MnOx Catalysts Research Articles

Insights into Reaction Conditions for Selective Catalytic Reduction of NOx with a MnAl Oxide Catalyst at Low Temperatures

ABSTRACT MnOx catalysts have been proven to have superior activity at low temperatures (LT) for selective catalytic reduction (SCR) of NOx. In this study, spherical mesoporous MnAlx catalysts with different molar ratios were prepared using a simple precipitation method. In addition, their NOx catalytic performance, along with relevant main factors (reaction temperature, molar ratio, particle size, space velocity, and O2 content), was investigated in a simulated fixed bed reactor. The relationship between the main factors and DeNOx efficiency was evaluated by analyzing two parameters: the NOx conversion rate and N2 selectivity. The results showed that the MnAl0.1 catalyst had the optimal low temperature DeNOx performance and had a relatively stable NOx conversion rate of more than 92% in a space velocity range of 14331 h–1 to 23885 h–1, where smaller catalyst particle size led to higher NOx conversion rate and N2 selectivity as high as 100%. In addition, the appropriate oxygen concentration was found to increase NOx conversion and N2 selectivity. This work would be beneficial to further optimization of LT SCR catalyst systems for practical applications.

Exploring an efficient manganese oxide catalyst for ozone decomposition and its deactivation induced by water vapor

A series of MnOx catalysts supported by carbon spheres were prepared by calcining mixtures of manganese acetate and carbon spheres under a nitrogen atmosphere, and their performance for ozone decomposition under high humidity conditions (RH = 90%) was evaluated.

Nickel oxide regulating surface oxygen to promote formaldehyde oxidation on manganese oxide catalysts

The surface oxygen of MnOx is effectively regulated by NiO addition, and the amount and reactivity of surface oxygen contributed to the activity of the NiO–MnOx catalyst, while surface oxygen mobility contributed to reaction stability.

Catalytic ozonation of ethyl acetate over mesoporous manganese oxides synthesized by a sonochemical method

AbstractEthyl acetate is one of typical volatile organic compounds (VOCs) emitting from solvent use, and removing ethyl acetate from exhaust air is vital to VOCs abatement. The mesoporous manganese oxide (MnOx) samples were sonochemically synthesized by the reduction of KMnO4 using Mn(NO3)2. The characterization shows that γ‐MnO2, β‐MnO2, and bixbyite‐like Mn2O3 were obtained by calcining the MnOx samples from 200°C to 500°C. The results show that γ‐MnO2 and β‐MnO2 possess more Mn4+ than bixbyite‐like Mn2O3. β‐MnO2 has the highest contents of surface oxygen species (Oads) and average oxidation state (AOS) among the MnOx catalysts. Based on a 7‐day test, β‐MnO2 performed the highest activity for the catalytic ozonation of ethyl acetate compared with the other MnOx catalysts. The results show that ethyl acetate is easy to be destroyed but difficult to be completely mineralized by catalytic ozonation. Lower gas hourly space velocity (GHSV) leads to higher CO2 selectivity and more intermediate products form in the catalytic ozonation of ethyl acetate under higher GHSV. A further study indicates that β‐MnO2 has a long‐term stability for catalytic ozonation of ethyl acetate because the accumulation and the decomposition of intermediate products are in a dynamic equilibrium during the catalytic ozonation.

Polydopamine mediated modification of manganese oxide on melamine sponge for photothermocatalysis of gaseous formaldehyde

It is an urgent need to develop environmentally friendly strategies with low energy consumption for gaseous formaldehyde (HCHO) purification. Herein, a sponge based MS/PDA/MnOx catalyst with plentiful 3D porosities was constructed. The dual-functional PDA layer not only promoted the MnOx loading (25 wt% MnOx in the composite), but also acted as a photothermal converter to absorb photo-irradiation to heat MnOx catalyst (~80 °C after 10 min irradiation). Moreover, the 3D network structure favored the mass transfer and effectively reduced the catalyst agglomeration to expose more active sites. As a result, the obtained MS/PDA/MnOx photothermocatalyst showed highly efficient performance for removal of HCHO within concentration of 40–320 ppm at room temperature under xenon light irradiation. This process followed a pseudo-second-order model, and the reaction rate of the MS/PDA/MnOx was 4.82 times of the MS/MnOx. Finally, a possible photothermocatalysis mechanism was proposed based on the intermediate examination via the in-situ DRIFTS investigation.

