Showing papers on "Thermal decomposition published in 2002"

NiCo2O4 Spinel: First Report on a Transition Metal Oxide for the Negative Electrode of Sodium-Ion Batteries

Abstract: This is the first time that a mixed transition metal oxide is used for the negative electrode of a sodium-ion battery. The NiCo2O4 spinel was prepared by thermal decomposition of oxalate precursors at 320 °C. The electrochemical reactions of this material with sodium lead to the complete reduction of the transition metals and the formation of Na2O. The new electrode material shows reversible capacities of ca. 300 mA h g-1 in sodium-ion cells versus NaxCoO2.

Thermal Stability of Quaternary Phosphonium Modified Montmorillonites

Abstract: Organically modified layered silicates (OLS) with high thermal stability are critical for synthesis and processing of polymer layered silicate nanocomposites (PLSN). In the current study, the non-oxidative thermal degradation chemistry of alkyl and aryl quaternary phosphonium-modified montmorillonites (P-MMT) was examined using TGA combined with pyrolysis/GC-MS. The morphology evolution at elevated temperature was investigated using in-situ high-temperature X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The onset decomposition temperature via TGA of these P-MMTs ranged from 190 to 230 °C. The initial degradation of the alkyl P-MMTs follows potentially two reaction pathways - ‚-elimination [E‚] and nucleophilic displacement at phosphorus [SN(P)] reflecting the multiple environments of the surfactant in the silicate. Aryl P-MMT decomposition proceeds via either a reductive elimination through a five-coordinate intermediate or radical generation through homologous cleavage of the P-phenyl bond. Overall, the interlayer environment of the montmorillonite has a more severe effect on stability of the phosphonium surfactant than previously reported for ammonium-modified montmorillonite (N-MMT). Nonetheless, the overall thermal stability of P-MMT is higher than that of N-MMT. These observations indicate that, in addition to their conventional purpose as stabilizers, phosphonium salts offer unique opportunities for melting processing polymer layered silicate nanocomposites.

Homogeneous precipitation of uniform hydrotalcite particles

Abstract: Hydrotalcite was synthesized by a homogeneous precipitation method utilizing urea hydrolysis. When the homogeneous aqueous solutions containing magnesium chloride, aluminum chloride, and urea were heated, hydrotalcite particles were obtained. Scanning electron micrographs revealed that the products were well-defined hydrotalcite particles. The particle sizes have been controlled from ca. 2 to 20 μm by the reaction temperature and the concentration of the reactants. The particle morphology of hydrotalcite was retained even after thermal decomposition, showing the possible application of the present well-defined hydrotalcite particles for the preparation of layered double hydroxide intercalation compounds by the reconstruction method.

Effects of melt-processing conditions on the quality of poly(ethylene terephthalate) montmorillonite clay nanocomposites†

Abstract: Organically modified montmorillonite was synthesized with a novel 1,2-dimethyl-3-N-alkyl imidazolium salt or a typical quaternary ammonium salt as a control. Poly(ethylene terephthalate) montmorillonite clay nanocomposites were compounded via melt-blending in a corotating mini twin-screw extruder operating at 285 °C. The nanocomposites were characterized with thermal analysis, X-ray diffraction, and transmission electron microscopy to determine the extent of intercalation and/or exfoliation present in the system. Nanocomposites produced with N,N-dimethyl-N,N-dioctadecylammonium treated montmorillonite (DMDODA-MMT), which has a decomposition temperature of 250 °C, were black, brittle, and tarlike resulting from DMDODA degradation under the processing conditions. Nanocomposites compounded with 1,2-dimethyl-3-N-hexadecyl imidazolium treated MMT, which has a decomposition temperature of 350 °C, showed high levels of dispersion and delamination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2661–2666, 2002

Isopropanol oxidation by pure metal oxide catalysts: number of active surface sites and turnover frequencies

