Influence of the Support Treatment on the Behavior of MnOx/Al2O3 Catalysts used in VOC Combustion

Alumina was treated with water and diluted nitric acid and then was used to prepare supported MnO _x /Al₂O₃ catalysts with two different loadings. The influence of the support treatment on the catalytic behavior in ethanol and toluene combustion was studied. The treatments modified the alumina physicochemical properties (porosity, surface area, isoelectric point, and surface acidity). The modification of these properties affected the interaction of the manganese oxide species with the support and increased the dispersion of the active phase. Catalysts prepared from treated supports showed the best catalytic performance in ethanol combustion. At high manganese loading, this better catalytic performance was related to the high capacity for adsorbing oxygen. While at low manganese loading, the great amount of dispersed surface manganese oxide species and/or the existence of surface defects were relevant in the catalytic activity. On the other hand, the reactivity of the catalysts in toluene combustion was roughly correlated with the reducibility of the surface manganese oxide species. On the basis of these observations, we conclude that the ethanol combustion occurs by a suprafacial mechanism whereas the toluene combustion proceeds through an intrafacial mechanism.     

Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion

For combustion with CO₂ capture, chemical-looping combustion (CLC) with inherent separation of CO₂ is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125-180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80-330 kg/MW_th and a needed recirculation flow of oxygen carrier of 4-8 kg/s, MW_th.     

Characterization of structure and electrochemical properties of lithium manganese oxides for lithium secondary batteries hydrothermally synthesized from δ-KxMnO2

Lithium manganese oxides have attracted much attention as cathode materials for lithium secondary batteries in view of their high capacity and low toxicity. In this study, layered manganese oxide (δ-K_xMnO₂) has been synthesized by thermal decomposition of KMnO₄, and four lithium manganese oxide phases have been synthesized for the first time by mild hydrothermal reactions of this material with different lithium compounds. The lithium manganese oxides were characterized by powder X-ray diffraction (XRD), inductively coupled plasma emission (ICPE) spectroscopy, and chemical redox titration. The four materials obtained are rock salt structure Li₂MnO₃, hollandite (BaMn₈O₁₆) structure α-MnO₂, spinel structure LiMn₂O₄, and birnessite structure Li_xMnO₂. Their electrochemical properties used as cathode material for secondary lithium batteries have been investigated. Of the four lithium manganese oxides, birnessite structure Li_xMnO₂ demonstrated the most stable cycling behavior with high Coulombic efficiency. Its reversible capacity reaches 155 mAh g⁻¹, indicating that it is a viable cathode material for lithium secondary batteries.     

One-pot synthesis of mesoporous CuOx/CeO2 co-loaded ZrO2–TiO2 nanocomposites via surfactant-free solvothermal method for catalytic removal of soot under NO/O2

A novel porous CuO_x/CeO₂ co-loaded ZrO₂–TiO₂ (ZT) nanocomposite with tunable pore structure and high surface area was prepared by a simple surfactant-free solvothermal method. The pore structure could be well controlled by adjusting the ratio of ethanol to H₂O during the solvothermal process. Both copper oxides and ceria species could be homogeneously loaded into porous ZT nanocomposite by either incorporation into ZT framework or dispersion into the pore channels. Two kinds of novel catalysts with different pore structures have been synthesized and exhibit excellent soot catalytic combustion performance, owing to the porous structure and the active components of CuO_x/CeO₂.     

Structural Reversibility of a Ternary CuO-ZnO-Al2O3 ex Hydrotalcite-Containing Material During Wet Pd Impregnation

A Cu-Zn-Al precursor was synthesized by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. CuO-ZnO-Al₂O₃ composite oxide support was obtained by calcination (673 K) of the Cu-Zn-Al precursor. Two palladium-modified CuO-ZnO-Al₂O₃ samples were prepared by impregnation of the mixed-oxide support and further calcination (673 K). The presence of remaining CO₃ ^2- anions in the CuO-ZnO-Al₂O₃ mixed oxide, as a result of incomplete Cu-Zn hydrotalcite phase decomposition, and the hydrothermal-like treatment during the Pd impregnation step, allow the partial reconstruction of the Cu-Zn hydrotalcite-type structure (memory effect). In addition, an enhancement in the CuO crystallinity was obtained for the Pd-modified oxides. A detailed characterization revealed that the hydrotalcite restoration enhances the crystallinity of the copper oxide as a consequence of a crystalline rearrangement of this oxidic phase.     

Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2O3 for the combustion of CO, ethyl acetate and ethanol

Combustion of CO, ethyl acetate and ethanol was studied over CuO_x/Al₂O₃, CuO_x–CeO₂/Al₂O₃, CuMn₂O₄/Al₂O₃ and Mn₂O₃/Al₂O₃ catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO_x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO_x–CeO₂/Al₂O₃ catalyst is more active than the CuMn₂O₄/Al₂O₃ catalyst for the oxidation of CO but the CuMn₂O₄/Al₂O₃ catalyst is more active for the combustion of ethyl acetate and ethanol.

Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuO_x–CeO₂/Al₂O₃ and CuMn₂O₄/Al₂O₃ catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed.

Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn₂O₄ and Mn₂O₃, respectively, on Al₂O₃.     

Carbon combustion synthesis of nanostructured perovskites

A novel, economical, and energy-efficient process to produce nanostructured particles of several perovskite oxides, such as ferroelectrics BaTiO₃, SrTiO₃ and LiNbO₃, is described. This process, referred to as carbon combustion synthesis of oxides (CCSO) is a modified SHS process that uses carbon as a fuel instead of a pure metal. In CCSO of nanostructured materials, the exothermic oxidation of carbon nanoparticles (∼5 nm) with a surface area of 80 m²/g generates a thermal reaction wave with temperature of up to 1200°C that propagates through the solid submicron reactant mixture, converting it to the desired complex oxide product. The carbon is not incorporated in the solid product since it is released in a gaseous form (CO₂) from the sample. The quenching front method combined with XRD and Raman spectroscopy revealed that crystalline tetragonal BaTiO₃ particles formed in the early stage of the combustion, before the temperature reached its maximum. A major difference between the thermal transport processes during CCSO and conventional SHS is the extensive emission of CO₂. The release of CO₂ enables synthesis of highly porous (up to 70%) powders having a particle size in the range of 60–80 nm with a surface area of up to 12.4 m²/g.      

Catalytic wet oxidation of p-chlorophenol over supported noble metal catalysts

Heterogeneous catalytic wet oxidation of aqueous p-chlorophenol (p-CP) solution was investigated at 453 K and 2.6 MPa in a slurry reactor. The performance of noble metal catalysts was compared with that of traditional manganese catalysts. Activated carbon supported catalysts showed significant higher activities for total organic carbon (TOC) reduction than those supported on alumina or cerium oxide. Pt was found to be the most active metal for p-chlorophenol oxidation. The activities of noble metal catalysts were found to correlate with heat of formation of metal oxides. CO₂ is the predominant oxidation product with formation of minor p-benzoquinone and acetic acid as intermediate compounds. Possible reaction pathway was also discussed.     

Catalytic combustion of methane on novel catalysts derived from Cu-Mg/Al-hydrotalcites

Novel Cu-Mg/Al mixed oxides (designated as i-CMAO-800) were prepared by calcinations of Cu-Mg/Al hydrotalcites [(Cu²⁺ +Mg²⁺)/Al³⁺= 3] at 800 °C. Their performance for the catalytic combustion of methane was investigated. The oxides and their precursors were characterized by XRD, TG-DSC, TPR and N₂ adsorption/desorption techniques. The results showed that BET surface areas and the stability of the resultant oxides were greatly influenced by the copper contents in hydrotalcite precursors, bringing about difference in their activities for methane catalytic combustion. XRD results indicated that Cu was highly dispersed in hydrotalcite precursors in case of low copper contents, (Cu 40 wt%). For higher Cu contents, Cu(OH)₂ was formed, and, consequently, a separate phase of CuO was detected in the oxide catalysts after calcination. As indicated by the TG-DSC results, different decomposition behaviors were observed for various hydrotalcites. Thermal calcination promoted the formation of copper aluminates and segregation of CuO from the bulk phases. TPR results showed 15CMAO-800 has the highest reduction rate, and the catalytic activities of iCMAO-800 mixed oxides depend on both the reduction rates and the amounts of copper ions in mixed oxides. The catalyst 15-CMAO-800 showed the best performance.     

