THE JOURNAL OF CHEMICAL PHYSICS 139, 164507 (2013)
Experimental inelastic neutron scattering spectrum of hydrogen hexagonal clathrate-hydrate compared with rigorous quantum simulations Milva Celli,1,a) Anna Powers,2,a) Daniele Colognesi,1 Minzhong Xu,2 Zlatko Baˇci´c,2,b) and Lorenzo Ulivi1,c) 1
Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy 2 Department of Chemistry, New York University, New York, New York 10003, USA
(Received 26 July 2013; accepted 8 October 2013; published online 25 October 2013) We have performed high-resolution inelastic neutron scattering (INS) measurements on binary hydrogen clathrate hydrates exhibiting the hexagonal structure (sH). Two samples, differing only in the ortho/para fraction of hydrogen, were prepared using heavy water and methyl tert-butyl ether as the promoter in its perdeuterated form. The INS spectrum of the translation-rotation (TR) excitations of the guest H2 molecule was obtained by subtracting the very weak signal due to the D2 O lattice modes. By means of a subtraction procedure, it has been possible to obtain separately the spectra of caged p-H2 and o-H2 . sH clathrates are comprised of three distinct types of cages, two of which, differing in shape and size, are each occupied by one H2 molecule only. Both contribute to the measured INS spectrum which is, therefore, rather complex and challenging to assign unambiguously. To assist with the interpretation, the INS spectra are calculated accurately utilizing the quantum methodology which incorporates the coupled five-dimensional TR energy levels and wave functions of the H2 molecule confined in each type of nanocage. The computed INS spectra are highly realistic and reflect the complexity of the coupled TR dynamics of the guest H2 in the anisotropic confining environment. The simulated INS spectra of p-H2 and o-H2 in the small and medium cages are compared with the experimental data, and are indispensable for their interpretation. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826451] I. INTRODUCTION
Inclusion compounds known as clathrate hydrates are common in nature. These are regularly ordered crystals, made of water molecules which form ice-like hydrogen-bonded framework with voids, referred to as cages, where different guest molecules are trapped. The guest molecules, often of a hydrocarbon gas, do not form chemical bonds with water, and are located in the cages that have various size and geometry. Clathrate hydrates generally assume one of the three main structures, cubic structure I (sI), cubic structure II (sII), and hexagonal structure (sH).1 Considerable effort has been devoted in recent years to the study of clathrate hydrates containing hydrogen, because of their potential as efficient and environmentally friendly materials for hydrogen storage. With water, hydrogen forms either simple clathrates, i.e., those made of only H2 O and H2 , at a pressure of about 2000 bars and temperature T 273 K,2, 3 or binary clathrates, which contain other guest molecules besides H2 . Binary clathrates containing tetrahydrofuran (THF) as the promoter, namely, formed by H2 O, THF, and H2 , are prepared at a much lower pressure but can still store a significant, although smaller, amount of hydrogen.4 Both simple
a) M. Celli and A. Powers contributed equally to this work. b) Electronic mail: [email protected] c) Electronic mail: [email protected]
0021-9606/2013/139(16)/164507/8/$30.00
and binary hydrogen clathrates can adopt the cubic sII crystal structure. This structure is constituted by small cages in the form of distorted dodecahedra (512 , in a usual notation, i.e., having 12 pentagonal faces), and large cages in the form of polyhedra (hexakaidecahedra) having 12 pentagonal and 4 hexagonal faces (512 64 , in short).5 In binary sII clathrates, the THF molecules reside in the large cages, and generally one hydrogen molecule is hosted in the small cages.6, 7 In simple clathrates, the 512 64 cages are multiply occupied by up to four H2 molecules, depending on the pressure.8 Several authors have investigated whether the other clathrate structures could also accommodate H2 molecules in some of their cages, and possibly serve as economical hydrogen storage materials. Attention has been given to clathrates having the hexagonal structure sH, which are stable with at least two kinds of guest molecules, in this case H2 and a larger promoter molecule.1 The sH clathrate structure, characterized by the space group P6/mmm, has a unit cell comprised of 34 H2 O molecules, which form three small 512 cages, formed by 20 water molecules and similar to those in the sII hydrates, two quite asymmetric medium 43 56 63 cages, also formed by 20 water molecules, and one 512 68 large cage formed by 36 water molecules.1 In clathrates having the sH structure, the large promoter molecules singly occupy the 512 68 cages, while small gas molecules can reside in both the small and medium cages. A representation of the small and medium cages is shown in Fig. 1. Single occupancy of these cages by hydrogen molecules has been demonstrated in
139, 164507-1
© 2013 AIP Publishing LLC
164507-2
Celli et al.
J. Chem. Phys. 139, 164507 (2013)
periment and the quantum simulations. Section V summarizes the paper.
