Tungsten modified MnOx–CeO2/ZrO2 monolith catalysts for selective catalytic reduction of NOx with ammonia

A Comprehensive Review of Solar Photocatalysis & Photothermal Catalysis for Hydrogen Production from Biomass: from Material Characteristics to Engineering Application

Biomass photoreforming is a promising way of producing sustainable hydrogen thanks to the abundant sources of biomass feedstocks. Solar energy provides the heat and driven force to initial biomass oxidation coupled with H2 evolution. Currently, biomass photoreforming is still far from plant-scale applications due to the lower solar energy utilization efficiencies, the low H2 yield, and the lack of appropriate photoreactors. The production of H2 from photoreforming of native biomass and platform molecules was summarized and discussed with particular attention to the prospects of scaling up the catalysis technology for mass hydrogen production. The types of photoreforming, including photocatalysis and photothermal catalysis, were discussed, consequently considering the different requirements for photoreactors. We also reviewed the photoreactors that support biomass photoreforming. Numerical simulation methods were implemented for the solid-liquid two-phase flow and inter-particle radiative transfer involved in the reaction process. Developing concentrated photothermal catalytic flowed reactors is beneficial to scale-up catalytic hydrogen production from biomass.

Low-Temperature NH3-SCR over Hierarchical MnO x Supported on Montmorillonite Prepared by Different Methods

Hierarchical MnO _x pillared or supported on montmorillonite were prepared by three methods, i.e., impregnation (IM), chemical precipitation (CP), and in situ deposition (SP). The catalysts were characterized by low-temperature N₂ adsorption (BET), XRD, XPS, SEM, TEM, H₂-TPR, NH₃-TPD, NO-TPD, TPSR, in situ DRIFTS, and evaluation of catalytic performance for NH₃-SCR. The best catalytic performance was obtained for catalysts prepared by SP in terms of activity and selectivity, obtaining >90% NO conversion with >95% selectivity to N₂ in 100-300 °C and GHSV of 70,000 h^-1. Compared to IM and CP, SP greatly simplified catalyst preparation, resulting in higher BET surface areas; a spongy pore structure; more highly dispersed, pillared MnO _x species; and higher density of acid sites distributed on catalysts surface, which all contributed to its superior performance for NH₃-SCR. The activity for low-temperature NH₃-SCR of manganese catalysts could be widely tailored by preparation methods.

Phosphotungstic Acid-Modified MnOx for Selective Catalytic Reduction of NOx with NH3

H3PW12O40-modified MnOx catalysts (denoted as Mn-HPW) were used for NOx elimination with co-fed NH3. The optimal Mn-HPW0.02 catalyst exhibited over 90% NOx conversion at 90–270 °C. The incorporation of HPW increased the amount of Lewis acid sites of the catalyst for adsorbing NH3, and accelerated the reaction between the adsorbed NH3 species and gas-phase NOx, thus, increasing the low-temperature catalytic activity. The oxidation ability of the Mn catalyst was decreased due to the addition of HPW, thus, mitigating the overoxidation of the adsorbed NH3 species and improving the de-NOx activity and N2 selectivity in the high-temperature region. DRIFT results revealed that the NH3 species on Lewis and Brønsted acid sites, bridged nitrate, and bidentate nitrate were important species/intermediates for the reaction. NH3-SCR over the Mn and Mn-HPW0.02 catalysts obeyed the Eley–Rideal and Langmuir–Hinshelwood mechanisms, simultaneously, at 120 °C.

Satisfactory Anti-Interference and High Performance of the 1Co-1Ce/Mn@ZSM-5 Catalyst for Simultaneous Removal of NO and Hg0 in Abominable Flue Gas

The removal performance of NO and Hg⁰, the operating temperature window, and the resistance of SO₂ and H₂O on Mn@ZSM-5 catalyst, which was synthesized by a one-step hydrothermal method with manganese oxide as the active component, were improved by doping different molar ratios of Co/Ce. Co and Ce doping increased the content of Mn⁴⁺ as well as of chemisorbed oxygen and promoted the NO and Hg⁰ removal performance, which reached 96.7 and 98.9%, respectively, in flue gas over the 1Co-1Ce/Mn@ZSM-5 catalyst. Furthermore, with SO₂ and H₂O addition, it decreased slightly to 88.4 and 89.3%, respectively, and then remained stable. The coexistence of SO₂ and H₂O had a synergistic poisoning effect on the activity of the catalyst, while the doping of Co and Ce had a positive influence on the tolerance to SO₂ and H₂O. The excellent anti-interference and high performance of the 1Co-1Ce/Mn@ZSM-5 catalyst in the abominable flue gas were mainly due to the outer surface modification of organosilane and because the sacrificial element Co protected the active sites of Ce and Mn from poisoning, which prevented the redox ability of the catalyst from getting affected.

