Abstract
Due to its essential use as a fertilizer, ammonia synthesis from nitrogen and hydrogen is considered to be one of the most important chemical processes of the last 100 years. Since then, an enormous amount of work has been undertaken to investigate and develop effective catalysts for this process. Although the catalytic synthesis of ammonia has been extensively studied in the last century, many new catalysts are still currently being developed to reduce the operating temperature and pressure of the process and to improve the conversion of reactants to ammonia. New catalysts for the Haber–Bosch process are the key to achieving green ammonia production in the foreseeable future. Herein, the history of ammonia synthesis catalyst development is briefly described as well as recent progress in catalyst development with the aim of building an overview of the current state of ammonia synthesis catalysts for the Haber–Bosch process. The new emerging ammonia synthesis catalysts, including electride, hydride, amide, perovskite oxide hydride/oxynitride hydride, nitride, and oxide promoted metals such as Fe, Co, and Ni, are promising alternatives to the conventional fused‐Fe and promoted‐Ru catalysts for existing ammonia synthesis plants and future distributed green ammonia synthesis based on the Haber–Bosch process.
Highlights
Due to its essential use as a fertilizer, ammonia synthesis from nitrogen and hydrogen is considered to be one of the most important chemical processes of the last 100 years
There is still a large volume of recent publications on the development of ammonia synthesis catalysts for the conventional Haber–Bosch process using a wide range of catalysts such as promoted-iron catalysts, supported ruthenium catalysts, and metal nitride catalysts
Current research is focusing on the development of new highly active ammonia synthesis catalysts using ruthenium, cobalt, nickel, and metal nitrides, the lessons learned from the extensive study of the promoted-iron catalyst, from its initial discovery to its widespread global use on an industrial scale, will help us improve the Haber– Bosch process through increased conversion by operating at reduced temperature and pressure
Summary
The proposed reaction mechanisms for the catalyzed reaction of nitrogen and hydrogen to form ammonia are as follows[13]. Www.advenergysustres.com of nitrogen on iron single-crystal surfaces for both Fe(100) and Fe(111) over a temperature range 140–1000 K to come to the conclusion that nitrogen adsorption is the rate-limiting step in the ammonia synthesis reaction.[24] Even after many experiments involving the adsorption of N2, there is still controversy over the exact mechanism of this step It is unknown whether the N2 dissociates directly to nitrogen atoms on the catalyst surface or whether this dissociation to nitrogen atoms takes place on intermediates. Most modern ammonia synthesis catalysts used in the Haber–Bosch process are reported to achieve a conversion rate of around 10–15% operating in the range of 425–450 C at pressures above 100 atm.[2] From this it can be seen that if catalysts can be developed to increase the rate of reaction at temperatures lower than this the conversion per pass of the ammonia synthesis loop will increase exponentially For this reason, the catalytic activity of the catalyst is of key importance. The ammonia synthesis loop consists of an ammonia convertor, which catalytically coverts the synthesis gas to ammonia; a compressor, which is used to achieve the desired pressure for the synthesis gas and to recycle the stream before it enters the convertor; and a condenser, which can be located either after or before
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