512
435663
FIG. 1. Perspective drawings of the small 512 and medium 43 56 63 cages of sH clathrate-hydrate. The large asymmetry of the medium cage is evident from the figure. A trapped H2 molecule inside each cage is also shown schematically (Figure realized with VESTA9 ).
two experimental papers via X-ray diffraction, Raman spectroscopy, and thermodynamic measurements.10, 11 In addition to their potential as hydrogen storage materials, hydrogen clathrate hydrates are also of considerable theoretical interest as prototypical examples of confined quantum particles. The authors have recently investigated both simple and binary sII hydrogen clathrates by Raman spectroscopy12, 13 and inelastic neutron spectroscopy (INS).14–18 The latter technique, in particular, has proven to be an extremely valuable tool for investigating at a microscopic level the properties of hydrogen molecules confined inside the nanocavities. The exceptionally large incoherent neutron scattering cross section of the proton, nearly two orders of magnitude greater than that of any other nucleus, makes INS a highly selective probe of the dynamics of encapsulated hydrogen molecules, since the bands in the spectra due to the host vibrational modes not involving hydrogen are typically quite weak. In addition, the neutron has the ability to induce spin flip in molecular nuclei, making it possible for INS to observe j = 1 rotational transitions which are forbidden in optical, infrared and Raman spectroscopy, since they involve conversion between para-hydrogen (p-H2 ) and orthohydrogen (o-H2 ). Recently, the spectroscopic features of hydrogen clathrates measured using INS have been interpreted with the help of a quantitative theory based on the rigorous treatment of the coupled translational-rotational dynamics of the guest hydrogen molecule.16, 17, 19 In this paper, we discuss the INS measurements obtained for a sH structure clathrate. Due to the presence of two distinct types of cages, small and medium, each occupied by one H2 molecule, we expect to observe spectral features arising from the rattling and rotational transitions of H2 confined in two substantially different environments. This results in the INS spectra which are significantly more complex than those of the binary sII hydrogen clathrates, where only the small cages are singly occupied by H2 . The INS spectra of H2 molecule in the small and medium cages, respectively, of the sH structure have been calculated and their combination has been compared to the experimental results. Sections II–IV of this paper discuss, in turn, the experiment and the information we can readily derive from it (Sec. II), the theory used to compute the INS spectra and its results, showing similarities with, and differences from, other clathrate structures (Sec. III), and finally, in Sec. IV, the conclusions we can reach from the comparison between the ex-
II. EXPERIMENT AND RESULTS A. Experimental details
The preparation of sH clathrates differs from that described in Refs. 14 and 16 in only a few aspects. The same high-pressure beryllium-copper vessel, adequate to hold hydrogen gas at pressures up to 3000 bars and at temperature down to 77 K, was used in this work as well. Among the promoter substances employed successfully to prepare sH hydrogen clathrates, as described in Ref. 10, we have decided to use methyl tert-butyl ether (MTBE), because it is available in the perdeuterated form (MTBE-d12). This compound is hardly soluble in water, so we have filled the berylliumcopper vessel with a known amount of finely ground D2 O ice, and subsequently added liquid MTBE-d12, in slight abundance with respect to the stoichiometric proportions, in order to compensate for its possible evaporation. The entire procedure was carried out under nitrogen atmosphere, in order to prevent the condensation of atmospheric H2 O on the sample. After closing the high pressure vessel, hydrogen gas was added at the pressure of p 1300 bars, and the sample was kept for 2–3 days under constant hydrogen pressure at a temperature T 0 ◦ C, well outside the stability region of the simple sII hydrogen clathrate. The sample was then quenched by immersing the pressure vessel in liquid nitrogen, the pressure was released and the clathrate powder was recovered to fill an aluminum slab cell. This operation was accomplished while keeping the sample close to liquid nitrogen temperature, in a dry-nitrogen atmosphere. The sample cells were finally shipped to ISIS, RAL (UK), in a Dewar flask at liquid nitrogen temperature. The neutron scattering measurements were carried out using TOSCA-II inelastic spectrometer. This is a crystalanalyzer inverse-geometry spectrometer,20 where the final neutron energy, E1 , is selected by two sets of pyrolytic graphite crystals placed in forward-scattering (at around 42.6◦ with respect to the incident beam) and in back-scattering (at about 137.7◦ with respect to the incident beam). This arrangement sets the nominal scattered neutron energy to E1 = 3.35 meV (forward-scattering), and to E1 = 3.32 meV (back-scattering). Higher-order Bragg reflections are filtered out by 120 mm-thick beryllium blocks wrapped in cadmium and cooled down to a temperature lower than 30 K. The incident neutron beam, on the other hand, spans a broad incoming energy (E0 ) range allowing to cover an extended energy transfer (E = E0 − E1 ) region: 3 meV
