Thermodynamics Of Decomposition Research Articles

Effect of Particle Size on Hydrogen Release from Sodium Alanate Nanoparticles

Density functional theory and the cluster expansion method are used to model 2-10 nm sodium alanate (NaAlH(4)) nanoparticles and related decomposition products Na(3)AlH(6), NaH, and Al. While bulk sodium alanate releases hydrogen in a two-step process, our calculations predict that below a certain size sodium alanate nanoparticles decompose in a single step directly to NaH, Al, and H(2) due to the effect of particle size on decomposition thermodynamics. This may explain why sodium alanate nanoparticles, unlike bulk sodium alanate, have been observed to release hydrogen in the operating temperature range of proton exchange membrane fuel cells. In addition, we identify low-energy surfaces that may be important for the dynamics of hydrogen storage and release from sodium alanate nanoparticles.

Step-by-step methodology of developing a solar reactor for emission-free generation of hydrogen

This study presents a methodology to develop a solar reactor based on the thermodynamics and kinetics of methane decomposition to produce hydrogen with no emissions. The kinetic parameters were obtained in the literature for two cases; methane laden with carbon particles and methane without carbon particles. Results show that there is significant difference in experimentally obtained and theoretically predicted methane conversion. The paper also presents a parametric study on the effects of temperature, pressure and the influence of inert gas composition, which is fed along with methane, on the thermodynamics of methane decomposition. Results show that there is significant effect of the inert gas presence in the feeding gas mixture on the equilibrium of methane conversion and product gas composition. Results also show that higher conversions are obtained when the carbon particles laden with methane. The step-by-step reactor design methodology for homogenous methane decomposition and the parametric study results presented in this paper can provide a very useful tool in guiding a solar reactor design and optimization of process operating conditions.

Quaternary Ammonium Borohydride Adsorption in Mesoporous Silicate MCM-48

AbstractInorganic borohydrides have a high gravimetric hydrogen density but release H2 only under energetically unfavorable conditions. Surface chemistry may help in lowering thermodynamic barriers, but inclusion of inorganic borohydrides in porous silica materials has proved hitherto difficult or impossible. We show that borohydrides with a large organic cation are readily adsorbed inside mesoporous silicates, particularly after surface treatment. Thermal analysis reveals that the decomposition thermodynamics of tetraalkylammonium borohydrides are substantially affected by inclusion in MCM-48. Inelastic neutron scattering (INS) data show that the compounds adsorb on the silica surface. Evidence of pore loading is supplemented by DSC/TGA, XRD, FTIR, and BET isotherm measurements. Mass spectrometry shows significant hydrogen release at lower temperature from adsorbed borohydrides in comparison with the bulk borohydrides. INS data from partially decomposed samples indicates that the decomposition of the cation and anion is likely simultaneous. These data confirm the formation of Si-H bonds on the silica surface upon decomposition of adsorbed tetramethylammonium borohydride.

Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide. 5. Thermodynamics and Kinetics of Unimolecular Decomposition

The unimolecular decomposition of the superoxide radical anion adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO), and 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) were computationally investigated by using PCM/BHandLYP/6-311G(d,p)//B3LYP/6-31G(d) and PCM/ROMP2/cc-PVDZ//B3LYP/6-31G(d) levels of theory. Results indicate that the O-O bond scission for nitrone-O(2)H to form the hydroxyl radical and the biradical is endoergic, and that the ring-opening step to form the nitrosoaldehyde is highly exoergic. The energy barriers for the O-O bond scission and ring-opening processes indicate that the former is the rate-limiting step of the reaction. The overall energetics for DMPO-O(2)H decomposition in the presence and absence of explicit water interactions was found to be the most preferred; however, no significant differences in the energetics of decomposition among the various isomers of AMPO-O(2)H, EMPO-O(2)H, and DEPMPO-O(2)H were observed.

Discovery of novel hydrogen storage materials: an atomic scale computationalapproach

Practical hydrogen storage for mobile applications requires materials that exhibit highhydrogen densities, low decomposition temperatures, and fast kinetics for absorption anddesorption. Unfortunately, no reversible materials are currently known that possess all ofthese attributes. Here we present an overview of our recent efforts aimed at developing afirst-principles computational approach to the discovery of novel hydrogen storagematerials. Such an approach requires several key capabilities to be effective: (i) accurateprediction of decomposition thermodynamics, (ii) prediction of crystal structures forunknown hydrides, and (iii) prediction of preferred decomposition pathways. Wepresent examples that illustrate each of these three capabilities: (i) prediction ofhydriding enthalpies and free energies across a wide range of hydride materials,(ii) prediction of low energy crystal structures for complex hydrides (such asCa(AlH4)2 CaAlH5,and Li2NH), and (iii) predicted decomposition pathways forLi4BN3H10 and destabilized systems based on combinations ofLiBH4,Ca(BH4)2 and metal hydrides. For the destabilized systems, we propose a set of thermodynamicguidelines to help identify thermodynamically viable reactions. These capabilities have ledto the prediction of several novel high density hydrogen storage materials and reactions.

