The soluble nature of traditional catalytic systems poses challenges for metal/ligand reuse and recovery. Single-atom heterogeneous catalysts (SACs) are considered to be more sustainable alternatives, bridging homogeneous and heterogeneous strategies. Unfortunately, understanding structure-function relationships and dynamics is an unmet challenge. By coupling droplet-based microfluidics and in-situ X-ray absorption spectroscopy (XAS), Thomas Moragues and colleagues from the deMello and Pérez-Ramírez groups at ETH Zürich have recently unlocked valuable insights into mechanisms of cross-coupling reactions over heterogeneous catalysts, focusing on the use of palladium SACs in Suzuki-Miyaura cross-coupling reactions.
Using XAS at both K and L edges, the team explored the electronic and coordination surroundings of palladium atoms within a palladium SAC during reaction. Interestingly, they found that palladium atoms remain in an oxidized state close to Pd(II) throughout the reaction, which goes against the expected cycle between Pd(0) and Pd(II) seen in homogeneous systems. This discovery highlights the distinctive behavior of SACs and how they adaptively coordinate with the carbon nitride host in response to their surroundings. Furthermore, the authors explored how ligands interact with palladium atoms, influencing their electronic structure and coordination environment. Specifically, at the K edge, XAS provides insights into the core electron transitions of the palladium atoms, shedding light on their interactions with ligands. Conversely, at the L edge, information regarding the bonding between palladium atoms and ligands can be extracted. Such a detailed examination sheds light on how ligands stabilize the palladium atoms and impact the catalytic activity of the palladium SAC. Elucidating these mechanisms advances the design and optimization of single-atom heterogeneous catalysts for a variety of organic transformations.
Written by Sarah Duclos Ivetich
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