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

Abstract

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 Ti 3+ -doped TiO 2 (Ti 3+ -TiO 2 ) was obtained via thermal reduction with the reductant NaBH 4 . Then, it was loaded on Fe-CNTs to obtain Ti 3+ -TiO 2 /Fe-CNTs. While characterizing the structure of these prepared catalysts, the photocatalytic coreduction of N 2 /CO 2 was performed for the synthesis of urea (CO(NH 2)2 ) in H 2 O. The findings were as follows: (i) the reductant NaBH 4 could lead to the formation of the brookite crystal TiO 2 ; (ii) the main product of the photocatalytic coreduction N 2 /CO 2 in H 2 O is CO(NH 2 ) 2 , and it also includes the gas products (H 2 , CO, and O 2 ) and other liquid products (NH 4+ , NO 2 − , NO 3 − , C 3 H 6 O 2 , and C 4 H 8 O 2 ); (iii) the performance of photocatalytic co-reduction of N 2 /CO 2 to CO(NH 2 ) 2 in H 2 O was related to the arrangement of Ti 3+ sites and oxygen vacancies on the surface of Ti 3+ -TiO 2 . The Ti 3+ sites and oxygen vacancies act as the active centers for N 2 and CO 2 molecules, respectively. The adsorption and activation converted the N 2 and CO 2 molecules into six-membered cyclic intermediates, which further transformed into the CO(NH 2)2 product. In addition, the CO(NH 2 ) 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 Ti 3+ -TiO 2 , which shows that the Fe-CNT support is beneficial for the photocatalysis. KEYWORDS: coal, carbon nanotubes, Ti 3+ -doped TiO 2 , photocatalytic co-reduction N 2 /CO 2 , urea

Photocatalyzed Production of Urea as a Hydrogen–Storage Material by TiO2–Based Materials

“…In a recent report by H. Maimaiti et al [47], the photocatalyzed synthesis of urea was performed on carbon nanotubes (CNTs) with Fe/Ti 3+ -TiO 2 composite as catalyst (Ti 3+ -TiO 2 /Fe-CNTs). This research group has established that urea synthesis from the simultaneous reduction of N 2 and CO 2 in H 2 O is related to the arrangement of Ti 3+ sites and oxygen vacancies on the catalyst surface.…”

“…The Ti 3+ sites act as the active center of N 2 and oxygen vacancies acts as the active center for CO 2 on the Ti 3+ -TiO 2 surface. H. Maimaiti et al [47] state that urea formation is achieved by adsorption and activation, converting the N 2 and CO 2 molecules into a cyclic intermediate that is transformed into two urea molecules. This research reports a urea yield of 710.1 µmol L −1 g −1 after 4 h of reaction using a 300 W high-pressure mercury lamp as the light source.…”

“…Although the trend indicates the use of photocatalysts fixed on a surface which enables better control over the reaction solution, and then the separation of the catalyst from the reaction solution, H. Maimaiti et al [47] propose using a catalyst supported on CNTs to improve the dispersion in water. However, the catalysts used have an Fe core, which allows their rapid and simple separation from the reaction solution using an external magnetic field.…”

“…In turn, CO2 is bent by oxygen Surface defects on TiO 2 are essential as active and adsorption sites for CO 2 and nitrogen compounds. H. Maimaiti et al [47] suggest that Ti 3+ sites and oxygen vacancies act as the active centers for N 2 and CO 2 molecules, respectively. Then, if N 2 and CO 2 are on the same plane, the photocatalysis may be activated simultaneously to form a C-N ring intermediate in situ and subsequently to form urea.…”

N-Formylation of Amines with Carbon Dioxide and Hydrogen Catalyzed by Ionic Liquid-Assisted Ru Complexes

“…However, CO 2 is also a nontoxic, abundant, cheap, and renewable C1 resource. , The transformation of CO 2 into various compounds such as urea, salicylic acid, carbonates, polymers, alcohols, and formic acid has been extensively studied. − The main obstacles for the utilization of CO 2 are the low energy content and the high chemical stability of CO 2 . Carbon capture, utilization, and storage (CCUS) is considered to be a technically and economically feasible method to reduce greenhouse gas emissions .…”