Moderate Activation Energy Research Articles

Solid solubility and transport properties of Ce1−xNdxO2−δ nanocrystalline solid solutions by a sol-gel route

Ce1−xNdxO2−δ (x = 0.05 to 0.55) nanocrystalline solid solutions were prepared by a sol-gel route. X-ray diffraction analysis showed that Ce1−xNdxO2−δ crystallized in a cubic fluorite structure. The lattice parameter for the solid solutions increased linearly with the dopant content. The solid solubility of Nd3+ in ceria lattice was determined to be about x = 0.40 in terms of the nearly constant lattice parameters at a dopant level larger than x = 0.40. First-order Raman spectra for Ce1−xNdxO2−δ at a lower dopant content exhibited one band associated with the F2g mode. At higher dopant contents, F2g mode became broadened and asymmetric, and a new broad band appeared at the higher frequency side of the F2g mode. This new band was assigned to the oxygen vacancies. The electron paramagnetic resonance spectrum for x = 0.05 showed the presence of O2− adsorbed on sample surface at g = 2.02 and 2.00 and of Ce3+ with a lower symmetry at g = 1.97 and 1.94. Further increasing dopant content led to the disappearance of the signals for O2−. Impedance spectra showed the bulk and grain boundary conduction in the solid solutions. The bulk conduction exhibited a conductivity maximum and an activation energy minimum with increasing dopant content. Ce0.80Nd0.20O2−δ was determined to give promising conduction properties such as a relatively high conductivity of σ700 °C = 2.44 × 10−2 S cm−1 and moderate activation energy of Ea = 0.802 eV. The variations of conductivity and activation energy were explained in terms of relative content of oxygen vacancies Vö and defect associations {CeCe ′Vö}/{NdCe ′Vö}.

XRD, SEM studies and electrical properties of metal complexes, including sodium oxalate ligand (Na 2C 2O 4)

The crystal structure, microstructure, and electrical properties of inorganic semiconductors were based on metal complexes, including sodium oxalate ligand. The crystal structure of samples at room temperature was examined using X-ray diffraction (XRD). X-ray diffraction showed that the three samples have different crystal structures. The microstructures of the samples have also been investigated by scanning electron microscopy (SEM). These samples have been found to be inorganic semiconductors having moderate conductivities and activation energies. Room temperature conductivities of O–Co, O–Ni, and O–Cu samples were 6.43×10 −9, 3.85×10 −7, and 2.10×10 −6 S/cm, respectively. The mobility and concentration of the carriers were calculated by means of current–voltage curve samples in the space charge limited region (SCLC).

Design and Control of Tubular Reactor Systems with Both Gas and Liquid Recycles

Previous studies of tubular reactor systems have explored flowsheets with either a gas recycle or a liquid recycle. In this paper, we extend this work to consider a flowsheet with two recycle streams: a large gas recycle from a separator drum and a small liquid recycle from a distillation column. Both the steady-state economics and the dynamic controllability of this dual-recycle system are compared with those of single-recycle processes. Three kinetic cases are considered: an irreversible reaction with moderate activation energy, an irreversible reaction with high activation energy, and a reversible reaction. Results demonstrate that the dual-recycle flowsheet is less expensive than the liquid-recycle system for the irreversible reaction case, particularly for the high activation energy where the total annual cost is 50% less. In the reversible reaction case, the liquid-recycle system is less expensive, because of the need for a high-purity recycle. Control structures are developed for all three flowsh...

Design and Control of a Gas-Phase Adiabatic Tubular Reactor Process with Liquid Recycle

Previous studies have explored the design and control of processes with gas-phase adiabatic tubular reactors that feature a gas recycle and a simple separation section consisting of a single ideal separator vessel. The gas recycle leads to high compressor capital and operating costs. This paper extends this work to the case in which a distillation column is required in the separation section and the recycle stream is liquid. The liquid recycle means that there are no compressor costs to counterbalance the reactor costs. However, there are large capital and energy costs associated with the vaporization/condensation of the recycle stream. For the numerical case studied, the liquid recycle process is more expensive than the gas recycle process, and it is more difficult to control. The basic reaction is A + B → C. Three reaction systems are considered: case 1 (irreversible with moderate activation energy), case 2 (irreversible with high activation energy), and case 3 (reversible). The optimum steady-state de...

