A study of the low temperature autoignition of methyl esters

The autoignition of a series of C₄ to C₈ fatty acid methyl esters has been studied in a rapid compression machine in the low and intermediate temperature region (650-850 K) and at increasing pressures (4-20 bar). Methyl hexanoate was selected for a full investigation of the autoignition phenomenology, including the identification and determination of the intermediate products of low temperature oxidation. The oxidation scheme and overall reactivity of methyl hexanoate has been examined and compared to the reactivity of C₄ to C₇n-alkanes in the same experimental conditions to evaluate the impact of the ester function on the reactivity of the n-alkyl chain. The low temperature reactivity leading to the first stage of autoignition is similar to n-heptane. However, the negative temperature coefficient region is located at lower temperature than in the case of the n-alkanes of corresponding reactivity. An evaluation of the distribution of esteralkyl radicals R and esteralkylperoxy radicals ROO gives an insight into the main reaction pathways.     

Characterisation of the Ru/MgF2 catalyst with adsorbed O2, NO, CO probe molecules by EPR and IR spectroscopy

Electron paramagnetic resonance (EPR) and infrared (IR) spectroscopy were used to study the formation of ruthenium and adsorbed species appearing on the catalyst during O₂, NO, and CO adsorption at room temperature on 1 wt% Ru/MgF₂ catalysts prepared from Ru₃(CO)₁₂ . Both EPR and IR results provided clear evidence for the interaction between surface ruthenium and probe molecules. No EPR signals due to ruthenium (Ru) species were recorded at 300 and 77 K after H₂-reduction of the catalyst at 673 K. However, at 4.2 K a very weak EPR spectrum due to low-spin (4d⁵) Ru³⁺ complexes was detected. A weak anisotropic O₂^- radicals signal with g_∣∣=2.017 and g_⊥=2.003 superimposed on a broad (ΔB_pp=120 mT), slightly asymmetric line at g=2.45(1) was identified after O₂ admission to the reduced sample. Adsorption of NO gives only a broad, Gaussian-shaped EPR line at g=2.43(1) indicating that the admission of NO, similarly to O₂ adsorption, brings about an oxidation of Ru species in the course of the NO decomposition reaction. Introduction of NO over the CO preadsorbed catalyst leads to EPR spectrum with parameters g_⊥=1.996, g_∣∣=1.895, and A_⊥^N=2.9 mT assigned to surface NO species associated with Ru ions. The IR spectra recorded after adsorption of NO or CO probe molecules showed the bands in the range of frequency characteristic of ruthenium nitrosyl, nitro, and nitrate/nitrite species and the bands characteristic of ruthenium mono-and multicarbonyls, respectively. Addition of CO after NO admission to the catalyst leads to appearance in the IR spectrum, beside the ones characteristic of NO adsorption, the bands which can be attributed to Ru-CO₂ and Ru-NCO species, indicating that the reaction between NO and CO occurs. These species were also detected after CO adsorption followed by NO adsorption, additionally to the band at 1850 cm⁻¹ being due to cis-type species.     

Monolayer aspects of high-resolution αs-plots

The problem of the estimation of high resolution comparative plots (α_s method) is revised. Adsorption isotherms of Ar (T = 87 K) are simulated (GCMC) on the bundles of closed and opened multiwalled carbon nanotubes. Those materials are chosen in order to observe the influence of porosity on Ar adsorption, and consequently, on the comparative plots. The introduction of the structural heterogeneity (defects in the external and internal walls of osculating and separated nanotubes) is also studied. The monolayer and multilayer parts of adsorption are extracted from the total amount adsorbed for all studied adsorbents and the reference one (the bundle of the ideal 3-layered carbon tubes with very wide diameters). The significant influence of the distance between carbon nanotubes and defects (in the external and internal walls) is observed on the monolayer α_s-plots. In the case of comparative plots calculated for the multilayer adsorption the differences are also visible; however, the expected linear dependence of the amount adsorbed on a porous solid plotted against the amount adsorbed on a reference nonporous solid (the layer-by-layer adsorption occurs on both solid surfaces) in a wide range of relative pressures is not observed. The obtained results demonstrate the complexity of α_s method when both microporosity and surface heterogeneity are present. A comparison of isotherms shows that for the studied adsorbents Ar is sensitive to the appearance of the geometrical and structural defects, even at low coverage.     

