The passage provides an in-depth discussion on the behavior of iron ions (Fe²⁺ and Fe³⁺) within an exfoliated gel-like material, specifically how they interact with the structure, including nickel oxide hydroxide (NiOxHy). Here’s a summary of the key points:
Formation and Anchoring:
- After creating the gel-like material, Fe ions can diffuse into its layers.
- They become anchored through condensation reactions between aquo-iron complexes and hydroxyl groups on the surface or within the layers.
Preconditioning Aquo-complexes:
- The aquo-complexes are modified to represent both Fe²⁺ and Fe³⁺ species by substituting water molecules in their coordination sphere with hydroxyl groups.
Interaction with NiOxHy:
- Iron ions may also interact directly with the NiOxHy surface or across two layers of this material.
Formation Energies:
- Calculations indicate that the formation energies for different configurations of Fe doping show that octahedral structures are generally more stable (lower energy) than tetrahedral ones for both Fe²⁺ and Fe³⁺ ions.
- Specifically, adsorption formation energies are approximately -2.5 eV for both octahedral and tetrahedral coordinated Fe²⁺, while for Fe³⁺ they are about -1.2 eV.
Proton Transfer Mechanism:
- Upon adsorption onto the surface, a proton transfers to the NiOOH layer along with electrons, leading to conversion from Fe²⁺ to Fe³⁺ or even further to Fe4+.
Intercalation vs. Adsorption:
- The energy required for intercalating these clusters into layers is about 1.0 eV higher than that required for simple adsorption.
- It was found that traditional ion exchange processes have high formation energies compared to other mechanisms being studied.
Adsorption Preferences:
- While isolated adsorption is preferred at low concentrations due to lower Gibbs free energy associated with configurational entropy, under high iron concentrations, dimer formations may occur among Fe³⁺ ions.
- Isolated Fe²⁺ exhibits lower adsorption energy compared to isolated Fe³⁺ but prefers isolation over forming clusters at low concentrations.
Conclusions:
- Both types of iron ions tend toward independent adsorption or intercalation rather than clustering in lower concentration scenarios.
- Following adsorption, they transform into higher oxidation states through a reduction process involving electron transfer from Ni3+ facilitated by protons.
This analysis highlights important thermodynamic factors governing iron’s interaction with layered materials which could be significant in contexts such as catalysis or battery technology where metal ion mobility and redox chemistry are crucial aspects.
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