Non-stoichiometric Oxygen Research Articles

Structure Analysis and Magnetic Properties of Nano-Sized Nb<SUB><I>x</I></SUB>Cu<SUB><I>y</I></SUB>Zn<SUB>1−<I>x</I>−<I>y</I></SUB>Fe<SUB>2</SUB>O<SUB>4</SUB> Powders Formed by Self-Propagating High-Temperature Synthesis and Ultrasonic Agitation Method

Nano-sized Nb0.12Cu0.45Zn0.43Fe2O4 ferrites with an average size of the agglomerated particles less than 300 nm were prepared by self-propagating high-temperature synthesis (SHS) followed by mechanical ball milling and classifying by ultrasonic floating agitation. The SHS reaction occurred in the powder mixture of iron and various oxides like NbO, CuO, ZnO, Fe2O3 at the oxygen pressure range of 0.1 to 1.0 MPa, which average combustion temperatures and combustion propagating rates were in the ranges of 1105 to 1343 K and 4.1 to 6.2 mm/s, respectively. The combustion temperature increased with the oxygen pressure. The combustion reaction did not occur above the uniaxial compacting pressure of 10.0 MPa, even though the oxygen pressure was 1.0 MPa. The neutron diffraction analysis revealed that the final product was Nb0.12Cu0.45Zn0.43Fe2O4 with lattice parameters of 0.8472 nm. The maximum magnetization (Ms), residual magnetization (M(r)), coercive force (iH0), and susceptibility of the Nb0.12Cu0.45Zn0.43Fe2O4 ferrites were 11.30 Wb/m2 Kg, 1.47 Wb/m2 Kg, 7321 A/m, and 0.02 m3/kg, respectively. The addition of niobium to copper-zinc ferrites resulted in increasing M(s), M(r), and iM(c) by about 49%, 68%, and 287%, respectively. The variation of the magnetic properties is related to the non-stoichiometric numbers and oxygen position of the niobium-copper-zinc ferrites.

Dielectric resonance effect with negative permittivity in a La1.5Sr0.5NiO4+δ ceramic

A polycrystalline sample of La1.5Sr0.5NiO4+δ was prepared by using a solid-state reaction. X-ray diffraction proved the sample to be a single phase with a tetragonal structure (space group: I4/mmm). By using an iodometric titration method to determine the non-stoichiometric oxygen concentration (δ) in La1.5Sr0.5NiO4+δ, we found δ = −0.017, which corresponds to a doping level of nh = x + 2δ = 0.466. Also, a strong increase of the magnetization in the M(T) curve at temperatures below the spin-ordering temperature (TSO ~ 100 K) was observed. The M(H) curves show very small magnetic moments, which proves the weak ferromagnetic nature of La1.5Sr0.5NiO4+δ. The dependences of the dielectric constant on the frequency and the temperature, e(ω, T) = e′(ω, T) + ie″(ω, T), was investigated in the frequency range of 1–13 MHz. At temperatures around room temperature, the maximum of the real part (e′) was higher than 105. Particularly, an abnormal dependence of the permittivity on frequency was observed. Depending on temperature, a dielectric resonance was observed at about 500 kHz or 8 MHz. Interestingly, we observed the dielectric-resonance effect with a negative permittivity. Such a feature is very similar to that observed in left-handed materials. The fitting of the experimental data for the dielectric constant at frequencies around the resonance frequency to the equations associated with an equivalent RLC series circuit proves that La1.5Sr0.5NiO4+δ belongs to the class of multiferroic materials.

Strong synergism between gold and manganese in an Au–Mn/triple-oxide-support (TOS) oxidation catalyst

A strong synergism between Au and Mn substantially increased the catalytic activity of the Au–Mn/TOS catalyst for the complete propane oxidation as compared to the activity of mono metal Au/TOS and Mn/TOS catalysts. The high activity of Au–Mn/TOS catalyst is a result of the formation of Au2Mn or Au5Mn2 phases and enhanced mobility of the oxygen in the support oxides. XPS, TPR, TEM and XRD studies were used to characterise the catalysts. Based on the results, a possible mechanism for the participation of the non-stoichiometric and lattice oxygen is also proposed. It is suggested that a joint effect of the Au and Mn increased the non-stoichiometric and lattice oxygen mobility of the Au–Mn/TOS catalyst, potentially being dominated by Mn.

