Photocatalytic mineralization of gaseous formaldehyde over silver-doped metal oxide/MOF heterostructure with a mediator-assisted hybridized step (MAH-S)-scheme charge configuration

Abstract

A novel mediator-assisted hybridized step (MAH-S)-scheme heterojunction has been developed by combining silver-doped tungsten trioxide (Ag-WO₃, denoted as AW) with an amine-functionalized metal-organic framework (NH₂-MIL-125, labeled as M). This heterostructure, coded as AWM-x (where 'x' indicates the AW/M weight ratios), serves as a photocatalytic filter within a portable air purifier to facilitate the synergistic adsorption-photocatalytic oxidation of formaldehyde (FA) in dry and humidified indoor air settings. AWM-10, with a prolonged charge-carrier lifetime (4.86 ns), a high photocurrent density (177.7 μA), and a low charge-transfer resistance (206.6 Ω), achieves 100 % removal of 5 ppm FA, with a clean air delivery rate of 11.48 L·min⁻¹ and an apparent quantum yield of 0.238 % (1 W UV light and 160 L·min⁻¹ flow). In situ diffuse reflectance infrared Fourier-transform spectroscopy and gas chromatography confirm the complete mineralization of FA (HCHO) to CO2 via multiple intermediates (e.g., CH2O2, HCOO-, and CO32-). The AWM-10 functions as an MAH-S scheme catalyst primarily due to the two-fold role of its Ag mediator. The Ag first acts as a conductive bridge that facilitates the rapid recombination of low-energy charges for efficient directional charge separation (relative to a conventional S-scheme). Second, it serves as a plasmonic photosensitizer to boost photocatalysis by generating high-energy "hot electrons" and extending light absorption.