Enrique Herrero

University of Alicante, Alicante, Valencia, Spain

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Publications (181)300.4 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Ethanol oxidation in 0.1 M NaOH on single-crystal electrodes has been studied using electrochemical and FTIR techniques. The results show that the activity order is the opposite of that found in acidic solutions. The Pt(111) electrode displays the highest currents and also the highest onset potential of all the electrodes. The onset potential for the oxidation of ethanol is linked to the adsorption of OH on the electrode surface. However, small (or even negligible) amounts of COads and carbonate are detected by FTIR, which implies that cleavage of the CC bond is not favored in this medium. The activity of the electrodes diminishes quickly upon cycling. The diminution of the activity is proportional to the measured currents and is linked to the formation and polymerization of acetaldehyde, which adsorbs onto the electrode surface and prevents further oxidation.
    ChemPhysChem 04/2014; · 3.35 Impact Factor
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    ABSTRACT: Thallium modified shape-controlled Pt nanoparticles were prepared and their electrocatalytic activity towards formic acid electrooxidation was evaluated in 0.5 M sulfuric acid. The electrochemical and in situ FTIR spectroscopic results show a remarkable improvement in the electrocatalytic activity, especially in the low potential region (around 0.1-0.2 V vs. RHE). Cubic Pt nanoparticles modified with Tl were found to be more active than the octahedral Pt ones in the entire range of Tl coverages and potential windows. In situ FTIR spectra indicate that the promotional effect produced by Tl results in the inhibition of the poisoning step leading to COads, thus improving the onset potential for the complete formic acid oxidation to CO2. Chronoamperometric experiments were also performed at 0.2 V to evaluate the stability of the electrocatalysts at constant potential. Finally, experiments with different concentrations of formic acid (0.05-1 M) were also carried out. In all cases, Tl-modified cubic Pt nanoparticles result to be the most active. All these facts reinforce the importance of controlling the surface structure of the electrocatalysts to optimize their electrocatalytic properties.
    Physical Chemistry Chemical Physics 03/2014; · 3.83 Impact Factor
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    ABSTRACT: The effect of the electrode potential in the reactivity of platinum stepped single crystal electrodes with (111) terraces toward CO oxidation has been studied. It is found that the CO adlayer is significantly affected by the potential at which the adlayer is formed. The electrochemical and FTIR experiments show that the adsorbed CO layer formed in acidic solution at 0.03 V vs SHE is different from that formed at −0.67 V vs SHE in alkaline solutions. The major effect of the electrode potential is a change in the long-range structure of CO adlayer. The adlayer formed in alkaline media presents a higher number of defects. These differences affect the onset and peak potential for CO stripping experiments. The stripping voltammogram for the adlayer formed at −0.67 V vs SHE always shows a prewave and the peak potential is more negative than that observed for the adlayer formed at 0.03 V vs SHE. This means that the apparent higher activity for CO oxidation observed in alkaline media is a consequence of the different CO adlayer structure on the (111) terrace, and not a true catalytic effect. The different behavior is discussed in terms of the different mobility of CO observed depending on the electrode potential. Also, the FTIR frequencies are used to estimate the pzc (potential of zero charge) for the Pt(111) electrode covered with a CO adlayer.
    The Journal of Physical Chemistry C. 01/2014; 118(4):1925–1934.
  • Rosa M. Arán-Ais, Enrique Herrero, Juan M. Feliu
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    ABSTRACT: Ethylene glycol (EG) oxidation has been studied on Pt(111) and its vicinal surfaces in acidic media by cyclic voltammetry and infrared spectroscopy. Even at Pt(111) the C 2
    Electrochemistry Communications 01/2014; 45:40–43. · 4.43 Impact Factor
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    ABSTRACT: Electrooxidation of CO at the Pt(hkl)-electrolyte interface in two different room temperature ionic liquids (RTILs) is probed to be surface-sensitive. Provided data reveal a specific surface structure, (110) sites, which selectively activate CO oxidation in RTILs. This new knowledge is crucial for designing the next generation of Pt nanosized electrocatalysts for the CO oxidation reaction by increasing that type of site on the catalyst surface.
