In this work, a step (S)-scheme heterojunction catalyst is synthesized by coupling acid-treated TiO2 (ATT) with graphitic carbon nitride nanosheet (g-C3N4: GCN) and used for photocatalytic mineralization of gaseous formaldehyde (FA). The ATT-GCN exhibits superior photodegradation performance against FA (e.g., removal efficiency of 86 %, the reaction rate of 70.2 nmole mg−1 min−1, the quantum yield of 5.1E-02 molec. photon-1, space–time yield of 5.1E-03 molec. photon-1 mg−1, clean air delivery rate of 5.2 L h−1, and specific clean air delivery rate of 515.4 L g-1 h−1) under optimum conditions (e.g., catalyst mass of 10 mg, 20 % relative humidity, and 21 % O2). The acid treatment of TiO2 proves to be an effective approach for improving the surface porosity of ATT with more abundant active sites for efficient catalytic reactions. Similarly, the S-scheme heterojunction displays superior redox potential (i.e., availability of free e- at conduction band of GCN with a reduction potential of −0.485 eV/NHE and free h+ at valance band of ATT with oxidation potential 2.79 eV/NHE). As such, the improved textural and optical properties of ATT-GCN offer outstanding adsorption and photocatalytic oxidation potential against FA molecules. In-situ Kelvin probe microscope analysis of ATT-GCN postulates the formation of a strong built-in electric field to improve the separation of charge carriers. Considerations on the degradation pathway and intermediate dynamics with the aid of in-situ DRIFTS analysis suggest that the oxidation and mineralization of FA molecules should proceed favorably under slightly wet conditions (e.g., 20 % relative humidity). The overall outcomes of this work are expected to help deliver new paths for the construction of advanced photocatalytic systems for upscaled applications in air quality management.