MnOx-CeO2 Catalysts Research Articles

Pd supported on SnO2–MnOx–CeO2 catalysts for low temperature CO oxidation

Pd deposited on CeO2, MnOx–CeO2 and SnO2–MnOx–CeO2 solid solution were tested for low temperature CO oxidation activity. Among them, Pd–MnOx–CeO2 (Pd–MC) and Pd–SnO2–MnOx–CeO2 (Pd–SMC) showed excellent low temperature activity and over 90% of CO conversion was obtained at room temperature. SO2 poisoning tests with 200ppm at room temperature showed that Pd–SMC could keep high CO conversion as compared to Pd–MC. The high CO oxidation activity of Pd–SMC at low temperatures could be attributed to strong oxidation ability of highly oxidized Pd species and interactions between Pd and solid solution support.

Activity of MnOx–CeO2 catalysts in combustion of low concentrated methane

Due to its warming potential over 20 times higher than that of CO2, methane is classified as one of the most dangerous pollutants. Removal of methane emitted particularly from low calorific sources, such as coal mine ventilation air streams or dumping grounds, falls thus in line with the most important challenges of environmental catalysis. Main goal of this work was to verify the efficiency and to understand the catalytic behavior of a series of coprecipitated MnOx–CeO2 mixed oxide samples, containing nominally 30–80mol% of MnO2. A plethora of structural (XRD, RS), textural (BET), and spectroscopic (EPR, XPS, and RS) methods have been applied for sample characterization. Catalytic activity has been determined by TPSR in the temperature range from 100 to 700C. It was shown that catalytic activity of the investigated MnOx–CeO2 samples is strongly related to their structural heterogeneity and reducibility of various manganese centers coexisting within the surface MnOx clusters interacting with nanocrystalline CeO2.

Oxidative dehydrogenation of ethane to ethylene in the presence of HCl over CeO2-based catalysts

This article reports a new catalytic route for the oxidative dehydrogenation of ethane to ethylene in the presence of HCl at moderate temperatures. CeO2 was found to be the most efficient catalyst for the production of ethylene from the variety of metal oxides examined in this work. CeO2 nanocrystals with rod and cube morphologies showed higher ethane conversions and ethylene selectivities than CeO2 nanoparticles. The modification of CeO2 by MnOx further enhanced the catalytic performance. Ethane conversion of 94% and ethylene selectivity of 69% were obtained after 2 h of reaction at 723 K over an 8 wt% MnOx–CeO2 catalyst. This catalyst was stable and the ethylene yield could be sustained at 65%–70% over 100 h of reaction. The presence of HCl played a key role in the selective production of C2H4, and some of the C2H4 was probably formed from chloroethane by dehydrochlorination.

Tailored temperature window of MnOx-CeO2 SCR catalyst by addition of acidic metal oxides

A MnOx-CeO2 catalyst was modified with various acidic metal oxides (Nb2O5, WO3, and MoO3) using a sol-gel method. The activities of the obtained catalysts were measured for the selective catalytic reduction (SCR) of NOx with NH3 to screen suitable acidic metal oxides for different temperature windows. The catalytic activities for NO and NH3 oxidation were also evaluated. The catalysts were characterized by X-ray diffraction, N2 adsorption, H2 temperature-programmed reduction, NH3/NOx temperature-programmed desorption analyses, and infrared spectroscopic measurements of NH3/NOx adsorption. The MnOx-CeO2 catalyst exhibited the greatest low-temperature (100–150 °C) activity. The addition of acidic metal oxides weakened the redox properties of the catalyst, resulting in inhibition of the partial oxidation of the adsorbed NH3 and NO2-assisted fast SCR reactions. Meanwhile, the oxidation of NH3 at relatively high temperatures (250–350 °C) was suppressed, and the adsorption of NH3 on Brönsted and Lewis acid sites was strengthened. Consequently, the temperature window of SCR reaction shifted to higher temperatures in the order Nb2O5 < WO3 < MoO3.

