Novel manganese oxide confined interweaved titania nanotubes for the low-temperature Selective Catalytic Reduction (SCR) of NO x by NH 3

Selective catalytic reduction of NOx with NH3 over TiO2 supported metal sulfate catalysts prepared via a sol–gel protocol

Metal sulfate catalysts exhibited high SO₂ tolerance in the NH₃-SCR reaction. The NH₃-SCR reaction mechanism on metal sulfate catalysts should follow the Eley–Rideal mechanism.

In Situ Hydrogen Uptake and NO x Adsorption on Bifunctional Heterogeneous Pd/Mn/Ni for a Low Energy Path toward Selective Catalytic Reduction

Facilitating catalyst accessibility of H2 and NO x at the catalyst surface remains a great challenge in catalytic selective catalytic reduction (SCR). The efficient conversion of NO x into N2 under mild conditions is an attractive pathway as SCR usually requires high operating temperature which consumes extra operating energy and restricts the possible locations of an SCR device. The H2 supply concentration of conventional H2-SCR is relatively sparse (0.5-2%), which leads to a relatively high operating temperature to activate H. We developed a H2-SCR process with the monolithic catalyst which combined with localized rarefied hydrogen enrichment enhanced by porous nickel and adsorption of NO x on Mn oxide with only 0.08, 0.25, and 0.42% palladium can achieve over 80% NO removal efficiency at 120, 100, and 90 °C. Maximizing the role of nickel foam-fixed hydrogen and Mn oxide in combination with NO can provide enriched NO x and H2 atmosphere for adjustable valence state Pd to yield positive catalytic behavior.

Novel metal-organic framework supported manganese oxides for the selective catalytic reduction of NOx with NH3: Promotional role of the support

A novel MnOx@MIL-125(Ti) catalyst was constructed for the selective catalytic reduction (SCR) of NOx with NH₃, in which MIL-125(Ti), a kind of metal-organic frameworks (MOFs), was used as the support because of the structural feature, large surface area and high thermal stability. The SCR results showed that MnOx@MIL-125(Ti) exhibited high deNOx ability and N₂ selectivity in a wide operating temperature range. Moreover, it exhibited better SO₂ resistance than MnOx/TiO₂(P25). Characterization results revealed that MIL-125(Ti) had resulted in a high dispersion of MnOx and a strong metal-support interaction for MnOx@MIL-125(Ti), which could promote the formation of the abundant Mn⁴⁺ and surface chemical oxygen, facilitating the activation of the reactants. In situ DRIFTS results suggested that NH₃-SCR over MnOx@MIL-125(Ti) followed both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanism. In addition, NO^- species were proved to be the important intermediates which were involved in SCR reaction.

Manganese-cerium mixed oxides supported on rice husk based activated carbon with high sulfur tolerance for low-temperature selective catalytic reduction of nitrogen oxides with ammonia

A rice husk-derived activated carbon supported manganese-cerium mixed oxide catalyst (Mn-Ce/RAC) was prepared by an impregnation method and tested for the selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR). 5 wt% Mn-Ce/RAC catalyst showed the highest activity, yielding nearly 100% NO x conversion and N2 selectivity at 240 °C at a space velocity of 30 000 h-1. Compared with commercial activated carbon supported manganese-cerium mixed oxide catalyst (Mn-Ce/SAC), a higher SCR performance with good SO2 tolerance could be observed in the tested temperature range over the Mn-Ce/RAC catalyst. The characterization results revealed that the Mn-Ce/RAC catalyst had a higher Mn4+/Mn3+ ratio, amount of chemisorbed oxygen and more Brønsted acid sites than the Mn-Ce/SAC catalyst. The XRD analysis indicated that Mn-Ce oxides were highly dispersed on the RAC support. These properties of Mn-Ce/RAC assisted the SCR reaction. Moreover, in situ DRIFTS results demonstrated that sulfate species formation on Mn-Ce/RAC was much less in the presence of SO2 than that of Mn-Ce/SAC, which might be ascribed to the reduced alkalinity of the catalyst by the presence of SiO2 in RAC.

