Catalysts For Low-temperature NH3-SCR Research Articles

High Resistance of SO2 and H2O over Monolithic Mn-Fe-Ce-Al-O Catalyst for Low Temperature NH3-SCR

Monolithic Mn-Fe-Ce-Al-O catalyst with honeycomb cordierite ceramic as a carrier was reported for the first time for low temperature deNOx application. In the reaction of selective catalytic reduction (SCR) of NO with NH3, a NO conversion of above 80% at 100 °C was obtained. Notably, the catalyst also showed excellent resistance against SO2 and H2O. About 60% NO conversion was maintained after successive operation in the mixed stream of SO2 and H2O for 168 h. The Brunner−Emmet−Teller (BET) measurement, SEM, EDS, thermogravimetric analysis (TG), FT-IR, and XPS results of the used catalysts indicated that certain amounts of ammonium sulfate was formed on the surface of the catalyst. XPS results revealed that partial of Fe2+ was oxidized to Fe3+ during the reaction process, and Fe2+ species have strong redox ability, which can explain the decrease in activity after reaction. In addition, SO2 and H2O induced a transformation of Ce from Ce4+ to Ce3+ on the surface of the catalyst, which increased the amount of chemisorbed oxygen. Owing to these factors, the addition of Ce and Fe species contributes to excellent resistance of the catalyst to SO2 and H2O.

The Effect of Tourmaline on SCR Denitrification Activity of MnOx/TiO2 at Low Temperature

Selective catalytic reduction (SCR) technology is the most widely used flue gas denitration technology at present. The stability of a catalyst is the main factor limiting the development of this technology. In this study, an environmentally friendly and highly efficient NH3-SCR catalyst was prepared by coprecipitation method from acidolysis residue of industrial waste and tourmaline. We found that the addition of tourmaline has an important impact on the denitration activity of the catalytic material. The NOx conversion exceeded 97% at 200 °C with the dosage of 10% tourmaline, which is about 7% higher than that without doping. The improvement of catalytic performance was mostly attributed to the permanent electrodes of tourmaline, which effectively promotes the dispersion of MnOx/TiO2 catalytic materials, increases the number of acidic sites and changes the valence distribution of manganese ions in products, which speeds up the diffusion of protons and ions, resulting in the acceleration of redox reaction. These as-developed tourmaline-modified MnOx/TiO2 materials have been demonstrated to be promising as a new type of highly efficient low-temperature NH3-SCR catalyst.

A Perspective on the Relationship Between Microstructure and Performance of Cu-Based Zeolites for the Selective Catalytic Reduction of NOx

Cu-exchanged zeolites are one kind of effective catalysts to eliminate NOx for Selective catalytic reduction with NH3(NH3-SCR) in oxygen-rich exhaust from diesel engines. The microstructure–performance relationships between supports and active sites is crucial to rationally design excellent catalysts for the effective elimination of NOx. Therefore, the effect of topology and element ratio on active species and performance, the new progress in reaction and chemical poisoning mechanism to clarify the nature of the catalytic reactions are reviewed in this article. Furthermore, some new research methods to further promote SCR performance are summarized, which is helping in guiding the rational design of Cu-zeolites catalysts. From the reviewed data, the following important viewpoints emerged: (i) topologies exert different influence on acid properties and the nature of Cu species, which further effect the NH3-SCR performance; (ii) the Si/Al and Cu/Al compositional ratios are key factors in effecting the performance of the Cu-zeolites catalysts; (iii) the degree of chemical inactivation depends on the content and type of impurities; (iv) the common modification methods include the doping of foreign cations or metal oxides and core–shell structures; (v) the active site in the low temperature NH3-SCR catalyst is a mobile transient ion pairs ([(NH3)2CuI-O2-CuI(NH3)2]).Only in the high temperature regime do the NH3-solvated Cu ions decompose to isolated Cu2+ ions that anchor on the zeolite framework and become immobilized.

