Understanding the chemisorption-based activation mechanism of the oxygen reduction reaction on nitrogen-doped graphitic materials

[EN] To optimize nitrogen-doped graphitic materials as metal-free catalysts for the oxygen reduction reaction mechanisms have to be better understood. Here, the role played by pyridinic nitrogen-dopants in the chemisorption-based activation of the target reaction is revealed. The study is centered on the monodentate chemisorption of molecular oxygen as the first step of the process. Several configurations of unclustrered nitrogen dopants in which there was always a nitrogen dopant in the edge of the material were tested using DFT. A clearly favorable chemisorbed state for molecular oxygen was found when the pyridinic nitrogen-dopant is hydrogenated and located at an armchair edge. The found chemisorbed state is further favored by additional available charge. By contrast, the chemisorbed state of oxygen is much less favorable when the hydrogenated pyridinic nitrogen-dopants are located at zigzag edges. Moreover, it was found that the charge involved in the hydrogenation of pyridinic nitrogen-dopants remains segregated, becoming available for reduction processes. Detailed reasons for the described facts are given, and an integrated model for the target activation mechanism is proposed including graphitic nitrogen-dopants effects. ©2016 Elsevier Ltd. All rights reserved.This work has been financially supported by the MICINN (Spain) (project 2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013).Ferre Vilaplana, A.; Herrero, E. (2016). Understanding the chemisorption-based activation mechanism of the oxygen reduction reaction on nitrogen-doped graphitic materials. Electrochimica Acta. 204:245-254. https://doi.org/10.1016/j.electacta.2016.04.039S24525420

Why nitrogen favors oxygen reduction on graphitic materials

[EN] Nitrogen-doped graphitic materials as promising catalysts for the oxygen reduction reaction in fuel-cells have been mainly investigated under the graphitic versus pyridinic nitrogen-dopant dichotomy approach. However, we show here that the active sites, reaction mechanism, selectivity and even the origin of each behavior can be better understood when the stability of the possible active site and the eventual contribution of charge from the surface are considered separately. The roles in the reaction played by specific nitrogen-dopants, the hydrogenation of pyridinic nitrogen-dopants and the solvation effect are all clarified. The investigated activity is much more closely linked to the edges, where certain carbon atoms are sufficiently unstable or can be destabilized by means of adjacent nitrogen-dopants, and where reaction intermediates can be better relaxed, than to the presence of specific nitrogen-dopants. Unfortunately, high overpotentials and the undesired production of hydrogen peroxide appear to be unavoidable in the oxygen reduction to water on these materials.This work has been financially supported by the MINECO (Spain) project No. CTQ2016-76221-PFerre Vilaplana, A.; Herrero, E. (2019). Why nitrogen favors oxygen reduction on graphitic materials. Sustainable Energy & Fuels. 3(9):2391-2398. https://doi.org/10.1039/c9se00262fS2391239839Bing, Y., Liu, H., Zhang, L., Ghosh, D., & Zhang, J. (2010). Nanostructured Pt-alloy electrocatalysts for PEM fuel cell oxygen reduction reaction. Chemical Society Reviews, 39(6), 2184. doi:10.1039/b912552cMorozan, A., Jousselme, B., & Palacin, S. (2011). Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes. Energy & Environmental Science, 4(4), 1238. doi:10.1039/c0ee00601gKuttiyiel, K. A., Sasaki, K., Choi, Y., Su, D., Liu, P., & Adzic, R. R. (2012). 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Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science, 323(5915), 760-764. doi:10.1126/science.1168049Qu, L., Liu, Y., Baek, J.-B., & Dai, L. (2010). Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano, 4(3), 1321-1326. doi:10.1021/nn901850uYang, L., Shui, J., Du, L., Shao, Y., Liu, J., Dai, L., & Hu, Z. (2019). Carbon‐Based Metal‐Free ORR Electrocatalysts for Fuel Cells: Past, Present, and Future. Advanced Materials, 31(13), 1804799. doi:10.1002/adma.201804799Singh, S. K., Takeyasu, K., & Nakamura, J. (2018). Active Sites and Mechanism of Oxygen Reduction Reaction Electrocatalysis on Nitrogen-Doped Carbon Materials. Advanced Materials, 31(13), 1804297. doi:10.1002/adma.201804297Kong, X.-K., Chen, C.-L., & Chen, Q.-W. (2014). Doped graphene for metal-free catalysis. Chem. Soc. 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Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science, 351(6271), 361-365. doi:10.1126/science.aad0832Mamtani, K., Jain, D., Zemlyanov, D., Celik, G., Luthman, J., Renkes, G., … Ozkan, U. S. (2016). Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CNx) in Acidic Media Using Phosphate Anion. ACS Catalysis, 6(10), 7249-7259. doi:10.1021/acscatal.6b01786Rao, C. V., Cabrera, C. R., & Ishikawa, Y. (2010). In Search of the Active Site in Nitrogen-Doped Carbon Nanotube Electrodes for the Oxygen Reduction Reaction. The Journal of Physical Chemistry Letters, 1(18), 2622-2627. doi:10.1021/jz100971vSheng, Z.-H., Shao, L., Chen, J.-J., Bao, W.-J., Wang, F.-B., & Xia, X.-H. (2011). Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis. 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Journal of the American Chemical Society, 136(25), 9070-9077. doi:10.1021/ja503347

