Mn-Ce Oxides Research Articles

Low-temperature SCR of NO with NH3 over biomass char supported highly dispersed Mn[sbnd]Ce mixed oxides

Series of catalysts with MnCe mixed oxides loaded onto biomass char (BC) modified by nitric acid were prepared via impregnation method. And these catalysts were used for the selective catalytic reduction (SCR) of NO with NH3. MnCe (7:3)/BC catalysts with loading 6% (mass ratio) MnCe oxides showed the best NO conversion ratio of 99.2% at 175 °C. The changes of the microstructure, phase composition, metal valence state and functional groups were investigated through SEM, BET, XRD, XPS and FT-IR. It showed that nitric acid modification could increase surface acidic functional groups of biomass char. And surface functional groups on BC could increase NH3 and NO adsorption capacity. In the denitration process, oxygen was transferred from CeO2 to Mn2O3, which could promote the cyclic catalytic reaction rate, then significantly enhanced the NO conversion in the MnCe/BC catalysts. Based on the experimental results and theoretical analysis, the synergetic mechanism model of surface functional groups on the surface of BC, Mn and Ce on the catalysts was set up.

Highly-efficient catalytic combustion performance of 1,2-dichlorobenzene over mesoporous TiO2–SiO2 supported CeMn oxides: The effect of acid sites and redox sites

High specific surface area CeMn/TiO2–SiO2 catalysts were synthesized and evaluated for the catalytic combustion of 1,2-dichlorobenzene (o-DCB). Mesoporous TiO2–SiO2 supports were prepared by evaporation-induced self-assembly (EISA) method to investigate the promotion of Si on the acid properties of TiO2. Pore structures of TiO2–SiO2 were modulated by controlling the amount of hydrochloric acid during the preparation process of mesoporous supports, and the influence of structures and physicochemical properties of CeMn/TiO2–SiO2 catalysts on the activity and stability of o-DCB catalytic combustion were thoroughly studied. The results showed that Mn cations incorporated into CeO2 fluorite structure caused the formation of more active oxygen species. The active oxygen species and high specific surface area of catalysts played significant roles in o-DCB oxidation reaction. The influence of water on o-DCB catalytic combustion was also studied, and two different observations were obtained at low or high content of water. Due to deposition of Cl species on the surface of catalyst, CeMn/TiO2–SiO2 loses partial activity during o-DCB catalytic combustion, and most of activity could be recovered through Deacon reaction.

Role of cerium in improving NO reduction with NH3 over Mn–Ce/ASC catalyst in low-temperature flue gas

Mn–Ce mixed oxide was loaded onto activated semi-coke (ASC) via impregnation method and the low temperature selective catalytic reduction (SCR) of NO with NH3 was investigated. The NO conversion and NO oxidation rates were both influenced by the ratios of Ce/(Mn+Ce), and Mn–Ce(0.3)/ASC with loading 5% (mass ratio) Mn–Ce oxides performed the highest catalytic conversion rate of 96.6% and NO oxidation rate of 18.4% at 250°C and GHSV=6000h−1. The changes of physical properties, microstructure, functional groups, crystal structure, surface atomic state, redox property and acid strength for the catalysts were characterized by BET, SEM, FT-IR, XRD, XPS, H2-TPR and NH3-TPD. The results showed that surface functional groups were increased after nitric acid treatment and abundant oxygen groups were observed on semi-coke surface, which were considered to be beneficial for the gas adsorption. Ce doping could raise the Mn4+/Mnx+ rate which played a vital role in catalytic reaction, enhancing the amount of weak and strong acid sites, increasing oxygen vacancies and accelerating the molecular oxygen turning into reactive oxygen species, which could enhance the NO oxidation activity thus contributed to the denitration efficiency.

