MnOx-CeO2 Catalysts Research Articles

Carbon sphere-assisted solution combustion synthesis of porous/hollow structured CeO2-MnOx catalysts

Solution combustion and hard template are traditional and classical approaches to synthesize porous and hollow structured metal oxides. However, their combination for the construction of porous/hollow structured bimetal oxides has seldom studied. In this work, a novel modified solution combustion method is proposed to synthesize bimetal oxides, e.g. CeO2-MnOx, in which carbon spheres as hard template are introduced into the solution combustion process. The experimental results reveal that the added carbon spheres not only act as hard template to create hollow spaces after removing them, but also burn off themselves in the solution combustion process, and thus affect the crystallinity and oxidation state of manganese oxide. Importantly, CeO2-MnOx catalyst harvested by using the carbon sphere-assisted solution combustion method exhibits the highest catalytic activity toward CO oxidation, compared with those prepared from traditional solution combustion and traditional hard template method. X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H2-TPR) analysis demonstrate that there is a strong interaction between CeO2 and MnOx, which is mainly responsible for its high catalytic activity toward CO oxidation. This work is expected to help us modify the solution combustion and design highly efficient catalysts.

New insights into the effect of morphology on catalytic properties of MnO x -CeO2 mixed oxides for chlorobenzene degradation.

We synthesized four CeO2–MnOx mixed oxides with different morphologies using simple hydrothermal methods. The catalytic activity for chlorobenzene (CB) degradation decreases in the following order: rod-CeO2–MnOx > plate-CeO2–MnOx > polyhedra-CeO2–MnOx > cube-CeO2–MnOx. CeO2 and MnOx in the mixed oxides are highly dispersed and two new phases of both todorokite (S.G.: P2/m:b) and vernadite (S.G.: I4/m) with a special tunnel-like structure are found. Both rod-CeO2–MnOx and plate-CeO2–MnOx exhibit increased lattice microstrains generated from lattice distortion and defects; further, there are more oxygen vacancies and more MnOx (Mn4+ and Mn2+) species on the surface, particularly when compared to cube-CeO2–MnOx. Therefore, this promotes deeper oxidation activity for CB. Moreover, the strong interaction between CeO2 and MnOx also promotes the redox ability of CeO2–MnOx mixed oxides, while their oxygen storage capacity (OSC) properties are not only intrinsic to their structures but also limited to their surfaces and by their particle sizes.

Active manganese oxide on MnOx–CeO2 catalysts for low-temperature NO oxidation: Characterization and kinetics study

MnOx–CeO2 catalysts were synthesized to investigate the active sites for NO oxidation by varying the calcination temperature. XRD and TEM results showed that cubic CeO2 and amorphous MnOx existed in MnOx–CeO2 catalysts. High temperature calcination caused the sintering of amorphous MnOx and transforming to bulk crystalline Mn2O3. H2-TPR and XPS results suggested the valence of Mn in MnOx–CeO2 was higher than pure MnOx, and decreased with the increasing calcination temperature. The turnover frequency (TOF) was calculated based on the initial reducibility according to H2-TPR quantitation and kinetic study. The TOF results indicated that the initial reducibility of amorphous MnOx with high valence manganese ions was equivalent to the active sites for NO oxidation. It can be inferred that the amorphous MnOx plays a key role in low-temperature NO oxidation.

Experimental and numerical performance analysis of a TC-Trombe wall

The present paper proposes a novel zero-energy solar application system combining the thermal catalytic technology with Trombe wall (TC-Trombe wall), which could realize indoor air purification and space heating simultaneously fully driven by solar energy. A full-day experiment was conducted to study the air heating performance and formaldehyde degradation performance of TC-Trombe wall. Results showed that the daily air heating efficiency was 41.3%. In our experiments, the generated total volume of fresh air and total formaldehyde degradation amount by TC-Trombe wall were 249.2m3/(m2day) and 208.4mg/(m2day), respectively. In addition, a photothermocatalytic synergetic effect exists in the solar light driven thermocatalytic oxidation process over MnOx-CeO2 catalysts. A dynamic numerical model was developed to predict system thermal properties. Based on the simulation results, the effects of the solar radiation intensity, ambient temperature and air layer thickness on the system thermal efficiency were discussed. Furthermore, the energy saving performance of TC-Trombe wall was evaluated in heating seasons in Hefei based the established system thermal model. Results showed that the total saving energy of up to 97.4kWh/m2 could be obtained. The saving energy for space heating and formaldehyde degradation were 64.3kWh/m2 and 33.1kWh/m2, respectively.

