Tri-phase photocatalysis for CO2 reduction and N2 fixation with efficient electron transfer on a hydrophilic surface of transition-metal-doped MIL-88A (Fe)

• Mn and Zn were doped in a spindle-shaped MIL-88A (Fe) via hydrothermal process. • As-prepared photocatalysts reduced CO 2 and fixed N 2 by water at ambient conditions. • Fe +2 sites facilitated photo-activity and were efficiently generated in the doped transition-metal MIL-88A. • The process was mass-transfer controlled, as suggested by contact angle measurement. Efficient transfer of charges at the interface of transition-metal and semiconductor can improve efficiency of photocatalyst. Herein, we examined photoactivity of Mn and Zn-doped MIL-88A for CO 2 reduction and N 2 fixation by water under UV–vis irradiation and ambient conditions. Gaseous-phase photo-reduction of CO 2 generated CO, H 2 , and CH 4 . Apparent quantum yield of Zn-MIL-88A was found to be 2.16-folds higher than that of pristine MIL-88A catalyst. Also, the highest NH 4 + generation rate ( 300 μ m o l g - 1 h - 1 ) was for Zn-MIL-88A catalyst. Dissecting the underlying mechanism discloses that electrons efficiently transferred from doped transition-metals to the Fe-O cluster of MIL-88A. The hydrophilic surface of the tested catalysts in a tri-phase photocatalyst system led the process to be controlled by mass-transport; which governed the product distribution and limited the kinetics of the process. Experimental results also unveil that the addition of transition metals improved stability, charge-separation, and efficiency of the resulting photocatalysts.