Mn、Fe取代六铝酸盐的结构和甲烷催化燃烧性能

采用化学可控共沉淀法制备了系列取代六铝酸盐LaMe_xAl_12-xO_19-δ（Me＝Fe、Mn）催化剂,研究了焙烧温度和Fe、Mn的离子取代量对催化剂比表面、结构及甲烷催化燃烧活性的影响.结果表明,催化剂前驱物经1000℃焙烧,催化剂中开始有六铝酸盐晶相生成; 当焙烧温度提高到1200℃时,样品主要以六铝酸盐晶相存在.增加Fe、Mn离子取代量可以提高六铝酸盐晶相的结晶度,但同时导致晶粒增大,引起比表面下降.由XPS和TPR分析表明,Mn在六铝酸盐结构中以+2价和+3价混合价态存在,而Fe以+3价形式存在.用Fe和Mn离子取代晶格中的Al³⁺大大提高了六铝酸盐对甲烷催化燃烧活性,当Mn离子的取代数为1,Fe离子的取代数为2时催化剂的活性最高.     

Catalytic combustion of methane over barium hexaferrites

Barium hexaferrite BaFe₁₂O₁₉ and iridium-containing barium hexaferrites have been prepared by the citrates gel method. Their catalytic activity in methane combustion has been evaluated. BaFe₁₂O₁₉ is an efficient catalyst for this reaction, and the introduction of iridium in the hexaferrite structure does not improve this activity. Mössbauer spectroscopy suggests that a part of the iridium ions are incorporated in the hexaferrite structure, however in crystallographic sites where they cannot interact with the gas phase. Infrared study of CO adsorption reveals the presence of two types of iridium particles in the surface: small Ir particles, in strong interaction with the hexaferrite structure, and some larger Ir particles which were not incorporated into the lattice.     

Mössbauer effect study of the Fe-substituted NASICON

Fe-substituted NASICON was prepared by co-precipitation method and the content of iron at different oxidation states was estimated by Mössbauer spectroscopy (MS). Compounds with different phase composition were obtained depending on the preparation procedure. In the case of samples sintered in reducing atmosphere, NASICON containing di- and trivalent iron was detected. The lattice constants were calculated by Rietveld method. Higher values of the unit cell parameters for Na₃Zr_1−yFe_1−y²⁺Fe_2y³⁺P₃O₁₂ compared to the structural analogue γ-Na₃Fe₂P₃O₁₂ indicated the partial replacement of smaller Fe³⁺ ions by bigger Fe²⁺ and Zr⁴⁺ in the NASICON lattice. The validation of Mössbauer spectroscopy as the useful tool in the calculation of iron content was also performed. The sintering in air led to the multiphase product, which was identified by XRD and MS.     

Surface structure and catalytic performance of supported PtSn catalysts

The valence states of tin in supported PtSn catalysts after reduction were tracked by in situ Mössbauer spectroscopy. Combined with catalytic evaluation, it is suggested that only when tin is in oxidative states, the catalytic performance can be greatly improved. The changes of surface structure of PtSn/Al₂O₃ during repeated coking-regeneration-reduction (CRR) cycles were also studied.     

Magnetic and Mössbauer behavior of the nanostructured MgFe2O4 spinel obtained at low temperature

This study investigated the solution combustion synthesis technique to obtain the nanostructured magnesioferrite (MgFe₂O₄) spinel powder. The reaction was performed in an electric muffle furnace. Considering the characteristics of the as-synthesized powders, the 30% fuel-deficient formulation was selected for synthesis temperature evaluation. This formulation was synthesized at different furnace temperatures. Powder characterization was carried out by X-ray diffraction (XRD) to evaluate crystallographic analysis and crystallite size; Transmission Electron Microscopy (TEM) was done to assess the morphology and crystallite size; and Mössbauer spectroscopy and vibrational sample magnetometer (VSM) were performed to obtain magnetic measurements. Crystallite sizes estimated from the XRD technique increased with furnace temperature values, which were consistent with the results obtained by TEM. The characterized samples of MgFe₂O₄ had an average crystallite size of 42.8 nm using the DRX method, average saturation magnetization of 25.6 emu/g and coercive field not higher than 11 Oe.     

In situ Mössbauer spectroscopy of carbon-supported iron catalysts at cryogenic temperatures and in external magnetic fields

A highly dispersed carbon-supported iron catalyst has been studied within situ Mössbauer spectroscopy at temperatures down to 5 K and with external magnetic fields. It is shown that measurements of spectra in the presence of large magnetic fields considerably improves the information obtained from Mössbauer spectra.     

