Photoreforming of plastic waste into valuable products and hydrogen using a high-entropy oxynitride with distorted atomic-scale structure

The persistent existence of plastic waste causes serious problems for the environment, directly and indirectly affecting the health of organisms and humans. Photoreforming is a nature-friendly method that only uses solar energy to convert plastic waste into green hydrogen (H₂) and valuable organic products. This study shows that a high-entropy oxynitride (HEON) photocatalyst, synthesized by the addition of nitrogen to a Ti-Zr-Hf-Nb-Ta-containing high-entropy oxide (HEO), exhibits a higher potential for the production of H₂, formic acid and acetic acid from polyethylene terephthalate (PET) photoreforming compared to the relevant HEO. Examination of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) by synchrotron light shows that, in addition to hybridization of 2p orbitals from oxygen and nitrogen, nitrogen atoms distort the structure and completely change the neighborhood of niobium and titanium (a main contributor to the conduction band), expands the atomic bonds of zirconium and tantalum, contracts the atomic bonds of hafnium and decreases the binding energy of titanium, niobium and tantalum. These electronic structure changes lead to a narrower bandgap and diminished electron-hole recombination, enhancing the photoreforming performance. This study introduces HEONs with distorted atomic-bond structures as efficient low-bandgap and stable catalysts for transforming plastics into high-value organic chemicals and H₂ by photocatalysis.