The inhibition effect of oxygen in the calcination atmosphere on the catalytic performance of MnOx–CeO2 catalysts for NO oxidation

In this study, the widely studied MnCe (MnOx–CeO2) catalyst prepared by the co-precipitation method was used for investigating the effects of oxygen in the calcination atmosphere (referred as Oca) on its catalytic performance for NO oxidation. The evaluation of catalytic performance showed that MnCe-O (calcined in O2/Ar) exhibited a much lower NO conversion in comparison with MnCe-A (calcined in Ar) catalyst. The characterization results revealed that Oca could cause the sintering of MnCe catalysts and then reduce the surface area, which decreased the active sites for catalytic reaction. Simultaneously, Oca inhibited the incorporation of Mn ions into CeO2 lattice, which could weaken the interaction between MnOx and CeO2 supports. This weak interaction decreased the amounts of active oxygen species and weakened the redox properties of MnCe catalyst. Furthermore, the nitrate species adsorbed on MnCe-O was more stable than that on MnCe-A. Therefore, the inferior catalytic performance of MnCe-O could be attributed to the collective effect of low concentration active sites and active oxygen species, poor redox properties and stable adsorbed nitrate species.

Evolution of oxygen vacancies in MnOx-CeO2 mixed oxides for soot oxidation

Oxygen vacancy (O-vacancy) is essential in catalytic oxidation but little is known about its insight. Herein, a series of MnOx-CeO2 catalysts with various Mn/(Mn+Ce) molar ratios were synthesized with citric acid complex method for O-vacancy study in soot catalytic combustion. The samples were characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption, O2-temperature programmed desorption (O2-TPD), H2-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy. It has been shown that MnOx(0.4)-CeO2 catalyst presented more O-vacancies, thus exhibiting the highest catalytic activities and redox properties. With the utilization of in situ Raman, two types of O-vacancies, including Frenkel-type oxygen vacancy (FOV) and intrinsic oxygen vacancy (IOV), were clarified. Furthermore, the transform relation between FOV and IOV was found. Those two types of O-vacancies favored to the migration and transformation of active species, enhancing further the oxidation-reduction cycle and the catalytic activity for soot oxidation. In addition, Mn4+/Mn3+(Mn2+), Olatt, Osur and Ce4+/Ce3+ were believed to play important roles in soot oxidation. Finally, evolution of O-vacancies was proposed, which is of significance for soot catalytic oxidation.

Manganese-cerium oxide (MnOx-CeO2) catalysts supported by titanium-bearing blast furnace slag for selective catalytic reduction of nitric oxide with ammonia at low temperature

ABSTRACTA series of manganese-cerium oxide (MnOx-CeO2) catalysts supported by Ti-bearing blast furnace slag were prepared by wet impregnation and used for low-temperature selective catalytic reduction (SCR) of NO with NH3. The slag-based catalyst exhibited high nitrogen oxide removal (deNOx) activity and wide effective temperature range. Under the condition of NO = 500 ppm, NH3 = 500 ppm, O2 = 7–8 vol%, and total flow rate = 1600 mL/min, the Mn-Ce/Slag catalyst exhibited a NO conversion higher than 95% in the range of 180–260 °C. The activity of Mn/Slag catalysts was greatly enhanced with the addition of CeO2. The results indicated that Ti-bearing blast furnace slag had suitable phase composition as good support of SCR catalyst.Implications: Ti-bearing blast furnace slag is a kind of industrial waste in China. Much slag was underused and piling up, which could cause many environmental issues, such as enormous waste of titanium and groundwater and soil contamination by heavy metals in leachates. The utilization of slag as the support of SCR catalyst will not only make use of solid waste but also cut down the NOx emitted from power plant.

A CeO2-MnOx core-shell catalyst for low-temperature NH3-SCR of NO

A CeO2-MnOx catalyst with a core-shell structure was prepared by a chemical deposition method and tested towards the low-temperature selective catalytic reduction of NO by NH3. The catalyst with a molar ratio of CeO2/MnOx=0.6, exhibited a relatively high conversion values of NO in the 110–220°C range and good resistance to SO2 and H2O at the space velocity of 40,000h−1. The excellent catalytic performance could be attributed to the presence of high crystalline α-MnO2, as well as high concentrations of Mn4+ and Ce3+. Notably, CeO2-MnOx catalyst showed relatively good resistance to SO2 due to its core-shell structure.

