Ru-based catalysts, the next generation of ammonia synthesis catalysts, are less susceptible to ammonia poisoning and more active than Fe-based catalysts . Ru is one of the best metals for ammonia synthesis based on a volcano plot . However, Ru-based catalysts are more sensitive to hydrogen partial pressure and more susceptible to hydrogen poisoning than Fe-based catalysts because the hydrogen order of Ru-based catalysts is often negative while that of Fe-based catalysts is positive .