Octahedral Molecular Sieve Research Articles

Efficient low-temperature CO removal from high-humidity sintering flue gas by combination Cu and OMS-2

The non-precious metal manganese-based catalysts currently show insufficient CO catalytic oxidation performance in actual sintering flue gas conditions, with increased water vapor levels causing catalyst deactivation. This study utilized a high-performance manganese dioxide octahedral molecular sieve (OMS-2). The effect of metal doping on the catalytic CO oxidation performance was investigated in preparing OMS-2 using the co-precipitation method. The experiments showed that Cu doping increased CO conversion efficiency more than other metals (Co, Ag, Zn, and Fe), with optimal performance achieved at a 1.91 wt% doping level. Besides, Cu doping significantly enhanced water resistance of the catalyst, enabling effective CO removal in high-humidity conditions. The study observed that Cu ions infiltrated the catalyst framework by substituting some of the Mn ions, creating additional active sites in the form of oxygen vacancies and improving surface oxygen mobility, thereby enhancing the performance of CO catalytic oxidation. Furthermore, Cu doping demonstrated selective absorption of water vapor, with CuxO in the catalyst, effectively adsorbing water vapor and protecting the initial active sites, thereby mitigating water vapor-induced poisoning. Even in 15 vol% H2O at 150 °C, 1.91%Cu-OMS-2 maintained total catalytic activity. Therefore, the co-precipitation method-prepared 1.91%Cu-OMS-2 catalyst holds excellent potential for CO removal in sintering flue gas and shows promise for practical applications.

Designing neighboring-site activation of single atom via tunnel ions for boosting acidic oxygen evolution

Realizing an efficient turnover frequency in the acidic oxygen evolution reaction by modifying the reaction configuration is crucial in designing high-performance single-atom catalysts. Here, we report a “single atom–double site” concept, which involves an activatable inert manganese atom redox chemistry in a single-atom Ru-Mn dual-site platform with tunnel Ni ions as the trigger. In contrast to conventional single-atom catalysts, the proposed configuration allows direct intramolecular oxygen coupling driven by the Ni ions intercalation effect, bypassing the secondary deprotonation step instead of the kinetically sluggish adsorbate evolution mechanism. The strong bonding of Ni ions activates the inert manganese terminal groups and inhibits the cross-site disproportionation process inherent in the Mn scaffolding, which is crucial to ensure the dual-site platform. As a result, the single-atom Ru-Ni-Mn octahedral molecular sieves catalyst delivers a low overpotential, adequate mass activity and good stability.

Facile synthesis of Cu-doped manganese oxide octahedral molecular sieve for the efficient degradation of sulfamethoxazole via peroxymonosulfate activation

Facile synthesis of Cu-doped manganese oxide octahedral molecular sieve for the efficient degradation of sulfamethoxazole via peroxymonosulfate activation

Selective and reversible carbon mono oxide gas detection using silver doped octahedral molecular sieves (OMS-2) nanorods

In this work a potable, resistive sensor is fabricated for the selective detection of CO gas using nanorods of Ag doped octahedral molecular sieves-2 (Ag-OMS-2). During exposure to carbon monoxide (CO) gas at room temperature, resistance of Ag-OMS-2 film was dropped from 557 kΩ to 352 kΩ in just 18 sec. However, exposed sample regained its initial resistance value in 25 sec when CO gas source was removed. Both sensing and recovery processes were carried out at room temperature. The sensor film showed excellent reproducibility during several cycles of CO gas exposure. Swift activation of oxygen molecules for the oxidation of CO by the silver present in the tunnel of manganese oxide network is supposed to be responsible for sensing activity of Ag-OMS-2 towards carbon monoxide gas.

