Materials to combat our changing climate
Reinventing catalysts, one atom at a time
Our planet is struggling on many fronts. Populations are booming, freshwater supplies are polluted and deforestation is widespread. We’re burning more fuel and using more energy than at any other point in history, and poor air quality is heralding a global health crisis. But, across the world, scientists are taking on these global challenges, and materials science lies at the heart of some of their most innovative solutions. In particular, a class of materials called catalysts have come to the fore in recent years; not because they’re new, but because materials scientists are completely reinventing them, one atom at a time.
The role of a catalyst is to speed up a chemical reaction. It does this by providing an alternative, easier route for atoms to bind to one other in a controlled way. But catalysts themselves aren’t changed by that reaction, and only a tiny quantity of catalyst is needed to kick-start each reaction. Because of this, catalysts are used in countless industrial applications, and are the basis of most of the manufactured products we rely on.
But as Einstein’s famous quote tells us, we can’t solve problems by using the same thinking we used when we created them. That’s where MacDiarmid researchers come in. They’re designing a whole host of low-cost, high-efficiency catalysts that could answer some of the biggest questions we’re facing, and they may just signal the start of a more sustainable age.
New Zealand’s water quality was once the envy of the world, but the agricultural intensification of more recent decades threatens to change that. The overuse of fertilisers, and increased quantities of animal waste, have resulted in more nitrate entering our waterways than ever before. Nitrate pollution in drinking water can be hazardous to human health, and because it promotes algal growth, nitrate can also starve a lake or river of oxygen, leading to a loss of biodiversity and, ultimately, death of the ecosystem.
Current denitrification processes are slow and energy-intensive, but Associate Investigator Dr Anna Garden from the University of Otago is designing new nanocatalysts that could solve this problem. “Catalytic denitrification is a particularly promising method because, in principle, it could convert nitrate into harmless nitrogen, quickly and without generating dangerous by-products,” she explained. Nanoparticles work well as catalysts because they have a high surface to volume ratio—they’re comprised of so few atoms that most of them are on their surface. This makes them uniquely reactive, and much less material is needed to produce the same reaction. But, with a virtually infinite number of compositions available, it is a challenge for lab scientists to find the ideal catalytic nanoparticle for any given application.
So Dr Garden uses advanced computational techniques to screen thousands of candidate materials, in order to identify those that display the highest catalytic activity. Collaborating with Professor Egill Skúlason from the University of Iceland, she is investigating the rate and products of this catalytic denitrification on the nanoscale, in order to design more efficient catalysts for cleaner water. In 2016 Dr Garden won a Marsden FastStart grant to work on nanotechnology to clean up waterways. Dr Garden’s goal is clear, “Ultimately, we’re trying to discover new catalysts that will help us move toward a more sustainable future.”
Principal Investigator and Massey University Professor Shane Telfer is also interested in cleaning up the environment, but his work focuses on air pollution. Working with his colleagues at Massey University, Professor Telfer has developed three dimensional metal-organic frameworks (or MOFs) that can soak up carbon dioxide (CO2) from the smoke stacks of coal-fired power plants.
Because MOFs act like molecular sponges, they can also be used to store gases. Vehicles powered by methane or hydrogen have been recognised as a more sustainable option than fossil fuels. And using MOFs like those being produced by Professor Telfer could reduce the pressures needed to safely store the gaseous fuels on-board, making these vehicles even more practical.
Nanoparticles work well as catalysts because they have a high surface to volume ratio.
Fueling the future
Sustainable fuel is the focus of other MacDiarmid Institute researchers too, including Principal Investigator Associate Professor Geoff Waterhouse (University of Auckland), Principal Investigator Professor Thomas Nann (Victoria University of Wellington), Associate Investigator Dr Vladimir Golovko (University of Canterbury), and Principal Investigator Professor Sally Brooker (University of Otago), and they’ve taken inspiration from the sun. Photocatalysts are compounds that use the sun’s energy to accelerate a reaction. We’ve all heard of photosynthesis, where in the presence of sunlight the light sensitive molecules chlorophyll helps plants produce their own food. The most famous photocatalyst in industry is titanium dioxide, used in everything from sunscreens to self-cleaning glass.
