We report the promoting effect of graphitic carbon nitride in Cu-catalyzed N-arylation. The abundance of pyridinic coordination sites in this host permits the adsorption of copper iodide from the reaction medium. The key to achieving high activity is to confine active Cu species on the surface, which is accomplished by introducing atomically-dispersed metal dopants to block diffusion into the bulk. The alternative route of incorporating metal during the synthesis of graphitic carbon nitride is ineffective as Cu is thermodynamically more stable in inactive subsurface positions. A combination of X-ray absorption, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, density functional theory, and Kinetic Monte Carlo simulations is employed to determine the location and associated geometry as well as the electronic structure of metal centers. N-arylation activity correlates to the surface coverage by copper, which varies during the reaction due to an interplay between site formation via adsorption from the reaction medium and deactivation by diffusion into the bulk of the material, and is highest when an Fe dopant is used that hinders movement through the lattice.