    ACS Catalysis 11/2013; 3(12):2935–2938. · 5.27 Impact Factor
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    ABSTRACT: The electrochemical reactivity of catechol-derived adlayers is reported at platinum (Pt) single-crystal electrodes. Pt(111) and stepped vicinal surfaces are used as model surfaces possessing well-ordered nanometer-sized Pt(111) terraces ranging from 0.4 to 12 nm. The electrochemical experiments were designed to probe how the control of monatomic step-density and of atomic-level step structure can be used to modulate molecule-molecule interactions during self-assembly of aromatic-derived organic monolayers at metallic single-crystal electrode surfaces. A hard sphere model of surfaces and a simplified band formation model are used as a theoretical framework for interpretation of experimental results. The experimental results reveal (i) that supramolecular electrochemical effects may be confined, propagated, or modulated by the choice of atomic level crystallographic features (i.e.monatomic steps), deliberately introduced at metallic substrate surfaces, suggesting (ii) that substrate-defect engineering may be used to tune the macroscopic electronic properties of aromatic molecular adlayers and of smaller molecular aggregates.
    Langmuir 10/2013; · 4.38 Impact Factor
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    ABSTRACT: The co-adsorption of CO and OH on two Pt stepped surfaces vicinal to the (111) orientation has been evaluated by means of density functional theory (DFT) calculations. Focusing on Pt(533) and Pt(221), which contain (100) and (111)-steps, respectively, we find that (111)-steps should be more reactive towards CO oxidation than surfaces containing (100)-steps. The DFT results are compared with electrochemical experiments on the CO adsorption and oxidation on these vicinal surfaces.
    Physical Chemistry Chemical Physics 09/2013; · 3.83 Impact Factor
  • Ruben Rizo, Enrique Herrero, Juan M Feliu
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    ABSTRACT: The oxygen reduction reaction (ORR) in 0.1 M NaOH on platinum single crystal electrodes has been studied using hanging meniscus rotating disk electrode configuration. Basal planes and stepped surfaces with (111) and (100) terraces have been employed. The results indicate that the Pt(111) electrode has the highest electrocatalytic activity among all the studied surfaces. The addition of steps on this electrode surface significantly diminishes the reactivity of the surface towards the ORR. In fact, the reactivity of the steps on the surfaces with wide terraces can be considered negligible with respect to that measured for the terrace. On the other hand, Pt(100) and Pt(110) electrodes have much lower activity than the Pt(111) electrode. These results have been compared with those obtained in acid media to understand the effect of the pH and the adsorbed OH on the mechanism. It is proposed that the surface covered by adsorbed OH is active for the reduction of the oxygen molecules.
    Physical Chemistry Chemical Physics 08/2013; · 3.83 Impact Factor
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    ABSTRACT: Ethanol oxidation on platinum nanoparticles with well-characterized surfaces is studied using cyclic voltammetry and FTIR techniques. Their behavior is compared with that obtained for platinum single crystal electrodes, in order to rationalize their performance and to understand the effects of the surface structure and anion adsorption on the reactivity. The results clearly demonstrate that there are strong effects of anion adsorption and surface structure on the measured current and oxidation mechanism. Thus, the main product of ethanol oxidation on (111) preferentially oriented Pt nanoparticles is acetic acid, and the amount of CO2 produced can be considered negligible. On the other hand, (100) preferentially oriented Pt nanoparticles are effective for the cleavage of the C–C bond yielding adsorbed CO, which eventually is oxidized to CO2. This nanoparticles electrode has the highest catalytic activity at high potentials, whereas (111) preferentially oriented Pt nanoparticles are more active at low potentials. In addition, no significant differences in the activity are reported by using different supporting electrolytes, which indicates that adsorbed acetate, which results from the adsorption of acetic acid, hinders ethanol oxidation.
    J. Mater. Chem. A. 05/2013; 1(24):7068-7076.
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    ABSTRACT: Electrochemical water splitting into H2 and O2 presents a significant and challenging energy loss due to the high overpotential required at the anode. Today, in industrially relevant applications, dimensionally stable anodes (DSA) based on the electrocatalytic active RuO2 are conventionally utilized. To enhance the resistance against corrosion, incorporation of TiO2 in the RuO2-coated electrodes is widely employed. In the present work we have used scanning electrochemical microscopy (SECM) to demonstrate that TiO2-doped RuO2-coated electrodes, in addition to being more durable, also show an electrocatalytic activity that is, on average, 13% higher as compared to the pure RuO2-coated electrodes. We also demonstrate that cracks in the pure RuO2 coating are the most active zones, probably because Ti from the Ti support has diffused into the first applied layer of the RuO2 coating. To reveal the nature of this enhanced activity for water oxidation displayed on TiO2-doped RuO2 electrodes, we have employed X-ray photoelectron spectroscopy (XPS) for material characterization. The results show that the electrocatalytic activity enhancement displayed on the mixed (Ru1–x:Tix)O2 coating is promoted through a charge transfer from the RuO2 to the TiO2, which provides new and more reactive sites designated as activated RuO2δ+.
