Simultaneous valorization of nitrate and formaldehyde waste streams to hexamethylenetetramine via photo-electro-thermo-catalysis

To remediate nitrate (NO 3 – ) pollution and recover resources from formaldehyde wastewater, this work presents a waste valorization strategy integrating photoelectrocatalytic nitrate reduction (PEC NO 3 RR) to ammonia (NH 3 ) with downstream chemical upgrading. Herein, a hollow MoO 2 @CuFe 2 O 4 @MnFe 2 O 4 Z-scheme-Schottky dual heterojunction nanoreactor array is developed to spatially separate proton and nitrate adsorption sites. The MoO 2 @CuFe 2 O 4 @MnFe 2 O 4 improves photogenerated charge separation and retaining robust redox capability, while the oxygen vacancy-rich MoO 2 layer exhibits favorable photothermal features to extend light harvesting, accelerate carrier migration and produce localized heating through localized surface plasmon resonance for reduced reaction energy barriers. Interfacial built-in electric field combined with spatial confinement jointly promote nitrate capture and enrichment. Fast hydrogen intercalation/deintercalation within MoO 2 create a dynamically *H-enriched surface to boost NO 3 RR-to-NH 3 conversion efficiency. This system resolves kinetic incompatibility between NO 3 – reduction and hydrogen supply, effectively suppressing NO 2 – accumulation and hydrogen evolution. It delivers an NH 3 yield rate of 5.96 mM h –1 cm –2 at –0.6 V vs. RHE with high selectivity and stability over 24 h. The in-situ generated NH 3 further transforms formaldehyde wastewater into high-value hexamethylenetetramine. This process enables simultaneous detoxification of two waste-streams and nitrogen/carbon feedstock upgradation, providing a closed-loop pathway for integrated pollution abatement and resource reclamation.