Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. I. H-chemisorbed case

Collision induced (CI) processes involving hydrogen atoms on a graphite surface are studied quantum mechanically within the rigid, flat surface approximation, using a time-dependent wave packet method. The Eley-Rideal (ER) reaction and collision induced desorption (CID) cross sections are obtained with the help of two propagations which use different sets of coordinates, a "product" and a "reagent" set. Several adsorbate-substrate initial states of the target H atom in the chemisorption well are considered, and CI processes are studied over a wide range of projectile energy. Results show that (i) the Eley-Rideal reaction is the major reactive outcome and (ii) CID cross sections do not exceed 4 A2 and present dynamic thresholds for low values of the target vibrational quantum number. ER cross sections show oscillations at high energies which cannot be reproduced by classical and quasiclassical trajectory calculations. They are related to the vibrational excitation of the reaction products, which is a rather steep decreasing function of the collision energy. This behavior causes a selective population of the low-lying vibrational states and allows the quantization of the product molecular states to manifest itself in a collisional observable. A peak structure in the CID cross section is also observed and is assigned to the selective population of metastable states of the transient molecular hydrogen.     

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Following previous investigation of collision induced (CI) processes involving hydrogen atoms chemisorbed on graphite [R. Martinazzo and G. F. Tantardini, J. Chem. Phys. 124, 124702 (2006)], the case in which the target hydrogen atom is initially physisorbed on the surface is considered here. Several adsorbate-substrate initial states of the target H atom in the physisorption well are considered, and CI processes are studied for projectile energies up to 1 eV. Results show that (i) Eley-Rideal cross sections at low collision energies may be larger than those found in the H-chemisorbed case but they rapidly decrease as the collision energy increases; (ii) product hydrogen molecules are vibrationally very excited; (iii) collision induced desorption cross sections rapidly increase, reaching saturation values greater than 10 A2; (iv) trapping of the incident atoms is found to be as efficient as the Eley-Rideal reaction at low energies and remains sizable (3-4 A2) at high energies. The latter adsorbate-induced trapping results mainly in formation of metastable hot hydrogen atoms, i.e., atoms with an excess energy channeled in the motion parallel to the surface. These atoms might contribute in explaining hydrogen formation on graphite.     

Green synthesis of banana flavor using different catalysts: a comparative study of different methods

ABSTRACT

Esterification of isoamyl alcohol with acetic acid was studied using different ion-exchange resins, namely Amberlyst 15 dry, Amberlyst 16 wet, Amberlite 120-IR. Esterification was carried out using different esterification methods that are quite new (ohmic, ultrasonic probe, and ultrasonic bath) and the results were compared with microwave-assisted esterification (MAE). The highest isoamyl acetate yield (99%) was obtained by MAE, using a mixture of acetic acid and isoamyl alcohol (mole ratio of 1:2) after 2?h of reaction time. In this process, 2% Amberlyst 15 dry was used. MAE had the least specific energy consumption (0.42?kWh/g isoamyl acetate) and specific CO₂ emission (34?g/g isoamyl acetate). According to the images obtained by scanning electron microscopy, lower amounts of Amberlyst 15 dry beads were destroyed by MAE method compared to other esterification methods. In conclusion, MAE proved to be an economic and environmentally-friendly method for esterification of different flavoring compounds.     

Reflection absorption infrared spectroscopy and temperature-programmed desorption studies of the adsorption and desorption of amorphous and crystalline water on a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two- and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.     

A comparative study of graphene materials formed by thermal exfoliation of graphite oxide and chlorine trifluoride-intercalated graphite

Graphene 3D materials GM1 and GM2 obtained by explosive exfoliation of graphite oxide and graphite intercalated with chlorine trifluoride, respectively, have been studied by elemental analysis, X-ray photoelectron spectroscopy, mass spectrometry, infrared and Raman spectroscopy, and scanning electron microscopy. The specific surface area, the pore size, and electrical conductivity of the materials have been measured. A comparative study has shown that the gas mixture produced during the preparation of GM1 is less hazardous than that in the case of GM2. However, GM2 exhibits a higher conductivity and a larger size of graphene crystallites. The feasibility of isolation of a suspension of graphene nanosheets from the test 3D materials has been demonstrated. Possible applications of these materials are discussed.     