In situ gasification of a lignite coal and CO2 separation using chemical looping with a Cu-based oxygen carrier

Chemical looping combustion (CLC) has the inherent property of separating CO₂ from flue gases. This paper is concerned with the application of chemical looping to the combustion of a solid fossil fuel (a lignite and its char) in a technique whereby the fuel is gasified in situ using CO₂ in the presence of a batch of supported copper oxide (the “oxygen carrier”) in a single reactor. As the metal oxide becomes depleted, the feed of fuel is discontinued, the inventory of fuel is reduced by further gasification and then the contents are re-oxidised by the admission of air to the reactor, to begin the cycle again. The choice of oxides is restricted because it requires an oxide which is exothermic during reduction to balance the endothermic gasification reactions. Copper has such oxides, but a key question is whether or not it can withstand temperatures at which gasification rates are significant (∼1173 K), particularly from the point of view of avoiding sintering and deactivation of the carrier in its reduced form. It was found that an impregnated carrier, made by impregnating a θ-alumina catalyst support (BET area 157 m²/g) with a saturated solution of copper and aluminium nitrates, acted as a durable carrier over 20 cycles of reduction and oxidation, using both Hambach lignite coal, and its char, and with air as the oxidising agent. During the course of the experiments, the BET surface area of the support fell from ∼60 m²/g, just after preparation, to around 6 m²/g after 20 cycles. However, this fall did not appear to affect the overall capacity of the oxygen carrier to react with fuels and its effect on the kinetics of the reaction with CO did not influence the outcome of the experiments, since the overall performance of the looping scheme is dominated by the much slower kinetics of the gasification reaction. The apparent kinetics of the gasification are faster in the presence of the looping agent: this is because the bulk concentration of CO in the presence of the looping agent is lower, and partly because the destruction of CO in the vicinity of a gasifying particle enhances the rate of removal of CO by mass transfer (and increases the local concentration of CO₂). There was little evidence to suggest a direct reaction between carbonaceous and carrier solids, other than via a gaseous intermediate. However, the observation of finite rates of conversion in a bed of active carrier, fluidised by nitrogen, is a scientific curiosity, which we have not been able to explain satisfactorily. At 1173 K, as used here, rates of gasification of Hambach lignite, and its char, are significant. The CuO in the carrier decomposes at 1173 K to produce gas-phase O₂ and Cu₂O: both can react with CO produced by gasification, whilst the O₂ can react directly with the char.     

Synergistic effect of Pd in methane combustion PdMnOx/Al2O3 catalysts

The catalytic methane combustion was investigated over alumina-supported monometallic and bimetallic palladium and manganese oxide catalysts. The catalytic activity of these systems showed that palladium incorporation on MnO_x/Al₂O₃ catalyst leads to an enhancement in methane combustion. The higher catalytic activity of the PdMn/Al₂O₃ catalysts is related to a greater mobility of lattice oxygen in manganese oxide in the presence of palladium. These bimetallic catalysts also showed a significant improvement in catalysts stability with respect the monometallic ones. Surface analysis of the used catalysts revealed less amount of coke and Mn/Al and Pd/Al atomic ratios almost unchanged, which is indication of absence of active phase sintering.     

High Catalytic Activity in CO Oxidation over MnO x Nanocrystals

Manganese oxides of various stoichiometry were prepared via Mn-oxalate precipitation followed by thermal decomposition in the presence of oxygen. A non-stoichiometric manganese oxide, MnO _x (x = 1.61…1.67) was obtained by annealing at 633 K and demonstrated superior CO oxidation activity, i.e. full CO conversion at room temperature and below. The activity gradually decreased with time-on-stream of the reactants but could be easily recovered by heating at 633 K in the presence of oxygen. CO oxidation over MnO _x in the absence of oxygen proved to be possible with reduced rates and demonstrated a Mars—van Krevelen—type mechanism to be in operation. A TEM structural analysis showed the MnO _x phase to form microrods with large aspect ratio which broke up into nanocrystalline manganese oxide (MnO _x ) particles with diameters below 3 nm and a BET specific surface area of 525 m²/g. Annealing at 798 K rather than 633 K produced well crystalline Mn₂O₃ which showed lower CO oxidation activity, i.e. 100% CO conversion at 335 K. The catalytic performance in CO oxidation of various Mn-oxides either studied in this work or elsewhere was compared on the basis of specific reaction rates.     