Plasma-catalytic removal of toluene over CeO2-MnOx catalysts in an atmosphere dielectric barrier discharge

The destruction of toluene in dielectric barrier discharge (DBD) reactor coupled with a series of CeO2-MnOx catalysts was investigated in this work. All catalysts prepared by sol-gel method were characterized in detail using N2 adsorption, XRD, TEM, H2-TPR, Raman, O2-TPD and XPS analysis technology. The effect of input power, molar ratio of Ce/Mn, gas flow rate and initial concentration of toluene on the removal efficiency of toluene and CO2 selectivity has been investigated. The plasma-catalytic system improves the removal efficiency and CO2 selectivity significantly compared with the plasma alone system. The catalytic performance of CeO2-MnOx catalysts are higher than pure CeO2 and MnOx, which might be contributed to the synergistic effect between CeO2 and MnOx (such as Mn3+/Mn2+ and Ce4+/Ce3+ redox couples) in the CeO2-MnOx catalysts. The highest removal efficiency of toluene (95.94%) and CO2 selectivity (90.73%) are acquired with Ce1Mn1 catalyst. The reason may be that: (1) the increased BET specific surface area and the decreased grain size can be obtained from CeO2-MnOx catalyst. (2) The formation of CeO2-MnOx solid solution resulted from the incorporation of Mn cations into the CeO2 lattice could generate more oxygen vacancies and improve the mobility of oxygen. On the basis of the analytic results of GC/MS, the pathways of toluene destruction in the plasma-catalysis system are discussed.

Promotional mechanisms of activity and SO2 tolerance of Co- or Ni-doped MnOx-CeO2 catalysts for SCR of NOx with NH3 at low temperature

Metals doped MnOx-CeO2 catalysts with a molar ratio of M:Mn:Ce=1:4:5 (M=Cu, Co, Cr, Ni, Fe, Sn, Mg) were prepared by a co-precipitation method for low-temperature selective catalytic reduction of NOx with NH3 (NOx-NH3-SCR). The results showed Co/Ni doped catalysts had high SCR performance and good tolerance to SO2. Solid solutions with higher surface area and smaller pore size were obtained over Co/Ni doped catalysts. XPS analysis presented Co/Ni doped catalysts had more chemisorbed oxygen, more active sites (Mn3+ and Mn4+) and higher ratio of Ce4+/Ce3+, which were favorable for improving the SCR activity accompanied by the fast-SCR due to a high activity for NO oxidation to NO2. The in-situ DRIFTS revealed NOx-absorbed species over Co/Ni doped catalysts tended to bidentate nitrate without influences by SO2, which further transformed to monodentate nitrate producing new Brønsted acid sites for adsorbing NH4+. While that over MnOx-CeO2 catalyst was more in favor of monodentate nitrite species, significantly inhibiting by SO2. Thermogravimetric analysis (TGA) presented the formations of ammonium sulfate and metal sulfate species over Co/Ni doped catalysts were markedly inhibited. The possible SCR reaction pathways and mechanisms of SO2 tolerance were proposed over Co/Ni doped catalysts contrasted to MnOx-CeO2 catalyst.

Influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for NH3-SCR at low temperature

This work examines the influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for selective catalytic reduction of NO by NH3 (NH3-SCR) at low temperature. Five different methods, namely, mechanical mixing, impregnation, hydrothermal treatment, co-precipitation, and a sol-gel technique, were used to synthesize MnOx-CeO2 catalysts. The catalysts were characterized in detail, and an NH3-SCR model reaction was chosen to evaluate the catalytic performance. The results showed that the preparation methods affected the catalytic performance in the order: hydrothermal treatment > sol-gel > co-precipitation > impregnation > mechanical mixing. This order correlated with the surface Ce3+ and Mn4+ content, oxygen vacancies and surface adsorbed oxygen species concentration, and the amount of acidic sites and acidic strength. This trend is related to redox interactions between MnOx and CeO2. The catalyst formed by a hydrothermal treatment exhibited excellent physicochemical properties, optimal catalytic performance, and good H2O resistance in NH3-SCR reaction. This was attributed to incorporation of Mnn+ into the CeO2 lattice to form a uniform ceria-based solid solution (containing Mn-O-Ce structures). Strengthening of the electronic interactions between MnOx and CeO2, driven by the high-temperature and high-pressure conditions during the hydrothermal treatment also improved the catalyst characteristics. Thus, the hydrothermal treatment method is an efficient and environment-friendly route to synthesizing low-temperature denitrification (deNOx) catalysts.