CO2 hydrogenation over rhodium cluster catalyst nucleated within a manganese oxide framework

Rhodium-based manganese oxide frameworks were explored as a prototype for carbon dioxide reactive capture and conversion. Three-dimensional frameworks of MnOx were utilized as support structures to isolate Rh metal centers. V, Na, and Zn were introduced as counterions to stabilize the structure and for their beneficial effect as promoters. With this multicomponent catalyst, Rh active centers with MnOxs and varied counterions, we were able to selectively tune the catalytic performance of the material via the choice of counterion and structure of the host material. With cryptomelane-type tunnel manganese oxides octahedral molecular sieve (OMS2), we found that Rh-V-OMS2 was highly stable even after 48 hours on stream with a reaction rate of around 1.5×10−4 mol CO2/gRh/s, surpassing the net reactivity of other initially more active combinations. Furthermore, during CO2 hydrogenation, in situ XAFS showed that single Rh atoms nucleated into nanoparticles/ sub-nanometer clusters with a coordination number of 5.5 or less. Our finding of the correlation between the reaction rate and particle size offers the potential for enhanced control over the reaction rate by tuning particle size. Our activity study with control experiments demonstrates that the activities of the catalysts are proved due to the unique metal support interaction offered by the Rh-X-MnO.

Efficient ozone decomposition in high humidity environments using novel iron-doped OMS-2-loaded activated carbon material.

This study effectively addresses the rapid deactivation of manganese-based catalysts in humid environments during ozone decomposition by introducing iron-doped manganese oxide octahedral molecular sieve (Fe-OMS-2) catalysts supported on activated carbon (AC). By optimizing the doping ratio of Fe-OMS-2, the Fe-OMS-20.5/AC catalyst achieves nearly 100% ozone decomposition efficiency across a wide range of relative humidity levels (0 to 60%), even at elevated air flow rates of 800 L·g-1·h-1, outperforming standalone AC, Fe-OMS-2, or a simple mixture of OMS-2 and AC. The Fe-OMS-20.5/AC catalyst features a porous surface and a mesoporous structure, providing a substantial specific surface area that facilitates the uniform distribution of the Fe-OMS-2 active phase on the AC surface. The incorporation of Fe3+ ions enhances electron transfer between valence state transitions of Mn, thereby improving the catalyst's efficiency in ozone decomposition. Additionally, the AC component protects catalytic sites and enhances the catalyst's humidity resistance. In conclusion, this research presents a novel strategy for developing highly efficient and cost-effective ozone decomposition catalysts that enhance dehumidification, significantly contributing to industrial ozone treatment technologies and advancing environmental protection.

Significant enhanced SO2 resistance of Ce-doped OMS-2 catalyst by building the CeO2 shell for the low-temperature SCR of NO with NH3

Developing highly SO2-resistance catalysts for the NOx reduction at low temperatures is a significant challenge. In this study, we fabricate a catalyst of Ce-doped manganese oxide octahedral molecular sieve by building the CeO2 shell (Ce-OMS-2@CeO2) to solve this problem. This catalyst showed high selective catalytic reduction with NH3 (NH3-SCR) activity and enhanced resistance to SO2-induced deactivation. Ce-OMS-2@CeO2 exhibited outstanding NH3-SCR activity and anti-sulfur performance at 200 °C. The mesoporous CeO2 shells exerted no influence on the transport of reactants to catalytic active sites. The NH3-SCR on Ce-OMS-2@CeO2 catalyst occurred via the Langmuir-Hinshelwood mechanism with adsorption of NH3 and NO on different sites, thus preventing competing adsorption. The CeO2 shell bound SO2 strongly in priority, thus protecting the Ce-OMS-2 core from SO2 poisoning. Moreover, a dynamic balance was established between NH4HSO4 formation and decomposition over our Ce-OMS-2@CeO2 catalyst at low temperatures, which also hindered the SO2 poisoning in NH3-SCR reaction.

