Showing papers on "Thermal decomposition published in 2009"

Shape and size effects of ZnO nanocrystals on photocatalytic activity.

Abstract: A wet-chemical method was employed to prepare zinc oxide nanocrystals having controlled morphology through thermal decomposition of a zinc precursor in self-assembled supramolecular structures in solvent under mild conditions. This solution method offers finer tailoring of the size and shape of the nanocrystals and is complementary to most reported physical methods. Understanding the morphological effects of pure or modified zinc oxide nanocrystals on photocatalytic activity is important in regard to enhanced solar energy capture and utilization but has been scarcely addressed in the past. The photocatalytic rate was found to have no dependence on ZnO particle size, but the shape factor seems to be of overriding importance. Hexagonal platelike nanocrystals were found to display at least 5 times higher activity than rod-shaped crystals, which clearly suggests that the polar (001) and (001) faces are more active surfaces than the nonpolar surfaces perpendicular to them.

Plasmonic Cu2−xS Nanocrystals: Optical and Structural Properties of Copper-Deficient Copper(I) Sulfides

Abstract: Cu2−xS (x = 1, 0.2, 0.03) nanocrystals were synthesized with three different chemical methods: sonoelectrochemical, hydrothermal, and solventless thermolysis methods in order to compare their common optical and structural properties. The compositions of the Cu2−xS nanocrystals were varied from CuS (covellite) to Cu1.97S (djurleite) through adjusting the reduction potential in the sonoelectrochemical method, adjusting the pH value in the hydrothermal method and by choosing different precursor pretreatments in the solventless thermolysis approach, respectively. The crystallinity and morphology of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which shows that most of them might be of pure stoichiometries but some of them are mixtures. The obtained XRDs were studied in comparison to the XRD patterns of previously reported Cu2−xS. We found consistently that under ambient conditions the copper deficient Cu1.97S (djurleite) is more stable than Cu2S (chalco...

Aromatic Production from Catalytic Fast Pyrolysis of Biomass-Derived Feedstocks

Abstract: The conversion of biomass compounds to aromatics by thermal decomposition in the presence of catalysts was investigated using a pyroprobe analytical pyrolyzer The first step in this process is the thermal decomposition of the biomass to smaller oxygenates that then enter the catalysts pores where they are converted to CO, CO2, water, coke and volatile aromatics The desired reaction is the conversion of biomass into aromatics, CO2 and water with the undesired products being coke and water Both the reaction conditions and catalyst properties are critical in maximizing the desired product selectivity High heating rates and high catalyst to feed ratio favor aromatic production over coke formation Aromatics with carbon yields in excess of 30 molar carbon% were obtained from glucose, xylitol, cellobiose, and cellulose with ZSM-5 (Si/Al = 60) at the optimal reactor conditions The aromatic yield for all the products was similar suggesting that all of these biomass-derived oxygenates go through a common intermediate At lower catalyst to feed ratios volatile oxygenates are formed including furan type compounds, acetic acid and hydroxyacetaldehyde The product selectivity is dependent on both the size of the catalyst pores and the nature of the active sites Five catalysts were tested including ZSM-5, silicalite, beta, Y-zeolite and silica–alumina ZSM-5 had the highest aromatic yields (30% carbon yield) and the least amount of coke

Controllable synthesis of mesoporous Co3O4 nanostructures with tunable morphology for application in supercapacitors.

Abstract: Flower power: Various mesoporous Co3O4 architectural structures (see figure) have been successfully prepared through a facile binary-solution route and sequential thermal decomposition at atmospheric pressure. The electrochemical experiments showed that the specific capacitance of Co3O4 nanosheets was higher than that of Co3O4 microspheres in a KOH electrolyte. Novel and complex mesoporous 2D and 3D architectures of the oxide semiconductor Co3O4, including nanosheets, nearly monodisperse microspheres that are self-assembled from nanosheets, and copper-coin-like nanosheets, have been synthesized through a facile binary-solution route and sequential thermal decomposition at atmospheric pressure. The influence of different reaction conditions on the morphology of the products has been discussed in detail. The results revealed that the volume ratio of H2O and ethanolamine (EA) play a crucial role in the morphology of the precursor. The thermal decomposition of the corresponding precursor leads to the formation of the mesoporous structure. The products have been characterized by X-ray diffraction techniques, field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and Raman spectroscopy. The electrochemical properties of the Co3O4 electrodes were investigated by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. The electrochemical experiments revealed that the specific capacitance of the Co3O4 nanosheets was higher than that of the Co3O4 microspheres in a KOH electrolyte solution (3 m). Furthermore, the Co3O4 nanosheet electrodes exhibited good rate capabilities, and maintained 93 % of the initial capacity at a current density of 5 mA cm−2 in a KOH (3 m) electrolyte solution. The results show that Co3O4 nanosheets might have potential applications as electrode materials for supercapacitors.

