Photo illustration by Jeffrey C. Chase January 26, 2022
Researchers demonstrate a method for making ammonia under milder conditions with less energy
Catalysts are the helpers that make possible many chemical reactions essential to food production, fuels, pharmaceuticals and other essential materials in our everyday lives. For example, refineries use catalysts to break crude oil into molecules used as gasoline.
However, catalysts and reactions have long been limited by their natural properties.
Researchers affiliated with the University of Delaware's Catalysis Center for Energy Innovation (CCEI), an Energy Frontier Research Center, and the RAPID manufacturing institute, found a way to break through these barriers in 2019, allowing catalysts to be far more effective and chemical reactions to exceed their natural limits. They called the theoretical approach dynamic catalysis.
Now, a UD-led team has demonstrated the method's effectiveness on a real-world reaction, the formation of ammonia. The research team’s computational model showed that it's possible to synthesize ammonia 10 times faster under milder conditions using dynamic catalysis and exceed the equilibrium conversion limit at which the building-block molecules, hydrogen and nitrogen, are converted to ammonia from 32% to 52%.
The results were reported in a paper in Science Advances on Wednesday, Jan. 26.
Co-authors on the paper include Gerhard Wittreich, the paper’s lead author and a UD doctoral student; Dionisios Vlachos, Unidel Dan Rich Chair in Energy, professor of chemical and biomolecular engineering and CCEI director; Shizhong Liu, a former postdoctoral researcher at UD; and Paul Dauenhauer, CCEI co-director, MacArthur Fellow and Lanny Schmidt Honorary Professor of Chemical Engineering and Materials Science at the University of Minnesota.
From theory to real-world reactions
The research team selected ammonia because the process for making it has been well studied in industry and because its manufacture is widely considered to contribute the most, among chemical processes, to global warming.
"We knew if we could show that you can actually change a reaction that people thought they knew and understood, then that could make a big mark," said Wittreich. "Our results show the principles of dynamic catalysis still hold when exposed to all the degrees of freedom that occur in real reactions."
Ammonia is the most highly produced chemical in the world. It is used in agricultural fertilizer, plastics, explosives, textiles and other chemicals. It affords dense storage for renewable energy as an interchangeable liquid fuel, too.
Making ammonia is difficult, though, and requires high temperatures and pressures typically only possible in chemical manufacturing plants.
The UD research team tested the computational model using ruthenium, a transition metal, as the catalyst to create ammonia. The model expanded on earlier theoretical work by Dauenhauer's research team, which showed if you could shift the binding energy to the catalyst at the right rate and at the right time, the catalytic reaction could go beyond the Sabatier limit, considered the natural "speed limit" for reactions.
At the atomic scale, catalysts contain large flat surfaces called terraces and steps or edges where two terraces overlap. For ammonia, Wittreich said, these steps and terraces play very specific roles in the reaction. The researchers wondered if stretching and compressing the ruthenium atoms on the catalyst surface would change the ability for things to bind to the reaction sites.
Liu helped the team understand how the different molecules would bind on steps and terraces of the catalyst under various strains. He found the atoms bind the strongest when there is no strain on the particle.
"So, this gave us credible information to say, if we could change the spacing of these atoms in the nanoparticle, the binding energy would change," Wittreich said.
Their mathematical model, meanwhile, confirmed that expanding and contracting the catalyst surface at a given frequency would make the reaction run 10 times faster at a given temperature.
Additionally, by oscillating the reaction surface, the research team found they could convert about 52% of the hydrogen and nitrogen molecules into ammonia (almost double the typical equilibrium conversion limit of 32%) without changing anything else. This means that it is possible to operate the reaction at lower temperatures and pressures and achieve the same amount of product more safely and efficiently, with less cost, using dynamic catalysis.
Less extreme conditions also open the door to potentially taking simpler, smaller reactors outside of chemical plants and using ammonia for other things — like on-site storage for renewable energy sources, such as wind or solar power.
Other research in this area has explored using electricity from a wind farm to generate hydrogen for fuel or energy by splitting the hydrogen and oxygen atoms apart, known as water electrolysis. Hydrogen storage has been challenging because hydrogen is notoriously difficult to contain without high pressure. Ammonia, however, can be stored at low pressures and easily be converted back to hydrogen and nitrogen on demand. With dynamic catalysis methods, perhaps the hydrogen could be converted and stored as ammonia on-site until needed.
"You could then ship the ammonia somewhere and turn it back into hydrogen for hydrogen-powered vehicles or other green energy," said Wittreich.
The researchers hope others will put their computational model to the test experimentally and explore whether dynamic catalysis can improve other reactions, too.
"We need people to do experiments, to figure out how to strain the surface of the catalyst and get these ruthenium atoms to pull apart and push together rhythmically," he said.
A non-traditional graduate student
Gerhard Wittreich isn't your typical graduate student. Wittreich earned his undergraduate degree from Lehigh University in chemical engineering in 1981 and spent his nearly 35-year career working for the DuPont Company and its spinoff companies, including Chemours, in both engineering and management roles.
As retirement loomed, Wittreich considered ways to remain involved with engineering beyond the workforce. He decided to take the professional engineering licensing exam and enrolled in a UD review course. John Richards, the adjunct professor teaching the class, suggested he consider getting an advanced degree.
Wittreich applied to UD after retiring from Chemours in 2016, but secretly wondered if he could keep up with the academic rigor of a doctoral program. He also worried whether his age — he was approaching his 60th birthday — would limit his ability to fit in.
"I hadn't been in an academic situation for 35 years, and here were all these super-smart young kids who just knew this stuff cold," said Wittreich, who is among the oldest students to pursue a UD doctoral degree in chemical and biomolecular engineering. "I wondered if I'd flounder, but the academics was great and my 25-year-old peers treated me like everybody else."
Now just months from defending his doctoral thesis, Wittreich is looking forward to participating in the doctoral hooding ceremony during Commencement in May. And after that?
"I'm not a kick back and relax kind of guy so, I think I'll be doing something in academics and research. I really like working with and mentoring young grad students," said Wittreich, a 2017 recipient of the University's Excellence in Graduate Student Teaching Award.