Direct electrochemical reduction of CO₂ to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO₂-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO₂ to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M-N _x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe-N-C and especially Ni-N-C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M-N _x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M-N _x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO₂ reduction catalysts.