Ethanol Electro-oxidation Reaction Selectivity on Platinum in Aqueous Media

Ethanol fuel cells require selective catalysts for complete oxidation of the fuel, which involves C–C bond cleavage. From experiments on well-defined surfaces and calculations, the mechanism controlling the ethanol electro-oxidation selectivity on platinum in aqueous media as a model system is elucidated. Adsorbed OH favors ethanol adsorption and conversion into adsorbed ethoxy, which favorably evolves to adsorbed COCH3. On Pt(111), adsorbed OH is also readily incorporated into adsorbed COCH3 to yield acetic acid. A higher barrier for this latter step on Pt(100) enables the COCH3 dehydrogenation to adsorbed COCH2, favoring C–C bond cleavage. As adsorbed OH plays an essential role as a reactant in this process, its adsorption properties have a decisive impact on this reaction. Furthermore, the adsorbed OH diffusion rate on the surface, which depends on the adsorbate/media/surface interaction at the interface, modulates the availability of this key reactant. These results highlight that the search for selective electrocatalysts requires holistic consideration of reactants, adsorbates, media, and substrate.This research was funded by Ministerio de Ciencia e Innovación (Spain) grant nos. PID2019-105653GB-I00 and FJC2018-038607-I and Generalitat Valenciana (Spain) grant no. PROMETEO/2020/063

Why Citrate Shapes Tetrahedral and Octahedral Colloidal Platinum Nanoparticles in Water

[EN] The performance of many advanced catalytic systems depends not only on the size and composition but also on the specific shape of the metal nanoparticles (NPs) from which they are assembled. In turn, the shape of colloidal NPs depends on the specific capping agent involved in their synthesis, though the mechanism is still poorly understood. Here, supported by electrochemical experiments, Fourier transform infrared spectra, and density functional theory calculations, on well-defined surfaces, we show how a specific capping agent determines the shape of colloidal NPs. Solvated citrate can become simultaneously adsorbed on the Pt(111) surface through three dehydrogenated carboxylic groups, with each one of them in bidentate configuration. On the other two basal planes, citrate can be adsorbed through only two of them. For this reason, under the synthesis conditions, citrate is more favorably adsorbed on the Pt(111) than on the other two basal planes of platinum. This adsorption behavior explains why colloidal platinum NPs of tetrahedral and octahedral shape are produced when citrate is used as the capping agent in water. The mechanism for citrate would also determine the shape of other pure face-centered cubic metals and can inspire the engineering of future capping agents.This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P.Gisbert-González, J.; Feliu, J.; Ferre Vilaplana, A.; Herrero, E. (2018). Why Citrate Shapes Tetrahedral and Octahedral Colloidal Platinum Nanoparticles in Water. The Journal of Physical Chemistry C. 122(33):19004-19014. https://doi.org/10.1021/acs.jpcc.8b05195S19004190141223

Understanding the Effect of the Adatoms in the Formic Acid Oxidation Mechanism on Pt(111) Electrodes

The engineered search for new catalysts requires a deep knowledge about reaction mechanisms. Here, with the support of a combination of computational and experimental results, the oxidation mechanism of formic acid on Pt(111) electrodes modified by adatoms of the p block is elucidated for the first time. DFT calculations reveal that some adatoms, such as Bi and Pb, have positive partial charge when they are adsorbed on the bare surface, whereas others, such as Se and S, remain virtually neutral. When the partial charge is correlated with previously reported experimental results for the formic acid oxidation reaction, it is found that the partial positive charge is directly related to the increase in catalytic activity of the modified surface. Further, it is obtained that such a positive partial charge is directly proportional to the electronegativity difference between the adatom and Pt. Thus, the electronegativity difference can be used as an effective descriptor for the expected electrocatalytic activity. This partial positive charge on the adatom drives the formic acid oxidation reaction, since it favors the formation and adsorption of formate on the adatom. Once adsorbed, the neighboring platinum atoms assist in the C–H bond cleavage. Finally, it is found that most of the steps involved in the proposed oxidation mechanism are barrierless, which implies a significant diminution of the activation barriers in comparison to that of the unmodified Pt(111) electrode. This diminution in the activation barrier has been experimentally corroborated for the Bi–Pt(111) electrode, supporting the proposed mechanism.This work has been financially supported by the MINECO (Spain) (project CTQ2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013)