Synthesis of mesoporous CeO2-MnOx binary oxides and their catalytic performances for CO oxidation

Mesoporous CeO2-MnOx binary oxides with different Mn/Ce molar ratios were prepared by hydrothermal synthesis and characterized by scanning electron microscopy (SEM), N2 sorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and H2 temperature-programmed reduction (H2-TPR). The characterization results indicated that the CeO2-MnOx catalysts exhibited flower-like microspheres with high specific surface areas, and partial Mn cations could be incorporated into CeO2 lattice to form solid solution. The CeO2-MnOx catalysts showed better catalytic activity for CO oxidation than that prepared by the coprecipitation method. Furthermore, the CeO2-MnOx catalyst with Mn/Ce molar ratio of 1 in the synthesis gel (Ce-Mn-1) exhibited the best catalytic activity, over which the conversion of CO could achieve 90% at 135 °C. This was ascribed to presence of more Mn species with higher oxidation state on the surface and the better reducibility over the Ce-Mn-1 catalyst than other CeO2-MnOx catalysts.

Characterization and activity of alkaline earth metals loaded CeO2–MOx (M = Mn, Fe) mixed oxides in catalytic reduction of NO

Nanocrystalline CeO2–MOx mixed oxides (M = Mn, Fe) with different M/(M + Ce) molar ratio are prepared by sol–gel combustion method. X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Temperature Programmed Reduction with H2 (H2-TPR) and N2-adsorption (BET) analyses are conducted to characterize the physical–chemical properties of the catalysts. The activity of catalysts for reduction of NOx with ammonia has been evaluated. The CeO2–MnOx catalysts showed better low temperature activity than CeO2–FeOx. The superior activity of CeO2–MnOx with Mn/(Mn + Ce) molar ratio of 0.25 respect to other catalysts (with 83% NO conversion and 68% N2 yield at 200 °C) is associated to nanocrystalline structure, reducibility at low temperature and synergistic effect between Ce and Mn that are observed by XRD, TEM and H2-TPR. The CeO2–FeOx catalysts were found to be active at high temperature, being Ce–Fe the best catalyst yielded 82% NO conversion at 300 °C. The effect of alkaline earth metals (Ca, Mg, Sr and Ba) loading on the structure and catalytic activity of cerium mixed oxides are also investigated. Loading of Ba enhanced the NO reduction activity of mixed oxides due to the increase of number of basic sites. Highest performance with 91% NO conversion and 80% N2 yield attained over CeO2–MnOx (0.25)-Ba (7%) catalyst at 200 °C.

Effect of preparation method on MnOx-CeO2 catalysts for NO oxidation

The NO oxidation reaction was studied over MnOx-CeO2 catalysts prepared by co-precipitation, impregnation and mechanical mixing method, respectively. It was found that the co-precipitation was the most active and a 60% NO conversion was achieved at 250 °C. X-ray diffraction (XRD), Brumauer-Emmett (BET), H2-temperature programmed reduction (H2-TPR) and oxygen storage capacity (OSC) techniques were employed to characterize the physical and chemical properties of the catalysts. XRD results showed that amorphous MnOx or Mn-O-Ce solid solution existed in co-precipitation and impregnation prepared sample, while crystalline MnOx was found in mechanical mixing catalyst. A larger surface area was observed on co-precipitation prepared catalyst compared to those prepared by impregnation and mechanical mixing. The strong interaction between MnOx and CeO2 enhanced the reducibility of the oxides and increased the amount of Mn4+ and activated oxygen, which are favorable for NO oxidation to NO2.