Preliminary performance studies of Al–Co–Mn mixed oxide and Ag-doping for the purpose of PGM-free DOC

Lean-burn engine powered vehicles are increasing through the world for better fuel combustion efficiencies. Diesel engines, taking lean-burn policy, essentially use platinum (Pt) as a catalyst for an after-treatment of their exhaust, called diesel oxidation catalyst (DOC). It is a great issue to reduce the use of platinum group metals (PGM) with respect to costly aspect. We are here reporting a preliminary oxidation performance results of a PGM-free mixed oxide of powdered Al1.8Co4.4MnO9.6+x with nonstoichiometric oxygen composition through a temperature ramp-up transient analysis. The mixed oxide was found having spinel structured mixed phase of Co3O4, Co2AlO4, and CoMn2O4 with excess oxygen weakly bonded to the transition metals at high oxidation states. The bare Al1.8Co4.4MnO9.6+x showed excellent activities for CO oxidation, which was greatly faster than 1.3%-Pt doped alumina, and HC oxidation was shown slightly slower than CO oxidation. Through the impregnation effect tests of precious metals such as Ag, Pd and Pt on the mixed oxide, 3% Ag-doped Al1.8Co4.4MnO9.6+x was found showing the highest oxidation activity for CO and HC oxidation. The results are providing a possibility to substitute for Pt-based DOC with a low cost PGM-free catalyst.

ZnO nanowire arrays synthesized on ZnO and GaN films for photovoltaic and light-emitting devices

The wide bandgap, one-dimensional zinc oxide (ZnO) nanowires (NWs) and their heterostructures with other materials provide excellent pathways for efficient photovoltaic (PV) and light-emitting devices. ZnO NWs sensitized with quantum dots (QDs) provide high-surface area and tunable bandgap absorbers with a directional path for carriers in advanced PV devices, while ZnO heterojunctions with other p-type wide bandgap materials lead to light-emitting diodes (LEDs) with better emission and waveguiding properties compared with the homojunction counterparts. Synthesis of the structures with the desired morphology is a key to device applications. In this work, ZnO NW arrays were synthesized using hydrothermal method on ZnO and GaN thin films. Highly crystalline, upright, and ordered arrays of ZnO NWs in the 50 to 250-nm diameter range and 1 μm in length were obtained. The morphology and optical properties of the NWs were studied. Energy dispersive x-ray spectroscopy (EDX) analysis revealed nonstoichiometric oxygen content in the grown ZnO NWs. Photoluminescence (PL) studies depicted the presence of oxygen vacancy and interstitial zinc defects in the grown ZnO NWs, underlining the potential for LEDs. Further, hydrophobically ligated CdSe/ZnS QDs were successfully incorporated to the NW arrays. PL analysis indicated the injection of electrons from photoexcited QDs to the NWs, showing the potential for quantum dot-sensitized solar cells.

Mass and Charge Transport Properties of Al Doped La2NiO4+δ

Mixed oxide-ion and electron conducting ceramic material La2NiO4+ d(LNO) have been reported to exhibit impressive oxide ion and p-type electronic conductivity, and high catalytic activity which make them an excellent component of some electrochemical devices such as fuel cell cathode, oxygen permeation membrane. The hyperstoichiometric LNO oxides show high oxygen diffusion coefficient and high oxygen ion conductivity, which are attributes of their crystal structure.The extent of oxygen nonstoichiometry significantly affects the electrochemical properties of LNO systems. The nonstoichiometric oxygen content may be varied by aliovalent doping or by substituting the host cation with smaller size cations[1,2]. The conduction mechanism in LNO-based system has been explained on the basis of both, the band conduction by delocalized electrons and polaron-hopping by localized electrons[3,4].The objective of this work is to study the effect of a donor cation (Al3+) on oxygen nonstoichiometry and resultant thermodynamic properties of LNO system. In this work we have prepared a non-transition metal cation Al3+-doped LNO system, La2Ni0.95Al0.05O4.025+ d, and presented an equilibrium defect model for this system. The variation of oxygen nonstoichiometry was measured by coulometric titration and the absolute value of oxygen nonstoichiometry was calculated from thermogravimetric analysis. Various thermodynamic quantities for LNAO were calculated from the experimental data and were compared with those for LNO in our previous work.Fig.1 shows pO2 dependence of oxygen nonstoichiometry at different temperature. As can be seen, the values of δ increased with decreasing temperature and with increasing pO2. The inset of Fig. 1 shows the relationship of log δ vs. log pO2, as can be seen, at isothermal condition the dependence of δ on the pO2exponent (m) is slightly less than 1/6. It is clear from the above observation that the oxygen exponent decreases with increasing oxygen activity, suggesting a positive deviation of the LNAO system from the ideal solution behavior[4].The best estimated values of Nv obtained from fitting the Eq. (1) with fitting parameters Kox, Kf, and Nv to the δ vs. log(pO2) data in Fig.(1). The density of states of the valence band varies from 1.87x1020 to 2.55x1020 in 800-1000 oC range. And , the effective mass mh *of holes is 1.02 - 1.21 times the rest mass mo. This indicates the occurrence of band-like conduction and allows the effect of conduction by a small degree of polaron hopping to be ignored. The behavior of holes starts deviates from that of ideal solution at a very low δ value and rh • increases with the increasing δ and reaches to ~7 at rh •≈0.08 and T=900°C. The calculation of excess partial molar quantities showed that incorporation of interstitial oxygen is less favorable in La2Ni0.95Al0.05O4.025 + δ in comparison to La2NiO4 + δ. Reference Nakamura T, Yashiro K, Sato K, Mizusaki, J. Solid State Ionics, 180, 368 (2009).Naumovich EN, Patrakeev MV, Kharton VV, Yaremchenko AA, Logvinovich DI, Marques FMB. Solid State Sciences, 7,1353 (2005).H. S. Kim, and H. I. Yoo, Solid State Ionics, 232, 129 (2013).H. S. Kim, and H. I. Yoo, Phys. Chem. Chem. Phys., 12, 4704 (2010).

