The solution to a 75-year-old material mystery could one day allow farmers in developing countries to produce their own fertilizer on demand, using sunlight and nitrogen from the air.
Thanks to an X-ray source specialized in the Lawrence Berkeley National Laboratory, researchers at the Georgia Institute of Technology have confirmed the existence of a long hypothesis interaction between nitrogen and titanium dioxide (TiOtwo) – a common photoactive material also known as titania – in the presence of light. It is believed that the catalytic reaction uses carbon atoms found as contaminants in titania.
If the nitrogen fixation reaction can be extended, it could one day help boost fertilizer production on a farm scale, which could reduce dependence on centralized, capital-intensive production facilities and costly distribution systems that increase costs for farmers in isolated areas of the world. . Most of the world's fertilizers are now produced with ammonia produced by the Haber-Bosch process, which requires large amounts of natural gas.
"In the United States, we have an excellent fertilizer production and distribution system, but many countries can not afford the Haber-Bosch plants and may not even have the proper transportation infrastructure to import fertilizers. regions, photocatalytic nitrogen fixation could be useful for the production of fertilizers on demand, "said Marta Hatzell, assistant professor in the Woodruff Mechanical Engineering School at Georgia Tech." Ultimately, this could be a low-cost process that could make fertilizer-based nutrients available to a wider range of farmers. "
Hatzell and collaborator Andrew Medford, an assistant professor in the School of Chemical and Biomolecular Engineering at Georgia Tech, are working with scientists at the International Fertilizer Development Center (IFDC) to study the possible impacts of the reaction process. The investigation was reported on October 29 in the Journal of the American Chemical Society.
The research began more than two years ago when Hatzell and Medford began collaborating on a mystery of materials that originated in a 1941 article published by Seshacharyulu Dhar, an Indian soil scientist who reported seeing an increase in ammonia emitted by the compost subjected to light. Dhar suggested that a photocatalytic reaction with minerals in the compost could be responsible for ammonia.
Since that article, other researchers have reported on nitrogen fixation in the production of titania and ammonia, but the results have not been systematically confirmed consistently.
Medford, a theoretician, worked with graduate research assistant Benjamin Comer to model the chemical pathways that would be necessary to fix nitrogen in titania to potentially create ammonia using additional reactions. The calculations suggested that the proposed process was highly unlikely in pure titania, and the researchers failed to obtain a grant they had proposed to use to study the mysterious process. However, they were given an experimental time in the Advanced Light Source at the Lawrence Berkeley National Laboratory of the US Department of Energy, which allowed them to finally test a key component of the hypothesis.
The specialized laboratory team allowed Hatzell and graduate student Yu-Hsuan Liu to use X-ray photoelectron spectroscopy (XPS) to examine the surface of the titania, since nitrogen, water and oxygen interacted with the surfaces under close pressure to the environment in the dark and in the interior. the light. At first, the researchers did not see any photochemical nitrogen fixation, but as the experiments continued, they observed a unique interaction between nitrogen and titania when light was directed to the surface of the minerals.
What explained the initial lack of results? Hatzell and Medford believe that surface contamination with carbon, probably from a hydrocarbon, is a necessary part of the catalytic process for nitrogen reduction in titania. "Before the test, the samples are cleaned to remove almost all the residual carbon from the surface, however, during the experiments, the carbon from various sources (gases and the vacuum chamber) can reintroduce the amount of carbon in the the sample, "explained Hatzell. "What we observed was that only reduced nitrogen species were detected if there was a degree of carbon in the sample."
The hydrocarbon contamination hypothesis would explain why previous investigations had provided inconsistent results. Carbon is always present in trace levels in Titania, but getting the right amount and type can be key to making the hypothetical reaction work.
"We believe that this explains the disconcerting results that had been reported in the literature, and we hope it gives us an idea of how to design new catalysts using this 75-year mystery," said Medford. "Often, the best catalysts are materials that are very pristine and are manufactured in a clean room, here it has the opposite: this reaction actually needs the impurities, which could be beneficial for sustainable applications in agriculture."
The researchers hope to confirm experimentally the role of carbon in upcoming tests at the National Laboratory of the Pacific Northwest (PNNL), which will allow them to directly detect carbon during the photocatalytic nitrogen fixation process. They also hope to learn more about the catalytic mechanism to better control the reaction and improve efficiency, which is currently less than one percent.
The research reported in the journal did not measure ammonia, but Hatzell and his students have detected it in laboratory tests. Because ammonia is currently produced at such low levels, researchers had to take precautions to avoid contamination based on ammonia. "Even the tape used in the equipment can create small amounts of ammonia that can affect the measurements," Medford added.
Although the amounts of ammonia produced by the reaction are currently low, Hatzell and Medford believe that with the improvements in the process, the advantages of producing fertilizers at the site under benign conditions could overcome this limitation.
"Although at first this may seem ridiculous from a practical perspective, if you really analyze the needs of the problem and the fact that sunlight and nitrogen from the air are free, in terms of costs, it seems more interesting," said Medford. . "If I could operate a small-scale ammonia production facility with enough capacity for a farm, it would have made a difference immediately."
Hatzell recognizes that the science of the vanguard surface finally provides an explanation for the mystery.
"Since previous researchers looked at this, significant advances have been made in the area of surface measurement and science," he said. "Most surface science measurements require the use of ultra-high vacuum conditions that do not mimic the catalytic environment that they intend to investigate." The environmental pressure near the XPS at Lawrence Berkeley National Laboratory allowed us to take a step closer to observe this reaction in their native environment. "
Nitrogen fixation in environmental conditions.
Benjamin M. Comer et al, The role of adventitious carbon in the photocatalytic nitrogen fixation by Titania, Journal of the American Chemical Society (2018). DOI: 10.1021 / jacs.8b08464