Ultrahigh surface density of Co-N₂C single-atom-sites for boosting photocatalytic CO₂ reduction to methanol

Cobalt species as active sites for photocatalytic reduction of CO$_{2}$ to valuable products such as methanol have received increasing attention, however, it remains a huge challenge to achieve the high activity. Herein, a pyrolysis-induced-vaporization strategy was successfully employed to fabricate Co/g-C$_{3}$N$_{4}$ single-atom catalysts (Co/g-C$_{3}$N$_{4}$ SACs) with surface Co atom loading up to 24.6 wt%. Systematic investigation of Co/g-C$_{3}$N$_{4}$ SACs formation process disclosed that concentrated-H$_{2}$SO$_{4}$ exfoliation of g-C$_{3}$N$_{4}$ nanosheets (g-C$_{3}$N$_{4}$ NSs) as the substrate followed by a two-step calcination process is essential to achieve ultrahigh metal loading. It was found that the ultrahigh-density of Co single-atom sites were anchored on the g-C$_{3}$N$_{4}$ substrate surface and coordinated with two nitrogen and one carbon atoms (Co-N$_{2}$C). These single dispersed Co-N$_{2}$C sites on the g-C$_{3}$N$_{4}$ surface were found to act not only as electron gathering centers but also as the sites of CO$_{2}$ adsorption and activation, subsequently, boosting the photocatalytic methanol generation during light irradiation. As a result, the methanol formation rate at 4 h (941.9 μmol g$^{-1}$) over Co/g-C$_{3}$N$_{4}$-0.2 SAC with 24.6 wt% surface Co loading was13.4 and 2.2 times higher than those of g-C$_{3}$N$_{4}$ (17.7 μmol g$^{-1}$) and aggregated CoOx/g-C$_{3}$N$_{4}$-0.2 (423.9 μmol g$^{-1}$), respectively. Simultaneously, H$_{2}$ (18.9 μmol g$^{-1}$ h$^{-1}$), CO (2.9 μmol g$^{-1}$ h$^{-1}$), CH$_{4}$ (3.4 μmol g$^{-1}$ h$^{-1}$), C$_{2}$H$_{4}$ (1.1 μmol g$^{-1}$ h$^{-1}$), C$_{3}$H$_{6}$ (1.4 μmol g$^{-1}$ h$^{-1}$), and CH$_{3}$OCH$_{3}$ (3.3 μmol g$^{-1}$ h$^{-1}$) products were detected over Co/g-C$_{3}$N$_{4}$-0.2 SAC. Besides, the photocatalytic activity of the Co/g-C$_{3}$N$_{4}$-0.2 SAC for the reduction of CO$_{2}$ to methanol was stable within 12-cycle experiments (~48 h). This work paves a strategy to boost the photoreduction CO$_{2}$ activity via loading ultrahigh surface density single atomically dispersed cobalt active sites.