NH3-SCR Of NOx Research Articles

Excellent performance of Ce doped CoMn2O4/TiO2 catalyst in NH3-SCR of NOx under H2O&SO2 conditions

Mn-based catalysts have become a research hotspot for low-temperature NH3-SCR, and the improvement of anti-SO2 poisoning performance is crucial to promote their industrial application for NH3 selective catalytic reduction of NOx. In this work, the CoMn2O4/Ce-TiO2 catalyst was able to maintain 95 % NOx conversion even in the presence of 10 vol% H2O+50 ppm SO2 for 24 h at 250 °C.The excellent performance was attributed to the large mesoporous structure, specific surface area, and the enhancement of acid and redox properties associated with the catalysts. The Ce doped CoMn2O4/Ce-TiO2 catalyst maintained Mn3+, Mn4+ and Co3+ in high concentrations on the surface, which improved the lattice oxygen mobility, and the electron transfer and interaction. The presence of more Ce4+ provided higher SCR activity due to the strong oxygen storage migration and also release ability. The reaction process of NH3-SCR for NOx removal on the surface of CoMn2O4/Ce-TiO2 catalyst mainly followed the Eley-Rideal (E-R) mechanism. The NH3 adsorbed on the catalyst surface reacted mainly with NO/NO2 species, even under H2O&SO2. The E-R reaction mechanism on the surface of CoMn2O4/Ce-TiO2 catalyst was less restricted by the inhibition of H2O&SO2. This is one of the primary reasons for the superior anti-sulfur toxicity performance of the catalyst. This work opens a new avenue for the design of efficient and environmentally friendly NH3-SCR catalysts with good application prospects.

Supercritical ion exchange: A new player in Cu-zeolite catalyst synthesis towards NH3-SCR of NOx

In this study, the effect of zeolite type, copper loading, and synthesis method/conditions on the performance of a range of Cu/Zeolites in the Standard Selective Catalytic Reduction (SCR) reaction was investigated via utilizing synthesis methods of Supercritical and Aqueous Ion Exchange (SCIE and AIE). Cu/MORS synthesized via SCIE outperformed the conventionally prepared catalyst, Cu-MORA. Cu/MORS and Cu/ZSM-5 exhibited higher NO conversion than Cu/SSZ-13 at low temperatures due to higher Cu loading. However, Cu/SSZ-13 ones surpassed the other catalysts in terms of NO conversion at temperatures above 450 °C. Selective nature of SCIE under different conditions resulted in an obvious difference between the catalyst’s performance while the SCIE temperature varied from 40 to 80 °C. The results of this study showed that synthesis method/condition affects directly the Cu speciation and catalyst performance in NH3-SCR, calling for developing new synthesis methods of Cu/Zeolites to modify the catalysts performance in after-treatment systems.

Al-rich Cu-SSZ-13 zeolite synthesized by interconversion from Y: Crystallization process resolution and application in the selective catalytic reduction of NO with NH3

Cu-SSZ-13 zeolite is a promising catalyst in the selective catalytic reduction (SCR) of NOx with NH3 and the interzeolite conversion is considered as a convenient method with relatively low cost to get the Al-rich SSZ-13 zeolite with great hydrothermal stability. However, the crystallization process of interzeolite conversion and its relation to the catalytic performance of Cu-SSZ-13 in the NH3-SCR of NOx are still poorly understood. Herein, the interzeolite conversion process of Y to SSZ-13 was well resolved and the effect of major synthesis factors on the hydrothermal stability and catalytic performance of Cu-SSZ-13 in the NH3-SCR of NOx was investigated. The results indicate that highly crystalline SSZ-13 can be synthesized by the interzeolite conversion from Y, using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as the structure-directing agent (SDA) and with a SDA/Si molar ratio of 0.06–0.2. The Y zeolite of FAU framework is first degraded and decomposed into the oligomeric aluminosilicate fragments quickly in the alkaline circumstance; thereafter, the oligomeric fragments develop into the crystal nuclei that arouse the rapid crystallization of the SSZ-13 zeolite of CHA framework with the help of SDA and solid Al(OH)3, during which the Al(OH)3 source is dissolved and restructured to tetrahedral Al species and incorporated into the CHA framework. The synthesized Cu-SSZ-13 zeolite exhibits excellent catalytic performance in the NH3-SCR of NO, achieving an NO conversion of nearly 90% at a rather low temperature of 150 °C and a gaseous hourly space velocity (GHSV) of 100,000 h−1. Moreover, the Cu-SSZ-13 catalyst displays great hydrothermal stability and remains highly active even after aging at 800 °C. These results help to clarify the mechanism of interzeolite conversion and provide an effective method to synthesize highly crystalline Al-rich SSZ-13 zeolite with great potential for application in the NH3-SCR of NOx.

