X-Ray Absorption Spectroscopy Study of Ni-Hexahydroxytriphenylene (HHTP) MOF Catalyst for Liquid Organic Hydrogen Carrier (LOHC) Systems

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The shift to green energy highlights Liquid Organic Hydrogen Carrier (LOHC) systems for hydrogen storage, leveraging existing fuel infrastructure. Conventional storage methods, like compression or cryogenics, demand high pressure and low temperatures, making them energy-intensive. LOHC systems chemically bind hydrogen to liquid organic carriers, enabling storage and release under ambient conditions. Electrocatalytic pathways facilitate hydrogenation and dehydrogenation under milder conditions than traditional catalysts. Metal-organic frameworks (MOFs) are ideal electrocatalysts due to their high surface area, tunable metal sites, and controlled environments, enhancing reaction selectivity and efficiency. The Ni HHTP MOF catalyst ( Ni hexahydroxytriphenylene Metal-Organic Framework) shows significant promise for improving the efficiency, stability, and selectivity in Liquid Organic Hydrogen Carrier applications. This catalyst can facilitate hydrogen release with higher efficiency, and to demonstrate its functionality, we use isopropanol (IPA) oxidation to acetone as model reactions, as IPA is a particularly attractive LOHC for specific mobile device applications. A deeper understanding of catalytically active species at low overpotential is essential to optimize processes further. The X-ray Absorption Spectroscopy (XAS) investigation is a beneficial technique for this task, where metal K edges and L edges help determine the electronic structure during heterogeneous catalysis and catalyst stability. Further, the Operando X-ray absorption spectroscopy technique can provide information on real-time changes in the electronic structure and the local environment during catalysis, which helps to interpret the mechanism accurately. Analyzing the Ni K edge of the pristine catalyst provides insightful ideas about the catalyst structure, such as the oxidation state of transition metal, coordination geometry, and the local environment around the Ni atom. Implementing metal L edge measurements after catalysis is beneficial because there are small changes that might not be sensitive to the K edges, as L edges are much more sensitive to the changes in d states, which is a good qualitative assessment of catalyst stability. Implementing operando XAS indicates shifts in Ni K edge spectra, indicating redox transitions during catalysis. Hence, operando XAS is crucial for identifying the active state during IPA oxidation and providing insights into catalytic pathways that stabilize the MOF.

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