Photocatalysis is incredibly common in nature—light sensitive molecules allow us to see, changing their shape in the retina when light hits them, and provide our skin cells with a way to make Vitamin D, a vital ingredient in bone development. These reactions are incredibly efficient, as Dr Vladimir Golovko explains, “Nature had millions of years to perfect natural catalysts. They work with atomic precision to make very specific biological products. Unfortunately, human civilisation has only had decades to play with catalysts, so most of them are poorly defined. We want to change that.”
So MacDiarmid scientists are developing a new breed of catalysts that will make it easier to produce cleaner fuels. “In order to replace fossil fuels, we need to offer a practical alternative for vehicles; one which makes the most of renewable energy sources,” said Professor Nann. “With photocatalysts we can, in principle, convert 30% of the sun’s energy into fuel.”
Turning CO2 into methane
With other teams focused on carbon dioxide (CO2) capture, Professor Nann, Associate Professor Waterhouse and Dr Golovko are investigating how low-cost photocatalysts could be used to turn this greenhouse gas into methane. This would not only remove CO2 from the atmosphere, but the resulting gas could be added directly into the natural gas grid, providing power to homes without needing to alter the infrastructure. Further down the line, these researchers also want to use photocatalysts to extract hydrogen gas from water.
Inspired by nature
This is a question that’s also keeping University of Otago’s Professor Sally Brooker busy. She is looking into photo- and electro-catalysts to develop a bioinspired approach to hydrogen production. Existing industrial catalysts used for this reaction tend to be based on rare metals like platinum, and are inefficient. But nature has a better solution—an enzyme called hydrogenase that relies on iron and/or nickel.
“Control of the metal centre is everywhere in naure—haemoglobin, the active iron-containing protein in red blood cells, for example. Without the precise environment that surrounds its iron core, you’d just get rust,“ explains Professor Brooker. “The same is true for catalysts—controlling the environment that the metal ions are in is absolutely critical to their performance, so that is our current focus.” Their work on hydrogen production is in its infancy, but early results from their international collaborations are extremely promising, and look likely to have an impact on the hydrogen economy.
With photocatalysts we can convert much more of the sun’s energy into fuel.
Professor Thomas Nann
Amongst all of the materials that we rely on in our modern world, arguably none have become more ubiquitous than plastics. Used in everything from banknotes and lunchboxes, to car parts and sewage pipes, their highly-tuneable properties have made them the go-to option for a range of industries. But the bulk of today’s plastics are made from petrochemicals, which makes them increasingly unsustainable. In addition, because many of them don’t biodegrade, their disposal poses a major environmental hazard. So, alongside her hydrogen work, Professor Sally Brooker and her team are investigating a catalyst-based approach to producing compostable bioplastics. Their source material is corn, which contains large quantities of a compound called lactide. With the aid of a suitable catalyst, single molecules of lactide building blocks join to form a long chain, to produce polylactide, a biodegradable plastic that can be used in a host of products.
“We’ve had a lot of success so far—our international collaborators have shown that our catalyst is hyper-active, which means it is very good at turning lactide into polylactide,” says Professor Brooker. “But we still have questions to answer.” With competing catalysts already on the market, her aim is to improve their catalyst even further. She’s also looking further ahead, towards biodegradable co-polymers, made from two or more compounds. “This will be a much bigger challenge, but if we get it right, industry will certainly stand up and take notice.”
Another key challenge for tomorrow’s changing energy landscape will be to match reliable energy generation with storage. Professor Maan Alkaisi from the University of Canterbury thinks that solar power will play a huge role in this. Photovoltaic panels installed on buildings across New Zealand may be the best option for converting sunlight into electricity, but they’re not all that efficient. Professor Alkaisi has designed a nanopatterned surface that improves the efficiency of commercial solar panels, by making it less reflective, and better at absorbing light. Its low cost and scalability means that it could also lead to a new generation of transparent photovoltaics, integrated directly into buildings. And for energy storage, Professor Alkaisi and other MacDiarmid researchers, including Professor Thomas Nann and Dr Vladimir Golovko, are involved in designing a system that combines a battery with a photocatalyst. In 2016, this team of MacDiarmid researchers from the University of Canterbury and Victoria University of Wellington, won an MBIE Smart Ideas grant for this work. “This work is only just beginning, but by bringing together the skills of scientists from multiple disciplines, we feel optimistic and excited for its future.”
The state of our planet will require more than one answer to the question of energy security and climate remediation. But with solutions like these being spearheaded by MacDiarmid researchers, the future looks a little brighter.
If we get it right, industry will certainly stand up and take notice.
Professor Sally Brooker