Enhancing CO catalytic oxidation performance over Cu-doping manganese oxide octahedral molecular sieves catalyst

The CO oxidation catalytic activity of catalysts is strongly influenced by the oxygen vacancy defects (OVDs) concentration and the valence state of active metal. Herein, a defect engineering approach was implemented to enhance the oxygen vacancy defects and to modify the valence of metal ions in manganese oxide octahedral molecular sieves (OMS-2) by the introduction of copper (Cu). The characterization and theoretical calculation results reveal that the incorporation of Cu2+ ion into the OMS-2 structure led to a rise in specific surface area and pore volume, weakening of Mn-O bonds, higher proportion of the low-coordinated oxygen species adsorbed in oxygen vacancies (Oads) and an increase in the average oxidation state of manganese. These structural modifications were discovered to considerably reduce the apparent activation energy (Ea), thus ultimately significantly enhancing the CO oxidation activity (T99 at 148 ℃at GHSV = 13,200 h−1) than the original OMS-2 (T99 = 215 ℃ at GHSV = 13,200 h−1). Furthermore, In-situ diffuse reflectance infrared Fourier transform (DRIFT) and In-situ near-ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS) results indicate that the bimetallic synergy enhanced by doping strategy accelerates the conversion of oxygen to chemisorbed oxygen species and the reaction rate of CO oxidation through Mn3++Cu2+↔Mn4++Cu+ redox cycle. The findings of this study offer novel perspectives on the design of catalysts with exceptional performance in CO oxidation.

Superior low temperature activity over α-MnO2/β-MnOOH catalyst for selective catalytic reduction of NO x with ammonia.

Manganese octahedral molecular sieves with an α-MnO2 crystal structure (OMS-2) and their related materials have attracted significant attention for the selective catalytic reduction of NO x using NH3 (NH3-SCR) at low temperatures. Further lowering their operating temperature should be an effective method to develop an environmentally friendly de-NO x system; however, their catalytic activity at low temperatures, especially below 100 °C, remains poor. This study describes a post-synthetic approach to develop Mn-based catalysts superior to those in the literature that operate at ultralow temperatures. Post-synthetic planetary ball milling for OMS-2 caused the partial conversion of OMS-2 into β-MnOOH. The obtained nanocomposite catalysts possessed abundant surface oxygen vacancies and strong surface acidity, allowing the milled catalyst to exhibit higher NO conversion at 90 °C (91%) than that in freshly prepared OMS-2 without planetary ball milling (29%). Lowering the operation temperature of OMS-2 catalysts contributed to the suppression of N2O evolution during NH3-SCR over manganese-based catalysts, resulting in high N2 selectivity over the milled OMS-2 catalyst (93%).

Structure sensitivity of Cu supported on manganese oxide catalysts in levulinic acid hydrogenation

Different synthesis methods were used to prepare a series of size-controlled copper nanoparticles supported on manganese oxide octahedral molecular sieves (OMS-2) catalysts.

Enhanced Oxygen Reduction Reaction Activity of Manganese Oxide via p−d Hybridization with Aluminum Group Element Dopants

AbstractDoping engineering is an effective strategy to improve the electrocatalytic activity of manganese oxides by enhancing their poor electrical conductivity and oxygen adsorption capacity. Herein, p‐block aluminum group metal ions (Al3+, Ga3+, and In3+) are introduced into cryptomelane‐type manganese oxide octahedral molecular sieves (OMS‐2), leading to p−d orbital hybridization between the p‐orbitals of the aluminum group metals and d‐orbitals of Mn, facilitating the oxygen reduction reaction. The aluminum group metal‐doped OMS‐2 exhibits excellent catalytic activity, rapid reaction kinetics, and favorable stability compared to commercial Pt/C. Among the three prepared catalysts, Ga‐doped OMS‐2 (Ga‐OMS‐2) has stronger oxygen reduction activity. Experimental and theoretical calculations show that the superiority of Ga‐OMS‐2 is attributed to p−d hybridization, which enriches the reaction sites and enhances the binding strength of the catalyst to the O2 reaction intermediates. As a proof of concept, Zinc−air batteries assembled with Ga‐OMS‐2 as a catalyst exhibit superior power density and cycle life to commercial Pt/C. This p−d hybridization strategy gives insight into the p‐block metal doping of catalysts prepared with other transition metals with excellent electrocatalytic activity and durability for energy storage and conversion.

Structured Cerium-Manganese Catalysts Supported on Nickel Foam for Toluene Oxidation by Electric Internal Heating.

