Catalysts For Low-temperature Selective Catalytic Reduction Research Articles

A green approach for preparation of MnFeTi Low-temperature NH3-SCR catalysts: utilizing spent V2O5–WO3/TiO2 Catalysts

V2O5–WO3/TiO2 catalysts are commonly used in industry to remove nitrogen oxides. However, spent V2O5–WO3/TiO2 are hazardous solid waste, and their improper disposal can lead to water and soil pollution risks. This study proposes a waste-to-resource approach that promotes the safe disposal of spent catalysts while reducing the production cost of denitrification catalysts. Specifically, vanadium-free low-temperature selective catalytic reduction catalysts are synthesized using spent catalysts. The optimized catalyst, named MnFeTi-22, exhibits a broad temperature activity range, with NO conversion exceeding 96 % from 100 to 300 °C. Even with 5 % H2O, the catalyst maintains a 95 % NO conversion (at approximately 200 °C). Characterization results confirm that the loading of TiO2 recycled from spent catalysts onto MnFe significantly optimizes the distribution of active species. Furthermore, TiO2 can adjust the distribution of acidic sites and the quantity of surface-adsorbed oxygen, promoting electron transfer between Mn4+/Fe2+ and Mn3+/Fe3+.

Nitration-Promoted Vanadate Catalysts for Low-Temperature Selective Catalytic Reduction of NOX with NH3.

Vanadium-based catalysts have been commercially used in selective catalytic reduction (SCR), owing to their high catalytic activity and effectiveness across a wide temperature range; however, their catalytic efficiency decreases at lower temperatures under exposure to SOX. This decrease is largely due to ammonium sulfate generation on the catalyst surface. To overcome this limitation, we added ammonium nitrate to the V2O5-WO3/TiO2 catalyst, producing a V2O5-WO3/TiO2 catalyst with nitrate functional groups. With this approach, we found that it was possible to adjust the amount of these functional groups by varying the amount of ammonium nitrate. Overall, the resultant nitrate V2O5-WO3/TiO2 catalyst has large quantities of NO3- and chemisorbed oxygen, which improves the density of Brønsted and Lewis acid sites on the catalyst surface. Furthermore, the nitrated V2O5-WO3/TiO2 catalyst has a high NOX removal efficiency and N2 selectivity at low temperatures (i.e., 300 °C); this is because NO3- and chemisorbed oxygen, generated by nitrate treatment, facilitated the occurrence of a fast SCR reaction. The approach outlined in this study can be applied to a wide range of SCR catalysts, allowing for the development of more, low-temperature SCR catalysts.

Cobalt-/pH-Modified V2O5-MoO3/TiO2 Catalyst with Enhanced Activity for the Low-Temperature Selective Catalytic Reduction Process

Currently, the elimination of gaseous pollutants—particularly nitrogen oxides—has emerged as a significant concern. Among various deNOx technologies, selective catalytic reduction (SCR) has gained prominence as the primary approach for NOx abatement, owing to its superior performance. In this study, novel low-temperature SCR catalysts were developed by regulating the pH value and doping cobalt based on a V2O5-MoO3/TiO2 (VMT) catalyst. The results show an increased SCR performance with 82.8% and 91.1% for catalysts after pH (=10) modification (VMT-10) and (1 wt%) Co/pH (=10) modification (1CoVMT-10), respectively. H2-TPR, NH3-TPD, XPS and DRIFTS confirmed that the pH regulation transformed polymerization V species into isolated V5+=O, thus leading to an increase in the number of acid sites, which enhanced the NH3 and NO2 adsorption capacity. Furthermore, the DRIFTS study indicated that the NH3-SCR reaction over 1CoVMT-10 followed the E–R and L–H mechanism.

Density Functional Study on Adsorption of NH3 and NOx on the γ-Fe2O3 (111) Surface.