A comparative computational study of hydrogen and lithium-bonded complexes

A computational study of hydrogen-bonded complexes of F(3)CH and C1H and of lithium-bonded complexes of F(3)CLi and CILi, with small molecules such as N(2) and H(2)O was undertaken at the MP2/6-311++G(d,p) level of theory. Bond extensions and redshifts were obtained for the Cl[Single Bond]H bond in the ClH complexes, while bond contractions and blueshifts were obtained for the C[Single Bond]H bond in the F(3)CH complexes. By contrast, bond extensions and blueshifts were obtained for all of the lithium-bonded species. These results were rationalized using a model derived from perturbation theory.     

Energetic potential of pyrolyzed biomass from different sources: a comparative study

Journal of Thermal Analysis and Calorimetry - The use of biomass has been a recent trend as a potential feedstock for char generation with improved energetic value. In this work, agricultural...     

A comparative study on the sorption and desorption of Hg,Th and U on clay

The sorption and desorption of uranium, thorium and mercury on a western Anatolian montmorillonite, obtained from the deposit located in Kula, were studied by application of a batch technique. The clay used is a tertiary clay originally found in a rather large geological formation of west Anatolia. It is nearly pure montmorillonite. Its cation exchange capacity (CEC) determination was performed for ammonium acetate by the Mehlich procedure. The mean CEC was found to be 83 meq/100 g, which, taking into account that CEC determinations were carried out on unfractionated material, is in good agreement with previously reported data. The concentration ranges were between 70–1500 ppm for mercury and 100–2000 ppm for thorium and uranium. The relative importance of test parameters, e.g., pH, clay particle size, groundwater composition, contact time and solid/water ratio, which determine the distribution coefficients was studied. The sorption coefficients varied between 2.7–6.4 ml/g for U, 0.22–1.59 ml/g for Th and 152.4–427.2 ml/g for Hg. The differences of distribution coefficients are discussed. The data could be fitted to Freundlich and Langmuir isotherms. The quantities of the sorbed and desorbed Th were much lower than its theoretical CEC's. This attitude was attributed to the blocking of montmorillonite by cation islands sorbed in the interlayer. Hg is sorbed most strongly. The experimental results indicate that the montmorillonites studied should be effective components of the buffer and backfill material and lead to eventual immobilization of these elements, which are environmentally dangerous.     

Effect of expanded graphite lattice in exfoliated graphite nanofibers on hydrogen storage

A graphite exfoliation technique, using intercalation of a concentrated sulfuric/nitric acid mixture followed by a thermal shock, has successfully exfoliated a herringbone graphite nanofiber (GNF). The exfoliated GNF retains the overall nanosized dimensions of the original GNF, with the exfoliation temperature determining the degree of induced defects, lattice expansion, and resulting microstructure. High-resolution transmission electron microscopy indicated that the fibers treated at an intermediate temperature of 700 degrees C for 2 min had dislocations in the graphitic structure and a 4% increase in graphitic lattice spacing to 3.5 A. The fibers treated at 1000 degrees C for 36 h were expanded along the fiber axis, with regular intervals of graphitic and amorphous regions ranging from 0.5 to >50 nm in width. The surface area of the starting material was increased from 47 m(2)/g to 67 m(2)/g for the 700- degrees C treatment and to 555 m(2)/g for the 1000- degrees C treatment. Hydrogen uptake measurements at 20 bar indicate that the overall hydrogen uptake and operative adsorption temperature are sensitive to the structural variations and graphitic spacing. The increased surface area after the 1000- degrees C treatment led to a 1.2% hydrogen uptake at 77 K and 20 bar, a 3-fold increase in hydrogen physisorption of the starting material. The uptake of the 700- degrees C-treated material had a 0.29% uptake at 300 K and 20 bar; although low, this was a 14-fold uptake over the starting material and higher than other commonly used pretreatment methods that were tested in parallel. These results suggest that selective exfoliation of a nanofiber is a means by which to control the relative binding energy of the hydrogen interaction with the carbon structure and thus vary the operative adsorption temperature.     

Non-convertional hydrogen bonding interaction of BH3NH3 complexes: a comparative theoretical study