Effect of preparation parameters on the catalytic performance of mesoporous NiO for the oxidative dehydrogenation of propane to propylene

Mesoporous NiO was prepared by the template synthesis method, using polyethylene glycol as a template, Ni(NO3)2·6H2O and Ni(CH3COO)2·4H2O as a nickel source. The major factors that influenced the formation of mesoporous NiO, such as nickel source, calcining temperature and molecular size of PEG, were estimated. As a result, the nickel source and calcination process could determine the crystallizability and organization of mesostructured NiO particles. The molecular size of PEG surfactant is not only relevant to the pore size of mesoporous NiO, but also of effect on the selectivity to propylene. The fine mesoporous NiO could be formed by using Ni(CH3COO)2·4H2O as nickel source and PEG400 as template, and then being calcined at 400–600 °C. The mesoporous NiO catalysts exhibit significant catalytic activities for the oxidative dehydrogenation of propane. The propylene yield of 14 % was obtained at propane conversion of 27 % at 350 °C. Furthermore, the selectivity to propylene can be maintained around 50 % even at 600 °C, indicating the high catalytic stabilities of mesoporous NiO catalysts in the present work. We proposed that a non-stoichiometric oxygen species on the surface is the catalytic active site and the mesoporous structure is the key factor for the high catalytic stabilities in the light of the characteristic results of mesoporous NiO catalysts.

Spin reorientation and giant dielectric response in multiferroic La1.5Sr0.5NiO4+δ

A multiferroic sample of La1.5Sr0.5NiO4+δ was prepared by conventional solid-state reaction. X-ray diffraction study revealed La1.5Sr0.5NiO4+δ exhibiting a single phase with a tetragonal structure (F4K2Ni-perovskite-type structure, and space group of I4/mmm). An iodometric titration method was used to determine non-stoichiometric oxygen concentrations (δ) in the La1.5Sr0.5NiO4+δ ceramic. We found δ = −0.017, which corresponds to a doping level of nh = x + 2δ = 0.466 (with x being Sr2+ content). The field-cooled (FC) magnetization curve indicated that there was a strong increase in spin ordering (SO) at temperature TSO ∼ 100 K. Below TSR ∼ 50 K, spins undergo a spin reorientation (SR) turning away from the stripe direction on cooling. Its dielectric constant ε (ω , T) = ε′(ω,T) − iε ″(ω,T) was estimated by basing on capacitance and conductance curves measured at different temperatures and frequencies. Dielectric relaxations of charge-ordered La1.5Sr0.5NiO4+δ ceramic were investigated in a broad temperature range. The giant dielectric constant (104–106) with a low dielectric loss of 10−2–10−1 was also found.