Aluminium-induced component engineering of mesoporous composite materials for low-temperature NH3-SCR

Supported Mn2O3 is useful in achieving high dinitrogen selectivity at low temperature during ammonia-selective catalytic reduction (SCR). However, its controlled synthesis is challenging when the supporting material is the conventional pure silicon SBA-15 mesoporous molecular sieve. Here we show that silicon and aluminium in fly ash, the solid waste produced by coal-fired power plants, can be used to synthesize an Al-SBA-15 mesoporous molecular sieve support, which can guide the growth of Mn2O3 in the as-synthesized Fe-Mn/Al-SBA-15 NH3-SCR catalyst. Its superior catalytic performance is demonstrated by the high NOx conversion (≥90%) and selectivity (≥86%) at low temperatures (150–300 °C). The combined theoretical and experimental results reveal that the introduction of Al induces the growth of Mn2O3 catalysts. Our findings, therefore, provide a strategy for the rational design of low-temperature NH3-SCR catalysts through dopant-induced component engineering of composite materials.

Design of practical Ce/CoMnAl-LDO catalyst for low-temperature NH3-SCR

A highly active Ce/CoMnAl-LDO catalyst for low-temperature NH3-SCR with good thermal stability and SO2/H2O resistance was designed from layered double hydroxides (LDHs). At 100–250 °C, Ce0.5/Co1Mn0.5Al0.5Ox-LDO showed practically ~100% NOx conversion compared to Ce-Co-Mn/γ-Al2O3 (63.4–90.3%) of the same metal composition prepared by the common wet-impregnation method. The LDO catalytic material showed very stable NOx conversion of 83.3% at 200 °C after 10 h test with 100 ppm SO2 in the feed stream, superior performance when compared to the control catalysts Co1Mn0.5Al0.5Ox (60.2%) and Ce-Co-Mn/γ-Al2O3 (28%). The thermal stability and resistance to water vapor were also investigated. Various studies suggested that the excellent performance of Ce0.5/Co1Mn0.5Al0.5Ox-LDO can be attributed to the high dispersion of active metal species, large specific surface area, moderate redox capacity and strong acidity.

Low-temperature selective catalytic reduction of NOx with NH3 over zeolite catalysts: A review

The selective catalytic reduction (SCR) of NOx by NH3 is one of the most mature technologies for NOx treatment. Catalysts are the main factors affecting denitrification efficiency. Zeolites as low-temperature NH3-SCR catalysts have been extensively studied in the past few years. In this work, the mechanism of zeolites for NH3-SCR reaction was reviewed and the denitrification performances of zeolite catalysts prepared by different methods were compared. The effects of sulfur and water poisoning on zeolite catalysts in NH3-SCR reaction were also analyzed. Several ways to address the problems in low-temperature NH3-SCR reaction were discussed. Hopefully, this review could provide a fundamental understanding of SCR reaction on zeolite catalysts and pave the way toward similar studies to realize its commercial applications.

In-situ DRIFTs study on different exposed facets of VOx-MnOx/CeO2 catalysts for low-temperature NH3-SCR

In order to achieve the low-temperature (200–250°C) NH3-SCR, highly dispersed VOx-MnOx/CeO2 catalysts with different exposed facets were produced. The NH3-SCR performance results indicate that the NO conversion over VOx-MnOx/CeO2-R with preferentially exposed {110} facets can achieve NO conversion >95% over a wide temperature span of 220–330°C. From the in-situ DRIFTs, gaseous NH3 and NO are favorable to be absorbed on the surface of the VOx-MnOx/CeO2-R catalyst with preferentially exposed {110} facets, which improves the efficiency of NO conversion. The mechanism study via in-situ DRIFTs demonstrates that the well dispersed vanadium species on CeO2{110} absorb NH3 to generate NH3(L) and NH+ 4(B) species, which in turn become highly reactive toward bridging nitrate and bidentate nitrate species to form N2 and H2O according to the Langmuir-Hinshelwood mechanism.

MnO2–GO-scroll–TiO2–ITQ2 as a low-temperature NH3-SCR catalyst with a wide SO2-tolerance temperature range

GO-scroll–TiO2–ITQ2 improved the steam-resistance and SO2-resistance of the low-temperature manganese-based NH3-SCR catalyst.

