Hydrotreating properties of mixed NbxMo1−xS2 alumina supported catalysts

Niobium-molybdenum disulfide solid solution (Nb_xMo_1−xS₂) has been prepared in a dispersed state on gamma alumina. The existence of this solid solution supported on alumina carrier has been proven with the help of EXAFS technique. The catalytic properties of these materials have been studied in hydrogenation and hydrodesulfurization reactions. Interestingly, as already observed for niobium sulfide, the activity of the Nb_xMo_1−xS₂ solid solution (HDS of DBT, P_tot=33 bar) is not decreased in the presence of H₂S up to p(H₂S)=200 Torr, at least up to x=0.4.     

Microwave dielectric properties and co-firing with copper of (Bi1−xCux)(Nb1−xWx)O4 ceramics

Effect of substitution of CuO and WO₃ on the microwave dielectric properties of BiNbO₄ ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ (x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Q_f values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ ceramics and copper electrode was co-fired under N₂ atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi_1−xCu_x)(Nb_1−xW_x)O₄ dielectric materials to be good candidates for LTCC applications with copper electrode.     

Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Electrical resistivity and thermopower of (La1−xSrx)MnO3 and (La1−xSrx)CoO3 at elevated temperatures

Electrical resistivity and Seebeck (S) measurements were performed on (La_1−xSr_x)MnO₃ (0.02x0.50) and (La_1−xSr_x)CoO₃ (0x0.15) in air up to 1073 K. (La_1−xSr_x)MnO₃ (x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La_1−xSr_x)CoO₃ (0x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO₃ showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase.     

Synthesis and properties of lanthanum hexaaluminate ceramic La1−xCaxAl11−y−zMgyTizO18

The aim of the present work is to obtain ceramic materials with a hexagonal structure and high density, hardness and mechanical strength at lower synthesis temperature. Ceramic samples with nominal composition La_1−xCa_xAl_11−y−zMg_yTi_zO₁₈ (x=0–1; y=0–3; z=0–3,5) are prepared. The samples are sintered at temperature 1500 °C by one-stage and two-stage ceramic technology. By X-ray diffraction and scanning electron microscopy, predominant phase LaAl₁₁O₁₈ and second phases LaAlO₃ and -Al₂O₃ are identified. Ceramic materials are characterized with high physico-mechanical properties and may be find application for production of mill bodies and materials for immobilization of nuclear waste.     

Electronic and structural properties of SnxTi1−xO2 solid solutions: a periodic DFT study

The structural and electronic properties of selected compositions of Sn_xTi_1−xO₂ solid solutions (x=0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard’s law in the Sn_xTi_1−xO₂ system. Our results also account for the fact that chemical decomposition in Sn_xTi_1−xO₂ system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of Sn_xTi_1−xO₂ solid solutions. Sn substitution for Ti in TiO₂ increases the oxidation–reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron–hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in Sn_xTi_1−xO₂ solid solutions.     

Sintering behaviour of La1−xSrxFeO3−δ mixed conductors

The sintering properties of La_1−xSr_xFeO_3−δ (x = 0.1, 0.25) mixed conductors have been investigated with particular emphasis on the effect of secondary phases due to cation non-stoichiometry (±5 mol% La excess and deficiency). Secondary phases, located at grain boundaries in cation non-stoichiometric materials, increased the sintering temperature compared to single-phase materials. Extensive swelling in final stage of sintering was observed in all materials, which resulted in micro-porous materials. The swelling was most pronounced in the phase pure and two-phase materials due to La-deficiency, while refractory secondary phases in La-excess materials inhibited both sintering, grain growth and swelling. In La-deficient materials, formation of molten secondary phases resulted in rapid swelling due to viscous flow. The present findings demonstrated the importance of controlling sintering temperature and time, as well as careful control of the cation stoichiometry of La_1−xSr_xFeO_3−δ in order to achieve fully dense and homogenous La_1−xSr_xFeO_3−δ ceramics.     