Formic acid oxidation on platinum electrodes: A detailed mechanism supported by experiments and calculations on well-defined surfaces

[EN] In spite of the fact that the formic acid oxidation reaction on electrode surfaces has been extensively investigated, a detailed mechanism explaining all the available experimental evidence on platinum has not been yet described. Herein, using a combined experimental and computational approach, the key elements in the mechanism of the formic acid oxidation reaction on platinum have been completely elucidated, not only for the direct path, through an active intermediate, but also for the CO formation route. The experimental results suggest that the direct oxidation path on platinum takes place in the presence of bidentate adsorbed formate. However, the results reported here provide evidence that this species is not the active intermediate. Monodentate adsorbed formate, whose evolution to the much more favorable bidentate form would be hindered by the presence of neighboring adsorbates, has been found to be the true active intermediate. Moreover, it is found that adsorbed formic acid would have a higher acid constant than in solution, which suggests that adsorbed formate can be originated not only from solution formate but also from formic acid. The CO formation path on platinum can proceed, also from monodentate adsorbed formate, through a dehydrogenation process toward the surface, during which the adsorbate transitions from a Pt-O adsorption mode to a Pt-C one, to form carboxylate. From this last configuration, the C-OH bond is cleaved, on the surface, yielding adsorbed CO and OH. The results and mechanisms reported here provide the best explanation for the whole of the experimental evidence available to date about this reaction, including pH, surface structure and electrode potential effects.This work has been financially supported by the MCINN-FEDER (Spain) and Generalitat Valenciana (Feder) through projects CTQ2016-76221-P and PROMETEOII/2014/013, respectively.Ferre Vilaplana, A.; Perales, JV.; Buso-Rogero, C.; Feliu, J.; Herrero, E. (2017). Formic acid oxidation on platinum electrodes: A detailed mechanism supported by experiments and calculations on well-defined surfaces. Journal of Materials Chemistry A. 5(41):21773-21784. https://doi.org/10.1039/c7ta07116gS2177321784541Bagotzky, V. S., Vassiliev, Y. B., & Khazova, O. A. (1977). Generalized scheme of chemisorption, electrooxidation and electroreduction of simple organic compounds on platinum group metals. 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Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes. Physical Chemistry Chemical Physics, 13(45), 20091. doi:10.1039/c1cp22498kCuesta, A., Cabello, G., Osawa, M., & Gutiérrez, C. (2012). Mechanism of the Electrocatalytic Oxidation of Formic Acid on Metals. ACS Catalysis, 2(5), 728-738. doi:10.1021/cs200661zChen, Y.-X., Heinen, M., Jusys, Z., & Behm, R. J. (2006). Bridge-Bonded Formate:  Active Intermediate or Spectator Species in Formic Acid Oxidation on a Pt Film Electrode?†. Langmuir, 22(25), 10399-10408. doi:10.1021/la060928qChen, Y. X., Heinen, M., Jusys, Z., & Behm, R. J. (2006). Kinetics and Mechanism of the Electrooxidation of Formic Acid—Spectroelectrochemical Studies in a Flow Cell. Angewandte Chemie International Edition, 45(6), 981-985. doi:10.1002/anie.200502172Chen, Y.-X., Heinen, M., Jusys, Z., & Behm, R. J. (2007). Kinetic Isotope Effects in Complex Reaction Networks: Formic Acid Electro-Oxidation. ChemPhysChem, 8(3), 380-385. doi:10.1002/cphc.200600520Joo, J., Uchida, T., Cuesta, A., Koper, M. T. M., & Osawa, M. (2013). Importance of Acid–Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum. Journal of the American Chemical Society, 135(27), 9991-9994. doi:10.1021/ja403578sJoo, J., Uchida, T., Cuesta, A., Koper, M. T. M., & Osawa, M. (2014). The effect of pH on the electrocatalytic oxidation of formic acid/formate on platinum: A mechanistic study by surface-enhanced infrared spectroscopy coupled with cyclic voltammetry. Electrochimica Acta, 129, 127-136. doi:10.1016/j.electacta.2014.02.040Brimaud, S., Solla-Gullón, J., Weber, I., Feliu, J. M., & Behm, R. J. (2014). Formic Acid Electrooxidation on Noble-Metal Electrodes: Role and Mechanistic Implications of pH, Surface Structure, and Anion Adsorption. ChemElectroChem, 1(6), 1075-1083. doi:10.1002/celc.201400011Perales-Rondón, J. V., Brimaud, S., Solla-Gullón, J., Herrero, E., Jürgen Behm, R., & Feliu, J. M. 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Glutamate adsorption on the Au(111) surface at different pH values