Novel MnaZrbCrcOx catalysts for low temperature NH3-SCR derived from high H2O content flue gas via natural gas combustion

A series of MnaZrbCrcOx catalysts with different Cr/Mn molar ratios were synthesized via co-precipitation for the selective catalytic reduction of NOx with NH3 (NH3-SCR). It was found that a Mn5Zr2Cr2.5Ox catalyst with a Mn:Zr:Cr ratio of 5:2:2.5 exhibited excellent low-temperature activity and the best levels of H2O resistance at a high weight hourly space velocity (WHSV) of 54,000 ml/(h·g). NO conversion levels were measured as greater than 87% within a temperature range of 150–250 °C, and catalytic performance levels reached 90% even 50 h after reactions occurred in the presence of 15% H2O at 230 °C. The characterization results show that strong interactions occurring between Mn, Zr and Cr oxides and higher quantities of Mn4+ and Lewis acid sites improved the redox properties and facilitated H2O resistance. In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) show that coordinated NH3 on Lewis acid sites and bidentate nitrates play important roles in SCR reactions and that such reactions follow both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. The competitive adsorption of H2O and NO may be the main cause of the adverse effects of H2O on catalytic performance.

Aftertreatment Protocols for Catalyst Characterization and Performance Evaluation: Low-Temperature Oxidation, Storage, Three-Way, and NH3-SCR Catalyst Test Protocols

A set of standardized and realistic aftertreatment catalyst test protocols have been developed by the Advanced Combustion and Emission Control Technical Team in support of the U.S. DRIVE Partnership. The protocols are intended to accelerate the pace of aftertreatment catalyst innovation by enabling the accurate evaluation and comparison of aftertreatment catalyst performance data from various testing and research facilities to maximize the impact of discovery-phase research occurring across the nation. The protocols address a need identified by the Partnership’s industry partners for consistent and accurate metrics for aftertreatment catalyst evaluation and comparison. The protocols consist of a set of standardized requirements and test procedures that sufficiently capture the performance capability of a catalyst technology in a manner that is adaptable in various laboratories. The protocols provide a detailed description of the necessary reactor system, the steps for achieving a desired aged state of the catalyst, all necessary sample pretreatments to be performed prior to testing, and realistic test conditions for evaluating performance. This article details four low-temperature catalyst test protocols applicable to (1) oxidation catalysts, (2) passive storage (and release) catalysts, (3) three-way catalysts, and (4) NH3-SCR catalysts. The catalyst test protocol descriptions are presented in five (5) sections: protocol general guidelines and the four individual catalyst test protocol descriptions. The general guidelines plus the individual protocol description forms the complete low-temperature catalyst test protocol for the application.

Promotion of Fe and Co doped Mn-Ce/TiO2 catalysts for low temperature NH3-SCR with SO2 tolerance

Fe and Co doped Mn-Ce/TiO2 catalysts for low temperature NH3-SCR with SO2 resistance were investigated via in-situ DRIFTS, SO2-TPD and FTIR. The activity results indicated that Fe and Co co-doped catalyst with an Fe/Co molar ratio of 2:4 (2Fe4Co-MCT) showed excellent SO2 resistance (98% NO conversion) at 200 °C when compared to the Mn-Ce/TiO2 catalyst. In-situ DRIFTS and SO2-TPD results showed that SO2 reacted with NH3 to form (NH4)2SO4 or NH4HSO4 as reversible deactivation blocking the active sites. Moreover, SO2-TPD results further implied that irreversible deactivation of Mn-Ce/TiO2 occurred by SO2 due to the formation of thermal stable sulfated species (MnSO4, Ce(SO4)2 or Ce2(SO4)3) to reduce active substances. The adsorption characteristics by in-situ DRIFTS revealed that more absorbed SO3, surface sulfite/sulfate and bulk-like sulfate species were formed on Mn-Ce/TiO2 catalyst, while Fe and Co addition to Mn-Ce/TiO2 significantly decreased the sulfur adsorption. The uniform distribution of Fe and Co on the Mn-Ce/TiO2 surface stopped SO2 diffusing to the inner layer of catalyst. In-situ DRIFTS also illustrated that the SCR reaction over 2Fe4Co-MCT catalysts mainly followed by Eley–Rideal (E–R) mechanism in the presence of SO2, which the adsorbed NH3 and NH4+ species reacted with gaseous NO species to produce unstable NH4NO2 and then rapidly decomposed into N2 and H2O.

