This study develops a biodegradable photocatalytic membrane with titanium dioxide (TiO2, p25) nanoparticles with a polylactic acid (PLA) support to remove volatile organic compounds (VOCs) from indoor air. It addresses two challenges simultaneously: (i) achieving effective gas-phase photocatalytic oxidation (PCO) of VOCs and (ii) enabling more sustainable fabrication of photocatalyst carriers. Conventional immobilization supports are often petroleum-based and processed with harsh solvents. Here, PLA-TiO2 membranes were prepared by phase inversion using Cyrene, a bio-based solvent that lowers environmental impact while yielding robust membranes. Scanning electron microscopy revealed a porous PLA structure with TiO2 well distributed on the surface, and optical measurements confirmed the expected UV response of the immobilized photocatalyst. To emulate a practical indoor-air scenario, ethanol (6800 ppm) was used as a model VOC in a quartz reactor irradiated by UV LEDs (with a total output power of 17 W at 365 nm). Time-resolved gas analysis with a quadrupole mass spectrometer – QMS, Hiden Analytical ExQ, Warrington, U.K. (now known as the QGA 2.0) – tracked the disappearance of ethanol and the appearance/consumption of intermediates (e.g., acetaldehyde, acetic acid, formaldehyde, formic acid) on the route to complete mineralization (CO2 + H2O). Prior to illumination, we established adsorption-desorption equilibrium across concentrations. The equilibrium data followed a Langmuir isotherm, indicating predominantly monolayer adsorption on a relatively homogeneous surface.
Figure 1. Hiden Analytical ExQ gas analysis system.
The results of a photocatalytic measurement with this membrane reveal a 14-min degradation time of 6800 ppm ethanol under UV light irradiation with the multiple-step mechanism. UV excites TiO2, promoting electrons to the CB and leaving holes in the VB. Electrons reduce O2 to superoxide (•O2-), holes oxidize adsorbed H2O to •OH. This active species can then break down adsorbed ethanol on the TiO2 surface. With O2 more abundant than H2O in gas phase, superoxide dominates. Notably, repeated cycles showed a counter-intuitive trend: apparent VOC removal accelerated over early runs. Microscopy and elemental analysis suggest that controlled exposure progressively unveils TiO2 active sites as a very thin top layer of PLA is oxidatively thinned, increasing accessible catalytic surface area. While this “self-unmasking” improves short-term activity, it also highlights a durability constraint for continuous use: photocatalytically assisted degradation of the biodegradable support gradually reduces mechanical robustness. Control experiments under UV light in the absence of TiO2 did not show comparable changes in the structure of PLA membranes, pointing to a catalysis-driven polymer alteration rather than simple photolysis. In practice, this means PLA-TiO2 is highly promising for sustainable, low-impact fabrication and effective VOC control, but lifetime engineering becomes essential. Future research should focus on stabilizing the polymer phase (e.g., mild cross-linking or blends with durable bio-polymers) to extend membrane life. Combining green processing with real-time QMS diagnostics has proven invaluable. Hiden’s QMS provided the sensitivity and temporal resolution needed to dissect mechanisms and guide materials choices.
Project Summary by: Hadis Mortazavi Milani, LumiLab, Department of Solid State Sciences, Ghent University, Ghent 9000, Belgium.
Paper Reference: Mortazavi Milani, H., Van Neste, B., Cosaert, E. and Poelman, D. (2024) ‘Assessing the Stability and Photocatalytic Efficiency of a Biodegradable PLA‐TiO2 Membrane for Air Purification.’ Advanced Sustainable Systems. Wiley, 9(1). DOI: 10.1002/adsu.202400594.
Hiden Product: ExQ (now known as the QGA 2.0).
