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Phase Transition of H2 in Subnanometer Pores Observed at 75 K
Raina J Olsen1, Andrew K Gillespie2, Cristian I Contescu1
1Materials Science and Technology Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
A novel phase transition in hydrogen (H₂) adsorbed in porous carbon occurs at 75 K. This high-temperature transition involves a shift from a low-density to a higher-density phase, influenced by hydrogen
Area of Science:
- Materials Science
- Physical Chemistry
- Adsorption Science
Background:
- Hydrogen (H₂) typically melts at 14 K in bulk, but surface interactions can elevate this.
- Previous studies show H₂ melting at 20 K on graphite due to surface stabilization.
- Subnanometer porous materials offer unique environments for gas adsorption and phase behavior.
Purpose of the Study:
- To investigate the phase transition of hydrogen adsorbed in subnanometer pores.
- To determine the conditions and characteristics of this phase transition.
- To understand the role of adsorbent structure and hydrogen's nuclear spin states.
Main Methods:
- Hydrogen adsorption isotherms were measured to determine film densities.
- Neutron backscattering experiments were conducted to probe the phase transition.
- Modeling was used to characterize the low-density and high-density phases.
Main Results:
- A phase transition was observed at 75 K and 77-200 bar in H₂ adsorbed in graphitic Saran carbon.
- The transition is from a low-density monolayer phase (para-H₂) to a higher-density bilayer phase (ortho-H₂).
- Neutron backscatter increased by a factor of 7.0 ± 0.1, with a discontinuous excitation mass increase.
Conclusions:
- The graphitic Saran carbon facilitates a significantly higher phase transition temperature for adsorbed H₂.
- The observed transition is attributed to the packing of ortho-H₂ in the higher-density phase.
- Collective nuclear spin excitations are associated with the orientationally ordered high-density phase.
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