Preparation,characterisation and activity of an iron/sodalite catalyst for the oxidation of methane to methanol

An iron sodalite catalyst similar to that reported to have good selectivity for the oxidation of methane to methanol by Lyons et al. has been prepared, tested and characterised. In a limited range of temperature at 34 bar total pressure the selectivity of the catalyst to methanol is a little better than that of an empty silica glass reactor. Before reaction, X-ray powder diffraction and Mössbauer spectroscopy confirm the presence of Fe(III) in the sodalite framework. After reaction Mössbauer spectroscopy identifies Fe(II) and also small particles of iron(III) oxide, < 1 m in size.     

Studies on the dispersion states of Fe2O3 on γ-Al2O3 by means of Mössbauer spectroscopy and XRD

XRD and Mössbauer spectroscopy studies show that Fe₂O₃/-Al₂O₃ made by impregnation-calcination method may have monolayer dispersed Fe₂O₃ and -Fe₂O₃ crystallites. There is a threshold monolayer dispersion capacity. If the Fe₂O₃ loading is lower than the threshold, the Fe₂O₃ will disperse on the surface of -Al₂O₃ as a monolayer, then the sample will give an XRD pattern with no crystalline Fe₂O₃ peaks and a Mössbauer spectrum of a doublet with large quadruple splitting (1.11 mm/s). When the Fe₂O₃ loading is higher than the threshold, in addition to the monolayer dispersed Fe₂O₃, crystalline -Fe₂O₃ will appear, then the sample will give an XRD pattern with crystalline -Fe₂O₃ peaks and a Mössbauer spectrum with a quadrupole splitting doublet superimposed on a magnetic splitting sextuplet. Monolayer dispersion capacity obtained by quantitative XRD phase analysis and Mössbauer spectra analysis are consistent with each other, having the same value of 0.052 g Fe₂O₃/100 m² -Al₂O₃ surface.     

Structural,magnetic and electrical properties of Ga-substituted NiCuZn nanocrystalline ferrite

In this work, we studied the substitution effect of iron by gallium on the structural, magnetic and electrical properties of the ferrite system; Ni_0.5Cu_0.25Zn_0.25Fe_2−xGa_xO₄ (x = 0–1.0), synthesized by using the urea combustion method. XRD patterns of the samples calcined at 700 °C show only cubic spinel ferrite with an average crystallite sizes in the range of 40–54 nm. The lattice parameters were slightly changed with increasing Ga content which can be explained on the basis of the relative ionic radii of Ga³⁺ and Fe³⁺ ions. FT-IR measurements show two fundamental absorption bands, assigned to the vibration of tetrahedral and octahedral complexes, which were slightly changed with increasing Ga content. Mössbauer measurements enable us to predict the possible cation distribution of the system. It was found that Ga³⁺ ion prefer to substitute Fe³⁺ ions located in the octahedral site. Superparamagnetic state was observed in the Mössbauer spectra of the samples with Ga content >0.5. The decrease of the magnetic hyperfine field with gallium concentration was explained on the basis of supertransferred hyperfine interaction. A semiconducting behavior was inferred for all samples and the conductivity values were found to decrease with increasing the Ga content. The conduction mechanism in the spinel ferrite compounds was explained in terms of the hopping conduction process. The dielectric constant measured as a function of frequency and temperature was found to be dependent on the Ga concentration. The determined transition temperature was found to decrease with increasing Ga content.     

Synthesis and characterization of chemically and electrochemically prepared conducting polymer/iron oxalate composites

Poly(3-octyl-thiophene) (POT) and polypyrrole (PPy) iron oxalate composites were synthesized through a post-polymerization oxidative treatment. The composite of the latter has been prepared also by electrochemical polymerization. The samples have been characterized by X-ray diffraction (XRD), impedance spectroscopy, scanning electron microscope (SEM) combined with energy dispersive X-ray (EDX) spectroscopy, Mössbauer spectroscopy, cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). In case of PPy, two peaks in the XRD spectra show the presence of iron containing composite, while with POT only the layered structure originating from the octyl side-chain interactions was modified by the composite formation. The assumption of the weakening of short- and long-range interactions was proven by the decrease in conductivity of the composite. The successful electrochemical synthesis resulted a composite of ∼5% iron content, determined by EDX. Mössbauer spectroscopy measurements evidenced a composite containing mixed valence iron oxalate doping ions, which supports the indirect EQCM data.     