    The Journal of Physical Chemistry C. 02/2013; 117(12):6126–6135.
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    ABSTRACT: This work reports experimental evidence that the adsorption of CO on Pt surfaces composed by (111) terraces and steps and/or kinks in alkaline media occurs faster on sites with (100) symmetry, followed by (110) sites. On the other hand, the (111) terrace has the lowest adsorption rate. CO electrooxidation demonstrates the existence of a close relationship between the preferential site CO occupancy and the peak multiplicity observed in CO stripping voltammetry. For the stepped Pt(554) and Pt(544) and kinked Pt(875) single crystal surfaces, the CO stripping process takes place at high potentials (0.80 V vs RHE) when only the (110) or (100) step sites are blocked by CO. However, when the terrace sites with (111) symmetry are fully occupied, new CO stripping peaks appear at lower potentials (<0.68 V vs RHE). For all surfaces, it is observed that the first released sites after partial CO oxidation are (111) terrace sites, followed by the step (110) sites and sites with (100) symmetry. The results of partial CO oxidation suggest that the diffusion of adsorbed CO from sites with (110) or (100) symmetry toward unoccupied (111) terrace sites is negligible. However, CO diffusion from terrace sites to step or kink sites cannot be discarded during the growth of the adsorbed CO adlayer, because of the preferential site occupancy for these latter sites. Due to the fact that the CO oxidation process on stepped Pt surfaces in alkaline media occurs at different potentials on different sites, the activation energies for CO oxidation on the different sites have been estimated. The results for full CO coverage and CO decorated stepped surfaces are in good agreement, indicating that the oxidation of CO on the different sites is not coupled. In the absence of CO on terrace sites, in situ FTIR spectroscopy shows that CO molecules on the (110) step sites are essentially linearly bonded, while on the (100) step sites both linearly and bridge bonded CO are observed. The comparison of these spectra with those obtained when a full coverage is attained shows that the band frequencies for CO on step sites are highly coupled with those on the terrace sites.
    The Journal of Physical Chemistry C. 02/2013; 117(6):2903–2913.
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    ABSTRACT: Dream island: Monoatomic, high‐platinum islands deposited on a Rh(1 1 1) electrode have an electrocatalytic activity for ethanol oxidation that is 20 times higher than that measured on platinum electrodes. On these islands, CO2 and acetic acid are the final products.
    ChemCatChem 01/2013; 5(6). · 5.18 Impact Factor
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    ABSTRACT: The electrocatalytic properties of palladium nanocubes towards the electrochemical oxidation of formic acid were studied in H(2)SO(4) and HClO(4) solutions and compared with those of spherical Pd nanoparticles. The spherical and cubic Pd nanoparticles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The intrinsic electrocatalytic properties of both nanoparticles were shown to be strongly dependent on the amount of metal deposited on the gold substrate. Thus, to properly compare the activity of both systems (spheres and nanocubes), the amount of sample has to be optimized to avoid problems due to a lower diffusion flux of reactants in the internal parts of the catalyst layer resulting in a lower apparent activity. Under the optimized conditions, the activity of the spheres and nanocubes was very similar between 0.1 and 0.35 V. From this potential value, the activity of the Pd nanocubes was remarkably higher. This enhanced electrocatalytic activity was attributed to the prevalence of Pd(100) facets in agreement with previous studies with Pd single crystal electrodes. The effect of HSO(4)(-)/SO(4)(2-) desorption-adsorption was also evaluated. The activity found in HClO(4) was significantly higher than that obtained in H(2)SO(4) in the whole potential range.
    Physical Chemistry Chemical Physics 06/2012; 14(29):10258-65. · 3.83 Impact Factor
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    ABSTRACT: Export Date: 3 July 2012, Source: Scopus
    Journal of chemical education 06/2012; 89(7):936-939. · 0.82 Impact Factor
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    ABSTRACT: Export Date: 3 July 2012, Source: Scopus
    ACS Catalysis. 01/2012; 2(5):901-910.
