Solid-state synthesis of undoped and Sr-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

The solid-state synthesis of undoped K_0.5Na_0.5NbO₃ (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 °C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 °C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 °C and finishes at 540 °C, and the second step has an onset at 540 °C and finishes with the end of the reaction between 630 and 675 °C, depending on the particle size distribution of the Nb₂O₅ precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 °C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 °C.     

Monitoring Silica Supported Molybdenum Oxide Catalysts at Work: A Raman Spectroscopic Study

The structure of silica SBA‐15‐supported molybdenum oxide catalysts is investigated during selective oxidation of propylene at 500 °C using operando Raman spectroscopy. The active catalysts contain mixtures of dispersed molybdenum oxide species exhibiting monooxo and dioxo structure. An increase in molybdenum oxide loading results in a decrease of the ratio of dioxo and monooxo species from 3.8 to 1.9, as determined by quantitative analysis of Raman spectra. Additional in situ Raman studies at 500 °C reveal that the dioxo/monooxo ratio increases in the presence of steam at higher molybdenum oxide loadings. The observed structural changes are assigned to shifts in the equilibrium between dioxo and monooxo species resulting from hydration/dehydration of the catalyst. This study demonstrates that the detailed structure of nanostructured molybdenum oxide catalysts depends on temperature, gas‐phase composition, and molybdenum oxide loading.     

Synthesis and properties of Mo and W ions co-doped porous nano-structured VO2 films by sol–gel process

Porous nano-structured vanadium dioxide (VO₂) films doped with Mo and W ions had been synthesized by sol gel process by employing a sol containing ammonium molybdate and ammonium tungstate with the addition of cetyltrimethyl ammonium bromide (CTAB). The effects of molybdenum and tungsten co-doping and CTAB addition on the structure, morphologies, crystalline and optical properties of VO₂ films were investigated systematically in this study. The composition and microstructure were detected by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The Mo and W ions co-doped porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), large transmittance difference (55 %) before and after the phase transition, low transition temperature (35 °C), wide hysteresis width (22 °C) and fast phase transition. The results suggest that such Mo and W ions co-doped porous nano-structured VO₂ film is an ideal fundamental material for optical data storage.     

Characterization of vanadium-based olefin polymerization catalyst precursors by solid-state 51v-nmr

Several VOCL₃-based ethylene polymerization catalyst precursors were prepared on silica and studied by solid-state ⁵¹V-NMR. The structure of the vanadium species in these samples, as determined by ⁵¹V-NMR, did not have any significant effect on the resultant polyethylene MI or MWD. This result is significant since conventional wisdom says the attachment of the transition metal to the silica plays a key role in polymer properties. VOCl₃ reacted with hexamethyldisilazane-treated silica and with 250°C dried silica results in double attachment of the vanadium to the silica, yet the catalysts which formed had different reactivities and produced polyethylene with different HLMIs. On the other hand, VOCl₃ reacted with 600°C dried silica results in single attachment of the vanadium to the silica, yet this catalyst had a similar reactivity and produced polymer properties similar to the doubly attached vanadium on 250°C dried silica. Two theories are offered to explain the lack of correlation between catalyst precursor structure and catalyst performance. © 1995 John Wiley & Sons, Inc.     

Non-Thermal Plasma Activation of Gold-Based Catalysts for Low-Temperature Water–Gas Shift Catalysis

Non-thermal plasma activation has been used to enable low-temperature water-gas shift over a Au/CeZrO₄ catalyst. The activity obtained was comparable with that attained by heating the catalyst to 180 °C providing an opportunity for the hydrogen production to be obtained under conditions where the thermodynamic limitations are minimal. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), structural changes associated with the gold nanoparticles in the catalyst have been observed which are not found under thermal activation indicating a weakening of the Au−CO bond and a change in the mechanism of deactivation.     

Non‐Thermal Plasma Activation of Gold‐Based Catalysts for Low‐Temperature Water–Gas Shift Catalysis

Non‐thermal plasma activation has been used to enable low‐temperature water‐gas shift over a Au/CeZrO₄ catalyst. The activity obtained was comparable with that attained by heating the catalyst to 180 °C providing an opportunity for the hydrogen production to be obtained under conditions where the thermodynamic limitations are minimal. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), structural changes associated with the gold nanoparticles in the catalyst have been observed which are not found under thermal activation indicating a weakening of the Au−CO bond and a change in the mechanism of deactivation.     

