Detection of Superoxide Anion Oxygen Reduction Reaction Intermediate on Pt(111) by Infrared Reflection Absorption Spectroscopy in Neutral pH Conditions

In this work, in situ external infrared reflection absorption spectroscopy (IRRAS) is successfully employed for the detection of intermediate species in the oxygen reduction reaction (ORR) mechanism on a flat and well-defined Pt surface. Superoxide anion species (O2–) are detected on the Pt(111) surface in an O2-saturated solution with a NaF/HClO4 mixture with pH 5.5 by the observation of a O–O vibration band at ca. 1080 cm–1. The observation of O2– without the use of any other additional method of signal enhancement is possible because in these experimental conditions O2– is the main ORR-generated intermediate and its reactivity is limited in this pH. This leads to the accumulation of O2– near the Pt surface, facilitating its identification.This work has been financially supported by MCINN (FEDER) (Spain) through project PID2019-105653GB-100

Borohydride electro-oxidation on Pt single crystal electrodes

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.This work has been financially supported by the MICINN (Spain) (project 2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013)

Insight into the role of adsorbed formate in the oxidation of formic acid from pH-dependent experiments with Pt single-crystal electrodes

Acknowledgments This research was funded by Ministerio de Ciencia e Innovación (Spain) (grant number PID2019-105653GB-I00), Generalitat Valenciana (Spain) grant number PROMETEO/2020/063. M.S. and A.C. gratefully acknowledge the support of the University of Aberdeen.Peer reviewedPublisher PD

The inhibition of hydrogen peroxide reduction at low potentials on Pt(111): Hydrogen adsorption or interfacial charge?

The hydrogen peroxide reduction reaction (HPRR) on Pt(111) has been studied for the first time in acid to neutral pH values in the absence of anion specific adsorption using the HMRDE configuration. The onset for the reduction is ca. 0.95 V (RHE) for the Pt(111), irrespective of the pH value. At more negative potential values, the reduction is inhibited. It has been found that the diminution of the activity on Pt(111) occurs at the same potential value in the SHE scale for the different pH values. This indicates that this deactivation is not dependent on the hydrogen adsorption process, as has been previously suggested. However, it appears to be related to the interface water reorganization and the potential of zero free charge of the metal surface. In addition, studies in alkaline conditions suggest that the presence of adsorbed OH species promotes the total conversion of hydrogen peroxide to water.This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P)

Effect of pH and Water Structure on the Oxygen Reduction Reaction on platinum electrodes

The oxygen reduction reaction (ORR) at different pH values has been studied at platinum single crystal electrodes using the hanging meniscus rotating disk electrode (HMRDE) configuration. The use of NaF/HClO4 mixtures allows investigating the reaction up to pH = 6 in solutions with enough buffering capacity and in the absence of anion specific adsorption. The analysis of the currents shows that the kinetic current density measured at 0.85 V for the Pt(111) electrode follows a volcano curve with the maximum located around pH = 9. This maximum activity for pH = 9 can be related to the effects of the electrode charge and/or water structure in the ORR. On the other hand, the catalytic activity for the other basal planes shows a monotonic behavior with a small dependence of the activity with pH. For stepped surfaces with (111) terraces, the behavior with pH changes gets closer to that of the Pt(111) surface as the terrace length increases. Additionally, the ORR curves show a dependence of the limiting diffusion current with pH. It is observed that the limiting current density diminishes as the pH increases in a potential region where hydrogen peroxide is readily reduced. These results suggest the existence of a bifurcation point in the mechanism previous to peroxide formation, in which OOH• is proposed as the bifurcation intermediate. The reduction of OOH• requires proton addition and would be more difficult at neutral pH values, justifying the diminution of the limiting currents.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. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P)

Bromide Adsorption on Pt(111) over a Wide Range of pH: Cyclic Voltammetry and CO Displacement Experiments

Bromide adsorption on Pt(111) is investigated by means of cyclic voltammetry and CO displacement experiments at different pH values. In acidic pH, bromide adsorption is strongly overlapped with hydrogen desorption process. However, as the pH increases, hydrogen adsorption process displaces toward negative potentials while bromide adsorption remains nearly in the same potential region. In consequence, both processes decouple at higher pH values. The structural transition from Pt(111)-(1×1) to Pt(111)(3×3)-4Br is pH independent, in the SHE scale, and not observed for pH > 9.1. Values of pztc are extracted from the combination of voltammetric and CO displaced charges. An alternative approach to obtain charge curves is based on the coincidence of the curves at the structural transition characteristic of the bromide adlayer completion. Pztc values obtained from different approaches with and without bromide are compared, and their dependence on pH discussed. A thermodynamic analysis is carried out to obtain hydrogen Gibbs excess and charge number from the Esin Markov analysis.This work has been financially supported by the MCINN-FEDER (Spain) through Project CTQ2016-76221-P. V.B.-M. thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176, Project CTQ2013-44803-P). G.A.B.M. expresses thanks for the postdoctorate fellowship from CNPq (Grant No. PDE 233268/2014-6)

