Tunability and Scalability of Single-Atom Catalysts Based on Carbon Nitride

Carbon nitrides are promising hosts for single-atom catalysts (SACs) based on small amounts of precious metals dispersed as isolated atoms, presenting tantalizing opportunities to reduce the cost and for higher efficiency as compared to traditional nanoparticle-based formulations. Heteroatom doping represents a straightforward method to tailor the (opto-)electronic properties of carbon nitrides and could, therefore, extend the tunability of SACs. This paper compares the impact of modifying graphitic carbon nitride with phosphorus, boron, sulfur, and fluorine on the interaction with palladium. As an aliovalent dopant, phosphorus is found to appreciably increase the electron density of carbon nitride, thereby lowering the oxidation state of the metal. The stability of single atoms depends on the dopant (D) content, with nanoparticle formation observed at higher concentrations (e.g., molar D:Pd ratio >1), which is linked to a weaker metal-host interaction. Evaluation in the three-phase semihydrogenation of 2-methyl-3-butyn-2-ol, an important building block in fine-chemical manufacturing, evidences an enhanced reaction rate (up to 5.4 times) upon doping with phosphorus that is governed by the P/Pd molar ratio. The selectivity to the desired product approaches 100%, outperforming the commercial Lindlar-type Pd-Pb/CaCO3 catalyst (78%). Looking toward the future implementation, scalability aspects of SACs based on carbon nitrides, such as the choice of precursor, synthesis conditions, and the trade-off between the host surface area and yield, are addressed. Extrapolation of the superior catalytic properties and robust stability are confirmed in a continuous-flow reactor. These findings identify key steps in the design of single-atom catalysts based on carbon nitride for large-scale application.