Comparative study of lithium ion conductors in the system Li1+xAlxA2−x (PO4)3 with A=Ti or Ge and 0≤x≤0·7 for use as Li sensitive membranes

Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li_1+xAl_xA_2−x^IV(PO₄)₃ (A^IV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li⁺ selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li⁺ migration than the Ge-based one. A grain conductivity of 10⁻³ S cm⁻¹ is obtained at 25°C in the case of Li_1·3Al_0·3Ti_1·7(PO₄)₃. A total conductivity of about 6×10⁻⁵ S cm⁻¹ is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.©     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Preparation, microstructure and microwave dielectric properties of ZrxTi1−xO4 (x=0.40–0.60) ceramics

Zr_xTi_1−xO₄ (x=0.40–0.60) ceramics sintered without additives were prepared from powders made by the coprecipitation of metal salts from aqueous solutions in order to investigate the existence range of a homogeneous phase and the relationships between composition, microstructure and the dielectric properties. XRD, TEM, SEM, EDS, and the dielectric measurements were used to characterize the products. A homogeneous solid solution was obtained. Its crystal structure was isomorphous with ZrTiO₄. The variation of the lattice parameters with TiO₂ content was discussed. The optimum sintering temperature of samples was dependent of composition. TiO₂ suppressed the densification and acted as a grain growth enhancer during the sintering process. With the increase in TiO₂ content the relative densities of the sintered bodies decrease, while the grain sizes increase. The dielectric properties at microwave frequency (1.8 GHz) in this system, especially Q value, were poor, due to low densification, impurities and lattice defects. The dielectric constant _r and Q value exhibited a significant dependence on the relative density and composition. Both _r and Q increased with the increase in relative density, but they were primarily influenced by the composition and the effect of the relative density could be ignored when the relative density was greater than 90% theoretical. _r increased slightly with increasing TiO₂ content, while Q value decreased.     

Synthesis, characterisation and properties of (Ba1 − xSrx) (Nd1/2Nb1/2) O3 ceramics for application as dielectric resonators in microwave circuits

The (Ba_{1 − x}Sr_x) (Nd_1/2Nb_1/2) O₃ ceramics have been prepared by the conventional ceramic route for different values of x. Addition of a small amount of CeO₂ (1 wt%) as a sintering aid increased the density of the samples. The structure and microstructure of the sintered samples are studied by X-ray diffraction and SEM methods. The dielectric properties of the samples are measured in the microwave frequency region as a function of composition. The dielectric constant decreases as x increases. The coefficient of thermal variation of resonant frequency decreases as the Sr content increases and goes to the negative side. The dielectric properties of (Ba_{1 − x}Sr_x) (Nd_1/2Nb_1/2) O₃ are in the range suitable for application as dielectric resonators in microwave circuits.     

Performance of La1−xSrxAl1−y−y′FeyMgy′O3−δ perovskites in methane combustion: effect of aluminum and magnesium content

Perovskites of different La_1−xSr_xAl_1−y−y′Fe_yMg_y′O_3−δ compositions (x=0, 0.1, 0.15, 0.2 and y=0.1, 0.3, 0.5, 0.8) were prepared from a reactive precursor slurry of hydrated oxides. Each sample was aged between 16 and 26 h up to 1473 K. Activity in methane combustion (1%/air) was determined in a plug-flow reactor, with 1 g catalyst and 24 l/h flowrate. Gradual decrease in activity due to thermal aging was observed, the degree of activity loss being composition dependent. Nevertheless, activity of samples aged at 1370 K was nearly independent of composition. The best thermal stability showed LaAl_0.65Fe_0.15Mg_0.2O₃ perovskite. None of the magnesium substituted perovskites performed better than a La_0.85Sr_0.15Al_0.87Fe_0.13O₃ reference sample.     

Microwave dielectric properties of rutile (Zn1/3Nb2/3)0.40(Ti1−xSnx)0.60O2 (0.15 ≤ x ≤ 0.30) ceramics

Microwave dielectric properties of (Zn_1/3Nb_2/3)_0.40(Ti_1−xSn_x)_0.60O₂ ceramics were investigated as a function of SnO₂ content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO₂ content, due to the larger ionic radius of Sn⁴⁺ (0.69 Å) than that of Ti⁴⁺ (0.605 Å) for octahedral site. Dielectric constant (K) of the sintered specimens was affected by the dielectric polarizability. Quality factor (Qf) was dependent on the degree of reduction of Ti⁴⁺ ion. With an increase of SnO₂ content, the temperature coefficient of resonant frequency (TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure.     