Growth of graphene on Ir(111)

Abstract: Catalytic decomposition of hydrocarbons on transition metals attracts a renewed interest as a route toward high-quality graphene prepared in a reproducible manner. Here we employ two growth methods for graphene on Ir(111), namely room temperature adsorption and thermal decomposition at 870–1470 K (temperature programmed growth (TPG)) as well as direct exposure of the hot substrate at 870–1320 K (chemical vapor deposition (CVD)). The temperature- and exposure-dependent growth of graphene is investigated in detail by scanning tunneling microscopy. TPG is found to yield compact graphene islands bounded by C zigzag edges. The island size may be tuned from a few to a couple of tens of nanometers through Smoluchowski ripening. In the CVD growth, the carbon in ethene molecules arriving on the Ir surface is found to convert with probability near unity to graphene. The temperature-dependent nucleation, interaction with steps and coalescence of graphene islands are analyzed and a consistent model for CVD growth is developed.

Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Abstract: Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2D-XRD), and transmission electron microscopy coupled with selected area electron diffraction (TEM-SAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (1014)calcite display the following topotactic relationships: (1) (1210)calcite//(110)CaO; (2) (1104)calcite⊥ (110)CaO; (3) (1018)calcite//(110)CaO; and (4) (0114)calcite⊥(110)CaO; with [841]calcite//[110]CaO in all four cases. At this stage, the reaction mechanism is independent of P CO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy E α = 176 ± 9 kJ/mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

Nanostructured α-Fe2O3 Thin Films for Photoelectrochemical Hydrogen Generation

Abstract: α-Fe2O3 thin film photoelectrodes were fabricated by aerosol-assisted chemical vapor deposition (AACVD) using a new hexanuclear iron precursor [Fe6(PhCOO)10(acac)2(O)2(OH)2]·3C7H8 (1) (where PhCOO = benzoate and acac = 2,4-pentanedionate). The precursor (1) designed for AACVD has a low decomposition temperature and sufficient solubility in organic solvents and was synthesized by simple chemical techniques in high yield. It was characterized by melting point, FT-IR, X-ray crystallography, and thermogravimetry (TGA). The TGA analysis proved that complex (1) undergoes facile thermal decomposition at 475 °C to give iron oxide residue. In-house designed AACVD equipment was used to deposit highly crystalline thin films of α-Fe2O3 on fluorine-doped SnO2 coated glass substrates at 475 °C in a single step. The material properties were characterized by XRD, XPS, and Raman spectroscopy, and the results confirmed that films were highly crystalline α-Fe2O3 and free from other phases of iron oxide. Further analysis of ...

Lignin Structure, Properties, and Applications

Abstract: Polymeric features of lignin and its potential as a bio-resource are reviewed, focusing on its characteristic structure and properties. Lignin is a random copolymer consisting of phenylpropane units having characteristic side chains. Lignin slightly crosslinks and takes an amorphous structure in the solid state. The molecular motion is observed as glass transition by thermal, viscoelastic and spectroscopic measurements. The hydroxyl group of lignin plays a crucial role in interaction with water. By chemical and thermal decomposition, a wide range of chemicals can be obtained from lignin that can be used as starting materials for synthetic polymers, such as polyesters, polyethers, and polystyrene derivatives. At the same time, a variety of polymers can be derived from lignin by simple chemical modification. The hydroxyl group acts as a reaction site for the above chemical reaction.

Carbon cluster formation during thermal decomposition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-triamino-2,4,6-trinitrobenzene high explosives from ReaxFF reactive molecular dynamics simulations.