γ-Fe2O3 is considered to be a promising catalyst for the selective catalytic reduction (SCR) of nitrogen oxide (NOx). In this study, first-principle calculations based on the density function theory (DFT) were utilized to explore the adsorption mechanism of NH3, NO, and other molecules on γ-Fe2O3, which is identified as a crucial step in the SCR process to eliminate NOx from coal-fired flue gas. The adsorption characteristics of reactants (NH3 and NOx) and products (N2 and H2O) at different active sites of the γ-Fe2O3 (111) surface were investigated. The results show that the NH3 was preferably adsorbed on the octahedral Fe site, with the N atom bonding to the octahedral Fe site. Both octahedral and tetrahedral Fe atoms were likely involved in bonding with the N and O atoms during the NO adsorption. The NO tended to be adsorbed on the tetrahedral Fe site though the combination of the N atom and the Fe site. Meanwhile, the simultaneous bonding of N and O atoms with surface sites made the adsorption more stable than that of single atom bonding. The γ-Fe2O3 (111) surface exhibited a low adsorption energy for N2 and H2O, suggesting that they could be adsorbed onto the surface but were readily desorbed, thus facilitating the SCR reaction. This work is conducive to reveal the reaction mechanism of SCR on γ-Fe2O3 and contributes to the development of low-temperature iron-based SCR catalysts.

Study on Denitration Performance of Mn-Ce/TiO2 Low-Temperature SCR Catalyst

Low sintering flue gas temperatures and large temperature fluctuations require the development of low-temperature and efficient SCR (selective catalytic reduction) catalysts suitable for the sintering process. It has been shown that modified Mn-Ce/TiO2 catalysts have good denitration capability and have potential commercial use. In this experiment, TiO2-loaded Mn and Ce SCR catalysts were prepared using the impregnation method, and a series of characterizations of the samples were carried out to illustrate the effect of the active material on the denitration efficiency. The kinetic analysis provides theoretical as well as data support for the subsequent optimization of the SCR catalysts. The results show that the denitration efficiency of the catalysts can reach 93.86% when the Mn content is 10% and the Ce content is 3%. The doping of active substances can increase the specific surface area, total pore volume and average aperture of the catalysts and improve the adsorption capacity of the catalysts.

Low-temperature selective catalytic reduction of NOx with NH3 over in-situ grown MnOx-Fe2O3/TiO2/Ti monolithic catalyst

The design and synthesis of highly efficient and low-temperature NOx reduction catalysts with long-term stability and good water resistance still remain a challenge in the field of gas catalysis. In this work, we demonstrated the integration of MnOx-Fe2O3/TiO2/Ti monolithic catalysts on flexible Ti mesh through plasma electrolytic oxidation technology, associated with hydrothermal reaction, and chemical bath deposition methods. The in-situ grown TiO2 nanosheet supports hold a very strong substrate adhesion with Ti mesh, and provide ideal nucleation sites for the following deposition of high density of MnOx-Fe2O3 active components. The brush-like nanostructure morphology of active MnOx-Fe2O3 catalysts enables the complete adsorption and reaction of gas molecules on the catalyst surface. The selective catalytic reduction of NOx with NH3 tests indicate that MnOx-Fe2O3/TiO2/Ti monolithic catalysts exhibit excellent low-temperature catalytic performance with 100% of NOx conversion at 137 °C, good stability and water resistance, showing great potential and broad application prospects in comparison with powder catalysts and honeycomb ceramic catalysts. The strategy of depositing multi-components active catalysts on in-situ grown ultra-thin TiO2 nanosheet supports on flexible Ti mesh paves a solid way toward the practical applications of novel low-temperature selective catalytic reduction catalysts in diverse fields.

Low-Temperature SCR Catalyst Development and Industrial Applications in China

In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NOx emissions in China. Currently, the most commonly used catalysts in industry are V2O5-WO3(MoO3)/TiO2, MnO2-based catalysts, CeO2-based catalysts, MnO2-CeO2 catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO2, moisture and alkali metals, which can affect the service life of SCR catalysts. This paper summarizes the mechanism of catalyst poisoning and aims to reduce the negative effect of NH4HSO4 on the activity of the SCR catalyst at low temperatures in industrial applications. It also presents the outstanding achievements of domestic companies in denitrification in the non-power industry in recent years. Much progress has been made in the research and application of low-temperature NH3-SCR, and with the renewed demand for deeper NOx treatments, new technologies with lower energy consumption and more functions need to be developed.

