Synergistic Effect on the Photocatalytic CO2 Hydrogenation to Methanol Using Dual Co–Cu Single Atom Poly(heptazine imide): Influence of Pressure on Product Selectivity

Single metal atom-doped materials are gaining importance in photocatalysis since they offer potential maximum atom economy in a system. Herein, the preparation of poly(heptazine imide) (PHI) carbon nitride materials having Cu2+ or Co2+ single atom sites or dual Cu2+ and Co2+ sites is reported. The materials have been characterized by chemical analysis, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), while the single-atom nature of the metal dopants is supported by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS). The latter also shows a pronounced Cu2+–Co2+ coordination. The resulting three metal-PHI samples were then explored as photocatalysts for the photocatalytic activation of CO2 reduction at various pressures from ambient to 35 bar. A drastic change in the products from CO and CH4 under ambient pressure to formic acid and methanol at high pressure was observed, with formic acid being the predominant product at intermediate pressures. The products derived from CO2 were firmly confirmed by 13C isotopic labeling monitored by gas chromatography-mass spectrometry (GC-MS) (gas products) or 1H NMR spectroscopy (liquid products). A synergy between Cu2+ and Co2+ was observed in the photocatalytic experiments, the activity following the order Co–Cu/PHI > Cu/PHI > Co/PHI and interpreted as derived from the complementary action of each cation, Cu promoting H2 activation better than Co and Co promoting hydrogenation of adsorbed CO at lower energy than Cu. These findings show the potential of synergistic effects among different single atoms on a semiconducting support to enhance photocatalytic activity. In addition, the data through light on the importance of pressure to control the product distribution in the photocatalytic CO2 hydrogenation toward the more valuable liquid products.