Selective Template Removal by Thermal Depolymerization to Obtain Mesostructured Molybdenum Oxycarbide

The carbon content of mesostructured organic‐inorganic hybrid material of a cylindrical block copolymer template of poly(2‐vinylpyridine)‐block‐poly(allyl methacrylate) (P2VP‐b‐PAMA) and ammonium paramolybdate (APM) could be reduced by thermal depolymerization. By calcination in vacuo at 320 °C the PAMA core can be completely removed while the remaining P2VP brush preserves the mesostructure. The P2VP‐APM composite can then be carburized in‐situ to MoO_xC_y in a second pyrolysis step without any additional carbon source but P2VP. The molybdenum oxycarbide nanotubes obtained, form hierarchically porous non‐woven structures, which were tested as catalyst in the decomposition of NH₃. They proved to be catalytically active at temperatures above 450 °C. The activation energy was estimated from an Arrhenius Plot to be 127 kJ · mol^–1.     

Low temperature synthesis of Li5La3Nb2O12 with cubic garnet-type structure by sol–gel process

A cubic Li₅La₃Nb₂O₁₂ phase with a garnet framework was synthesized by the sol–gel process, in which lithium hydroxide, niobium oxide and acetic lanthanum were used as starting materials, while water was used as solvent. Pure garnet-like Li₅La₃Nb₂O₁₂ powders were obtained after heating the gel precursor at 700 °C for 6 h with 10 % excess lithium salt. The calcination temperature is nearly 250 °C lower than that by the solid state reaction. The phase transforms from cubic to tetragonal symmetry with loss of lithium at 717 °C, but the garnet framework remains stable to above 900 °C. A pellet annealed at 900 °C for 6 h had a room-temperature Li⁺-ion conductivity σ_Li (22 °C) = 1.0 × 10^?5 S cm^?1, a little higher than that attained by solid-state synthesis. The Li₅La₃Nb₂O₁₂ compound was chemically stable against two commonly used cathode materials, LiMn₂O₄ and LiCoO₂, up to 900 °C and against metallic lithium.     

Calcination temperature-dependent morphology of photocatalytic ZnO nanoparticles prepared by an electrochemical–thermal method

ZnO nanoparticles (NPs) with tunable morphologies were synthesized by a hybrid electrochemical–thermal method at different calcination temperatures without the use of any surfactant or template. The NPs were characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction, dynamic light scattering, thermogravimetry–differential thermal analysis, scanning electron microscope and N₂ gas adsorption–desorption studies. The FT-IR spectra of ZnO NPs showed a band at 450 cm^?1, a characteristic of ZnO, which remained fairly unchanged at calcination temperatures even above 300 °C, indicating complete conversion of the precursor to ZnO. The products were thermally stable above 300 °C. The ZnO NPs were present in a hexagonal wurtzite phase and the crystallinity of ZnO increased with an increasing calcination temperature. The ZnO NPs calcined at lower temperature were mesoporous in nature. The surface areas of ZnO NPs calcined at 300 and 400 °C were 51.10 and 40.60 m² g^?1, respectively, which are significantly larger than commercial ZnO nanopowder. Surface diffusion has been found to be the key mechanism of sintering during heating from 300 to 700 °C with the activation energy of sintering as 8.33 kJ mol^?1. The photocatalytic activity of ZnO NPs calcined at different temperatures evaluated by photocatalytic degradation of methylene blue under sunlight showed strong dependence on the surface area of ZnO NPs. The ZnO NPs with high surface area showed enhanced photocatalytic activity.     

Vanadium complex with tetradentate [O,N,N,O] ligand supported on magnesium type carrier for ethylene homopolymerization and copolymerization

Immobilization of 1,2‐cyclohexylenebis(5‐chlorosalicylideneiminato)vanadium dichloride on the magnesium support obtained in the reaction of MgCl₂·3.4EtOH with Et₂AlCl gives a highly active precursor for ethylene homopolymerization and its copolymerization with 1‐octene. This catalyst exhibits the highest activity in conjunction with MAO, but it is also highly active with AlMe₃ as a cocatalyst. On the other hand, when combined with chlorinated alkylaluminum compounds, Et₂AlCl and EtAlCl₂, it gives traces of polyethylene. Moreover, its catalytic activity is strongly affected by the reaction temperature: it increased with rising polymerization temperature from 20 °C to 60 °C. The kinetic curves obtained for the supported vanadium catalyst, in contrast to its titanium analogue, are of decay type, yet the reduction in the polymerization rate is rather moderate in the early stages of polymerization, and then it is relatively very slow. The vanadium catalyst gives copolymers at a lower yield than the titanium one does, but with the significantly higher 1‐octene content. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 471–478, 2010     

