Inelastic neutron scattering study of acetonitrile adsorbed on Raney nickel

A vibrational study of acetonitrile adsorbed on Raney nickel has been performed using inelastic neutron scattering. The chemisorption is associative and no C-H bond breaking is observed up to 393 K. Acetonitrile is found to be adsorbed parallel to the surface. The vibrational modes of the molecule are not very perturbed, this indicates a weak interaction with the surface. The coverage of the metal surface by acetonitrile is small, which has also been observed in the hydrogenation reaction in the gas phase. The excellent selectivity for the primary amine formation can be explained by this weak chemisorption.     

Inelastic neutron scattering spectra of some English coals: a preliminary study

The applications of incoherent inelastic neutron scattering (IINS) spectroscopy to the study of coal are discussed in the light of preliminary data (350–2000 cm^?1) obtained from three English coals of various ranks.     

Density functional theory (DFT) and microcalorimetric investigations of CO adsorption on Pt clusters

Microcalorimetric measurements were conducted at 573 K of CO adsorption on Pt clusters supported in L-zeolite. The measured heat of CO adsorption is 175 kJ/mol, and the heat decreases to 90 kJ/mol near saturation coverage. Quantum chemical calculations were performed using density functional theory to study the interaction of CO with 10-atom Pt clusters. The heat of CO adsorption on atop-sites is calculated to be 209 kJ/mol, while a lower heat of 142 kJ/mol is calculated for CO on bridge-sites. These values decrease to 197 and 102 kJ/mol for population of two atop-sites and two bridge-sites, respectively, on the same Pt₁₀ cluster. The heat of adsorption decreases to 157 kJ/mol when six CO molecules adsorb on six atop-sites of the cluster. The calculated initial heat of CO adsorption on Pt₁₀ clusters is in agreement with experimental and theoretical values reported for CO adsorption on Pt single-crystal surfaces. The higher heat of CO adsorption at atop-sites may be caused by more σ-donation from CO to sp orbitals of Pt for atop-sites. The heat of CO adsorption on bridge-sites becomes higher on negatively charged platinum clusters. The calculated C-O stretching frequencies for charged and neutral platinum clusters agree with experimental data. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sub-nanometer characterization of activated carbon by inelastic neutron scattering

Inelastic neutron scattering spectra are calculated for hydrogen molecules adsorbed on activated carbon. The slit-shaped pore model is used to calculate the adsorption potentials of a hydrogen molecule in pores of variable width. The motion of the hydrogen molecules is quantized perpendicular to the plane of the pore and both rotational and vibrational transition energies are found. The perturbation of adjacent hydrogen molecules on the transition energies is discussed. Form factors and Debye–Waller factors are calculated for momentum transfer parallel and perpendicular to the pore; this anisotropy is particularly pronounced in pores where there is nearly enough room for two adsorbed layers. A spectrum composed of a uniform distribution of pore sizes agrees only qualitatively with experimental results for activated carbon. Reasons for this disparity are discussed, including the possibility that transitions in the translational motion parallel to the adsorption plane contribute significantly to the spectra. In addition, the positioning of the center of gravity of the first rotational transition is discussed, with several factors contributing to shift it from the 14.7 meV of the gas phase. Our results suggest that inelastic neutron scattering can be valuable as a complementary sub-nanometer pore characterization technique.     

A small-angle neutron scattering study of activated carbon fibers

Phenolic resin based activated carbon fibers are widely used as filter materials in industry. We studied the fiber type ACFY-0204-3-18 which is applied to recover 2-propanol from air. The fibers were characterised by measuring the chemical composition, the apparent density, the wide angle X-ray diffraction data, and the sorption isotherms for nitrogen at 77.4 K and 2-propanol at 308.2 K. The porous structure was studied by small-angle neutron scattering. The two-dimensional contour patterns show an anisotropic intensity distribution I(q) at scattering vectors |q|<0.4 nm⁻¹. In this q-range I(q) is perpendicular to the fiber axis due to a refraction of the neutrons by the fibers, and I(q) increases abruptly with decreasing q. At |q|>0.4 nm⁻¹ scattering is isotropic and I(q) shows a weak interference peak due to the presence of micropores. The mean pore size of ∼2 nm was determined by fitting a monodisperse and a polydisperse Percus-Yevick hard sphere model to the experimental I(q) data. The microstructure of ACFY differs basically from the microstructure of polyacrylonitrile and pitch based carbon fibers.     