A new type of hydrogen bond, called a dihydrogen bond, has recently been introduced. In this bond hydrogen is donated to (hydridic) hydrogen. In this paper, ab initio HF, MP2 and DFT(B3LYP) levels of theory with different basis sets in combination with counterpoise procedure for basis set superposition error correction have been applied to BH₃NH₃ dimer and BH₃NH₃ complexes of methane, hydrogen cyanide, ammonia, water, methanol and hydrogen fluoride to understand the features of dihydrogen bond. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The structures obtained at different computational levels are in agreement with each other. Dihydrogen bond does not occur in both BH₃NH₃⋯CH₄ and BH₃NH₃⋯NH₃ complexes. Apart from the B–H⋯H–N dihydrogen bond found in the BH₃NH₃ crystal and dimmer, the B–H⋯H–X (XC, O, F) dihydrogen bonds have been observed in the BH₃NH₃⋯HCN, BH₃NH₃⋯H₂O, BH₃NH₃⋯CH₃OH and BH₃NH₃⋯HF complexes, while the classic H bonds also exist in the last three complexes. As for the complexes in which only dihydrogen bonds appear the strength of dihydrogen bonds ranges from 17.9 to 18.9 kJ mol⁻¹ at B3LYP/6-311++g(d,p) level. Binding energies obtained from the MP2 and B3LYP optimized structures are more sensitive to basis sets than those from the HF method. Larger basis functions generally tend to produce slightly longer intermolecular distances, and the B3LYP and MP2 methods generate shorter intermolecular distances though they usually produce longer bond lengths compared with those at the HF level. The infrared spectrum frequencies, IR intensities and the vibrational frequency shifts are reported. Finally the solution phase studies on BH₃NH₃⋯HF complex are also carried out using the Onsager reaction field model with a range of dielectric constants from 2 to 80 at B3LYP/6-311++g(d,p) level.     

Sorption and desorption of neptunium(V) on calcareous soil and its solid components: A comparative study

The sorption and desorption isotherms of untreated calcareous soil and three treated soils to remove CaCO₃, organic matter (OM) and both CaCO₃ and OM were determined and analyzed with the Freundlich equation at pH 7.8, moderate concentrations of NpO₂ ⁺ (~10^-5mol/l), in the presence of 0.01 mol/l CaCl₂ and under ambient aerobic conditions. The relative contribution of CaCO₃ and OM to the neptunium(V) sorption on calcareous soil and the sorption/desorption hysteresis is discussed. The effects of adding fulvic acid (FA) and carbonate in to the solution on the sorption of neptunium(V) on the soils were also studied. The sorption and desorption characteristics of NpO₂ ⁺, Zn²⁺, Sr²⁺ and Cs⁺ on the soils are compared.     

Phenol adsorption on different nano-sized carbon materials: first comparative study

Carbon nanofibres and multi-branched carbon nanotubes are synthesized by using a new, proposed in this study method. The acid–base, adsorption and geometric properties of nanomaterials are characterised. Next we present first comparative adsorption and calorimetric studies of phenol uptake from aqueous solutions on mentioned nano-sized carbon materials and on nanotubes having similar diameter. The comparison of carbon nanofibres, multi-branched carbon nanotubes, and multi walled carbon nanotubes shows that for non-porous carbon nanomaterials the concentration of basic surface functionalities determines the mechanism of phenol adsorption. In consequence, larger phenol adsorption is recorded for the most basic nanomaterials (nanofibres and multi walled nanotubes) and smaller for multi branched nanotubes having the smallest surface concentration of basic groups. Possible explanation of differences between enthalpy of phenol displacement on graphite and on studied carbon adsorbents is also given.     

Ca-intercalated graphite as a hydrogen storage material: Stability against decomposition into CaH2 and graphite

We have used calculations based on density functional theory to investigate the energetics of hydrogen absorption in calcium-intercalated graphites. We focus particularly on the absorption energy and the stability of the hydrogenated material with respect to decomposition into graphite and calcium hydride, which is essential if this material is to be used for practical H₂ storage. The calculations are performed with two commonly used approximations for the exchange-correlation energies. Our calculations confirm earlier predictions that the absorption energy is approximately −0.2 to −0.4 eV, which is favourable for practical use of Ca-intercalated graphite as a hydrogen storage medium. However, we find that the hydrogenated material is strongly unstable against decomposition. Our results therefore explain recent experiments which show that H₂ does not remain stable in CaC₆ but instead forms a hydride plus graphite.     

Calculation of ionization potentials of small molecules: a comparative study of different methods

With the help of various theoretical methods, ionization potentials (IPs) have been computed for a panel of small molecules containing atoms of group 14, 15, or 16 and representing different singly, doubly, or triply bonded systems with or without an interacting heteroatom lone pair. Comparison of experimental IP values to theoretical results indicates that (i) the standard outer valence green function (OVGF), density functional theory (DFT), and DeltaSCF methods lead to rather accurate values, (ii) the CASPT2 method systematically underestimates IPs, (iii) the method of deducing IPs from a shift of some standard DFT eigenvalue spectrum is a straightforward approach leading to rather accurate IPs, (iv) the eigenvalue spectrum obtained with the so-called statistical average of different orbital model potential (SAOP) exchange-correlation model potential is an efficient approach leading directly to quite accurate IPs, and (v) a good prediction of the IP spectrum can be obtained from the shifted excitation spectra of the system calculated by the time-dependent DFT (TD-DFT) method. It is also shown that the TD-DFT calculations of the ionized species bring a significant improvement over the calculations of the neutral molecules, indicating that a great part of the electronic relaxation is already taken into account (in a similar way for all ionizations). Finally, in the case of TD-DFT calculations of neutral molecules, the statistical average of different orbital model potential (SAOP) functional does not lead to significantly better results than the B3LYP functional.     