Hg0 removal by palygorskite (PG) supported MnOx catalyst

The MnOx/PG catalysts were prepared by supporting Mn oxides to palygorskite (PG) and used to remove Hg0 in simulated flue gas, which were analyzed and characterized by the specific surface area analysis (BET), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the co-effect of MnOx and PG significantly enhances the Hg0 removal performance; the MnOx/PG catalyst with 8% MnOx loading has the highest Hg0 removal activity. The Hg0 removal efficiency can maintain above 95% at 210°C with a space velocity of 6000 h−1 and an inlet Hg0 concentration of 80 μg/m3 for 400 min. O2 can promote the Hg0 removal by MnOx/PG catalysts, while SO2 and H2O have inhibitory effects. In the presence of O2, the inhibitory effect of SO2 can be obviously weakened. The results of XRD, XPS and temperature programmed desorption experiments (TPD) indicate that the active components MnOx disperse well on the surface of PG. The removal process of Hg0 on the MnOx/PG catalyst includes the steps of adsorption, oxidation and reaction, and HgO and HgSO4 are generated and adsorbed on the catalyst.

Barium promoted manganese oxide catalysts in low-temperature methane catalytic combustion

In this study, a series of BaO-MnOx mixed oxide catalysts were synthesized by the mechanochemical method and employed in lean methane catalytic combustion (MCC) at low temperatures. The synthesized catalysts were characterized by XRD, BET, TGA, FT-IR, H2-TPR, O2-TPD, and FESEM analyses. The results indicated that the 10 wt% BaO-MnOx catalyst with a BET surface area of 25 m2 g−1 possessed the best catalytic performance. The higher activity of the 10 wt% BaO-MnOx catalyst was due to the higher ability to supply oxygen through the components during the MCC process. The light-off temperature corresponding to 50% of the methane conversion was about 330 °C, which was about 50 °C lower than the pure MnOx. Moreover, for the BaO(10)-MnOx catalyst, the 10 and 90% of methane conversion temperatures were about 305 and 427 °C, respectively. Also, the 10 wt% BaO-MnOx catalyst exhibited high catalytic stability under dry feed condition at 450 °C for 50 h. Furthermore, the influence of various parameters such as calcination temperature, feed ratio, GHSV, pretreatment condition, and presence of water vapor in the feedstock was studied on the catalytic performance.

Tailoring manganese oxide catalysts for the total oxidation of pollutants in gas and liquid phase

Bare and ceria-promoted MnOx catalysts (0 ≤ χMn≤1) were prepared by redox-precipitation reactions of Mn(VII), Mn(II) and Ce(III) or Ce(IV) precursors in slightly acidic (pH, 4.5) or basic (pH, 8.0) environment to assess genesis, nature, and functionality of surface active sites. Both synthesis protocols yield nanostructured materials with large surface area and exposure of Mn sites, featuring high activity in the CO oxidation and the phenol wet-air-oxidation (CWAO) model reactions (T, 423 K). High oxide dispersion prompts an extensive incorporation of Mn(II) ions into ceria substitutional solid-solution structures, forming oxygen-vacancies with stronger oxidation activity than surface Mn(IV) sites. Basic structure-activity relationships indicate that the superior CO oxidation performance of the pristine α-MnO2 system relies on large exposure of very reducible Mn(IV) active sites, while O2-adsorption onto Mn(II)/O-vacancy active centres generates very reactive surface oxygen-species boosting the efficiency of composite MnCeOx catalysts in the CWAO of phenol.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

Effect of the Conditions of Solution Combustion Synthesis on the Properties of Monolithic Pt–MnOx Catalysts for Deep Oxidation of Hydrocarbons

Catalysts based on manganese oxides, doped with Pt, and supported on ceramic monoliths with a honeycomb structure were produced by impregnation and solution combustion synthesis (SCS). In SCS, glycine was used as a fuel additive, ensuring different conditions of the combustion of depleted (φ 1) fuel mixtures. The catalysts were studied by TGA, XRD, HRTEM, TPR-H2, BET, and differential dissolution. The catalytic properties of the samples were investigated in the deep oxidation of butane and methane. It was shown that, under SCS conditions, the active components form as a finely dispersed particles of metallic platinum and manganese oxides Mn3O4 in the near-surface layers of the support. Unlike this, after the thermal treatment of the impregnated catalyst, the formed manganese oxides are enriched with Mn(IV) cations and primarily localized in the bulk of the support, forming with it an interaction phase. The high activity of the SCS catalysts in the oxidation of butane and methane is manly determined by the presence of reduced forms of manganese oxide and the accessibility of the active components for the reactants in the near-surface layers of the support.