Interactions between CO oxidation and selective catalytic reduction of NO with NH3 over Mn-based catalysts

The α-MnO2-Cu catalyst has a high NO and CO removal rate. CO reduced the NH3-SCR activity by inhibiting NO adsorption. NH3 negatively affected CO oxidation by minimizing the formate intermediate. NO decomposed intermediates to promote CO oxidation.

Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review

Recently, manganese oxides (MnOx)/cerium(IV) oxide (CeO2) composites have attracted widespread attention for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), which exhibit outstanding catalytic performance owing to unique features, such as a large oxygen storage capacity, excellent low-temperature activity, and strong mechanical strength. The intimate contact between the components can effectively accelerate the charge transfer to enhance the electron–hole separation efficiency. Nevertheless, MnOx/CeO2 still reveals some deficiencies in the practical application process because of poor thermal stability, and a low reduction efficiency. Constructing MnOx/CeO2 with other semiconductors is the most effective strategy to further improve catalytic performance. In this article, we discuss progress in the field of MnOx/CeO2-based ternary composites with an emphasis on the SCR of NOx by NH3. Recent progress in their fabrication and application, including suitable examples from the relevant literature, are analyzed and summarized. In addition, the interaction mechanisms between MnOx/CeO2 catalysts and NOx pollutants are comprehensively dissected. Finally, the review provides basic insights into prospects and challenges for the advancement of MnOx/CeO2-based ternary catalysts.

Sb-Containing Metal Oxide Catalysts for the Selective Catalytic Reduction of NOx with NH3

Sb-containing catalysts (SbZrOx (SbZr), SbCeOx (SbCe), SbCeZrOx (SbCeZr)) were prepared by citric acid method and investigated for the selective catalytic reduction (SCR) of NOx with NH3 (NH3-SCR). SbCeZr outperformed SbZr and SbCe and exhibited the highest activity with 80% NO conversion in the temperature window of 202–422 °C. Meanwhile, it also had good thermal stability and resistance against H2O and SO2. Various characterization methods, such as XRD, XPS, H2-TPR, NH3-TPD, and in situ diffuse reflectance infrared Fourier transform (DRIFT), were applied to understand their different behavior in NOx removal. The presence of Sb in the metal oxides led to the difference in acid distribution and redox property, which closely related with the NH3 adsorption and NO oxidation. Brønsted acid and Lewis acid were evenly distributed on SbCe, while Brønsted acid dominated on SbCeZr. Compared with Brønsted acid, Lewis acid was slightly active in NH3-SCR. The competition between NH3 adsorption and NO oxidation was dependent on SbOx and metal oxides, which were found on SbCe while not on SbCeZr.

Selective catalytic reduction of NOx by NH3 over CeVO4-CeO2 nanocomposite

In this study, it was found that the CeVO₄-CeO₂ nanocomposite possessed remarkably selective catalytic reduction (SCR) performance and wider active temperature scope. And, the promotion principle was explored based on BET, XRD, XPS, H₂-temperature-programmed reduction, NH₃-temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform (DRIFT) techniques. The characterization outcomes manifested that the CeVO₄-CeO₂ nanocomposite could inhibit its crystallinity and enhance the concentrations of chemisorbed oxygen species and Ce³⁺, which was advantageous to the SCR process. Moreover, the in situ DRIFT technique manifested that the NH₃-SCR reaction over Ce_0.75V_0.25O_y was enhanced effectively through the mechanism of L-H.

The enhancement of NH3-SCR performance for CeO2 catalyst by CO pretreatment

CO pretreatment was found to effectively improve the SCR performance of CeO₂, with over 90% at about 300 °C. The larger specific area and the decrease of CeO₂ crystallization indicated the modification of the surface structure after CO pretreatment. Abundant Ce³⁺ species and active oxygen, better reducibility, and the higher surface adsorption capacity were mainly responsible for its enhanced SCR performance. DRIFT analysis revealed the presumed coexistence of two reaction routes that the L-H mechanism was related to the reaction temperature, while the reaction rate of E-R route was almost directly proportional to the NO concentration at a certain temperature, based on the kinetic calculation. In addition, the CO-pretreated CeO₂ also exhibited a better poisoning tolerance for SO₂ and H₂O and excellent thermal stability and circularity. Graphical abstract The process of NH₃-SCR reaction over CeO₂-CO catalyst.

