Hydrogen spillover on carbon-supported metal catalysts studied by inelastic neutron scattering. Surface vibrational states and hydrogen riding modes

Hydrogen spillover on carbon-supported precious metal catalysts has been investigated with inelastic neutron scattering (INS) spectroscopy. The aim, which was fully realized, was to identify spillover hydrogen on the carbon support. The inelastic neutron scattering spectra of Pt/C, Ru/C, and PtRu/C fuel cell catalysts dosed with hydrogen were determined in two sets of experiments: with the catalyst in the neutron beam and, using an annular cell, with carbon in the beam and catalyst pellets at the edge of the cell excluded from the beam. The vibrational modes observed in the INS spectra were assigned with reference to the INS of a polycyclic aromatic hydrocarbon, coronene, taken as a molecular model of a graphite layer, and with the aid of computational modeling. Two forms of spillover hydrogen were identified: H at edge sites of a graphite layer (formed after ambient dissociative chemisorption of H-2), and a weakly bound layer of mobile H atoms (formed by surface diffusion of H atoms after dissociative chernisorption of H-2 at 500 K). The INS spectra exhibited characteristic riding modes of H on carbon and on Pt or Ru. In these riding modes H atoms move in phase with vibrations of the carbon and metal lattices. The lattice modes are amplified by neutron scattering from the H atoms attached to lattice atoms. Uptake of hydrogen, and spillover, was greater for the Ru containing catalysts than for the Pt/C catalyst. The INS experiments have thus directly demonstrated H spillover to the carbon support of these metal catalysts.

Hydrogen adsorption in a copper ZSM5 zeolite: an inelastic neutron scattering study

Currently microporous oxidic materials including zeolites are attracting interest as potential hydrogen storage materials. Understanding how molecular hydrogen interacts with these materials is important in the rational development of hydrogen storage materials and is also challenging theoretically. In this paper, we present an incoherent inelastic neutron scattering (INS) study of the adsorption of molecular hydrogen and hydrogen deuteride (HD) in a copper substituted ZSM5 zeolite varying the hydrogen dosage and temperature. We have demonstrated how inelastic neutron scattering can help us understand the interaction of H-2 molecules with a binding site in a particular microporous material, Cu ZSM5, and by implication of other similar materials. The H-2 molecule is bound as a single species lying parallel with the surface. As H-2 dosing increases, lateral interactions between the adsorbed H-2 molecules become apparent. With rising temperature of measurement up to 70 K (the limit of our experiments), H-2 molecules remain bound to the surface equivalent to a liquid or solid H-2 phase. The implication is that hydrogen is bound rather strongly in Cu ZSM5. Using the simple model for the anisotropic interaction to calculate the energy levels splitting, we found that the measured rotational constant of the hydrogen molecule is reduced as a consequence of adsorption by the Cu ZSM5. From the decrease in total signal intensity with increasing temperature, we were able to observe the conversion of para-hydrogen into ortho-hydrogen at paramagnetic centres and so determine the fraction of paramagnetic sites occupied by hydrogen molecules, ca. 60%. (c) 2006 Elsevier B.V. All rights reserved.

Dihydrogen in cation-substituted zeolites X - an inelastic neutron scattering study

An inelastic neutron scattering (INS) study of the rotational - vibrational spectrum of dihydrogen sorbed by zeolite X having substituted sodium, calcium and zinc cations is reported. The rotational - vibrational spectrum of H-2 was observed at low energy transfer ( below ca. 25 meV, 202 cm(-1)); the vibration was that of the H-2 molecule against the binding site (H-2 - X, not H - H). The vibration frequency was proportional to the polarising power of the cation (Na+ < Ca2+ < Zn2+). Polarisation of the H-2 molecule dominated the interaction of H-2 with this binding site. The total scattering intensity was proportional to the dihydrogen dose. However the vibrational intensities became constant at ca. 0.3 wt% showing that the H-2 binding sites had saturated. Additional dihydrogen appeared as unbound or weakly bound dihydrogen exhibiting recoil.

Dihydrogen in zeolite CaX: an inelastic neutron scattering study

We report an inelastic neutron scattering (INS) study of the rotational–vibrational spectrum of dihydrogen sorbed by zeolite CaX. In the low energy (<200 cm−1) INS spectrum of adsorbed H2 we observe the rotational–vibrational spectrum of H2, where the vibration is that of the H2 molecule against the binding site (i.e. H2–X, not H–H). We have observed for the first time the vibrational overtones of the hydrogen molecule against the adsorption surface up to sixth order. These vibrations are usually forbidden in INS spectroscopy because of the selection rules imposed by the spin flip event required. In our case we are able to observe such a vibration because the rotational transition J(1 ← 0) convolutes the vibrational spectrum. This paper reports the effect for the first time.

