Catalyzing Ortho-to-Para Hydrogen: A Material Breakthrough

Hydrogen is a promising alternative energy source to fossil fuels, but its liquefaction and storage pose significant challenges. One of them is the conversion of ortho-hydrogen to para-hydrogen, which are two different forms of hydrogen molecules. In this article, we will explain what ortho-hydrogen and para-hydrogen are, why they need to be converted, and how researchers have discovered new catalysts that can accelerate this process.

What are ortho-hydrogen and para-hydrogen?

Hydrogen molecules (H2) consist of two hydrogen atoms that share a pair of electrons. However, these electrons can have different orientations, or spins, relative to the nuclei of the atoms. If the spins are parallel, the molecule is called ortho-hydrogen. If the spins are antiparallel, the molecule is called para-hydrogen.

Under normal conditions, ortho-hydrogen and para-hydrogen are present in a 3:1 ratio, with ortho-hydrogen being slightly more energetically unstable than para-hydrogen. However, at low temperatures, the thermodynamic equilibrium shifts towards para-hydrogen, which becomes the dominant form. This is because para-hydrogen has a lower quantum mechanical energy level than ortho-hydrogen, and therefore is more stable at low temperatures.

Why do ortho-hydrogen and para-hydrogen need to be converted?

To liquefy hydrogen, it needs to be cooled down to below -253°C under high pressure. However, if the hydrogen gas contains a large amount of ortho-hydrogen, the conversion to para-hydrogen will continue during the cooling process, releasing heat and causing partial vaporization of the liquid hydrogen. This results in significant loss of hydrogen and energy.

To prevent this problem, it is desirable to convert most of the ortho-hydrogen to para-hydrogen before liquefaction. This can be done by using catalysts that facilitate the spin change of the hydrogen molecules. Catalysts are substances that increase the rate of a chemical reaction without being consumed or changed by it. By using catalysts, the conversion of ortho-hydrogen to para-hydrogen can be completed faster and more efficiently.

How have researchers discovered new catalysts for ortho-hydrogen to para-hydrogen conversion?

A research team consisting of NIMS and the Tokyo Institute of Technology has evaluated the ability of more than 170 solid materials—including metals and ionic crystals—to catalyze ortho-to-para conversion. The team found that manganese oxide (Mn3O4) and cobalt oxide (CoO) exhibited significantly higher catalytic performance than conventional iron oxide-based catalysts.

The team also identified major factors influencing the catalytic activities of these materials in accelerating ortho-to-para conversion. They found that the magnetic properties of the materials played an important role in facilitating the spin change of the hydrogen molecules. The team hypothesized that the magnetic fields generated by the materials could align with the spins of the hydrogen molecules and induce their transition from ortho- to para- state.

The team believes that these catalysts should be essential for the spread of mass-transportation/storage of liquid hydrogen, which is crucial for long-distance hydrogen transportation by sea from major hydrogen producers/exporters (such as Australia and the Middle East) to hydrogen importers (such as Japan). Liquid hydrogen has many advantages over compressed hydrogen gas, such as higher energy density, lower safety risks, and lower transportation costs.

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