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO₃ (A = La, Nd, Sm) and Sm_1−xSr_xMnO₃ (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm³ g⁻¹ h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v.

Specific surface areas of perovskites were in the range 13–20 m² g⁻¹. XRD analysis showed that LaMnO₃, NdMnO₃, SmMnO₃ and Sm_1−xSr_xMnO₃ (x = 0.1) are single phase perovskite type oxides. Traces of Sm₂O₃ besides the perovskite phase were detected in the Sm_1−xSr_xMnO₃ catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO₃. The fraction of Mn(IV) in the Sm_1−xSr_xMnO₃ samples increased with x. TPR measurements on AMnO₃ showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO₃ > NdMnO₃ > SmMnO₃. In Sm_1−xSr_xMnO₃ the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO₂ below 1023 K. The activation energies of the AMnO₃ perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO₃ perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.     

Fabrication and thermoelectric properties of heavily rare-earth metal-doped SrO(SrTiO3)n (n = 1, 2) ceramics

To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT = S²σTκ⁻¹) of Ruddlesden–Popper phase SrO(SrTiO₃)_n (or Sr_n+1Ti_nO_3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ), of (Sr_1−xRE_x)_n+1Ti_nO_3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr_1−xRE_x)_n+1 Ti_nO_3n+1 evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the σ values simply depended on the dopant concentration, indicating that both La³⁺ and Nd³⁺ ions act as electron donors. The |S| values increased with temperature due to decrease in the chemical potential. Significant reduction of the κ values was observed as compared to cubic-perovskite SrTiO₃. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr_0.95La_0.05)₃Ti₂O₇.     

Interaction of NO and NO2 with the surface of CexZr1−xO2 solid solutions – Influence of the phase composition

Structural (XRD) and spectroscopic (EPR, IR and Raman) investigations were performed to elucidate the influence of CeO₂ content on the phase composition and surface chemistry of Ce_xZr_1−xO₂ solid solutions (x = 0.10–0.85), interacting with NO and NO₂ in the absence and presence of oxygen. Strong influence of ceria loading on the adsorption modes of both nitrogen oxides and the nature of the resultant surface species was revealed. Adsorption of NO led to formation of mononitrosyl complexes, dimers and N₂O, whereas interaction of NO₂ with the ceria–zirconia catalyst resulted in the adsorbate disproportionation or coupling, depending on the sample composition.     

Structure and microwave dielectric properties of Nd(2−x)/3LixTiO3

The structure evolution, and microwave dielectric properties of Nd_(2−x)/3Li_xTiO₃ ceramics (0 ≤ x ≤ 0.5) were investigated in this paper. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results show that samples with x = 0.2–0.4 exhibit single phase. Multi-phases of Nd₂Ti₂O₇, Nd_2/3TiO₃ and Nd₂Ti₄O₁₁ were observed when x = 0 and 0.1. The concentration and ordering degree of A-site decrease with the increase of x value. The dielectric constant increases up to x = 0.2 and then decreases with the further increase of x value. The Qf value decreases with the increase of x value. The temperature coefficient of resonant frequency exhibits negative value and the absolute value decreases greatly with the decrease of x value.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.     

Effect of the method of obtaining Li2CO3---LiyNi1−yO mixtures on the densification of the resulting LixNi1−xO solid solutions

The weight and plaque diameter change following trermal treatment, and the density and lattice constant of lithium-rich Li_xNi_1−xO solid solutions obtained by heat treatment at 750°C of Li₂CO₃---Li_yNi_1−yO (y < x) mixtures were measured as a function of time. It was found that densification of the solid solutions depends on the method of obtaining the Li₂CO₃---Li_yNi_1−yO mixtures. When a large amount of carbonate is put into the starting mixtures Ni---Li₂CO₃-binder, and the powders are milled together, sintering occurs; in contrast, when the carbonate is added after the formation of Li_yNi_1−yO solid solution, so that the interstices of the plaque are filled with molten carbonate, the resulting mixtures exhibit no densification. For densification to occur, Li₂CO₃ must be in intimate contact with the lithium-doped NiO grains.