Temperature Dependent Relaxation Behavior of Polycrystralline LuFe2O4 Studied By Dielectric Spectroscopy

Polycrystalline LuFe2O4 ceramics was synthesized by solid state reactions at 1200°C with a mixed gas flow of CO2 and H2. The temperature dependence of zero-field cooled and field-cooled magnetization measurements indicates a clear paramagnetic to ferromagnetic transition in our LuFe2O4 samples. The temperature relaxation behavior of LuFe2O4 ceramics was investigated by an impendence analyzer in the frequency range from 20 Hz to 1 MHz. The contributions of grain boundaries, bulks, and electrodes to the ac conductivity and relaxation processes were identified by fitting the impendence data with an equivalent circuit model. When the temperature is lower than around 220 K, the bulk effect dominates over the contributions of grain boundaries and electrodes on the dielectric relaxation process. However, when the temperature is higher than 220 K, the electrode effect becomes dominant. The polycrystalline LuFe2O4 also show a temperature dependent activation energies for ac conductivities which may relate to non-stoichiometric oxygen deficiency in our samples.

Spectrophotometric method for rapid estimation of the oxygen stoichiometry in YBCO superconducting samples

Abstract Abstract Detailed investigations on the possibility for determination of the oxygen stoichiometry in YBa2Cu3Oy superconducting samples by measuring the absorbance of the colored Co(III)-EDTA and Cu(II)-EDTA complexes in one solution have been performed. The absorptivity of the complexes at pH 2.5 and 4.9 as well as Beer’s law have been studied. The content of the solution for the sample dissolving has been established. The stoichiometry of the oxidation-reduction reaction between Cu(III) from the sample and Co(II) in the solution has been proved. The possibility to dissolve the sample in air has been tested. As a result a simple and rapid spectrophotometric method for the determination of the δ non-stoichiometric oxygen coefficient in YBa2Cu3O6.5+δ samples has been developed without precise measurement of the sample mass. The standard deviation of the δ- and y-coefficient results is 0.017. The reproducibility of the y-values in relative units (sr ) is 0.2%. The accuracy of the results has been confirmed by the method, which is based on the absorbance measurement of the I 3−-starch compound. Graphical abstract

Overview of the recent performance of DI-BSCCO wire

Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires since the discovery of Bi-based superconductors. DI-BSCCO (Dynamically-Innovative BSCCO) is the high performance wires produced with the controlled-overpressure (CT-OP) sintering technique. The recent R&D activities have enabled to produce the 1000m-long wires with Ic of 200A and the maximum Ic reached 250A at 77K by short sample. Besides, the fine control of the carrier density with the non-stoichiometric oxygen enhanced the in-field Ic performances at lower temperature. At a magnetic field of 3T applied normal to wire surface, the Ic of 280A at 30K, and 420A at 20K were successfully achieved. To meet the growing needs for various high temperature superconducting applications, Type HT wire with high mechanical strength and Type G wire with low thermal conductivity have been developed.

Structure Analysis and Magnetic Properties of Nano-Sized NbxNiyZn1−x−yFe2O4 Powders Formed by Combustion Synthesis and Mechanical Milling

Nano-sized Nb(x)Ni(y)Zn1-x-yFe2O4 ferrites with average particle size of less than 100 nm were prepared by using self-propagating high-temperature synthesis and mechanical ball milling. Average ferrites size of the SHS products were less than about 100 nm after 20 minutes mechanical milling. The average combustion temperatures and the combustion propagating rates were in the ranges of 1145 to 1543 K and 4.1 to 7.2 mm/s, respectively. Rietveld refinement of the pattern converged to good agreement (chi2 = 4.87). Final product of SHS was Nb0.13Ni0.41Zn0.46Fe2O4 with Fd3m structure and the lattice parameter of 0.83623 nm. Maximum magnetization (Ms), residual magnetization (Mr), coercive force (iHc) and susceptibility of the Nb0.013Ni0.41Zn0.46Fe2O4 ferrites formed at the oxygen pressure of 0.25 MPa were 12.30 Wb/m2 Kg, 1.57 Wb/m2 Kg, 6321 A/m, and 0.02 m3/Kg, respectively. Niobium addition to nickel-zinc ferrites resulted in increasing Ms, Mr and iMc about 59%, 78% and 387%, respectively. Neutron diffractometry revealed that the variation of magnetic properties was related to non-stoichiometric number and oxygen position of the niobium-nickel-zinc ferrites due to the competitive reduction reaction among niobium, nickel and zinc oxides.

