PW9V3 confined in MIL-101(Cr) via V–O–Cr interfacial linkages for enhanced photocatalytic nitrogen reduction

Photocatalytic nitrogen fixation can convert N 2 to NH 3 under mild conditions. However, the high bond energy of the N≡N bond, the tendency of photogenerated charge carriers to recombine, and a lack of active sites severely limit the reaction. POM@MOF composites, which combine tunable electronic structures with spatial confinement effects, have been used in research on photocatalytic nitrogen fixation. In this study, vanadium-substituted Keggin-type polyoxometalates (POMs) PW 12−X V X were synthesized to regulate the electronic structure of phosphotungstic acid and were subsequently confined within the cages of MIL-101(Cr) through an in-situ encapsulation strategy. The optimized PW 9 V 3 @MIL-101(Cr) photocatalyst achieved an NH 3 production rate of 38.31 μmol g -1 cat h −1 under simulated sunlight irradiation in air without sacrificial agent, approximately 5.8 times that of pristine MIL-101(Cr). In-situ XPS, EPR, and PL/TRPL measurements confirme that S-scheme heterojunction formed in this system, photoelectrochemical measurements suggest promoted interfacial charge redistribution and charge separation. IR spectroscopy, ground-state XPS analysis, and DFT calculation indicate that PW 9 V 3 preferentially interacts with MIL-101(Cr) through a V–O–Cr interfacial linkage, and the V site shows stronger N 2 adsorption, larger electron donation, and greater N–N bond elongation than W sites. Differential charge density and Bader charge analyses suggest electron transfer from the V-centered interfacial region to adsorbed N 2 , resulting in N≡N bond weakening. In situ DRIFTS measurements provide spectroscopic evidence for possible nitrogen-reduction intermediates and support the proposed photocatalytic N 2 reduction pathway. DFT calculations also indicate that the energy barrier for the first-step hydrogenation of the V site is low, whereas the energy barrier for the formation of N 2 H 4 at the V site is high, further confirming that the reaction follows a long-range reaction pathway.