Experimental g factor of hydrogenlike silicon-28

Using the first principles calculations, the mixed Au _n Zn _m ⁺ (n + m ?? 6) cluster cations and their monocarbonyls Au _n Zn _m CO⁺ have been investigated at the PW91 level. For the small Au _n Zn _m ⁺ , most ground-state isomers are planar structures. A significant odd-even oscillation of the highest occupied-lowest unoccupied molecular orbital energy gaps with the number of Au atoms is observed. Upon CO adsorption, the top site and C head-on adsorptions are most favorable in energy. Moreover, the optimized geometries indicate that the CO molecule prefers binding to Au atom of the Au _n Zn _m ⁺ clusters, which can be understood by the frontier molecular orbital theory in detail. From the theoretical calculations, the CO charge population, CO binding energy (BE) and the Gibbs free-energy change ??G generally decrease with the increase of the Zn content. It is found that the BE is highly related to the electron transfer between CO and the cluster cations. Furthermore, a linear correlation between ??G and the CO BE is found. The red shift in the CO stretching frequency is sensitive to the cluster size and composition. Our calculation suggests that CO reactive collision on Au₃Zn⁺ and Au₂Zn ₂ ⁺ may lead to the dissociation of the clusters with a Zn atom loss.     

Paramagnetic centers in single-walled carbon nanotubes encapsulated with palladium and their interaction with hydrogen at H/C ≥ 1.0

The nature of paramagnetic centers in a nanostructure based on single-walled carbon nanotubes (SWCNTs) encapsulated with Pd was studied by EPR spectroscopy at 77 and 293 K. It was found that strong charge-transfer π complexes of the (Pd-C _x ) type, which manifested themselves as a narrow resonance (ΔH = 6–8 G and g = 2.002 at T = 77 k), were formed in the Pd-SWCNT composite along with impurity centers (Fe₃O₄ nanoparticles within the nanotubes), which were responsible for a broad EPR signal (ΔH = 75 G and g = 2.065 at T = 293 K). These complexes were found to be predominant adsorption sites responsible for a high gravimetric density of hydrogen (H/C ≥ 1.0) within the single-walled carbon nanotubes.     

Calculation of refractive indices for the (NH<Subscript>2</Subscript>CH<Subscript>2</Subscript>COOH)<Subscript>2</Subscript> · HNO<Subscript>3</Subscript> crystal

The refraction R of the diglycine nitrate (DGN) crystal, (NH₂CH₂COOH)₂ · HNO₃, in the para-and ferroelectric phases has been calculated in the model of noninteracting diatomic chemical bonds of the elementary unit cell of the crystal on the basis of the longitudinal and transversal polarizabilities of these bonds. The calculated magnitudes of the principal refractive indices n _p , n _m , and n _g and the orientations of the optical indicatrix of the crystal agree satisfactorily with experimentally observed values. Introducing the coefficient of Lorenz-Lorentz interaction x into the corresponding formula permits better agreement of the calculated and experimental refractive indices of DGN crystal to be obtained. The temperature changes of these x coefficients upon the ferroelectric phase transition in the DGN crystal have been analyzed.     

Evaluation of the adsorption capacity of carbon nanofibers with the use of distribution functions

The sorption properties of carbon nanosystems are investigated using appropriate distribution functions. The adsorption capacities of carbon nanofibers for hydrogen are calculated in the singlet approximation under the assumption that adsorbent molecules elastically interact with walls. The optimum interplanar distances determined for carbon nanofibers are as follows: 0.39 ± 0.07 nm for the reduced density n ₀ = 0.15 and 0.46 ± 0.04 nm for the reduced density n ₀ = 0.3.     

Sulphur overlayers on Ir(100) and its effect on the adsorption of CO: a DFT study

Ordered sulphur overlayers adsorbed on Ir(100) surface are studied with differentcoverage ranging from 0.11 to 1.0 ML. Calculations indicate that atomic S adsorbsfavourably in hollow sites, forming strong covalent bonds with the substrate surface andthe adsorption energy is nearly unchanged at lower coverages(θ _S ≤ 0.50 ML). In good agreement withexperimental observations, the p(2 × 2)-S and c(2 × 2)-Sare predicted to be the most stable overlayers. The obtained surface electronic structuremodifications induced by sulphur adsorption are coverage-dependent and the results are inaccordance with the rectangular band and the Hammer-Nørskov models. Moreover, the effectof sulphur on the adsorption of CO is discussed in the p(2 × 2)?(S + CO)overlayer on Ir(100).     

