Ammonia-treated activated carbon has been studied as a support of Ru-Ba catalyst for ammonia synthesis. It is shown that the introduction of nitrogen leads to a decrease of ammonia synthesis activity for the catalysts with a low Ba/Ru molar ratio, while no significant changes are obtained for the catalysts with a high Ba/Ru molar ratio, confirming that electronegative impurities suppress the activity in ammonia synthesis and consume part of the promoters.