On the hunt for a catalyst capable of efficient hydrogen production

It’s generally agreed that changes to our energy mix can help tackle climate change. While nuclear power is considered to be a low CO₂ emission technology for generating electricity, one suggestion is to use hydrogen to fuel our cars and heat our homes. Hydrogen can be generated from fossil fuels or by splitting water. While this latter method can be done using electrolysis, it is energy intensive. An alternative is to use ionising radiation.

You would think that the best way to generate hydrogen from water using radiation would be to simply irradiate as much of it as possible but some studies show that spreading an incredibly thin layer over a surface can actually generate more hydrogen than simply irradiating a large volume of it.

Bulk water radiolysis

When water is irradiated it can split apart into different combinations of electrons, hydroxide, peroxide and proton (e^-, OH^-, H₂O₂ and H⁺ respectively). These different species undergo a series of ultra-fast reactions that depend on the ability of these chemical fragments to diffuse through the water. When an individual gamma ray passes through bulk water it creates a track of ionised water molecules that spread out as they begin to react. Gamma rays travel very far in water which leads to a lot of water being ionised.

Some of the reactions in water radiolysis. Primary yields are commonly quoted and refers to the initial interaction of ionising radiation with matter; the period where a track, and spurs from the track, are produced.  Primary yields from the radiolysis of pure water are found by including a scavenger, a chemical that will quickly react with a targeted water fragment before it diffuses into bulk solution, effectively preventing it from reacting with the species of interest. The primary yield of H₂ can be found by including KBr; the bromide ions prevent the reaction of hydroxyl radical with molecular hydrogen. Credit: L Leay, adapted from various sources.

 

Alpha particles can also lead to the splitting of water. The alpha particles don’t travel as far as gamma rays. They still create tracks the way that gamma rays do and the ionised water molecules will still diffuse and react with other water molecules that weren’t hit by the alpha particle. Since the track is shorter, the energy is deposited into a much smaller volume of water, and so many more water fragments are created in a smaller space. A higher density of water fragments can lead to more chemical reactions and so the amount of hydrogen produced can increase. The key to generating hydrogen from water radiolysis lies in generating many fragments in a small volume and so increasing the chances that they can react in the right way. 

Now, take that water and spread it over a surface and you’ll see something quite different. 

Catalysis on a surface

Spreading water over a surface means that the water fragments don’t have as much space to diffuse; some of the reactions that may happen in the glass of water don’t happen as often. There are dozens of reactions happening between the water fragments; some lead to the generation of molecular hydrogen and some prevent it. Some of those reactions can stop hydrogen gas from forming.

The chemical make-up of the material that you use to make your surface is also important. Some materials such as tenorite (CuO) tend to generate less hydrogen gas than the glass of water does while zirconia generates more hydrogen. Some materials like silica don’t change the amount of hydrogen produced. No one knows for sure why this is but it could have something to do with the material also absorbing some of the radiation then transferring it to the water.

The nanostructure of the material is important too and studies from related fields can inform the search for a catalyst. For example, calcium-silicate-hydrates in cement pastes, which are usually studied to understand how nuclear waste might evolve over the associated long timescales, lead to nm thick layers of water sandwiched between sheets of calcium and silicate. In this material the calcium ions can trap electrons and these ions just so happen to sit in regions rich in structural water and OH groups. Since much of the hydrogen gas is formed by reactions that involve solvated electrons, it seems that this could be an excellent way of producing hydrogen.

Outlook

So what might the future technology to drive the hydrogen economy look like? Given that many nations already use nuclear power to generate electricity, it could be that this by-product is simply captured from the reactor core. This might give rise to questions about another Fukushima-like incident but it’s important to remember that the explosion at Fukushima-Daichi was caused by an unexpectedly severe event and, as its sister plant, Fukushima-Daini showed, was preventable. The nuclear industry, even with a safety record that surpasses many other industries, has learned a lot from that event.

It might also be possible to build some sort of water processing plant that spreads water over a surface and passes it through a radiation field. The radiation could be delivered by radioactive material, a particle accelerator or an x-ray generator.  This sort of conveyor-belt for water radiolysis may seem a bit far-fetched, but if climate change is to be halted, it would seem that future technologies may have to get radical (radiation chemistry pun intended).

 

 

For more information on water radiolysis see S. Le Caer’s 2011 article in Water.

For a detailed analysis of radiolytic hydrogen production from cement pastes, see S. Le Caer et al, 2017 in Cement and Concrete Research.