Enhancing the deNOx performance of MnOx/CeO2-ZrO2 nanorod catalyst for low-temperature NH3-SCR by TiO2 modification

In the present work, we chose MnOx/CeO2-ZrO2 nanorod (i.e., Mn/CZ-NR) as a benchmark catalyst, and used TiO2 as a modifier with the purpose of inhibiting the formation of N2O (by-product) as well as further enhancing the catalytic activity and H2O + SO2 tolerance of Mn/CZ-NR catalyst. These samples were characterized by TEM, HRTEM, XRD, Raman, H2-TPR, XPS, NH3-TPD, and in situ DRIFTS. DeNOx performance and H2O + SO2 tolerance of these samples were evaluated by low-temperature NH3-SCR reaction. The obtained results show that the TiO2 modified catalyst (i.e., Mn-Ti/CZ-NR) exhibits higher catalytic activity than Mn/CZ-NR catalyst due to larger amount of oxygen vacancy accompanied with more Ce3+, higher ratios of Mn4+ and surface adsorbed oxygen species, as well as the improvement of surface acidity. Furthermore, TiO2 modification effectively inhibits the non-selective oxidation of NH3 to N2O through appropriately weakening the redox property of Mn/CZ-NR catalyst, which is beneficial to the enhancement of N2 selectivity. Finally, Mn-Ti/CZ-NR catalyst exhibits excellent H2O + SO2 tolerance, which indicates that it has the potential to be used for practical low-temperature deNOx application.

Effect of Ti4+ and Sn4+ co-incorporation on the catalytic performance of CeO2-MnOx catalyst for low temperature NH3-SCR

A novel Ti4+ and Sn4+ co-incorporated CeO2-MnOx based catalyst was synthesized by an inverse co-precipitation method with the purpose of inhibiting the formation of N2O and enhancing the NOx conversion of CeO2-MnOx catalyst for NH3-SCR at low temperature. The obtained catalysts were characterized by a series of XRD, Raman, N2-physisorption, NH3-TPD, H2-TPR, XPS, and in situ DRIFTS techniques in details. The experiment results suggested that CeO2-MnOx-TiO2-SnO2 (Ce-Mn-Ti-Sn) catalyst displayed the optimal catalytic performance with an enhancement of 30% compared with CeO2-MnOx (Ce-Mn) in the range of 225–275 °C. Moreover, the N2O formation was inhibited remarkably compared to Ce-Mn (152 ppm), and there was no N2O (0 ppm) detected even at 250 °C. This is linked to moderate weakening of redox property, more generation of acid sites, enhancement of acidic strength, as well as large amount of Ce3+, oxygen vacancies, and absorbed oxygen species, which are contributed to improving the chemical property of Ce-Mn-Ti-Sn catalyst, and further enhancing the catalytic performance and inhibiting the generation of N2O.

Enhancing DeNOx performance of CoMnAl mixed metal oxides in low-temperature NH3-SCR by optimizing layered double hydroxides (LDHs) precursor template

LDHs templates (marked as CoAl-MnO2-LDH and CoMnAl-CO3-LDH) were fabricated via ion-exchange/redox reaction and hexamethylenetetramine (HMT) hydrolysis methods respectively. CoMnAl mixed metal oxides (marked as MnO2/CoAl-LDO and CoMnAl-LDO) from the as-prepared LDHs templates were novelly prepared and tested as low-temperature NH3-SCR catalysts. Catalytic evaluation pronouncedly affirmed that the catalytic performances of the as-acquired catalysts were vulnerable to the preparation method of precursor template, where MnO2/CoAl-LDO presented preferable DeNOx activity (over 90% NOx conversion), N2 selectivity and SO2 resistance in much broader working temperature window (90–300 °C) than CoMnAl-LDO. Multiple characterizations revealed that the excellent catalytic performance of the MnO2/CoAl-LDO could be attributed to its higher specific surface area which could promote dispersion of abundant Mn4+ and Co3+ species, and resultantly afforded stronger redox ability and more acid sites. Moreover, dynamic calculation confirmed that the relatively lower activation energy on MnO2/CoAl-LDO for low-temperature NH3-SCR reaction should be closely related to the occurrence of a more rapid redox cycle and in agreement with the excellent DeNOx performance.

Self-Prevention of Well-Defined-Facet Fe2O3/MoO3 against Deposition of Ammonium Bisulfate in Low-Temperature NH3-SCR.

