A new source of cheap agricultural fertilizer?

The solution to a seventy-five-year-old mystery may one day allow farmers in developing countries to produce their own fertilizer as needed, using sunlight and nitrogen from the air

Thanks to a special X-ray device, researchers from the Georgia Institute of Technology have verified the existence of the interrelationship that had been only a hypothesis for many years between nitrogen and titanium dioxide (TiO2) - a common photocatalytic material known as titania - in the presence of light. The researchers believe that the catalytic reaction makes use of carbon atoms that are in the form of impurities within the titania.

If it is possible to expand the scale of this nitrogen fixation reaction, it may help in the future for a clean production of fertilizer that can reduce its financial costs, especially in agricultural farms located in isolated areas. Most of the fertilizer produced today is produced using ammonia as part of a process called Haber-Bosch, which is required for large quantities of natural gas. "In the USA, there is an excellent system for the production and distribution of fertilizer. At the same time, many countries are not able to build factories based on this process (Haber-Bush), and they also lack the appropriate transport infrastructures to import fertilizer. For such areas, the photocatalytic fixation of nitrogen may be useful for the production of fertilizer on demand," said one of the researchers. "Ultimately, the process may be a cheap process that can produce fertilizer-based nutrients so that they are available to a wider spectrum of farmers."

The research began over two years ago when the researchers began collaborating on a mystery that originated in an article published in 1941 by an Indian soil scientist who reported an observation in which an increase in the amount of ammonia emitted from a compost pile that was exposed to light was demonstrated. The Indian scientist suggested that a photocatalytic reaction with minerals present in the manure could be responsible for obtaining the ammonia.

Since the article, other researchers have also reported fixing nitrogen on top of titania and producing ammonia as a result, but the results have never been reliably proven in scientific experiments. One of the researchers from the Georgia Institute of Technology specializes in theoretical chemistry and he was involved in building a computer model that could describe the chemical pathways that would be required in order to fix nitrogen on titania and potentially produce ammonia while utilizing additional reactions.

The calculations implied that the proposed process was impractical over pure titania, and the researchers failed to receive a research grant aimed at testing the unknown process. Despite the rejection of their research proposal, they did receive trial time on advanced equipment that allowed them, ultimately, to test a key component of their hypothesis.
The special equipment allowed the researchers to use the XPS (X-ray Photoelectron Spectroscopy) method to examine the surface of titania as nitrogen, water and oxygen react with it in the dark and in the light. Initially, the researchers did not observe any photochemical nitrogen fixation, but as the experiments progressed, they observed a unique reaction between nitrogen and biotitania when light was shone directly onto the surface of the mineral. What is the explanation for the lack of these results in their previous experiments? The researchers believe that contamination of the surface with carbon atoms - most likely from hydrocarbon substances - is an essential part of the catalytic process of nitrogen reduction on titania. "Before starting the experiments, the samples undergo a thorough cleaning to remove all carbon residues from their surface, however, during the test a residual amount of carbon from a variety of sources (gases and vacuum chamber) may enter back into the sample," explains the lead researcher. "We discovered that cases where recycled nitrogen forms were observed only occurred when there was a certain level of carbon within the sample." The hypothesis of hydrocarbon contamination could explain why previous studies have provided inconsistent results. Carbon is always present in residual levels on titania, but leaving the right amount and type may be key to the success of the reaction.
"We believe that our findings well explain the surprising results that have been reported over the years in the scientific literature, and we hope that they provide insights into the preparation of new catalysts by solving a 75-year-old mystery," adds the lead researcher. "Usually the best catalysts are extremely pure materials produced in clean rooms. In this case it is exactly the opposite - this reaction actually requires the presence of impurities, a result that may be beneficial for sustainable applications in agriculture."
The study published in the journal did not measure the presence of ammonia, but the same researchers discovered amounts of it in laboratory tests they conducted since then. In view of the fact that the ammonia obtained now is at such low levels, researchers have to take precautions to avoid contamination from the ammonia itself. "Even the adhesive tape used in the equipment can create tiny amounts of ammonia that can affect the measurements," explains the lead researcher. The researchers believe that thanks to improvements in the process, the advantages of producing fertilizer at the consumption site itself under normal conditions will be able to overcome the limitations of the method. "If we can develop a small-scale ammonia production facility with a capacity sufficient for one farm, a production facility capable of meeting the needs of nitrogen and oxygen from the free air around us, then we will immediately provide a significant change to that farm," explains the lead researcher.

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