The adsorption of hydrogen and hydrogen sulphide on tungsten sulphide; isotherm and neutron scattering studies

Adsorption isotherms for hydrogen and hydrogen sulphide on a WS₂ surface have been measured at 400°C at pressures up to 1 at. Inelastic neutron scattering spectra have been recorded for the materials produced. Both hydrogen and hydrogen sulphide adsorb dissociatively to produce S-H bonds. Hydrogen sulphide adsorption is smaller than that of hydrogen and it is suggested that this is because dissociation leads also to sulphidation of the surface vacancies at which the adsorption occurs. In both cases the hydrogen atoms produced as a consequence of the dissociation are able to diffuse over the surface to form bonds to available sulphur atoms.     

Inelastic neutron scattering study on the influence of after-treatments on different technical cokes of varying impurity level and their sp/sp character

Technical cokes from the surfaces of Pd/SiO₂ and Pt/Al₂O₃ catalysts obtained from large scale chemical reactors, waste products, diesel soot and pyrocarbon containing different amounts of inorganic/metallic contaminants or corrosion products were compared utilizing the inelastic incoherent neutron scattering (INS) technique. Different sensitivities of material from high speed coking and low speed coking towards a treatment with hydrogen were observed dependent on the relative amounts of inorganic contaminants throughout the coke. Metallic contaminants are of a pronounced catalytic relevance not only during the deposition of these cokes by means of a chemical vapour deposition process and catalytic transformations of the carbon structure but also in the first interactions between coke and hydrogen in a controlled decomposition process. The internetwork linking between sp² and sp³ type carbonaceous entities was affected and degraded in after-treatments using hydrogen or air. The sp² entities themselves remained largely unaffected. However, the formation of new metal-organic structures was observed only after a first hydrogasification step. A species similar to [Fe(H₂O)Cl₅]²⁻ is an important intermediate in dew point corrosion. As expected, methane-derived pyrocarbon from Pt/Al₂O₃ catalysts showed a much higher resistance to the influence of hydrogen than soot particles from diesel fuel or various low temperature cokes. INS is useful to improve the understanding of catalyst deactivation under industrial conditions by the formation of coke together with the occurrence and properties of corrosion products.     

Microcalorimetric, FTIR, and DFT studies of the adsorption of isobutene on silica

Microcalorimetric measurements, infrared spectroscopic studies(FTIR), and quantum-chemical calculations based on density-functional theory(DFT) were made of the interactions of isobutene with silica at 300 K. On the basis of DFT calculations and FTIR spectra, most of isobutene adsorbs reversibly on silica at 300 K, involving the interaction of the π-bond with hydroxyl groups on the surface. The average energy of these interactions is ~45kJ/mol at a surface coverage of ~400 μmol of isobutene per gram of silica. The formation of butoxy and 2-methyl-1-propoxy species upon reaction of isobutene with silanol groups appears to be limited kinetically at 300 K. While the enthalpies of formation of these species from gaseous isobutene and silanol groups are calculated to be -55 and -40 kJ/mol, respectively, the activation energies for the formation of these species from adsorbed isobutene are estimated from DFT to be 172 and 226 kJ/mol, respectively. These high activation barriers are caused by the required localization of positive charge in the corresponding transition states, which is made difficult by the weak acidity of silica. Minor amounts of surface butoxy might form on silica at 300K, perhaps on defect or impurity sites, and these species may be responsible for the higher heats of adsorption (56±2 kJ/mol) measured at low surface coverages on silica. This revised version was published online in July 2006 with corrections to the Cover Date.     

