Synergistic integration of Bi metal and phosphate defects on hexagonal and monoclinic BiPO4: Enhanced photocatalysis and reaction mechanism

Abstract

Bi metal deposited hexagonal BiPO4 with the exposure of {102} facet (Bi-HBPO-102) and Bi metal deposited monoclinic BiPO4 with the exposure of {120} facet (Bi-MBPO-120) were prepared by chemical deposition method and solvothermal approach, respectively. The as-prepared catalysts presented more efficient photocatalytic activity of NOx removal than pure BiPO4 (2.0% for BiPO4, 51.4% for Bi-HBPO-102 and 36.2% for Bi-MBPO-120) under visible light irradiation, which can be attributed to the synergistic effects endowed by the phosphate defect, the surface plasmon resonance (SPR) effect of Bi metal and the facet effect. The existence of phosphate defect was confirmed by the XPS and solid state EPR technique. The DFT calculation revealed the position of phosphate and the phosphate defect induced the formation of an intermediate level within the forbidden band to allow efficient charge transfer from valence band to conduction band. Moreover, the Bi metal would act as the electron contributor and electron conductor which facilitated the charge carriers separation. Therefore, a new charge transfer pathway can be certified on account of the fact that the covalent loop was evidently generated both at the interface and along with the path of [Bi2O2]2+ → Bi metal → PO43− on the Bi@BiPO4. More importantly, the Bi-HBPO-102 with exposure of {102} facet exhibited higher photocatalytic activity than the Bi-MBPO-120 with exposed {120} facet. The {102} facet with the stronger distorted PO4 tetrahedron and the lower potential energy barrier (-17.5eV) contributed to the contacted interface with the more efficient charge transfer, which promoted the generation of active radicals on {102} facet. Additionally, for Bi-HBPO-102, the reaction intermediate NO+ can be observed with in situ DRIFTS, which facilitated the activation of NO via the formation of NO+ to promote the oxidation of NO into final products. Herein, a new strategy for tailoring the charge transfer pathway was developed to enhance the photocatalytic performance and a new photocatalytic reaction mechanism for photocatalytic NOx removal was proposed. This work could provide new insights into the modification of photocatalysts and mechanistic understanding of the gas-phase photocatalytic reaction mechanism.