Development and future view of ultra-small metal cluster catalysts for ammonia synthesis

Abstract

Ammonia is expected to be used on a much larger scale as hydrogen carrier and fuel that does not emit carbon dioxide at the time of combustion in the next 30 years. Industrially, ammonia is produced under severe reaction conditions (350–550 °C and 15–35 MPa) using the Haber-Bosch process. The development of new catalysts for ammonia synthesis under milder conditions is urgently necessary. We have developed a novel supported ultra-small molybdenum metal cluster catalyst with a size of less than 1 nm on average. This metal cluster has been prepared from molybdenum chloride cluster supported on HY zeolite, followed by removal of all the chloride ligands by hydrogen heat treatment. The resulting metal cluster effectively cleaves N≡N bond in dinitrogen (N₂) by taking advantage of the multiple molybdenum sites, and constantly produces ammonia even at 200 °C under 5 MPa. The stability of the chloride cluster precursor in air and the easy availability of the metal cluster catalyst by the simple hydrogen treatment are advantageous for industrial use. This report describes the development and future view of the ultra-small molybdenum cluster catalyst for ammonia synthesis.