Synthesis and properties of M2V8O21 (M = K, Tl) octavanadates and K2?xTlxV8O21 (0 ≤ x ≤ 2) solid solutions

Formation parameters and properties of M₂V₈O₂₁ octavanadates where M = K and Tl were studied using X-ray powder diffraction, microscopy, thermogravimetry, vibrational spectroscopy, EPR, and voltammetry. Original synthesis processes for these compounds were developed; their thermal stability parameters were determined. Potassium and thallium(I) octavanadates were shown to form complete solid solutions with each other. Potassium octavanadate in air is stable to 450°C; above this temperature, it transforms, on account of partial reduction of vanadium, to vanadium bronze by the reaction K₂V₈O₂₁ ↔ K₂V₈O_20.8 + 0.1O₂. The K₂V₈O_20.8 percentage in the sample increases with rising temperature. The substitution of small thallium amounts for potassium (x ≥ 0.025) enhances the stability of the phase until it melts at ∼525°C.     

Alumina nanofibers obtained via electrospinning of pseudo-boehmite sol/PVP solution

Alumina nanofibers were fabricated by calcination of the polyvinylpyrrolidone (PVP)/pseudo-boehmite nanocomposite precursor fibers formed by electrospinning PVP/ethanol solution of dispersed pseudo-boehmite nanoparticles with and without additive of silica. The evolution of the phase, mechanical property and morphological features of the calcined fibers were studied and the effect of adding SiO₂ on the phase transformation of alumina was discussed. Adding SiO₂ can retard the phase transformation of γ-Al₂O₃ to α-Al₂O₃ and therefore inhibit the growth of alumina grains during calcination. Upon calcining the precursor fibers with 4 wt% SiO₂ additive at 1,300 °C, continuous alumina nanofibers with diameter ranging from 300 to 800 nm were obtained. These continuous nanofibers exhibited good flexibility and could be very promising for applications in filtration and catalyst support.     

Thermal stability of nanocrystalline ε-Fe2O3

Thermal behavior of highly crystalline ε-Fe₂O₃ nanoparticles of different apparent crystallite sizes was characterized using thermogravimetry, differential thermal analysis, and analysis of evolved gas by mass spectrometry. Phase composition of the samples was monitored ex situ by X-ray powder diffraction. The results show that the thermal stability of this metastable iron oxide polymorph decreases with increasing particle size. For the particle diameter of 19(2) nm, the transformation temperature was equal to 794(5) °C, while for 28(2) nm only 755(10) °C. Surface of the nanoparticles contained adsorbed water and carbon dioxide. Elimination of these species proceeds in two steps. Water is removed at temperatures below 200 °C and CO₂ in the temperature range between 200 and 450 °C.     

Precipitation sequence during ageing in β1 phase of Cu–Al–Ni shape memory alloy

The shape memory alloys based on the ternary system Cu–Al–Ni are able to produce a memory effect at high temperatures. However, if the material undergoes an accidental overheating, a transformation process leads to progressive loss of its characteristics. In this study, the effect of ageing on the metastable β₁ (austenite) phase of a Cu–13.3 %Al–4 %Ni shape memory alloy was investigated. In addition, the effects of heating rate between 450 and 580 °C on the structural transformations of austenite after cooling to room temperature were studied. Observation by transmission electron microscopy of the structure that has undergone an isothermal ageing shows that the precipitation process depends on the maximum ageing temperature. Furthermore, calorimetric analysis shows that precipitates dissolution is possible when rapid heating between 450 and 580 °C. This behaviour is observed on the cooling diagram which shows a martensitic transformation.     

