Understanding the Catalytic Site Structure in Metal Poly(Heptazine Imide) – Crystalline Carbon Nitride

Abstract

Metal poly(heptazine imide) (M-PHI), a crystalline carbon nitride, has emerged as a highly promising platform for surface and single-atom catalysis (SAC). However, the precise structure and coordination environment of its catalytically active sites remain unresolved. In this work, we elucidate the structure of a representative M-PHI catalytic site, nickel poly(heptazine imide) (Ni-PHI), through a synergistic experimental–computational approach integrating synthesis, characterization, and CO probe-assisted diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) with DFT-based static and molecular dynamics simulations combined with vibrational density-of-states analysis of CO bond stretching (CO-VDOS). By systematically assessing different nickel loadings, examining the formation of nickel single atoms and nickel-based nanoparticles, comparing static and dynamic DFT energetics, and correlating the experimentally measured CO-DRIFTS spectra with the simulated CO-VDOS features, we identify highly probable Ni-PHI catalytic site structures that account for nickel single-atom and nickel-based nanoparticle cases. This work provides a general and efficient experimental–computational strategy for molecularly understanding the catalytic sites in metal poly(heptazine imide) - crystalline carbon nitrides.