A Novel Porous Ceramic Membrane Supported Monolithic Cu-Doped Mn–Ce Catalysts for Benzene Combustion

Porous ceramic membranes (PCMs) are considered as an efficient hot gas filtration material in industrial systems. Functionalization of the PCMs with high-efficiency catalysts for the abatement of volatile organic compounds (VOCs) during dust elimination is a promising way to purify the industrial exhaust gases. In this work, we prepared PCMs (porosity: 70%) in a facile sintering process and integrated Cu-doped Mn–Ce oxides into the PCMs as monolithic catalysts by the sol–gel method for benzene oxidation. Through this method, the catalysts are dispersed evenly throughout the PCMs with excellent adhesion, and the catalytic PCMs provided more active sites for the reactant gases during the catalytic reaction process compared to the powder catalysts. The physicochemical properties of PCMs and catalytic PCMs were characterized systematically, and the catalytic activities were measured in total oxidation of benzene. As a result, all the prepared catalytic PCMs exhibited high catalytic activity for benzene oxidation. Significantly, the monolithic catalyst of Cu0.2Mn0.6Ce0.2/PCMs obtained the lowest temperature for benzene conversion efficiency of 90% (T90) at 212 °C with a high gaseous hourly space velocity of 5000 h−1 and showed strong resistance to high humidity (90 vol.%, 20 °C) with long-term stability in continuous benzene stream, which is caused by abundant active adsorbed oxygen, more surficial oxygen vacancy, and lower-temperature reducibility.

Spatially Confined Tuning the Interfacial Synergistic Catalysis in Mesochannels toward Selective Catalytic Reduction

Low-temperature selective catalytic reduction of nitrogen oxides (NO _x) with NH₃ (NH₃-SCR) has been identified as a promising strategy to mitigate the pollution of NO _x. The fine control of synergistic effect and the suppression of aggregation of the active component, however, are still the challenge because of the weak interaction between the active component and matrix. In this work, a series of Ce-promoted Mn-based heterogeneous catalysts supported on mesoporous silica (SBA-15) with different Mn contents were prepared by two separated impregnation processes. Low-temperature NH₃-SCR activity demonstrates that the Mn content in the catalyst has a great influence on the activity of the NH₃-SCR reaction. The 20% MnO _x-CeO _x/SBA-15 catalyst exhibited the best catalytic performance in a broad temperature window. Moreover, it exhibits enhanced resistance to SO₂ and H₂O and long-term durability during 72 h reaction. The highly dispersive active phase, the formation of solid solution, the high ratio of Ce³⁺, and the spatial confinement effect largely contribute to the outstanding activity and durability of the 20% MnO _x-CeO _x/SBA-15 catalyst. Finally, a monolithic catalyst fabricated by the 20% MnO _x-CeO _x/SBA-15 catalyst powder and cordierite substrate show promising industrial application.

The synergistic effects between Ce and Cu in CuyCe1−yW5Ox catalysts for enhanced NH3-SCR of NOx and SO2 tolerance

Non-manganese-based metal oxides are promising catalysts for the NH₃-SCR (selective catalytic reduction) of NO_x at low temperatures.

Low-temperature co-purification of NOx and Hg0 from simulated flue gas by CexZryMnzO2/r-Al2O3: the performance and its mechanism

In this study, series of Ce_xZr_yMn_zO₂/r-Al₂O₃ catalysts were prepared by impregnation method and explored to co-purification of NO_x and Hg⁰ at low temperature. The physical and chemical properties of the catalysts were investigated by XRD, BET, FTIR, NH₃-TPD, H₂-TPR, and XPS. The experimental results showed that 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃ yielded higher conversion on co-purification of NO_x and Hg⁰ than the other prepared catalysts at low temperature, especially at 200-300 °C. 91% and 97% convert rate of NO_x and Hg⁰ were obtained, respectively, when 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃ catalyst was used at 250 °C. Moreover, the presence of H₂O slightly decreased the removal of NO_x and Hg⁰ owing to the competitive adsorption of H₂O and Hg⁰. When SO₂ was added, the removal of Hg⁰ first increased slightly and then presented a decrease due to the generation of SO₃ and (NH₄)₂SO₄. The results of NH₃-TPD indicated that the strong acid of 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃ improved its high-temperature activity. XPS and H₂-TPR results showed there were high-valence Mn and Ce species in 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃, which could effectively promote the removal of NO_x and Hg⁰. Therefore, the mechanisms of Hg⁰ and NO_x removal were proposed as Hg (ad) + [O] → HgO (ad), and 2NH₃/NH₄⁺ (ad) + NO₂ (ad) + NO (g) → 2 N₂ + 3H₂O/2H⁺, respectively. Graphical abstract ᅟ.

