Platinum/nitrogen-co-doped TiO₂ as photocatalyst and light-free catalytic adsorbent for gaseous formaldehyde

Platinum and nitrogen co-doped titanium dioxide (Pt/N-TiO₂, with 1 wt% Pt and an N/Ti molar ratio of 1) has been synthesized. This Pt/N co-doping strategy creates Schottky junctions, reduces the bandgap energy (3.25 to 2.12 eV), and introduces a new energy level (N 2p). The modified catalyst exhibits dual functionality, serving as both a photocatalyst under light irradiation (λ = 365 nm, 32 W) and a light-free catalytic adsorbent against gaseous formaldehyde (FA). The Pt/N-TiO₂ catalysts are immobilized on ceramic bead supports, placed in a tubular reactor system, and tested under controlled operating conditions, including FA concentrations (100-500 ppm), oxygen levels (0-21%), relative humidity (RH; 0-100%), and gas flow rates (100-500 mL min^-1). The Pt/N-TiO₂ achieves a photocatalytic oxidation efficiency of 94.2% (reaction rate of 9.24 μmol mg^-1 h^-1 and apparent quantum yield of 5.58%) against 200 ppm FA (100% RH). The catalyst's efficiency stems from a synergistic dual mechanism, as evidenced by molecular simulation using density functional theory. First, N doping enhances light absorption and extends the charge carrier lifetime, while the Pt as a co-catalyst promotes charge separation by acting as an electron sink. Second, the catalyst's ability to efficiently trap H₂O and O₂ molecules also contributes to the efficient mineralization of FA through the facile generation of reactive oxygen species. This dual functionality extends to dark conditions as a catalytic adsorbent, achieving a FA removal efficiency of 78.9% with a CO₂ yield of 57%. In-situ diffuse reflectance infrared Fourier transform spectroscopy analysis confirms this mechanism by identifying the generation of Pt-OH hydroxylation and ^•O₂^- radicals from H₂O vapor and O₂, respectively. Overall, this research provides a practical guideline for constructing an advanced VOC abatement platform.