Fischer–Tropsch Synthesis: Changes in Phase and Activity During Use

Four iron catalysts (unpromoted, K-promoted, Si-promoted and K,Si-promoted) were activated and subjected to common Fischer–Tropsch synthesis conditions. At increasing times on stream, samples were withdrawn from the continuously stirred tank reactor in the reactor wax while keeping the sample blanketed with an inert gas. Mössbauer spectra were recorded for various samples and the iron phases of the catalyst were compared to the catalytic activity. A simple model based on bulk composition of the catalyst is not related to the catalytic activity during the course of the run.     

Some physical properties of anhydrous and hydrated Brownmillerite doped with NaF

Different samples of Brownmillerite (the ferrite phase of cement clinker) doped with 0, 1 or 3 wt.% NaF were prepared. At first, the oxide mixture of Brownmillerite was prepared according to the following composition: 4 mol CaO, 1 mol Al₂O₃ and 1 mol Fe₂O₃ in addition to 1 or 3 wt.% NaF. Each mixture was mixed very well, introduced into an electric furnace at 1300 °C for 1 h in a platinum crucible, and then quenched in air. The product was divided into four portions mixed with 40 wt.% distilled water to form Brownmillerite paste, except for one portion which was left dry. Each paste was molded into two molds; after 24 h, they were immersed in a distilled water and withdrawn after 1 or 3 days of hydration, respectively. The pastes were ground again. The anhydrous powders of Brownmillerites and the hydrated samples were prepared for a.c. conduction measurements by pressing it to be in pellets form. The two surfaces of each pellet were coated with silver paste. The a.c. conductivity and dielectric constant for different samples were measured using four-probe method. The data was collected from 320 up to 670 K. Mössbauer spectra and X-ray diffraction patterns were measured for each sample (anhydrous and hydrated) to confirm the formation of Brownmillerite, identify the iron states and the magnetic properties. The results showed that NaF addition to Brownmillerite expedites the hydration reaction rate. The superparamagnetic relaxation, which appeared in the anhydrous Brownmillerite spectra due to the small particle size, decreases with increasing the hydration time. Also, the Fe³⁺(Oh) state increases while Fe³⁺(Td) decreases with the time of hydration. The a.c. conductivity value at fixed frequency for anhydrous and hydrated samples was found to increase with NaF addition. The a.c. conductivity and Mössbauer measurements can be used as good tools to verify the purity of Brownmillerite phase and, accordingly, the purity of cement.     

Structural and magnetic properties of NiFe2−xBixO4 (x = 0, 0.1, 0.15) nanoparticles prepared via sol-gel method

NiFe_2−xBi_xO₄ (x = 0, 0.1, 0.15) nanopowders were synthesized via sol-gel method. The precursor gels were calcined at 773 K in air for 1 h to obtain the pure nanostructured NiFe_2−xBi_xO₄ spinel phase. The crystal structure and magnetic properties of the substituted spinel series of NiFe_2−xBi_xO₄ have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy, transmission electron microscopy and alternating gradient force magnetometry. Mössbauer spectroscopic measurements revealed that Bi³⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite, i.e., the crystal-chemical formula of the as-prepared nanoparticles may be written as: (Fe)[NiFe_1−xBi_x]O₄ (x = 0, 0.1, 0.15), where parentheses and square brackets enclose cations on sites of tetrahedral and octahedral coordination, respectively. Selective area electron diffraction studies provided evidence that the samples of the NiFe_2−xBi_xO₄ series, independently of x, exhibit the cubic spinel structure. The values of the saturation magnetization and the coercive field of NiFe_2−xBi_xO₄ nanoparticles were found to decrease with increasing degree of bismuth substitution.     

SrxCa1-xNiAl11O19催化剂的制备及其催化甲烷燃烧活性研究

用异丙醇盐水解法制备SrxCa1-xNiAl11O19(x=1.0,0.75,0.5,0.25,0)催化剂,通过X射线衍射、比表面积分析等实验技术及甲烷燃烧,对催化剂的结构和性质进行了考察。研究了掺杂不同量的钙离子对催化剂的结构以及对甲烷催化燃烧活性的影响。结果表明,催化剂在1 200℃焙烧后可以形成完整的六铝酸盐晶型,同时具有较高的催化性能和高温稳定性,不同量的Ca离子掺杂对于催化剂的比表面积有较大的影响。Sr0.25Ca0.75NiAl11O19催化剂具有较好活性,其起燃温度(T10%/℃)为590℃,完全转化温度(T90%/℃)为730℃。     

