The solution to a mystery 75-year-old materials might one day allow farmers to develop their own fertilizer on demand, using sunlight and nitrogen from the water.
Thanks to a specialized X-ray source at Lawrence Berkeley National Laboratory, the Georgia Institute of Technology researchers have confirmed the existence of a hypothetized interaction between nitrogen and titanium dioxide (TiO2) – a common photoactive material also known as titania – – in the presence of light. The catalytic reaction is believed to use carbon atoms found as contaminants on the titania.
If the nitrogen fixing reaction can be scaled up, it might help one day clean clean-scale fertilizer production that can reduce dependence on capital-intensive centralized production facilities and cost distribution systems that drive costs for farmers in isolated areas of the world . Most of the world's fertilizer is now made using ammonia produced by the Haber-Bosch process, which requires large amounts of natural gas.
"In the United States, we have an excellent production and distribution system for fertilizer. However, many countries are not able to afford to build Haber-Bosch plants, and may have adequate infrastructure for import fertilizers. For these regions, photocatalytic "nitrogen fixation is useful for on-demand fertilizer production," said Marta Hatzell, an assistant professor at Georgia Tech's Woodruff School of Mechanical Engineering. "Ultimately, this might be a low-cost process that could be available to a broader array of farmers."
Hatzell and collaborator Andrew Medford, an assistant professor in Georgia Tech's School of Chemical and Biomolecular Engineering, are working with scientists at the International Fertilizer Development Center (IFDC) to study the potential impacts of the reaction process. The research was reported October 29 in the Journal of the American Chemical Society.
Research began more than two years ago when Hatzell and Medford began collaborating on a 1941 paper published by Seshacharyulu Dhar, an Indian soil scientist who reported an increase in ammonia emitted from compost subjected to light. The suggested that a photocatalytic reaction with minerals in the compost could be responsible for the ammonia.
Since that paper, other researchers have reported nitrogen fixation on titania and ammonia production, but have not been consistently experimentally confirmed.
Medford, a theoretician, worked with graduate research assistant Benjamin Comer to the chemical pathways model that would be needed to fix nitrogen on titania to potentially create ammonia using additional reactions. The calculations suggested by the proposed process that is highly unlikely on pure titania, and the research process for the grant process have been proposed. However, they were awarded an experimental time on the Advanced Light Source at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, which allowed them to finally test a key component of the hypothesis.
Specialized equipment at the lab is allowed by Hatzell and graduate student Yu-Hsuan Liu to use X-ray photoelectron spectroscopy (XPS) to examine the surface of nitrogen, water and oxygen interacted with the surfaces under near ambient pressure in the dark the light. At first, the researchers saw no chemical nitrogen fixation, but as the experiments continued, they observed a unique interaction between nitrogen and titania when directed at the minerals surface.
What accounted for the initial lack of results? Hatzell and Medford believe that surface contamination with carbon – is likely from a hydrocarbon – is a necessary part of the catalytic process for nitrogen reduction on the titania. "Prior to testing, but during experiments with carbon sources from various sources (gases and vacuum chambers) can introduce the amount of carbon trace back to the sample," Hatzell explained. "What we observed was that reduced nitrogen was a degree of carbon on the sample."
The hydrocarbon contamination hypothesis would explain why earlier research had provided inconsistent results. Carbon is always present at trace levels on titania, but the right amount and type may be key to making the hypothetical reaction work.
"We think this explains the puzzle and the new catalysts using this 75-year-old mystery," Medford said. "Often the best catalysts are materials that are very pristine and made in a clean room. Here you have just the opposite – this is actually needs the impurities, which can be beneficial for sustainable applications in farming."
The researchers hope to experimentally confirm the role of carbon at the National Northwest National Laboratory (PNNL), which will allow them to directly probe the carbon during the photocatalytic nitrogen fixation process. So, they can better control the reaction to improve efficiency, which is currently less than one percent.
The research reported in the ammonia journal did not measure, but Hatzell and her students have since detected it in lab scale tests. Because the ammonia is currently produced by low levels, the researchers have to take precautions to avoid ammonia-based contamination. "Even tape used on equipment can make small quantities of ammonia that can affect the measurements," Medford added.
Though the amounts of ammonia are produced by the process of improvement, Hatzell and Medford believe that the advantages of on-site fertilizer production under benign conditions could overcome that limitation.
"While this may be a ridiculous sound from a practical perspective at first, if you actually look at the problem and the fact that sunlight is free, on a cost basis it starts to look more interesting," Medford said . "If you could operate a small-scale ammonia production facility with enough capacity for one farm, you have immediately made a difference."
Hatzell cutting-edge credits.
"Since earlier investigators looked at this, there have been significant advances made in the area of measurement and surface science," she said. "Most surface science measurements at Lawrence Berkeley National Lab, allowed to take a step closer to observing this reaction in its native environment. "