MnOx Catalysts Research Articles

MnOx Catalysts Modified By Nonthermal Plasma For NO Catalytic Oxidation

Nonthermal plasma modification (NTPM) has been applied to MnOx catalysts for the catalytic oxidation of nitrogen oxide (NO) at low temperature (50–250 °C), and much higher NO conversion was obtained over the nonthermal plasma-treated catalysts. These modified catalysts were characterized in different ways—scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), and Fourier transform infrared (FT-IR)—and the results can help us to understand the effect of the plasma treatment. The results showed that the surface active sites of these catalysts were enhanced with the treatment of plasma, which improved the dispersion of catalysts. In addition, after NTPM, a considerable amount of deposition with limited depths appeared on the MnOx surface, the average pore size was slightly enlarged, and the pore size distributions of the MnOx became wider. Also, XPS results revealed that the NTPM could increase the oxygen, nitrogen, and kalium functi...

Oxidation of formaldehyde over Pd/Beta catalyst

The role of catalyst support in the complete oxidation of HCHO has been investigated over the metal oxide- and zeolite-supported Pd catalysts. The 0.25Pd/Beta catalyst exhibited the highest intrinsic activity for the complete oxidation of HCHO among other monometallic catalysts, mainly due to the high HCHO adsorption capacity of Beta zeolite support and the fast surface reaction for HCHO oxidation on Pd as determined by TPD and TPSR studies, respectively. Formate and dioxymethylene have been identified as major reaction intermediates by an in situ FTIR study. These reaction intermediates are the most abundant surface species on the Pd/Beta catalyst during HCHO oxidation, so their oxidation is the rate determining step of the complete oxidation of HCHO. When Mn was incorporated into the 0.25Pd/Beta catalyst, the HCHO oxidation activity was further improved, especially in the low temperature region; HCHO was completely oxidized to CO2 over the 0.25Pd/20Mn/Beta catalyst at 40°C and 50,000h−1. The enhanced oxidation activity of the 0.25Pd/20Mn/Beta catalyst could be understood by a kinetic synergism between MnOx and Pd for the oxidation of HCHO. Oxygen adsorbs on and diffuses through MnOx along an oxygen concentration gradient to Pd, while Pd consumes oxygen via HCHO oxidation to create the oxygen concentration gradient in MnOx on the surface of the Beta zeolite. A possible reaction pathway has been proposed that could elucidate the enhanced HCHO oxidation activity over the bimetallic 0.25Pd/20Mn/Beta catalyst by the extended Mars–van Krevelen mechanism involving direct and indirect surface reactions over the bimetallic Pd–MnOx catalyst system.

Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting

AbstractIncipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnOx/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as‐prepared state of a 6.5 wt. % MnOx/CNT sample could be comprehensively characterized by comparison to an unsupported MnOx reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as‐prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnOx compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post‐treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5 wt. % MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state‐of‐art MnOx catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions.

Supported manganese dioxide catalyst for seawater flue gas desulfurization application

In seawater flue gas desulfurization (FGD) process, ceramic supported MnOx catalysts were developed to improve SO2 removal efficiency of the scrubber by increasing S(IV) oxidation rate at the low pH values. These supported MnOx catalysts were prepared by impregnation–precipitation method, calcined at different temperatures, and exposed to simulated flue gases to investigate the catalytic oxidation of sulfur dioxide in seawater buffering system. Compared with “blank” ceramic, the most active MnOx/ceramic catalyst, 623MnOx/ceramic, that was prepared by doping (NH4)2CO3 as the precipitant and calcined at 623K, exhibited up to 17% higher SO2 conversion at the GHSV 3000h−1 in the industrial feasible temperature range of 318–358K. XPS spectra revealed manganese was in the state of oxidation +4 on the surface of 623MnOx/ceramic. XRD patterns and SEM-EDS spectra indicated that the high activity of 623MnOx/ceramic for S(IV) oxidation at low temperatures could be attributed to oxygen activation and transfer ability of highly dispersed MnO2 on ceramic support. H2-TPR profiles showed the low temperature reduction peak was shifted to higher temperature when the calcination temperature increased from 573K to 723K, and low temperature reducible manganese species were related to catalytic activity for S (IV) oxidation. The good performance of MnO2/ceramic catalyst on the SO2 removal appeared to be promising in seawater FGD application when high sulfur coals were burned.