Catalytic Oxidation of NO over MnOx-CeO₂ and MnOx-TiO₂ Catalysts.

A series of MnOx–CeO2 and MnOx–TiO2 catalysts were prepared by a homogeneous precipitation method and their catalytic activities for the NO oxidation in the absence or presence of SO2 were evaluated. Results show that the optimal molar ratio of Mn/Ce and Mn/Ti are 0.7 and 0.5, respectively. The MnOx–CeO2 catalyst exhibits higher catalytic activity and better resistance to SO2 poisoning than the MnOx–TiO2 catalyst. On the basis of Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and scanning transmission electron microscope with mapping (STEM-mapping) analyses, it is seen that the MnOx–CeO2 catalyst possesses higher BET surface area and better dispersion of MnOx over the catalyst than MnOx–TiO2 catalyst. X-ray photoelectron spectroscopy (XPS) measurements reveal that MnOx–CeO2 catalyst provides the abundance of Mn3+ and more surface adsorbed oxygen, and SO2 might be preferentially adsorbed to the surface of CeO2 to form sulfate species, which provides a protection of MnOx active sites from being poisoned. In contrast, MnOx active sites over the MnOx–TiO2 catalyst are easily and quickly sulfated, leading to rapid deactivation of the catalyst for NO oxidation. Furthermore, temperature programmed desorption with NO and O2 (NO + O2-TPD) and in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTS) characterizations results show that the MnOx–CeO2 catalyst displays much stronger ability to adsorb NOx than the MnOx–TiO2 catalyst, especially after SO2 poisoning.

MnOx-CeO2 shell-in-shell microspheres for NH3-SCR de-NOx at low temperature

A novel MnOx-CeO2 catalyst with shell-in-shell microsphere structure was successfully prepared by a one-step hydrothermal method for the first time. The obtained catalyst was characterized by SEM, TEM, XRD, N2 adsorption and desorption, XPS, H2-TPR and NH3-TPD in detail, and its catalytic activity was investigated by selective catalytic reduction of NOx with NH3. The results showed that the MnOx-CeO2 microsphere catalyst presented high catalytic activity at low temperatures (lower than 210°C), which was much superior to the counterpart MnOx-CeO2 catalyst without shell-in-shell microsphere structure prepared by co-precipitation method.

Dynamic active sites over binary oxide catalysts: In situ/operando spectroscopic study of low-temperature CO oxidation over MnOx-CeO2 catalysts

The determination of the dynamic active sites over binary oxide catalysts is of great challenge in heterogeneous catalysis. In this work, the origin of active sites toward low-temperature CO oxidation (<200°C) were thoroughly studied using MnOx-CeO2 composite oxide catalysts with different Mn/Ce molar ratios, synthesized by a redox co-precipitation method. The optimum Mn1Ce1 catalyst (T100=190°C), which showed excellent activity, has found to be composed of three phases: (1) CeO2; (2) amorphous MnOx; (3) MnOx-CeO2 solid solution (active sites). With the combination of kinetics and characterization results, including Temperature-Programmed-Desorption/Reduction (TPD/TPR), operando Raman spectroscopy and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS), the dynamic structures of catalysts were rationalized with the identification of the interface of MnOx and CeO2. The mechanism for CO oxidation over MnOx-CeO2 in the temperature range 100–190°C were proposed that the direct and the formate routes were followed at T<130°C, and the carbonate route became dominant at T>130°C. Notably, the Mars-van Krevelen mechanism was proceeded in the whole temperature range. We speculate that cheap binary oxides will substitute for noble metal as catalysts for the removal of CO and other toxic gases, especially operating under mild conditions.

Catalytic Removal of Volatile Organic Compounds

The degradation of air quality by the release of volatile organic compounds (VOCs) into the air particularly harms human health and our environment. [...]

Catalytic oxidation of NO by O2 over CeO2–MnOx: SO2 poisoning mechanism

The catalytic oxidation of NO by O2 was performed over a series of CeO2–MnOx catalysts with different molar ratios of Mn/Ce, which were prepared by the sol–gel method.