Theoretical Study on σ-Bond Activation of (HO)2B−XH3 by M(PH3)2 (X = C, Si, Ge, or Sn; M = Pd or Pt). Noteworthy Contribution of the Boryl pπ Orbital to M−Boryl Bonding and Activation of the B−X σ-Bond

Oxidative addition of (HO)2B−XH3 to M(PH3)2 (X = C, Si, Ge, or Sn; M = Pd or Pt) was theoretically investigated with MP2-MP4(SDQ) and CCSD(T) methods. (HO)2B−XH3 easily undergoes oxidative addition to Pt(PH3)2 with a moderate activation energy for X = C and either a very small barrier or no barrier for X = Ge, Si, and Sn. Also, (HO)2B−SiH3, (HO)2B−GeH3, and (HO)2B−SnH3 undergo oxidative addition to Pd(PH3)2 with either a very small barrier or no barrier. Only the oxidative addition of (HO)2B−CH3 to Pd(PH3)2 cannot take place, but the reductive elimination of (HO)2B−CH3 from Pd(CH3)[B(OH)2](PH3)2 occurs with no barrier. The transition states (TS) of these oxidative additions are nonplanar except for the nearly planar TS of the oxidative addition of (HO)2B−CH3 to Pt(PH3)2. This TS structure is very sensitive to steric and electronic factors; for instance, the TS becomes nonplanar by substituting PH2(C2H5) for PH3, to decrease the steric repulsion between (HO)2B−CH3 and PH2(C2H5). A noteworthy feature of these reactions is that the TS is much stabilized by the charge-transfer interaction between M d and B(OH)2 pπ orbitals, which is the main reason for the high reactivity of (HO)2B−XH3 in the oxidative addition reaction. Pt−B(OH)2 and Pd−B(OH)2 bonds are much stronger than Pt−XH3 and Pd−XH3 bonds, respectively. This is because the M−B(OH)2 bond is stabilized by the π-back-donating interaction between the empty pπ orbital of B(OH)2 and the doubly occupied dπ orbital of Pt and Pd. Also, it should be noted that the trans influence of the boryl group is stronger than the very strong trans influence of silyl group.

Four-Electron Reduction of Diazo Compounds at a Single Tungsten Metal Center: A Theoretical Study of the Mechanism

A possible mechanism for the unprecedented one-metal reduction of diazobenzene by the [W(OC6HPh3-η6-C6H5)(OAr)(PMe2Ph)] (OAr = 2,3,5,6-tetraphenyloxide) complex was characterized by hybrid Becke3LYP DFT calculations on the model system [W(OH)2(C6H6)(Ph3)]+ N2H2. The model reaction is shown to be strongly exothermic, and takes place via a multistep process with moderate activation energy barriers. We discuss how this specific set of ancillary ligands permits the reaction to occur on a one-metal system in place of the more usual two-metal systems.

Oxide ion conductivity in Bi2W1−xMExO6−x/2 (ME = Nb, Ta)

Oxygen deficient Bi2WO6 phases have been synthesized by substitution for WVI with NbV and TaV, in this m = 1 member of the Aurivillius series. The composition range of the solid solutions is limited to 0 < x < 0.15 according to a formula Bi2W1−x(Nb or Ta)xO6−x/2. The maximum oxygen ion conductivities are obtained when x = 0.05 for both phases with moderate activation energies. The performances are intermediate between those of the high conductor BIMEVOX phases and those of doped higher members of the Aurivillius series m = 2, 3 and 4. These results provide the first experimental evidence of the predominant role of the Bi3+ 6s2 lone pair in the oxide ion diffusion process within these layered phases.