Shielding ceria based catalysts from SO2 poisoning in NH3-SCR reaction: Modification effect of acid metal oxides

Modifying CeO2 with acid sites is a common strategy for designing efficient catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx). However, the effect of acid metal oxide modification on the SO2 resistance of CeO2 has not been well revealed. In this work, it was found that the SO2 resistance of CeO2 modified with several acid metal oxides (i.e., MoO3, WO3 and Nb2O5), followed an order of Mo/CeO2 ≥ W/CeO2 > Nb/CeO2. Further systematic characterizations revealed that Mo-OH and W-OH on CeO2 as Brønsted acid sites could better inhibit the SO2 adsorption and the sulfation of active sites than highly dispersed NbOx mainly as Lewis acid sites because NH4+ coordinated to Brønsted acid sites could help trap SO2 to form ammonium sulfates rather than metal sulfates. The findings in this work provided important guidance for the design of efficient catalysts with superior SO2 resistance performance.

Regeneration of PbO-deactivated Mn-Ce/Ti-bearing blast furnace slag SCR catalyst: Washing and Mn-Ce reloading

The NH3-SCR of NOx from sintering flue gas can be hindered by PbO, deactivating catalysts and making its regeneration a significant challenge. To address this issue, a novel MnOx-CeO2/Ti-bearing blast furnace slag catalyst was developed and its Pb poisoning mechanism was previously elucidated. In this work, regeneration of Pb-poisoned catalyst was systematically evaluated, including washing with different agents and replenishing MnOx-CeO2 components. The optimal conditions for catalyst cleaning were 1.5 mol/L H2SO4 pickling at 30 °C for 2 h. The doped SO42− could interact with catalyst ingredients, generating sulfate substances that partly transformed into various active sulfur-bearing species. These species enriched the catalyst surface and led to the formation of new S-OH/S = O chemical bonds. Loading MnOx-CeO2 further improved the catalyst's redox capacity by facilitating electronic conversions and increasing chemisorbed oxygen Oα content. The regeneration process also enhanced catalyst surface acidity by generating new N = O and N-OH bonds (N = Mn, Ce), intensifying the adsorption and activation of gaseous NH3. The present work offers new insights into the regeneration of MnOx-CeO2/Ti-bearing blast furnace slag catalyst, particularly for their application in flue gas streams with high Pb content.

New CHA-Type aluminoborosilicates as efficient catalysts for MTO and NH3-SCR of NOx reactions

The small-pore CHA-type zeolites have received intensive attentions due to their outstanding performance in both of the methanol-to-olefin (MTO) process and ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) reactions. However, the costly synthesis of CHA-type aluminosilicate is a great issue for the practical application. In this study, a novel and cheap organic structure-directing agent of N,N,N-trimethylisobutylammonium hydroxide was developed and used to synthesize CHA-type zeolites (name as ECNU-25) with boric acid as the supporting agent. The Si/Al ratio in the synthetic gel could be tailored in a wide range of 50 - ∞. [Al,B]-ECNU-25 zeolite thus synthesized served as an effective catalyst for the MTO reaction. After Cu2+-exchange, the Cu-[Al,B]-ECNU-25 was also highly active in the NH3-SCR of NOx.

