New durable and low-cost catalyst reduces carbon footprint

A cheap and chemically stable catalyst for the synthesis of ammonia.

The Haber-Bosch process, often used to synthesize ammonia (NH3)—the basis for synthetic nitrogen fertilizers—by combining hydrogen (H2) and nitrogen (N2) over a catalyst at high pressures and temperatures is one of the most significant scientific breakthroughs, which helped increase crop yields and increase food production worldwide.

However, due to the high temperature and pressure requirements of the process, large fossil fuel energy costs are required. The hydrogen used in this method is obtained from natural gas (mainly methane). This hydrogen production process consumes a lot of energy and emits a lot of CO2. To solve these problems, several catalysts have been created so that the reaction can take place under milder conditions using hydrogen produced by the electrolysis of water using renewable energy sources. Among them are nitride-based catalysts containing active nanoparticles of metals such as nickel and cobalt (Ni, Co) supported on lanthanum nitride (LaN) supports.

Both the support and the active metal participate in the formation of NH3 in these catalysts. The active metal splits H2, while the crystal structure of the LaN support contains nitrogen vacancies and nitrogen atoms that adsorb and activate nitrogen (N2). Although these catalysts are cheap (as they do not require expensive ruthenium), their catalytic performance deteriorates when exposed to moisture, and the LaN support is converted to lanthanum hydroxide (La(OH)3).

Now in a new study published in Angewandte Chemieresearchers from China and Japan led by Professor Hideo Hoson of Tokyo Institute of Technology (Tokyo Tech), Japan, have developed a chemically stable catalyst that is stable in the presence of moisture. Inspired by stable rare-earth compounds containing chemical bonds between a rare-earth metal (in this case La) and a metal, they incorporated aluminum atoms into the LaN structure and synthesized a chemically stable La3AlN support containing La-Al bonds that prevent the lanthanum atoms from reacting with moisture .

The La-Al-N support, along with active metals such as nickel and cobalt (Ni, Co), was able to produce NH3 at rates similar to conventional metal nitride catalysts and could maintain stable production under nitrogen feed. – containing moisture. “Nickel or co-loaded La-Al-N catalysts showed no significant degradation after exposure to 3.5% moisture,” says Professor Hosana.

While the Al atoms stabilized the support, the lattice nitrogen and nitrogen defects present in the doped substrate enabled the synthesis of ammonia in a manner similar to conventional active metal nitride and rare earth metal catalysts. “Lattice nitrogen as well as nitrogen vacancies in La-Al-N play a key role in N2 adsorption, with the La-Al-N support and the active metal Ni responsible for N2 and H2 uptake and activation, respectively,” explains Professor Hosono.

The Haber-Bosch process is an energy-intensive chemical reaction that accounts for about 1% of the world’s annual carbon dioxide emissions. While alternative environmentally friendly approaches to NH3 production are being explored, the introduction of low-cost catalysts could bring immediate benefits by allowing the process to operate under milder conditions.

Reference: “An Approach to Chemically Stable Lanthanum Nitride-Based Nickel-Cobalt Catalysts for Ammonia Synthesis” by Prof. Yangfang Lu, Prof. Tian-Nang Ye, Dr. Jiang Li, Zichuang Li, Haoqiang Guan, Dr. Masato Sasase, Dr. Yasuhiro Niwa, Prof. Hitoshi Abe , Prof. Tian Li, Prof. Fusheng Pan, Prof. Masaaki Kitano and Prof. Hideo Hosano, 26 September 2022, Angewandte Chemie International Edition.
DOI: 10.1002/anie.202211759

The research was funded by the Ministry of Education, Culture, Sports, Science and Technology, Japan Society for the Promotion of Science, Japan Science and Technology Agency, Chongqing University, Chongqing Municipal Key Research and Development Program of China, National Natural Science Foundation of China, Shanghai Municipal Science and Technology Commission and Shaoxing Research Institute of Renewable Energy and Molecular Engineering.