A comparative study on the Mn/TiO2-M(M = Sn, Zr or Al) Ox catalysts for NH3-SCR reaction at low temperature

A series of TiO₂-M(M = Sn, Zr or Al) O_x were prepared and manganese oxide (MnO_x) was supported on the carrier by the traditional impregnation method for low-temperature selective catalytic reduction (SCR) of NO_x with ammonia as a reductant. The obtained catalysts were characterized by XRD, BET, high-resolution transmission electron microscope (HRTEM), H₂-TPR, NH₃-TPD, X-ray photoelectron spectroscopy (XPS) and in situ Fourier-transform infrared (FT-IR) and their catalytic activities for NO_x reduction with NH₃ in the presence of SO₂ were investigated comparatively. The results showed that the highest NO_x conversion of over 90% could be obtained with the Mn/Ti-Sn catalyst at a wide range of temperature window of 150-270°C. The combination of characterization techniques, such as BET, XRD and HRTEM, revealed that manganese oxides were well dispersed on Ti-Sn. H₂-TPR suggested that Ti-Sn and Ti-Zr supports could enhance the reduction ability of catalysts. Accordingly, Mn/Ti-Al exhibited worse activity at low temperature. XPS results were in good agreement with H₂-TPR results, and Mn/Ti-Sn had more surface-reducible species of Mn⁴⁺ ions and more surface-adsorbed oxygen species, which was conducive to SCR reaction. The in situ FT-IR spectra of NH₃ adsorption indicated that all the modified catalysts had more Lewis acid sites and the amide species at 1506 cm^-1 had a certain influence on the catalytic reaction at low temperature. Mn/Ti-Zr showed a stronger resistance to SO₂ but Mn/Ti-Al was affected more adversely and all the catalysts could not be restored to the initial catalytic activity after stopping feeding SO₂. NH₃-TPD revealed that the total acid amount of the Mn/Ti-Sn sample was larger than other samples, which indicated that the Ti-Sn solid solution could provide more surface acid sites over the catalyst.

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. Mn-based oxides can be classified into four types: (1) single manganese oxide (MnOx), (2) supported manganese oxide (MnOx/support), (3) composite manganese oxides (MnOx-X), and (4) special crystalline manganese oxides (S-MnOx). These Mn-based oxides have been widely used as catalysts for the elimination of gaseous pollutants. This review aims to describe the environmental applications of these manganese oxides and provide perspectives. It gives detailed descriptions of environmental applications of the selective catalytic reduction of NOx with NH₃, the catalytic combustion of volatile organic compounds, Hg⁰ oxidation and adsorption, and soot oxidation, in addition to some other environmental applications. Furthermore, this review mainly focuses on the effects of structure, morphology, and modified elements and on the role of catalyst supports in gaseous heterogeneous catalytic reactions. Finally, future research directions for developing manganese oxide catalysts are proposed.

Comparison of titania nanotubes and titanium dioxide as supports of low-temperature selective catalytic reduction catalysts under sulfur dioxide poisoning

