Conflicting Roles of Coordination Number on Catalytic Performance of Single-Atom Pt Catalysts

Tailoring the coordination number (CN) of metal atoms has been increasingly recognized as one of the strategies to enhance the catalytic performance of single-atom catalysts (SACs). We here present the single-atom Pt loaded onto a semiconductor SiC substrate (Pt1/SiC) with a high loading of up to 9.6 wt % and a precise control of its CN from 3 to 5. The CN tuning was enabled by binding organic linkers on the substrate surface and retaining the metal-linker bonds after photoreduction and mild thermal treatment from 80 to 160 °C. At a higher temperature, Pt became coordinated with additional oxygen atoms from the surface Si–OH groups and organic linkers. This resulted in the increase of the CN from 3 for Pt1 treated at 80 °C to 5 at 160 °C. The Pt1/SiCs with varying CNs effectively broke C–Br bonds in the model brominated compounds through both thermocatalysis using H2 and photocatalysis using H+ as the source for strongly reducing atomic hydrogen (Hatom). The thermocatalytic debromination kinetics increased with the decreasing CN. However, photocatalytic debromination kinetics were independent of the CN, contradictory to the prevalent understanding in literature. We attribute the differential CN effects on these two catalytic schemes to the differences in the pathways for the formation of Hatom as well as the rate-limiting step of the overall reaction pathways. Our study presents a unique and important example as to how the performance of SACs and the role of CN can significantly vary depending on the catalytic schemes.