Kinetics and thermodynamics of the aluminum hydride polymorphs

Polymorphs of AlH 3 were prepared by organometallic synthesis. We demonstrate that freshly synthesized, nonsolvated AlH 3 releases approximately 10 wt% H 2 at desorption temperatures less than 100 °C. The decomposition kinetics, measured by isothermal hydrogen desorption between 30 and 140 °C, suggest that the rate of H 2 evolution is limited by nucleation and growth of the aluminum phase. The H 2 evolution rates for small crystallites of α and γ-AlH 3 (undoped) meet the DOE full flow target for a 50 kW fuel cell (1 gH 2/s) above 114 °C (based on 100 kg AlH 3). The decomposition thermodynamics were measured using differential scanning calorimetry and ex situ X-ray diffraction. The decomposition of the less stable polymorph, γ-AlH 3, occurs by an exothermic transformation to the α phase (∼100 °C) followed by the decomposition of α-AlH 3. A formation enthalpy of approximately −10 kJ/mol AlH 3 was measured for α-AlH 3, which is in good agreement with previous experimental and calculated results.

Effect of the Phosphoryl Substituent in the Linear Nitrone on the Spin Trapping of Superoxide Radical and the Stability of the Superoxide Adduct: Combined Experimental and Theoretical Studies

A new phosphorylated linear nitrone N-(4-hydroxybenzyliene)-1-diethoxyphosphoryl-1-methylethylamine N-oxide (4-HOPPN) was synthesized, and its X-ray structure was determined. The spin trapping ability of various kinds of free radicals by 4-HOPPN was evaluated. Kinetic study of decay of the superoxide spin adduct (4-HOPPN-OOH) shows the half-life time of 8.8 min. On the basis of the X-ray structural coordinates, theoretical analyses using density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level were performed on spin-trapping reactions of superoxide radical with 4-HOPPN and PBN and three possible decay routes for their corresponding superoxide adducts. The comparative calculations on the spin-trapping reactions with superoxide radical predicted that both spin traps share an identical reaction type and have comparable potency when spin trapping superoxide radical. Analysis of the optimized geometries of 4-HOPPN-OOH and PBN-OOH reveals that an introduction of the phosphoryl group can efficiently stabilize the spin adduct through the intramolecular H-bonds, the intramolecular nonbonding attractive interactions, as well as the bulky steric protection. Examination of the decomposition thermodynamics of 4-HOPPN-OOH and PBN-OOH further supports the stabilizing role of the phosphoryl group to a linear phosphorylated spin adduct.

Electronic structure and energetics of the quaternary hydride Li4BN3H10

Li 4 B N 3 H 10 has been synthesized recently from LiNH2∕LiBH4 mixtures and its crystal structure determined. We have calculated the electronic structure of this complex hydride and investigated its thermodynamic stability and decomposition energetics. We find that its enthalpy of formation is −708kJ∕mole with respect to the elemental constituents and −8kJ∕mole relative to a 3:1 molar LiNH2∕LiBH4 mixture, in qualitative agreement with experiment. Reaction enthalpies computed for several decomposition pathways suggest Li4BN3H10→Li3BN2+12Li2NH+12NH3+4H2 as the likely dehydriding route.

Thermodynamics of thermal decomposition of group 13 metal trihalide adducts with piperidine: A combined theoretical and experimental study

Three major decomposition pathways of complexes formed by MX 3 (M = Al, Ga; X = Cl, I) and piperidine (pip) have been theoretically considered at the B3LYP/LANL2DZ(d,p) level of theory. Hydrogen gas elimination with formation of gaseous MX 3 · pyridine complexes becomes thermodynamically favorable above 500 K and is expected as a primary thermolysis product of MX 3 · pip. Alternative routes, which include dissociation into components or hydrogen halide elimination, require much higher temperatures, despite of the exothermic dimerization energies of the MX 2C 5H 10N monomers. This theoretical conclusion is in qualitative agreement with present experimental thermolysis study of the GaCl 3 · pip complex.