A series of iron-manganese oxide catalysts supported on TiO₂ and titanium nanotubes (TNTs) were studied for low temperature selective catalytic reduction (SCR) of NO with NH₃ in the presence of SO_2. The results showed that the specific surface area and the amount of Brønsted acid sites were highly correlated. The results also demonstrated that higher Mn⁴⁺/Mn³⁺ ratios and larger specific surface areas might be the main reasons for the excellent performance of MnFe-TNTs catalyst after SO₂ poisoning. The SO₂ poisoning effect could be minimized by reducing the GHSV, increasing the reaction temperature, or increasing the [NH₃]/[NO] molar ratio. The results also indicated that the formation of ammonium sulfate had a stronger effect on the NO conversion efficiency as compared to the formation of metal sulfate. Thus operating the low temperature SCR at above 230 ^oC to avoid the formation of ammonium sulfate would be the priority choice when SO₂ poisoning is a concerned issue. Implications: Low-temperature selective catalytic reduction (SCR) has attracted increasing attention due to that it can reduce the energy consumption for the SCR process employed in industries such as steel plants and glass manufacturing plants. However, it also suffers from the sulfur dioxide (SO₂) poisoning problem. This study investigates the possibility of using titania nanotubes (TNTs) as the support of Mn/Fe bimetal oxide catalysts for low-temperature SCR to reduce the SO₂ poisoning. The results indicated that the MnFe-TNT catalyst can tolerate SO₂ for a longer time as compared with the MnFe-TiO₂ catalyst.

A review of Mn-containing oxide catalysts for low temperature selective catalytic reduction of NOx with NH3: reaction mechanism and catalyst deactivation

The reactions over Mn-containing selective catalytic reduction (SCR) catalysts.

Effect of the calcination temperature of cerium–zirconium mixed oxides on the structure and catalytic performance of WO3/CeZrO2 monolithic catalyst for selective catalytic reduction of NOx with NH3

The calcined temperature of the carrier obviously affected SCR activity of catalysts, WO₃/Ce_0.68Zr_0.32O₂-500 showed the best low-temperature NH₃-SCR activity due to its more Lewis acid sites and stronger redox property.

Influence of tungsten on the NH3-SCR activity of MnWOx/TiO2 catalysts

A series of bulk MnWO_x and supported MnWO_x/TiO₂ catalysts with MnWO₄ structure were prepared via self-propagating high-temperature synthesis (SHS), co-precipitation and impregnation methods.

Simultaneous removal of elemental mercury and NO from simulated flue gas using a CeO2 modified V2O5–WO3/TiO2 catalyst

The redox cycle (V⁴⁺ + Ce⁴⁺ ↔ V⁵⁺ + Ce³⁺) over V₂O₅–WO₃/TiO₂–CeO₂ plays a key role in Hg⁰ oxidation and NO conversion.

Oxidation of elemental mercury by modified spent TiO2-based SCR-DeNOx catalysts in simulated coal-fired flue gas

In order to reduce the costs, the recycle of spent TiO2-based SCR-DeNOx catalysts were employed as a potential catalytic support material for elemental mercury (Hg(0)) oxidation in simulated coal-fired flue gas. The catalytic mechanism for simultaneous removal of Hg(0) and NO was also investigated. The catalysts were characterized by Brunauer-Emmett-Teller (BET), scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) method. Results indicated that spent TiO2-based SCR-DeNOx catalyst supported Ce-Mn mixed oxides catalyst (CeMn/SCR1) was highly active for Hg(0) oxidation at low temperatures. The Ce1.00Mn/SCR1 performed the best catalytic activities, and approximately 92.80% mercury oxidation efficiency was obtained at 150 °C. The inhibition effect of NH3 on Hg(0) oxidation was confirmed in that NH3 consumed the surface oxygen. Moreover, H2O inhibited Hg(0) oxidation while SO2 had a promotional effect with the aid of O2. The XPS results illustrated that the surface oxygen was responsible for Hg(0) oxidation and NO conversion. Besides, the Hg(0) oxidation and NO conversion were thought to be aided by synergistic effect between the manganese and cerium oxides.

Effect of water vapor on NH3-NO/NO2 SCR performance of fresh and aged MnOx-NbOx-CeO2 catalysts

A MnOx-NbOx-CeO2 catalyst for low temperature selective catalytic reduction (SCR) of NOx with NH3 was prepared by a sol-gel method, and characterized by NH3-NO/NO2 SCR catalytic activity, NO/NH3 oxidation activity, NOx/NH3 TPD, XRD, BET, H2-TPR and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that the MnOx-NbOx-CeO2 catalyst shows excellent low temperature NH3-SCR activity in the temperature range of 150-300°C. Water vapor inhibits the low temperature activity of the catalyst in standard SCR due to the inhibition of NOx adsorption. As the NO2 content increases in the feed, water vapor does not affect the activity in NO2 SCR. Meanwhile, water vapor significantly enhances the N2 selectivity of the fresh and the aged catalysts due to its inhibition of the decomposition of NH4NO3 into N2O.