| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Energy Harvesting & Storage | Monday Sessions |
| Session EH-MoM |
| Session: | Surfaces & Interfaces for Environmental Processes |
| Presenter: | Katsuyuki Fukutani, University of Tokyo, Japan |
| Authors: | G. Nakamura, University of Tokyo, Japan K. Takeyasu, University of Tokyo, Japan S. Ohno, University of Tokyo, Japan D. Ivanov, University of Tokyo, Japan S. Ogura, University of Tokyo, Japan K. Fukutani, University of Tokyo, Japan |
| Correspondent: | Click to Email |
Hydrogen is a clean energy source, and storing hydrogen in a liquid form is a high-density storage method. Molecular hydrogen exists in nuclear-spin isomers of ortho and para species according to the total nuclear spin [1]. Since these species are correlated to the rotational states with even and odd rotational quantum numbers (J) because of the symmetry of the total wavefunction with respect to the permutation of the two nuclei, the stable state at the liquid hydrogen temperature is the para state. Although the ortho-para conversion in an isolated state is strictly forbidden, the conversion is promoted at solid surfaces [1], which has been used for efficient hydrogen liquefaction. Due to the symmetry of the H2 molecule, on the other hand, the molecule-surface interaction is anisotropic depending on the molecular orientation, which can be analyzed with the J-state selective temperature-programmed desorption (TPD) [2]. In the present work, we have investigated the potential anisotropy of H2 on a Pd(210) surface with resonance-enhanced multiphoton ionization (REMPI). With the anisotropic potential, furthermore, we propose that the ortho and para species can be separated by surface scattering, which is potentially used for efficient hydrogen liquefaction. We demonstrate such nuclear-spin state filtering on a TiO2(110) surface.
When the Pd(210) surface is exposed to H2 at ~30 K, H2 is molecularly chemisorbed on a H-covered surface, which is revealed by a TPD peak at ~70 K [3]. When this desorption is J-state selectively detected, the J=0 para H2 is desorbed at 74 K, while the J=1 ortho H2 is desorbed at a higher temperature of 78 K. This indicates that the adsorption energy of the ortho species is larger than that of the para species. This is due to the lifting of the degeneracy of the ortho state induced by the potential anisotropy. From the population ratio of the ortho and para states, the ortho-para conversion time in this adsorption state is found to be shorter than ~10 s, which is much faster than Ag surfaces [1]. By applying a pulsed supersonic H2 beam, the angular distribution of H2 scattered from TiO2(110) is state-selectively measured. Whereas the J=1 ortho H2 reveals a broad distribution in the near-specular direction, the J=0 para H2 shows a slightly shifted distribution. We discuss the scattering of H2 via an anisotropic potential and resultant nuclear spin-state filtering.
References
[1] K. Fukutani, T. Sugimoto, Prog. Surf. Sci. 88, 279 (2013).
[2] T. Sugimoto, K. Fukutani, Phys. Rev. Lett. 112, 146101 (2014).
[3] P. K. Schmidt et al., Phys. Rev. Lett. 24 87, 096103 (2001).