A study by high energy transfer inelastic neutron scattering spectroscopy of the mineral fraction of ox femur bone

We have used high energy transfer (HET) inelastic neutron scattering spectroscopy to measure the vibrational modes in the spectra of hydroxyapatite, bone and brushite to confirm our earlier work that only a fraction of the hydroxyl groups in bone mineral are substituted. The HET spectra are better observed due to the higher scattering cross section of hydrogen compared with the other elements in the calcium phosphate compounds. (C) 2003 Elsevier Science B.V. All rights reserved.

Relationship between phonons and thermal expansion in Zn(CN)2 and Ni(CN)2 from inelastic neutron scattering and ab initio calculations

Zn(CN)2 and Ni(CN)2 are known for exhibiting anomalous thermal expansion over a wide temperature range. The volume thermal expansion coefficient for the cubic, three dimensionally connected material, Zn(CN)2, is negative (alpha(V) = −51  10(-6) K-1) while for Ni(CN)2, a tetragonal material, the thermal expansion coefficient is negative in the two dimensionally connected sheets (alpha(a) = −7  10(-6) K-1), but the overall thermal expansion coefficient is positive (alpha(V) = 48  10(-6) K-1). We have measured the temperature dependence of phonon spectra in these compounds and analyzed them using ab initio calculations. The spectra of the two compounds show large differences that cannot be explained by simple mass renormalization of the modes involving Zn (65.38 amu) and Ni (58.69 amu) atoms. This reflects the fact that the structure and bonding are quite different in the two compounds. The calculated pressure dependence of the phonon modes and of the thermal expansion coefficient, alpha(V), are used to understand the anomalous behavior in these compounds. Our ab initio calculations indicate that phonon modes of energy approx. 2 meV are major contributors to negative thermal expansion (NTE) in both the compounds. The low-energy modes of approx.8 and 13 meV in Zn(CN)2 also contribute significantly to the NTE in Zn(CN)2 and Ni(CN)2, respectively. The measured temperature dependence of the phonon spectra has been used to estimate the total anharmonicity of both compounds. For Zn(CN)2, the temperature-dependent measurements (total anharmonicity), along with our previously reported pressure dependence of the phonon spectra (quasiharmonic), is used to separate the explicit temperature effect at constant volume (intrinsic anharmonicity).

New insights into the compressibility and high-pressure stability of Ni(CN)2: a combined study of neutron diffraction, Raman spectroscopy, and inelastic neutron scattering

Nickel cyanide is a layered material showing markedly anisotropic behaviour. High-pressure neutron diffraction measurements show that at pressures up to 20.1 kbar, compressibility is much higher in the direction perpendicular to the layers, c, than in the plane of the strongly chemically bonded metal-cyanide sheets. Detailed examination of the behaviour of the tetragonal lattice parameters, a and c, as a function of pressure reveal regions in which large changes in slope occur, for example, in c(P) at 1 kbar. The experimental pressure dependence of the volume data is fitted to a bulk modulus, B0, of 1050 (20) kbar over the pressure range 0–1 kbar, and to 124 (2) kbar over the range 1–20.1 kbar. Raman spectroscopy measurements yield additional information on how the structure and bonding in the Ni(CN)2 layers change with pressure and show that a phase change occurs at about 1 kbar. The new high-pressure phase, (Phase PII), has ordered cyanide groups with sheets of D4h symmetry containing Ni(CN)4 and Ni(NC)4 groups. The Raman spectrum of phase PII closely resembles that of the related layered compound, Cu1/2Ni1/2(CN)2, which has previously been shown to contain ordered C≡N groups. The phase change, PI to PII, is also observed in inelastic neutron scattering studies which show significant changes occurring in the phonon spectra as the pressure is raised from 0.3 to 1.5 kbar. These changes reflect the large reduction in the interlayer spacing which occurs as Phase PI transforms to Phase PII and the consequent increase in difficulty for out-of-plane atomic motions. Unlike other cyanide materials e.g. Zn(CN)2 and Ag3Co(CN)6, which show an amorphization and/or a decomposition at much lower pressures (~100 kbar), Ni(CN)2 can be recovered after pressurising to 200 kbar, albeit in a more ordered form.