Synthese en milieu hydrothermal et caractérisation de l'oxyhydroxyde de vanadium V3+OOH et d'une nouvelle variété allotropique du dioxyde VO2

Vanadium oxyhydroxide containing mostly trivalent vanadium was synthesized under hydrothermal conditions at 200°C and 2 k bar by hydrolysis of NaVO₃ previously reduced under hydrogen. The structure of VOOH is of diaspore type, isotypic with A1OOH. Oxidation of VOOH at 80°C in air gives in a few days a new metastable phase of vanadium dioxide VO₂; the structure of this new phase is also of the diaspore type, the transformation VOOH → VO₂ being topotactic. Characterization of these phases was done by X-ray diffraction, thermogravimetry, and magnetic, and electrical measurements. Both phases show semiconductor behavior and no electronic transition is observed in VO₂ (diaspore) up to 200°C.     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

Substituents effect on thermal electrocyclic reaction of dihydroazulene–vinylheptafulvene photoswitch: a DFT study to improve the photoswitch

Thermal cyclization for a series of substituted vinylheptafulvenes (VHFs) to dihydroazulenes (DHAs) was studied at PBE0 method of density functional theory in the gas phase and in the acetonitrile solvent (through PCM). Judicious control of the thermal reaction through substituent is quite necessary to design thermally robust DHA–VHF photoswitches. For most of the substituents, DHA was predicted thermodynamically stable over VHF except for amino (in gas phase and solvent) and hydroxyl (in acetonitrile), where DHA isomers were calculated thermally unstable compared to VHF. Activation barriers for thermal electrocyclic reaction in both media showed positive correlation with Taft’s σ _R values at positions 7 and 5, however, a negative correlation was observed at position 4 and 6. The latter unprecedented behavior is proposed to arise from the delocalization of negative charges on the seven membered ring. Activation barriers for amino-substituted VHFs were generally lower than expected from Taft’s σ _R. A fluoro group at the position 7 was quite effective in imparting very high activation barrier (31.73 kcal mol^?1) for the thermal cyclization in the gas phase. However, in the acetonitrile solvent, the highest activation barriers were observed for electron withdrawing CHO (28.10 kcal mol^?1) and NO₂ (28.13 kcal mol^?1) groups at positions 7.     

Hydrothermal Synthesis and Characterization of a New Arsenic–Vanadium Compound with a β-[As<Subscript>8</Subscript>V<Subscript>14</Subscript>O<Subscript>42</Subscript>(SO<Subscript>4</Subscript>)]<Superscript>6−</Superscript> anion

A new arsenic vanadium compound (NH₄)₂[N(CH₃)₄]₄[β-As₈V₁₄O₄₂(SO₄)] 1 has been synthesized under hydrothermal conditions and characterized by single crystal X-ray diffraction, IR spectrum, elemental analysis and thermogravimetic (TG) analysis. Compound 1 crystallizes in monoclinic system with space group P2(1)/c, a = 11.4631(5) Å, b = 11.3539(5) Å, c = 24.6265(10) Å, β = 93.6620(10)°, V = 3198.6(2) ų, Z = 2, R1 = 0.0421 and wR2 = 0.1044. The molecule structural analysis reveals that compound 1 has a β-[As₈V₁₄O₄₂(SO₄)]^6? arsenic–vanadium cluster anion.     

Syngas production from ethanol dry reforming over Rh/CeO2 catalyst

Carbon dioxide reforming of ethanol over Rh/CeO₂ catalyst was deeply investigated at different reaction temperatures of 450–700 °C and reactant ratios (CO₂/ethanol from 1 to 3) under atmospheric pressure. The obtained results indicated that Rh/CeO₂ catalyst presented a promising activity and stability for syngas production from renewable bio-ethanol instead of conventional methane. Typically, CO₂-rich conditions (CO₂/ethanol = 3) were favorable for reaction process and dynamic coke cleaning, which led to remarkably stable performance over 65 h on stream. The strong redox capacity of CeO₂ support might also accelerate CO₂ activation and prevent the carbon accumulation over the catalyst surface. Additionally, tunable H₂/CO ratios were available by changing the CO₂/ethanol ratios. The results from characterization of samples before and after catalytic tests allowed to establish the relationship between textural properties and catalytic performance.     

Following the crystallisation of Bi2Mo2O9 catalyst by combined XRD/QuEXAFS

The formation ofβ-phase Bi₂Mo₂O₉ catalyst from a precursor precipitate has been studied using thein situ combined XRD/QuEXAFS technique and DSC during calcination. Accordingly the precursor was observed to undergo a number of changes in both the molybdenum (VI) coordination and long-range ordering during this heating. Initially the two other forms of bismuth molybdate (α-andγ-phases) were observed to form from the poorly crystalline precursor at about 230°C, however, theβ-phase eventually crystallised after prolonged heating at 560°C. Dedicated to Professor C N R Rao on his 70th birthday