Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Herein, coordination of copper cations within a crystalline carbon nitride, i.e., poly(triazine imide) or CNx, was found to yield two ordered structures for Cu-CNx wherein one or two Cu(I) cations coordinate to its intralayer N-triazine groups. The crystallites electrophoretically deposit from aqueous particle suspensions and yield current densities of ~10 to 50 mA/cm2 with a concomitant and increasing reduction of CO2 and H2O. Reduction of CO2 increases for smaller particles as mechanistic calculations reveal its catalysis mediated by two intralayer Cu atoms.