Hydrogen is widely regarded as a clean and renewable alternative to fossil fuels. However, its transportation and storage pose significant challenges due to its low density and high volatility. One of the solutions is to liquefy hydrogen at very low temperatures, but this requires a fast and complete conversion of ortho-hydrogen to para-hydrogen, two isomeric forms of hydrogen molecules that differ in their nuclear spin states.
What is ortho-to-para conversion?
Hydrogen molecules (H2) consist of two hydrogen atoms that can have their nuclear spins aligned either parallel (ortho-H2) or antiparallel (para-H2). Under normal conditions, ortho-H2 and para-H2 are present in a 3:1 ratio, with ortho-H2 being slightly more energetically unstable than para-H2. When hydrogen is cooled down to its liquefaction temperature (below -253°C), all ortho-H2 molecules convert to para-H2 molecules, resulting in stable liquid hydrogen.
However, if hydrogen is cooled down rapidly under high pressure, as needed for industrial liquefaction, the conversion of ortho-H2 to para-H2 is delayed, leaving a large amount of ortho-H2 in the liquid hydrogen. This residual ortho-H2 will continue to convert to para-H2 during storage, releasing heat and causing partial vaporization of the liquid hydrogen. This leads to significant loss of hydrogen and energy.
To prevent this problem, catalysts are needed to accelerate the conversion of ortho-H2 to para-H2 before liquefaction. However, existing catalysts, such as iron oxide-based materials, are not very effective and require large amounts of catalysts and long contact times.
What are the new materials?
A research team consisting of NIMS and the Tokyo Institute of Technology has identified materials that can catalyze the conversion of ortho-H2 to para-H2 with much higher efficiency than conventional catalysts . The team screened more than 170 solid materials, including metals and ionic crystals, and found that manganese oxide (Mn3O4) and cobalt oxide (CoO) exhibited significantly higher catalytic performance than iron oxide-based catalysts.
The team also identified the major factors that influence the catalytic activity of these materials. They found that the materials need to be non-metallic and have cations with ionic radii smaller than the bond length of H2. These factors affect the electronic structure and magnetic properties of the materials, which in turn affect their ability to interact with H2 molecules and induce spin conversion.
Why are these materials important?
The new materials developed by the research team are expected to enable efficient hydrogen liquefaction and storage, which are essential for the widespread use of hydrogen as an energy source. Hydrogen liquefaction can reduce its volume by a factor of 800, making it suitable for long-distance transportation by sea or land. Hydrogen storage can ensure a stable supply of hydrogen for various applications, such as fuel cells, power generation, and chemical synthesis.
The new materials can also reduce the cost and environmental impact of hydrogen liquefaction and storage. By using less catalysts and shorter contact times, the new materials can save energy and resources. By avoiding partial vaporization of liquid hydrogen, the new materials can prevent greenhouse gas emissions and fire hazards.
The research team hopes that their findings will contribute to the advancement of hydrogen economy, which aims to use hydrogen as a clean and renewable energy carrier. The team plans to further optimize the new materials and test them under realistic conditions.
How do these materials work?
The mechanism behind the catalytic activity of these materials is not fully understood yet, but some hypotheses have been proposed based on experimental observations and theoretical calculations. One possible explanation is that these materials have a high density of surface defects or vacancies that can act as active sites for H2 adsorption and dissociation. Another possible explanation is that these materials have a strong magnetic anisotropy that can align the nuclear spins of H2 molecules along a preferred direction, facilitating spin conversion.
The research team intends to conduct further studies to elucidate the exact mechanism behind the catalytic performance of these materials. They also aim to explore other potential candidates for ortho-to-para conversion catalysts based on their design guidelines.
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