Adsorbed amino acids can modulate the behavior of metal nanoparticles in advanced applications. Using a combination of electrochemical experiments, FTIR spectroscopy, and DFT calculations, glutamate species interacting with the Au(111) surface in solution are here investigated. Electrochemical results indicate that the adsorption behavior depends on the solution pH (which controls the glutamate ionization) and on the charge of the surface. Glutamate adsorption starts at potentials slightly negative to the potential of zero charge. The thermodynamic analysis of these results indicates that two electrons are exchanged per molecule, implying that both carboxylic groups become deprotonated upon adsorption. The FTIR spectra reveal that carboxylate groups are bonded to the surface in the bidentate configuration (with both oxygen atoms attached to the surface). Plausible adsorbed configurations, consistent with the whole of these insights, were found using DFT. -Additionally, it was observed that glutamate oxidation only takes place when the surface is oxidized, which suggests that this oxidation process involves the transfer of an oxygen group to the molecule, though, according to the FTIR spectra, the main chain remains intact.Financial support from Ministerio de Ciencia e Innovación (Project PID2019-105653GB-100) and Generalitat Valenciana (Project PROMETEO/2020/063) is acknowledged

An Aza-Fused pi-Conjugated Microporous Framework Catalyzes the Production of Hydrogen Peroxide

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/page/policy/articlesonrequest/index.html"[EN] In order to produce hydrogen peroxide in small-scale electrochemical plants, selective catalysts for the oxygen reduction reaction (ORR) toward the desired species are required. Here, we report about the synthesis, characterization, ORR electrochemical behavior, and reaction mechanism of an aza-fused pi-conjugated microporous polymer, which presents high selectivity toward hydrogen peroxide. It was synthesized by polycondensation of 1,2,4,5-benzenetetramine tetrahydrochloride and triquinoyl octahydrate. A cobalt-modified version of the material was also prepared by a simple postsynthesis treatment with a Co(II) salt. The characterization of the material is consistent with the formation of a conductive robust porous covalent laminar polyaza structure. The ORR properties of these catalysts were investigated using rotating disk and rotating disk ring arrangements. The results indicate that hydrogen peroxide is almost exclusively produced at very low overpotentials on these materials. Density functional theory calculations provide key elements to understand the reaction mechanism. It is found that, at the relevant potential for the reaction, half of the nitrogen atoms of the material would be hydrogenated. This hydrogenation process would destabilize some carbon atoms in the lattice and would provide segregated charge. On the destabilized carbon atoms, molecular oxygen would be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state, which would have the characteristics of a superoxide species, would be only slightly stable, promoting the formation of hydrogen peroxide.This work has been financially supported by the MCINN-FEDER (projects CTQ2016-76221-P, MAT2013-46753-C2-1-P, and MAT2014-52305-P) and Generalitat Valenciana (project PROMETEO/2014/013).Briega-Martos, V.; Ferre Vilaplana, A.; De La Peña, A.; Segura, J.; Zamora, F.; Feliu, J.; Herrero, E. (2017). An Aza-Fused pi-Conjugated Microporous Framework Catalyzes the Production of Hydrogen Peroxide. ACS Catalysis. 7(2):1015-1024. https://doi.org/10.1021/acscatal.6b03043S101510247

Cleavage of the C-C bond in the ethanol oxidation reaction on platinum. Insight from experiments and calculations

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b03117, see http://pubs.acs.org/page/policy/articlesonrequest/index.html".[EN] Using a combination of experimental and computational methods, mainly FTIR and DFT calculations, new insights are provided here in order to better understand the cleavage of the C–C bond taking place during the complete oxidation of ethanol on platinum stepped surfaces. First, new experimental results pointing out that platinum stepped surfaces having (111) terraces promote the C–C bond breaking are presented. Second, it is computationally shown that the special adsorption properties of the atoms in the step are able to promote the C–C scission, provided that no other adsorbed species are present on the step, which is in agreement with the experimental results. In comparison with the (111) terrace, the cleavage of the C–C bond on the step has a significantly lower activation energy, which would provide an explanation for the observed experimental results. Finally, reactivity differences under acidic and alkaline conditions are discussed using the new experimental and theoretical evidence.This work has been financially supported by the MINECO (Spain) (project CTQ2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013).Ferre Vilaplana, A.; Buso-Rogero, C.; Feliu, JM.; Herrero, E. (2016). Cleavage of the C-C bond in the ethanol oxidation reaction on platinum. Insight from experiments and calculations. Journal of Physical Chemistry C. 120(21):11590-11597. https://doi.org/10.1021/acs.jpcc.6b03117S11590115971202