Low-temperature selective catalytic reduction of NO by NH3 (NH3-SCR) is a promising technology for controlling NO emission from various industrial boilers, but it remains challenging because unavoidable deposition of ammonium bisulfates (ABS) in the stack gases containing both SO2 and H2O inevitably results in deactivation of catalysts. Here we developed a stable low-temperature NH3-SCR catalyst by supporting Fe2O3 cubes on surfaces of MoO3 nanobelts with NH4+-intercalatable interlayers, which enables Fe2O3/MoO3 to spontaneously prevent ABS from depositing on the surfaces. Using in situ synchrotron X-ray diffraction, 1H magic angle spinning nuclear magnetic resonance, and temperature-programmed decomposition procedure, the results demonstrate that NH4+ of ABS was initially intercalated in the interlayers of MoO3, leading to a NH4+-HSO4- cation-anion separation by conquering their strong electrostatic interactions, and subsequently the separated NH4+ was consumed by taking part in low-temperature NH3-SCR. Meanwhile, the surface HSO4- separated from ABS oxidized the reduced catalyst during the NH3-SCR redox cycle, concomitant with release of SO2 gas, thereby resulting in decomposition of ABS. This work assists the design of stable low-temperature NH3-SCR catalysts with strong resistance against deposition of ABS.

Improvement of activity, selectivity and H2O&SO2-tolerance of micro-mesoporous CrMn2O4 spinel catalyst for low-temperature NH3-SCR of NOx

Cr-Mn mixed-oxide catalysts were prepared by citric acid method for low-temperature NH3-SCR of NOx. Mn(3)Cr(2)Ox (molar ratio) catalyst has an excellent NOx conversion of nearly 100% at 100–225 °C and good N2 selectivity above 70% at 100–200 °C. Results of XRD, BET, and XPS suggested that the micro-mesoporous CrMn2O4 spinel with high specific surface area, more active sites (Mn3+ and Mn4+) and effective electron transfer (Cr5+ + 2Mn3+ ↔ Cr3+ + 2Mn4+) were the important promotion factors for outstanding SCR performance. In-situ DRIFTS experiments indicated the SCR reaction pathway over CrMn2O4 spinel mainly followed the typical ER mechanism at the temperature around 200 °C. The higher Mn–O binding energy and lower dehydrogenation ability were the main reasons for CrMn2O4 spinel with low N2O by-product than Mn3O4. The CrMn2O4 spinel has satisfying tolerance to SO2 and H2O (about 72% NOx conversion at 200 °C for 20 h). In-situ DRIFTS showed SO2 could completely inhibit the adsorption of NO and weaken the adsorption of coordinated NH3 to Lewis acid sites but promote the adsorption of NH4+ to Brønsted acid sites. Analysis of TGA and FTIR results indicated that the sulfation of metal might be the main reason for the decline of SCR activity with SO2. The formation of Cr(III) sulfate could play an important role in protecting Mn active sites away from sulfating. Besides, the transform of HSO3 and SO42− to (H⋯SO42−) can provide new Brønsted acid sites for ionic NH4+, enhancing the SCR activity via fast-SCR (NO2 + NH4+ → NH4NO2 → N2 + H2O).

Synthesis and catalytic performance of Cu1Mn0.5Ti0.5Ox mixed oxide as low-temperature NH3-SCR catalyst with enhanced SO2 resistance

A new type of low-temperature NH3-SCR catalyst with a chemical composition of CuwMnyTi1-yOx was prepared from layered double hydroxides precursors for the first time. The purpose of this novel design is to improve the De-NOx efficiency and SO2 resistance of Mn-based catalysts. The Cu1Mn0.5Ti0.5Ox catalyst achieved a NOx conversion as high as 90% at 200 °C, which is much higher than that of the control catalysts Cu-Mn/TiO2 (86.1%) and Mn/TiO2 (80.7%). The properties of catalysts were characterized in detail using a series of physico-chemical techniques including XRD, BET, FTIR, SEM, TEM, H2-TPR, NH3-TPD, TGA, and XPS analyses. The excellent catalytic performance of Cu1Mn0.5Ti0.5Ox catalyst can be associated with its higher specific surface area and surface acidity, and more active MnO2 and CuO species. Besides, when copper oxide is introduced, the catalysts showed significant resistance to 100 ppm SO2 and 5% H2O. Finally, the poisoning mechanism and the regenerability of Cu1Mn0.5Ti0.5Ox catalyst was proposed. In short, the newly designed Cu1Mn0.5Ti0.5Ox catalyst was found to have higher catalytic activity and excellent SO2 and H2O resistance compared to the control catalysts of Cu-Mn/TiO2 and Mn/TiO2.