Catalytic and stoichiometric multielectron reduction of hydrazine to ammonia and acetylene to ethylene with clusters that contain the MFe 3S 4 cores (MMo, V). Relevance to the function of nitrogenase

A functional model for nitrogenase is currently sought in our laboratory in reactivity studies using various single cubane clusters that possess the [MFe 3S 4] n+ core, (MMo, n = 3; MV, n = 2). These clusters are used as catalysts for the reduction of enzymatically relevant substrates. Substrates such as hydrazine and acetylene are catalytically reduced by (NEt 4) 2[(Cl 4-cat) (CH 3CN) MoFe 3S 4Cl 3], I, to ammonia and ethylene respectively, in the presence of added protons and reducing equivalents. Hydrazine also is catalytically reduced by the (NEt 4) [(DMF) 3 VFe 3S 4Cl 3] cubane under similar conditions. Gas chromatography was employed to monitor the reduction of acetylene to ethylene and a trace of ethane. Catalysis in excess of 100 turnovers (for hydrazine reduction) and in excess of 15 turnovers (in acetylene reduction) has been demonstrated over a period of 24 h. A study of the acetylene reduction reveals saturation kinetics to be operating at high substrate concentrations. A variable temperature kinetic study of acetylene reduction shows a moderate activation energy ( E act = 9(1) kcal mol −1) but a large entropy of activation ( ΔS ‡ = −32(2) cal K −1 mol −1 ) which extrapolates to a significant Gibbs free energy ( ΔG ‡ = 19(1) kcal mol −1 ). These results are consistent with an ordered transition state. Considerable evidence has been amassed which directly implicates the Mo and V atoms as the primary catalytic sites. Replacement of the Mo or V bound, kinetically labile, solvent molecules with non-labile ligands acts to suppress the observed rates of reaction. The Fe sites on I are totally inactive in the reduction of hydrazine, however they have been found to effect acetylene reduction albeit at a markedly reduced rate compared to the Mo site. Catalyst integrity has also been demonstrated by a variety of techniques, primarily EPR spectroscopy which identifies the characteristic S = 3 2 signals of the Mo and V cubanes after at least 18 h reaction time.

On the electronic structure of the palladium monoxide and the methane adsorption: Density functional calculations

Electronic structure of the palladium monoxide and its interaction with a methane molecule has been investigated by means of density functional theory. The two triplets, 3Π and 3Σ−, lie very close in energy, with the indication at the 3Π ground state of the oxide. A methane molecule interacts with the open shell PdO and forms two stable adsorption complexes: in collinear on palladium and bridging conformations. The scission of the C–H bond in adsorbed methane requires moderate activation energy of 24.5 kcal/mol and the dissociation product is very stable, however, the singlet–triplet crossing occurs at the transition state.

The initiation of detonation from general non-uniformly distributed initial conditions

The chemical evolution of a hot gas subject to initial non-uniformities in velocity, pressure, temperature and reactant mass concentration is studied for moderate activation energies. It is demonstrated that such initial non-uniformities generate gradients in the distribution of chemical ignition times for each fluid particle, resulting in the creation of a high-speed, shockless reaction wave. If these gradients are sufficiently large, a transition from the high speed reaction wave to a strong detonation occurs. Time-dependent generalizations of the spontaneous flame concept, where the evolution of each fluid particle is determined by integration along particle paths only, are derived for initial velocity and pressure disturbances under the assumption of slowly varying initial disturbances. The unsteady structure of the high-speed reaction wave arising due to reaction from initial non-uniformities is investigated for the moderate activation energy used.

Electrical and photoconductive properties of orientation-controlled chloroaluminumphthalocyanine thin films

Structure dependence of electrical properties was investigated for amorphous and epitaxial thin films of chloroaluminumphthalocyanine vacuum deposited on both glass and KCl. The extrinsic conductivity in the dark was improved by elevating substrate temperatures, depending on the morphological change from granular, discontinuous films deposited at a substrate temperature of 25 °C to uniform, continuous ones at 250 °C. The photocurrents under monochromatic light illumination increased in proportion to the amount of adsorbed O2 in the amorphous films on glass. A regular doping form of O2 in the epitaxial film on KCl could enhance photocarrier generation. The granular, discontinuous morphology in the films caused a frequent charge-carrier recombination at the grain boundaries and defect sites. In the intrinsic region the amorphous films exhibited a higher dark conductivity due to the π-electron interaction between the randomly arranged molecules and gave a moderate activation energy for conduction. The epitaxial film with a regular orientation exhibited a significantly higher activation energy for intrinsic dark conduction because of a weak molecular interaction between the standing molecular columns.