Green Ce-based honeycomb catalyst with excellent water and sulfur dioxide resistances for low-temperature selective catalytic reduction of NOx with ammonia

Low-temperature selective catalytic reduction (SCR) is effective and promising for converting NOx in flue gas into N2, with wide application prospects. However, low-temperature denitration (DeNOx) catalysts are susceptible to SO2 and H2O poisoning. Here, we prepared a monolithic Mn-Fe-Ce/Al2O3 catalyst that exhibited excellent DeNOx performances at low temperatures. At 110 °C, the NO conversion rate of the catalyst was> 90%, exhibiting excellent resistances to H2O and SO2·H2O and SO2 are introduced at the same time, the efficiency is as high as 73%. Furthermore, The catalyst has a porous structure with abundant chemisorbed oxygen spices and exposed active, the reaction pathway on the catalyst proved to be the L-H mechanism based on in situ DRIFTS. This study also explored the mechanisms of low-temperature SCR catalyst poisoning by SO2 and H2O, providing insights into the effects of SO2 and H2O and theoretical support for the development of poisoning-resistant low-temperature SCR catalysts.

Understanding the mechanism of selective catalytic reduction on spinel TiMn2O4(001) surface

Manganese based spinel oxides is one of the most promising catalysts for low temperature selective catalytic reduction (SCR) process due to its high activity and tunable composition. Understanding the molecular-level reaction mechanism of SCR on Mn-containing spinel oxides is of great importance for design and development of appropriate catalyst for low temperature SCR process. In this paper, density functional theory calculations were performed to address the detail SCR reaction pathways between NH3 and NO on spinel TiMn2O4(001) surface. The full SCR process was found to be divided into four sequential stages, including the NH3 chemisorption (stage i), NH2NO formation (stage ii), H migration (stage iii), and products desorption (stage iv). Notably, we found that water molecules could serve as a hydrogen exchanger and play a positive role in promoting the hydrogen migration during stage i and stage iii of the SCR process. Thermodynamically, the overall SCR process is highly exothermic with the reaction energy of -3.054 eV. Kinetically, the formation of NH2NO intermediate was identified as the rate determining step with a moderate activation barrier of 1.064 eV. Our results suggest that the spinel TiMn2O4 is a promising catalyst for low temperature SCR. Moreover, the promoting effect of water on hydrogen diffusion should be also applicable to other chemical reactions involving hydrogen transfer.

Molybdenum oxide as an efficient promoter to enhance the NH3-SCR performance of CeO2-SiO2 catalyst for NOx removal

Selective catalytic reduction (SCR) of NOx with NH3 has been widely used for the removal of NOx. Because of the inevitable disadvantages of conventional V2O5-WO3(MoO3)/TiO2 catalysts such as poor low-temperature SCR activity and toxicity of vanadium, as well as the high-cost and over-strong NH3 adsorption of zeolite catalysts, the development of efficient and non-toxic metal oxide SCR catalysts is highly demanded. Previously, we have developed an environmentally-benign CeO2-SiO2 mixed-oxide SCR catalyst (CeSi2), which exhibited superior SO2 resistance ability. However, the low-temperature SCR activity on CeSi2 was not that satisfactory. In this work, we proposed a new strategy of Mo doping to improve the low-temperature SCR activity on CeSi2, which was very crucial for its practical application. By a simple co-precipitation method, a homogenous Mo-Ce-Si mixed-oxide catalyst was prepared. The Mo doping could significantly enhance the NH3-SCR activity on CeSi2 below 250 °C, and the optimal catalyst was Mo0.1CeSi2, which could achieve 80% NOx conversion at 200 °C. Mo0.1CeSi2 also exhibited superior N2 selectivity and resistance to SO2/H2O poisoning. Via a series of characterizations, it was found that the redox properties and surface acidity on CeSi2 by Mo doping, which accounted for the enhanced low-temperature NH3-SCR activity on Mo0.1CeSi2. The reaction mechanism on Mo0.1CeSi2 was also fully revealed by in situ DRIFTS experiments. This work provided a new insight for the development of efficient low-temperature SCR catalysts with superior SO2 resistance ability.

Sulfur-Resistant Ceria-Based Low-Temperature SCR Catalysts with the Non-bulk Electronic States of Ceria.