Electron-density studies on hydrogen bonding in chromone derivatives. Part II: comparative study

The experimental electron density of a chromone derivative was determined from a multipole refinement of 100 K X-ray synchrotron data and complemented by theoretical calculations with experimental and optimized geometry. Atomic and topological properties were obtained using the Quantum Theory of Atoms in Molecules approach. The examination of topological parameters unambiguously showed π-delocalization within the H-bonded ring. The application of source function analysis confirmed the intramolecular N–H···O hydrogen bond to be a resonance-assisted hydrogen bond. The topological study confirmed the covalent nature of N–H···O interaction and the electrostatic nature of weak C–H···O interactions.     

Capillary electrophoresis using copolymers of different composition as physical coatings: a comparative study

In this work, a comparative study on the use of different polymers as physically adsorbed coatings for CE is presented. It is demonstrated that the use of ad hoc synthesized polymers as coatings allows tailoring the EOF in CE increasing the flexibility of this analytical technique. Namely, different polymers were synthesized at our laboratory using different percentages of ethylpyrrolidine methacrylate (EpyM) and N,N-dimethylacrylamide (DMA). Thus, by modifying the percentage of EpyM and DMA monomers it is possible to manipulate the positive charge of the copolymer, varying the global electrical charge on the capillary wall and with that the EOF. These coated capillaries are obtained by simply flushing a given EpyM-DMA aqueous solution into bare silica capillaries. It is shown that by using these coated capillaries at adequate pHs, faster or more resolved CE separations can be achieved depending on the requirements of each analysis. Moreover, it is demonstrated that these coated capillaries reduce the electrostatic adsorption of basic proteins onto the capillary wall. Furthermore, EpyM-DMA coatings allow the reproducible chiral separation of enantiomers through the partial filling technique (PFT). The EpyM-DMA coated capillaries are demonstrated to provide reproducible EOF values independently of the pH and polymer composition with%RSD values lower than 2% for the same day. It is also demonstrated that the coating procedure is reproducible between capillaries. The compatibility of this coating protocol with CE in microchips is discussed.     

Linear chain surfactants at a planar interface: a comparative Monte Carlo study of several lattice models

Linear chain surfactants in a densely packed arrangement (such as alkane chains in lipid monolayers in the “uniform tilt” structures) are described by a crude coarse-grained model where the endgroups grafted on the interface form a regular lattice and the chains are described by the bond fluctuation model with chains containing N = 4 effective monomers only. Square-well interactions between the monomers are studied for both the attractive and repulsive case for three choices of the interaction range. None of these models exhibits a structure with uniform tilt. For attractive interactions the last bond has a strong tendency to fold back thus leading to a very high density close to the interface. Only when an intrachain-potential favoring stiff chain configurations also is included one can obtain configurations with uniform tilt order. Although related models (with much longer chain lengths and lower grafting densities) are very useful for the study of polymer brushes, the present case of very short chains in a high-density state clearly is plagued by various lattice artefacts and it is concluded that for modelling linear chain surfactants one should use an off-lattice model even on a coarse-grained level.     

Effect of adsorbed species on hydrogen desorption from niobium

Surface-controlled hydrogen permeation experiments were carried out on niobium under conditions such that the permeation of hydrogen was controlled by the surface reactions at the exit surface. Particular attention was paid to the effects of structure and chemistry on these surface reactions and hence on the permeation rate. Structural defects were introduced in the near-surface region by irradiation with low energy argon ions. Traps which were introduced by this process were shown to decrease the lattice diffusivity of hydrogen in the near-surface region by introducing hydrogen traps with binding energies of 10–100 kJ (mol H)₋₁. The effects of the adsorption of oxygen, nitrogen, chlorine, SO₂, CO and H₂S on the exit surface were studied. The observed decreases in the hydrogen permeation rates were interpreted on the basis of a desorption model in which the adsorbed species served to block surface sites from participating in the hydrogen desorption process. A model which describes the kinetics of permeation and desorption under these surface-controlled conditions was developed.