Unveiling the Cerium(III)/(IV) Structures and Charge-Transfer Mechanism in Sulfuric Acid

Abstract

The Ce3+/Ce4+ redox couple has a charge transfer (CT) with extreme asymmetry and a large shift in redox potential depending on electrolyte composition. The redox potential shift and CT behavior are difficult to understand because neither the cerium structures nor the CT mechanism are well understood, limiting efforts to improve the Ce3+/Ce4+ redox kinetics in applications such as energy storage. Herein, we identify the Ce3+ and Ce4+ structures and CT mechanism in sulfuric acid via extended X-ray absorption fine structure spectroscopy (EXAFS), kinetic measurements, and density functional theory (DFT) calculations. We show EXAFS evidence that confirms that Ce3+ is coordinated by nine water molecules and suggests that Ce4+ is complexed by water and three bisulfates in sulfuric acid. Despite the change in complexation within the first coordination shell between Ce3+ and Ce4+, we show that the kinetics are independent of the electrode, suggesting outer-sphere electron-transfer behavior. We identify a two-step mechanism where Ce4+ exchanges the bisulfate anions with water in a chemical step followed by a rate-determining electron transfer step that follows Marcus theory (MT). This mechanism is consistent with all experimentally observed structural and kinetic data. The asymmetry of the Ce3+/Ce4+ CT and the observed shift in the redox potential with acid is explained by the addition of the chemical step in the CT mechanism. The fitted parameters from this rate law qualitatively agree with DFT-predicted free energies and the reorganization energy. The combination of a two-step mechanism with MT should be considered for other metal ion CT reactions whose kinetics have not been appropriately described.

A trifunctional N-doped activated carbon–ceria shell, derived from covalent porphyrin polymers for promoting Pt activity in fuel cell cathode performance

“…70 In addition, the cerium 4f-electron is available for charge transfer in redox activity or bonding, thus facilitating the conversion within Ce( iv ) and Ce( iii ). 71 This characteristic may also promote a surface cooperative effect with the Pt in the CeO 2 –Pt interface through electron transfer reactions via redox mediation. The reduction peak of Ce was observed at 0.6 V under alkaline conditions, as seen in Fig.…”

Study of rutile TiO2(110) single crystal by transient absorption spectroscopy in the presence of Ce4+ cations in aqueous environment. Implication on water splitting

“…One of the early metal cations used together with the semiconductor under photo-excitation is a system composed of TiO 2 (n-type semiconductor) and Ce cations [7][8][9]. Since then many have investigated such a system, mostly in the solid state, however [10,11].…”

Strengthening the Stability of the Reconstructed NiOOH Phase for 5-Hydroxymethylfurfural Oxidation

“…For example, Yan et al reported that W-doped α-Ni­(OH) 2 presents outstanding electrocatalytic activity, and W could work as the active site for the water splitting reaction. Ce commonly exhibits the reversible reaction accompanied by the redox pairs, and it is a class of dopant ions with the desirable compatibility . Moreover, the radius of the Ce 3+ ion (102 pm) is significantly larger than that of the Ni 2+ ion (69 pm), so the dopant Ce would lead to the increase of the Ni–O bond length and the d -spacing, where the looser lattice structure might solve the stability problems caused by phase transition. , Therefore, it is feasible to introduce Ce ions in Ni­(OH) 2 to establish a stable system for HMF oxidation.…”