Experimental inelastic neutron scattering spectrum of hydrogen hexagonal clathrate-hydrate compared with rigorous quantum simulations Milva Celli,1,a) Anna Powers,2,a) Daniele Colognesi,1 Minzhong Xu,2 Zlatko Baˇci´c,2,b) and Lorenzo Ulivi1,c) 1
Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy 2 Department of Chemistry, New York University, New York, New York 10003, USA
(Received 26 July 2013; accepted 8 October 2013; published online 25 October 2013) We have performed high-resolution inelastic neutron scattering (INS) measurements on binary hydrogen clathrate hydrates exhibiting the hexagonal structure (sH). Two samples, differing only in the ortho/para fraction of hydrogen, were prepared using heavy water and methyl tert-butyl ether as the promoter in its perdeuterated form. The INS spectrum of the translation-rotation (TR) excitations of the guest H2 molecule was obtained by subtracting the very weak signal due to the D2 O lattice modes. By means of a subtraction procedure, it has been possible to obtain separately the spectra of caged p-H2 and o-H2 . sH clathrates are comprised of three distinct types of cages, two of which, differing in shape and size, are each occupied by one H2 molecule only. Both contribute to the measured INS spectrum which is, therefore, rather complex and challenging to assign unambiguously. To assist with the interpretation, the INS spectra are calculated accurately utilizing the quantum methodology which incorporates the coupled five-dimensional TR energy levels and wave functions of the H2 molecule confined in each type of nanocage. The computed INS spectra are highly realistic and reflect the complexity of the coupled TR dynamics of the guest H2 in the anisotropic confining environment. The simulated INS spectra of p-H2 and o-H2 in the small and medium cages are compared with the experimental data, and are indispensable for their interpretation. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826451] I. INTRODUCTION
Inclusion compounds known as clathrate hydrates are common in nature. These are regularly ordered crystals, made of water molecules which form ice-like hydrogen-bonded framework with voids, referred to as cages, where different guest molecules are trapped. The guest molecules, often of a hydrocarbon gas, do not form chemical bonds with water, and are located in the cages that have various size and geometry. Clathrate hydrates generally assume one of the three main structures, cubic structure I (sI), cubic structure II (sII), and hexagonal structure (sH).1 Considerable effort has been devoted in recent years to the study of clathrate hydrates containing hydrogen, because of their potential as efficient and environmentally friendly materials for hydrogen storage. With water, hydrogen forms either simple clathrates, i.e., those made of only H2 O and H2 , at a pressure of about 2000 bars and temperature T 273 K,2, 3 or binary clathrates, which contain other guest molecules besides H2 . Binary clathrates containing tetrahydrofuran (THF) as the promoter, namely, formed by H2 O, THF, and H2 , are prepared at a much lower pressure but can still store a significant, although smaller, amount of hydrogen.4 Both simple
a) M. Celli and A. Powers contributed equally to this work. b) Electronic mail: [email protected] c) Electronic mail: [email protected]
0021-9606/2013/139(16)/164507/8/$30.00
and binary hydrogen clathrates can adopt the cubic sII crystal structure. This structure is constituted by small cages in the form of distorted dodecahedra (512 , in a usual notation, i.e., having 12 pentagonal faces), and large cages in the form of polyhedra (hexakaidecahedra) having 12 pentagonal and 4 hexagonal faces (512 64 , in short).5 In binary sII clathrates, the THF molecules reside in the large cages, and generally one hydrogen molecule is hosted in the small cages.6, 7 In simple clathrates, the 512 64 cages are multiply occupied by up to four H2 molecules, depending on the pressure.8 Several authors have investigated whether the other clathrate structures could also accommodate H2 molecules in some of their cages, and possibly serve as economical hydrogen storage materials. Attention has been given to clathrates having the hexagonal structure sH, which are stable with at least two kinds of guest molecules, in this case H2 and a larger promoter molecule.1 The sH clathrate structure, characterized by the space group P6/mmm, has a unit cell comprised of 34 H2 O molecules, which form three small 512 cages, formed by 20 water molecules and similar to those in the sII hydrates, two quite asymmetric medium 43 56 63 cages, also formed by 20 water molecules, and one 512 68 large cage formed by 36 water molecules.1 In clathrates having the sH structure, the large promoter molecules singly occupy the 512 68 cages, while small gas molecules can reside in both the small and medium cages. A representation of the small and medium cages is shown in Fig. 1. Single occupancy of these cages by hydrogen molecules has been demonstrated in
139, 164507-1
© 2013 AIP Publishing LLC
164507-2
Celli et al.
J. Chem. Phys. 139, 164507 (2013)
periment and the quantum simulations. Section V summarizes the paper.
