MnOx CeO2 Mixed Oxide Research Articles

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

The cerium-based catalysts were investigated for the catalytic combustion of trichloroethylene (TCE) and exhibit a surprising catalytic activity. MnOx was doped into CeO2 by a citric acid (CA) sol-gel method, and the effect of Mn content on the physicochemical properties and catalytic activities of MnOx-CeO2 mixed oxides was investigated systemically. The introduction of MnOx into CeO2 can evidently improve the catalytic activity and reaction stability of the CeO2 catalyst, and the catalytic activity can be maintained for more than 100 h at 250 °C under air atmosphere. Moreover, a possible reaction mechanism and a catalytic cycle for the catalytic combustion of TCE over the MnOx-CeO2 mixed oxide catalysts are proposed.

Dual-Site Cooperation for High Benzyl Alcohol Oxidation Activity of MnO2 in Biphasic MnOx–CeO2 Catalyst Using Aerial O2 in the Vapor Phase

Selective oxidation of benzyl alcohol to benzaldehyde in the vapor phase has drawn growing interest recently. In this work, MnOx–CeO2 mixed oxide compositions have been prepared by a coprecipitation method and tested for their oxidation activities of benzyl alcohol to benzaldehyde in the vapor phase. Detailed structural analyses have indicated that MnOx–CeO2 catalysts contain two phases, namely, α-MnO2 and fluorite CeO2 phases. The benzyl alcohol oxidation activity of pure MnO2 is more than 7 times higher compared to that of CeO2, indicating a much higher intrinsic oxidation ability of the MnO2 phase. Further, enhancement of the benzyl alcohol oxidation rate over MnO2 in 10%MnOx–CeO2 catalyst by 13 times is observed in relation to pure MnO2. The role of CeO2 in the MnOx–CeO2 catalyst has also been investigated, which indicates that the oxidation activity is almost independent of CeO2. However, stronger adsorption of benzyl alcohol over the MnOx–CeO2 catalysts compared to that of MnO2 points to the role of CeO2 in adsorption. Thus, both the CeO2 and the MnO2 components have different roles in the catalytic process—adsorption of benzyl alcohol on the CeO2 surface and its oxidation on MnO2 at the interface. The cooperation between the two sites toward oxidation could happen due to jumping of adsorbed benzyl alcohol from the surface of the CeO2 phase to the closest Mn4+ site in the MnO2 phase at the contact surface with MnO2 by thermal motion.

Mesoporous MnOx-CeO2 composites for NH3-SCR: the effect of preparation methods and a third dopant.

In this study, different preparation methods including an oxalate route, a nano-casting strategy and a traditional co-precipitation route were applied to obtain MnOx–CeO2 mixed oxides for selective catalytic reduction (SCR) of NO with NH3. The catalyst prepared from the oxalate route showed improved performance for NOx conversion and SO2 + H2O durability. To further improve the SO2 and H2O resistance of catalysts, ternary oxides were prepared from the oxalate route. The catalysts were studied by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), NH3 temperature-programmed desorption (NH3-TPD), SO2 temperature-programmed desorption (SO2-TPD), and in situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS). The nickel–manganese–cerium ternary oxide showed the best SO2 and H2O durability. The reason can be ascribed to its smaller pores, amorphous structure, and moderate amount of surface Mn3+/oxygen species, which could decrease chemical adsorption of SO2.

MnOx–CeO2 mixed oxides for diesel soot oxidation: a review

Catalytic diesel particulate filter is an efficient equipment for particulate matters control downstream of a diesel engine. MnOx–CeO2 mixed oxides are a potential alternative of the high cost commercial platinum catalyst. However, the poor thermal stability and sulfur resistance are two issues challenging its application. Additionally, the mechanism of synergistic effect between manganese oxide and ceria still remained debatable. Here, the catalysis mechanisms in the presence/absence of NOx were reviewed. The origins of synergistic effects were discussed based on whether the solid solutions formed or not. Meanwhile, different kinds of metal modifications were summarized and their promotion influences on the thermal stability and sulfur resistance were clarified in detail.

MnOx-CeO2 mixed oxides as the catalyst for NO-assisted soot oxidation: The key role of NO adsorption/desorption on catalytic activity

The role of NO adsorption/desorption on soot oxidation activity of MnOx-CeO2 mixed oxides was investigated in this work. NO adsorption/desorption on MnOx-CeO2 results in a visible promotion of the low-temperature reactivity. Physico-chemical characterization reveals that the formation of NO adsorbed species, mainly nitrates, leads to changes in the surface Ce/Mn states and in an increase of oxygen vacancies. Moreover, TPD-MS indicates that the decomposition of surface nitrates accelerates the desorption of oxygen in the catalyst. Soot-TPR-MS confirms that the oxygen species desorbed from nitrates decomposition play an important role during soot oxidation. The isothermal experiments evidence that NO adsorption/desorption enhances soot oxidation rate, with active oxygen playing a key role in a cooperative C + NO2 + O2 reaction with the catalyst.