Direct Synthesis of Novel Sponge-Like Porous MnOx Catalysts Derived from Mn-MOFs for High-Efficiently Eliminate o-Dichlorobenzene by Catalytic Combustion

A series of novel lamellar sponge-like MnOx porous catalysts derived from Mn-MOFs have been prepared via modulator-assisted synthesis method and their catalytic performance has been evaluated for o-dichlorobenzene (o-DCB) catalytic combustion reaction. The catalysts were characterized by BET, XRD, Raman, SEM, TEM, XPS, H2-TPR, O2-TPD and other techniques to test the physicochemical properties of the catalysts. The results showed that MnOx had different morphology and properties. The active oxygen species and large specific surface area of MnOx played an important role in the catalytic combustion of o-dichlorobenzene. The T90 of the laminar catalyst MnOx-D, MnOx-D/M, MnOx-D/H/M, and MnOx-D/H was 321, 341, 346, and 355 °C, respectively. MnOx-D has good catalytic activity for the catalytic combustion of o-DCB, which confirmed that surface lattice oxygen played a crucial role in o-DCB combustion. Additionally, they had good hydrothermal stability, thermal stability and water resistance.

Sacrificial template-directed fabrication of mesoporous manganese oxide based catalysts: Relationship between the ordering degree of pore structure and Fenton-like catalytic activity

In this study, novel mesoporous MnOx-based catalysts were prepared via the hard template method by using the mesoporous silica as a template. The performances of these catalysts were evaluated in catalytic wet H2O2 oxidation for Rhodamine B (RhB) in high-salinity wastewater (10% Na2SO4 solution containing RhB). The microstructure characterization indicated that the order of pore structures and the MnOx crystallinity in mesoporous materials improved as the number of repetitions of the immersion and calcination–molding process increased. Given good selective adsorption performance for RhB and rich Mn3+/Mn4+ couples, MnOx mesoporous materials with ordered regular pores exhibited excellent Fenton-like catalytic performance and the highest removal rate of RhB in high-salinity wastewater reached approximately 90% after 150 min of degradation. Moreover, because of the accelerated generation of the free radicals catalyzed by the Fe3+/Fe2+ couples, the loading of Fe species increased the performance of the MnOx ordered catalysts for RhB degradation. Furthermore, the Fe-loading MnOx catalyst exhibited only slight activity loss within the five cycles of catalytic degradation (the removal rate for RhB within the five cycles all exceeded 95%), due to the morphological stability of the MnOx ordered catalyst and the presence of stable Fe3+/Fe2+ couples during the RhB degradation process.

Interfaces in MOF-Derived CeO2–MnOX Composites as High-Activity Catalysts for Toluene Oxidation: Monolayer Dispersion Threshold

A series of CeO2–MnOX catalysts with different Ce contents was prepared using Mn–BTC MOF as a sacrificial template for toluene oxidation. Interestingly, the performance of CeO2–MnOX increased rapidly only when the Ce content lower than 5%. The 1%-, 3%- and 10%-Ce-content samples exhibited the T90 value of 325 °C, 291 °C and 277 °C, respectively. XRD shows that the catalyst phase changes significantly before (Mn3O4 only) and after (Mn2O3, Mn3O4 and CeO2) 3% Ce loading. All other results indicated that the Ce–Mn interface properties of different Ce content composite oxides was quite distinguishable in terms of removal and energy efficiency. XRD and XPS results further showed that there a Ce monolayer dispersion threshold existed on the interface of MnOX (3.2 wt%, confirmed by XPS), which caused the difference in performance increment. The dispersed Ce could be divided into a monolayer dispersion state (1–3%) and a crystalline phase state (>3%), according to the existence form, which corresponded to the significant and minor enhancements of toluene conversion rate. Importantly, the Ce in monolayer dispersion state obviously improved the redox properties of catalysts interface, while the Ce in crystal state not. The interfaces with monolayer dispersion Ce result in more abundant metal ion states, oxygen vacancies, better electron transfer performance and catalytic activity.

