One-pot photothermal upcycling of polylactic acid to hydrogen and pyruvic acid

ABSTRACT While biodegradable polymers such as polylactic acid (PLA) are widely used as alternatives to non-biodegradable polymers to address the plastic crisis, their biodegradation is difficult to control, and the process emits carbon. Conversion of PLA waste into value-added products via thermal and photocatalytic pathways is a promising solution. Herein, we describe a one-pot photothermal catalytic method that efficiently converts PLA into hydrogen and other valuable chemicals without requiring a concentrated KOH solution. Examining the catalytic properties of Pt/A-V-PCN (atomic-layered g-C3N4), Pt/TiO2, and Pt/CdS for the photothermal reforming of lactic acid (LA) indicated superior hydrogen production for all the employed catalysts owing to the promotion effects of external heat on the excitation and utilization of carriers, as characterized by electrochemical impedance spectroscopy Nyquist and transient photocurrent (I-t) tests. We found that the cadmium sulfide (CdS) possesses excellent selectivity for converting LA into pyruvic acid (PyA). Using in situ electrochemical electron spin resonance spectroscopy experiments and density functional theory computations, we found that the high PyA selectivity for LA oxidation on the CdS surface was attributed to the different affinities of the active sites for C- and O-adsorbed species, resulting in enhanced dehydrogenation and hindered C–C cleavage. Furthermore, Pt/CdS demonstrated increased reactivity and achieved 98.1% selectivity for high-value pyruvic acid in liquid products during the one-pot photothermal upcycling of commercial PLA plastics. Hence, this study provides a strategy for developing more efficient catalytic routes for upcycling PLA and other polyesters.