Low-temperature abatement of toluene over Mn-Ce oxides catalysts synthesized by a modified hydrothermal approach

Mn-Ce oxides catalysts were synthesized by a novel method combining redox-precipitation and hydrothermal approach. The results indicate that the ratio between manganese and cerium plays a crucial role in the formation of catalysts, and the textual properties as well as catalytic activity are remarked affected. Mn0.6Ce0.4O2 possesses a predominant catalytic activity in the oxidation of toluene, over 70% of toluene is converted at 200°C, and the complete conversion temperature is 210°C. The formation of Mn-Ce solid solution markedly improves the surface area as well as pore volume of Mn-Ce oxide catalyst, and Mn0.6Ce0.4O2 possesses the largest surface area of 298.5m2/g. The abundant Ce3+ and Mn3+ on Mn0.6Ce0.4O2 catalyst facilitate the formation of oxygen vacancies, and improve the transfer of oxygen in the catalysts. Meanwhile, it is found that cerium in Mn-Ce oxide plays a key role in the adsorption of toluene, while manganese is proved to be crucial in the oxidation of toluene, the cooperation between manganese and cerium improves the catalytic reaction process. In addition, the reaction process is investigated by in situ DRIFT measurement, and it is found that the adsorbed toluene could be oxidized to benzyl alcohol as temperature rises around 80–120°C that can be further be oxidized to benzoic acid. Then benzoic acid could be decomposed to formate and/or carbonate species as temperature rises to form CO2 and H2O. In addition, the formed by-product phenol could be further oxidized into CO2 and H2O when the temperature is high enough.

Porous microspheres of manganese-cerium mixed oxides by a polyvinylpyrrolidone assisted solvothermal method

Mixed cerium manganese oxides were synthesized using a polyvinylpyrrolidone (PVP) assisted solvothermal method. Materials obtained after calcination at 400°C were characterized by X-ray diffraction, scanning and transmission electron microscopies, electron paramagnetic resonance (EPR), Raman spectroscopy, thermal analysis and nitrogen adsorption/desorption isotherms. The influence of the synthesis parameters on the oxide structure, such as the Mn:Ce ratio or the amount of PVP, was discussed. Micrometric spheres of mixed Mn-Ce oxides, resulting from the aggregation of 100nm porous snowflakes, were successfully synthesized. These snowflakes were formed from the aggregation of smaller oriented crystallites (size 4nm). The hydrothermal stability of these materials was also investigated.

Scanning tunneling microscopy studies of Mn-doped CeOx(111) interfaces

The growth and structure of Mn on reducible ceria were investigated under ultrahigh vacuum conditions with X-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy techniques. Submonolayer coverage of Mn was deposited on well-ordered CeOx(111) (1.5<x<2) thin films by physical vapor deposition method at room temperature. Mn is oxidized and formally in the +2 oxidation state on both fully oxidized and partially reduced ceria. Oxidation of Mn is accompanied with the reduction of ceria. STM results show the presence of atomic-layer high islands of Mn forming Mn-O-Ce linkages uniformly distributed on the ceria surface. With heating to higher temperatures, these MnO features can coalesce to form slightly larger two-dimensional structures. Our combined spectroscopy and microscopy studies demonstrate a strong interaction between Mn and ceria supports. Introduction of Mn can form mixed Mn-Ce oxides at the interface which can represent model metal-doped ceria surfaces for further examination of the chemical behavior.

Low temperature catalytic combustion of o-dichlorobenzene over supported Mn–Ce oxides: effect of support and Mn/Ce ratio

Mn9Ce1/cordierite represents the lowest reduction temperature among three supports (cordierite, TiO2and Al2O3) and higher combustion activity foro-DCB due to containing a higher Mn3+species concentration and surface hydroxides.

Post-plasma catalytic removal of methanol over Mn–Ce catalysts in an atmospheric dielectric barrier discharge