Intrinsic Activity of MnOx-CeO2 Catalysts in Ethanol Oxidation

MnOx-CeO2 mixed oxides are considered efficient oxidation catalysts superior to the corresponding single oxides. Although these oxides have been the subject of numerous studies, their fundamental performance indicators, such as turnover frequency (TOF) or specific activity, are scarcely reported. The purpose of the present work is to investigate the effect of catalyst composition on the concentration of active sites and intrinsic activity in ethanol oxidation by the employment of temperature-programmed desorption and oxidation of isotopically-labelled ethanol, 12CH313CH2OH. The transformation pathways of preadsorbed ethanol in the absence of gaseous oxygen refer to dehydrogenation to acetaldehyde followed by its dissociation combined with oxidation by lattice oxygen. In the presence of gaseous oxygen, lattice oxygen is rapidly restored and the main products are acetaldehyde, CO2, and water. CO2 forms less easily on mixed oxides than on pure MnOx. The TOF of ethanol oxidation has been calculated assuming that the amount of adsorbed ethanol and CO2 produced during temperature-programmed oxidation (TPO) is a reliable indicator of the concentration of the active sites.

Hollow MnOx-CeO2 mixed oxides as highly efficient catalysts in NO oxidation

A series of hollow MnOx-CeO2 mixed oxide catalysts were prepared by employing carbon as hard templates. The activity tests reveal that the catalytic performance of MnOx-CeO2(CS) samples are more active than that of MnOx-CeO2(AC) sample, indicating carbon sphere (CS) as a more effective carbon template. Among these samples, MnOx-CeO2(CS1) sample exhibits the best activity, for which a maximal NO conversion (∼82%) could be attained at 220°C when GHSV=120,000h−1. In addition, the NO conversion could be further improved with lower NO initial concentration or higher O2 content. Under the simulated flue gas condition, the activity of MnOx-CeO2(CS1) sample is barely affected by the presence of CO2, but the influence of H2O and SO2 couldn’t be neglected. The characterization result proves that the core-shell hollow structure has been successfully synthesized over MnOx-CeO2 binary oxides. For the hollow MnOx-CeO2 sample, the mesopore structures are formed and the MnOx oxides are highly dispersed on CeO2 support. Among these samples, MnOx-CeO2(CS1) sample possesses the largest SBET and Vtotal values. The result of H2-TPR and oxygen pulse measurement indicates that MnOx-CeO2(CS) samples, especially MnOx-CeO2(CS1) exhibits the best redox property. Moreover, more amount of high valence Mn as redox active sites and more oxygen vacancies exist in MnOx-CeO2(CS1) proved by XPS technique. Overall, these properties may contribute to the excellent activity.

Selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over MnOx–CeO2 composite catalysts

Non-noble metal catalysts, MnOx–CeO2 mixed oxides, were prepared by a co-precipitation method and used in the direct aerobic oxidation of HMF to FDCA, showing excellent catalystic activity and stability.

Effects of WO3 doping on stability and N2O escape of MnOx–CeO2 mixed oxides as a low-temperature SCR catalyst

MnOx–WOx–CeO2 catalysts synthesized using a sol–gel method were investigated for the low-temperature NH3-SCR reaction. Among them, W0.1Mn0.4Ce0.5 mixed oxides exhibited above 80% NOx conversion from 140 to 300°C. In addition, this catalyst exhibited high stability and CO2 tolerance in a 50h activity test at 150°C. Substantially reduced N2O production and enhanced N2 selectivity were achieved by WO3 doping, which was due to the weakened reducibility and increased number of acid sites. The decreased SO2 oxidation activity as well as the reduced formation of ammonium and manganese sulfates resulted in a high SO2 resistance of this catalyst.

