The solution to a mystery of 75-year-old materials could one day allow farmers in developing countries to produce their own fertilizers on demand, using sunlight and nitrogen from the air.
Thanks to a specialized X-ray source from the Lawrence Berkeley National Laboratory, researchers at the Georgia Institute of Technology have confirmed the existence of a long-standing interaction between nitrogen and titanium dioxide (TiO).2) – a common photoactive material also called titanium oxide – in the presence of light. It is thought that the catalytic reaction uses carbon atoms found as contaminants on titanium oxide.
If the nitrogen fixation reaction can be amplified, this could someday help fuel a farmer 's own fertilizer production, which could reduce the dependence on it. Capital-intensive centralized production facilities and expensive distribution systems that drive up costs for farmers in remote areas of the world. . Most of the world's fertilizers are now made from 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. However, many countries can not afford to build Haber-Bosch plants and may not even have the transportation infrastructure to import fertilizer. For these regions, fixing nitrogen could be useful for the production of fertilizer on demand, "said Marta Hatzell, assistant professor at Georgia Tech's Woodruff School of Georgia Engineering." By the end of account, it could be an inexpensive process that could make fertilizer nutrients available to more farmers. "
Hatzell and Associate Andrew Medford, Assistant Professor at the School of Chemical Engineering and Biomolecular Engineering at Georgia Tech, Collaborate with Scientists from the International Fertilizer Development Center (IFDC) to Investigate the Potential Impacts of the Reaction Process . The research 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 born of a 1941 article published by Seshacharyulu Dhar, an Indian soil scientist who had observed an observed increase in ammonia emitted by compost subjected to light. Dhar suggested that a photocatalytic reaction with minerals in the compost could be responsible for ammonia.
Since this article, other researchers have reported a fixation of nitrogen on the production of titanium oxide and ammonia, but the results have not been confirmed so systematic.
Medford, a theoretician, worked with Benjamin Comer, a postgraduate research assistant, to model the chemical pathways that would be needed to fix nitrogen on titanium oxide in order to potentially create l / 39. ammonia using additional reactions. The calculations suggested that the proposed process was highly unlikely on pure titania and that the researchers had failed to obtain a grant that they had proposed to use to study the mysterious process. However, they obtained experimental time on the advanced light source of the Lawrence Berkeley National Laboratory of the US Department of Energy, which allowed them to finally test a key element of the hypothesis.
The laboratory's specialized equipment allowed Hatzell and graduate student Yu-Hsuan Liu to utilize X-ray Photoelectron Spectroscopy (XPS) to examine the surface of titanium oxide when the X-ray photoelectron spectroscopy (XPS) nitrogen, water and oxygen interacted with the ambient pressure surface in the light. At first, the researchers did not see any photochemical fixation of nitrogen, but over the course of experiments, they observed a unique interaction between nitrogen and titanium oxide when the light was directed towards the surface of minerals.
What explains the initial lack of results? Hatzell and Medford believe that surface contamination by carbon – probably from a hydrocarbon – is a necessary part of the catalytic process of reducing nitrogen on titanium oxide. "Before testing, the samples are cleaned to remove almost all traces of carbon from the surface, but during the experiments, carbon from various sources (gas and vacuum chamber) can reintroduce traces of carbon into the surface. sample, "said Hatzell. "What we observed was that reduced nitrogen species were only detected when there was a degree of carbon on the sample."
The hypothesis of oil contamination would explain why previous research had provided inconsistent results. Carbon is always present in trace amounts on titanium oxide, but getting the right amount and type can be the key to the success of the supposed reaction.
"We think this explains the surprising results that have been reported in the literature and we hope this will give insights on how to design new catalysts using this 75-year-old mystery," said Medford. "The best catalysts are often very pure materials and are manufactured in a clean room – here you have exactly the opposite – this reaction actually requires impurities, which could be beneficial for sustainable agricultural applications."
The researchers hope to experimentally confirm the role of carbon in future tests at the Pacific Northwest National Laboratory (PNNL), which will allow them to directly probe carbon during the process of photocatalytic nitrogen fixation. They also hope to learn more about the catalytic mechanism in order to better control the reaction in order to improve efficiency, which is currently less than 1%.
The research reported in the newspaper did not measure ammonia, but Hatzell and his students have since detected it during laboratory tests. Since ammonia is currently produced at such low levels, researchers have had to take precautions to avoid ammonia contamination. "Even the adhesive tape used on the equipment can create small amounts of ammonia that can affect the measurements," added Medford.
Although the quantities of ammonia produced by the reaction are currently low, Hatzell and Medford believe that with the improvement of the processes, the benefits of on-site fertilizer production under favorable conditions could overcome this limitation.
"Although it may seem ridiculous from a practical point of view at first, if you really examine the needs of the problem and the fact that sunlight and nitrogen from the air are free, in terms of costs, it's starting to look more interesting, "said Medford. . "If you could operate a small ammonia production facility of sufficient capacity for a farm, you immediately made a difference."
Hatzell believes that advanced surface science has finally provided an explanation for the mystery.
"Since the researchers examined this issue, important progress has been made in the field of measurement and surface science," she said. "Most surface measurements require the use of very high vacuum conditions that do not reproduce the catalytic environment you want to explore. The near-ambient pressure XPS from Lawrence Berkeley's National Laboratory has taken us one step closer to observing the native environment. "
Nitrogen fixation under ambient conditions
Benjamin M. Comer et al., The role of adventitious carbon in the photocatalytic fixation of nitrogen by Titania, Journal of the American Chemical Society (2018). DOI: 10.1021 / jacs.8b08464