Залізовмісні похідні графену

• Дата: 2025
• Автори: Кремень, О. С., Лобанов, В. В., Картель, М. Т.
• Формат: Стаття
• Мова: Англійська
• Опубліковано: Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine 2025
• Теми: теорія функціоналу густини; залізовмісні електрокаталізатори; рентгенофотоелектронні спектри; остівні рівні N1s; порфіриноподібні фрагменти; реакція відновлення кисню (РВК); чотириелектронний механізм РВК; ступінь допування графену азотом; аномальна дифузія
• Онлайн доступ: https://surfacezbir.com.ua/index.php/surface/article/view/795
• Назва журналу: Surface

The review focuses on the results of quantum-chemical calculations of the properties of electrocatalysts based on iron-containing carbons, mainly graphene. Depending on the nitrogen atom states in electrocatalysts produced by pyrolysis of a mixture of carbon-, nitrogen-, and iron-containing precursors, a method for assigning N1s peaks in XPS was described. The different types of nitrogen atoms in ORR electrocatalysts enable a semi-quantitative determination by combining data from experimentally obtained XPS spectra with quantum-chemical calculations of chemically induced shifts in N1s core-level energies.Quantitative analysis of the EXAFS and XANES spectra regions of FeN-C catalysts, which are free or nearly free of Fe crystalline structures, revealed the presence of porphyrin-like FeN4C12 fragments. Electrochemical studies showed that these FeN4C12 fragments catalyze the four-electron reduction of O2 to water. Such porphyrin-like fragments can form either within highly disordered graphene sheets or between the zigzag edges of graphene, leading to the formation of micropores. FeN-C catalysts subjected to Ar- and NH3-pyrolysis exhibit significantly different ORR activities. The increased ORR activity associated with FeN4C12-type fragments results from highly basic N-groups generated during pyrolysis with NH3.A detailed kinetic and thermodynamic analysis of ORR on FeN4-G type catalyst with all pyrrole nitrogen atoms showed that the activation energy of dissociation of the adsorbed O2 molecule is very high, regardless of its adsorption type on FeN4-G catalyst in the Pauling or Griffiths model configuration.The calculated ORR free energy change diagrams indicate that for all its elementary stages via the four-electron mechanism, the free energy changes (ΔG) are negative at low electrode potentials (up to 0.41 V). The rate-limiting stage for the entire ORR is the reduction of OH(ads) to H2O(ads), with Eact = 1.02 eV.The first self-consistent comparison of the activity of several potential structures of edge defects in the active center of iron-containing catalysts based on graphene nanocarbon showed that, depending on the synthesis conditions, the most stable Fe-containing defects are structures with four or three nitrogen atoms. It is assumed that both of these structures can coexist. Cluster structures of the FeN3 (Fe2N5) type are capable of cleaving the bond in the O2 molecule with a zero activation barrier and, therefore, can direct ORR along the dissociative route. This route is expected to be more selective, without H2O2 formation, due to the excess binding of ORR intermediates. Ab initio molecular dynamics data indicate that this spontaneous reaction is likely to be unaffected by solvation, as the solvent does not seem to alter the stability of the considered edge defects.The DFT results showed that as the nitrogen doping level of graphene-FeNx (x = 4, 3, 2, 1) increases, their activity in the hydrochlorination reaction increases sequentially. The following order of Eact for the catalytic reaction of the graphene-FeNx catalyst series is obtained: graphene-FeN1 > graphene-FeN2 > graphene-FeN3 > graphene-FeN4.The Fe atom embedded in the graphene network activates the methane molecule with an activation energy of 25.7 kcal/mol without applying an external electric field. The stability of the adsorption complexes, transition states, and products changes significantly under the influence of the direction and strength of the applied electric field. A positive electric field destabilizes the adsorption complexes, while the transition state and products are more stable compared to the case without a field. The activation energy decreased significantly from 25.7 to 17.5 kcal/mol when an electric field of +0.015 a.u. was applied. The results indicate that an applied external electric field can control the catalytic activity of graphene when iron is added. Using aberration-corrected TEM, we show that the diffusion of single Fe atoms at graphene edges depends on the edge type (zigzag and armchair), with subdiffusion occurring at armchair edges and superdiffusion occurring at zigzag edges. Theoretical calculations show that this difference is due to different diffusion barriers between stable states. The anomalous diffusion behaviour can be expected to affect the growth/catalysis kinetics of synthetic sp2 nanomaterials grown using metal catalysts. The in situ observations and theoretical studies (MD and DFT) provide key insights into the fundamental growth processes of sp2 nanostructures, such as graphene and carbon nanotubes, on metal catalysts.