Preparation of MDEA derived nitrogen-doped CQDs nanofluids and their CO2 capturing performance and mechanism investigation

Nanofluids acting as the third-generation absorbents are raising concern. However, the complex preparation method, high material cost, and instability hinder their further promotion. Herein, N-methyldiethanolamine (MDEA) as a classic organic amine solution was first used to prepare nitrogen-doped carbon quantum dots (CQDs) by a mild method, and the effective CQDs yield was determined by TEM, FT-IR, XPS, and fluorescence spectrometry. Generated nitrogen-doped CQDs have abundant surface function groups, a narrow size distribution (1-10 nm), and stable fluorescent properties. Moreover, CQDs nanofluids stability was comprehensively eval-uated by Turbiscan stability analyzer, Zeta potential analyzer, and solid content analysis. The results indicated a high stability of CQDs in the amine solution. The CO2 capture performance of CQDs nanofluids was evaluated by a bubble reactor, and the maximum absorption enhancement index reached 1.27 in the mixed amine solution. The desorption process was also improved and nearly 6 % heat duty was saved. Finally, the enhancement mechanism of CQDs nanofluids was discussed. In contrast to other studies focusing on the mass transfer effect of nanofluids on macroscopic kinetics, the microscopic interaction effect between CQDs and CO2 was discussed by in-situ FT-IR and XPS. The results prove that apart from the physical adsorption effect, a chemical reaction also exists to capture CO2 by the nitrogen-doped structure and basic groups on CQDs surface, which endow the unique enhancement effect for CO2 capture system by CQDs. In general, the advantageous performance ensured the potential of CQDs nanofluids as advanced absorbents to achieve a low manufacturing cost and scale application.