Kinetic and activity study of CO oxidation over CuO–MnOx–CeO2 catalysts

Three CuO–MnOx–CeO2 catalysts with different impregnation sequences (i.e. MnOx/CuO/CeO2, CuO/MnOx/CeO2 and CuO–MnOx/CeO2) were prepared and the effects of impregnation sequences on the structures and catalytic behaviors of these catalysts were investigated. It was found that the MnOx/CuO/CeO2 possessed the largest amount of oxygen vacancies but the lowest reducibility; the CuO/MnOx/CeO2 had the largest Cu+ contents but the lowest amount of oxygen vacancies; the CuO–MnOx/CeO2 catalyst had the highest CuO dispersion and the best reducibility, along with moderate amount of oxygen vacancies and Cu+ contents on the surface. The kinetic studies revealed that the apparent activation energies of CO oxidation over the CuO–MnOx/CeO2, MnOx/CuO/CeO2 and CuO/MnOx/CeO2 were 49.5, 51.8 and 73.8 kJ mol−1, in order, and the activities followed an order of CuO–MnOx/CeO2 > MnOx/CuO/CeO2 > CuO/MnOx/CeO2. The highest performance of the CuO–MnOx/CeO2 was ascribed to the highly dispersed CuO species and the mobility of lattice oxygen.

Catalytic performance of manganese doped CuO–CeO2 catalysts for selective oxidation of CO in hydrogen-rich gas

Manganese doped CuO–CeO2 catalysts (CuO–MnOx–CeO2) with Mn/Cu molar ratio of 1:5 and variable calcination temperatures were prepared by a hydrothermal method and used for selective oxidation of CO in hydrogen-rich gas, characterized by various techniques. An appropriate calcination temperature shows an essential stimulative effect on CuO–MnOx–CeO2 catalysts for the CO-PROX. The catalyst calcined at 500°C displays the highest low-temperature catalytic activity (T50%=74°C) and the broadest operating temperature “window” (CO conversion >99.0%, 110–140°C). XRD and UV-Raman results reveal that an appropriate calcination temperature can promote the formation of Cu–Mn–Ce–O ternary oxide solid solution, adjust the degree of crystallinity of CeO2 and enhance the formation of oxygen vacancies. H2-TPR and XPS demonstrate that calcining at 500°C improves the active reducing species in both bulk and surface of the catalyst. In situ DRIFTS suggests that Cu+ species which possess strong interaction with ceria also can be facilitated with a suitable calcination temperature.

Effect of Y on improving the thermal stability of MnOx-CeO2 catalysts for diesel soot oxidation

A series of MnOx-CeO2-Y2O3 catalysts with different Y loadings (0, 1, 3, 6, and 10 wt%) were prepared by a co-precipitation method and investigated for NOx-assisted soot oxidation. The thermal stabilities of these catalysts were evaluated by treating them at 800 °C for 12 h under dry air flow. The catalysts were characterized by X-ray diffraction, N2 adsorption-desorption, Raman spectroscopy, H2 temperature-programmed reduction, oxygen storage capacity, NO temperature-programmed oxidation, X-ray photoelectron spectroscopy, and soot temperature-programmed oxidation. The addition of Y led to decreased BET surface areas and poor low-temperature reduction abilities and oxygen storage capacities, which affected NO and soot oxidation activities. However, after aging, the doping of Y had effectively enhanced the stability of the catalytic activities for NO and soot oxidations, where the addition of 6%–10% Y achieved the optimum result because the maximal soot oxidation rate temperature was increased by only 34–35 °C. Additionally, the catalytic activity and deactivation of MnOx-CeO2-containing catalysts were closely related to the presence of Mn4+ and oxygen species on the surface.

MnOx-CeO2 catalysts synthesized by solution combustion synthesis for the low-temperature NH3-SCR

MnOx-CeO2 catalysts with different compositions have been investigated as catalysts for low temperature SCR by preparing a series of samples by the solution combustion synthesis (SCS) method. Variable amounts of two different organic fuels (glycine and citric acid) with respect to the stoichiometry of synthesis were investigated. The samples were characterized by XRD, BET, H2-TPR, XPS, FESEM and their catalytic activity tested for NOx abatement in the range of temperature of interest (120–350°C). By varying the synthesis parameters it was possible to obtain catalysts different in terms of structure, morphology, specific surface area, Mn average oxidation state as well as superficial content of Mn, Ce and O atoms. These features were correlated with the resulting catalytic performances in the SCR reaction and compared to pure MnOx phases obtained through the same synthesis method. All the developed catalysts were considerably active in the temperature range investigated and could be considered suitable for the low-temperature SCR process. The sample with the higher content of manganese oxide obtained with glycine in below-stoichiometric amount showed the best performance in terms of NOx conversion and N2 yield, likely due to the high reducibility as well as the presence of the Mn3O4 phase.