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    ABSTRACT: The oxidation of adsorbed CO on Pt single crystal electrodes has been studied in alkaline media. The surfaces used in this study were the Pt(111) electrode and vicinal stepped and kinked surfaces with (111) terraces. The kinked surfaces have either (110) steps broken by (100) kinks or (100) steps broken by (110) kinks and different kink densities. The voltammetric profiles for the CO stripping on those electrodes show peaks corresponding to the oxidation of CO on the (111) terraces, on the (100) steps/kinks and on the (110) steps/kinks at very distinctive potentials. Additionally, the stripping voltammograms always present a prewave. The analysis of the results with the different stepped and kinked surfaces indicates that the presence of the prewave is not associated with defects or kinks in the electrode surface. Also, the clear separation of the CO stripping process in different peak contributions indicates that the mobility of CO on the surface is very low. Using partial CO stripping experiments and studies at different pH, it has been proposed that the low mobility is a consequence of the negative absolute potential at which the adlayers are formed in alkaline media. Also, the surface diffusion coefficient for CO in these media has been estimated from the dependence of the stripping charge of the peaks with the scan rate of the voltammetry.
    Physical Chemistry Chemical Physics 08/2011; 13(37):16762-71. · 3.83 Impact Factor
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    ABSTRACT: The initial oxidation of Pt surfaces is an important process that could determine the reactivity of catalysts in a wide range of reactions, from electrocatalytic oxidation of organics to oxygen reduction, but the understanding of electrochemical Pt oxidation has been hindered by a lack of surface-structural definition. We have investigated the process at surfaces vicinal to Pt(111) and show that these oxidize in successive stages depending on site geometry as well as the adsorption behavior of the electrolyte anion. Step sites of {100} orientation slowly oxidize at low potential (0.7 V vs RHE) in a region overlapping that of the “butterfly” peak seen at Pt(111) in the absence of specific electrolyte anion adsorption. Almost regardless of the latter, both {110} and {100} steps also oxidize between 0.9 and 1.2 V, causing voltammetric peaks with shapes that are characteristic of step orientation. The complex oxidation behavior of Pt(111) in perchloric acid, ranging from 0.6 V to the onset of O2 evolution at 1.5 V, is mostly suppressed in sulfuric acid. However, if steps are introduced, then the oxidation at lower potential is again facilitated, probably by breaking the ordered protective sulfate adlayer along the steps. It was found that {100} steps oxidize to the extent of one electron per step atom, whereas {110} steps show two consecutive oxidations, amounting to a total of two electrons per step atom. At the onset of O2 evolution, Pt(111) terraces are oxidized with two electrons per surface atom.
    The Journal of Physical Chemistry C. 07/2011; 115(31).
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    ABSTRACT: Ethanol oxidation on platinum stepped surfaces vicinal to the (111) pole modified by tin has been studied to determine the role of this adatom in the oxidation mechanism. Tin has been slowly deposited so that the initial stages of the deposition take place on the step, and deposition on the terrace only occurs when the step has been completely decorated. Voltammetric and chronoamperometric experiments demonstrate that tin on the step catalyzes the oxidation. The maximum enhancement is found when the step is completely decorated by tin. FTIR experiments using normal and isotopically labeled ethanol have been used to elucidate the effect of the tin adatoms in the mechanism. The obtained results indicate that the role of tin is double: (i) when the surface has sites capable of breaking the C-C bond of the molecule, that is, when the step sites are not completely covered by tin, it promotes the oxidation of CO formed from the molecular fragments to CO(2) through a bifunctional mechanism and (ii) it catalyzes the oxidation of ethanol to acetic acid.
    Physical Chemistry Chemical Physics 07/2011; 13(26):12163-72. · 3.83 Impact Factor
  • ChemPhysChem 06/2011; 12(9):1641-4. · 3.35 Impact Factor
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    ABSTRACT: Cited By (since 1996): 1, Export Date: 6 February 2012, Source: Scopus
    Electrochemistry Communications. 01/2011; 13(5):502-505.

Publication Stats

668 Citations
300.40 Total Impact Points

Institutions

  • 1993–2014
    • University of Alicante
      • • Departamento de Química Física
      • • Instituto Universitario de Electroquímica
      Alicante, Valencia, Spain
  • 2008–2013
    • Pontificia Universidad Catolica de Puerto Rico
      Ponce, Ponce, Puerto Rico
    • Universidade Estadual de Alagoas
      APQ, Alagoas, Brazil
    • Leiden University
      • Leiden Institute of Chemistry
      Leyden, South Holland, Netherlands
  • 2011
    • Universidade Federal de Alagoas
      Bom Successo, Minas Gerais, Brazil
  • 1998–2010
    • Cornell University
      • Department of Chemistry and Chemical Biology
      Ithaca, New York, United States
  • 2008–2009
    • University of São Paulo
      • Instituto de Química de São Carlos (IQSC)
      São Paulo, Estado de Sao Paulo, Brazil
  • 2003
    • University of Chichester
      Chichester, England, United Kingdom