.75Zr0.25Oy/ZSM-5 catalyst presented the highest catalytic activity for the removal of NO, corresponding to the broadest active window of 175–468 °C. The cerium and zirconium addition enhanced the activity of catalysts, and the cerium-rich catalysts exhibited more excellent SCR activities as compared to the zirconium-rich catalysts. XRD and TEM results indicated that zirconium additions improved the copper dispersion and prevented copper crystallization. According to XPS and H2-TPR analysis, copper species were enriched on the ZSM-5 grain surfaces, and part of the copper ions were incorporated into the zirconium and/or cerium lattice. The strong interaction between copper species and cerium/zirconium improved the redox abilities of catalysts. Furthermore, the introduction of zirconium abates N2O formation in the tested temperature range.

Synergistic effect of VOx and MnOx surface species for improved performance of V2O5/Ce0.5Ti0.5−xMnxO2−δ catalysts in low-temperature NH3-SCR of NO

Inserting adjacent Mn³⁺/Mn²⁺ and VO³⁺/VO²⁺ redox couples in Ce_1−xTi_xO₂ improves catalytic performance.

Single-atom iron catalyst with single-vacancy graphene-based substrate as a novel catalyst for NO oxidation: a theoretical study

Nitric oxide (NO) emitted from coal-fired power plants has raised global concerns.

A PGM-free NOx adsorber + selective catalytic reduction catalyst system (AdSCR) for trapping and reducing NOx in lean exhaust streams at low temperature

Novel AdSCR systems are able to trap and reduce NO_x in lean exhausts at low temperature.

Combined fast selective reduction using Mn-based catalysts and nonthermal plasma for NOx removal

In this study, the concept of fast SCR for NO reduction with NH₃ as reducing agent is realized via the combination of nonthermal plasma (NTP) with Mn-based catalyst. Experimental results indicate that 10% wt. Mn-Ce-Ni/TiO₂ possesses better physical and chemical properties of surface, resulting in higher NO removal efficiency if compared with 10% wt. Mn-Ce/TiO₂ and 10% wt. Mn-Ce-Cu/TiO₂. Mn-Ce-Ni/TiO₂ of 10% wt. achieves 100% NO_x conversion at 150 °C, while 10% wt. Mn-Ce/TiO₂ and 10% wt. Mn-Ce-Cu/TiO₂ need to be operated at a temperature above 200 °C for 100% NO_x conversion. However, NO conversion achieved with 10% wt. Mn-Ce-Ni/TiO₂ is significantly reduced as H₂O_(g) and SO₂ are introduced into the SCR system simultaneously. Further, two-stage system (SCR with DBD) is compared with the catalyst-alone for NO_x conversion and N₂ selectivity. The results indicate that 100% NO_x conversion can be achieved with two-stage system at 100 °C, while N₂ selectivity reaches 80%. Importantly, NO_x conversion achieved with two-stage system could maintain >95% in the presence of C₂H₄, CO, SO₂, and H₂O_(g), indicating that two-stage system has better tolerance for complicated gas composition. Overall, this study demonstrates that combining NTP with Mn-based catalyst is effective in reducing NO_x emission at a low temperature (≤200 °C) and has good potential for industrial application.