Agents of change

When Professor James Wilton-Ely (Chemistry 1994, PhD 1998) ran an experiment on catalysts recently, he started on eBay. There, his team bought an assortment of old phone SIM cards. They were cheap, with 30 cards costing around £1.30. Yet for Wilton-Ely’s purpose, they were invaluable.

“That shiny golden contact button on every SIM card has a very high gold content,” says the Professor of Inorganic Chemistry at Imperial. The team ground up the cards, separated out the plastic and extracted the gold using an environmentally friendly organic compound containing sulfur. The result was not pure gold, but a gold compound.

Then, they tested the recovered gold in reactions that the pharmaceutical industry uses to manufacture drugs such as anti-inflammatory and pain relief medications. “To our surprise, the compound turned out to be a very effective catalyst for a whole host of important reactions without needing any further transformations,” says Wilton-Ely.

Catalysts – from the Greek καταλύειν, meaning ‘loosen’ – are the hidden miracle workers of science. Simply put, they are substances – often metals – that jump-start chemical reactions. In ways that even scientists don’t always fully understand, a catalyst enables other elements and compounds to form chemical bonds that would otherwise require a lot of energy or time to happen, and the catalyst itself emerges unchanged at the end.

A more sustainable way to create gold catalysts

Freshly mined elemental gold

Gold is an exceptionally effective catalyst, but mining 1kg of gold produces 12,500kg of carbon dioxide.

Gold compounds recycled from SIM cards

The Imperial team extract gold from previously discarded sources, such as SIM cards, as a more sustainable alternative.

Anti-inflammatory medicines

The resulting gold compound is used as a catalyst for reactions that could be applied to future medicines.

“It’s often said that 90 per cent of all man-made products have a catalytic step in their production,” says Wilton-Ely. Catalysts nudge inedible plant oils to form luscious margarine; they break down wood to produce paper; and they change petrochemical compounds into plastic. The human body, too, runs on catalysts – although we call them enzymes. They help us digest, breathe and move.

At Imperial, scientists, including Wilton-Ely, are now finding ways to use these chemical miracle workers to push the world towards better sustainability. They are doing so by creating innovative catalysts from waste products, by making green energy more affordable, and by using catalysts to jump-start an “artificial photosynthesis” that pulls climate-damaging carbon dioxide straight from the air.

Many of the best catalysts are rare metals such as palladium, rhodium and gold. They are often only found in certain parts of the world, such as Russia, making supply chains problematic. Mining these metals tends to heavily damage the environment, leaching toxins into the soil and climate-changing gases into the air. “The mining of one kilogram of gold produces 12,500kg carbon dioxide,” says Wilton-Ely.

Ninety per cent of man-made products have a catalytic step in their production
Professor James Wilton-Ely

This is particularly troubling since the world has already mined vast amounts of these metals – only to carelessly throw them away. In Europe alone, every person produces 20kg of e-waste a year – inclusing computers, printer cartridges and phones. And the numbers keep growing, even though our old gadgets are full of precious substances. “The amount of gold found in computers is many times higher than in even the richest seams of mining,” says Wilton-Ely. “You have to process 30 to 40 tonnes of rock to find as much gold as in 20 or 30 computers.” Yet very little e-waste gets recycled.

So when one of his collaborators, Angela Serpe at the University of Cagliari in Italy, developed a low-energy process that recovered gold from e-waste using environmentally benign reagents, Wilton-Ely set out to find a use for it. Serpe’s recovery method results in a compound of gold – not elemental gold – that would cost too much energy to reuse in electronics. But the compound works as well, or even better, than catalysts derived from newly mined gold in pharmaceutical reactions, Wilton-Ely found.

His group is also working to repurpose another precious catalyst metal: palladium, which sits in the catalytic converters of vehicles and reduces harmful emissions. “Every car with a combustion engine drives around with about four grams of palladium. That’s 200 times the concentration in mined ore,” says Wilton-Ely.

