The challenges lie in the tremendous amount of energy and extremely low temperatures required for liquefying hydrogen, which hamper large-scale production and long-distance transportation. Enter ammonia, a readily liquifiable hydrogen carrier that could be transported and then decomposed, with the aid of a catalyst, to produce pure nitrogen and the coveted hydrogen gas.
Research over the years has pointed to the beneficial role basic oxide support plays in the ammonia decomposition process, particularly in conjunction with non-precious metal catalysts such as nickel (Ni). Equally promising is the capacity for nitrogen-bearing support materials to encourage ammonia catalysis. Despite their potential, both options present obstacles: the former demands high operating temperatures for catalysts, while the latter is extremely sensitive to air and water, risking irreversible deactivation.
Undeterred, a team of researchers led by Professor Masaaki Kitano from Tokyo Institute of Technology (Tokyo Tech) recently achieved a significant breakthrough, presented in Advanced Energy Materials. They developed a highly active Ni-based ammonia decomposition catalyst, reinforced with hexagonal barium titanium oxynitride (h-BaTiO3-xNy). This innovative, precious metal-free catalyst shows a remarkable ammonia decomposition rate, functioning at lower operating temperatures than conventional Ni-based catalysts.
"Precious metals such as ruthenium are often used as ammonia decomposition catalysts but are prohibitively expensive. Our study delivers a Ni-based alternative with an impressive hydrogen production rate at low temperatures, a challenge considering the weak attraction between nitrogen and Ni below certain temperatures," explains Prof. Kitano.
The Tokyo Tech team delved into perovskite-type oxynitrides, a group of materials renowned for their stability and capacity to form nitrogen vacancies. These materials had not been previously utilized as support for low-temperature ammonia decomposition catalysis. The researchers created the new Ni/h-BaTiO3-xNy catalyst by reacting nitrogen gas and Ni/h-BaTiO3-xHy oxyhydride under mild conditions. This catalyst was subjected to ammonia decomposition tests to evaluate its reaction rates and effectiveness, while a range of analytical examinations and mathematical calculations were carried out to decipher its catalysis mechanism.
The study found that substituting O2- sites on the BaTiO3 lattice with N3- ions led to a significant reduction of over 140oC in the operating temperature for the Ni-based catalyst. This impressive result outstripped both conventional Ni-based ammonia decomposition catalysts and its oxyhydride precursor.
Detailed investigations, including isotope experiments and Fourier transform-infrared spectroscopy measurements, suggested that N3- vacancies serve as active sites for the decomposition reaction at the metal support interface, where Ni assists the nitrogen gas's desorption from the support. Moreover, the team discovered that Ni/h-BaTiO3-xNy demonstrated water resistance, maintaining its catalytic activity despite exposure.
This ground-breaking research not only uncovers the underlying catalysis mechanism but also underscores the role of N3 ion substitution in enhancing the catalysis of ammonia decomposition. The findings pave the way for the creation of a variety of highly active non-precious metal catalysts consisting of nickel, cobalt, and iron, promising to boost the viability of hydrogen fuel production from ammonia, contributing to a cleaner, greener world, as Prof. Kitano asserts.
Research Report:Ammonia decomposition over water-durable hexagonal BaTiO3-xNy-supported Ni catalysts
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