What was the progress of ammonia catalyst research abroad in the 1930s?

The catalytic activity of ruthenium for ammonia synthesis was first reported by Zenghelis and Stathis in the 1930s, but at that time ruthenium was not as catalytically active as iron. It was not until 1972 that Aika et al. found that the catalyst with ruthenium as active component, potassium metal as promoter and activated carbon as carrier had high activity for ammonia synthesis, and its activation energy reached 69.1 kJ/mol, thus creating a precedent in the research of ruthenium catalysts. Subsequently, scholars from various countries invested a lot of energy in developing ruthenium-based catalysts to replace the traditional iron-based catalysts.