Showing papers on "Thermal decomposition published in 1995"

Thermal decomposition and reformation of PdO catalysts; support effects

Abstract: The thermal decomposition of PdO and the reformation of Pd to PdO is dependent on the support upon which they are dispersed indicating significant metal (oxide) support interactions. The study defines temperature regions of different stabilities for Pd:O species. The nature of the Pd:O structure is of great importance for a large number of environmental catalytic applications including the gasoline automobile catalytic converter, ozone decomposition catalysts, abating emissions from natural gas fueled vehicles, primary combustion catalysts, etc. The ZrO2 support shows the largest hysteresis effect between the temperature of decomposition and reformation. In contrast both TiO2 and CeO2 have small hysteresis effects because of a significant increase in the temperature for reformation of the PdO. There exists a large region of temperature stability of the PdO when dispersed on these two carriers. This is quite favorable for those catalytic reactions where the oxide state is important such as the primary catalytic combustion of natural gas. Further evidence of the importance of PdO for methane oxidation is presented.

HMX and RDX - Combustion mechanism and influence on modern double-base propellant combustion

Abstract: Experimental investigations of the burning wave structures of HMX and RDX were carried out by microthermocouple technique at a pressure of 1-70 (90) atm. Burning-surface temperature, heat release in solid, heat feedback from gas to solid, and heat-release rate in gas near the surface were obtained. The data obtained show that in the condensed phase of combustion waves of nitramines, evaporation, and thermal decomposition occur simultaneously, with heat release in the solid being negative at p 5-10 atm. Activation energies of HMX and RDX decomposition in combustion waves were evaluated. Published results of investigation s of the thermal burning wave structures of double-base propellants (DBF) containing HMX and various catalysts were used to establish the influence of additional nitramines on the unified dependencies of simple DBFs. It was found that the gasification law, heat-release law in the solid, and heatfeedback law do not change, but that the dark zone laws and dependencies for temperature sensitivities change significantly when HMX is added.

From minerals to materials: synthesis of alumoxanes from the reaction of boehmite with carboxylic acids

Abstract: Reaction of pseudo-boehmite, [Al(O)(OH)]n, with carboxylic acids (RCO2H) results in the formation of the carboxylatoalumoxanes, [Al(O)x(OH)y(O2CR)z]n where 2x + y + z = 3 and R = C1–C13. The physical properties of the alumoxanes are highly dependent on the identity of the alkyl substituents, R, and range from insoluble crystalline powders to powders which readily form solutions or gels in hydrocarbon solvents, from which films may be readily spin-coated. The physical and chemical changes that occur during the reaction of boehmite with carboxylic acids, and the resulting alumoxanes, have been characterized by scanning electron and transmission electron microscopy (SEM and TEM), IR and multinuclear NMR spectroscopy, and thermogravimetric/differential thermal analysis (TG/DTA). The carboxylatoalumoxanes reported herein are spectroscopically similar to analogues prepared from small molecule precursors. Based on the IR and NMR spectra of the alumoxanes as well as comparison with the aluminium carboxylate compounds [Me2Al(µ-O2CR)]2 and Al(O2CR)(salen)(R = CH3, n-C5H11), a model structure of the alumoxanes is proposed, consisting of a boehmite-like core with the carboxylate substituents bound in a bridging mode. Furthermore, the alumoxane particles appear as rod or sheet-like particles, not linear polymers. This is proposed to be due to the destruction of hydrogen bonding within the mineral as hydroxide groups are removed and replaced with acid functionalities. All of the alumoxanes decompose under mild thermolysis to yield alumina. Mass spectral studies indicate that upon thermolysis the volatile decomposition products are water and the carboxylic acid.

The structure and chemistry of ch3 and ch radicals adsorbed on ni(111)

Abstract: A detailed analysis of the vibrational spectra of CH3, CH2D, and CD3 adsorbed on Ni(111) and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH4, CH3D, or CD4 is effected by molecular beam techniques. The ability to measure these spectra by high‐resolution electron energy loss spectroscopy (HREELS) at higher resolution (35 cm−1) and higher sensitivity (5×106 counts/s) has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH3 radical is adsorbed with C3v symmetry on a threefold hollow site. The symmetric C–H stretch mode of CH3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH2D is a superposition of those from the decomposition of CH3 and CD3. The adsorption site of...

27Al and 29Si MAS NMR Study of Kaolinite Thermal Decomposition by Controlled Rate Thermal Analysis

Abstract: The processing of kaolinite under controlled rate thermal analysis conditions (CRTA) provides homogeneously transformed samples at defined dehydroxylation rates. The kaolinite-to-metakaolinite transformation has been monitored by solid-state NMR and X-ray diffraction. The metakaolinite product is an amorphous material with no resolved XRD pattern and is based on a disordered polymerized silicon/aluminum network in which aluminum preferably occupies four- and five-fold structural positions in the ratio 60% AlVI, 30% AlV, and 10% AlVI. This complex dehydroxylation process has been viewed at different scales, with the aluminum NMR tracing a site-by-site destructon of the octahedral sheet and the silicon NMR and XRD following a more rapid destruction of the longrange order.