The kinetics of dehydroxylation and mullitization of zettlitz kaolin in the presence of calcium(II) as an ingredient

The method of differential thermal analysis and Kissinger's approach were used in this study to investigate the kinetics of dehydroxylation and mullitization. The relatively high resistance to the breaking of structural bonds and the diffusion to water molecules through the lamellar gas layer of water vapour limits the activation energy to 178 kJ mol −1, as determined in the experimental work. It is assumed that the relatively moderate activation energy value of mullitization, 487 kJ mol −1, obtained during the exothermic reaction is the result of a moderately defective structure and of the influence of calcium(II) which decreases the activation energy.

Shock‐implanted noble gases II: Additional experimental studies and recognition in naturally shocked terrestrial materials

Abstract— Several experimentally and naturally shocked silicate samples were analyzed for noble gas contents to further characterize the phenomenon by which ambient gases can be strongly implanted into silicates by shock and to evaluate the possible importance of this process in capturing planetary atmospheres in naturally shocked samples. Gas implantation efficiency is apparently mineral independent, as mono‐mineralic powders of oligoclase, labradorite, and diopside and a powdered basalt shocked to 20 GPa show similar efficiencies. The retentivity of shock‐implanted gas during stepwise heating in the laboratory is defined in terms of two parameters: activation energy for diffusion as determined from Arrhenius plots, and the extraction temperature at which 50% of the gas is released, both of which correlate with shock pressure. These gas diffusion parameters are essentially identical for radiogenic 40Ar and shock‐implanted 40Ar in oligoclase and labradorite shocked to 20 GPa, suggesting that the two 40Ar components occupy analogous lattice sites. Our experiments indicate that gas implantation occurs through an increasing production of microcracks/defects in the lattice with increasing shock pressure. The ease of diffusive loss of implanted gas is controlled by the degree of annealing of these microcracks/defects.Identification of a shock‐implanted component requires relatively large concentrations of implanted gas which is strongly retained (i.e., moderate activation energy) in order to separate implanted gas from surface adsorbed gases. Literature data on shocked terrestrial samples indicate only weak evidence for shock‐implanted gases, with an upper limit for 40Ar of ∼ 10−6 cm3STP/g. New analyses of shocked samples from the Wabar Crater indicate the presence of shock‐implanted Ar, having concentrations (∼ 10−4 cm3STP/g) and activation energies for diffusive loss which are essentially that expected from experimental studies. Lack of sufficient target porosity or the presence of ground water may explain the sparse evidence for shock‐implanted gas at other terrestrial craters. Although Wabar Crater may represent an unusually favorable environment on Earth for shock‐implanting gases, surfaces of other planetary bodies, such as Mars, may frequently provide such environments. Analyses of returned samples from old Martian terraines may document temporal changes in earlier atmospheric composition.

Inelastic neutron scattering study of the rotational potential of NH4 + in (NH4)2PdCl4 and (NH4)2PtCl4

A consistent description of the rotational excitations in (NH4)2PdCl4 and (NH4)2PtCl4 is given in the model of a one-dimensional rotation in a nearly fourfold rotational potential, which is supported by the crystal structure. In such a potential, large tunnel splittings are combined with high-lying librations and moderate activation energies. The estimated model potentials are V( phi )=70 cos 4 phi +0.0004 cos 2 phi (meV) for (NH4)2PdCl4 and V( phi )=66 cos 4 phi +0.0008 cos 2 phi (meV) for (NH4)2PtCl4.

