The resolution of a 75-year-old mystery could constitute a new source of agricultural fertilizer

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 supposedly long-standing interaction between nitrogen and dioxide of titanium (TiO ₂ ). a common photoactive material also known as titanium oxide – in the presence of light. It is believed that the catalytic reaction utilizes carbon atoms present as contaminants on titanium oxide.

If the nitrogen fixation reaction can be amplified, it could someday help fuel a farmer's own fertilizer production, which could reduce the reliance on a high capital intensity. Centralized production facilities and costly distribution systems that increase 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 system of fertilizer production and distribution, but many countries can not even afford to build Haber-Bosch plants and could even do not have adequate transport infrastructure to import fertilizers.For these regions, photocatalytic fixation with nitrogen could be useful for the production of fertilizer on demand, "said Marta Hatzell , badistant professor at Georgia Tech's Woodruff Mechanical Engineering School. "Ultimately, it could be an inexpensive process that would put fertilizer nutrients at the disposal of more farmers." . "

Hatzell and his collaborator Andrew Medford, an badistant professor at the Georgia Tech School of Chemical Engineering and Biomolecular Engineering work with scientists from the International Fertilizer Development Center (IFDC) to study the potential impacts of the process of reaction. The research was reported Oct. 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 material mystery born of a 1941 article published by Seshacharyulu Dhar, an Indian soil scientist, reported having observed a 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 this article, other researchers have reported a fixation of nitrogen on titanium oxide and ammonia production, but the results have not been systematically confirmed by experience.

Medford, a theoretician, worked with Benjamin Comer, a graduate research badistant, to model the chemical pathways that would be required to fix nitrogen on titanium oxide in order to eventually create the desired pathway. ammonia with the help of 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, the American laboratory Lawrence Berkeley of the US Department of Energy gave them time to experiment on the advanced light source, which allowed them to finally test a key element of the hypothesis.

The specialized equipment of the laboratory allowed Hatzell and the graduate student Yu -Hsuan Liu will use X-ray photoelectron spectroscopy (XPS) to examine the surface of titanium oxide while Nitrogen, water and oxygen interacted with almost ambient pressure surfaces in the dark and in the light. At first, the researchers did not see any photochemical fixation of nitrogen, but over the course of the experiments, they observed a unique interaction between nitrogen and titanium oxide when the light was directed towards the surface of the minerals.

How to explain 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 puzzling results that have been reported in the literature and hopefully explains 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 needs impurities, which could be beneficial for sustainable applications in agriculture."

The researchers hope to find out more. experimentally confirms the role of carbon in future tests at Pacific Northwest National Laboratory (PNNL), which will allow them to directly probe carbon during the photocatalytic nitrogen fixation process. They also hope to learn more about the catalytic mechanism in order to better control the reaction to improve efficiency, which is currently less than 1%.

Research reported in the newspaper did not measure ammonia, but Hatzell and his students have since already detected in laboratory scale 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 amounts of ammonia produced by the reaction are currently low, Hatzell and Medford estimate that, thanks to improved processes, the production of fertilizer on site under favorable conditions could to overcome this limitation.

"This may sound ridiculous from a practical point of view at first, if you actually examine the needs of the problem and the fact that sunlight and nitrogen from the air are free on a cost basis, 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 researchers have examined this issue, significant 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 almost ambient pressure XPS from Lawrence Berkeley's National Laboratory allowed us to take a step closer to observing this reaction in its native environment. "

Explore Further:
Nitrogen fixation under ambient conditions

More information:
Benjamin M. Comer et al., The role of adventitious carbon in the photo-catalytic fixation of nitrogen by Titania, Journal of the American Chemical Society (2018). DOI: 10.1021 / jacs.8b08464