The Effect of Different Morphologies on the Nh3-Scr of Nox for Cu-Ce/Tio2 Catalysts and its In-Situ Drifts Study

The Effect of Different Morphologies on the Nh3-Scr of Nox for Cu-Ce/Tio2 Catalysts and its In-Situ Drifts Study

NH3-Selective Catalytic Reduction of NOx to N2 over Ceria Supported WOx Based Catalysts: Influence of Tungsten Content

A series of HPW/CeO2 catalysts generated from 12-tungstophosphoric acid, H3PW12O40 (HPW), supported on ceria and presenting different tungsten loadings (2, 4.5, 9, 16, and 40 wt% W) were prepared and characterized by N2 physisorption, XRD, IR, Raman, and UV-Vis. The different characterization techniques suggested that low loading of tungsten resulted in mainly isolated sites, while high tungsten loading produced polymeric or tungsten clusters. Those materials exhibited high activity in NH3-SCR of NOx into N2. Moreover, the series of experiments indicated that low loading in tungsten (2% HPW/CeO2) displayed the highest activity with a remarkable N2 selectivity (99%) at medium-high temperature (300–515 °C), owing to the high amount of monomeric tungstate coverage on the catalyst surface.

Cost-effective fast-synthesis of chabazite zeolites for the reduction of NOx

The costly synthesis of conventional CHA zeolites for ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) has hindered their widespread applications. In this study, highly crystalline CHA zeolites (CHA-Y and CHA-B) were successfully synthesized from low-cost NaY and Al-rich beta precursors and alternately low cost or low content structure directing agents through the inter-zeolite conversion route. After ion exchange with Cu, the Cu−CHA-Y and Cu−CHA-B showed superior performance in the NH3-SCR of NOx reaction, which can be attributed to their high surface area, abundant acidic sites. The in situ analysis by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed the reaction intermediates of NH3-SCR of NOx over the low-cost prepared Cu−CHA catalysts. The as- prepared low-cost Cu−CHA catalysts are promising candidates for the reduction of NOx conforming to the economic production of deNOx catalysts.

Influence of Aluminum Sources on Synthesis of SAPO-34 and NH3-SCR of NOx by as-Prepared Cu/SAPO-34 Catalysts

Influence of Aluminum Sources on Synthesis of SAPO-34 and NH3-SCR of NOx by as-Prepared Cu/SAPO-34 Catalysts

Influence of Aluminum Sources on Synthesis of SAPO-34 and NH3-SCR of NOx by as-Prepared Cu/SAPO-34 Catalysts

The full article will be published in the English version of the journal "Catalysis in Industry" No. 1, 2021.The synthesis of the microporous SAPO-34 molecular sieve goes from a combination of three templates: triethylamine, tetraethylammonium hydroxide, and morpholine under hydrothermal conditions. Two aluminum sources, namely aluminum hydroxide, and aluminum isopropoxide, were used exclusively to synthesize SAPO-34 zeolites. The effects of aluminum sources on the crystallization and physicochemical properties of SAPO-34 were studied thoroughly. The synthesized samples were characterized by using different characterization methods, including XRD, FE-SEM, N2 isotherm, EDS, and NH3-TPD. The results illustrate that the various sources of aluminum used for the synthesis of SAPO-34 materials extremely affect the crystallinity, morphology, and density of acid sites. Besides, the influence of aluminum sources on the performance of NH3-SCR technology was studied with Cu/SAPO-34 catalysts in a fixed-bed flow reactor. The two Cu/SAPO-34 catalysts promoted different NO and NH3 conversions between 200–600 °C though they share similar Cu content, which was loaded by the ion-exchange method in aqueous solution. In addition, the different Cu species in the two catalyst samples are surveyed by H2-TPR, while the EPR method is also used to assess the coordination of the copper element in the two catalysts.

Effect of Formaldehyde in Selective Catalytic Reduction of NOx by Ammonia (NH3-SCR) on a Commercial V2O5-WO3/TiO2 Catalyst under Model Conditions.

The impact of formaldehyde (HCHO, formed in vehicle exhaust gases by incomplete combustion of fuel) on the performance of a commercial V2O5-WO3/TiO2 catalyst in NH3-SCR of NOx under dry conditions has been analyzed in detail by catalytic tests, in situ FTIR and transient studies using temporal analysis of products (TAP). HCHO reacts preferentially with NH3 to a formamide (HCONH2) surface intermediate. This deprives NH3 partly from its desired role as a reducing agent in the SCR and diminishes NO conversion and N2 selectivity. Between 250 and 400 °C, HCONH2 decomposes by dehydration (major pathway) and decarbonylation (minor pathway) to liberate toxic HCN and CO, respectively. HCN was proven to be oxidized by lattice oxygen of the catalyst to CO2 and NO, which enters the NH3-SCR reaction.