X-Ray Spectral Studies of the Interface Interaction in CuO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/MWCNTs Nanocomposite

Results of a comprehensive study of the interface interaction of a nanostructured CuO_x and multiwalled carbon nanotubes (MWCNTs) in CuO_x/MWCNT nanocomposite by X-ray absorption spectroscopy (XANES, NEXAFS) and X-ray photoelectron spectroscopy (XPS) methods using a synchrotron radiation are presented. It is established that a nanostructured CuO_x in CuO_x/MWCNT nanocomposite is predominantly formed by CuO and has the form of flakelike particles 200–500 nm in size uniformly dispersed over an array of nanotubes. A chemical interaction of CuO_x and nanotubes with formation of covalent carbon–oxygen bonds, which does not lead to a significant destruction of the outer layers of carbon nanotubes, is observed at the interfaces of the nanocomposite.     

Classification of two-shell nanotubes with commensurate structures of shells

A classification of two-shell carbon nanotubes with commensurate structures of shells is proposed. The classification is based on the concept of equivalence classes as a set of shells with chiral indices of the (kf, kg) type, where f and g are the chiral indices of the equivalence class and k is the index of the shell diameter. All two-shell nanotubes with commensurate shells that are characterized by the chiral indices (k₁f₁, k₁g₁) and (k₂f₂, k₂g₂), where k₁ and k₂ are integral numbers, make up a family of nanotubes of different radii but with equal geometric parameters (such as the intershell distance, the unit cell length of the nanotube, and the difference between the chiral angles of the shells). The geometric parameters of nanotubes are calculated for a number of families, the distribution of different types of two-shell nanotubes with commensurate shells over the outer-shell radii is determined, and the threshold forces required to induce relative motion of nanotube shells are evaluated. The possible use of two-shell nanotubes with commensurate shells in nanostructures is discussed.     

The electronic structure and reactivity of the oxygen-modified Mo2C(0 0 0 1) surface

Oxygen adsorption on Mo₂C(0 0 0 1) has been investigated with angle-resolved photoemission spectroscopy (ARPES). When the surface is reacted with O₂, the O 2p-induced states are formed at 4.1 and 5.3 eV at the point. The emissions around the Fermi level are also intensified by oxygen adsorption, which is due to the formation of a partially filled state. It is found that the reactivity of the surface toward H₂O adsorption is much enhanced by pre-adsorption of oxygen. The reactivity is found to be maximized at θ_O ∼ 0.2.     

One-nucleon spectroscopy of light nuclei

The capabilities and limitations of the conventional many-particle shell model and modern potential cluster models are discussed. New revaluated and more accurate calculations of one-nucleon spectroscopic characteristics of the light nuclei of 1p shell are presented. In many-particle shell model for nuclei with A = 7, 9, 11, 13, and 15 nucleon partial widths of highly excited states with the isotopic spin T = 3/2 were calculated both for “allowed” and “forbidden” transitions. One-nucleon spectroscopic factors were calculated in threebody multicluster models of ⁶Li{αnp}, ⁸Li{αtn}, and ⁹Be{ααn} nuclei. For isobar-analogue nuclei ⁷Li and ⁷Be, the spectroscopic proton S _p and neutron S _n factors for transitions to the ground and excited states of corresponding residue nuclei of the triplet ⁶Li-⁶He-⁶Be were calculated in the framework of binary potential αtand ατ models. Integral, differential and polarization characteristics of photonuclear processes ⁷Li(γ, n ₀)⁶Li, ⁶He(p, γ_{0 + 1})⁷Li, ⁷Li(γ, p ₀)⁶He, and ⁹Be(γ, p _{0 + 1})⁸Li were calculated in this approach.     

