Browsing by Author "Zagal, Jose H."

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    Cover Picture: Electron Spin-Dependent Electrocatalysis for the Oxygen Reduction Reaction in a Chiro-Self-Assembled Iron Phthalocyanine Device (Angew. Chem. Int. Ed. 4/2024)
    (2024) Scarpetta-Pizo, Laura; Venegas, Ricardo; Barrias, Pablo; Munoz-Becerra, Karina; Vilches-Labbe, Nayareth; Mura, Francisco; Mendez-Torres, Ana Maria; Ramirez-Tagle, Rodrigo; Toro-Labbe, Alejandro; Hevia, Samuel; Zagal, Jose H.; Onate, Ruben; Aspee, Alexis; Ponce, Ingrid
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    Effect of Plasma Argon Pretreatment on the Surface Properties of AZ31 Magnesium Alloy
    (2023) Montero, Cecilia; Ramirez, Cristian Gino; Munoz, Lisa; Sancy, Mamie; Azocar, Manuel; Flores, Marcos; Artigas, Alfredo; Zagal, Jose H.; Zhou, Xiaorong; Monsalve, Alberto; Paez, Maritza
    Climate change has evidenced the need to reduce carbon dioxide emissions into the atmosphere, and so for transport applications, lighter weight alloys have been studied, such as magnesium alloys. However, they are susceptible to corrosion; therefore, surface treatments have been extensively studied. In this work, the influence of argon plasma pretreatment on the surface properties of an AZ31 magnesium alloy focus on the enhancement of the reactivity of the surface, which was examined by surface analysis techniques, electrochemical techniques, and gravimetric measurements. The samples were polished and exposed to argon plasma for two minutes in order to activate the surface. Contact angle measurements revealed higher surface energy after applying the pretreatment, and atomic force microscopy showed a roughness increase, while X-Ray photoelectron spectroscopy showed a chemical change on the surface, where after pretreatment the oxygen species increased. Electrochemical measurements showed that surface pretreatment does not affect the corrosion mechanism of the alloy, while electrochemical impedance spectroscopy reveals an increase in the original thickness of the surface film. This increase is likely associated with the high reactivity that the plasma pretreatment confers to the surface of the AZ31 alloy, affecting the extent of oxide formation and, consequently, the increase in its protection capacity. The weight loss measurements support the effect of the plasma pretreatment on the oxide thickness since the corrosion rate of the pretreated AZ31 specimens was lower than that of those that did not receive the surface pretreatment.
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    Electrocatalytic oxidation of hydrazine in alkaline media promoted by iron tetrapyridinoporphyrazine adsorbed on graphite surface
    (2008) Dantas, Luiza M.F.; dos Reis, Alaecio P.; Tanaka, Sonia Maria C.N.; Zagal, Jose H.; Chen Carrillo, Yo-Ying Adriana; Tanaka, Auro A.
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    Electrocatalytic oxygen reduction on metalloporphyrins. Second coordination sphere substituents on the ligand: Electronic and steric effects through pocket and extended structures and their effect on the electrocatalytic activity
    (2024) Silva, Carlos P.; Candia, Carolina P.; Paredes, Mauricio; Imbarack, Elizabeth; Martinez-Gomez, Fabian; Olguin, Camila F.; Jara, Geraldine; Zagal, Jose H.; Pavez, Jorge; Agurto, Nicolas
    The rational design of new electrocatalysts remains a continuous challenge in pursuing high-performance fuel cells free of precious metals. In this work, we report MN4-type metal-complex catalyst based on bio-inspired systems, i.e., metalloporphyrins complexes that include the second coordination sphere as the central axis for a relevant improvement in catalysis. Two isostructural systems, tetra-ferrocenyl-triazole metalloporphyrin (pocket vs. extended structures), were synthesized to study the role of the second coordination sphere on the electrochemical oxygen reduction reaction (ORR). The electrocatalysis results show that both porphyrin systems promote the ORR via four electrons. The system that presents a pocket structure (o-Fc4TPP-FeIII) exhibits a foot of the wave 72 mV more positive than its analog with an extended structure (p-Fc4TPP-FeIII). Rotating ring-disc experiments evidenced that the presence of H-bonding and distal redox groups would be directly related to faradaic efficiency for the 4-electron reduction to water, with an 86 % for the pocket structure and 76 % for the extended system. This study allows us to confirm the molecular scaffolding function by comparing two systems that present the same functional groups but in different chemical environments, providing valuable information regarding the steric-spatial parameters that play a role in the ORR.
