The superior mineralization potential of a graphitic carbon nitride/titanium dioxide composite and its application in the construction of a portable photocatalytic air purification system against gaseous formaldehyde

A portable air purification (AP) system has been built using filters coated with TiO₂/g-C₃N₄ (CNT-n) as an S-scheme heterojunction photocatalyst (n (in CNT-n) as the g-C₃N₄:TiO₂ molar ratio of 0.02 to 1). The AP (CNT-0.02) is identified as the best performer with the highest mineralization rate for formaldehyde (FA) by efficiently harnessing the intrinsic redox capabilities of each n-type semiconductor through the formation of an internal electric field at their interface. The superiority of AP (CNT-0.02) is validated in terms of clean air delivery rate [CADR] of 13.3 L min⁻¹ and quantum yield [QY] of 2.74 × 10⁻³ molecules photon⁻¹ against 5 ppm FA in dry air under low UV-A LED (1 W) light irradiation conditions. Its remarkable stability (e.g., over 5 reuse cycles) even at high FA levels (e.g., at 100 ppm) may also come from the synergistic adsorption-photocatalysis of FA molecules through the S-scheme charge transfer pathway to efficiently preserve their reactive intermediates. The noticeable reduction in performance is also observed with increasing moisture levels (e.g., in terms of CADR (L min⁻¹): 7.47 (at 30% relative humidity) vs. 13.3 (in dry air)) to reflect the combined effects of multiple processes (e.g., competitive adsorption, surface blocking, and alteration in reaction pathways), as supported by an in situ DRIFTS analysis. Nonetheless, the high FA conversion efficiency of CNT-0.02 under such humid conditions (CO₂ yield: 99.2%) may reflect the potential of H₂O vapor as co-reactant in stabilizing CH₂O₂/HCOO⁻ intermediates generated over the catalytic surface. These findings should help deliver a new path to upscale the design and process efficiency of S-scheme photocatalyst for practical applications.