Porous TiO2 aerogel-modified SiC ceramic membrane supported MnOx catalyst for simultaneous removal of NO and dust

As the main atmospheric pollutants, the nitrogen oxides (NOx) and dust from industrial exhaust have caused a series of environmental problems. Catalytic membrane is one of the most important technologies for integrated denitration and dust removal, which can achieve the efficient treatment of multiple pollutants. In order to achieve the simultaneous removal of NO and dust in low-temperature exhaust, a novel MnOx/TiO2/SiC catalytic membrane was developed. The using of TiO2 aerogel as the transition layer could increase the loading capacity of MnOx by more than twice, but has little effect on gas permeance of SiC ceramic membrane. The MnOx/TiO2/SiC catalytic membrane also exhibit excellent filtration performance with dust rejection rate exceeding 99.97% and thus preventing adverse effects of dust on the catalyst. In the NO and dust coexistence system, the obtained catalytic membrane exhibits good catalytic activity owing to its high loading capacity and outstanding filtration performance, which lead to the high NO conversion rate above 80% with the temperature range of 120–180 °C. Meanwhile, the catalytic membrane exhibits remarkable stability in simulation environment, and the NO conversion rate could maintain at 90% within 65 h. The novel MnOx/TiO2/SiC catalytic membrane showed great promise for low-temperature exhaust gas purification.

Flaming combustion and smoldering of active impregnated cigarette butts: A novel method for synthesis of nanostructured MnOx catalysts for NOx reduction

In the present study, a novel combustion method was proposed to synthesize nanomaterials, and nano-sized MnOx was prepared by this method as example. Being different from the traditional solution combustion synthesis method, a cigarette butt (CB, a typical municipal solid waste), rather than the pure soluble organics (e.g., glycine, urea, citric acid, etc.), was used as the fuel. In this process, the cellulose acetate-based cigarette butt also acted as the support or template, because the manganese nitrate in the solution will be adsorbed and dispersed uniformly on the surface of the cellulose acetate filters. After burning the impregnated cigarette butts, the nano-sized MnOx was obtained. Both smoldering and flaming combustion with different ignition temperatures were considered in the combustion synthesis process. Thermogravimetric (TG) analysis was conducted to study the combustion behavior of the pure and manganese nitrate-loaded cigarette butts. XRD, FE-SEM, TEM, XPS, H2-TPR and N2 adsorption–desorption were conducted to detect the physical and chemical characteristics. The product obtained via this combustion method was main in the Mn3O4 phase. The prepared nanoparticles with different ignition temperatures have a uniform particle size distribution (40–50 nm). The MnOx synthesized by smoldering showed better porosity than the flaming-combustion-synthesized MnOx. The H2-TPR and XPS characterization showed that both of the catalysts synthesized by either the smoldering or flaming combustion process, possessed similar redox properties. The nano-MnOx catalyst was applied for the selective catalytic reduction of NO, and the results suggested that MnOx synthesized by smoldering had more stable activity.

Challenges and opportunities for manganese oxides in low-temperature selective catalytic reduction of NOx with NH3: H2O resistance ability

Manganese oxides (MnOx) have attracted increasing attention in selective catalytic reduction of NOx with NH3 (NH3-SCR) due to their excellent low-temperature catalytic activities. However, MnOx still suffer from poor resistance to SO2 and H2O in NH3-SCR. In particular, the irreversible poisoning of SO2 limits their practical applications in NH3-SCR. With the new regulations concerning the emission of SO2 gas, MnOx may be used for control over NOx (i.e. NO and NO2) emission in NH3-SCR. This mini-review was devoted to challenges and opportunities of MnOx catalysts in NH3-SCR, which may be helpful for its practical application in NH3-SCR system.

The insight into the role of Al2O3 in promoting the SO2 tolerance of MnOx for low-temperature selective catalytic reduction of NOx with NH3

This research aims to enhance the SO2 tolerance of manganese oxide (MnOx) catalysts for low-temperature selective catalytic reduction (SCR) de-NOx. Based on the deactivation mechanism of MnOx, we target at the promotion of the byproduct (NH4HSO4) decomposition and the inhibition of the byproduct (MnSO4) formation. To achieve this target, thermogravimetric (TG) and temperature programmed desorption of SO2 (SO2-TPD) are performed upon as many as 21 kinds of single metal oxides to explore the potentially effective chemical composition as the additive of MnOx catalysts. The TG result indicates that Al2O3 can remarkably decrease the thermal stability of NH4HSO4. In addition, the SO2-TPD analysis reveals that the formed species after SO2 adsorption are weakly adsorbed on Al2O3. Thus, Al2O3 is selected to mix with MnOx to optimize its SO2 tolerance. Further experimental results demonstrate that the proper introduction of Al2O3 can effectively enhance the low-temperature SCR de-NOx activity and the SO2 tolerance of MnOx. It is revealed that the introduction of Al2O3 into MnOx can not only facilitate the decomposition of NH4HSO4 but also lead to reduced thermal stability of the adsorbed SO2 species to some extent. As compared to the pristine MnOx, the slight introduction of Al2O3 as the additive is beneficial to the low-temperature SCR de-NOx activity and SO2 tolerance of MnOx.