Efficiency of Phosphotungstic Acid Modified Mn-Based Catalysts to Promote Activity and N2 Formation for Selective Catalytic Reduction of NO with Ammonia

In this work, the modified Mn-based NH3-SCR (NH3 low-temperature selective catalytic reduction) catalysts with excellent NO conversion and N2 selectivity be designed. N2 yield was hardly more than 75 % over MnOx/TiO2 for NH3-SCR reaction, whereas the NH3-SCR performance has been significantly improved by using 50 wt.% HPW (H3PW12O40)-MnOx/TiO2. 100 % NO conversion and more than 95 % N2 yield was obtained in wide operating temperature window (150–400°C), suggesting that the addition of HPW could effectively improve the NO reduction conversion. After that, the catalysts were further characterized by XRD, H2-TPR, XPS and in situ DRIFT. DRIFT analysis implied that the introduction of HPW significantly improve the capacity of NH4 + species adsorbed on Brønsted acid sites accompanied with inhibiting the formation and consumption of nitrite species. It proved that the non-selective catalytic reduction reaction over HPW-MnOx/TiO2 catalysts are restrained. HPW could accelerate the formation and consumption of NH4 + species adsorbed on Brønsted acid sites with deactivation of nitrate species. In addition, NH3(ad) could be hardly oxidized to NH species and then reacted with nitrate species (L-H mechanism) and gaseous NO (E-R mechanism). More importantly, the oxidation of NH3 was also suppressed, which plays a dominate role to form N2O above 300°C. Besides, the deactivation of potassium poisoning on the SCR activity significantly weakened for modified samples compared to parent catalyst.

Insights over Titanium Modified FeMgOx Catalysts for Selective Catalytic Reduction of NOx with NH3: Influence of Precursors and Crystalline Structures

Titanium modified FeMgOx catalysts with different precursors were prepared by coprecipitation method with microwave thermal treatment. The iron precursor is a key factor affecting the surface active component. The catalyst using FeSO4 and Mg(NO3)2 as precursors exhibited enhanced catalytic activity from 225 to 400 °C, with a maximum NOx conversion of 100%. Iron oxides existed as γ-Fe2O3 in this catalyst. They exhibited highly enriched surface active oxygen and surface acidity, which were favorable for low-temperature selective catalytic reduction (SCR) reaction. Besides, it showed advantage in surface area, spherical particle distribution and pores connectivity. Amorphous iron-magnesium-titanium mixed oxides were the main phase of the catalysts using Fe(NO3)3 as a precursor. This catalyst exhibited a narrow T90 of 200/250–350 °C. Side reactions occurred after 300 °C producing NOx, which reduced the NOx conversion. The strong acid sites inhibited the side reactions, and thus improved the catalytic performance above 300 °C. The weak acid sites appeared below 200 °C, and had a great impact on the low-temperature catalytic performance. Nevertheless, amorphous iron-magnesium-titanium mixed oxides blocked the absorption and activation between NH3 and the surface strong acid sites, which was strengthened on the γ-Fe2O3 surface.

Deactivation Mechanism of Multipoisons in Cement Furnace Flue Gas on Selective Catalytic Reduction Catalysts

Increasing numbers of cement furnaces have applied selective catalytic reduction (SCR) units for advanced treatment of NO in the flue gas. However, the SCR catalysts may face various poisons, such as acidic, alkaline, and heavy metal species, in the fly ash. In this work, we studied the deactivation mechanisms of multipoisons (Ca, Pb, and S) on the CeO₂-WO₃/TiO₂ catalyst, using the in situ diffuse reflectance infrared Fourier transform spectroscopy method. Calcium promoted the conversion of Ce(III) to Ce(IV) and, thus, (i) suppressed the redox cycle, (ii) decreased the NO adsorption (monodentate NO₃^- and bridged NO₂^-), and (iii) enriched the Lewis acid sites. Pb(IV) blocked Ce₂(WO₄)₃, aggravating the electronegativity of W⁶⁺, which inhibited (i) the binding stability of tungsten and ammonia species, (ii) bridged NO₃^- (bonded to tungsten), and (iii) the Brønsted acid sites. The multipoisoning processes enriched O^2- by repairing partial surface oxygen defects, which suppressed O₂^2- and O^-. Sulfur occupied the surface base sites and formed PbSO₄ after Ce₂(WO₄)₃ was saturated.

