Single-Atom Titanium on Mesoporous Nitrogen, Oxygen-Doped Carbon for Efficient Photo-thermal Catalytic CO2 Cycloaddition by a Radical Mechanism.

Developing efficient and earth-abundant catalysts for CO2 fixation to high value-added chemicals is meaningful but challenging. Styrene carbonate has great market value, but the cycloaddition of CO2 to styrene oxide is difficult due to the high steric hindrance and weak electron-withdrawing ability of the phenyl group. To utilize clean energy (such as optical energy) directly and effectively for CO2 value-added process, we introduce earth-abundant Ti single-atom into the mesoporous nitrogen, oxygen-doped carbon nanosheets (Ti-CNO) by a two-step method. The Ti-CNO exhibits excellent photothermal catalytic activities and stability for cycloaddition of CO2 and styrene oxide to styrene carbonate. Under light irradiation and ambient pressure, an optimal Ti-CNO produces styrene carbonate with a yield of 98.3 %, much higher than CN (27.1 %). In addition, it shows remarkable stability during 10 consecutive cycles. Its enhanced catalytic performance stems from the enhanced photothermal effect and improved Lewis acidic/basic sites exposed by the abundant mesopores. The experiments and theoretical simulations demonstrate the styrene oxide⋅+ and CO2⋅- radicals generated at the Lewis acidic (Tiδ+) and basic sites of Ti-CNO under light irradiation, respectively. This work furnishes a strategy for synthesizing advanced single-atom catalysts for photo-thermal synergistic CO2 fixation to high value products via a cycloaddition pathway.

BiOBr/Bi2S3 heterojunction with S-scheme structureand oxygen defects: In-situ construction and photocatalytic behavior for reduction of CO2 with H2O.

Constructing heterojunction has been considered as an efficient strategy to promote the separation and transfer of photogenerated carriers and improve redox ability of single photocatalyst. Herein, S-scheme BiOBr/Bi2S3 heterojunction with surface oxygen vacancies (OVs) was synthesized in situ by a facile hydrothermal method. The as-prepared photocatalyst show high activity for CO2 photoreduction with pure water. The yields of product CO and CH4 are as high as 100.8 and 8.5 µmol g-1h-1, which are 17.5 and 13.5 times higher than that of the pristine Bi2S3, and 2.3 and 4.7 times higher than that of the pristine BiOBr respectively. The excellent activity of the BiOBr/Bi2S3 heterojunction is attributed to both the S-scheme electron structure and the surface OVs of the component BiOBr. The S-scheme structure can enhance utilization of sunlight and improve the separation and transfer of photogenerated electron/hole pairs. The surface OVs of BiOBr can serve as active sites of CO2 and H2O in the photocatalytic process. This work provides some novel insights of S-scheme heterojunction with defects for photocatalytic CO2 reduction.

Oxygen vacancy-rich hierarchical BiOBr hollow microspheres with dramatic CO2 photoreduction activity.

Conversion of carbon dioxide into useful chemicals has attracted great attention. However, the significant bottlenecks facing in the field are the poor conversion efficiency of CO2 and low selectivity of products. Herein, hierarchical BiOBr hollow microspheres are fabricated by a solvothermal method using ethylene glycol (EG) as solvent in presence of polyvinyl pyrrolidone (PVP). The hollow BiOBr microspheres prepared at 120 °C exhibit the best performance for CO2 photoreduction. The evolution rates of product CO and CH4 are up to 88.1 µmol g-1h-1 and 5.8 µmol g-1h-1, which are 8.8 times and 5.8 times higher than that of plate-like BiOBr respectively. The hollow microspheres possess larger specific area and generate multiple reflections of light in the cavity, thus enhancing the utilization efficiency of light. The modulated electronic structure by oxygen vacancy (OVs) is beneficial to the transfer of photogenerated electrons and holes. Especially, the enriched charge density of BiOBr by OVs is conductive to the adsorption and activation of CO2, which could lower the overall activation energy barrier of CO2 photoreduction. In summary, the synergistic effect of the hollow structure with OVs plays a vital role in boosting the photoreduction of CO2 for BiOBr. This work provides a new opportunity for designing the high efficiency catalyst by morphology engineering with defects at the atomic level for CO2 photoreduction.