Effect of Silicon Content on Thermodynamics of Austenite Decomposition in C-Si-Mn TRIP Steels

Some numerical models such as central atoms model (CAM) and superelement model were used to simulate the thermodynamics of austenite decomposition in the Fe-C-Mn-Si TRIP (transformation induced plasticity) steels. Thermodynamic calculations were carried out under a para-equilibrium (PE) condition. The results show that certain silicon content can accelerate the polygonal ferritic transformation and increase the volume fraction and stability of retained austenite by retarding the precipitation of carbides during the bainitic transformation.

Optimisation of an asymmetric manganese oxide/activated carbon capacitor working at 2 V in aqueous medium

The charge storage mechanism of manganese oxide and activated carbon has been studied in aqueous medium in order to optimise an asymmetric (or hybrid) supercapacitor based on these two materials as positive and negative electrode, respectively. Amorphous manganese oxide can be polarised up to potentials of 1.2 V in neutral medium. Under negative polarisation, a pseudocapacitive behaviour of activated carbon has been demonstrated, that is related with reversible hydrogen adsorption in the pores. It allows carbon to be polarised at potential values far from the thermodynamic decomposition of the electrolyte. Balancing the mass of these two materials with pseudocapacitive properties results in a practical cell voltage of 2 V in aqueous medium, with energy densities close to the values obtained with electric double layer capacitors working in organic electrolytes, while avoiding their disadvantages.

Hysteresis and related irreversible phenomena in CeNi 5-based intermetallic hydrides: Effect of substitution of Co for Ni

The phase relationships and thermodynamics of hydride formation and decomposition in the CeNi 5− x Co x –H 2 ( x = 1, 1.5, 2.5) systems have been studied by means of pressure–composition isotherm measurement and XRD analysis. Special attention has been paid to the first hydrogenation. A significant reduction of hysteresis, especially within the first cycle, has been observed in all studied alloys as compared with non-substituted CeNi 5. The effect of temperature and reaction direction on the isotherm shape and plateau splitting has been revealed for CeNi 3.5Co 1.5. The CeNi 2.5Co 2.5–H 2 system has demonstrated an exceptional phenomenon of oscillating dynamics of equilibrium achievement within the hydrogen concentration range of 3–5 atoms H per formula unit. Some suggestions on the CeNi 5− x Co x peculiarities have been proposed.

Interfacial Oxidation of Ultrathin Nickel and Chromium Films on Yttria-Stabilized Zirconia

The substrate-induced oxidation upon prolonged annealing in UHV of ultrathin films of Ni and Cr vapor deposited on yttria-stabilized zirconia YSZ(100) was studied by X-ray photoelectron spectroscopy (XPS) to obtain information about the oxidation mechanism, determine the available quantity of reactive oxygen in YSZ, and investigate the thermal stability of the thin oxide films. Up to about 0.8 ML of Ni deposited at room temperature was oxidized to NiO at a constant rate at 650 K via the substrate, whereas at slightly higher coverage, the oxidation rate under identical conditions was drastically reduced. In contrast to Ni, up to 4.8 ML of Cr deposited at 275 K could be oxidized via the substrate to Cr2O3 upon extensive UHV annealing at increasing temperature up to 820 K, indicating a reactive oxygen content of at least 4 x 10(-6) with respect to the lattice oxygen in the YSZ specimen. The Cr2O3 decomposed to metallic Cr above about 800 K, whereas NiO was stable up to the maximum temperature of 875 K. These results indicate that the oxidation via the substrate is kinetically analogous to the gas-phase oxidation of bulk Ni and Cr. The reactive oxygen content of the single-crystal YSZ is larger than expected, and part of it is accommodated at the surface of the substrate. The thermal stability of the thin oxide films is determined by the oxygen exchange with YSZ and not by the respective bulk oxide thermodynamic decomposition temperature.

Phase diagrams of decomposing nanoalloys

The thermodynamics of nucleation and decomposition in small isolated particles are considered. There exist three possibilities: phase separation, prohibition of decomposition and a metastable state. We investigate the peculiarities of phase diagrams related to depletion of the nanosize parent phase even at the nucleation stage. For small particles the equilibrium diagram becomes split (and shifted and size dependent). Concentration, size and temperature hystereses take place. Size-dependent ‘critical supersaturation’, increasing with decreasing size, has been analysed.

