New photocatalyst produces ammonia from atmospheric nitrogen at room temperature without fossil fuels

The research team was inspired by nature and developed a iron-metalated porphyrin based metal-organic frame photocatalyst. Credit: City University of Hong Kong
Ammonia (NH 3 ), a key component of fertilizer, is a promising carbon-free carrier of energy. Ammonia production is estimated to consume around 2 percent of total global energy production, and it releases 500 million tons of carbon dioxide each year. Scientists at City University of Hong Kong (CityU), developed a new type of photocatalyst capable of producing ammonia from atmospheric Nitrogen at room temperature by using sunlight. The new method is more efficient than the traditional one, which produces huge carbon emissions. According to the research team, such sustainable ammonia production technology would help advance the future nitrogen economy.

Professor Leung Hi Michael, Shun Hing Education and Charity Fund Professor of Energy and Environment, and assistant Professor Dr. Shang Jin from CityU's School of Energy and Environment, as well as an Australian scholar, led the research. The title of their findings was published in the scientific journal ACS Nano as "Atomically Dispersed Iron Metal site in a Porphyrin–Based Metal-Organic Framework For Photocatalytic Nitrogen Fixation."

Ammonia is an emerging fuel that could replace oil and coal when it comes to generating electricity

Ammonia is an important component of fertilizers and food. The majority of artificial ammonia used in agriculture is to make fertilizers. The industrial uses of ammonia include the production of detergents and refrigerants. Ammonia is a good source of hydrogen and can be transported more easily than hydrogen. This is why it has been a popular choice in recent years. Ammonia can also be used as a fuel to generate power, in addition to petroleum and coal. There is a great demand for ammonia.

After achieving artificial nitrogen fixation powered with sunlight, the team creates a photocatalyst that can produce ammonia at ambient temperature and pressure. Credit: City University of Hong Kong

Current production methods: Very harmful to the environment

Before making ammonia, it is important to "fix" nitrogen. Although nitrogen makes up 80 percent of the atmosphere, it cannot be used unless converted into nitrogen-containing substances. "Nitrogen fixation" is the process of converting nitrogen into useful compounds.

There are two ways to fix nitrogen. The artificial method is usually referred to the HaberBosch industrial process at elevated temperature and pressure that uses iron as a catalyst to make ammonia from hydrogen and nitrogen. Ammonia production relies heavily on the HaberBosch process. However, it is not sustainable as it uses a lot of fossil fuels and produces large amounts of carbon dioxide.

Professor Leung and Dr. Jin work together to find a sustainable method of producing ammonia. They bring together their respective teams to create a process for nitrogen fixation in ambient conditions with water and renewable energy. The most difficult task for the team was to create a catalyst that allows the multi-step nitrogen fixation reaction.

This illustration shows how the photocatalyst triggers a nitrogen reduction reaction that produces ammonia. Credit: City University of Hong Kong

Biomimetic photocatalyst now available

Iron in nitrogenase, a type of enzyme, favorably binds to and activates nitrogen. Porphyrin, a type of organic compound, in chlorophyll harvests sunlight. The team created a iron-metalated porphyrin based metal-organic frame (MOFs), photocatalyst.

The biomimetic photocatalyst is only 15-25 nm thick and can produce ammonia once artificial nitrogen fixation has been achieved using sunlight and water as the reducing agents, at ambient temperature, pressure.

Because MOFs provide more sites for the adsorption of and activation nitrogen, the team used them in their photocatalyst. This increases the efficiency of the nitrogen reduction process.

This photocatalyst was tested and it produced ammonia. "We created a photocatalyst that could achieve the highest photocatalytic nitrogen fixation performance within the MOFs-based category. Professor Leung said that it has one of the highest ammonia yields, and the best MOF hydrolytic stability. The photocatalyst's good hydrolytic stability allows it to be used repeatedly.

The team investigated the photocatalytic reduction reaction of photocatalysts on their biomimetic photocatalyst through this research. Dr. Shang stated that the results of this research would help in the rational design and construction of next-generation MOFs based photocatalysts. Their findings, he believed, would allow them to create various porphyrin-based MOFs that can be used as photocatalysts in a variety of energy and environmental applications.

This pioneering study was intended to inspire engineers and scientists in the field of catalysis, to develop MOFs-based photocatalysts that can catalyze other chemical reactions at ambient temperatures and pressures.

"Fossil-fuel-free energy production and commodity chemicals are the best ways to achieve carbon neutrality. The technology was developed to make ammonia using atmospheric nitrogen and water. It can be harvested from the sun. Professor Leung concluded that we were able to sustainably obtain carbon-free energy." Their findings were believed to help address the growing energy crisis and other environmental issues.

Continue reading The new alchemy of carbon neutrality: Making water into ammonia using only renewable energy

Additional information: Shanshan Shang et al., Atomically Dispersed iron metal site in a Porphyrin–Based MetalOrganic Framework to Photocatalytic Nitrogen Fixation. ACS Nano (2021). Information from the Journal: ACS Nano Shanshan Shang et al. Atomically Dispersed iron Metal Site in a Porphyrin Based MetalOrganic Framework For Photocatalytic Nitrogen Fixation. (2021). DOI: 10.1021/acsnano.0c10947

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