Co-Exchange of Mn: A Simple Method to Improve Both the Hydrothermal Stability and Activity of Cu–SSZ-13 NH3–SCR Catalysts

A series of Cu–Mn–SSZ-13 catalysts were obtained by co-exchange of Mn and Cu into SSZ-13 together (ion exchange under a mixed solution of Cu(NO3)2 and Mn(NO3)2) and compared with Cu–SSZ-13 catalysts on the selective catalytic reduction (SCR) of nitric oxide (NO) by ammonia. The effects of total ion exchange degree and the effect of Mn species on the structure and performance of catalysts before and after hydrothermal aging were studied. All fresh and aged catalysts were characterized with several methods including temperature-programmed desorption with NH3 (NH3-TPD), X-ray diffraction (XRD), 27Al and 29Si solid-state nuclear magnetic resonance (NMR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and low-temperature N2 adsorption–desorption techniques. The results showed that the increase of the total ion exchange degree can reduce the content of residual Brønsted acid sites of catalysts, thus relieved the dealumination and the decrease of crystallinity of the catalyst during hydrothermal aging. The moderate addition of a Mn component in Cu–Mn–SSZ-13 catalysts significantly increased the activity of NO conversion at low temperature range. The selected Cu(0.2)Mn(0.1)–SSZ-13 catalyst achieved a high NO conversion of >90% in the wide and low temperature range of 175–525 °C and also exhibited good N2 selectivity and excellent hydrothermal stability, which was related to the inhibition of the Mn component on the aggregation of Cu species and the pore destruction of the catalyst during hydrothermal aging.

A Review of Low Temperature NH3-SCR for Removal of NOx

The importance of the low-temperature selective catalytic reduction (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

Synthesis and Evaluation of Copper-Supported Titanium Oxide Nanotubes as Electrocatalyst for the Electrochemical Reduction of Carbon Oxide to Organics

Carbon dioxide (CO2) is considered as the prime reason for the global warming effect and one of the useful ways to transform it into an array of valuable products is through electrochemical reduction of CO2 (ERC). This process requires an efficient electrocatalyst with high faradaic efficiency at low overpotential and enhanced reaction rate. Herein, we report an innovative way of reducing CO2 using copper-metal supported on titanium oxide nanotubes (TNT) electrocatalysts. The TNT support material was synthesized using alkaline hydrothermal process with Degussa (P-25) as a starting material. Copper nanoparticles were anchored on the TNT by homogeneous deposition-precipitation method (HDP) with urea as precipitating agent. The prepared catalysts were tested in a home-made H-cell with 0.5 M NaHCO3 aqueous solution in order to examine their activity for ERC and the optimum copper loading. Continuous gas-phase ERC was carried out in a solid polymer electrolyte (SPE) reactor. The 10% Cu/TNT catalysts were employed in the gas diffusion layer (GDL) on the cathode side with Pt-Ru/C on the anode side. Faradaic efficiencies for the three major products namely methanol, methane, and CO were found to be 4%, 3%, and 10%, respectively at −2.5 V with an overall current density of 120 mA/cm2. The addition of TNT significantly increased the catalytic activity of electrocatalyst for ERC. It is mainly attributed to their better stability towards oxidation, increased CO2 adsorption capacity and stabilization of the reaction intermediate, layered titanates, and larger surface area (400 m2/g) as compared with other support materials. Considering the low cost of TNT, it is anticipated that TNT support electrocatalyst for ECR will gain popularity.

Three-dimensionally ordered macroporous K0.5MnCeOx/SiO2 catalysts: facile preparation and worthwhile catalytic performances for soot combustion

A series of novel catalysts with three-dimensionally ordered macroporous structures and active-component nanoparticles, exhibiting excellent catalytic performance for soot combustion, were fabricated.

Sulfate promotion of selective catalytic reduction of nitric oxide by ammonia on ceria

Selective catalytic reduction of nitric oxide by ammonia (NH₃-SCR) is a promising technology for NO_x emission control.

Green synthesis of mesoporous MnNbOx oxide by a liquid induced self-assembly strategy for low-temperature removal of NOx

A highly porous MnNbO_x with excellent low-temperature NO_x reduction was fabricated by a facile, sustainable ionic liquid induced self-assembly strategy.

A Comparative Study of the NH3-SCR Reactions over an Original and Sb-Modified V2O5–WO3/TiO2 Catalyst at Low Temperatures

