Promotion effects in ammonia synthesis over ruthenium catalysts: A review

Abstract

Haber-Bosch (HB) process for industrial NH₃ synthesis operated at high temperature (500 °C) and pressure conditions (100–300 atm) with iron ore catalysts. Ru-based catalysts promoted by 1A and 2A elements, particularly Ba and Cs, show promise in electrolysis-driven HB operating under intermittent conditions. However, the roles of these promoters, traditionally classified as structural and electronic promoters based on trial-and-error study, remain controversial and lack a comprehensive literature review. This paper systematically examined these promotion effects, integrating conventional knowledge with recent experimental and computational advancements. It highlighted new insights into the composition and intrinsic roles of Ba, Cs, and other 1A/2A elements. The reaction mechanism of thermo-catalyzed NH₃ synthesis, including recent theories on spin promotion effects at active sites, is extensively discussed. This review provides a thorough understanding of promotion effects and guides the rational design of advanced metal catalysts with promoters for industrial NH₃ synthesis and various energy-related processes.