Photocatalytic Synthesis of Urea (CO2/N2/H2O) on Coal-Based Carbon Nanotubes with the Fe-Core-Supported Ti3+-TiO2 Composite Catalyst

In this study, carbon nanotubes with Fe cores (Fe-CNTs) were successfully prepared by the vapor deposition method, which used raw coal as a carbon source. The brookite crystal Ti3+-doped TiO2 (Ti3+-TiO2) was obtained via thermal reduction with the reductant NaBH4. Then, it was loaded on Fe-CNTs to obtain Ti3+-TiO2/Fe-CNTs. While characterizing the structure of these prepared catalysts, the photocatalytic co-reduction of N2/CO2 was performed for the synthesis of urea (CO(NH2)2) in H2O. The findings were as follows: (i) the reductant NaBH4 could lead to the formation of the brookite crystal TiO2; (ii) the main product of the photocatalytic co-reduction N2/CO2 in H2O is CO(NH2)2, and it also includes the gas products (H2, CO, and O2) and other liquid products (NH4+, NO2–, NO3–, C3H6O2, and C4H8O2); (iii) the performance of photocatalytic co-reduction of N2/CO2 to CO(NH2)2 in H2O was related to the arrangement of Ti3+ sites and oxygen vacancies on the surface of Ti3+-TiO2. The Ti3+ sites and oxygen vacancies act as the active centers for N2 and CO2 molecules, respectively. The adsorption and activation converted the N2 and CO2 molecules into six-membered cyclic intermediates, which further transformed into the CO(NH2)2 product. In addition, the CO(NH2)2 yield of the composite photocatalyst can reach 710.1 μmol/(L g) in a 4 h reaction, with 4.6 times as the single Ti3+-TiO2, which shows that the Fe-CNT support is beneficial for the photocatalysis.