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.