Ba-MCM-49分子筛上1-丁烯催化裂解性能的研究

以Ba-MCM-49分子筛为催化剂,纯1-丁烯为原料,考察了钡的含量,反应温度,原料空速及分压对烯烃催化裂解制丙烯、乙烯反应性能的影响。实验表明,适量的钡修饰可以在一定程度上抑制氢转移及芳构化副反应的发生,从而提高了目标产物丙烯和乙烯的选择性。     

Synthesis of sodium ferrite by precursor and combustion methods: A comparative study

Thermal decomposition of sodium ferricarboxylate precursors, Na₃[Fe(L)₆].xH₂O (L = formate, acetate, propionate, butyrate) has been carried out in flowing air atmosphere from ambient temperature to 850 °C. Various physico-chemical techniques i.e. TG, DTG, DSC, XRD, IR, Mössbauer spectroscopy, etc. have been employed to characterize the intermediates and end products. After dehydration, the anhydrous complexes undergo decomposition to yield various intermediates i.e. sodium propionate/oxalate/carbonate and α-Fe₂O₃ in the temperature range 260–285 °C. A subsequent decomposition of sodium carbonate leads to the formation of sodium oxide in the successive stages. Finally, nanosized ferrites of the stoichiometry, NaFeO₂ have been obtained above 760 °C as a result of solid-state reaction between α-Fe₂O₃ and Na₂O. The same ferrite has also been prepared by the combustion method at comparatively lower temperature (400 °C) and in less time than that of precursor/conventional ceramic methods.     

Interaction between ammonium sulfate-iron oxide shown by ESR and Mössbauer spectroscopy

The ammonium sulfate-iron oxide system treated at different temperatures has been characterized by ESR, IR and Mössbauer spectroscopy. Results show that during the heat treatment some interaction between ammonium sulfate and iron oxide takes place. As a result of this interaction a surface sulfato complex of iron is formed. The complex is involved in the sites responsible for activity for acid catalytic esterification. Above 573 K, the sulfato complex will gradually decompose with a further increase in temperature. A new structure scheme of the sulfato complex is proposed.     

Monte Carlo simulation studies of the catalytic combustion of methane

A Monte Carlo (MC) simulation is carried out of the catalytic oxidation of methane based on the pseudo-Langmuir-Hinshelwood (LH) mechanism proposed by Iglesia et al., that attempts to interpret the behavior of this reaction, which from experiments is assumed to be of the Mars-van Krevelen type. The results interpret reasonably some experimental findings (reaction order with respect to H₂O, CH₄, activity and structural sensitivity) if certain criteria based on the experiment and some laboratory data for the kinetic and thermodynamic parameters from the literature are considered.     

微型燃烧器内甲烷预混催化燃烧的数值研究

微尺度燃烧存在热量损失大、易熄火、燃烧不完全、转化效率不高等问题,因此对微型燃烧器内甲烷的燃烧采取预混催化燃烧方式来提高燃烧的稳定性和转化效率,为微型发动机碳氢燃料燃烧技术奠定基础。采用连续介质层流有限速率模型和二阶离散方法对微型燃烧器微流道内的催化燃烧、流动和传热进行了三维数值模拟。结果表明,甲烷质量流量和过量空气系数对催化转化效率有一定影响,壁面温度是影响催化转化效率的主要因素。甲烷质量流量、壁面温度与最佳过量空气系数之间具有一定的变化关系。可根据催化温度选择富燃料或富氧燃烧方式来提高微尺度催化转化效率。恒壁温边界条件下,催化燃烧主要发生在燃烧腔的下壁面。     

Oxidative desulphurisation of sulphur-rich coal

Process of oxidative desulphurisation of sulphur-rich containing Ukrainian coal was carried out in the temperature range of 623-723 K by air/steam mixture in the fluidised bed reactor. Mössbauer spectroscopy was applied to study the reaction products in form of iron compounds after every step of thermal treatment. The pyritic sulphur that constitutes most of total sulphur oxidises selectively at these conditions. At lower temperature (623 K) the ferrous sulphates and pyrite oxidised mainly to the ferric sulphate. At 673 K and higher temperatures α-Fe₂O₃ and SO₂ were basic products of FeS₂ oxidation. The transformation of FeS₂ into pyrrhotite as the first stage of desulphurisation was observed at 698 K independently of the partial pressure of oxygen. The iron monosulphide (FeS) was not found at any stage of desulphurisation suggesting the pyrrhotite Fe_1−xS oxidation before the sequential FeS₂→Fe_1−xS→FeS transformation.