Developing solar-powered catalysts produce nitrogen fertilisers

Developing solar-powered catalysts to produce nitrogen fertilisers

8:21 AM, 12th April 2017
Developing solar-powered catalysts to produce nitrogen fertilisers
Stanford researchers lead an effort to sustainably produce nitrogen-rich fertiliser. (File photo)

STANFORD, US: Engineers at Stanford University, researchers at SLAC National Accelerator Laboratory and SUNCAT are leading a multi-year effort to produce nitrogen-based fertilisers (growth booster) in a sustainable way, by inventing a solar-powered chemistry technology that can make this fertiliser right on the farm and apply it directly to crops, drip irrigation style.

“Our team is developing a fertiliser production process that can feed the world in an environmentally sustainable way,” said chemical engineer Jens Norskov, director of the SUNCAT Center for Interface Science and Catalysis, a partnership between researchers from Stanford Engineering and the SLAC National Accelerator Laboratory.

This eight-year SUNCAT project is supported by a $7 million grant from the Villum Foundation, an international scientific and environmental philanthropy. The sustainable nitrogen effort is part of a broader, $20 million Villum-backed initiative to bring Stanford researchers together with Danish scientists to develop sustainable technologies to produce not just fertilisers, but fuels and other vital industrial chemicals.

“One common thread across these projects is the need to identify catalysts that can promote chemical processes powered by sunlight, instead of relying on the fossil fuels now commonly used as energy sources and, often, as feedstock for reactions,” said Norskov.

Catalysts are compounds that spur reactions without being consumed – have been used on an industrial scale for more than a century. Developing a low-energy, solar-based process to make nitrogen fertilisers could benefit billions of people, particularly those in the developing world. But to get there SUNCAT researchers will have to break ground in the science of catalysis.

Nitrogen and life

Nitrogen is literally woven into the fabric of life. Through chemical combinations with carbon, hydrogen and oxygen, nitrogen helps form amino acids, which are themselves the building blocks of proteins, that versatile family of molecules vital to every living thing. We can thank soil bacteria for making nitrogen usable.

We don’t know when farmers first discovered the benefits of fertilisation but the practice is ancient. 6,000 years ago, farmers sought to boost yields by fertilising crops with animal waste – now known to contain nitrogen-rich urea (ammonia plus carbon).

By the first decade of the 20th century, German chemist Fritz Haber, working with chemical engineer Carl Bosch, discovered how to mass produce ammonia in giant vats using natural gas, which was the starting point or feedstock of the process- the Haber-Bosch technology.

Under extreme pressure and heat, chemical catalysts could crack natural gas molecules, liberating the hydrogen atoms and joining them to nitrogen from air to form NH3 or synthetic ammonia that could be readily absorbed by plants.

Scale and environmental impact

Tom Jaramillo, deputy director of the SUNCAT Center and a member of the nitrogen synthesis project, put annual fertiliser production into perspective.

“Each year we produce more than 20 kilogrammes of ammonia per person for every person on the planet, and most of that ammonia is used for fertiliser,” said Jaramillo.

But this massive fertiliser output has several costs, starting with production. Due to the heat and pressure required by the Haber-Bosch process, ammonia catalysis accounts for approximately 1 percent of all global energy use. On top of that, between 3 percent and 5 percent of the world’s natural gas is used as a feedstock to provide the hydrogen for ammonia synthesis.

Then comes the environmental costs. Today’s fertilisers are mass produced in centralised plants, delivered to farms and administered using mechanised spreaders. Rain and irrigation water can wash excess fertiliser into waterways. This accumulation can spur the hyper growth of water-borne plants, creating a negative environmental spiral in which the plants can suffocate marine life to create dead zones.

SUNCAT researchers aim to provide the benefits of fertilisation without any of these costs. The idea is to replace the centralised, fossil-fuel based Haber-Bosch process with a distributed network of ammonia-on-demand production modules run off renewable energy.

These modules would use solar power to pull nitrogen from the atmosphere and also to catalyse the splitting of water molecules to get hydrogen and oxygen. The catalytic processes would then unite one nitrogen atom to three hydrogen atoms to produce ammonia, with oxygen as a waste product.

Next-generation catalysis

Developing a solar-powered technology to produce nitrogen-based fertilisers is an enormous challenge that begins with designing the necessary catalysts.

Catalysts are chemistry’s multitaskers: They must target specific molecules, break certain chemical bonds and, often, create new bonds to remake from the atomic jumble whatever end molecule is desired. It is understandably rare to find a chemical agent that can perform all this breaking and make without becoming exhausted.

“While the catalyst must bind strongly enough to the target molecule to do the work required, it also has to release the end product,” said Stacey Bent, a professor of chemical engineering at Stanford and a key member of the SUNCAT team.

“We have to design catalysts that can make and break bonds with atomic precision, and we have to ensure these materials can be mass produced at the necessary scales and price points and are durable and simple to use in the fields,” added Bent.

Computation, visualisation, experimentation

In addition to Norskov, Jaramillo and Bent, other participating Stanford researchers include chemical engineering faculty Zhenan Bao and Matteo Cargnello. SLAC collaborators include Thomas Bligaard, senior staff scientist and deputy director of theory at SUNCAT, and staff scientist Frank Abild-Pedersen. A group of Danish researchers led by professor Ib Chorkendorff at the Technical University of Denmark are key members of the project.

The ultimate goal is to create a catalytic process that can spur the various ammonia-producing chemical reactions with no inputs other than air, water and sunlight. Moreover, these inexhaustible catalysts, and indeed every component in these ammonia-production modules, must be inexpensive to mass produce, durable in the field and easy to operate. It’s a tall order but the potential payoff is huge.

“Sustainable nitrogen production will only become possible with the cross-disciplinary collaboration of people working in fields such as materials science, chemical engineering and computer science,” Bent said. “It could literally change the world.”

If the project’s goal seems worth the effort, the same is true for its research methodology. The team-based discovery that combines theoretical insight, atomic-level visualisation and computational simulation can be applied to designing other sustainable processes to create fuels and industrial chemicals, as envisioned by the broader Villum initiative.

© Stanford University news 

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