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    Electron Spin-Dependent Electrocatalysis for the Oxygen Reduction Reaction in a Chiro-Self-Assembled Iron Phthalocyanine Device
    (2023) Scarpetta-Pizo, Laura; Venegas, Ricardo; Barrias, Pablo; Munoz-Becerra, Karina; Vilches-Labbe, Nayareth; Mura, Francisco; Mendez-Torres, Ana Maria; Ramirez-Tagle, Rodrigo; Toro-Labbe, Alejandro; Hevia, Samuel; Zagal, Jose H.; Onate, Ruben; Aspee, Alexis; Ponce, Ingrid
    The chiral-induced spin selectivity effect (CISS) is a breakthrough phenomenon that has revolutionized the field of electrocatalysis. We report the first study on the electron spin-dependent electrocatalysis for the oxygen reduction reaction, ORR, using iron phthalocyanine, FePc, a well-known molecular catalyst for this reaction. The FePc complex belongs to the non-precious catalysts group, whose active site, FeN4, emulates catalytic centers of biocatalysts such as Cytochrome c. This study presents an experimental platform involving FePc self-assembled to a gold electrode surface using chiral peptides (L and D enantiomers), i.e., chiro-self-assembled FePc systems (CSAFePc). The chiral peptides behave as spin filters axial ligands of the FePc. One of the main findings is that the peptides ' handedness and length in CSAFePc can optimize the kinetics and thermodynamic factors governing ORR. Moreover, the D-enantiomer promotes the highest electrocatalytic activity of FePc for ORR, shifting the onset potential up to 1.01 V vs. RHE in an alkaline medium, a potential close to the reversible potential of the O2/H2O couple. Therefore, this work has exciting implications for developing highly efficient and bioinspired catalysts, considering that, in biological organisms, biocatalysts that promote O2 reduction to water comprise L-enantiomers.
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    Elucidating the electronic synergetic effects in heteroatomic doped FeN4-C-N-R (R= -F, -Cl, -Br) oxygen reduction catalysts
    (2023) Escobar, Gonzalo; Venegas, Ricardo; Ponce, Ingrid; Toro-Labbe, Alejandro; Zagal, Jose H.; Recio, F. Javier; Munoz-Becerra, Karina
    The structural and electronic characteristics of FeN4 are the determining factors in the catalytic performance of heat-treated Fe-N-C materials, as they serve as active sites. The insertion of heteroatoms as co-dopants (B, S, halogens) can induce electronic effects in the carbon matrix that improves their ORR catalytic activity. Therefore, it has become essential to combine experimental studies with DFT approaches to rationally design this type of catalyst. In this work, we evaluated by means of first principle DFT approaches, the ORR activity for the Fe (phen)2N2 moiety including atoms/functionalities with different atomic radii and electronegativity, to resemble co-doped Fe-N-C-R catalysts. The results showed that the inclusion of halogens heteroatoms (-F, -Cl, and -Br) in the graphitic N-C surrounding the FeN4 core could improve its ORR activity in terms of Fe-O2 binding energy that is related to the Fe(III)/Fe(II) formal potential and, in consequence, with the on-set potential for the ORR. The high expected ORR activity is obtained for bromide co-doped FeN4 catalyst (FeN4-C-Br) since -Br atoms act synergistically, inducing long- and short-range electronic effects over both the FeN4 unit and N-pyridinic-like functions that change the electronic distribution over the aromatic N-C structure modulating the Fe acidity, FeO2 binding, and Fe-O2 orbital interaction.
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    Using reactivity predictors for enhancing the electrocatalytic activity of MN4 molecular catalysts for the oxygen reduction reaction: The role of the N-pyridinium functional group in the porphyrazine-derivative ligands
    (2023) Scarpetta-Pizo, Laura; Venegas, Ricardo; Munoz-Becerra, Karina; Munoz, Lisa; Toro-Labbe, Alejandro; Darwish, Nadim; Matute, Ricardo; Onate, Ruben; Zagal, Jose H.; Ponce, Ingrid
    Using reactivity predictors to enhance or control the electrocatalytic activity of materials is a fascinating concept. This is especially true for the development of alternative platinum metal group-free materials as it facilitates the rational design of active catalytic materials for the oxygen reduction reaction (ORR). In previous work, we have found that the peripheral and non-peripheral electron-withdrawing effects and the electron-pull effect from axial extraplanar ligand in iron-phthalocyanine (FePc) are key factors in improving the binding energy between the active Fe site and O2 resulting in an increase of the electrocatalytic activity of FePcs for the ORR. In this work, we have utilized fundamental principles of electrocatalysis and DFT calculations to design and synthesize FeN4 molecular catalysts to increase their catalytic performance for the ORR through the "pull" effect. To achieve this, by chemical synthesis, we have incorporated pyridinium functional groups (N+py) in peripheral and non -peripheral positions into the porphyrazine cyclic ligands. In this fashion we obtain the porphyrazinium molec-ular catalysts, [Fe(II)2,3-(TMe)TPyPz]4+ and [Fe(II)3,4-(TMe)TPyPz]4+. Because these new compounds are not commercially available and, to the best of our knowledge, they have not been tested for ORR. In order to determine their effectiveness, we have compared porphyrazinium with neutral analog porphyrazine compounds (Fe(II)TPyPz) and perfluorinated and perchlorinated iron phthalocyanines, which are currently the best molecular catalysts for ORR. The electrocatalytic activity was determined for each molecular catalyst deposited on the edge plane of a graphite electrode (EPG) surface in an alkaline medium. Only for the purpose of comparison we include two Fe porphyrins studied previously, which show low activity for ORR. Although the DFT theoretical analysis of porphyrazinium complexes suggests a high activity for these catalysts, our experimental findings revealed the opposite trend. Therefore, this finding makes us reconsider the interfacial effects, such as the counter-ions effects on N+py that could influence the electron-pull effect, opening new insights for designing molecular catalysts considering interface engineering. Moreover we report for the first time, the reactivity linear relationship between the metal-centered redox potential gap (E degrees Fe(III)/(II) - E degrees Fe(II)/(I))) with the electrocatalytic activity for ORR for all catalysts studied, emerging this potential gap as a possible and promising new reactivity descriptor for ORR in MN4 catalyst.