<p>Ammonia (NH₃) is the second most produced chemical globally and is one of the largest chemical industries by volume. However, although being well-established, it requires high temperature and pressure conditions, which result in a large energy consumption and carbon dioxide emissions. To address this issue, the development of novel catalysts with high activity under mild reaction conditions has become a prominent area of research. Although the effect of Cs as the promoter for Fe and Ru catalysts has been well described, the effect of Cs doping on the Co catalyst performance in ammonia synthesis has not been well studied yet. Here, we studied the effect of Cs doping on the physicochemical properties and activity of the Co catalysts. It was found that caesium doping was detrimental to the catalytic activity. The ammonia synthesis rate decreased with the increasing amount of caesium added. The Cs species were not thermally stable and evaporated from the surface of the catalysts during the reductive activation. In addition, the Cs doping contributed to the catalyst sintering, resulting in the irreversible loss of active sites. This was the primary reason for the observed poor activity of the Cs-doped Co catalysts in ammonia synthesis.</p>