Conjugated organic component-functionalized hourglass-type phosphomolybdates for visible-light photocatalytic Cr(VI) reduction in wide pH range

Utilizing efficient and durable photocatalysts for the removal of carcinogen Cr(VI) in wide pH water environment is of great significance for environment remediation. Herein, three conjugated organic component-functionalized hourglass-type phosphomolybdate hybrids with the formulas (H 2 DBQ)[Fe(H 2 O) 3 ] 2 {Fe[P 4 Mo 6 O 31 H 7 ] 2 }·7H 2 O ( 1 ), (H 2 DBQ) 2 (H 2 DBP)[Fe(H 2 O) 4 ]{Fe[P 4 Mo 6 O 31 H 7 ] 2 }·4H 2 O ( 2 ), (H 2 DBP) 2 {Fe[P 4 Mo 6 O 31 H 9 ] 2 }·10H 2 O ( 3 ) (DBQ = 2,2'-dimehyl-6,6'-biquinoline; DBP = 4,4'-diaminobiphenyl) were synthesized via hydrothermal method as photocatalysts for reducing Cr(VI) under visible light illumination. Structure analysis showed that hybrids 1–3 consist of hourglass-type [Fe(P 4 Mo 6 O 31 H 6 ) 2 ] 10− (abbr. Fe{P 4 Mo 6 } 2 ) polyanionic clusters functionalized with different proportional of conjugated organic components (DBQ and DBP) effectively regulated their visible-light absorption capabilities and electron structures, which could be employed to reflect the structure–activity relationship of materials. When used as catalysts for reducing Cr(VI), hybrids 1–3 displayed distinct photocatalytic activities with Cr(VI) removal efficiencies of 91%, 86% and 81% with only 10 W of white light being used as illumination, which are superior to that of pristine Fe{P 4 Mo 6 } 2 salt (62%). Moreover, hybrid 1 also maintained high Cr(VI) removal efficiencies above ~ 85% and high structural stabilities in solutions with wide pH ranges of 2.5–10.0. Mechanism investigation showed that the higher organic component conjugation levels could promote the optical absorption and facilitate the electron transfer between Fe{P 4 Mo 6 } 2 and organic components, thus resulting in effective Cr(VI) reduction photocatalytic efficacy. This work provides a viable way to design the photocatalysts based on polyoxometalates for removing wastewater pollutants. Graphical abstract