Small-angle neutron scattering and cyclic voltammetry study on electrochemically oxidized and reduced pyrolytic carbon

The electrochemical double layer capacitance and internal surface area of a pyrolytic carbon material after electrochemical oxidation and subsequent reduction was studied with cyclic voltammetry and small-angle neutron scattering. Oxidation yields an enhanced internal surface area (activation), and subsequent reduction causes a decrease of this internal surface area. The change of the Porod constant, as obtained from small-angle neutron scattering, reveals that the decrease in internal surface area is not caused merely by a closing or narrowing of the pores, but by a partial collapse of the pore network.     

Energetics of single-wall carbon nanotubes as revealed by calorimetry and neutron scattering

Bundles of (10,10) single-wall carbon nanotubes (SWCNTs) have been studied by high-temperature oxidation calorimetry and inelastic neutron scattering to obtain standard formation enthalpies and entropies at 298 K. SWCNTs are found to be only moderately less stable than graphite, and are significantly more stable than their fullerene counterparts. They are 7 kJ mol⁻¹ metastable in terms of enthalpy relative to graphite, and just 5 kJ mol⁻¹ less stable than diamond. Despite striking differences in vibrational dynamics of carbon atoms in SWCNTs and graphite, their thermodynamic properties at room and higher temperatures are dominated by the same set of high energy vibrations, reflected in very similar vibrational entropies. However, the energetics of SWCNTs are governed by the diameter-dependent enthalpic contributions, but not the specifics of phonon density of states.     

Structural investigations of pyrogenic silica-epoxy composites: Combining small-angle neutron scattering and transmission electron microscopy

Small-angle neutron scattering (SANS) and transmission electron microscopy (TEM) were used to investigate the structure of pyrogenic silica in epoxy-based sub-microcomposites. Pyrogenic silica exhibits a fractal-like structure that can be characterized accurately by SANS. Two series of networks were synthesized. In the first series the specific surface area of pyrogenic silica varied from 50 to 300 m²/g. In the second series, the silica surface was modified in order to be able to react covalently with the precursors of the epoxy network.     

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Raman investigations of NH3 adsorption on TiO2, Nb2O5, and Nb2O5/TiO2

A study has been carried out of the interactions of NH₃ with TiO₂, Nb₂O₅, and Nb₂O₅/TiO₂. Raman spectroscopy was used to characterize the structure of the adsorbed NH₃ and perturbations of species present at the surface of the adsorbent. On each oxide, NH₃ adsorbs predominantly at Lewis acid sites. Hydrogen-bonding occurs between the adsorbed NH₃ and OH groups present on the surface of TiO₂. A small concentration of NH ₄ ⁺ is observed, consistent with the relatively low concentration of Brønsted acid sites compared to Lewis acid sites on each of the samples investigated. Exposure of Nb₂O₅/TiO₂ to NH₃ at temperatures up to 500°C does not result in partial reduction of the supported niobia.     

Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.     

In situ formation of onion-like carbon from the evaporation of ultra-dispersed nanodiamonds

We report the in situ formation of onion-like carbon (OLC) by evaporation from a nanodiamond source under ultra-high vacuum conditions. The OLC is characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) and is found to be highly defective but completely separated. The absence of any signature in XPS, Raman spectra and TEM associated with nanodiamond in the film suggests that the OLC is formed from carbon vapor or by the direct evaporation of only the smallest particles resulting from nanodiamond graphitization. The method thus provides a route to the formation of individually separated OLC nanoparticles.     

Oxidative dehydrogenation of ethylbenzene to styrene over ultra-dispersed diamond and onion-like carbon

The catalytic properties of sp³-hybridized ultra-dispersed diamond and sp²-hybridized onion-like carbon in the oxidative dehydrogenation of ethylbenzene to styrene were investigated, highlighting the structure sensitivity of the reaction. The sp³-carbon led initially to C-C cleavage and benzene formation, while a switchover of the main reaction pathway into the styrene formation occurred with time on stream due to the formation of surface sp² carbon, required for the selective styrene formation. This was confirmed by the behavior and the high stable styrene selectivity shown by onion-like carbons. High temperature oxygen pre-treatment created catalytically active species at the sp² carbon surface, confirming that a high thermal stability carbon-oxygen complex was the active surface site for forming styrene.     