Structural,electronic, and optical absorption properties of TiO2 nanotube adsorbed with Cu n clusters

The structural,electronic,and optical absorption properties of TiO2 nanotube(TiO2NT)with Cun clusters(n=1–4)adsorbed on its surface have been investigated based on density functional theory calculations.The TiO2NT is constructed by rolling up a(101)sheet of anatase TiO2 around the[1 01]direction;the ground states of Cun/TiO2NT systems are determined by analyzing the average adsorption energies.Calculation results show that odd-even oscillations occur for the average adsorption energy,the Cu–O bond length,and the amount of transferred electrons,with the increase in Cun cluster size;and the Cun/TiO2NTs with odd n’s demonstrate stronger interaction between the Cun cluster and the TiO2NT.Also,the impurity states introduced by the Cun cluster to the band gap of TiO2NT cause an obvious redshift of the optical absorption spectrum toward the visible light region,especially for the even n cases.     

Hydrogen chemisorption on <Emphasis Type="BoldItalic">sp</Emphasis><Superscript><Emphasis Type="Bold">2</Emphasis></Superscript>-bonded carbon: Influence of the local curvature and local electronic effects

We report on the interaction of hydrogen with sp²-bonded carbon which has been investigated on graphite (0001), single-walled carbon nanotubes and C₆₀ multilayer films. These substrates have been chosen to represent a large range of curvature in the carbon network. The photoelectron spectroscopy study of the samples treated with atomic hydrogen and low-energy hydrogen ions reveals that hydrogen is chemisorbed on the basal plane of the sp²-bonded carbon networks, as evidenced by the lowered emission from -derived states and a lowering of the electron work function of up to 1.3 eV. The hydrogen adsorption energy barrier is found to strongly depend on the local curvature of the carbon network whereby the barrier is lowered with increasing curvatures. Whereas in the case of C₆₀ and single-walled carbon nanotubes, hydrogen chemisorption can be achieved by exposure to atomic hydrogen, the chemisorption on graphite (0001) requires hydrogen ions of low kinetic energy (1 eV). Furthermore, the adsorption energy barrier is found to increase with hydrogen coverage.The scanning tunnelling microscopy study of individual adsorption sites on the graphite (0001) surface reveals long-ranged (5 nm) electronic effects observed as a (sqrt(3)×sqrt(3))R30° superstructure in the local density of states. It is shown that this superstructure is due to the scattering of delocalized electron wavefunctions at the point defects. The resulting standing waves induce a redistribution of the local density of states which is directly related to the point-like Fermi surface of graphite. PACS 68.43.-h; 71.20.Tx; 68.37.Ef     

A comparative DFT study of adsorption and catalytic performance of Au nanoparticles at anatase and brookite TiO2 surfaces

We have compared the adsorption properties of small Au_n (n = 1–8) nanoparticles on the defect-free (stoichiometric) and defective (partially reduced) brookite TiO₂(210) and anatase TiO₂(101) surfaces using density functional theory calculations. The interaction between Au atoms and anatase TiO₂(101) was determined to be quite weak and small Au_n particles grown at defects (O vacancies) prefer extended 2D structures. By contrast, dispersion and 3D configurations appear to be favored at brookite TiO₂(210) for Au_n nanoparticles due to their strong interaction. Calculations of CO oxidation at Au_n (n = 6–8) particles supported at defective brookite TiO₂(210) show that occurrence of protruding low-coordinated Au atoms is essential for favorable CO adsorption and subsequent reaction with O₂. In particular, the configuration of the Au_n nanoparticles can determine the energetics in the formation of active Au atoms, and their mobility also affects the reaction between CO and O₂ (or O).     

Electronic excitations on clean and adsorbate-covered oxide surfaces

We have studied electronic excitations at the surfaces of NiO (100), Cr₂O₃(111), and Al₂O₃(111) thin films with Electron Energy Loss Spectroscopy (EELS). On NiO (100) we observe surface electronic excitations in the energy range of the band gap which shift upon adsorption of NO. Ab initio cluster calculations show that these excitations occur within the Ni ions at the oxide surface. The (111) surface of Cr₂O₃ is characterized by distinct excitations which are also strongly influenced by the interaction with adsorbates. Temperature-dependent measurements show that two different states of the surface exist which are separated by an activation energy of about 10 meV. For Al₂O₃(111) we present data for a CO adsorbate. The oxide is quite inert with respect to CO adsorption as indicated by desorption temperatures between 38 K and 67 K. Due to the weak interaction with the substrate the a³II valence excitation of CO shows a clearly detectable vibrational splitting which has not been observed previously for a CO adsorbate in the (sub)monolayer coverage range. For several different adsorption state the lifetimes of the a³II state could be estimated from the halfwidths of the loss peaks, yielding values between 10^–15 s for the most strongly bound species and 10^–14 s for the CO multilayer.     