Mixed Conductivity, Nonstoichiometric Oxygen, and Oxygen Permeation Properties in Co-Doped Sr3Ti2O7−δ

Electrical conductivity and oxygen permeation rates in Co-doped Sr(3)Ti(2)O(7-δ) with Ruddesden-Popper type structures were investigated. The effects of metal dopants (M) in the Ti site of Sr(3)Ti(2)(M)O(7-δ) on the mixed conductivity were also studied. Doping of Sr(3)Ti(2)O(7-δ) with Co was found to be effective for improving the electrical conductivity as well as the oxygen permeation rate, which could be assigned to the increased oxygen vacancy concentration by doping Co(3+) into Ti(4+) sites. The nonstoichiometric oxygen of these oxides was measured by using a thermal gravimetric method. The creation of oxygen vacancies, which is compensated with Co(3+) doping, leads to higher oxide ion conductivity. The oxygen permeation rate monotonously increased with increasing amounts of Co in the Ti site. Sr(3)Ti(0.8)Co(1.2)O(7-δ) exhibited high oxide ion conductivity and a large oxygen permeation rate. The highest oxygen permeation rate achieved a value of 2.02 cc min(-1) cm(-2) at 1273 K for Sr(3)Ti(0.8)Co(1.2)O(7-δ). Neutron diffraction analysis and redox titration suggests that the oxygen diffusion occurs through oxygen vacancies in the perovskite block, but not through excess oxygen in the rock salt block.

Assessment of perovskite-type La0.8Sr0.2ScxMn1−xO3–δ oxides as anodes for intermediate-temperature solid oxide fuel cells using hydrocarbon fuels

Composites formed by the infiltration of 40wt% La0.8Sr0.2ScxMn1−xO3–δ (LSSM) oxides (x=0.1, 0.2, 0.3) into 65% porous yttria-stabilized zirconia (YSZ) are investigated as anode materials for intermediate-temperature solid oxide fuel cells for hydrocarbon oxidation. The oxygen non-stoichiometry and electrical conductivity of each LSSM–YSZ composite are determined by coulometric titration. As the concentration of Sc increases, the composites show higher phase stability and the electrical conductivity of LSSM is significantly affected by the Sc doping, the non-stoichiometric oxygen content, and oxygen partial pressure (p(O2)). To achieve better electrochemical performance, it is necessary to add ceria-supported palladium catalyst for operation with humidified CH4. Anode polarization resistance increases with Sc doping due to a decrease in electrical conductivity. An electrolyte-supported cell with a LSSM–YSZ composite anode delivers peak power densities of 395 and 340mWcm−2 at 923K in humidified (3% H2O) H2 and CH4, respectively, at a flow rate of 20mLmin−1.

Wet-chemical dip-coating preparation of highly oriented copper–aluminum oxide thin film and its opto-electrical characterization

Transparent p-type semiconducting copper aluminum oxide thin film has been synthesized by a wet-chemical route. CuCl and AlCl 3, dissolved in HCl, are taken as starting materials. pH value of the solution is controlled by adding a measured amount of NaOH into it. Films are deposited by dip-coating technique on glass and Si substrates followed by annealing in air at 500 °C for 3 h. XRD pattern confirms the crystalline CuAlO 2 phase formation in the film and also indicates a strong (0 0 6) orientation. UV–vis spectrophotometric measurements show high transparency of the film in the visible region with a direct allowed bandgap of 3.94 eV. Electrical measurements depict the thermally activated conduction within the films. Thermoelectric measurements confirm the p-type nature of the films. Compositional analysis shows the presence of excess (nonstoichiometric) oxygen within the material, which is incorporated during the air-annealing of the film. According to defect equilibrium, this excess oxygen is predicted to cause the p-type conductivity of the film. This type of cost-effective solution-based technique is very useful for volume production of this kind of technologically important material for transparent electronic and other diverse applications.

Electrical conductivity, Seebeck coefficient, and defect structure of oxygen nonstoichiometric Nd 2− xSr xNiO 4+ δ

To elucidate the electronic state and the conduction mechanism of Nd 2NiO 4+ δ series oxides at high temperatures, the electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of Nd 2− x Sr x NiO 4+ δ ( x = 0, 0.2, 0.4) were measured as a function of the Sr content, temperature, and oxygen partial pressure. The hole mobility is estimated from the electrical conductivity and the hole concentration which is defect chemically determined. The mobility slightly decreases as temperature increases as in metals at high temperatures. The relationships between the Seebeck coefficient, electrical conductivity, and hole concentration can be explained by Mott's equation, which expresses the Seebeck coefficient for metals. Semi-quantitative analyses strongly indicate that the electron or hole is itinerant in Nd 2− x Sr x NiO 4+ δ , and the conduction mechanism is metal-like band conduction at high temperatures. Based on the experimental results, schematics for energy level and band structure are proposed. At high temperatures, free holes in the σ x2− y2 band composed of d x2− y2 orbitals contribute to metallic conduction.