NO oxidation to NO2 over manganese-cerium mixed oxides

There is a recognized need for lowering greenhouse gas emissions from mobile sources in order to address concerns regarding global climate change. Diesel engines offer superior fuel efficiency and greenhouse gas reduction potential; however, the highly loaded Pt for converting NO to NO2 in diesel aftertreatment catalysts is one of the major hurdles in the broad implementation of highly fuel efficient diesel engine vehicles. Here, we reported that manganese-cerium mixed oxides coated on monolith substrates demonstrated a superior NO oxidation to NO2 activity, showing a promise for PGM thriftting for diesel or lean burn engines emission control catalysts. The mixed oxide (MnOx-CeO2) shows a much higher NO oxidation to NO2 rate and is nearly 3 and 10 times active of that MnOx and CeO2 at 200°C, respectively. Significantly lower reduction temperature of the mixed oxide (MnOx-CeO2) compared with MnOx and higher Mn4+ to Mn3+ reduction peak intensity suggest that ceria helps stabilize higher oxidation state of Mn. The MnOx-CeO2 mixed oxide shows higher activity for NO oxidation due to the enhanced rate of nitrites decomposition to NO2. The structure of the MnOx-CeO2 mixed oxide maintained during a cyclic treatment of oxidative/reductive gas conditions at 650°C. Furthermore, the incorporation of ZrO2 into MnOx-CeO2 mixed oxide not only increased NO oxidation activity but also the sulfur tolerance. About 70% of the activity of the MnOx-CeO2-ZrO2 was recovered via regeneration in a H2 rich stream.

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.

Formation of BaMnO3 in Ba/MnOx–CeO2 catalyst upon the hydrothermal ageing and its effects on oxide sintering and soot oxidation activity

MnOx–CeO2 mixed oxides were synthesized using a sol–gel method and the barium-loaded catalyst was prepared by a wet impregnation. Both catalysts were hydrothermally aged at 800°C for 10h. The fresh and aged catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), high-resolution transmission electron microscopy (HRTEM), H2 temperature-programmed reduction (H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). Compared with MnOx–CeO2, the Ba-modified mixed oxides show a much higher hydrothermal stability. Barium exists in the form of orthorhombic carbonate and cubic nitrate on the fresh sample. BaMnO3 nanoparticles are identified to form at the interface of CeO2 crystallites upon the hydrothermal ageing. The formation of the perovskites prohibits the sintering of ceria significantly, and consequently maintains the redox property of the aged mixed oxides to a great degree. Additionally, the nanoscaled perovskites themselves also contribute to the catalytic activity of the aged oxides catalyst.

A strategy for improving deactivation of catalytic combustion at low temperature via synergistic photocatalysis

For practical catalytic combustion in heterogeneous catalysis, non-noble metal oxides with excellent low temperature activity as well as performance sustainability still remain challenging. Herein, semiconductor photocatalysis was introduced to address the problems. With greatly enhanced spectral response and low temperature reducibility, MnOx–CeO2 mixed oxide achieved convincing photo/thermo-catalysis synergy in formaldehyde abatement. This synergy is promising for utilizing infrared (IR) energy from sunlight and idle heat from daily illuminants. Moreover, the exhausted material after prolonged dark reaction presented inspiring self-remediation under in situ light irradiation. Catalytic cycles were thoroughly studied, unveiling the underlying mechanism based on photo-induced remediation. Roles of high valence metal ions, reaction intermediates and surface hydroxyls (OH) have been in-depth and newly cognized. Given the day and night alternations in nature, our results may be instructive in designing reliable catalysts for efficient and sustainable catalytic combustion.

Preparation of MnOx–CeO2–Al2O3 mixed oxides for NOx-assisted soot oxidation: Activity, structure and thermal stability

Three kinds of MnOx–CeO2–Al2O3 mixed oxides were prepared by introducing Al2O3 to MnOx–CeO2 at different stages of a sol–gel method. MnCeAl and MnCe/Al were received by adding aluminum nitrate and alumina powders to the Mn–Ce precursor solution, respectively, while MnCe+Al was obtained by mixing the sol–gel-synthesized MnOx–CeO2 and Al2O3 powders mechanically. The catalysts were calcined in air at 800°C for 20h to obtain the thermally aged samples. The catalysts were characterized by soot and NO temperature-programmed oxidation (soot-TPO and NO-TPO), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), extended X-ray absorption fine structure (EXAFS), energy dispersive spectroscopy (EDS) and H2 temperature-programmed reduction (TPR). The introduction of Al2O3 degrades the soot oxidation activity of the fresh catalysts compared with MnOx–CeO2 mixed oxides, but it helps to maintain relatively high catalyst surface area and high MnOx dispersion after the rigorous aging treatment. Among the catalysts prepared, MnCe/Al exhibits the highest thermal stability with a small increase of Tm by 17°C for soot oxidation after aging. The active surface area, which refers to the exposure area of MnOx on the catalyst, is suggested to determine the NO oxidation and soot oxidation activities of the aged catalysts.