Although ceria-based catalysts serve as an appealing alternative to traditional V2O5-based catalysts for selective catalytic reduction (SCR) of NOx with NH3, the inevitable deactivation caused by SO2 at low temperatures severely hampers the ceria-based catalysts to efficiently control NOx emissions from SO2-containing stack gases. Here, we rationally design a strong sulfur-resistant ceria-based catalyst by tuning the electronic structures of ceria highly dispersed on acidic MoO3 surfaces. By using Ce L3-edge X-ray absorption near edge structure spectra in conjunction with various surface and bulk structural characterizations, we report that the sulfur resistance of the catalysts is closely associated with the electronic states of ceria, particularly expressed by the Ce3+/Ce4+ ratio related to the size of the ceria particles. As the Ce3+/Ce4+ ratio increases up to or over 50%, corresponding to CeO2/MoO3(x %, x ≤ 2.1) with the particle size of approximately 4 nm or less, the non-bulk electronic states of ceria appear, where the catalysts start to show strong sulfur resistance. This work could provide a new strategy for designing sulfur-resistant ceria-based SCR catalysts for controlling NOx emissions at low temperatures.

The poisoning mechanism of gaseous HCl on low-temperature SCR catalysts: MnOx−CeO2 as an example

The degradation of catalyst properties, rather than the deposition of NH4Cl, was the main reason for the deactivation of the MnCe low-temperature SCR (selective catalytic reduction) catalyst under HCl atmosphere. Cl ions prefer to be located at the oxygen vacancies around Mn cations on the MnCe surface. The adsorbed Cl ions could further promote the formation of oxygen vacancies. Therefore, the active Mn sites on the surface of MnCe would be quickly covered by Cl ions when HCl was present and thus promote the formation of Brønsted acid sites. However, the ionic NH4+ adsorbed on the Brønsted acid sites of MnCe and MnCe − Cl were not fully utilized in the SCR reaction. Meanwhile, the reactivity of the coordinated NH3 bonded with Lewis acid sites was significantly restrained by HCl due to the significant inhibition of the redox performance. Overall, HCl could obviously restrain the low-temperature SCR activity of MnCe.

DeNOx of Nano-Catalyst of Selective Catalytic Reduction Using Active Carbon Loading MnOx-Cu at Low Temperature

With the improvement of environmental protection standards, selective catalytic reduction (SCR) has become the mainstream technology of flue gas deNOx. Especially, the low-temperature SCR nano-catalyst has attracted more and more attention at home and abroad because of its potential performance and economy in industrial applications. In this paper, low-temperature SCR catalysts were prepared using the activated carbon loading MnOx-Cu. Then, the catalysts were packed into the fiedbed stainless steel micro-reactor to evaluate the selective catalytic reduction of NO performance. The influence of reaction conditions was investigated on the catalytic reaction, including the MnOx-Cu loading amount, calcination and reaction temperature, etc. The experimental results indicate that SCR catalysts show the highest catalytic activity for NO conversion when the calcination temperature is 350 °C, MnOx loading amount is 5%, Cu loading amount is 3%, and reaction temperature is 200 °C. Under such conditions, the NO conversion arrives at 96.82% and the selectivity to N2 is almost 99%. It is of great significance to investigate the influence of reaction conditions in order to provide references for industrial application.

Review of Researches on SCR Catalyst with Low Temperature and high Sulfur Tolerance and Theoretical Design

Selective catalytic reduction (SCR) of nitrogen oxides (NOx) using ammonia (NH3) is currently the main technology for flue gas denitration. However, the currently widely used commercial catalysts (such as V2O5-WO3 / TiO2, V2O5-MoO3 / TiO2, etc.) have the disadvantages of high operating temperature, narrow active temperature window, and high catalytic cost. Therefore, in recent years, researchers have devoted themselves to the development of low-cost and efficient low-temperature SCR catalytic materials. This paper summarizes the research progress of low-temperature (less than 250 °C) selective catalytic reduction of NOx by unsupported metal oxide catalysts, supported metal oxide catalysts, precious metals, and molecular sieve catalysts. Among them, manganese-based catalysts show good low-temperature selectivity and stability, and have good application prospects. Finally, the research directions of manganese low temperature SCR catalysts are prospected and theoretically designed based on the existing problems.