Abstract: The objective of the present study was to determine the number of active surface sites and their nature, redox or acidic, for bulk metal oxide catalysts using isopropanol as a chemical probe molecule. Isopropanol oxidation activity on the following metal oxides was investigated: MgO, CaO, SrO, BaO, Y 2 O 3 , La 2 O 3 , CeO 2 , TiO 2 , ZrO 2 , HfO 2 , V 2 O 5 , Nb 2 O 5 , Ta 2 O 5 , Cr 2 O 3 , MoO 3 , WO 3 , Mn 2 O 3 , Fe 2 O 3 , Co 3 O 4 , Rh 2 O 3 , NiO, PdO, PtO, CuO, Ag 2 O, Au 2 O 3 , ZnO, Al 2 O 3 , Ga 2 O 3 , In 2 O 3 , SiO 2 , SnO 2 and Bi 2 O 3 . On average, the number of active surface sites for isopropanol dissociative adsorption on these catalysts was ∼2–4 μmol/m 2 . The number of active surface sites enabled quantification of the turnover frequency (TOF) for these catalysts. The TOF values for the various pure metal oxides were normalized at 200 °C. The TOFs of catalysts showing redox activity vary by six-orders of magnitude (10 2 to 10 −4 s −1 ). For catalyst showing acidic activity, the TOFs varied by over eight-orders of magnitude (10 1 to 10 −7 s −1 ). The reaction products from isopropanol oxidation at low conversions reflected the nature of the active surface sites, redox or acidic, on these catalysts. Redox surface sites yield acetone and acidic surface sites yield propylene. Small amounts of isopropyl ether formation are sometimes also observed via bimolecular recombination of surface isopropoxide species on acidic surface sites. All catalysts with the exception of Fe 2 O 3 and TiO 2 , exhibited extremely high selectivity to either redox or acidic products. Except for the sharp decrease in TOFs towards redox products with increasing bulk M–O heats of formation at low −Δ H f , no correlations were found between the TOFs and bulk metal oxide properties (TPR–H 2 and −Δ H f ). However, an inverse relation was found between the TOFs (redox) and the surface isopropoxide intermediate decomposition temperature at low decomposition temperatures. At moderate and high decomposition temperatures, the TOFs (redox) were almost independent of the surface isopropoxide decomposition temperature. The selectivity of the metal oxide catalysts was found to be independent of the TOFs.

The thermal decomposition of Mg-Al hydrotalcites: effects of interlayer anions and characteristics of the final structure.

Abstract: The thermal decomposition of hydrotalcites (HTs) with different interlayer anions in the 298 ± 523 K re- gion has been investigated by using transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and IR, 27 Al MAS-NMR and X-ray absorp- tion near-edge structure (XANES) spec- troscopy. The thermal stability of the HT with interlayer oxalate was remarkably higher than that of HTs with interlayer carbonate; the onset temperatures for decomposition were 523 K and 473 K, respectively. It is proposed that the basicity of the interlayer anion is the key factor in the onset of dehydroxyla- tion of the brucite-like layers: the lower the basicity, the more thermally stable the HT compound. After heat treatment at 723 K, small Mg(Al)O domains ( 5 nm) within the HT crystallites cause broadening of the XRD reflec- tions. The electron diffraction pattern consists of spots and rings, due to non- randomly oriented crystalline material. Formation of disordered bonds, caused by nonideal packing between the de- composing adjacent cation layers in the (1 1 1) direction, could explain the ex- panded d value in the resulting MgO- like phase observed with XRD and electron diffraction. The orientation of the Mg(Al)O domains in the heat-treat- ed material may be crucial for the so- called ™memory effect∫ of HTs.

Synthesis of spherical FePd and CoPt nanoparticles

Abstract: Co48Pt52 nanoparticles were prepared by the simultaneous chemical reduction of platinum acetylacetonate and the thermal decomposition of cobalt tricarbonyl nitrosyl. Similarly, Fe50Pd50 nanoparticles were prepared by the simultaneous reduction of palladium acetylacetonate and the thermal decomposition of iron pentacarbonyl. The average diameter for the Co48Pt52 particles was 7 nm, while the average diameter for the Fe50Pd50 particles was 11 nm. Films of the particles were cast onto silicon wafers from hydrocarbon solution. Only after annealing at 700 °C for three hours did the Co48Pt52 films transform to the L10 phase. However there was no evidence for the tetragonal phase for the films containing Fe50Pd50 particles after annealing at 700 °C for three hours.

Mechanism of OH formation from ozonolysis of isoprene: a quantum-chemical study.

Abstract: The formation and unimolecular reactions of primary ozonides and carbonyl oxides arising from the O(3)-initiated reactions of isoprene have been investigated using density functional theory and ab initio molecular orbital calculations. The activation energies of O(3) cycloaddition to the two double bonds of isoprene are found to be comparable (3.3-3.4 kcal mol(-1)), implying that the initial two O(3) addition pathways are nearly equally accessible. The reaction energies of O(3) addition to isoprene are between -47 and -48 kcal mol(-1). Cleavage of primary ozonides to form carbonyl oxides occurs with a barrier of 11-16 kcal mol(-1) above the ground state of the primary ozonide, and the decomposition energies range from -5 to -13 kcal mol(-1). OH formation is shown to occur primarily via decomposition of the carbonyl oxides with the syn-positioned methyl (alkyl) group, which is more favorable than isomerization to form dioxirane (by 1.1-3.3 kcal mol(-1)). Using the transition-state theory and master equation formalism, we determine an OH yield of 0.25 from prompt and thermal decomposition of the carbonyl oxides.

