Catalysts Save the Planet from Global Warming - Ammonia Synthesis and Decomposition Catalysts Attract Worldwide Attention for Achieving a Decarbonized Society

Professor Katsutoshi Nagaoka
Department of Chemical Systems Engineering
Graduate School of Engineering
Nagoya University
 

The Haber-Bosch process^*1, developed in the early 20th century, enabled the industrial-scale production of ammonia using catalyst^*2 technology. Ammonia plays an indispensable role in our daily lives, serving as a key component in fertilizers essential for food production, as well as in material for clothing and pharmaceuticals. In recent years, amid accelerating effort to realize a decarbonized society, ammonia has been gaining attention as a next-generation energy source that does not emit carbon dioxide when burned, and as an energy carrier for transporting hydrogen.

We spoke with Professor Katsutoshi Nagaoka of the Department of Chemical Systems Engineering at Nagoya University’s Graduate School of Engineering, who is conducting research on ammonia synthesis and decomposition. In particular, his research focuses on developing innovative ammonia synthesis methods that aim to surpasses the conventional Haber-Bosch process.

It would not be an exaggeration to say that modern life would be inconceivable without catalysts. The development of the Haber-Bosch process in the early 20th century marked a turning point in industrial chemistry, enabling large-scale ammonia synthesis. This breakthrough allowed for the mass production of nitrogen-based chemical fertilizers, helping to overcome the global food crisis. Today, catalyst technologies derived from the Haber-Bosch process are utilized across a wide range of fields – from environmental management and energy resources to advanced manufacturing.

Catalysts are substances that make what once seemed impossible, possible. For instance, placing a mixture of oxygen and hydrogen in a sealed beaker at a 2:1 ratio yields no reaction on its own. However, introducing platinum nanoparticles into the mixture triggers an explosive reaction that produces water. This same principle is applied in fuel cells, which generate clean energy by emitting only water, without releasing CO₂ or air pollutants.

At my laboratory, we focus on catalyst research for ammonia. We design combinations of materials that promote both the decomposition and synthesis of ammonia, and even create catalysts from scratch. Of the approximately 120 elements in the periodic table, about 60 elements (excluding radioactive and harmful substances and rare gases) are suitable for catalytic application. We typically use a combination of 5 to 10 elements in a single catalyst. A critical aspect of catalyst design is ensuring that the material remains stable under the intended reaction conditions.

Synthesizing a single catalyst typically required two to four days. For example, if cobalt nitrate or other cobalt salts or complexes, are dissolved in a solvent and mixed with magnesium oxide (MgO) as a base, the MgO surface contains strong absorption sites that anchor the cobalt. After evaporating the solvent and thermally treating the mixture at around 500°C to eliminate nitric acid and other nitrates, the catalyst is ready. Initially produced in powder form, the catalyst is then shaped into particles of the appropriate size for the intended application. At the lab scale, where the flow rate is low, particle diameters range from 250 to 500 micrometer. In industrial-scale plants, where higher gas throughputs are required, catalysts are manufactured in millimeter- to centimeter-scale particle diameter to ensure gas flow passage.

When I began my research on catalysts in the 1990s, the global mainstream was on reacting methane (CH₄), the main component of city gas and natural gas. My own research started with binding two CH₄ molecules to produce ethylene (a raw material for polyethylene). Once that project had reached substantial research results, I consulted my lab professor about pursuing a new direction. That’s when I was given a completely unexplored topic within our lab: reacting methane with carbon dioxide (CO₂). This led to further research on production carbon monoxide (CO) and hydrogen by reacting methane with CO₂, or with steam (H₂O) – in other words, generating hydrogen from fossil resources. At that time, hydrogen itself wasn’t the main focus; the idea was that with CO and hydrogen as raw materials, you could synthesize a wide range of useful compounds. But just as I was working on this, fuel cells were starting to gain public attention, prompting me to focus more specifically on hydrogen production.

Then in 2008, I was approached by a company with an intriguing request: Could ammonia be used as a hydrogen source through catalytic decomposition? That opportunity marked a turning point in my research. With my background in hydrogen production, I felt confident in this new direction, especially since my university professor specialized in ammonia synthesis. This gave me the confidence to tackle ammonia decomposition as well. Additionally, my desire to explore uncharted territory as a researcher also pushed me to make the shift towards ammonia.