Performance and exhaust emission characteristics of a spark ignition engine using ethanol and ethanol-reformed gas

Since ethanol is a renewable source of energy and has lower carbon dioxide (CO₂) emissions than gasoline, ethanol produced from biomass is expected to be used more frequently as an alternative fuel. It is recognized that for spark ignition (SI) engines, ethanol has the advantages of high octane and high combustion speed and the disadvantage of ignition difficulties at low temperatures. An additional disadvantage is that ethanol may cause extra wear and corrosion of electric fuel pumps. On-board hydrogen production out of ethanol is an alternative plan.Ethanol has been used in Brazil as a passenger vehicle fuel since 1979, and more than six million vehicles on US highways are flexible fuel vehicles (FFVs). These vehicles can operate on E85 - a blend of 85% ethanol and 15% gasoline.This paper investigates the influence of ethanol fuel on SI engine performance, thermal efficiency and emissions. The combustion characteristics of hydrogen enriched gaseous fuel made from ethanol are also examined.Ethanol has excellent anti-knock qualities due to its high octane number and a high latent heat of evaporation, which makes the temperature of the intake manifold lower. In addition to the effect of latent heat of evaporation, the difference in combustion products compared with gasoline further decreases combustion temperature, thereby reducing cooling heat loss. Reductions in CO₂, nitrogen oxide (NOx), and total hydrocarbons (THC) combustion products for ethanol vs. gasoline are described.     

Effects of cobalt,copper, manganese and titanium oxide additions on the microstructures of zinc containing soft porcelain glazes

Different types of glazes, which are nearly all based upon silicate compositions, are used to meet a wide range of requirements in service. Many artistic effects are achieved by departing from a clear, smooth, transparent system. Coloured glazes are produced by several means such as the inclusion of colouring oxides, addition of stains, dispersing finely divided particles and the use of precious metals, applied in the form of lines or bands, or even screen-printed patterns. Colouring oxides commonly used include iron, copper, cobalt, chromium, manganese, nickel, vanadium, cadmium and selenium. Zinc oxide has a beneficial effect in many coloured glazes amongst which crystalline ones are more noteworthy. With this paper the effects of CoO, CuO, MnO₂ and TiO₂ additions into zinc oxide containing crystal glazes differentially heat-treated are described on micro-scale appearances. Experimental techniques used were X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDX).     

Studying the composition and structure of films obtained by thermal oxidation of copper

The composition and structure of copper oxide thin films obtained by thermal oxidation of copper layers with a thickness of 100 nm, deposited onto quartz glass by high-frequency magnetron sputtering, have been studied. The dependences of the film thickness and content of CuO and Cu₂O oxides in it on the temperature–time conditions of thermal treatment are obtained. The studies have shown that depending on the preparation modes, the films can be multiphase and contain different copper oxides, predominantly, CuO and Cu₂O, which possess different bandgaps and p-type conductivity.     

Synthesis and Characterization of Manganese Oxide Catalysts for the Total Oxidation of Ethyl Acetate

Manganese oxide catalysts were synthesized by direct reaction between manganese acetate and permanganate ions, under acidic and reflux conditions. Parameters such as pH (2.0–4.5) and template cation (Na⁺, K⁺ and Cs⁺) were studied. A pure cryptomelane-type manganese oxide was synthesized under specific conditions, and it was found that the template cation plays an important role on the formation of this kind of structure. Cryptomelane was found to be a very active oxidation catalyst, converting ethyl acetate into CO₂ at low temperatures (220 °C). This catalyst is very stable at least during 90 h of reaction and its performance is not significantly affected by the presence of water vapour or CO₂ in the feed stream. The catalyst performance can be improved by the presence of small amounts of Mn₃O₄.     