Structured catalysts are widely used in catalytic oxidation of gaseous pollutants, hot catalysis is usually needed to assist the reaction in the catalytic process. Herein, a Ce-modified manganese oxide octahedral molecular sieves (Ce-OMS-2) structured catalyst supported on foam nickel was prepared through impregnation process. A systematically quantitative testing on the toluene catalytic oxidation effectiveness of this structured catalyst was conducted through catalyst evaluation device, combining a series of characterization methods, such as XRD and SEM, the structure-activity relationship was established. Assisted with electric internal heating and ozone oxidation environments, this structured catalyst exhibits excellent catalytic oxidation performance for oxidative decomposition of toluene even under high humidity conditions. The results showed that the ozone-coupled structured nickel foam catalyst increased the decomposition efficiency of toluene from 25 % (without catalyst and heating) to 55 % (with catalyst and without heating) and the electric internal heating can significantly improve the reactivity and moisture resistance of the structured nickel-foam catalyst, at 90 % RH and 40000 h-1, 50000 ppb O3 and 40 mg/m3 toluene was maintained 100 % catalytic efficiency. The high-efficiency non-precious metal-based electrothermal catalyst prepared herein is expected to have certain enlightenment for the purification of VOCs.

Intensifying levulinic acid hydrogenation using mechanochemically prepared copper on manganese oxide catalysts

A series of copper nanoparticles supported on manganese oxide octahedral molecular sieves (OMS-2) were prepared using mechanochemical (Ball-Mill) and conventional wet-impregnation (Wet-Imp) methods. All catalysts prepared were thoroughly characterized using ICP-OES elemental analysis, X-ray diffraction (XRD), N2 sorption, H2 temperature programmed reduction (TPR) and transmission electron microscopy (TEM) techniques. The catalyst preparation methods greatly affected the size of the Cu nanoparticles. TEM images showed that 5 wt% Cu/OMS-2 (Ball-Mill) catalyst had a narrow particle size distribution with an average Cu nanoparticle size of 2.1 nm, while the corresponding 5 wt% Cu/OMS-2 catalyst prepared using wet-impregnation method had an average Cu nanoparticle size of 19.2 nm. The structural features of the catalysts were corelated with the catalytic activity using the liquid phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL), as an exemplar process. In LA hydrogenation at 190 °C and 20 bar H2 pressure, the ball milled catalysts achieved higher LA conversion, and greater GVL yield, as compared to the corresponding catalysts prepared by wet-impregnation method, reinforcing that Cu nanoparticle size and metal dispersion are important tool to intensify the catalytic activity. For instance, 5 wt% Cu/OMS-2 (Ball-Mill) catalyst achieved almost twice the turnover frequency (TOF), 24.7 h−1 as compared to the 5 wt% Cu/OMS-2 (Wet-Imp) catalyst, TOF 11.8 h−1, under identical reaction conditions. The results of this study demonstrate that ball milling is a superior method for Cu/OMS-2 catalyst preparation than wet impregnation.

Cu incorporating into OMS-2 lattice creates an efficient Hg0 removal sorbent in natural gas with ambient-temperature H2S/H2O tolerance

Removing mercury in wet natural gas containing acidic components is essential to developing the natural gas purification industry and mitigating corrosion in aluminum heat exchangers. In this work, the Cu-doped manganese oxide octahedral molecular sieve (OMS-2) nanorods were designed for oxidation of Hg0 in wet natural gas. The Hg0 removal efficiency over Cu(0.05)-OMS-2 reached 98.2 % under a high space velocity of 24 × 104 h−1 and a complex atmosphere containing H2O, CO2, and H2S. Surface reactive oxygen species produced by strong electronic interactions owing to Mn-O-Cu hybridization was the main active sites for Hg0 oxidation. The reactive oxygen species on Cu(0.05)-OMS-2 reacted with H2S to form active sulfur species, which could combine with Hg0 to form HgS. Cu(0.05)-OMS-2 exhibited excellent ambient-temperature water resistance, which was attributed to the good redox properties caused by Cu incorporation. This work provided a new strategy for designing efficient sorbents to remove Hg0 from wet natural gas.