512
435663
FIG. 1. Perspective drawings of the small 512 and medium 43 56 63 cages of sH clathrate-hydrate. The large asymmetry of the medium cage is evident from the figure. A trapped H2 molecule inside each cage is also shown schematically (Figure realized with VESTA9 ).
two experimental papers via X-ray diffraction, Raman spectroscopy, and thermodynamic measurements.10, 11 In addition to their potential as hydrogen storage materials, hydrogen clathrate hydrates are also of considerable theoretical interest as prototypical examples of confined quantum particles. The authors have recently investigated both simple and binary sII hydrogen clathrates by Raman spectroscopy12, 13 and inelastic neutron spectroscopy (INS).14–18 The latter technique, in particular, has proven to be an extremely valuable tool for investigating at a microscopic level the properties of hydrogen molecules confined inside the nanocavities. The exceptionally large incoherent neutron scattering cross section of the proton, nearly two orders of magnitude greater than that of any other nucleus, makes INS a highly selective probe of the dynamics of encapsulated hydrogen molecules, since the bands in the spectra due to the host vibrational modes not involving hydrogen are typically quite weak. In addition, the neutron has the ability to induce spin flip in molecular nuclei, making it possible for INS to observe j = 1 rotational transitions which are forbidden in optical, infrared and Raman spectroscopy, since they involve conversion between para-hydrogen (p-H2 ) and orthohydrogen (o-H2 ). Recently, the spectroscopic features of hydrogen clathrates measured using INS have been interpreted with the help of a quantitative theory based on the rigorous treatment of the coupled translational-rotational dynamics of the guest hydrogen molecule.16, 17, 19 In this paper, we discuss the INS measurements obtained for a sH structure clathrate. Due to the presence of two distinct types of cages, small and medium, each occupied by one H2 molecule, we expect to observe spectral features arising from the rattling and rotational transitions of H2 confined in two substantially different environments. This results in the INS spectra which are significantly more complex than those of the binary sII hydrogen clathrates, where only the small cages are singly occupied by H2 . The INS spectra of H2 molecule in the small and medium cages, respectively, of the sH structure have been calculated and their combination has been compared to the experimental results. Sections II–IV of this paper discuss, in turn, the experiment and the information we can readily derive from it (Sec. II), the theory used to compute the INS spectra and its results, showing similarities with, and differences from, other clathrate structures (Sec. III), and finally, in Sec. IV, the conclusions we can reach from the comparison between the ex-
II. EXPERIMENT AND RESULTS A. Experimental details
The preparation of sH clathrates differs from that described in Refs. 14 and 16 in only a few aspects. The same high-pressure beryllium-copper vessel, adequate to hold hydrogen gas at pressures up to 3000 bars and at temperature down to 77 K, was used in this work as well. Among the promoter substances employed successfully to prepare sH hydrogen clathrates, as described in Ref. 10, we have decided to use methyl tert-butyl ether (MTBE), because it is available in the perdeuterated form (MTBE-d12). This compound is hardly soluble in water, so we have filled the berylliumcopper vessel with a known amount of finely ground D2 O ice, and subsequently added liquid MTBE-d12, in slight abundance with respect to the stoichiometric proportions, in order to compensate for its possible evaporation. The entire procedure was carried out under nitrogen atmosphere, in order to prevent the condensation of atmospheric H2 O on the sample. After closing the high pressure vessel, hydrogen gas was added at the pressure of p 1300 bars, and the sample was kept for 2–3 days under constant hydrogen pressure at a temperature T 0 ◦ C, well outside the stability region of the simple sII hydrogen clathrate. The sample was then quenched by immersing the pressure vessel in liquid nitrogen, the pressure was released and the clathrate powder was recovered to fill an aluminum slab cell. This operation was accomplished while keeping the sample close to liquid nitrogen temperature, in a dry-nitrogen atmosphere. The sample cells were finally shipped to ISIS, RAL (UK), in a Dewar flask at liquid nitrogen temperature. The neutron scattering measurements were carried out using TOSCA-II inelastic spectrometer. This is a crystalanalyzer inverse-geometry spectrometer,20 where the final neutron energy, E1 , is selected by two sets of pyrolytic graphite crystals placed in forward-scattering (at around 42.6◦ with respect to the incident beam) and in back-scattering (at about 137.7◦ with respect to the incident beam). This arrangement sets the nominal scattered neutron energy to E1 = 3.35 meV (forward-scattering), and to E1 = 3.32 meV (back-scattering). Higher-order Bragg reflections are filtered out by 120 mm-thick beryllium blocks wrapped in cadmium and cooled down to a temperature lower than 30 K. The incident neutron beam, on the other hand, spans a broad incoming energy (E0 ) range allowing to cover an extended energy transfer (E = E0 − E1 ) region: 3 meV