A post-plasma catalysis system has been developed for the removal of methanol over Mn–Ce oxide catalysts with different Mn/Ce molar ratios at low temperatures. The Mn50Ce50 oxide catalyst (Mn/Ce=1:1) shows the best performance in terms of methanol removal efficiency and energy efficiency of the plasma-catalytic process. The maximum methanol removal efficiency of 95.4% can be achieved at a discharge power of 15W and a gas flow rate of 1L/min, while the highest energy efficiency of the plasma-catalytic process is 47.5g/kWh at 1.9W. The combination of plasma and Mn–Ce catalysts significantly reduces the formation of major by-products (methane, formaldehyde and formic acid) based on the Fourier transform infrared spectra. Possible reaction mechanisms and pathways of the post-plasma catalytic removal of methanol are also proposed. A three-layer back propagation artificial neural network (ANN) model has been developed to get a better understanding of the roles of different process parameters on methanol removal efficiency and energy efficiency in the post-plasma catalytic process. The predicted data from the ANN model show a good agreement with the experimental results. Catalyst composition (i.e. Mn/Ce ratio) is found to be the most important factor affecting methanol removal efficiency with a relative importance of 31.53%, while the discharge power is the most influential parameter for energy efficiency with a relative weight of 30.40%. These results indicate that the well-trained ANN model provides an alternative approach for accurate and fast prediction of the plasma-catalytic chemical reactions.

Binary Ce–Mn oxides confined in carbon nanotubes as efficient catalysts for ethylbenzene dehydrogenation in the presence of carbon dioxide

The promoting effect of Mn species of binary Ce–Mn oxides confined in CNTs on the catalytic performance of ethylbenzene dehydrogenation in the presence of CO2 was investigated in detail.

Interaction of Mn with reducible CeO2(1 1 1) thin films

The structure and interaction of Mn/ceria interfaces were investigated under ultrahigh vacuum conditions with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques. Mn was deposited on well-ordered reducible CeO2(111) thin films by thermal evaporation at room temperature. Deposition of Mn on both oxidized and reduced ceria surfaces can form the mixed Mn–Ce oxide at the interface. XPS data indicate that Ce4+ is partially reduced to Ce3+, while Mn is formally in the +2 oxidation state. STM results show the formation of two-dimensional manganese oxide features on ceria, which is consistent with the thermodynamics data. Our study demonstrates that Mn can modify both electronic and structural properties of ceria.

Catalytic oxidation of benzene over Ce–Mn oxides synthesized by flame spray pyrolysis

Flame spray pyrolysis (FSP) was utilized to synthesize Ce–Mn oxides in one step for catalytic oxidation of benzene. Cerium acetate and manganese acetate were used as precursors. The materials synthesized were characterized using X-ray diffraction (XRD), N2 adsorption, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, and H2-temperature programmed reduction (H2-TPR) and their benzene catalytic oxidation behavior was evaluated. Mn ions were evidenced in multiple chemical states. Crystalline Ce–Mn oxides consist of particles with size <40nm and specific surface areas (SSA) of 20–50m2/g. Raman spectrums and H2-TPR results indicated the interaction between cerium and manganese oxides. Flame-made 12.5%-Ce–Mn oxide exhibited excellent catalytic activity at relatively low temperatures (T95 about 260°C) compared to other Ce–Mn oxides with different cerium-to-manganese ratios. Redox mechanism and strong interaction conform to structure analysis that Ce–Mn strong interaction formed during the high temperature flame process and the results were used to explain catalytic oxidation of benzene.

Synergetic effect of thermo-photocatalytic oxidation of benzene on Pt-TiO2/Ce-MnOx

Pt-TiO2/Ce-MnOx catalysts were obtained by depositing TiO2 and platinum, respectively, on the Ce-Mn oxides prepared by co-precipitation method. The phases of CeO2 and anatase TiO2 were observed in the catalysts from X-ray diffraction (XRD) patterns. X-ray photoelectron spectroscopy (XPS) revealed that lattice oxygen and surface active oxygen were found to be the major components of O1s. The experiment results showed that the kinetic constant of thermo-photocatalysis was 7.6 times of the kinetic constant of single photocatalysis, and was 2.29 times of the kinetic constant sum of photocatalytic and thermal catalytic reaction.

