Dimensional reduction enhances photocatalytic carbon dioxide reduction performance of metal-organic frameworks

Metal-organic frameworks (MOFs) have shown significant potential as photocatalysts. It has been widely assumed that all catalytic active sites within MOFs are functional in photocatalytic reactions but for a three-dimensional MOF, whether the internal catalytic active sites can effectively absorb light and actively contribute to photocatalytic reactions remains to be explored. In this context, we synthesized a two-dimensional nanosheet MOF (2D-MOF) and a three-dimensional bulk MOF (3D-MOF) composed of Zr₆ clusters and tetracarboxylic porphyrin (TCPP) by the approach described in the literature. Re(bpy)(CO)₃Cl (bpy = 2,2’-bipyridine), which has remarkable CO₂ photoreduction ability, was introduced to the two MOFs to create two new photocatalysts 2D-MOF-Re and 3D-MOF-Re, respectively. Photocatalytic CO₂ reduction experiments show that based on the equal number of catalytic active sites, the CO turnover number (TON) of 2D-MOF-Re reaches 27.8 in 6 h, which is 50 times that of 3D-MOF-Re. The result shows that certain catalytic active sites inside the bulk MOF are inactive due to the inability to absorb light. This study illuminates the potential of the dimensional reduction approach in the design of photocatalysts to exploit the capabilities fully.