Amorphous manganese oxide as highly active catalyst for soot oxidation.

A series of highly active amorphous manganese oxide catalysts for soot combustion were synthesized using colloidal solution combustion synthesis (CSCS) method. The surface morphological and structural properties were systematically tested via various techniques: X-ray diffraction, N2 adsorption-desorption, temperature-programmed reduction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Manganese precursors and calcination temperatures affect the crystal structure, redox properties, and surface properties of MnOx. With the calcination temperature increasing from 550 to 850°C, the crystalline structure of manganese oxides changed from amorphous phase to crystal phase. In general, the amorphous MnOx with a hierarchical porous structure showed better catalytic activity for soot oxidation than the crystal ones (T10 as indicator), which can be ascribed to the improved low-temperature reducibility, more surface active oxygen species, and abundant surface Mn4+ ions. The presence of NO in O2 also promoted soot oxidation which follows the NO2-assisted mechanism. Our work may provide a rational comparison between high-efficient amorphous and crystal MnOx catalysts for soot oxidation.

Comparative investigation on catalytic ozonation of VOCs in different types over supported MnOx catalysts

This paper conducted catalytic ozonation of CB (chlorobenzene) over a series of MnOx based catalysts with different supports (Al2O3, TiO2, SiO2, CeO2, and ZrO2) at 120 °C. Mn/Al2O3 exhibited highest CB conversion efficiency, ca. 82.92 %, due to its excellent textual properties, O2 desorption, redox ability, and desirable surface adsorbed oxygen species and acidity. O3 conversion all approached nearly 100.0%, with residual <10 ppm. Mn/Al2O3 was further employed to investigate effect of temperature, O3/CB, and space velocity on CB conversion. Hereafter, catalytic ozonation of single and binary VOCs in different types was performed, i.e., CB, DCE (dichloroethane), DCM (dichloromethane), and PhH (Benzene). Conversion results demonstrated aromatics degraded easier than alkanes and more carbon atoms decreased difficulty, as CB∼PhH > DCE∼DCM, and DCE > DCM; but chlorinated substitution increased difficulty, as PhH > CB. Catalytic co-ozonation of CB/DCE indicated that DCE significantly improved CB conversion to reach totally degradation at low O3 input, but inhibited DCE conversion, especially at higher ratio of DCE/CB. Co-ozonation improved ozone utilization efficiency, and maintained the original property of catalyst. By contrast, CB/PhH co-ozonation displayed very mild effects. Finally, critical intermediates during catalytic CB ozonation, i.e., DCM, carboxyl and formic acid, were detected from mass spectrum results.

Titania-Samarium-Manganese Composite Oxide for the Low-Temperature Selective Catalytic Reduction of NO with NH3.

A novel Ti-doped Sm-Mn mixed oxide (TiSmMnOx) was first designed for the selective catalytic reduction (SCR) of NOx with NH3 at a low temperature. The TiSmMnOx catalyst exhibited a superior catalytic performance, in which NOx conversion higher than 80% and N2 selectivity above 90% could be achieved in a wide-operating temperature window (60-225 °C). Specially, the catalyst also showed high durability against the large space velocity and excellent SO2/H2O resistance. Ti incorporation can efficiently inhibit MnOx crystallization and tune the MnOx phase during calcination at a high temperature. Subsequently, a high specific surface area as well as an increased amount of acid sites on the TiSmMnOx catalysts were produced. Further, the reducibility of the Sm-doped MnOx catalyst was modulated, facilitating NO oxidation and inhibiting NH3 nonselective oxidation. Consequently, a superior SCR activity was achieved at a low temperature and the operating temperature window of the TiSmMnOx catalyst was significantly widened. These findings may provide new insights into the reasonable design and development of the new non-vanadium catalysts with a high NH3-SCR activity for industrial application.