Mechanistic Probes of Zeolitic Imidazolate Framework for Photocatalytic Application

In this work, we report a zeolitic imidazolate framework (ZIF-67) with remarkable activity in the hydrogen evolution reaction (HER): 40 500 mu mol H-2/g of metal organic framework (MOF). This is, to the best of our knowledge, the highest activity achieved by any MOF system. This result necessitated assessment of the atomic-scale mechanistic function of ZIF-67 in HER, using advanced spectroscopy techniques, including time-resolved optical (OTA) and in situ X-ray-absorption (XAS) spectroscopy. Through the correlation of the OTA results with the catalytic performance, we demonstrated that the electron transfer (ET) pathway, rather than the energy transfer (ENT) pathway, between the photosensitizer and ZIF-67 is the key factor that controls the efficiency of HER activity, because HER activity that undergoes the ET pathway is 3 orders of magnitude higher than that which undergoes the ENT process. Using in situ XAS, we unraveled the spectral features for key intermediate species, which are likely responsible for the rate-determining process under turnover conditions. This work represents an original approach to study porous ZIF materials at the molecular level using advanced spectroscopic techniques, providing unprecedented insights into the photoactive nature of ZIF frameworks.