Abstract: We report molecular dynamics (MD) simulations using the first-principles-based ReaxFF reactive force field to study the thermal decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) at various densities and temperatures. TATB is known to produce a large amount (15−30%) of high-molecular-weight carbon clusters, whereas detonation of nitramines such as HMX and RDX (1,3,5-trinitroperhydro-1,3,5-triazine) generate predominantly low-molecular-weight products. In agreement with experimental observation, these simulations predict that TATB decomposition quickly (by 30 ps) initiates the formation of large carbonaceous clusters (more than 4000 amu, or 15−30% of the total system mass), and HMX decomposition leads almost exclusively to small-molecule products. We find that HMX decomposes readily on this time scale at lower temperatures, for which the decomposition rate of TATB is about an order of magnitude slower. Analyzing the ReaxFF MD results leads to the detailed atomistic structure of this carbon-rich phase of TATB and allows characterization of the kinetics and chemistry related to this phase and their dependence on system density and temperature. The carbon-rich phase formed from TATB contains mainly polyaromatic rings with large oxygen content, leading to graphitic regions. We use these results to describe the initial reaction steps of thermal decomposition of HMX and TATB in terms of the rates for forming primary and secondary products, allowing comparison to experimentally derived models. These studies show that MD using the ReaxFF reactive force field provides detailed atomistic information that explains such macroscopic observations as the dramatic difference in carbon cluster formation between TATB and HMX. This shows that ReaxFF MD captures the fundamental differences in the mechanisms of such systems and illustrates how the ReaxFF may be applied to model complex chemical phenomena in energetic materials. The studies here illustrate this for modestly sized systems and modest periods; however, ReaxFF calculations of reactive processes have already been reported on systems with 106 atoms. Thus, with suitable computational facilities, one can study the atomistic level chemical processes in complex systems under extreme conditions.

Decomposition of hydrocarbons to hydrogen and carbon

Abstract: This review assesses technologies and catalysts pertaining to the catalytic decomposition of hydrocarbons for the CO2-free generation of hydrogen for fuel cell applications through a single-step cracking (decomposition, decarbonization, dehydrogenation, pyrolysis, splitting, or dissociation) of hydrocarbons. It discusses and systematically categorizes the options for hydrocarbon decomposition to hydrogen and carbon. This decomposition helps to reduce green house gases by co-producing valuable carbon products such as carbon black or graphite-like carbon (carbon nanotubes or carbon filaments). The catalytic approach comprises metal and carbon-based catalysts while plasma-based decomposition depends on thermal or non-thermal methods. Almost all the proposed processes are applicable to a variety of gaseous and liquid hydrocarbon fuels, and some of these processes can potentially produce a stream of high-purity hydrogen. There have been successful attempts to use catalysts to reduce the maximum temperature of the thermal decomposition of hydrocarbons. Common catalysts used are noble and transition metals such as Ni, Fe, Pd, Co, Mo, etc., supported on high surface area ceramic substrates such as A12O3 and SiO2, etc. Several other publications disclose the use of carbon-based materials as catalysts for decomposition of hydrocarbons into H2 and carbon. The other non-catalytic decomposition methods include non-thermal low-temperature plasmas such as RF (radio frequency), dc (direct current) generators, microwave plasmatrons, and arc plasma jet.

Thermal decomposition of carbonates in fault zones: Slip-weakening and temperature-limiting effects

Abstract: [1] During an earthquake, the heat generated by fault friction may be large enough to activate the devolatilization of minerals forming the fault rocks. In this paper, we model the mechanical effects of calcite thermal decomposition on the slip behavior of a fault zone during an earthquake. To do so, we introduce the coupled effects of calcite volume loss, heat consumption, and CO2 production in the theoretical analysis of shear heating and thermal pressurization of pore fluids. We consider a rapidly deforming shear band consisting of a fluid-saturated carbonate rock. The equations that govern the evolution of pore pressure and temperature inside the band and the mass of emitted CO2 are deduced from the mass and energy balance of the multiphase-saturated medium and from the kinetics of the chemical decomposition of calcite. Numerical simulation of seismic slip at depths of 5 to 8 km show that decarbonation has two critical consequences on fault slip. First, the endothermic reaction of calcite decomposition limits the coseismic temperature increase to less than ∼800°C (corresponding to the initiation of the chemical reaction) inside the shear band. Second, the rapid emission of CO2 by decarbonation significantly increases the slip-weakening effect of thermal pressurization. The pore pressure reaches a maximum and then decreases due to the reduction of solid volume, causing a restrengthening of shear stress. Our theoretical study shows, on the example of decarbonation, that the thermal decomposition of minerals is an important slip-weakening process and that a large part of the frictional heat of earthquakes may go into endothermic devolatilization reactions.

