Unveiling the Superior Activity of Fe-O Single-Atom Catalysts in Ascorbic Acid Oxidation: A Gibbs Free Energy Analysis

This study delves into the superior activity of Fe-O single-atom catalysts in the oxidation of ascorbic acid (AA) to dehydroascorbic acid (DHA). To rationalize this exceptional performance, Gibbs free energy diagrams were calculated for the oxidation of AA to DHA for four different metal single-atoms at U = 0 V and 1 V (Fig. 4d and Fig. S13). The reaction commences with the formation of adsorbed ascorbate by deprotonation of AA on the Fe single-atom catalyst, as illustrated in Fig. 4e. Subsequently, an electrophilic adsorption process occurs between the metal cation, followed by HA- anion deprotonation and electron loss. Finally, a process of metal and DHA desorption takes place. Calculations revealed a negative Gibbs free energy for step I for all metals, indicating that this process can be spontaneous. The Gibbs free energy diagram exhibited that Step II is the rate-determining step (RDS) for all metal oxide catalysts. The C, O, and H electron transfer in the enol-like structures of HA- and metal-adsorbed HA- were analyzed using the Bader charge calculation method (Figs. S14-15). Our findings indicate that the charge of H in the adsorbed HA- is higher than that in the initial HA-, suggesting a weaker polarization of -OH and shorter -OH bond lengths. Notably, the amount of charge transfer and bond length change of the adsorbed HA- on the FeOδ single-atom catalyst is more pronounced compared to the other three metal oxide single-atom catalysts. Furthermore, it was demonstrated that the Fe catalyst could promote the dissociation of the hydroxyl group on the other side of the enol structure. These results suggest that the strong adsorption of metal cations with HA- anions leads to a weaker p-π conjugation effect of C-O in the enol structure, resulting in a weaker polarization of the hydroxyl group on the other side. Consequently, the -OH group in the enol structure of the adsorbed HA- is more difficult to hydrolyze than that of the initial HA-, making this step the rate-determining step of the overall reaction. In summary, the superior activity of Fe-O single-atom catalysts in the oxidation of ascorbic acid to dehydroascorbic acid can be attributed to the strong adsorption of metal cations with HA- anions, leading to a weaker p-π conjugation effect of C-O in the enol structure and a weaker polarization of the hydroxyl group on the other side. This makes the -OH group in the enol structure of the adsorbed HA- more difficult to hydrolyze, making the electron transfer process the rate-determining step of the overall reaction. These findings provide valuable insights into the design and development of highly efficient single-atom catalysts for various catalytic reactions.