Enhancing the Water Resistance of Mn-MOF-74 by Modification in Low Temperature NH3-SCR

In this study, Mn-MOF-74 was successfully synthesized and further modified via two paths for enhanced water resistance. The structure and morphology of the modified samples were investigated by a series of characterization methods. The results of selective catalytic reduction (SCR) performance tests showed that polyethylene oxide-polypropylene-polyethylene oxide (P123)-modified Mn-MOF-74 exhibited outstanding NO conversion of up to 92.1% in the presence of 5 vol.% water at 250 °C, compared to 52% for Mn-MOF-74 under the same conditions. It was concluded that the water resistance of Mn-MOF-74 was significantly promoted after the introduction of P123 and that the unmodified P123-Mn-MOF-74 was proven to be a potential low-temperature SCR catalyst.

Iron doped effects on active sites formation over activated carbon supported Mn-Ce oxide catalysts for low-temperature SCR of NO

To clarify the Fe doped effects over activated carbon (AC) supported Mn-Ce oxide catalysts and study the influence of Fe addition on the AC, several Mn/Ce/Fe mixed oxide catalysts prepared via an impregnation method supported on AC were investigated for low-temperature selective catalytic reduction (SCR) of NO with NH3. The Mn-Ce-Fe/AC catalyst with 5% (mass ratio) loading exhibited the highest catalytic activity and yielded above 90% NO conversion at 125 °C with a space velocity of 12,000 h−1. The Fe addition could obviously reduce the destruction of AC surface area. Also, the metal ions could insert into graphite crystallite structures of AC, splitting it into smaller graphene-like sheets. After doping with Fe species, the relative ratios of Mn4+/Mnn+, Ce3+/Cen+ and the surface adsorbed oxygen greatly enhanced in Mn-Ce-Fe/AC catalyst. Additionally, both weak acid and medium acid amount increased significantly after the Fe species introduced in Mn-Ce/AC, which could be attributed to the more exposed active sites of acid due to Fe species or its influence on Mn/Ce species. Relying on the obtained results, a L-H mechanism was proposed, owing to the Fe doping, the average valence state of Mn ions and surface adsorbed oxygen both increased on the Mn-Ce-Fe/AC catalyst, specially accompanying with the surface acid sites promotion, therefore remarkably promoting the denitration efficiency due to all the cumulative effect.

Research progress of low-temperature SCR denitration catalysts

Nitrogen oxide NOx (NO, NO_2, and N_2O) is one of the major pollutants in the air pollution, it can cause environmental problems such as photochemical smog, acid rain, and ozone layer destruction, which has posed threat to people's living environment and quality of life, and attracted great attention from the world. Countries made stricter emission standards for burning emissions from both fixed and mobile sources. The major denitrification technologies include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), oxidative denitrification, and activated carbon adsorption and denitrification at present. The SNCR has higher conditions in industrial applications, the main factors affecting successful operation are temperature, ammonia-nitrogen ratio, distribution of ammonia gas in the flue gas and residence time, so there were certain limitations in industrial application of SNCR. Compared with other denitrification technologies, SCR denitration technology is more widely used in industrial application, in which denitration is mostly arranged after dust removal and desulfurization, at this time, the temperature is mostly between 100~250℃. The performance of SCR denitrification in low-temperature must be improved, which is one of the most promising flue gas DeNOx technology. In this paper, the recent works on low-temperature SCR catalysts were reviewed on manganese-based catalysts, vanadium-based catalysts and carbon-based catalysts. Single-component Mn-based catalysts, supported Mn-based catalysts and composite Mn-based catalysts were reviewed, the effects of preparation of V-based catalysts on the de-dumping and denitrification mechanisms were described. The effect of transition metal doping on C-based catalysts was reviewed. The influence of H_2O and SO_2 resistance on low-temperature NH_3-SCR catalytic activity and reaction mechanism were also discussed. Finally, the virtues advantages and defects of low temperature SCR catalysts were summarized, and the future development direction was also given out. Key learning points: (1) Catalysts for different systems were reviewed. The problems of manganese-based catalysts, vanadium-based catalysts and carbon-based catalysts were analyzed. The preparation methods were used to improve the water and sulfur resistance and catalytic activity of the catalysts. (2) Denitrification mechanism. From the currently reported denitration mechanism to analyze the activity of the catalyst, it is expected that the mechanism rsearches will improve the low temperature performance of the catalyst. (3) Water and sulfur resistance of low temperature catalyst. The presence of SO_2 at low temperatures tends to result in ABS deposition.