Theoretical study of CO and Pb adsorption on the Ni(1 1 1) and Ni3Al(1 1 1) surfaces

Adsorption of CO molecules and Pb atoms on the Ni(1 1 1) and Ni₃Al(1 1 1) substrates is studied theoretically within an ab initio density-functional-theory approach. Stable adsorption sites and the corresponding adsorption energies are first determined for stoichiometric surfaces. The three-fold hollow sites (fcc for Pb and hcp for CO) are found most favourable on both substrates. Next, the effect of surface alloying by a substitution of selected topmost substrate atoms by Pb or Ni atoms on the adsorption characteristics is investigated. When the surface Al atoms of the Ni₃Al(1 1 1) substrate are replaced by Ni atoms, the Pb and CO adsorption energies approach those for a pure Ni(1 1 1) substrate. The Pb alloying has a more substantial effect. On the Ni₃Al(1 1 1) substrate, it reduces considerably adsorption energy of CO. On the Ni(1 1 1) substrate, CO binding strengthens slightly upon the formation of the Ni(1 1 1)p(2×2)-Pb surface alloy, whereas it weakens drastically when the Ni(1 1 1)-Pb surface alloy is formed.     

Theoretical study of the adsorption of CO2 on tungsten carbide nanotubes

The adsorption of CO₂ on the single-walled tungsten carbide nanotubes has been investigated employing density functional theory method. The center of a hexagon of tungsten and carbon atoms in sites on tungsten carbide nanotube surfaces is the most stable adsorption site for CO₂ molecule, with a binding energy of −1.68 eV (−38.72 kcal/mol) and a WO binding distance of 1.95 Å. Furthermore, the adsorption of CO₂ on the single-walled carbon nanotubes has been investigated. Our first-principles calculations predict that the CO₂ adsorptive capacity of tungsten carbide nanotubes is about quadruple that of carbon nanotubes. This might have potential for greenhouse gas detection and bioremediation.     

Screening of strongly charged macroparticles in liquid electrolyte solutions

A modified Poisson-Boltzmann model has been proposed which makes it possible to describe the screening of strongly charged macroparticles in liquid electrolyte Z: Z solutions in the case when parameter B= ZeQ₀/εRT?1(Q₀ is the surface electric charge, T is the temperature, ε is the solution permittivity, and Z is the valence of ions) provided that the solution is dilute: κR ≡ (8πZ²e²n_i0/εT)^1/2R?1 (n_i0 is the equilibrium number density of ions). It is assumed that the charge Q₀ of a macroparticle appears as a result of adsorption of ions of a certain polarity on its surface. Quantitative criteria of division of dissolved ions into capable and incapable of adsorption are formulated. For aqueous solutions, the adsorption mechanism always leads to values of B ? 1. It is shown that the charge inversion effect predicted by other authors on the basis of different models must be observed for such solutions for all Z ≥ 1. The effect of Brownian movement of macroparticles on their screening is considered. It is shown that viscous forces emerging during such movement lead to peripheral destruction (“washing out”) of the screening ionic shell of macroparticles and, as a result, to violation of their electroneutrality. This results in the emergence of two types of oppositely charged compound particles with small radii close to R and with radii much larger than R, the charge polarity of the latter being opposite to the polarity of Q₀. It is found that both types of ions of compound particles obey the “law of distribution” of the mean energy of their electric field, expressed by formula (29). The problem of ionic screening of gas bubbles accompanied by the formation of bubstons (bubbles stabilized by ions) is considered separately. It is shown that the bubston radius R in pure water and in aqueous solutions of electrolytes is equal to 14 nm irrespective of the ion number density n_i0. The value of n_i0 determines the number density n _b of bubstons themselves, which are formed spontaneously under equilibrium conditions.