Cu-SSZ-13 zeolite–metal oxide hybrid catalysts with enhanced SO2-tolerance in the NH3-SCR of NOx

The relatively low SO2-tolerance of Cu-SSZ-13 zeolite limits its broader applications in NH3-SCR. In this study, Cu-SSZ-13 zeolite-metal oxide hybrids with different metal oxide components (Mn, Co, Ni, Zn) were prepared using the sol-dispersion method. After poisoning with 50 ppm SO2, ZnTi10Ox-Cu-SSZ-13(1:5) catalyst shows a NO conversion of 73 %, which is significantly higher than in Cu-SSZ-13. Regeneration tests together with TGA and SO2-TPD results suggest that ZnTi10Ox in the hybrid catalyst might serve as a sacrificial component that undergoes preferential reaction with SO2, thereby sparing the Cu2+ active sites from sulfur deactivation. ZnTi10Ox-Cu-SSZ-13(1:5) prepared using the sol-dispersion method exhibits higher NO conversion after SO2 poisoning versus mechanically mixed and co-precipitated hybrid catalysts, which may be due to stronger interactions between Cu2+ and the SSZ-13 support. Our results suggest that ZnTi10Ox-Cu-SSZ-13 may be a promising hybrid catalyst for deNOx due to its high reactivity and decreased sensitivity towards SO2.

Improvement of Low-Temperature Activity of FeBeta Monolith Catalyst in NH3-SCR of NOx

FeBeta||Mn–Ce/FeBeta catalyst was prepared by coating cordierite monolith with FeBeta zeolite followed by an impregnation of its downstream part by Mn–Ce solution. The resulting dual-zone catalyst demonstrates improved catalytic performance in the selective catalytic reduction of NOx by NH3 (NH3-SCR) as compared to the parent Fe-Beta. The catalyst exhibits 80–90% NOx conversion and 100% selectivity to N2 within wide temperature ranges: 180–500 °C (at GHSV = 18,000 h−1) and 260–500 °C (at GHSV = 54,000 h−1). Moreover, the dual-zone catalyst provides favorable NH3-slip removal efficiency even at NH3/NO = 1.60.

Nb-Modified Ce/Ti Oxide Catalyst for the Selective Catalytic Reduction of NO with NH3 at Low Temperature

Recently, great attention has been paid to Ceria-based materials for selective catalytic reduction (SCR) with NH3 owing to their unique redox, oxygen storage, and acid-base properties. Two series of bimetallic catalysts issued from Titania modified by Ce and Nb were prepared by the one-step sol-gel method (SG) and by the sol-gel route followed by impregnation (WI). The resulting core-shell and bulk catalysts were tested in NH3-SCR of NOx. The impregnated Nb5/Ce40/Ti100 (WI) catalyst displayed 95% NOx conversion at 200 °C (GHSV = 60,000 mL·g−1·h−1, 1000 ppm NOx, 1000 ppm NH3, 5% O2/He) without forming N2O. The catalysts were characterized by various methods including ICP-OES, N2-physisorption, XRD, Raman, NH3-TPD, DRIFTS, XPS, and H2-TPR. The results showed that the introduction of Nb decreases the surface area and strengthens the surface acidity. This behavior can be explained by the strong interaction between Ceria and Titania which generates Ce-O-Ti units, as well as a high concentration of amorphous or highly dispersed Niobia. This should be the reason for the excellent performance of the catalyst prepared by the sol-gel method followed by impregnation. Furthermore, Nb5/Ce40/Ti100 (WI) has the largest NH3 adsorption capacity, which is helpful to promote the NH3-SCR reaction. The long-term stability and the effect of H2O on the catalysts were also evaluated.