The catalytic oxidation of organic air pollutants. Part 3. Catalysts from manganese and copper sulphates

Manganese sulphate and copper sulphate were evaluated as the basis of catalysts for the control of organic air pollutants responsible for malodorous process emissions. Tests were made with low concentrations, 100 volumes × 10^?6 in air, (i.e. 100 parts × 10^?6 by volume), of methyl mercaptan or mercaptan + n-butanal as a function of temperature and time to determine efficiencies for destructive oxidation. The complex pattern of odorant removal and product formation observed using manganese sulphate is discussed in terms of active sites which convert mercaptan to SO₂ but are inhibited by the onset of dimethyl disulphide formation at higher temperatures; reactions between the odorants may also occur. Copper sulphate formed dimethyl disulphide at the lower temperatures but only SO₂, SO₃, CO and CO₂ were detected above 300°C; after a 100 h durability test at 400°C, the odour removal efficiency determined by dynamic dilution olfactometry was 99.9%. Hence copper sulphate is preferable to manganese sulphate and is equal in performance to the copper oxide tested previously (Part 1) but with the advantage of a stable performance from the outset under sulphating conditions.     

Titania-supported iron oxide as oxygen carrier for chemical-looping combustion of methane

Chemical-looping combustion is a two-stage process proposed as an alternative for the combustion of carbonaceous materials, such as natural gas or coal gas, for almost complete CO₂ capture. In the reduction stage, the structural oxygen contained in the lattice of a reducible inorganic oxide, is used for combustion of the carbonaceous material. In the regeneration stage the oxygen carrier, found in a reduced state after the reduction stage, is regenerated with pure air to recover the physical and chemical properties of the carrier, ready to reinitiate a new cycle reduction-regeneration. In a typical multicycle reactor test, the carriers are subjected to accumulative chemical and thermal stresses and the performance will, probably, decay progressively with the number of cycles. The occurrence of some side reactions may limit the efficiency of the overall process in CO₂ capture. In this paper, titania-supported iron oxides with different iron loadings have been tested in multicycle tests in a fixed-bed reactor at 900 °C and atmospheric pressure, as oxygen carriers for the chemical-looping combustion of methane. The study shows that the available oxygen for methane combustion in the reduction stage is lower than expected since the active phase interacts with the support forming FeTiO₃ ilmenite. The reactivity of these iron based carriers in the reduction stage is independent on the iron oxide content but lower than that exhibited by other tested carriers, such as CuO or NiO. However, iron carriers are cheaper no showing any tendency to carbon deposition.     

Investigation of oxygen ion transport and surface exchange properties of PrBaFe2O5+δ

The oxygen transport properties and chemical stability of PrBaFe₂O_5+δ (PBF) double-perovskite oxide were systematically investigated as a chemically stable, highly oxygen-permeable membrane and solid oxide fuel cell (SOFC) electrode under a CO₂-containing or reducing atmospheres. The oxygen permeation flux of 0.7 mm-thick samples and the oxygen ion conductivity were 4.7 × 10^?1 mL?cm^?2 min^?1 and 0.12 S?cm^?1 at 900 °C, respectively, which are comparable to those of PrBaCo₂O_5+δ, exhibiting the most superior performance among oxides with a double-perovskite structure. Moreover, the bulk diffusion and surface exchange coefficients estimated from the electrical conductivity relaxation analysis were generally comparable to those of PrBaCo₂O_5+δ. The characteristic thickness estimated from the membrane and conductivity relaxation tests was ～0.6 mm at 900 °C. The results indicate the significant influence of the surface exchange reaction on the permeability within a thickness of 0.5–1.7 mm. The PBF double-perovskite oxide exhibited superior chemical stability, compared to typical oxides such as PrBaCo₂O_5+δ under a CO₂-containing atmosphere. All results suggest that PrBaFe₂O_5+δ exhibits high oxygen diffusivity with high chemical stability under CO₂-containing or reducing atmospheres.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.