■ Title: Molecular Photocatalysts in HX-Splitting Solar Energy Conversion
■ Speaker: Prof. Seung Jun Hwang (POSTECH)
■ Date and time: 11/19 (Tue) 16:00
■ Venue: Applied Engineering Building (W1) 1st Floor, Multimedia Lecture Room
■ Host : Prof. EunAe Cho
■ Abstract :
We have developed bimetallic catalysts that mediate the energy-storing photochemical splitting of HCl to H2 and Cl2. We have demonstrated HCl-splitting photocatalysis with two closely related bimetallic Rh2 catalysts (Rh2[I,III] and Rh2[II,II]). The critical chemical step with either catalyst is the energy-storing photoelimination of Cl2. Using transient absorption (TA) spectroscopy, a time-resolved photochemical experiment, halogen elimination chemistry from each of these catalysts was shown to proceed through a common reaction intermediate (μ-Cl). The TA spectrum alone, however, does not provide sufficient structural information to assign the identity of the critical photointermediate. In collaboration with scientists at ChemMatCARS, we have carried out low-temperature steady-state photo-crystallography experiments that have established that chloride-bridged intermediates are the structural lynchpins of halogen elimination reactions during catalysis. The illustrated photodifference map shows migration of Cl(3) towards a bridging position. The results of these studies led to the development of a second generation of photocatalysts, which are approximately 10 times more active for photochemical production of H2 than the first generation.
A major focus of my research efforts has been transitioning from second- and third-row transition metal complexes to inexpensive and earth-abundant first-row transition elements for photochemical HX splitting platforms. These efforts have hinged on developing strategies to overcome the inherently short excited state lifetimes of first-row transition metal based complexes in order to avoid the fast energy wasting internal conversion processes. High-yielding, endothermic Cl2 photoelimination chemistry from mononuclear Ni(III) complexes have been developed utilizing a new strategy based on secondary coordination sphere effects to suppress undesired rapid back reaction. Building upon the success of our previous photocrystallography experiments, we have applied steady-state photocrystallography techniques to the NiX3 complexes. Steady-state photocrystallography of Ni complex shows substantial elongation of apical Ni–Br(1) bond from 2.464(2) Å in the ground state to 3.70(4) Å in the photoinduced structure whereas the two basal Ni–Br bonds are crystallographically unchanged (2.3615(7) Å (dark), and 2.35(5) Å (photoinduced)) showing that the apical halide that is extruded in the photoelimination process.