But palladium, whose natural deposits are mostly located in Russia, is also an important catalyst in the manufacturing of pharmaceuticals and agricultural chemicals such as fertilisers and fungicides. And like the gold from SIM cards, the palladium compound that Wilton-Ely recovered from exhaust emission converters has proven to be “a very well-behaved catalyst”, he says. “We don’t see any difference in durability compared to metals sourced from mining. We can reuse them again and again and again.”

Besides mining and geopolitical complexities, rare metal catalysts have another downside: they are expensive. Platinum, for example, trades at close to $1,000 per ounce. It’s used as a catalyst in hydrogen fuel cells, a sought-after form of green power, in which hydrogen gas reacts with oxygen gas to create water and electricity.

“Currently, around 60 per cent of the cost of a fuel cell is the platinum for the catalyst,” says Professor Anthony Kucernak in the Department of Chemistry. “To make fuel cells a viable alternative to, for example, fossil fuel-powered vehicles, we need to bring that cost down.” Kucernak has discovered that the much more common and affordable iron can replace the platinum – provided you finesse its configuration.

Many rare metals are effective catalysts because they’re built in a way that attracts neighbouring atoms enough to make them break their existing bonds with other atoms – but not so much that they will then bond with the catalyst itself. Rather, the newly freed atoms go on to react with other chemical partners. And a lot of these metals “sit in this Goldilocks zone,” says Kucernak. But he and his team have found that if they disperse iron into a single-atom structure, it acts similarly. “We started thinking about what was holding it back from being a usable catalyst. It seemed to us that we needed to modify the chemistry to allow for more active sites,” he says.

An important trick was to prevent iron atoms from forming clusters, “which aren’t really active for reactions”, so the team has developed a process called transmetallation, which disperses individual iron atoms within an electrically conducting carbon matrix. Once this was achieved, the common iron started to rival platinum’s performance as a fuel cell catalyst.

Kucernak’s approach resembles the way catalysts work in the human body. “The body doesn’t use rare elements for catalytic processes,” he says. “Instead, it modifies the electronic structure around much more ubiquitous metals to change the properties. It’s very similar to what we are doing.”

A diagram linking rocks of platinum, iron and an electric car, showing the opportunity to create cheaper catalysts for renewable energy powered solutions.

Expensive platinum: currently, 60% of the cost is the platinum used as a catalyst in a single fuel cell.

Single-atom iron: the team has created a catalyst using iron, carbon and nitrogen – cheaper and more readily available materials.

Hydrogen fuel cell car: the cheaper catalyst design could allow deployment of significantly more renewable energy systems that use hydrogen as fuel.

Currently, Kucernak is working on increasing iron’s catalytic durability. If the approach can be commercialised, “our cheaper catalyst design should allow deployment of significantly more renewable energy systems that use hydrogen as fuel, ultimately reducing greenhouse gas emissions and putting the world on a path to net-zero emissions.”

Kucernak’s method could also prove useful for something that interests Camille Petit, a Professor in the Department of Chemical Engineering. Petit is working to pull carbon out of the air or more concentrated sources to use it for new chemicals and fuels. “Typically, the carbon in our everyday products comes from fossil fuels. But we have all this carbon dioxide in the air, so we’re asking: ‘Can we use this instead?’” says Petit.

Already, she and other researchers have found materials that can efficiently capture carbon dioxide (CO2) from factory flue stacks. Now she wants to add a second step that converts the harvested carbon in a process which resembles the photosynthesis of plants. “In actual photosynthesis, you convert CO2 and water to carbohydrates and oxygen. We use CO2 with hydrogen or water, and convert it to carbon-based fuels or chemicals,” says Petit.

Since CO2 is a highly stable molecule, the conversion requires a catalyst. Petit is testing possible candidates, including nitrides, metal organic frameworks and hypercrosslinked polymers. She wants her catalyst to be photoactive, meaning that just like plant-based photosynthesis, her artificial one will only need the sunlight to function.

It’s early days. Petit has already managed to get the process to work on a small scale in the lab, and says it’s only a matter of time before the output is big enough to make a difference. And once it does, catalysts will become even bigger miracle workers – transforming climate pollution into a source for sustainable and carbon-neutral living.

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This story was published originally in Imperial 55/Winter 2023–24.