Preparation of La-Mn-O Perovskite Catalyst by Microwave Irradiation Method and its Application to Methane Combustion

La-Mn-O perovskite catalyst was successfully prepared by microwave irradiation processing using only several minutes for heat treatment, and characterized by XRD, SEM, XPS, H2-TPR and O2-TPD. The catalyst by microwave irradiation processing with 6 min, mainly non-stoichiometric excess oxygen LaMnO3.15 phase, shows excellent catalytic performance. Compared with the sample prepared by conventional method, it possesses relative higher surface area, the increase in Mn4+ and oxygen vacancy content and the enhancement in the mobility of lattice oxygen which play an important part in the methane combustion.

Ni–Me–O mixed metal oxides for the effective oxidative dehydrogenation of ethane to ethylene – Effect of promoting metal Me

This work examines the effect of Li, Mg, Al, Ga, Ti, Nb and Ta on the properties and catalytic behavior of Ni-based mixed metal oxides in the ethane oxidative dehydrogenation reaction. In all mixed oxides, with the exception of Ta, the dopants modify the NiO lattice, which contracts or expands depending on the radius of the cation, indication for the formation of solid solutions. This systematic study of doping metals varying from low (+1) to high valence (+5) elements clearly demonstrates the large effect of the dopants’ valence. O 2-TPD experiments showed that increasing the valence of the foreign cation reduces the non-stoichiometric oxygen due to the cation deficient nature of the host oxide, in complete agreement with the principle of controlled valence. Temperature-programmed isotopic oxygen exchange measurements portray furthermore that not only the quantity but also the lability of the oxygen species reduces with increasing dopant valence. These electrophilic oxygen species catalyze the total oxidation reactions of ethane to CO x , and thus their reduction/elimination favors the selective oxidation to ethylene. Ni–Nb–O exhibited the best catalytic performance with a 46% ethene yield at 400 °C, while Ni–Li–O demonstrated the worst results with a respective yield of 8.42%. Based on the results of the present work, it can be inferred that the catalytic properties of NiO in ethane ODH can be tuned with doping, based on the nature of the dopant in Me-promoted NiO catalysts. Depending on the valence of the foreign species, the dopants can increase or decrease the unselective electrophilic oxygen radicals of NiO, leading to, respectively, reduced or enhanced ethane ODH activity.

Electrical Conductivity and Thermoelectric Power of La2-xSrxNiO4+δ

Electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of La2-xSrxNiO4+δ (x = 0, 0.2, 0.4) were measured as a function of Sr content, temperature, and oxygen partial pressure. At above around 873 K, mobility of hole decrease as temperature increase. The temperature dependence of hole mobility indicate large polaron conduction. The relationship between Seebeck coefficient and electrical conductivity is explained by metal like band conduction model. The behavior of electrical conductivity and Seebeck coefficient strongly suggests that electron (hole) is itinerant in La2-xSrxNiO4+δ at high temperatures.

Methyl ethyl ketone combustion over La-transition metal (Cr, Co, Ni, Mn) perovskites

A series of LaBO3 (B=Cr, Co, Ni, Mn) and La0.9K0.1MnO3+δ perovskites have been prepared and tested as catalysts in the combustion of methyl ethyl ketone (MEK) at two concentration levels in air. Complete MEK conversion can be achieved for the most concentrated stream (1250ppmv, WHSV=425h−1) at temperatures between 270°C (manganite) and 345°C (chromite). Activity is governed by the nature of the cation in position B and related to reducibility, being comparable for manganite activity with that of the much more expensive Pt-supported catalysts. Doping with K of lanthanum manganite produces an increase in surface area, as well as the formation of non-stoichiometric oxygen and a greater proportion of Mn4+ on the surface. All these factors may have a role in increasing its activity for catalytic combustion. Catalytic results suggest a marked influence of MEK concentration on the combustion rate. MEK oxidation to CO2 goes through acetaldehyde as intermediate product; methyl vinyl ketone and diacetyl (2,3-butanedione) were also formed, albeit in very low amounts. Nevertheless, acetaldehyde yield is zero at complete conversion, so the combustion of MEK can be carried out over these perovskite systems with 100% selectivity for CO2.