Low temperature SCR reaction over Nano-Structured Fe-Mn Oxides: Characterization, performance, and kinetic study

Fe-Mn oxides was prepared by thermal activation of Mn-rich limonite at different temperatures, and then developed as catalysts for low-temperature NH3-SCR. The obtained samples were characterized by XRD, XRF, TEM, XPS, BET, H2-TPR, NH3-TPD, NH3-SCR model reaction and in situ DRIFTS. The results indicated that Fe-Mn oxides catalyst derived from the thermal treatment at 300 °C (H300) exhibited an excellent NO conversion which was higher than 90% at a temperature range from 130 to 300 °C. The large surface area (95.9 m2/g), large amount of acid sites, suitable reduction behavior, and abundant surface hydroxyl positively responsed to the excellent performance of H300 for NH3-SCR. The results of in situ DRIFTS and kinetic study demonstrated that when the SCR reaction temperature reached to 100 °C, both the Eley–Rideal mechanism and the Langmuir-Hinshelwood mechanism might happen over H300. However, the Langmuir-Hinshelwood mechanism could be approximately neglected at higher reaction temperatures. This study favors the understanding of NH3-SCR mechanism of the prepared provides Fe-Mn oxides and explores new application field for Mn-rich limonite in low-temperature NH3-SCR.

Inductive Effect Boosting Catalytic Performance of Advanced Fe1–xVxOδ Catalysts in Low-Temperature NH3 Selective Catalytic Reduction: Insight into the Structure, Interaction, and Mechanisms

A series of vanadium doped Fe2O3 catalysts were synthesized using the homogeneous precipitation method and subjected to laboratory evaluation for selective catalytic reduction of NOx with NH3 (NH3-SCR). The best Fe0.75V0.25Oδ catalyst with a Fe/V mole ratio of 3/1 exhibited superior catalytic performance, achieving 100% NOx conversion at 200 °C over a wide temperature window from 175 to 400 °C, believed to be the best Fe-based low-temperature NH3-SCR catalyst identified to date. The Fe0.75V0.25Oδ catalyst also showed prominent resistance to high gas hourly space velocity (GHSV; 200 000 h–1) and strong durability to SO2 and H2O. Doping of V was shown to remarkably boost the catalytic activity, due to enhancement of the redox ability and surface acidity. XRD, Raman, and morphology results revealed that the incorporation of V had led to the formation of amorphous FeVO4 and Fe2O3. Coupling XPS and UV–vis diffuse reflectance spectra (DRS) results with DFT, it was discovered that the electron inductive effect b...

Composite catalytic systems: A strategy for developing the low temperature NH3-SCR catalysts with satisfactory SO2 and H2O tolerance

Mn-based catalysts were believed to be promoting low temperature NH3-SCR catalysts for their satisfactory catalytic activity, whereas their poor SO2 and H2O tolerance inhibited their applications. In this work, utilizing poor SO2 tolerance of Mn oxides, composite catalytic systems containing Mn oxides as deSOx catalyst was designed and synthesized, which exhibited excellent low temperature NH3-SCR performance in the presence of SO2 and H2O. The deSOx mechanism of Mn oxides was systematically investigated. In results, SO2 was catalytic oxidized on the surface of Mn oxides. Oxidized sulfur species could flow away from Mn oxides surface through feed gas flow or form MnSO4. When NH3 and H2O were introduced into the feed gas, the oxidized sulfur species were completely deposited as NH4HSO4 instead of forming MnSO4. Thus, SO2 was removed from feed gas entirely resulting in the superb low temperature NH3-SCR performances. This work provided a general strategy to enhance SO2 and H2O tolerance of NH3-SCR catalysts at low temperature by removing SO2 using NH3 and H2O who existed in the exhaust gas from stationary sources.