Water Dissociation Mechanism in MOF-74

Water dissociation represents one of the most important reactions in catalysis, essential to surface and nano sciences. Combining in-situ IR spectroscopy and first-principles calculations, we demonstrate, for the first time, that water starts to dissociate at the metal centers of MOF-74 at temperatures as low as 150~$^{\circ}$C. The fingerprint of this reaction is a sharp band in the IR spectrum at 970~cm$^{-1}$ when D$_2$O is introduced into the MOF. Surprisingly, this fingerprint is not detected by experiments with H$_2$O. To explain this peculiar finding, we perform DFT simulations of the reaction, utilizing vdW-DF to capture the important van der Waals interactions. Our calculations show that, once the D$_2$O molecule is adsorbed at the metal center, the D atom is transferred to the oxygens of the linker (phenolate group), producing the notable O-D absorption band at 970~cm$^{-1}$, while the OD remains at the open metal sites. Even though we find that H$_2$O undergoes an analogous dissociation reaction, the corresponding O-H mode is strongly coupled to MOF vibrations and cannot easily be detected by experiments. Overall, this work elucidates water interactions with cation-exposed surfaces and aids in the development of more efficient catalysts for water dissociation.