Considering the practical requirements for continuous operation under part load condition, the commercial honeycomb selective catalytic reduction (SCR) catalyst was modified with Sb addition. Experiments were performed to investigate the effect of modification on long-time SCR performance under part load condition. Characterizations for the original and modified catalysts were also conducted to analyze the changes of the catalysts. The results indicated that the activity of modified catalyst was obviously enhanced in the temperature range of 275–325 °C and it achieved about 64.5% removal efficiency during the 30 h stability test at 275 °C. The characterization results indicated that the ammonium sulfate was chemically adsorbed on the catalyst surface at low temperatures, which led to the decrease of the specific surface area, pore volume, and V4+/V5+ ratio of the catalysts. These are the reasons for the decrease of the catalyst activity at low temperatures, while the deposition amount of ammonium sulfate was relatively small over the modified catalyst. In addition, the decomposition temperature of the ammonium sulfate was reduced in the modified catalyst compared with the original one. NH4+ ions decomposed at 275 °C by reacting with the NOx in the flue gas, and the dynamic equilibrium of this reaction was achieved on the modified catalyst after a short period of time. Therefore the modified catalyst can be continuously and stably operated at this temperature, and the part load operation of the SCR system in the coal-fired power plant can be realized.

Fabrication of manganese-based Zr-Fe polymeric pillared interlayered montmorillonite for low-temperature selective catalytic reduction of NOx by NH3 in the metallurgical sintering flue gas

A series of Zr-Fe (Zr/Fe = 4:0, 3:1, 2:2, 1:3, 0:4) polymeric pillared interlayered montmorillonite loading 10 wt.% MnO_x (Mn/Zr-Fe-PILM) were investigated for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) in metallurgical sintering flue gas. The X-ray diffraction (XRD), N₂ adsorption-desorption isotherm, scanning electron microscope (SEM), and ammonia temperature-programmed desorption (NH₃-TPD) were used to analyze the physicochemical property. The Fe polymerized with Zr exchanged to montmorillonite can improve the Mn/Zr-Fe-PILM low-temperature NO_x conversion and N₂ selectivity. The Mn/Zr-Fe-PILM (1:3) shows the highest NO_x conversion between 140 and 180 °C. The XRD results suggest that the growth of crystalline ZrO₂ phase is intensely restrained for the Fe₂O₃ migration into the ZrO₂ lattice. The ZrO₂ and MnO_x have an excellent dispersion in montmorillonite. The N₂ adsorption result illustrates that the increase of Fe molar content in the Zr-Fe-PILM support increases the catalyst-specific surface area. The NH₃-TPD results elucidate that the Mn/Zr-Fe-PILM (1:3) has the most total acid sites. Therefore, the low-temperature catalytic activity of the Mn/Zr-Fe-PILM (1:3) has been assigned to the large specific surface area, abundant acid sites, and the dispersion of metallic oxides.

NOx Removal by Selective Catalytic Reduction with Ammonia over a Hydrotalcite-Derived NiFe Mixed Oxide

A series of NiFe mixed oxide catalysts were prepared via calcining hydrotalcite-like precursors for the selective catalytic reduction of nitrogen oxides (NOx) with NH3 (NH3-SCR). Multiple characterizations revealed that catalytic performance was highly dependent on the phase composition, which was vulnerable to the calcination temperature. The MOx phase (M = Ni or Fe) formed at a lower calcination temperature would induce more favorable contents of Fe2+ and Ni3+ and as a result contribute to the better redox capacity and low-temperature activity. In comparison, NiFe2O4 phase emerged at a higher calcination temperature, which was expected to generate more Fe species on the surface and lead to a stable structure, better high-temperature activity, preferable SO2 resistance, and catalytic stability. The optimum NiFe-500 catalyst incorporated the above virtues and afforded excellent denitration (DeNOx) activity (over 85% NOx conversion with nearly 98% N2 selectivity in the region of 210–360 °C), superior SO2 resistance, and catalytic stability.

Research Status and Prospect on Vanadium-Based Catalysts for NH₃-SCR Denitration

Selective catalytic reduction of NO_x with NH₃ is one of the most widely used technologies in denitration. Vanadium-based catalysts have been extensively studied for the deNO_x process. V₂O₅/WO₃(MoO₃)TiO₂ as a commercial catalyst has excellent catalytic activity in the medium temperature range. However, it has usually faced several problems in practical industrial applications, including narrow windows of operation temperatures, and the deactivation of catalysts. The modification of vanadium-based catalysts will be the focus in future research. In this paper, the chemical composition of vanadium-based catalysts, catalytic mechanism, the broadening of the temperature range, and the improvement of erosion resistance are reviewed. Furthermore, the effects of four major systems of copper, iron, cerium and manganese on the modification of vanadium-based catalysts are introduced and analyzed. It is worth noting that the addition of modified elements as promoters has greatly improved the catalytic performance. They can enhance the surface acidity, which leads to the increasing adsorption capacity of NH₃. Surface defects and oxygen vacancies have also been increased, resulting in more active sites. Finally, the future development of vanadium-based catalysts for denitration is prospected. It is indicated that the main purpose for the research of vanadium-based modification will help to obtain safe, environmentally friendly, efficient, and economical catalysts.