Improvement of Activity and SO2 Tolerance of Sn-Modified MnOx–CeO2 Catalysts for NH3-SCR at Low Temperatures

The performances of fresh and sulfated MnOx-CeO₂ catalysts for selective catalytic reduction of NOx by NH₃ (NH₃-SCR) in a low-temperature range (T < 300 °C) were investigated. Characterization of these catalysts aimed at elucidating the role of additive and the effect of sulfation. The catalyst having a Sn:Mn:Ce = 1:4:5 molar ratio showed the widest SCR activity improvement with near 100% NOx conversion at 110-230 °C. Raman and X-ray photoelectron spectroscopy (XPS) indicated that Sn modification significantly increases the concentration of oxygen vacancies that may facilitate NO oxidation to NO₂. NH₃-TPD characterization showed that the low-temperature NH₃-SCR activity is well correlated with surface acidity for NH3 adsorption, which is also enhanced by Sn modification. Furthermore, as compared to MnOx-CeO₂, Sn-modified MnOx-CeO₂ showed remarkably improved tolerance to SO₂ sulfation and to the combined effect of SO₂ and H₂O. In the presence of SO₂ and H₂O, the Sn-modified MnOx-CeO₂ catalyst gave 62% and 94% NOx conversions as compared to 18% and 56% over MnOx-CeO₂ at temperatures of 110 and 220 °C, respectively. Sulfation of SnO₂-modified MnOx-CeO₂ may form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO₂ during NH₃-SCR at T > 200 °C.

Analysis and Evaluation of Active Based on MnOx-CeO&lt;sub&gt;2&lt;/sub&gt; Catalysts in Process of Particle Combustion for Diesel Particulate Filter

In order to reduce effectively the particulate ignition temperature of Diesel Particulate Filter containing MnOx-CeO2 catalysts，the physical and mathematical calculation models of MnOx-CeO2 catalysts were established on Diesel Particulate Filter，and according to bench tests, the activity measurements of the five different concentrations were processed for MnOx-CeO2 catalyst. The results indicated that when concentration is 20ppm, the ignition temperature can be droped about 105°C, activity measurements of Kp、Kpm and ΔKp decrease 10%, 8% and 7% respectively. Meanwhile, regeneration time can be shortened to 7min. It shows that Diesel Particulate Filter which contains concentration of this type not only has higher regeneration rate and level , but also decrease effectively exhaust back pressure.

Catalytic formaldehyde removal by “storage-oxidation” cycling process over supported silver catalysts

Catalytic removal of indoor HCHO was proposed to proceed in a “storage-oxidation” cycling process. Two kinds of supported silver catalysts, namely Ag/HZSM-5 and Ag–MnOx–CeO2, were investigated as catalysts for this cycling process. Due to the highly dispersed silver clusters formed and its good redox properties, the Ag–MnOx–CeO2 catalyst showed better HCHO oxidation properties in both the storage phase (HCHO partial oxidation to HCOO− at room temperature) and oxidation-regeneration phase (total oxidation of the formates into CO2 and H2O at elevated temperatures). The presence of H2O (RH=50%, 25°C) was found to enhance the HCHO storage capacity for Ag–MnOx–CeO2 catalyst, while competitive adsorption of HCHO with H2O was observed over Ag/HZ catalyst. The results of DRIFTS indicate that the partial oxidation of HCHO into HCOO− is accelerated by the presence of H2O over the Ag–MnOx–CeO2 catalyst. In addition, the catalyst saturated with HCHO can be in situ regenerated without production of secondary pollutants and can be used repeatly. It is suggested that this is a promising approach for indoor HCHO elimination.

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.

Screening of doped MnOx–CeO2 catalysts for low-temperature NO-SCR

The effect of different dopants including niobium, iron, tungsten and zirconium oxide on the low-temperature activity of MnOx–CeO2 catalysts for the selective catalytic reduction (SCR) of NOx with ammonia has been studied with coated cordierite monoliths in model gas experiments. A clearly higher activity and particularly superior nitrogen selectivity was obtained with the niobium-doped catalyst in comparison with the MnOx–CeO2 reference system. At 200°C, the DeNOx was 80% while the N2 selectivity reached more than 96%. In contrast, a decrease of the SCR activity was observed when iron, zirconium or tungsten oxides were added to MnOx–CeO2. However, the addition of niobium oxide did not improve the resistance of the catalyst against SO2 poisoning. A strong and irreversible deactivation occurred after exposure to SO2.

