Metal−organic frameworks (MOFs) are in many ways ideal precursors to make electrochemical single atomic catalysts (E-SACs) with high loading of accessible active sites. However, the current pyrolysis requires a long high temperature process, which commonly accompanied with serious aggregation of metal atoms. The formation of metal aggregation reduces the density of single atom sites and disturbs the designed activity and selectivity of the SACs. Our group reports for the first time, a microwave heating strategy, to transform the designed metal sites in a MOF to single atomic catalytic sites without aggregation. Therefore, all the associated tedious post-treatment can be omitted. This concept was demonstrated with the fabrication of Cu-based electrochemical single atomic catalysts (Cu-E-SAC), which has been challenging to fabricate with traditional pyrolysis due to the fast diffusion of Cu. The achieved Cu-E-SAC synthesized from the Cu-porphyrin-based MOF precursor exhibited excellent catalytic activity and selectivity in reduction of molecular oxygen to water in both acidic and alkaline media. Very interestingly, it is found that the selectivity to water via a direct 4e pathway in acidic media is very different compared to Cu-E-SAC reported previously, possibly due to the unique electronic/geometric structures of the Cu-E-SAC. Furthermore, the catalytic activity and mechanism/pathways can be changed by simply changing the microwave irradiation time, indicating the local electronic and molecular structures around the SAC centers can be further fine-tuned. This capability of fine-tuning chemical environment around active centers will provide a new platform resembling those metal complex based catalysts for in-depth probing the catalytic mechanism and structure/function relationships of SACs. Combined with the merits of simplicity, rapidness to fabricate different E-SACs with molecular level tunability, this approach may serve to tackle some of the critical challenges of precious-metal-free single atom catalysts.

Transformation of the designed metal centers in a MOF to single atom catalytic sites via microwave heating

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.