Electron–proton and electron–methyl exchanges in pyrolysis of polyacetylene and polymethylacetylene

AbstractPyrolysis of polyacetylene is marked by high yields of proton‐enriched products methane, ethane, ethylene, propane, polypylene, butadiene, cyclopentadiene, 1,3‐pentadiene, and toluene in total amounts exceeding benzene. The activation energies for their formation are low. Polyacetylene doped with AsF5 and iodine produced these products in even higher yields of two to 17 times of undoped polymers. The dominant mechanism is thought to be random‐chain scission followed by electron–proton exchange reactions. Polymethylacetylene is thermally less stable than polyacetylene. Pyrolysis gave mesitylene as the expected main product. However, as in the case of polyacetylene, large amounts of proton‐enriched products were formed with moderate activation energies. (The yields of methane, propylene, and propane are nearly the same in the pyrolysis of polymethylacetylene as compared to that of polyacetylene at 923°K referenced to mesitylene and benzene, respectively.) By analogy, mechanisms involving both electron‐proton and electron–methyl exchange reactions were proposed to account for the formation of all the pyrolyzates of polymethylacetylene. These reactions, not observed in the pyrolysis of polypropylene and polyisoprene, are attributable to the conjugated backbone permitting facile migrations of electrons, protons, and methyl groups.

Theoretical aspects of the radiation effects of polymers

AbstractThe Millar–Wall–Charlesby empirical rule of radiation effects on polymers was theoretically examined. The theoretically calculated adiabatic potential curves of the main chain scission and side chain cleavage revealed that the effects of radiation on polymers are clearly interpreted as chemical reactions in the excited states; namely, that degradation occurs when little or no activation energy is required in the main chain cleavage reaction in the excited states, or, in other words, stabilization of the excited polymer molecule leads to the main chain cleavage. Crosslinking occurs when moderate or large activation energy is required in the main chain cleavage reaction in any electronic state and little or no activation energy is needed in the CH bond cleavage reaction in the excited states. Therefore stabilization of the excited polymer molecule leads to the formation of a polymer radical that crosslinks. It was concluded that Millar–Wall–Charlesby's rule is exact only when the shape of the adiabatic potential curve in the ground state reflects those in the excited states.

Mechanisms of dihydrogen activation by zirconocene alkyl and hydride derivatives : A molecular orbital analysis of a ‘direct hydrogen transfer’ reaction mode

The formation of an adduct between a dihydrogen and a d 0 species of the general type (C 5R 5) 2Zr(R′) 2 (R,R′  H or CH 3) is suggested to involve a process analogous to that in CO adduct formation, and is studied by an extended Hückel molecular orbital analysis. Back donation into σ* H 2 orbitals arises from high-lying MR′ bonding orbitals and determines the stability of alternative adduct geometries. Two isomeric H 2 adducts can interconvert by an intramolecular hydrogen shift. With D 2, this reaction sequence leads to hydrogen isotope exchange. The transition state of this reaction mode contains a three-centre (H⋯H⋯H) − ligand configuration with bonding properties similar to those of an allyl ligand; it does not involve an oxidative change in the charge on the metal nor generation of a positively charged, protonic hydrogen species by heterolytic H 2 cleavage. The term ‘direct hydrogen transfer’ is proposed for this type of elementary reaction. For H 2-induced alkane elimination from group IV metallocene alkyl derivatives, an analogous reaction mechanism, involving formation of a H 2 adduct and a subsequent intramolecular hydrogen shift toward the alkyl ligand, is predicted to proceed with moderate activation energies.

Oxygen chemisorption on tungsten ribbons

Thermionic and flow-technique studies of oxygen chemisorption on tungsten ribbons exhibiting (100) planes have shown the presence of three surface states in the range of coverages up to a monolayer (10 15 atom cm -2 ). At coverages up to half a monolayer, the adsorbed state is characterized by a decreasing sticking probability, a low dipole moment, a low activation energy and a low polarizability. The second state (coverages between 5 and 8.5 x l0 14 atom cm -2 ) shows a constant sticking probability, moderate activation energy (20 kJ mole -1 ), high dipole moment and high polarizability. The third state exhibits zero dipole moment. The formation of tungsten oxides is not apparent until coverages of a monolayer have been reached. Under some conditions, thermionic emission in the second state exhibits the unusual phenomenon of a negative Schottky effect.