A new approach for the determination of As, Cu, and Pb in seawater samples using manganese oxide octahedral molecular sieve as a sorbent for dispersive solid-phase microextraction

This study introduces a novel method for preconcentrating As, Cu, and Pb from seawater samples using manganese oxide octahedral molecular sieve (OMS-2), as a sorbent, and the analysis by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The OMS-2 nanomaterial was synthesized and characterized using X-ray diffraction and scanning electron microscopy, revealing a crystallite size of 20.9 nm and a typical needle-like morphology of cryptomelane structure. To optimize the ICP-OES operating conditions and the preconcentration process, a central composite design was used. The optimal conditions for ICP-OES analyses were 1200 W and 0.7 L min−1 for the levels of the radio frequency potential (RF) and nebulization gas flow rate, respectively. The optimal conditions for the adsorption process were achieved at a pH of 6.5, 30 mg of OMS-2, and 35 min of stirring time, in the presence of the sample matrix. The enrichment factors obtained were 66, 45, and 21, and a limit of detection of 0.3, 0.1, and 2.1 μg L−1 for As, Cu, and Pb, respectively. The recovery tests ranged from 80 % to 120 %. The method was successfully applied to determine As, Cu, and Pb in seawater samples.

Size Effect of Platinum Nanoparticles over Platinum-Manganese Oxide on the Low-Temperature Oxidation of Toluene.

The effect of size of Pt nanoparticles has an important influence on the performance of supported Pt-based catalysts for the elimination of toluene. Herein, uniform Pt nanoparticles with average sizes of 1.5, 2.0, 2.5, 2.9, and 3.6 nm were obtained and supported on manganese oxide octahedral molecular sieves (OMS-2), and their catalytic performances for toluene oxidation were evaluated. Benefiting from the moderate interfacial interaction between nanoparticles and manganese oxide support, Pt/OMS-2-3 with the Pt particle size of 3.0 nm showed the best catalytic performance owing to the highest content of Pt2+ species. It also facilitates the formation of more abundant Mnδ+ (Mn2+ and Mn3+) and oxygen vacancies than that of the other sizes of the OMS-2-supported Pt nanoparticles, which can be filled by a large amount of adsorbed oxygen and converted into reactive oxygen species. We further showed that the resulting surface synergetic oxygen vacancies (Pt2+-Ov-Mnδ+) play a decisive part in catalyzing the complete oxidation of toluene. The result will provide new insights for designing efficient Pt-based catalysts for deep purification of toluene.

TiN nanoparticles/OMS-2 nanorods/carbonized wood composites for simultaneous solar water evaporation and abatement of volatile organic compound from undrinkable water

Severe air pollution, water pollution, and climate change have led to an increasing shortage of clean water. Solar interface water evaporation is becoming important to obtain distilled water from undrinkable water such as seawater and contaminated water. However, the as-obtained distilled water usually contains excessive volatile organic compounds (VOCs), which are serious harmful to human health. In this article, a novel bi-functional material is prepared by depositing manganese oxide octahedral molecular sieve (OMS-2) nanorods on carbonized wood (CW), which can simultaneously realize solar water evaporation and VOCs abatement. OMS-2 nanorods synthesized at lower hydrothermal temperature possess higher oxygen vacancy defects, which can improve not only the solar absorption and solar water evaporation rate but also solar driven photothermo-catalytic activity for the oxidation of VOCs. Doping TiN nanoparticles into CW/OMS-2 can further enhance the solar water evaporation performances and VOCs removal efficiencies because of its excellent solar absorption ability, which can further increase the temperature of OMS-2 catalyst and accelerate the solar water evaporation and the toluene degradation. The as-designed bifunctional composites possess an outstanding water evaporation rate over 1.436 kg m−2 h−1 and a toluene removal efficiency of 92.96% under 2.5 sun irradiation, indicating exciting prospects for efficiently producing clean water from undrinkable water with VOCs.

