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 was 13.4 and 2.2 times higher than those of g-C 3 N 4 (17.7 μmol g −1 ) and aggregated CoO x /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.