Dehydrated UiO-66(SH)(2): The Zr-O Cluster and Its Photocatalytic Role Mimicking the Biological Nitrogen Fixation

This work reports the dehydrated Zr-based MOF UiO-66(SH)(2) as a visible-light-driven photocatalyst to mimic the biological N-2 fixation process. The N-15(2) and other control experiments demonstrated that the new photocatalyst is highly efficient in converting N-2 to ammonia. In-situ TGA, XPS, and EXAFS as well as first-principles simulations were used to demonstrate the role of the thermal treatment and the changes of the local structures around Zr due to the dehydration. It was shown that the dehydration opened a gate for the entry of N-2 molecules into the [Zr6O6] cluster where the strong N equivalent to N bond was broken stepwise by mu-N-Zr type interactions driven by the photoelectrons aided by the protonation. This mechanism was discussed in comparison with the Lowe-Thorneley mechanism proposed for the MoFe nitrogenase, and with emphasis on the [Zr6O6] cluster effect and the leading role of photoelectrons over the protonation. The results shed new light on understanding the catalytic mechanism of biological N-2 fixation and open a new way to fix N-2 under mild conditions.