CO Adsorption by Carbon Quantum Dots/Metal Ferrites (M = Co, Ni, and Zn): Electrochemical and Theoretical Studies.

In this study, we investigated the adsorption of CO by carbon quantum dot-based ferrites (MFeO, M = Co, Ni, and Zn) in the context of industrial CO emissions and global warming. The ferrites have been characterized using various analytical techniques [X-ray powder diffraction, FTIR, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS)], showing cubic spinel for CoFeO, reverse cubic spinel for NiFeO, and typical spinel for ZnFeO. A TGA study revealed a significant weight loss around 740-780 °C, indicating structural change occurred with increasing temperature. SEM and TEM images displayed spherical particles with sizes ranging from 10 to 50 nm. XPS confirmed the presence of C, O, and Fe atoms with specific cations (Co, Ni, and Zn). Electrochemical impedance Nyquist diagrams suggest a linear relationship between Z″ (ohm) and Z' (ohm) at low frequencies, but the semicircular loop obtained was significantly increased at higher frequencies. This suggests that the charge transfer resistance ( ) at the electrode boundaries (interface) is much higher than at low frequencies, indicating the resistance per area was 1853 Ω cm for carbon paste electrodes (CPE)/CoFeO and it decreased to 1652 Ω cm for CPE/NiFeO and 1672 Ω cm for CPE/ZnFeO. However, improved electron transfer with lower resistance was seen due to the composite nature of the samples (CQDs@MFeO), revealing a lower resistance (1163 Ω cm) for CQD@MFeO-CO as compared to 1567 Ω cm for MFeO. Thus, the adsorption of CO was studied electrochemically, and interaction between ferrates with CO was enhanced by the presence of CQDs in the samples. This is consistent with the adsorption of CO with the samples as it follows the Langmuir pseudo-second-order kinetics ( = 4.9, = 121.93 for CQD@CoFeO, = 2.9, = 156.52 for CQD@NiFeO, and = 3.0, = 141.71 for CQD@ZnFeO), and the data show that the adsorption efficiency has been decreased by around 1.0% after 7-8 cycles. Lastly, density functional theory analysis demonstrated the interaction of CO on the surface of the ferrites, deforming the CO linearity, which leads to a subsequent C-O interaction to form carbonate.