Synthesis of Highly Effective CeO x –MnO y –BaO Catalysts for Direct NO Decomposition

Ce–Mn mixed oxides with a Mn/(Ce + Mn) molar ratio of 0.25 were prepared by solvothermal (ST-1) and co-precipitation (CP) methods, and Ba was loaded on the Ce–Mn oxides. In addition, CeO2–MnO x –BaO catalysts with various compositions were directly prepared by the solvothermal (ST-2) method. The NO decomposition activities of these catalysts were examined. Among the catalysts examined, the ST-2 catalyst having a nominal composition of Ce0.8Mn0.15Ba0.05O x exhibited the highest activity; 77% NO conversion to N2 was attained at 800 °C. These catalysts were characterized by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The Raman and XPS results indicate that the CP catalyst had larger amounts of the BaMnO3-δ and/or Mn3O4 phases. The ST-1 and ST-2 catalysts had highly dispersed Ba species on the surface. The ST-2 catalyst had Mn species with the lowest binding energy of Mn 2p and also had a high population of oxygen vacancies in the ceria lattice, suggesting that Mn species with a low oxidation state contributes to the formation of oxygen vacancies, which play an important role in this reaction.

Mn–Ce oxide as a high-capacity adsorbent for fluoride removal from water

A novel Mn–Ce oxide adsorbent with high sorption capacity for fluoride was prepared via co-precipitation method in this study, and the granular adsorbent was successfully prepared by calcining the mixture of the Mn–Ce powder and pseudo-boehmite. High-resolution transmission electron microscopy (TEM) image showed that the Mn–Ce adsorbent consisted of about 4.5nm crystals, and X-ray diffraction (XRD) analysis indicated the formation of solid solution by Mn species entering CeO2 lattices. The surface hydroxyl group density on the Mn–Ce adsorbent was determined to be as high as 15.3mmolg−1, mainly responsible for its high sorption capacity for fluoride. Sorption isotherms showed that the sorption capacities of fluoride on the powdered and granular adsorbent were 79.5 and 45.5mgg−1 respectively at the equilibrium fluoride concentration of 1mgL−1, much higher than all reported adsorbents. Additionally, the adsorption was fast within the initial 1h. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that the hydroxyl groups on the adsorbent surface were involved in the sorption of fluoride. Both anion exchange and electrostatic interaction were involved in the sorption of fluoride on the Mn–Ce oxide adsorbent.

Effect of Mn content on physical properties of CeO x–MnO y support and BaO–CeO x–MnO y catalysts for direct NO decomposition

A series of manganese–cerium mixed oxides were prepared by a glycothermal method, and the NO decomposition activities of the Ba-loaded Ce–Mn oxides were examined. Among the catalysts examined, the highest NO conversion was obtained on the BaO/Ce–Mn oxide catalyst with a Mn/(Ce+Mn) ratio of 0.25. The X-ray diffraction and Raman analyses indicated the formation of Ce–Mn oxide solid solutions with a cubic fluorite structure. The electron spin resonance analysis indicated the presence of paramagnetic Mn 2+ species in the composite catalysts. Incorporation of Mn 2+ in the fluorite structure of CeO 2 causes an increase in the concentration of oxygen vacancies, which play an important role in the NO decomposition activity of the catalysts. The catalysts were also characterized by X-ray photoelectron spectroscopy and temperature-programmed reduction techniques. Based on the results obtained, the relationship between the physical properties of the catalysts and their NO decomposition activities was discussed.

Complete catalytic oxidation of o-xylene over Mn–Ce oxides prepared using a redox-precipitation method

A series of Mn–Ce oxides were prepared using a redox-precipitation method and the complete catalytic oxidation of o-xylene was examined. Catalytic activity was evaluated in terms of both o-xylene conversion and CO 2 yield. The effects of the Mn at/Ce at atomic ratio and calcination temperature on the features of catalyst structure and catalytic behavior were examined. When the Mn at/Ce at ratio was 1.5 and the catalyst was calcined at 400 °C, the conversion of o-xylene was 100% and the CO 2 yield was 100% at 240 °C. X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H 2-TPR) studies revealed that the catalysts prepared via the redox-precipitation method possessed a homogenous dispersion of amorphous MnO 2 and CeO 2 nanoparticles; in the main active phase, MnO 2 provided available oxygen species and CeO 2 enhanced oxygen mobility. The synergistic effects between MnO 2 and CeO 2 potentiated the catalytic activity necessary for the complete catalytic oxidation of o-xylene.

