Photocatalytic Formamide Synthesis from Plastic Waste and Ammonia via C─N Bond Construction Under Mild Conditions

Abstract

Photocatalytic upcycling of polyethylene terephthalate (PET) waste into value-added chemicals represents a sustainable route for plastic valorization. However, previous studies have solely focused on generating oxygenated compounds through intramolecular C─O bond transformations. Achieving heteroatom-containing important chemicals via intermolecular coupling reactions is still challenging. Herein, we report the first example of the direct photocatalytic C─N coupling of PET-derived ethylene glycol (EG) with ammonia to produce formamide (FA) and H₂ over a Pt₁Au/TiO₂ catalyst. Pt₁Au/TiO₂ delivers a FA yield rate of ∼7.1 mmol g_cat⁻¹ h⁻¹, as well as H₂ evolution (∼22.8 mmol g_cat⁻¹ h⁻¹). Photoelectrochemical analyses, including time-resolved transient absorption spectroscopy (TAS) and in situ X-ray photoelectron spectroscopy (XPS), reveal that monatomic Pt sites can effectively trap electrons, while Au nanoparticles act as hole sinks, thereby synergistically enhancing charge separation efficiency. Mechanistic investigations via in situ electron paramagnetic resonance (EPR) and Fourier transform infrared spectroscopy (FTIR) elucidate the reaction pathway: EG undergoes hole-mediated oxidation to yield an aldehyde intermediate, which is subsequently attacked nucleophilically by hole-derived •NH₂, inducing sequential C─C bond cleavage and C─N bond formation. This work provides critical mechanistic insights for the sustainable synthesis of high-value heteroatom-containing chemicals from plastic waste.