Formation and Decomposition Mechanisms for Clathrate Hydrates

Large-scale nanosecond classical molecular dynamics calculations have been employed to simulate initial clathrate hydrate formation. Preferential formation of small (5 12) cages in the initial stages of clathrate formation is not found. This observation is compared to the recent NMR observations on the formation of small cages preceding the crystallization process. The result seems to support experimental observations that the formation of structure II SF 6 hydrate does not require occupation of small cages. The decomposition mechanism of gas hydrates has been investigated using classical molecular dynamics. The ‘preservation effect’ that inhibits decomposition of gas hydrates at temperatures above its thermodynamic melting or decomposition points may be explained with a phenomenological model assuming the formation of a thin ice crust layer.

A DFT Study of CHx Chemisorption and Transition States for C−H Activation on the Ru(112̄0) Surface

The thermodynamics of methane decomposition on the ruthenium (1120) surface has been investigated with ab initio periodic calculations. All surface intermediates are more stable than the gas-phase methane even if the last step of the decomposition path: CH → C + H, is highly endothermic. Among all of the surface species, CH2 appears to be the most stable. All of the surface species (CHx, x = 3−1 and H) adsorb on bridge-up sites, while atomic C prefers top-down sites. The transition states of the elementary reactions for the dissociation of methane on the ruthenium (112̄0) surface have been investigated with nudged elastic band method (NEB). The calculated barriers are 56 kJ mol-1 for methane decomposition, 11 kJ mol-1 for methyl decomposition, and 52 kJ mol-1 for methylene decomposition, respectively. The decomposition of CHads requires the highest activation energy from the series with 95 kJ mol-1.

Catalyzed alanates for hydrogen storage

The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH 4⇔1/3Na 3AlH 6+2/3Al+H 2⇔NaH+Al+3/2H 2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125–165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti–Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications.

The prediction of the hydriding thermodynamics of Pd-Rh-Co ternary alloys

A dilute solution model (with respect to the substitutional alloying elements) has been utilized to accurately predict the hydride formation and decomposition thermodynamics and hydrogen storage capacities of dilute Pd-Rh-Co alloys. The effect of varying the rhodium and cobalt compositions on the thermodynamics of hydride formation and decomposition and hydrogen capacity of several palladiumrhodium-cobalt ternary alloys has been investigated using pressure-composition (PC) isotherms. Alloying in the dilute regime (<10 at. pct) causes the enthalpy for hydride formation to linearly decrease with increasing alloying content. Cobalt has a stronger effect on the reduction in enthalpy than rhodium for equivalent alloying amounts. Also, an increasing alloying content of cobalt reduces the hydrogen storage capacity. The plateau thermodynamics are strongly linked to the lattice parameters of the alloys. A near-linear dependence of the enthalpy of hydride formation on the lattice parameter was observed for both the binary Pd-Rh and Pd-Co alloys, as well as for the ternary Pd-Rh-Co alloys. The Pd-5Rh-3Co (at. pct) alloy was found to have similar plateau thermodynamics to a Pd-10Rh alloy; however, this ternary alloy had a diminshed hydrogen storage capacity relative to Pd-10Rh.

Compound Mechanism of the Endo-Exothernic and Exo-Endothermic ± Balanced Blowing Agent

The effects of auxiliary blowing agent were investigated. Based on the results, new combined blowing agents were studied. The difference between the kinetics and thermodynamics of decomposition of the typical blowing agents were compared. Compared with the typical single blowing agents, the new combined blowing agents had the below characteristics: (1) The endothermic enthalpy equals exothermic-enthalpy; (2) The rate of gas evolution of the combined blowing agents is relatively rapid but not abrupt to form more regular and small cell structure; (3) They comprised of blowing agent, nucleation agent and blowing auxiliary agent. The blowing fratures in actual processing by Brabender extruder were verified. The data and the results are valuable to flowing research and can provide the theoretical basis for producing cellular PVC material by extruding.

A theoretical study of CHx chemisorption on the Ru(0001) surface

Abstract The thermodynamics of methane decomposition on the ruthenium (0001) surface has been investigated with ab initio periodic calculations for coverages of 25% and 11%. All surface intermediates are more stable than the gas-phase methane even if the last step of the decomposition path (CH→C+H) is highly endothermic. Amongst all of the surface species, CH appears to be the most stable, in agreement with experiments. All of the surface species (CH x =3,0 and H) adsorb on three-fold sites. Short-range lateral interactions between CH x =3,0 and H are also considered and are found to be mostly repulsive.