Microwave Characteristics of BaO‐TiO2 Ceramics Prepared via a Citrate Route

Abstract: Microwave dielectrics of the TiO2-rich BaO-TiO2 system (BaTi4O9 and Ba2Ti9O20) were prepared by the citrate route. Pure and well-crystallized BaTi4O9 and Ba2Ti9O20 particles of nanometer size (30–50 nm) could be obtained by thermal decomposition of citrate gel precursors. After sintering at 1200°–1350°C (for 2–10 h), dense compounds with >90% of theoretical density could be obtained. Dielectric properties of disk-shaped sintered specimens, in the microwave frequency region, were measured in the TE01δ mode. They were found to have excellent microwave characteristics: for BaTi4O9, er= 36, Q= 4900 at 10.3 GHz, and τf= 16 ppm/°C; and for Ba2Ti9O20, er= 37, Q= 5300 at 10.7 GHz, and τf=−6.0 ppm/°C.

Thermal decomposition kinetics of protonated peptides and peptide dimers, and comparison with surface-induced dissociation.

Abstract: Rate constants for the unimolecular decomposition of peptide monomer and dimer ions by thermal and surface-induced dissociation (SID) are measured and compared. Rate constants for thermal dissociation are measured in a heated wide-bore capillary flow reactor attached in front of the capillary leading into the mass spectrometer. Thermal decomposition of the leucine enkephalin ion (YGGFL)H+ is observed between 600 and 680 K with rate constants of 20-200 s-', and yields many of the same fragments as SID at 35 eV, although with different relative intensities. The thermal decomposition yields the Arrhenius parameters E,, = 38.3 kcal/mol, log A = 15.7. The decomposition of the monomer and dimer ions are also observed by using SID on C18 and fluorinated hydrocarbon surfaces, with rate constants of 2 X lo4 to 40 X lo4 s-'. The SID activated monomer ions are assigned equivalent temperatures of 710-840 K by extrapolation of the thermal activation parameters. The protonated dimer ion (YGGFL)2 H+ decomposes thermally at 500-540 K to yield the monomer ion. The dimer also decomposes by SID at low collision energies 10-20 eV on both surfaces to yield the monomer ion, and at much higher energies of 60-80 eV to yield fragments identical to the decomposition of the monomer. The large energy requirement for fragmentation from the dimer is due to energy deposition into more degrees of freedom plus the additional energy required for dissociation of the dimer to the monomer. It is assumed that the energy deposition is linear with collision energy up to 80 eV, and that the energy becomes randomized throughout the dimer, including energy flow through the hydrogen bond($. These mechanistic assumptions are supported quantitatively by the SID energy relations between monomer and dimer fragmentation. Thermal decomposition of the larger, multiply protonated melittin ion [M + 3HI3+ occurs at substantially higher temperatures, between 810-840 K, than those required for thermal decomposition of (YGGFL)H+, to yield many of the same sequence ions as produced by SID at 135 eV on a fluorinated surface. The fragmentation of molecular ions or protonated molecules is essential for mass spectrometric structure determination, while the decomposition of gas-phase adducts may allow the determination of non-covalent binding energies. We previously studied the thermal decomposition kinetics of carbonium ions and protonated alcohols and ethers,'-4 and the thermal decomposition of non-covalent , hydrogen-bonded dimers is observed routinely in the reverse processes of clustering equilibria. This work extends unimolecular kinetics to larger biomolecules. A similar application was presented more recently by Busman et al. for the decomposition of melittin ions.' The present work also extends previous work of the authors in ion pyrolysis and surface-induced dissociation (SID) to compare the energetics of these two processes. Surface-induced dissociation, developed originally by Cooks and coworkers,6 has proved to be a powerful technique for the investigation of the mass spectral fragmentation of protonated pep tide^."'^ In addition to practical applications, e.g. sequencing peptides, SID has the potential to provide energetic data that can be used to describe the mechanisms of peptide fragmentation. We have recently reported, for example, that fragmentation efficiencies (% fragmentation vs. collision energy) can be correlated with the basicities of peptides and with peptide size.13 Although these qualitative trends are themselves important , it is desirable to

Potential reaction intermediates of NOx reduction with propane over Cu/ZSM-5

Abstract: Adsorption complexes that are formed by exposing Cu/ZSM-5 to nitric oxides have been identified by FTIR. Some of them are thermally decomposed below 200°C, but others, though much more stable thermally, are reduced at low temperature by chemical interaction with gaseous propane. The FTIR spectra suggest that the latter complexes are nitro and/or nitrate groups associated with Cu ions. Their formation and reduction when Cu/ZSM-5 is exposed to NO + O2 + C3H8 seems consistent with the assumption that they are intermediates in catalytic NOx reduction. Mass spectrometric analysis of the gases that are released when these complexes react with propane reveals formation of N2, whereas thermal decomposition at higher temperatures only yields NO2, NO, H2O, traces of O2, but no N2. The results suggest that adsorbed NOy, groups (y ⩾ 2) are able to abstract H atoms from hydrocarbon molecules.