Dynamic light scattering and small-angle neutron scattering studies on phenolic resin solutions

Solution properties of unfractionated phenolic resins prepared by polycondensation of phenol and formaldehyde using oxalic acid as a catalyst were investigated by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The hydrodynamic radius, obtained by DLS experiments with 1 vol % solution in acetone, and the correlation length, ξ, of the Ornstein-Zernike equation, obtained by SANS experiments with 10 vol % solution in tetrahydrofuran, obey a power-law relation as a function of z-average molecular weight estimated by gel permeation chromatography, with scaling exponents of 0.57 and 0.27, respectively. These behaviors are unaffected by polymerization conditions, such as initial phenol-to-formaldehyde molar ratio in the range from 0.9 to 1.5, catalyst concentration with oxalic acid-to-phenol molar ratio from 0.01 to 0.1, and reaction time within the period in which the polymer remains soluble. SANS curves for polymers prepared under different conditions are sufficiently superimposed onto a single curve with the reduced variables, ξ⁻²I(q) and ξq. These results indicate that unfractionated phenolic resins have a self-similar structure with respect to the molecular weight.     

Surface-enhanced Raman scattering study of nano-sized organic carbon particles produced in combustion processes

Surface-enhanced Raman scattering (SERS) has been employed for the first time to characterize nano-sized organic carbon (NOC) particles produced in ordinary combustion processes. Ag-coated glass microparticles, used as SERS substrate, provide a Raman scattering cross section enhancement up to five orders of magnitude, which allows sample investigation at low concentration level. The observed spectral features supply a deeper insight on the chemical properties of the investigated combustion product. In addition, the high sensitivity of the SERS technique might be useful to test and characterize the toxicity of NOC particles.     

Characterization of accessible and inaccessible pores in microporous carbons by a combination of adsorption and small angle neutron scattering

We determine the pore size distribution for five activated carbons (comprising carbide derived as well as commercial activated carbon samples) by the interpretation of experimental small angle neutron scattering (SANS) intensity profiles, based on the primary assumption of an infinitely dilute solution of hollow spherical particles. The interpretation yields the pore size distribution of the carbon samples that have predominantly micropore populations (size <20 Å), but not for carbons which have significant mesopore populations of sizes up to 48 Å and high mass fractal degrees. The pore size distribution (PSD) results based on SANS data reveal significant populations of micropores of size <6.1 Å, and mesopores of size >20 Å, which are not present in the PSD results based on adsorption isotherms of either Ar at 87 K or CO₂ 273 K. This inaccessible porosity becomes accessible to CO₂ and Ar on heat treatment, leading to increase in the adsorption based pore volume. However, the surface area does not commensurately increase, indicating the inaccessible microporosity to predominantly comprise surface defects and roughness that are removed on heat treatment or activation. This finding sheds the light onto the evolution of porosity of activated carbons during gasification or post synthesis-treatment.     

Synthesis,characterization, and DFT Raman study of pure and Mn-doped LiTaO3 nanofibers

Pure and Mn-doped lithium tantalate nanofibers, with Mn concentrations of 1%, 2.5%, and 5%, were synthesized by the electrospinning method. The morphology, microstructure, and crystal structure of as-spun and annealed composite nanofibers were characterized by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. Raman spectroscopy has shown to be a powerful tool to detect either local variations or changes of the whole structure. Position and width of one Raman line can be used as markers of a structural change. Some vibrational modes are especially associated with the site of Li or Ta ions and so, they can be affected by the introduction of dopant ions. Any damages or local changes in the microstructure can be detected by a line broadening. With the use of Raman spectroscopy, the sites where Mn ions enter the doped structures were established by recording the shift and broadening of peaks in Mn-doped structures with respect to pure lithium tantalate. Thus it was proven that Mn ions enter the Li sites for low Mn concentration and, on the other hand, for higher concentrations, the dopant substitutes Li and Ta sites. First-principles calculations were performed within the density functional theory, including lattice-dynamic calculations of the phonon modes at the zone center (Γ point), for the pure structure, to find the irreducible representation of the modes.