Steering oxidation pathways via Au-mediated transition from type-II to Z-scheme for high-efficiency NO deep oxidation with near-zero NO₂ emission

Fundamentally, the type-II and Z-scheme heterojunctions exhibit identical band alignments but diverge in the charge carriers transfer mechanisms. Here, we demonstrate that the Au-mediated heterojunction transition from type-II to Z-scheme dictates the subsequent photocatalytic NO reaction pathway to obtain excellent activity and selectivity. As proven by density functional theory (DFT) calculations, Kelvin probe force microscopy (KPFM), and in-situ X-ray photoelectron spectroscopy (XPS), the type-II to Z-scheme heterojunction transition is regulated by incorporating Au nanoparticles as electron bridges within in-situ fabricated NH₂-MIL-125/TiO₂ through controllable hydrolysis. This transition maintains robust redox potentials to generate more reactive active species through effectively separated charge carriers under the high-efficient built-in electric field. As a result, the Z-scheme (NH₂-MIL-125/Au/TiO₂) exhibits an impressive NO removal efficiency of 82.0%, surpassing the original NH₂-MIL-125 by 3.7 times and the type-II (NH₂-MIL-125/TiO₂) by 1.2 times, while shows a selectivity of almost 100% toward NO₂⁻/NO₃⁻. The in-situ Fourier transform infrared (FT-IR) and DFT reveal that, in comparison with the type-II favored NO⁺ intermediates, the Z-scheme favors NO⁻ intermediates with enhanced O₂/H₂O activation, enabling ideal Gibbs free energy for NO-to-NO₃⁻ conversion. This study achieves a metal-nanoparticle-mediated strategy for precisely engineering metal-organic framework (MOF)-based heterojunction architectures, which regulates the NO reaction pathways for efficient environmental purification.