Ozone Catalytic Oxidation of HCHO in Air over MnOx at Room Temperature

The complete oxidation of HCHO in air with O3 over MnOx catalysts at room temperature was studied. The MnOx catalysts were prepared by a redox method. The catalysts showed amorphous patterns in X-ray diffraction characterization. Formaldehyde in simulated air containing 137 mg/m3 HCHO with 56% relative humidity (RH, 25 °C) was totally oxidized to CO2 by 642 mg/m3 O3 over the MnOx catalysts at GHSV (gas hourly space velocity) = 2 × 105 h−1. The formaldehyde conversion and CO2 selectivity were maintained at ∼ 100% during 150 min time-on-stream. The effect of the molar ratio of O3 to HCHO was also investigated. At a O3 to HCHO molar ratio of 2:3, which was significantly lower than the stoichiometric ratio, a CO2 selectivity of 100% was still achieved. No byproduct was detected during HCHO oxidation with O3 over the MnOx catalysts using an online Fourier transform infrared spectrometer. 采用氧化还原法制备了 MnOx 催化剂, Ｘ射线衍射结果表明其主要为无定形结构. 在甲醛和臭氧浓度分别为 137 和 642 mg/m3, 相对湿度为 56% (25 oC), GHSV 为 2 × 105 h条件下, MnOx 催化剂上 O3 可将甲醛全部氧化为 CO2, 反应 150 min 内甲醛转化率和 CO2 选择性一直保持在–100%. 另外, 当臭氧与甲醛的摩尔比约为 2:3, 即显著低于化学计量比时, CO2 选择性仍可达–100%. 采用傅里叶变换红外光谱仪在线分析了甲醛氧化反应产物, 未检测到任何副产物, 从而确认了 MnOx 催化剂上 O3 对甲醛的完全氧化.

Low overpotential water oxidation to hydrogen peroxide on a MnOx catalyst

There is a current intense interest in water electrolysis as a means of generating or storing energy as hydrogen. The water oxidation step typically requires high overpotentials, creating a significant energy loss in the process. In this work we report for the first time water oxidation over a MnOx electrocatalyst at low overpotentials (∼150 mV) in butyl ammonium bisulfate electrolytes (aqueous 0.4 M and hydrated ionic liquid 50/50). The catalyst–electrolyte combination produces currents of the order of 1 mA cm−2 at 150 mV overpotential. A similar experiment in an inorganic electrolyte produces only negligible currents on this catalyst. The product is shown by permanganate oxidation to be predominantly hydrogen peroxide dissolved in the electrolyte. This was confirmed by the preparation of model solutions directly from hydrogen peroxide which were shown to behave similarly to the electrochemically prepared solutions. Ab initio calculation of possible solvated hydrogen peroxide structures in the electrolyte suggests that the hydrogen peroxide is stabilised by a hydrogen bonded complex involving the amine. The hydrogen peroxide solution is shown to be stable for several hours, to days, depending on the concentration, but can be rapidly decomposed into oxygen by a MnO2 catalyst. The process could thus be of interest both as part of a water splitting scheme as well as a route to hydrogen peroxide for chemical use.

Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Fe–Mn Mixed-Oxide Catalysts Containing Fe3Mn3O8 Phase

Novel Fe–Mn mixed-oxide catalysts were prepared for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. It was found that Fe(0.4)–MnOx catalyst showed the highest activity, yielding 98.8% NOx conversion and 100% selectivity of N2 at 120 °C at a space velocity of 30 000 h–1. XRD results suggested that a new crystal phase of Fe3Mn3O8 was formed in the Fe–MnOx catalysts. TPR and Raman data showed that there was a strong interaction between the iron oxide and manganese oxide, which is responsible for the formation of the active center―Fe3Mn3O8. Intensive analysis of fresh, used, and regenerated catalysts by XPS revealed that electron transfer between Fen+ and Mnn+ ions in Fe3Mn3O8 may account for the long lifetime of the Fe(0.4)–MnOx catalyst. In addition, the SCR activity was suppressed a little in the presence of SO2 and H2O, but it was reversible after their removal.

MOx (M = Mn, Fe, Ni or Cr) improved supported Co3O4 catalysts on ceria–zirconia nanoparticulate for CO preferential oxidation in H2-rich gases

This approach dealt with the modification of our developed Co3O4/Ce0.85Zr0.15O2 catalysts with transition metal oxides MOx (M=Mn, Fe, Ni or Cr) for CO preferential oxidation (CO PROX) in H2-rich gases. Results showed that just MnOx modification remarkably broadened the temperature window of 100% CO conversion, also better than those in the report about Co-Mn composite catalysts from another research group. And even in the presence of H2O and CO2, the 175–225°C temperature range for almost 100% CO conversion can be achieved over the optimized catalyst. Moreover, the addition of MnOx significantly improved the catalytic activity and selectivity, which was dramatically better than those of the supported MnOx or Co3O4 catalysts on ceria–zirconia nanoparticulate, indicating the existence of the remarkable Co-Mn synergistic effect. The MnOx modified supported Co3O4 catalysts exhibited outstanding catalytic properties for CO PROX reaction, which was strongly dependent on the reducibility and dispersion of the active species affected by the Co/Mn atomic ratio and loading. The XRD and H2-TPR techniques were performed to reveal the effect of structure–activity relationship on the catalytic properties. The analytic results presented that Co3+ was the main active species for CO PROX reaction, and the existence of Mn4+ and Mn3+ was also favorable to the reaction. The XRD and H2-TPR characterization results affirmed the existence of strong interaction between Co and Mn, increasing the ratio of Co3+/Co2+ resulted from the electron transfer between Co2+ and Mn4+. Besides the improvement of Co3O4 dispersion and the enlargement of the Co3+/Co2+ ratio through Co-Mn interaction, the addition of MnOx could improve the stability of the ceria–zirconia support through the metal–support interaction. As a result, the CO selective oxidation reaction was efficiently improved. The 16wt.%Co3O4-MnOx/Ce0.85Zr0.15O2 (Co/Mn=8:1) could be a quite potential catalyst for eliminating trace CO from H2-rich gases.

Development of carbon-supported hybrid catalyst for clean removal of formaldehyde indoors

A novel hybrid catalyst for a clean removal of formaldehyde indoors with a long lifetime was developed via a deposition of manganese oxide (MnOx) catalysts on a polyacrylonitril-based activated carbon nanofiber (PAN-ACNF) support. The combination of MnOx with PAN-ACNF induced synergic effects on the formaldehyde removal performance; twofold performance improvements were obtained as compared with PAN-ACNF alone not only in dry condition but also in the extremely humid condition at room temperature without an irradiation of the UV light. The two-staged removal process, the adsorption of formaldehyde in the PAN-ACNF micropores followed by the oxidative decomposition by the deposited MnOx nanoparticles, was found to afford the superior formaldehyde removal performance of the developed hybrid catalyst, MnOx@PAN-ACNF.

Nano-Materials Coupled with Non-Thermal Plasma Application in Environmental Protection

Nano-catalyst was prepared in the lab. Non-thermal plasma was generated by dielectric barrier discharge (DBD). Through nano-catalyst coupled with non-thermal plasma, a series of experiments for toluene decomposition were carried out. Based on reactor input energy density and removal efficiency and energy efficiency and inhibition for O3 formation, the load amount MnOx catalyst on the surface of γ-Al2O3 pellets were compared in the experiment. The results show the catalysis performance of 10 wt% MnOx/γ-Al2O3 coupled with non-thermal plasma resulted in higher removal efficiency of toluene and better energy efficiency. At the same time, 10 wt% MnOx/γ-Al2O3 operated on a better inhibition for O3 formation in the gas exhaust.

