Carbon supports with a low ratio of micropore volume to total pore volume can be prepared by KOH activation of thermally modified carbon. KOH treatment of carbons in nitrogen led to the creation of basic sites as well as acidic surface groups, whereas KOH treatment in air resulted in more acidic groups. Steam treatment quickly increased the surface area and pore volume, but did not create surface-reactive groups. The increase in surface areas, micropores, and surface functional groups of carbon supports led to a decrease of the particle sizes, which not only improved the ability of hydrogen to migrate from Ru metal to the remote promoter precursors, but also increased the amount of adsorbed hydrogen and nitrogen. The properties of carbon combined with the size of the Ru particles affected the ammonia-synthesis activity.