Charge effects on the behavior of CTAB adsorbed on Au(111) electrodes in aqueous solutions

The behavior of adsorbed CTAB on Au(111) electrodes has been studied using electrochemical and FTIR experiments in different aqueous solutions. The results show that the adsorbed layer is stable in acidic solutions in the whole potential range of study. The observed electrochemical and FTIR behavior is compatible with the formation of a membrane of CTA+ on the electrode surface with the polar amino groups in contact with the surface. When the electrode charge is negative, the polar groups are attracted to the surface, so that the capacitance of the electrode is smaller than that recorded for the unmodified Au(111) electrode. As the charge becomes positive, the membrane detaches from the surface and water molecules permeate through it, changing the capacitance of the electrode and giving rise to characteristic peaks in the voltammetric profile. At potentials higher than these peaks, the behavior of the electrode is comparable to that observed for the unmodified electrode. The stability of the membrane is facilitated by the incorporation of anions of the supporting electrolyte. Those anions remain on the membrane even when the electrode is transferred to a different solution, as the electrochemical behavior shows.Financial support from Ministerio de Ciencia e Innovación (Project PID2019-105653GB-100 ) and Generalitat Valenciana (Project PROMETEO/2020/063 ) is acknowledged

Spectroelectrochemical Studies of CTAB Adsorbed on Gold Surfaces in Perchloric Acid

The behaviour of CTAB adsorbed on polycrystalline gold electrodes has been studied using a combination of spectroelectrochemical methods. The results indicate that the formation of the layer is the consequence of the precipitation of the CTAB micelles on the electrode surface as bromide ions, which stabilize the micelles, are replaced by perchlorate anions. This process leads to the formation of CTA+ layers in which perchlorate ions are intercalated, in which the adlayer suffers a continuous rearrangement that leads to the formation of micro-dominions of different types of hydrogen-bonded water populations throughout the adlayer. After prolonged cycling, a stable situation is reached. Under these conditions, water molecules permeate through the adlayer toward the electrode surface at potentials positive of the potential of zero charge, due to the repulsion between the CTA+ layer and the positive charge of the electrode.This research was funded by Ministerio de Ciencia e Innovación (Spain) grant number PID2019-105653GB-I00), Generalitat Valenciana (Spain) grant number PROMETEO/2020/063. A.C. gratefully acknowledges the support of the University of Aberdeen

Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species

The degradation of Pt-containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO4 by combining operando high-energy surface X-ray diffraction, online mass spectrometry, and density functional theory. Our atomic-scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe-like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO2 and starts to grow when the coverage of the stripe-like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential-independent after the stripe-like oxide has reached its saturation coverage.Funding is acknowledged from Deutsche Forschungsgemeinschaft for OMM and SC (project number 418603497), for OMM by the German Federal Ministry of Education and Research (BMBF) via project 05K19FK3, and for DAH by the NSERC (grant RGPIN-2017-04045). FCV acknowledges that the grants PID2021-127957NB-I00 and TED2021-132550B-C21 were funded by MCIN/AEI/ 10.13039/501100011033 and by the European Union. The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences, with financial support by NWO. Open Access funding enabled and organized by Projekt DEAL

Understandings on the Inhibition of Oxygen Reduction Reaction by Bromide Adsorption on Pt(111) Electrodes at Different pH Values

Oxygen reduction reaction (ORR) is studied on Pt(111) in the presence of different concentrations of bromide anions at different pH values ranging from very acidic to neutral solutions. While adsorbed bromide inhibits the ORR, the strength of the inhibition decreases when the pH is increased. This is a consequence of the lower relative adsorption energy of bromide at higher pH values, caused by the lower absolute applied potential. This is reflected in a shift of the onset of the ORR (as measured with the hanging meniscus rotating disc electrode, HMRDE) to higher values as the pH is increased. HMRDE measurements reveal that the limiting current density (jlim) coincides with the theoretical value for two electrons only at very acidic solutions. However, when pH is increased, jlim tends toward the value for a four electrons reaction. From pH > 3 jlim coincides both in the presence and in the absence of bromide despite the specific anion adsorption. Experiments in solutions with different ionic strength and hydrogen peroxide reduction measurements suggest that the formation of a reaction intermediate different from H2O2 is favored at neutral pH values.This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P). GABM thanks the post-doctorate fellowship from CNPq (grant no. PDE 233268/2014-6)