Sluggish charge kinetics and high NO₂ selectivity are still the bottleneck issues that restrict the efficient NOₓ photo-oxidation and removal using polymeric carbon nitride (PCN). Herein, approximately 2 times higher NOₓ-removal performance and almost complete NO₂-inhibition can be synchronously achieved by maximizing the versatility of the coupled hydroxyl groups. The experimental and computational results reveal the roles of different types of coupled-hydroxyl groups in PCN in the enhancement of photocatalytic activity in terms of NOₓ removal and product selectivity. The spontaneous polarization effect caused by the introduction of the structural hydroxyl into the C₆N₇ framework accounts for the improved separation efficiency of the carriers. The mediated role of product selectivity was attributed to the process of surface hydroxyl grafting. Different conversion pathways for NOₓ photo-oxidation over pristine and hydroxylated-PCN were proposed. These findings provide molecular-level insight into the multiple roles of the coupled hydroxyls and can help design efficient NOₓ-removal photocatalysts. Copyright © 2023 Elsevier B.V. All rights reserved.