Nitric Oxide Reduction of Heavy-Duty Diesel Off-Gas by NH3-SCR in Front of the Turbocharger

This paper investigates the impacts and effects of higher pressure on NH3-SCR of NOx corresponding to positioning the SCR catalyst in front of the exhaust gas turbine, which is an option in heavy-duty and off-road diesel engine applications (1–4). The influence of applied pressure up to 500 kPa on the selective catalytic reduction (SCR) of nitrogen oxides by NH3 over commercial V2O5/WO3-TiO2-type catalysts (VWT) is studied experimentally and numerically. The model includes a detailed reaction mechanism for standard and fast SCR and a two-dimensional description of the flow field and diffusion in the honeycomb-structured catalyst. The pressure effect on catalytic conversion, residence time, and mass transfer are examined for varying temperature and two different channel sizes, i.e., 25 and 300 cpsi (channels per square inch) at constant mass flux from the engine. Pre-turbo positioning of the catalyst leads to a significant increase in NOx conversion due to higher pressure aside from the positive temperature effect. However, with increasing channel size, diffusional mass transport limits the reaction rate, and reduces the benefit of increased residence time due to the decrease in diffusion rates with increasing pressure. The achievable increase in conversion is transformed into a length reduction of the catalyst.

Plasma-treated Ce/TiO2–SiO2 catalyst for the NH3-SCR of NOx

ABSTRACTCe/TiO2–SiO2 catalysts with different Ti/Si molar ratios are prepared by the incipient impregnation method and their NH3-SCR activities are evaluated at 100–500°C on a fixed reactor. The Ce/TiO2–SiO2 (3/1) catalyst, modified by non-thermal plasma (NTP) treatment and then activated by thermal treatment at 500°C for 4 h, exhibits best performance. Comprehensive deNOx performance of the catalyst is evidently improved and its efficiency reaches up to 99.21% at 350°C. NO conversion efficiency of the treated catalyst doped with K remains about 90.23% at 300°C and the catalyst also shows improved activity at lower temperatures. Various characterization methods show that the activity enhancement is correlated only with NTP treatment, as it increases the number of Ce3+ species, which generates more chemisorbed oxygen, leads to improved dispersion of Brønsted and Lewis acidic sites and finally has an inherent etching effect.

ChemInform Abstract: Recent Advances in Automotive Catalysis for NOx Emission Control by Small‐Pore Microporous Materials

AbstractReview: 211 refs.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials.

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

A comparison of hydrothermal aging effects on NH3-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts

The impacts of hydrothermal aging on the selective catalytic reduction (SCR) performance of both Cu-SSZ-13 and Cu-SAPO-34 were compared. Upon 750°C hydrothermal aging for 16h, the NO conversions on both materials were somewhat maintained, however, a significant difference appeared after hydrothermal aging at 800°C. The Cu-SSZ-13 sample resulted in remarkably lower NO conversions at all temperatures tested while the Cu-SAPO-34 still maintained its high SCR activity. In fact, the NO conversions at low temperatures (below 350°C) over Cu-SAPO-34 were even enhanced upon high temperature treatment, suggesting an increase in active sites, which is possibly due to a post solid state ion exchange process driven by high temperatures. Consistently, for the Cu-SAPO-34 sample, NH3-TPD results suggest an increase in Lewis acid sites after hydrothermal aging at 750 and 800°C as compared with the fresh one. However, for the Cu-SSZ-13 sample, DRIFTs characterization results suggest a decrease in acid sites (Brønsted and Lewis), and a significant reduction in the amount of exchanged Cu2+ sites upon aging at 800°C, leading to a drastically decreased SCR activity. XRD results also showed the CHA (chabazite) structure collapses for Cu-SSZ-13 after 800°C hydrothermal aging of Cu-SSZ-13, but still remains for Cu-SAPO-34 upon the same aging. Besides SCR performance, critical functions of the SCR catalyst including NH3 oxidation activity and selectivity to NOx were also evaluated. Over Cu-SSZ-13, initial aging at 750°C results in increased NH3 oxidation and selectivity to NOx, suggesting the aggregation of isolated Cu2+ sites into CuO particles, while further aging at 800°C leads to decreased NH3 oxidation activity due to structure collapse as evidenced by XRD. On the other hand, for Cu-SAPO-34, no significant change in NH3 oxidation was observed after hydrothermal aging. All SCR performance test, NH3 oxidation and characterization results consistently indicate that Cu-SAPO-34 is more robust than Cu-SSZ-13 towards hydrothermal aging for the formulations and conditions examined here.