Thermal degradation of poly(lactic acid) measured by thermogravimetry coupled to Fourier transform infrared spectroscopy

Abstract: Thermal decomposition of poly(lactic acid) (PLA) has been studied using thermogravimetry coupled to Fourier transform infrared spectroscopy (TGA-FTIR). FTIR analysis of the evolved decomposition products shows the release of lactide molecule, acetaldehyde, carbon monoxide and carbon dioxide. Acetaldehyde and carbon dioxide exist until the end of the experiments, whereas carbon monoxide gradually decreases above the peak temperature in that the higher temperature benefits from chain homolysis and the production of carbon dioxide. A kinetic study of thermal degradation of PLA in nitrogen has been studied by means of thermogravimetry. It is found that the thermal degradation kinetics of PLA can be interpreted in terms of multi-step degradation mechanisms. The activation energies obtained by Ozawa–Flynn–Wall method and Friedman’s method are in good agreement with that obtained by Kissinger’s method. The activation energies of PLA calculated by the three methods are 177.5 kJ mol−1, 183.6 kJ mol−1 and 181.1 kJ mol−1, respectively.

Lithium‐Catalyzed Dehydrogenation of Ammonia Borane within Mesoporous Carbon Framework for Chemical Hydrogen Storage

Abstract: Ammonia borane (AB) has attracted tremendous interest for on-board hydrogen storage due to its low molecular weight and high gravimetric hydrogen capacity below a moderate temperature. However, the slow kinetics, irreversibility, and formation of volatile materials (trace borazine and ammonia) limit its practical application. In this paper, a new catalytic strategy involved lithium (U) catalysis and nanostructure confinement in mesoporous carbon (CMK-3) for the thermal decomposition of AB is developed. AB loaded on the 5% Li/CMK-3 framework releases similar to 7 wt% of hydrogen at a very low temperature (around 60 degrees C) and entirely suppresses borazine and ammonia emissions that am harmful for proton exchange membrane fuel cells. The possible mechanism for enhanced hydrogen release via catalyzed thermal decomposition of AB is discussed.

Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes with Electrical Contact to Metallic Substrates Enabled by Thermal Decomposition of the Carbon Feedstock

Abstract: Growth of vertically aligned carbon nanotube (CNT) carpets on metallic substrates at low temperatures was achieved by controlled thermal treatment of ethylene and hydrogen at a temperature higher than the substrate temperature. High-resolution transmission electron microscopy showed that nanotubes were crystalline for a preheating temperature of 770 °C and a substrate temperature of 500 °C. Conductive atomic force microscopy measurements indicated electrical contact through the CNT carpet to the metallic substrate with an approximate resistance of 35 kΩ for multiwall carpets taller than two micrometers. An analysis of the activation energies indicated that thermal decomposition of the hydrocarbon/hydrogen gas mixture was the rate-limiting step for low-temperature chemical vapor deposition growth of CNTs. These results represent a significant advance toward the goal of replacing copper interconnects with nanotubes using CMOS-compatible processes.

Carbon-rich SiCN ceramics derived from phenyl-containing poly(silylcarbodiimides)

Abstract: Novel phenyl-containing polysilylcarbodiimides were synthesized and their thermolysis and crystallization behavior up to 2000 °C was investigated. The Si/C ratio of the preceramic polymer was varied in a defined way by starting from dichlorosilanes with different organic substituents, namely R and R′ with R = phenyl and R′ = H, phenyl, methyl or vinyl. Several techniques were employed to study the structural features of the polymers and their thermolysis products. The temperature of crystallization depends on the carbon content of the precursors. Thus, in the sample with the highest carbon content the separation of β-SiC from the amorphous SiCN matrix is observed at T > 1500 °C, resulting in the highest temperature of thermal stability against crystallization ever reported for a SiCN ceramic derived from polysilylcarbodiimides. Moreover, no crystallization of β-Si3N4 was observed.