Advanced oxidation processes for treatment of effluents from a detergent industry

Ozonation, catalytic ozonation, Fenton’s and heterogeneous Fenton‐like processes were investigated as possible pretreatments of a low biodegradable and highly toxic wastewater produced by a detergent industry. The presence of a Mn–Ce–O catalyst in ozonation enhances the biodegradability and improves the degradation at low pH values. However, a high content of carbonyl compounds adsorbed on the recovered solid indicates some limitations for real‐scale application. A commercial Fe2O3–MnOx catalyst shows higher activity as well as higher stability concerning carbon adsorption, but the leaching of metals is larger than for Mn–Ce–O. Regarding the heterogeneous Fenton‐like route with an Fe–Ce–O catalyst, even though a high activity and stability are attained, the intermediates are less biodegradable than the original compounds, indicating that the resulting effluent cannot be conducted to an activated sludge post‐treatment. The highest enhancement of effluent biodegradability is obtained with the classic homogeneous Fenton’s process, with the BOD5/COD ratio increasing from 0.32 to 0.80. This process was scaled up and the treated effluent is now safely directed to a municipal wastewater treatment plant.

Catalytic performance of manganese-promoted nickel catalysts for the steam reforming of tar from biomass pyrolysis to synthesis gas

We investigated the performance of Ni+MnOx/Al2O3 catalysts in the steam reforming of tar from the pyrolysis of cedar wood. Performance of Ni+MnOx/Al2O3 catalyst with optimum composition was much higher than those of the corresponding monometallic Ni and MnOx catalysts. This tendency is also supported by the activity test in the steam reforming of toluene as a model compound of tar. Based on the catalyst characterization results, the surface of Ni metal particles was partially covered with MnOx, and the interaction between Ni metal and MnOx can play an important role on the enhancement of the catalytic activity and the suppression of coke deposition in the steam reforming of tar. Excess MnOx addition decreased the catalytic activity by decreasing the number of the surface Ni atoms.

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.

Simultaneous removal of particulates and NO by the catalytic bag filter containing MnOx catalysts

A catalytic bag filter which can remove particulates and NOx simultaneously was prepared and tested in a laboratory and a pilot plant. Manganese oxides (MnOx), active for the selective NO reduction with NH3 at low temperatures, was utilized as the catalyst. This MnOx-coated bag filter showed 92.6% NOx conversion at 423 K with a space velocity of 400,000 h−1.

Catalytic Decomposition of Ozone in Ground Air by Manganese-Based Monolith Catalysts

A highly active manganese dioxide material with deficient oxygen atoms (MnOx, x = 1.6–2.0) was prepared by calcining man-ganese carbonate, and a SiO2-Al2O3 support with large surface area was prepared by the peptizing method. A series of Pd-MnOx/SiO2-Al2O3 catalysts for ozone (O3) decomposition with different MnOx contents were prepared by incipient wet impregnation and characterized by activity tests, N2 adsorption, and life tests. The results of activity tests indicated that catalytic performance increased with MnOx content, but the MnOx catalyst without the support fell off easily and its catalytic performance decreased. At gas hourly space velocity (GHSV) = 360000 h−1, the complete conversion temperature (T100) on the Pd-MnOx/SiO2-Al2O3 catalyst with 80% MnOx content was 30 °C. At GHSV = 660000 h−1, the catalysts with 80%–90% MnOx had the best performance with T100 = 45–50 °C. Touching temperature between air and the water tank of an automobile was satisfied in this temperature range. At 45 °C and GHSV = 510000 h−1, O3 conversion on the catalyst with 80% MnOx still kept better standard (>90%), which indicates that the catalyst had very high capacity to resist deactivation after 95 h of life test.