The promoting effect of CeO2@Ce-O-P multi-core@shell structure on SO2 tolerance for selective catalytic reduction of NO with NH3 at low temperature

A series of CeO2@Ce-O-P “multi-core@shell” catalysts were synthesized in this paper for selective catalytic reduction (SCR) of NO with NH3. The experimental results had showed that CeO2@Ce-O-P-30:3 yielded best SO2 tolerance of an over 70% deNOx efficiency at 250 °C in the presence of 100 ppm SO2 for 20 h, which was much higher compared to pure Ce-O-P and CeO2 samples. Further characterization results indicated that Ce-O-P coating layer could somewhat inhibit sulfur depositing on the catalysts during SCR reaction in the presence of SO2, thereby protecting the active sites from SO2 poisoning. Especially, O2-TPD results illustrated that a great amount of active oxygen species were retained on used CeO2@Ce-O-P catalyst after a long term reaction. The synergetic effect of “multi-core@shell” structure could be attributed to such enhanced performances. The “core” CeO2 devoted abundant active oxygen sites to fulfill the SCR reaction. And the “shell” Ce-O-P could not only provide acid sites, but also protect the active oxygen species by avoiding the over-adsorption of SO2 on the catalyst. This work could provide a new way to increase the sulfur resistance for low temperature SCR catalysts.

De-NOx Performance and Mechanism of Mn-Based Low-Temperature SCR Catalysts Supported on Foamed Metal Nickel

A series of manganese-based catalysts supported on foamed metal nickel (FMN) with various Mn/Ni ratios was prepared for low-temperature selective catalytic reduction (SCR) of NO with NH3 (NH3-SCR). The effects of calcination temperature, amount of added Mn, optimal operating conditions, and H2O on the elimination of nitrogen oxides (de-NOx) performance of catalysts were studied. The catalysts were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed desorption experiments of NH3 analyses. The experimental results revealed that the Mn7.5/FMN catalyst calcined at 350 oC exhibited the best NO conversion that was ca. 100% at 120-200 oC. Moreover, it had excellent H2O tolerance. The superior activity of the Mn7.5/FMN catalyst, which was calcined at 350 oC, was attributed to the presence of amorphous manganese oxide, more unsaturated Ni atoms and structural defects, an increase in NH3 adsorbance, and the number of surface acid sites. Based on these studies, we established that the reaction of the NH3-SCR with Mn7.5/FMN catalyst mainly exhibits an Eley-Rideal mechanism.

Experimental Study on Preparation and Operating Conditions over a Promising Monolithic Catalysts for NO<sub>x</sub> Removal: MnO<sub>x</sub>/TiO<sub>2</sub>/Cordierite

The samples of MnOx/TiO2 catalysts supported on cordierite honeycomb ceramics were prepared by a sol-gel-impregnation method, and evaluated for low-temperature (353-473 K) selective catalytic reduction (SCR) of NOx with NH3. The influences of pretreatment on cordierite and catalyst dosage were investigated at first and optimized as follows: pickling for cordierite honeycomb ceramics with 1 mol/L HNO3 for 3 h prior to loading procedure as well as the catalyst dosage of 3-5 wt.%. The activity results indicated that there was an optimum working condition for MnOx/TiO2/cordierite catalysts: NH3/NO molar ratio=1.1, [O2]=3 vol.%, GHSV=5514 h-1, the highest activity of nearly 100% NO conversion could be obtained. As a comparison, the performances of commercialized vanadium-based honeycomb catalyst were also employed, which revealed the narrower scope of application of GHSV and the higher active temperature window. In conclusion, it turns out that the prepared MnOx/TiO2/cordierite catalysts are more applicable as a low-temperature SCR catalyst for NOx removal in a more complicated application environment.