2:00 PM - 2:15 PM
▼ [15p-424-4] H2 Hindered Rotation and Otrho-Para Conversion on Pd(210)
Keywords:hydrogen storage, ortho-para separation and conversion, nuclear spin
H2 has a total nuclear spin I=0 or I=1 for even and odd values of its angular momentum J, respectively. In the lowest energy states, I=1 species, called ortho-H2 is associated with the 3-fold () degenerate anti-symmetric spin eigenfunctions, while I=0, called para-H2 has a symmetric spin eigenfunction. Far away from the surface, and in the absence of external perturbations, the rotational energies for different values of J can be readily calculated as, (B is the rotational constant) which is equal to 0 for J=0 (para) and 14.7 meV for J=1(ortho). When H2 is adsorbed molecularly on a surface, its rotation becomes hindered and the degeneracies of the J>1 states are lifted. The surface-molecule interaction that hinders the rotational motion results in bound species that desorb at different temperatures, usually measured by temperature programmed desorption (TPD) and other similar experiments.
In terms of adsorption, H2 is found to dissociate spontaneously on most transition metal surfaces and chemisorption is only restricted to stepped surface facets[1-3]. Further, previous studies[2,3] have shown that H2 behaves as a quantum two-dimensional (2D) rotor on Cu(510) due to relatively large perpendicular anisotropic potential on this surface. In this study, we investigate H2 adsorption on Pd(210) surface with the emphasis on the nuclear spin species separation and conversion using a combination of density functional theory (DFT)-based and model Hamiltonian calculations. It is known that molecular hydrogen adsorption on stepped Pd(210) is only possible after the accumulation of H atoms on the surface[4]. Our results show that H2 molecule favors adsorption on top of Pd atoms located at the step-edge regardless of orientation and atomic H coverage. Further, we find that H2 adsorbed in parallel orientation (defined as parallel to the []-[] plane of the unit cell) with respect to the surface has higher adsorption energy compared to the one adsorbed perpendicularly at low atomic H coverage (Θ<3ML). From these molecular orientation-dependent adsorption energies, we obtain the anisotropy potential that hinders rotation of H2 on the surface and calculate its rotational state energies. We also find that ortho-hydrogen has higher desorption energy compared to para-hydrogen [5]. Using 2nd order perturbation theory, we calculate the conversion probability from ortho to para hydrogen adsorbed on the surface. We find that the hydrogen molecule adsorbed perpendicularly tend to have higher conversion probability than the parallel-oriented H2.
In terms of adsorption, H2 is found to dissociate spontaneously on most transition metal surfaces and chemisorption is only restricted to stepped surface facets[1-3]. Further, previous studies[2,3] have shown that H2 behaves as a quantum two-dimensional (2D) rotor on Cu(510) due to relatively large perpendicular anisotropic potential on this surface. In this study, we investigate H2 adsorption on Pd(210) surface with the emphasis on the nuclear spin species separation and conversion using a combination of density functional theory (DFT)-based and model Hamiltonian calculations. It is known that molecular hydrogen adsorption on stepped Pd(210) is only possible after the accumulation of H atoms on the surface[4]. Our results show that H2 molecule favors adsorption on top of Pd atoms located at the step-edge regardless of orientation and atomic H coverage. Further, we find that H2 adsorbed in parallel orientation (defined as parallel to the []-[] plane of the unit cell) with respect to the surface has higher adsorption energy compared to the one adsorbed perpendicularly at low atomic H coverage (Θ<3ML). From these molecular orientation-dependent adsorption energies, we obtain the anisotropy potential that hinders rotation of H2 on the surface and calculate its rotational state energies. We also find that ortho-hydrogen has higher desorption energy compared to para-hydrogen [5]. Using 2nd order perturbation theory, we calculate the conversion probability from ortho to para hydrogen adsorbed on the surface. We find that the hydrogen molecule adsorbed perpendicularly tend to have higher conversion probability than the parallel-oriented H2.