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.

Carbon-supported manganese oxide nanoparticles as electrocatalysts for oxygen reduction reaction (orr) in neutral solution

Manganese oxides (MnOx) catalysts were chemically deposited onto various high specific surface area carbons. The MnOx/C electrocatalysts were characterised using a rotating disk electrode and found to be promising as alternative, non-platinised, catalysts for the oxygen reduction reaction (ORR) in neutral pH solution. As such they were considered suitable as cathode materials for microbial fuel cells (MFCs). Metal [Ni, Mg] ion doped MnOx/C, exhibited greater activity towards the ORR than the un-doped MnOx/C. Divalent metals favour oxygen bond splitting and thus orientate the ORR mechanism towards the 4-electron reduction, yielding less peroxide as an intermediate.

Elucidation of deactivation or resistance mechanisms of CrOx, VOx and MnOx supported phases in the total oxidation of chlorobenzene via ToF‐SIMS and XPS analyses

AbstractVOx, CrOx and MnOx supported on TiO2 are all efficient catalysts in the total oxidation of benzene. However, in the oxidation of chlorobenzene, they exhibit different behaviors in terms of their resistance to deactivation by the chlorinated reactant and/or products of the reaction (Cl2, HCl). Precisely: VOx catalysts present a very good resistance; conversely, MnOx catalysts present a huge deactivation and CrOx catalysts exhibit an intermediate behavior. This contribution leads to a better understanding of the mechanisms and origins of the respective deactivation (or resistance to deactivation) of the catalysts via a postmortem characterization by ‘X‐ray photoelectron spectroscopy’ (XPS) and ‘time of flight—secondary ion mass spectroscopy’ (ToF‐SIMS). (i) The different behaviors are correlated to the atomic ratio of chlorine/transition metal at the surface of the used catalysts and (ii) the nature of the chlorinated species responsible for the deactivation is elucidated: namely, CrOx catalysts deactivate because of a firm adsorption at the surface of the chlorinated volatile organic compounds (VOC) or of chlorinated intermediates of reaction, while MnOx catalysts deactivate because of the formation of (oxy)chlorides at their surface. Copyright © 2008 John Wiley & Sons, Ltd.

Manganese oxide catalysts for NOx reduction with NH3 at low temperatures

Manganese oxide catalysts prepared by a precipitation method with various precipitants were investigated for the low temperature selective catalytic reduction (SCR) of NOx with NH3 in the presence of excess O2. Various characterization methods such as N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and X-ray absorption near edge structure (XANES) were conducted to probe the physical and chemical properties of MnOx catalysts. The active MnOx catalysts, precipitated with sodium carbonate and calcined in air at moderate temperatures such as 523 K and 623 K, have the high surface area, the abundant Mn4+ species, and the high concentration of surface oxygen on the surface. The amorphous Mn3O4 and Mn2O3 were mainly present in this active catalyst. The carbonate species appeared to help adsorb NH3 on the catalyst surface, which resulted in the high catalytic activity at low temperatures.

Low-temperature catalytic reduction of nitrogen oxides with ammonia over supported manganese oxide catalysts

MnOx supported on γ-Al2O3, TiO2, Y-ZrO2 and SiO2 was prepared by an impregnation and a deposition-precipitation method, and their catalytic activities for the low-temperature selective catalytic reduction (SCR) of NOx with NH33 in the presence of excess O2 were examined. The catalytic activity of the catalysts prepared by a deposition-precipitation method was higher than that of catalysts prepared by an impregnation method. The activity follows in the order: MnOx/TiO2 ≈ MnOx/γ-Al2O3 > MnOx/SiO2 > MnOx/Y-ZrO2. Supported MnOx catalysts prepared by a deposition-precipitation method appeared to have an amorphous manganese oxide phase and those prepared by an impregnation method exhibited a crystalline MnO2 phase, respectively. The addition of SO2 with H2O in the feeding gas slightly deactivates the SCR activity of MnOx/TiO2 catalysts.