The characterization of organic modified clay and clay-filled PMMA nanocomposite

Abstract: Recently, polymer–clay hybrid materials have received considerable attention from both a fundamental research and application point of view.1–3 This organic–inorganic hybrid, which contains a nanoscale dispersion of the layered silicates, is a material with greatly improved physical and mechanical characteristics. These nanocomposites are synthesized through in situ polymerization or direct intercalation of the organically modified layered silicate (OLS) into the polymer matrix. Thus, understanding the relationship between the molecular structure and the thermal stability (decomposition temperature, rate, and the degradation products) of the OLS is critical. In this study, modern thermal analysis techniques combined with infrared spectroscopy and mass spectrometry (TGA-FTIR-MS) were used to obtain information on the thermal stability and degradation products of organic modified clay. Furthermore, the thermal and mechanical properties of clay-filled PMMA nanocomposites were determined by using TGA and DSC. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1702–1710, 2002

Thermal decomposition of ethanol. I. Ab Initio molecular orbital/Rice–Ramsperger–Kassel–Marcus prediction of rate constant and product branching ratios

Abstract: The unimolecular decomposition of CH3CH2OH has been investigated at the G2M (RCC2) level of theory. The decomposition reaction was found to be dependent strongly on pressure and temperature. Among the eight product channels identified, the H2O-elimination process (1) via a four-member-ring transition state is dominant below 10 atm in the temperature range of 700–2500 K. At the high—pressure limit and over 1500 K, cleavage of the C–C bond by reaction (2) producing CH3+CH2OH is predicted to be dominant while the CH3CH2+OH channel (8) also becomes competitive. The predicted high-pressure rate constants for the two major product channels can be given by k1=7.0×1013 exp(−34 200/T) and k2=3.7×1026 T−2.95 exp(−45 600/T) s−1, which compare reasonably with earlier data and with our preliminary experimental result obtained in a shock tube and static cell study. At the internal energy corresponding to the O(1D)+C2H6 reaction (140.7 kcal/mol above C2H5OH), the predicted branching ratios for the production of CH3, C2H...

Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO-synthesis, physical, chemical and spectroscopic properties, gas phase, and solid state structure.

Abstract: Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO, is obtained in high yield by the solvolysis of K[B(CF3)4] in concentrated sulfuric acid. The in situ hydrolysis of a single bonded CF3 group is found to be a simple, unprecedented route to a new borane carbonyl. The related, thermally unstable borane carbonyl, (C6F5)3BCO, is synthesized for comparison purposes by the isolation of (C6F5)3B in a matrix of solid CO at 16 K and subsequent evaporation of excess CO at 40 K. The colorless liquid and vapor of (CF3)3BCO decomposes slowly at room temperature. In the gas phase t1/2 is found to be 45 min. In the presence of a large excess of 13CO, the carbonyl substituent at boron undergoes exchange, which follows a first-order rate law. Its temperature dependence yields an activation energy (EA) of 112 kJ mol-1. Low-pressure flash thermolysis of (CF3)3BCO with subsequent isolation of the products in low-temperature matrixes, indicates a lower thermal stability of the (CF3)3B fragment, than is found for (CF3)3BCO. Towa...

Photooxidative and thermal degradation of polyethylenes: interrelationship by chemiluminescence, thermal gravimetric analysis and FTIR data

Abstract: The thermal and photooxidative behaviour of polyethylenes of different manufacturing histories (linear low, metallocene and high density) have been investigated by FTIR spectroscopy and the data related to the chemiluminescence (CL) and thermal analysis of the polymers. The CL intensity and activation energies for thermal decomposition were found to be dependent on the rates of oxidation of the polymers under light and heat. On thermal oxidation, the activation energies were found to decrease with time and followed the order HDPE>m-PE>LLDPE, which correlated with the results obtained by means of CL analysis, where a higher intensity of CL at low temperature was found for HDPE. On photooxidation, the order followed their instability, i.e. m-PE>HDPE>LLDPE. The rates of thermal oxidation in each case were found to be the same irrespective of the nature of the oxidation product, i.e. hydroperoxide, vinyl and carbonyl groups. The influence of branching in the polymer appears to be important, showing a decrease of thermal stability as branching in polyethylene increases.