Geologic hydrogen production has long been thought to fuel deep ecosystems. Hydrogen-producing reactions known to take place between water and rock at high temperatures and pressures, however, have only been recapitulated in the laboratory at very slow rates, perhaps too slow to support Earth’s ubiquitous deep biosphere. In a report published in the October 2013 edition of American Mineralogist, DCO scientists Muriel Andreani, Isabelle Daniel, and Marion Pollet-Villard of University Claude Bernard Lyon 1 show that the rate of one such reaction, serpentinization, can be increased by an order of magnitude using aluminum oxide as a catalyst.
Serpentinization reactions take place naturally under hydrothermal conditions, such as those found in and around deep ocean vent systems, and result in the production of molecular hydrogen (H2). High-energy molecular hydrogen is liberated from water, and the rocky, ultramafic substrate is simultaneously modified to produce serpentine. Such reactions have been reproduced in laboratories around the world for several decades, but proceed slowly, over the course of weeks or months. This sluggish reaction would likely be unable to support the thriving, sunlight-deprived, deep microbial ecosystems present on our planet today.
“For the first time we understand why and how we have H2 produced at such a fast rate. When you take into account aluminum, you understand the amount of life flourishing on hydrogen,” said study co-author Isabelle Daniel.
By adding aluminum oxide to olivine and water in a ‘low-pressure’ diamond anvil cell, and exposing these components to high temperature and pressure, Andreani, Daniel, and Pollet-Villard were surprised to discover that the serpentinization reaction took place over night; almost 50 times faster than without aluminum.
Hydrogen fuel on Earth and other planets
This finding has important implications, both in our basic understanding of deep geochemical energy production and for long-term industrial applications. Since aluminum oxides are abundant in Earth’s crust, these experimental results are strikingly relevant to natural serpentinization reactions. And the potential for clean, green, fuel production, while several decades from becoming a technological reality, is a tantalizing prospect.
DCO co-founder Jesse Ausubel, of The Rockefeller University, New York, USA, commented that current methods for commercial hydrogen production for fuel cells or rocket fuel “usually involve the conversion of methane (CH4), a process that produces the greenhouse gas carbon dioxide (CO2) as a byproduct. Alternatively, we can split water molecules at temperatures of 850 degrees Celsius or more — and thus need lots of energy and extra careful engineering. Aluminum’s ability to catalyze hydrogen production at a much lower temperature could make an enormous difference. The cost and risk of the process would drop a lot.”
Moreover, water-rock interactions such as the serpentinization reaction may be at the root of the origins of life on Earth. Both energy and molecular precursor production through these reactions were likely prolific on young Earth, and may well be taking place elsewhere in our solar system and in the universe.
“We believe the serpentinization process may be underway on many planetary bodies — possibly in the subsurface of Mars if liquid water is present,” added Daniel. “The reaction may take one day or one million years but it will occur whenever and wherever there is some warm water present to react with olivine — one of the most abundant minerals in the solar system.”
Références :
Aluminum speeds up the hydrothermal alteration of olivine. Andreani M, Daniel I, Marion Pollet-Villard (2013) American Mineralogist 98:1738-1744
Le communiqué du Deep Carbon Observatory
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