Combined promoting effects of platinum and MnOx–CeO2 supported on alumina on NOx-assisted soot oxidation: Thermal stability and sulfur resistance

MnOx–CeO2 mixed oxides were introduced into Pt/Al2O3 catalyst to reduce the amount of platinum and improve the catalytic activity for soot oxidation in presence of NOx. The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), CO chemisorption, thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), H2 temperature-programmed reduction (H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). Due to the superior redox property, the introduction of MnOx–CeO2 mixed oxides effectively enhanced the catalytic activities of Pt/Al2O3 catalyst for NO oxidation and hereby for soot oxidation in NO+O2. The Pt/MnOx–CeO2/Al2O3 catalyst also exhibited high activities after the thermal ageing and sulfur poisoning, which are related to the intrinsic characteristics of MnOx–CeO2/Al2O3 and Pt as well as their synergistic effects. The deactivation mechanisms of different catalysts (Pt/Al2O3,MnOx–CeO2/Al2O3 and Pt/MnOx–CeO2/Al2O3) were discussed.

Textural–structural properties and soot oxidation activity of MnO -CeO2 mixed oxides

Abstract A series of MnOx-CeO2 mixed oxides were prepared using a sol-gel method followed by calcinations at various temperatures. The soot oxidation activities were measured in different NO concentrations under loose contact conditions. A quasi-parabolic curve is observed for the apparent dependency of activity on the catalyst surface area. As the surface area is below 8 m2/g, the soot oxidation is limited by the availability of NO2 due to the separation and sintering of the mixed oxides. The rate-determining step turns to be NO2 reaction with soot for the high-surface-area catalysts.

Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx–CeO2 mixed oxides

MnOx–CeO2, MnOx and CeO2 catalysts were synthesized by the sol–gel method. The structural, redox and adsorption properties of the individual and mixed oxides were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), CO temperature-programmed reduction (TPR), NO temperature-programmed oxidation (TPO), NOx temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The TPO tests were performed with the soot–catalyst mixture under loose contact conditions to evaluate the catalytic activity of the oxide catalysts for soot oxidation. The MnOx–CeO2 mixed oxide catalyst presents the lowest soot oxidation temperature among the catalysts investigated in the presence of NO and O2. The synergetic effect between manganese oxide and ceria restrains the growth of oxide crystallites, increases the specific surface area and improves the low-temperature redox property. Especially, the activity of MnOx–CeO2 mixed oxides for NO oxidation and its capacity for NO2 storage in the form of surface nitrates are greatly enhanced. Not only the NO2 released from decomposition of surface nitrates but also that formed by catalytic oxidation of gaseous or adsorbed NO on the catalyst is confirmed important for soot oxidation. It is found by the in situ DRIFTS tests with the soot–catalyst mixture that the generation of surface oxygen complexes (SOCs), such as carboxylic anhydrides, lactones, quinine, ceto-enol groups, ethers and phenols, occurs at about 100°C lower temperature with exposure to NO than in the absence of NO, which is an important step for soot oxidation.

Catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

A series of MnOx–CeO2 mixed oxide catalysts with different compositions prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of volatile organic compounds of chlorinated aromatics. MnOx–CeO2 catalysts with different ratios of Mn/Ce+Mn were found to possess high catalytic activity in the catalytic combustion of CB, and MnOx(0.86)–CeO2 was identified as the most active catalyst, on which the temperature of complete combustion of CB was 254°C. Effects of systematic variation of reaction conditions, including space velocity and inlet CB concentration on the catalytic combustion of CB were investigated. Additionally, the stability and deactivation of MnOx–CeO2 catalysts were studied by various characterization methods and other assistant experiments. MnOx–CeO2 catalysts with high Mn/Ce+Mn ratios present a stable high activity, which is related to their high ability to remove the adsorbed Cl species and a large amount of active surface oxygen.

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.

Promoting effect of copper on the catalytic activity of MnOx–CeO2 mixed oxide for complete oxidation of benzene

Abstract The promoting effect of copper on the catalytic activity of MnO x –CeO 2 for complete oxidation of benzene was investigated. The results showed that the addition of copper could significantly improve the catalytic activity of the mixed oxide and the complete conversion of benzene was achieved at 523 K. Structural analysis by X-ray powder diffraction (XRD) and H 2 -temperature-programmed reduction (H 2 -TPR) measurements indicated that the copper species was highly dispersed on the surface of the mixed oxide, which greatly promoted the redox properties of the catalyst leading to the at low temperatures. X-ray photoelectron spectra (XPS) and FTIR characterizations of the Cu/MnO x –CeO 2 catalyst further revealed that the addition of copper significantly increased the generation of surface O β species and also created more active sites for benzene adsorption.