Cryptomelane nanocrystals: Pseudosymmetry causes anisotropic peak broadening in Rietveld refinement

Cryptomelane (□2-xKx)[Mn8O16] is an octahedral molecular sieve with the hollandite framework, a strongly correlated electron system with tunnel structure. Rietveld refinement using powder X-ray diffraction patterns of a nanocrystalline sample was undertaken to assess both structural and morphological details which might be tuned for specific applications. The distribution of peak widths was highly anisotropic but successful refinement of the tetragonal structure could be achieved assuming both grain shape and microstrain anisotropy. The microstrain hypothesis appeared flawed from high uncertainties and correlations and was in conflict with the negative slope of a Williamson-Hall plot. As an alternative, lattice desymmetrization from tetragonal to monoclinic was considered. Comparison of calculated and observed hkl dependence of line broadening confirmed that the pseudosymmetry hypothesis was more appropriate, improving results for both structure and particle shape. This is the first time that lattice desymmetrization was used to explain anisotropic diffraction line broadening, a major issue in nanomaterials where peaks are intrinsically broad and the new model helps to abate correlations which afflict Rietveld refinement in these systems.

Magnesium doping to improve the light to heat conversion of OMS-2 for formaldehyde oxidation under visible light irradiation

Mg-doped manganese oxide octahedral molecular sieve (Mg-OMS-2) catalysts were prepared by hydrothermal method. The photothermal degradation performance of these catalysts for formaldehyde (HCHO) in batch system and continuous system was investigated. The light absorption of OMS-2 was increased by Mg-doped, especially for near infrared light, which promoted surface temperature reach a maximum of 214.8 °C under xenon irradiation. At this temperature, the reinforced surface lattice oxygen and oxygen vacancy that formed by lattice distortion via Mg-doped were activated. The best HCHO elimination efficiency was achieved over Mg0.2/OMS-2 catalyst with Mg2+/Mn2+ = 1/5, which could reduce HCHO from 250 ppm to 10 ppm within 20 min. The in situ DRIFTS was also carried out to monitor the changes in the content of reaction intermediates and analyze the degradation paths of HCHO. It was found the HCHO was attacked by formed •OH and •O2− to generate formate species and carbonate species, and finally transformed to CO2 and H2O. This photothermal catalytic oxidation process exhibited a high efficiency purification of HCHO without the help of extra energy consumption.

Synthesis of Ag-OMS Catalyst for Sunlight-Assisted Photodegradation of Crystal Violet Dye

The contamination of water with organic pollutants, such as dyes, has become a serious threat to the environment. Therefore, the development of a cost-effective, eco-friendly, proficient, and visible-light-driven catalyst for the treatment of organic dye-contaminated wastewater has been a burning issue recently. Photocatalysis is suggested as a potential treatment technique for the eradication of organic pollutants. The 1D tunnel-structured manganese oxide octahedral molecular sieve (OMS) is a suitable substance to be tested as a visible-light-driven photocatalyst for the degradation of organic contaminants. However, the fast recombination of photoinduced charges (h+/e−) limits its photocatalytic application. The development of heterojunctions between OMS and other metals, such as Ag, is a suitable technique for improving the photocatalytic performance of OMS. In this study, Ag-OMS with plasmon-enhanced photocatalytic activity is reported for the photodegradation of crystal violet dye. Manganese oxide OMS was prepared by an acidic precipitation method using potassium permanganate, manganese acetate, and nitric acid as precursor materials. Ag nanoparticles were deposited on OMS using leaf extracts of Calotropis gigantea. The deposition of Ag enhanced the photocatalytic performance of OMS from 68 to 95%. The effects of Ag contents, catalyst dosage, and concentration of crystal violet dye on catalytic performance were explored as well. Approximately 100, 95, and 75% photodegradation of 50, 100, and 150 mg/L crystal violet dye was observed in 90, 120, and 120 min in the presence of 10% Ag-OMS, respectively. Excellent photocatalytic performance, low dose utilization, and reusability proved that Ag-OMS might have practical environmental applications.