Low-temperature catalytic combustion of chlorobenzene over MnO –CeO2 mixed oxide catalysts

MnOx–CeO2 mixed oxide catalysts prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of chlorinated aromatic volatile organic compounds (CVOCs). MnOx–CeO2 catalysts with the different ratio of Mn/Ce + Mn were found to possess high catalytic activity for catalytic combustion of CB, and MnOx(0.86)–CeO2 was the most active catalyst, on which the complete combustion temperature (T90%) of chlorobenzene was 236 °C. The stability of MnOx–CeO2 catalysts in the CB combustion was investigated. MnOx–CeO2 catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen.

VOC oxidation over MnOx–CeO2 catalysts prepared by a combustion method

MnOx–CeO2 catalysts were prepared by a urea combustion method and their performance in the oxidation of ethanol, ethyl acetate and toluene was evaluated. XRD, XPS, H2-TPR and N2 physisorption were employed in catalyst characterization. Mn2+ and Mn3+ ions are present in the catalysts. In ceria-rich materials, crystalline manganese oxide phases are absent and Mn ions are homogeneously distributed between the bulk and the surface suggesting incorporation of Mn ions in ceria structure. In Mn-rich materials, segregation of a Mn3O4 phase takes place. The mixed oxides get reduced by H2 at lower temperatures than the corresponding single oxides and Mn ions promote reduction of ceria. The surface area of MnOx–CeO2 catalysts is larger than the one of single oxides prepared with the same method. The larger surface area of MnOx–CeO2 catalysts counterbalances their smaller specific activity allowing complete conversion of the examined VOCs at lower temperatures compared to the single oxides.

Synthesis of large surface area MnOx-CeO2 using CTA and its catalytic activity for soot combustion

Large surface area (n)MnOx-(1-n)CeO2 (0≤n≤1) catalysts were synthesized using a hybrid organic/inorganic route and characterized by XRD, N2 adsorption and catalytic test. The addition of aqueous ammonia to mixed solutions of cerium and manganese nitrates in the presence of CTAB (cetyltrimethyl ammonium bromide) yielded gelatinous hydrous Ce-Mn oxide/CTA composites (MnCeCTA). As calcined MnOx-CeO2 (n=0.25, 450°C) exhibited a largest surface area of 223 m2/g, which is quite larger than 80 m2/g for the corresponding oxide prepared from conventional co-precipitation route (MnCeCP). As synthesized MnOx-CeO2 catalysts were tested for soot oxidation by O2. The catalytic activity of MnCeCP was highest at n=0.25-0.5, whereas the catalytic activity of MnCeCTA was lower and increased monotonically with an increase of the Mn content (n). The different catalytic behavior seems to be associated with the activity of surface oxygens and lower oxidation state of MnCeCTA.

Low-temperature catalytic combustion of methane over MnO x –CeO2 mixed oxide catalysts: Effect of preparation method

The effect of preparation method on MnOx–CeO2 mixed oxide catalysts for methane combustion at low temperature was investigated by means of BET, XRD, XPS, H2-TPR techniques and methane oxidation reaction. The catalysts were prepared by the conventional coprecipitation, plasma and modified coprecipitation methods, respectively. It was found that the catalyst prepared by modified coprecipitation was the most active, over which methane conversion reached 90% at a temperature as low as 390 °C. The XRD results showed the preparation methods had no effect on the solid solution structure of MnOx–CeO2 catalysts. More Mn4+ and richer lattice oxygen were found on the surface of the modified coprecipitation prepared catalyst with the help of XPS analysis, and its reduction and BET surface area were remarkably promoted. These factors could be responsible for its higher activity for methane combustion at low temperature.