X-ray photoelectron spectroscopy (XPS) reveals the presence of C, N, and Co elements, and allows for the surface chemical states of CN and Co-CN to be analyzed (Figure , Supplementary Information). The high-resolution C 1s XPS spectrum (Table , Supplementary Information) reveals a component at 288.3 eV, which is characteristic of the sp 2 -hybridized carbon (N-C=N) of the graphitic carbon nitride support. Two additional components at 284.7 eV and 286.2 eV are related to the adventitious carbon present on the sample holder, and its corresponding oxygen species, respectively. The N 1s region can be deconvoluted into three main peaks at 398.7 eV, 399.9 eV, and 401.15 eV, corresponding to pyridinic N (C=N-C), amino N ((C)2-NH), and graphitic N((C)3-N) species, respectively (Table , Supplementary Information). The analysis of the N 1s region gives information about the CN structure and points to a high polymerization degree of a condensed heptazine-based structure. A C/N molar ratio of 0.754 was estimated based on the XPS results, which is close to the theoretical value of (0.75). The Co 2p XPS region presents two peaks at 781.0 eV and 796.8 eV with their corresponding satellites, which are attributed to the Co 2p3/2 and Co 2p1/2 components of Co 2+ , respectively. Consistent with XPS results, the X-ray absorption near-edge structure (XANES) of Co-CN indicates that Co is retained on the CN matrix as Co 2+ species (Figure ). The phase-uncorrected Fourier-transform (FT) extended X-ray absorption fine structure (EXAFS) spectrum only shows one peak at 1.5 Å that can be ascribed to Co-N bonds (Figure ). We note that contributions from Co-O species, such as residual hydration of the Co sites, could not be excluded. The absence of peaks in the region between 2. EPR peak associated with ferromagnetic Co clusters or nanoparticles was not observed (Figure ). The EPR signal at g = 2.003 is ascribed to localised unpaired electrons hosted in the p orbital belonging to a sp 2 hybridised C atom (Figure ), while the absence of a EPR signal from single Co 2+ sites, even at 77 K, could be attributed to a short relaxation time that is in agreement with previous reports. The optical properties of Co-CN were investigated using diffuse reflectance UV-Vis ((DR)UV-Vis) spectroscopy (Figure and Figure , Supplementary Information). The material exhibits absorption in the visible region, with a calculated energy gap of 2.78 eV (Figure , Supplementary Information). Beyond the semi-hydrogenation of acetylene, we also studied the photocatalyst's versatility demonstrating that it is competent for the conversion of phenylacetylene to styrene (Figure , Supplementary Information). This indicates the applicability of Co-CN to other hydrogenation reactions.