With research funds provided by the company, we built two test units, and within a year, my lab students screened around 100 catalyst samples. That hands-on exploration gave us a strong feel for ammonia decomposition catalysts, and more importantly, it gave me the conviction that we could succeed.

Around that time, we were approached by the New Energy and Industrial Technology Development Organization (NEDO^＊3), which expressed strong interest in research focused on generating hydrogen instantaneously, prompting me to take on the challenge. Utilizing hydrocarbons in the development, we tried an approach that defied conventional thinking, leading to a world-first discovery.

By loading precious metal nanoparticles onto ceria-zirconia oxide, a material commonly used in automotive exhaust purification catalyst, and applying a simple pretreatment, we found that introducing hydrocarbons and oxygen at room temperature resulted in hydrogen production. This research outcome was so far removed from conventional wisdom that it was initially met with skepticism. But through persistent research, we were able to clarify the reaction mechanism: when　the catalyst is in a reduced state, contact with oxygen triggers heat generation, raises the temperature, and initiates the catalytic reaction. This eventually gained us recognition.       

Next, we were asked whether a similar approached could instantly generate hydrogen from ammonia and oxygen, and we applied the hydrocarbons method to this research. Once again, by testing a method that conventional wisdom suggested wouldn`t work, we made another new discovery.  

We knew that using a catalyst with ruthenium nanoparticles on an alumina support^*4 would trigger a catalytic reaction upon heating to a certain temperature. Additionally, with supports like ceria-zirconia, if the catalyst is in a reduced state, exposure to oxygen would generate heat, raise the temperature, and trigger the catalytic reaction. Based on this knowledge, we tested the catalytic reaction using an alumina support, initially believing that the reaction wouldn't occur due to the lack of reduction. To our surprise, the reaction occurred, and hydrogen was generated instantly. Upon investigation of the mechanism, we discovered that even without prior reduction, the adsorption of ammonia onto the catalyst surface produced enough heat to trigger the reaction. This discovery was published in Science Advances and helped secure significant research funding.

Some barriers can’t be overcome by logic only. There’s an element of luck, of course – but as a researcher, I’ve always strived not to do the same as others. I maintain a broad perspective and am always willing to try various approaches. I believe that the willingness to try things – especially those that defy expectations – is what leads to breakthrough discoveries.

Based on my own experience, I encourage my students to develop instinct for challenging established norms, to value creative insights, and to pursue wide-ranging research without fear of failure. Careful observation and open discussion are essential. In fact, many of the ideas we pursue often come from the unexpected results students uncover in their diverse explorations.

Ammonia decomposition efficiency has now reached nearly 100% at temperatures above 600°C.  The current research challenges include simplifying the pretreatment process, utilizing non-precious metals, and enabling reactions under milder conditions. Equilibrium-wise, ammonia decomposition can achieve nearly 100% decomposition at around 400°C. In practice, using the precious metal ruthenium allows decomposition at around 400°C. However, with non-precious metals, the required reaction temperature rises to 600-700°C. The challenge, then, is to discover materials that can lower this temperature thresholds. To address this, we are exploring new approaches that integrate both catalyst design and process optimization – particularly by developing catalysts based on non-precious metals. By tackling the problem from both materials and process optimization sides, we aim to expand the possibilities for more efficient and practical ammonia decomposition.

The synthesis efficiency of ammonia via the Haber-Bosch process involves synthesizing ammonia from nitrogen under conditions of 500°C and over 200 bar of pressure, achieving about a 30% conversion. Our goal is to develop a synthesis method that exceeds the Haber-Bosch process in efficiency and feasibility, particularly under milder temperature and pressure conditions.

The nitrogen conversion in ammonia synthesis decreases as the temperature increases, according to the graph of equilibrium state versus temperature. Therefore, chemically, synthesis should be performed at lower temperatures. However, the iron catalyst used in the Haber-Bosch process performs poorly at low temperatures, achieving only around a 30% conversion at 500°C and 200 bar. Theoretically, higher nitrogen conversion those achieved by the Haber-Bosch process can be obtained at 300-350°C and 50-70 bar. We are conducting daily research to develop catalysts that can synthesize ammonia at lower temperatures to realize this potential.