Low-temperature selective catalytic reduction of NO with NH 3 over Mn [sbnd]Ce oxides supported on TiO 2 and Al 2O 3: A comparative study

Mn Ce oxides were supported on TiO 2 and Al 2O 3 by an ultrasonic impregnation method and used for selective catalytic reduction (SCR) of NO with NH 3 at low-temperature (80–220 °C). Mn Ce/TiO 2 showed a relatively higher SCR activity than Mn Ce/Al 2O 3 at the temperature range of 80–150 °C. When the reaction temperature was higher than 150 °C, Mn Ce/Al 2O 3 exhibited superior SCR activity to Mn Ce/TiO 2. NH 3 temperature programmed desorption study proved that Mn Ce/TiO 2 was mainly Lewis acidic, while Mn Ce/Al 2O 3 could provide more Brönsted acid sites. These acid sites play an important role in SCR according to in situ diffuse reflectance infrared transform spectroscopy (DRIFT) analysis. The main SCR reaction was a typical Eley–Rideal mechanism on Mn Ce/TiO 2, which took place between coordinated NH 3 / NH 4 + and gas-phase NO. For Mn Ce/Al 2O 3, the reaction mainly occurred via another pathway when the temperature exceeded 150 °C, which commenced with the adsorption and oxidation of NO and was followed by reaction between NO 2 or NO 2-containing compounds and NH 3 adspecies. This reaction pathway makes a significant contribution to the improved NO conversion for Mn Ce/Al 2O 3 at higher temperature.

Low-temperature CO Oxidation over Coprecipitated Co–Ce–Mn Oxides

Abstract Co3O4–CeO2–MnOx catalysts prepared by coprecipitation are very active for CO oxidation at low temperature, and 100% CO conversion is obtained at −70 °C over the best catalyst; Adding a small mount of MnOx in CoOx–CeO2 can improve the dispersion of cobalt oxide and ceria effectively, which leads to high CO oxidation activity.

Low-temperature combustion of 2,4,6-trichlorophenol in catalytic wet oxidation with nanocasted Mn–Ce-oxide catalyst

The essentially complete mineralization (>95% total organic carbon elimination) of 2,4,6-trichlorophenol by a CWO is reported at a very mild operating conditions (120 °C, P O 2 = 10 bar ), with the use of a nanocasted Mn–Ce-oxide catalyst. This unique catalyst was prepared by fabrication of a thin nanometric layer of Mn–Ce oxides inside the tubular mesopores of SBA-15 silica using internal gelation of corresponding metal chlorides. Crystallization of this oxide layer at 700 °C, followed by removal of silica matrix, yielded a nanostructured mixed-oxide catalyst with surface area up to 316 m 2/g. It demonstrated superior textural, structural, and surface oxygen properties compared with a reference co-precipitated bulk Mn–Ce-oxide catalyst. This is a result of reduction and equalization of the crystal size of both oxide-phases Mn 2O 3 and CeO 2 at the level of 2–3 nm, providing the full availability of their surface for reacting molecules. The performance measured with the novel catalyst indicates a major improvement in the efficient application of CWO processes for complete purification of complex waste streams.

Cerium incorporated ordered manganese oxide OMS-2 materials: Improved catalysts for wet oxidation of phenol compounds

Cryptomelane type manganese oxide OMS-2 material was synthesized by redox reaction between potassium permanganate and manganese sulphate in acidic medium under reflux or hydrothermal crystallization conditions. Ce was added by ion-exchange, impregnation or directly at crystallization stage. The chemical composition, structure, texture, morphology and thermal stability of the materials were measured by EDX spectroscopy, XRD, N2-adsorption, SEM and TGA. Results of the catalytic wet oxidation of phenol at 100°C indicate best performance of the well-crystallized octahedral molecular sieve with cryptomelane structure where all the accessible potassium ions were exchanged for cerium cations. OMS-2 materials containing pure CeO2 phase and excess of active oxygen species had lower stability, activity and capacity for oxidative reactive adsorption of phenol. Implementation of Ce-exchanged crystalline OMS-2 catalyst improves the wastewater treatment capacity by a factor of 1.5–2 and 3 compared with co-precipitated Mn–Ce-oxide and activated carbon, respectively.