The critical role of isolated Ti3+ sites in MIL-125 for photocatalytic nitrate reduction: performance enhancement and deactivation mechanism

Abstract

Photocatalytic reduction of nitrate (NO₃⁻) to ammonia (NH₃) represents a sustainable route for wastewater treatment and chemical production. Metal–organic frameworks (MOFs) such as MIL-125 are promising catalysts, yet their performance often decays during reaction. This study reveals that the aggregation state of Ti³⁺ sites in MIL-125 is a critical factor governing its photocatalytic efficiency. We demonstrate that the as-synthesized MIL-125, featuring isolated Ti³⁺ sites, achieves a high NH₃ production rate of 738.32 µmol g⁻¹ h⁻¹ under full-spectrum light without sacrificial agents. Comprehensive characterization and theoretical calculations indicate that these isolated Ti³⁺ sites elevate the material's conduction band and enhance NO₃⁻ adsorption, thereby lowering the energy barrier for the rate-determining step. However, prolonged photoreaction leads to linker deficiency, resulting in agglomeration of Ti³⁺ sites. This structural evolution causes a negative shift in the conduction band potential, impedes charge separation, and increases the reaction barrier, leading to a noticeable loss in activity over cycles. This work underscores the pivotal role of maintaining isolated metal sites for designing stable and efficient MOF-based photocatalysts for nitrogen cycling.