Hydrogen (H2) is a zero-carbon and high-energy-density fuel promising to replace fossil fuels for power generation and clean energy. However, hydrogen still faces enormous challenges in terms of production, transportation, and storage. Ammonia (NH3) is a promising H2 (17.7 wt %) carrier that easily overcomes the difficulties associated with H2 storage and transport. However, for the NH3 decomposition hydrogen production reaction, the biggest challenge at present is to achieve complete conversion of ammonia under a relatively high space velocity (about 30,000 mL·gcat –1·h–1) at low-temperature conditions (about 350 °C) with reasonable price catalysts. At present, the most efficient ammonia decomposition catalyst is a Ru-based catalyst doped with K, Ba, and Cs and supported on various carbon supports and metal oxides. Otherwise, the catalysts that exhibited the most outstanding activity among non-noble metal catalysts are nickel-based, and because of their low cost, nickel is regarded as a reasonable alternative candidate material for NH3 decomposition. Advances in the study of reaction kinetics of ammonia decomposition reactions and whether the rate-determining step of the ammonia decomposition reaction is the cleavage of the first N–H bond or the desorption of nitrogen gas are also discussed. This review provides a comprehensive consideration of the recent development of Ru-based and Ni-based catalysts and proposed mechanisms of ammonia decomposition on them are examined. The effects of preparation methods, support, and promoters on catalyst activity were studied and theoretical bases for the design of future catalysts are presented. At last, a brief introduction to catalytic membrane reactor technology in recent years is given. This review can serve as a comprehensive work for designing novel catalysts.

In order to study the poisoning effect of the alkaline earth metal on Cu/SSZ-13 and reveal the poisoning mechanism, Cu/SSZ-13 zeolite catalysts were synthesized by the ion-exchange method and poisoned by different concentrations of Mg and Ca. The results of SCR tests and characterizations showed that the SCR activity of Cu/SSZ-13 catalysts decreased obviously after Mg or Ca poisoning, which is because Mg and Ca reduced the Cu 2+ species and Brønsted acid sites in Cu/SSZ-13, while increasing the content of Cu x O y . Furthermore, Mg and Ca on the catalyst surface could block the pores of the zeolite and then decrease the specific surface area of the catalysts, and Ca could further cause partial collapse of the zeolite skeleton. Through this article, the effect and mechanism of alkali earth metal poisoning of Cu/SSZ-13 were revealed, which provided data and theory support for the development of efficient Cu/SSZ-13 NH 3 -SCR catalysts with high alkali earth metal resistance.

With the energy transition looming, the search for carbon-free fuels is imperative. Hydrogen is an excellent alternative to fossil fuels, but there are significant challenges in transport and production. Ammonia is an excellent hydrogen carrier with 17.6 wt % hydrogen content and zero carbon, and the infrastructure for its production, storage, and transport is already well-established. Currently, ammonia decomposition for hydrogen only reacts at high temperatures; therefore, the challenge is to optimize highly active catalysts for ammonia decomposition at low temperatures. This review will start from the background of ammonia decomposition for hydrogen, thoroughly introduce the ammonia catalytic decomposition systems, mainly covering ruthenium-based, iron-based, cobalt-based, nickel-based, metal nitride, metal carbide, and new alkali metal amide-imide systems, analyze in detail the influence of different catalyst preparation methods, supports, promoters, and other methods on the catalytic activity, and discuss the general rules for optimizing the efficiency of catalysts. Moreover, the reaction kinetics of different catalysts, including reaction mechanisms, reaction-determining steps, and activation energies, are presented, and future prospects on ammonia decomposition catalysts are proposed.

No abstract