Porous Materials as a Platform for Highly Uniform Single-Atom Catalysts: Tuning the Electronic Structure for the Low-Temperature Oxidation of Carbon Monoxide

The selective oxidation of carbon monoxide (CO) remains a challenge in many research areas. Here we propose using porous hybrid organic-inorganic frameworks as tunable single-metal site catalysts which can be utilized for the oxidation reaction. Low-valent zinc ions were embedded on ZSM-5 zeolite and metal organic framework (MOF) supports to be used as nonprecious metal catalysts. Unlike the case with ionic compensation between the metal and zeolite, linker-functionalized MOFs exhibit a delocalized electron of the metal residing on both the organic linker and the metal atom. This electron delocalization plays a crucial role in the catalytic activity by assisting charge transfer of the metal site to break the O-O bond. By adding electron-donating groups to the linker, we found the activation energy to be dramatically decreased to similar to 4 kcal/mol compared with 18 kcal/mol for the zeolite. This is essential for oxidation at low temperature. Moreover, the donating groups result in a more stable Zn active site on the supports. We also show relationships between activation energy and (a) charge, (b) ionization energy of Zn atom, and (c) the highest occupied molecular orbital energy. Those parameters can therefore be related to how well the electron transfer from Zn to the 02 molecule proceeds and can be used to construct a model for activation energy prediction which is useful for catalyst design and screening. We believe that such a customizable platform of catalysts could open a new avenue for optimizing both activity and selectivity of catalysts for low-temperature preferential CO oxidation applications.