HORIBA's multi-gas analyzer has been beneficial for measuring the concentration of synthesized ammonia. Before introducing the analyzer, we manually measured the ammonia by reacting it with sulfuric acid and testing its electrical conductivity. Moreover, the concentration of sulfuric acid must be adjusted according to the concentration of synthesized ammonia, making both complicated and potentially hazardous. We also tried using gas chromatography, but the tailing effect on the graphs prevented satisfactory measurement results. The introduction of HORIBA's multi-gas analyzer has allowed us to perform continuous and safe analysis without laborious and dangerous procedures. Today, as we operate multiple reactors to expand the number of experiments, the system has become an invaluable part of our research.

⇒ Link to the product pages: Multi-Component Gas Analyzer - VA-5000 Series - HORIBA
Performance Evaluation of Ammonia Synthesis and Decomposition Catalyst

The Japanese government has declared its commitment to achieving net-zero greenhouse gas emission by 2050, aiming to realize a fully decarbonized society. As a potential solution, ammonia gas been drawing significant attention, and demand is expected to grow substantially in the coming years. Two main approaches are being explored for using ammonia as a fuel: co-firing, where ammonia is mixed with coal, and mono-firing, which involves burning ammonia alone. It is estimated that 20% co-firing (80% coal, 20% ammonia) could reduce annual CO₂ emissions by 40 million tons, and replacing current power generation with mono-fired ammonia combustion could halve the CO₂ produced during electricity generation in Japan. Co-firing at 20% requires 20 million tons of ammonia. However, Japan currently produces less than 1 million tons, necessitating at least a twentyfold increase. We hope to contribute to meeting this growing demand through our research and development efforts.

In the early 20th century, the Haber-Bosch process enabled the artificial synthesis of ammonia, helping humanity overcome the food crisis through nitrogen-based chemical fertilizers. Even today, we continue to rely on this breakthrough developed over a century ago on the other side of the world. As a Japanese researcher, I find it frustrating that we still depend on technology created 110 years ago. That’s why our lab is fully committed to develop next-generation catalyst technologies that surpass the Haber-Bosch process.

I believe that ammonia is one of the key substances that can help save humanity from the climate crisis in the 21st century and pave the way for the future. We intend to advance research and development in close collaboration partnership with industry and bring practical solutions to the real world.

(Interview Date: June 2023)
Note: All names of organizations, affiliations, and positions mentioned in the article are as of the interview date.
 

1. Haber-Bosch Process: A method for synthesizing ammonia using nitrogen, hydrogen, and primarily iron catalysts. Invented in 1906 by German chemists Fritz Haber, Carl Bosch, and Alwin Mittasch. It is considered one of the greatest inventions of the 20th century. This invention enabled the mass production of nitrogen fertilizers, significantly increasing food production and supporting population growth since the 20th century.
2. Catalyst: A substance that accelerates the reaction rate in relatively small quantities compared to the reactants, without being consumed during the reaction.
3. NEDO: Acronym for the New Energy and Industrial Technology Development Organization, a national research and development agency.
4. Support (Carrier): A material that itself does not exhibit catalytic activity but serves as a base to fix other substances which show adsorption and catalytic activity. Materials such as porous substances, diatomaceous earth, pumice, and alumina are used.

Katsutoshi Nagaoka
Professor
Department of Chemical Systems Engineering, Graduate School of Engineering, Nagoya University
 

Career History

• April 2002 – January 2004: Postdoc, Technische Universität München, Germany
• February 2004 – March 2005: Lecturer, Associate Professor, Faculty of Engineering, Oita University
• April 2005 - March 2017: Associate Professor, Faculty of Engineering, Oita University
• April 2017 - March 2019: Associate Professor, Faculty of Science and Engineering, Oita University
• April 2019 - Present: Professor, Graduate School of Engineering, Nagoya University

Educational Background

• April 1992 - March 1996: Bachelor’s Degree, Applied Chemistry Course, Department of Chemical Engineering, Tokyo Institute of Technology
• April 1996 - March 1998: Master's Degree, Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science & Engineering, Tokyo Institute of Technology
• April 1998 - March 2001: Doctorate Degree, Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science & Engineering, Tokyo Institute of Technology

Performance Evaluation of Ammonia Synthesis and Decomposition Catalyst