Differential and partial cross sections of elastic and inelastic positronium-helium-atom scattering

Scattering of positronium (Ps) by a helium atom has been investigated in a three-Ps-state coupled-channel model including Ps(1s,2s,2p) states using a recently proposed time-reversal-symmetric regularized electron-exchange model potential. Specifically, we report results of differential cross sections for elastic scattering and target-elastic Ps excitations. We also present results for total and different partial cross sections and compare them with experiment and other calculations.

Close-coupling calculations of elastic and inelastic positron-helium scattering

Elastic and inelastic positron-helium scattering have been investigated in different partial waves at medium energies using the close-coupling approximation with realistic wavefunctions employing the following states: He(1s1s), He(1s2s), He(1s2p), He(1s3s), He(1s3p), Ps(1s), Ps(2s) and Ps(2p). All excitations of the helium atom are in the spin-singlet electronic state. Calculations are reported of cross sections to He(1s1s), He(1s2s), and He(1s2p) transitions for incident positron energies up to 200 eV. These cross sections are in good agreement with experimental results.

Encapsulation of paclitaxel into a bio-nanocomposite. a study combining inelastic neutron scattering to thermal analysis and infrared spectroscopy

The anticancer drug paclitaxel was encapsulated into a bio-nanocomposite formed by magnetic nanoparticles, chitosan and apatite. The aim of this drug carrier is to provide a new perspective against breast cancer. The dynamics of the pure and encapsulated drug were investigated in order to verify possible molecular changes caused by the encapsulation, as well as to follow which interactions may occur between paclitaxel and the composite. Fourier transformed infrared spectroscopy, thermal analysis, inelastic and quasi-elastic neutron scattering experiments were performed. These very preliminary results suggest the successful encapsulation of the drug.

Soft edge of hadron scattering and minijet models for the total and inelastic pp cross sections at LHC and beyond

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Differential cross sections for elastic and inelastic positronium-hydrogen-atom scattering

A time reversal symmetric regularized electron exchange model was used to elastic scattering, target elastic Ps excitations and target inelastic excitation of hydrogen in a five state coupled model. A singlet Ps-H-S-wave resonance at 4.01 eV of width 0.15 eV and a P-wave resonance at 5.08 eV of width 0.004 eV were obtained using this model. The effect on the convergence of the coupled-channel scheme due to the inclusion of the excited Ps and H states was also analyzed.

Low energy elastic and electronically inelastic electron scattering from biomolecules.

Reactions initiated by collisions with low-energy secondary electrons has been found to be the prominent mechanism toward the radiation damage on living tissues through DNA strand breaks. Now it is widely accepted that during the interaction with these secondary species the selective breaking of chemical bonds is triggered by dissociative electron attachment (DEA), that is, the capture of the incident electron and the formation of temporary negative ion states [1,2,3]. One of the approaches largely used toward a deeper understanding of the radiation damage to DNA is through modeling of DEA with its basic constituents (nucleotide bases, sugar and other subunits). We have tried to simplify this approach and attempt to make it comprehensible at a more fundamental level by looking at even simple molecules. Studies involving organic systems such as carboxylic acids, alcohols and simple ¯ve-membered heterocyclic compounds are taken as starting points for these understanding. In the present study we investigate the role played by elastic scattering and electronic excitation of molecules on electron-driven chemical processes. Special attention is focused on the analysis of the in°uence of polarization and multichannel coupling e®ects on the magnitude of elastic and electronically inelastic cross-sections. Our aim is also to investigate the existence of resonances in the elastic and electronically inelastic channels as well as to characterize them with respect to its type (shape, core-excited or Feshbach), symmetry and position. The relevance of these issues is evaluated within the context of possible applications for the modeling of discharge environments and implications in the understanding of mutagenic rupture of DNA chains. The scattering calculations were carried out with the Schwinger multichannel method (SMC) [4] and its implementation with pseudopotentials (SMCPP) [5] at di®erent levels of approximation for impact energies ranging from 0.5 eV to 30 eV. References [1] B. Boudai®a, P. Cloutier, D. Hunting, M. A. Huels and L. Sanche, Science 287, 1658 (2000). [2] X. Pan, P. Cloutier, D. Hunting and L. Sanche, Phys. Rev. Lett. 90, 208102 (2003). [3] F. Martin, P. D. Burrow, Z. Cai, P. Cloutier, D. Hunting and L. Sanche, Phys. Rev. Lett. 93, 068101 (2004). [4] K. Takatsuka and V. McKoy, Phys. Rev. A 24, 2437 (1981); ibid. Phys. Rev. A 30, 1734 (1984). [5] M. H. F. Bettega, L. G. Ferreira and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993).