Performance of Modified La xSr1- xMnO3 Perovskite Catalysts for NH3 Oxidation: TPD, DFT, and Kinetic Studies

The modified perovskites (La _xSr_{1- x}MnO₃) were prepared using the selective dissolution method for the selective catalytic oxidation (SCO) of NH₃. We found that more Mn⁴⁺ cations and active surface oxygen species formed on the catalyst's surface with increasing the dissolution time (dis). The 1h-dis catalyst exhibited excellent NH₃ conversion, and it performed well in the presence of SO₂ and H₂O. The 10h-dis and 72h-dis catalysts produced considerable N₂O and NO at high temperatures, while they were not detected from the fresh catalyst. Both temperature-programmed experiments and density functional theory calculations proved that NH₃ strongly and mostly bonded to the B-site cations of the perovskite framework rather than A-site cations: this framework limited the bonding of SO₂ to the surface. The reducibility increased superfluously after more than 10 h of immersion. The adsorptions of NH₃ on Mn⁴⁺ exposed surface were stronger than that on La³⁺ or Sr⁴⁺ exposed surfaces. The selective catalytic reduction, nonselective catalytic reduction, and catalytic oxidation reactions all contributed to NH₃ conversion. The formed NO from catalytic oxidation preferred to react with -NH₂/-NH to form N₂/N₂O.

NH3-SCR Performance and Applicability of Mn-Based Spinels over TiO2 Catalyst

A series of Mn-based spinels over TiO2 catalysts have been prepared with the impregnation method. Catalysts were comprehensively characterized using XRD, FESEM, H2-TPR, and the activity evaluation of NH3-SCR, while long-time stability tests and the effect of H2O on NH3-SCR were also investigated. Meanwhile, K poisoning effect was studied by preparing K-doped catalysts (K-Mn/TiO2, K-Cu-Mn/TiO2, K-Mg-Mn/TiO2 and K-Co-Mn/TiO2). According to the characterizations, Cu-Mn/TiO2, Mg-Mn/TiO2 and Co-Mn/TiO2 catalysts exhibited superior low-temperature SCR activity, stability, K resistance and H2O resistance due to the formation of spinels (MgMn2O4, CoMn2O4, CuMn2O4).

The promotion effect of manganese on Cu/SAPO for selective catalytic reduction of NO x with NH3

The activity and hydrothermal stability of Cu/SAPO and xMn–2Cu/SAPO for low-temperature selective catalytic reduction of NO_x with ammonia were investigated. An ion-exchanged method was employed to synthesize xMn–2Cu/SAPO, which was characterized by N₂ adsorption, ICP-AES, X-ray diffraction (XRD), NH₃-temperature programmed desorption (NH₃-TPD), NO oxidation, X-ray photoelectron spectrum (XPS), UV-vis, H₂-temperature programmed reduction (H₂-TPR) and diffuse reflectance infrared Fourier transform spectra (DRIFTS). 2Mn–2Cu/SAPO and 4Mn–2Cu/SAPO showed the best SCR activity, in that at 150 °C NO conversion reached 76% and N₂ selectivity was above 95% for the samples. NO oxidation results showed that the 2Mn–2Cu/SAPO had the best NO oxidation activity and the BET surface area decreased as manganese loading increased. XRD results showed that the metal species was well dispersed. NH₃-TPD showed that the acid sites have no significant influence on the SCR activity of xMn–2Cu/SAPO. H₂-TPR patterns showed good redox capacity for xMn–2Cu/SAPO. UV-vis and H₂-TPR showed that the ratio of Mn⁴⁺ to Mn³⁺ increased as manganese loading increased. XPS spectra showed a significant amount of Mn³⁺ and Mn⁴⁺ species on the surface and addition of manganese increased the ratio of Cu²⁺. The promotion effect of manganese to 2Cu/SAPO comes from the generation of Mn³⁺ and Mn⁴⁺ species. Deduced from the DRIFTS spectra, the Elay–Rideal mechanism was effective on 4Mn–2Cu/SAPO.

The activity and N₂ selectivity of Cu/SAPO and xMn–2Cu/SAPO for low-temperature selective catalytic reduction of NO_x with NH₃ were investigated.