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

Disentangling Catalytic Activity at Terrace and Step Sites on Selectively Ru-Modified Well-Ordered Pt Surfaces Probed by CO Electro-oxidation

In heterogeneous (electro)catalysis, the overall catalytic output results from responses of surface sites with different catalytic activities, and their discrimination in terms of what specific site is responsible for a given activity is not an easy task. Here, we use the electro-oxidation of CO as a probe reaction to access the catalytic activity of different sites on high-Miller index stepped Pt surfaces with their {110} steps selectively modified by Ru at different coverages. Data from in situ Fourier transform infrared spectroscopy and cyclic voltammetry evidence that Ru deposited on {110} steps modifies the surface in a nontrivial way, favoring only the electrocatalytic oxidation of CO over {111} terraces. Moreover, these {111} terraces become catalytically active throughout a large potential window. On the other hand, after the deposition of Ru on {110} steps, the partial oxidation of a CO adlayer (by stripping voltammetry and in situ FTIR potential steps) shows that those {110} steps that remain free of Ru seem not to be influenced by the presence of this metal. As a result, the remaining CO adlayer is oxidized on these Ru-free {110} steps at potentials identical to those observed in steps of pure stepped Pt surfaces (in the absence of Ru). First, these findings suggest that COads behaves as a motionless species during its oxidation. Second, they evidence that the impact caused by the presence of Ru in the catalytic activity of Pt(s)-[(n–1)(111)×(110)] stepped surfaces depends on the crystallographic orientation of Pt sites. These results help us to shed new light on the role of Ru in the mechanism of oxidation of CO and allow a deeper understanding regarding the CO tolerance in Pt–Ru catalysts.M.J.S.F. acknowledges financial support from the CNPq (Brazil) (Grants 200390/2011-2 and 313402/2013-2). G.A.C. acknowledges financial assistance from CNPq (Grants 305494/2012-0, 309176/2015-8, and 405695/2013-6) and FUNDECT (Grant 23/200.583/2012). J.M.F. thanks the MINECO (Spain) (Project CTQ2013-44083-P)

Requirement of initial long-range substrate structure in unusual CO pre-oxidation on Pt(111) electrodes

The activation pathway of CO electro-oxidation at low potentials is frequently favored on a catalyst with surface defects. Here, we report a discovery in which an initial long-range substrate structure is required for the activation of an unusual CO pre-oxidation reaction pathway on Pt(111) surfaces which have been flame annealed and then cooled in a CO atmosphere. Different to current understanding about the oxidation of CO on Pt, the activation of this reaction pathway is inhibited as the (111) planes become defect-rich.M.J.S.F and A.A.T. thanks the CAPES (Brazil) Finance Code 001. J.M.F. thanks the MINECO (Spain) CTQ2013-44083-P project

Hydroxyurea electrooxidation at gold electrodes. In situ infrared spectroelectrochemical and DFT characterization of adsorbed intermediates

The oxidation of hydroxyurea (H2NCONHOH, HU) at Au(100), Au(111) and Au(111)–25 nm thin film electrodes is studied spectroelectrochemically in perchloric acid solutions. HU, which in agreement with DFT results interacts weakly with the gold surfaces, oxidizes irreversibly at gold electrodes irrespective of the surface orientation. The in situ infrared external reflection spectra prove the formation of dissolved carbon dioxide and adsorbed cyanate as products of the HU electrooxidation reaction. A band at ca. 2230 cm−1 can be related both to dissolved isocyanic acid coming from the protonation of adsorbed cyanate or to nitrous oxide coming from the oxidation of hydroxylamine, which is formed (together with adsorbed cyanate) upon the chemical decomposition of hydroxyurea. ATR-SEIRAS experiments allow the observation of other adsorbate bands that can be tentatively ascribed to reaction intermediates that conserve the NCN skeleton and are bonded to the metal by the nitrogen atoms at near on-top positions. Bonding to the surface can be either unidentate or bidentate, involving covalent-type bonds or dative bonding through the lone pairs of the N atoms. Some of the signals of the experimental spectra, in particular those appearing around 1800 cm−1, can be assigned to the CO stretch of adsorbed intermediates having a nitrosyl group formed by oxidation of the NOH moiety (namely, adsorbed nitrosoformamide or its deprotonated form). The bands observed around 1650 cm−1 can correspond either to the NO stretching mode of the former species or to the CO stretching modes of adspecies conserving the NOH group.The authors acknowledge the funding by Ministerio de Economía y Competitividad through projectsCTQ2016-76221-P (AIE/FEDER, UE) and CTQ2016-76231-C2-2-R (AEI/FEDER, UE) and by the University of Alicante (VIGROB-263). William Cheuquepán is grateful for the award of a F.P.I. grant associated to project CTQ2009-13142

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

On the electrochemical behavior of formamidine disulfide on gold electrodes in acid media

The adsorption and reactivity of formamidine disulfide (FDS) were studied at gold single crystal and thin film electrodes in perchloric acid solutions and the results of cyclic voltammetry and in situ infrared spectroscopy (SNIFTIR and ATR-SEIRAS) experiments compared with those previously obtained with thiourea (TU). In addition, optimized geometries and theoretical harmonic vibrational frequencies were obtained for adsorbed FDS, TU and thioureate from plane-wave DFT calculations using periodic surface models. These results were compared in the case of TU and thioureate to those previously obtained with a model Au cluster surface with (111) orientation. In the case of FDS, weak adsorption takes place with the S–S bond essentially parallel to the metal surface. No specific bands for adsorbed FDS can be identified in the experimental ATR-SEIRA spectra. These latter spectra suggest that adsorbed thioureate species are spontaneously formed upon dissociative adsorption of FDS when dosing this molecule at open circuit. Some of the adsorbed thioureate species undergo reductive protonation giving rise to a mixed adlayer formed by adsorbed thioureate and thiourea in a surface process which is faster when a controlled potential of 0.70 V is applied. In agreement with the observations reported when dosing TU, the ratio between TU and thioureate adsorbates is found to depend on the electrode potential, being higher for potentials close to the hydrogen evolution limit and decreasing for higher potential values.The authors acknowledge the finance by Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and the University of Alicante. William Cheuquepán is grateful for the award of a F.P.I. grant associated to project CTQ2009-13142

Squaric acid adsorption and oxidation at gold and platinum electrodes

The adsorption and oxidation of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione, H2C4O4, SQA) at platinum and gold electrodes were studied spectroelectrochemically in perchloric acid solutions. Voltammetric experiments demonstrate that reversible adsorption takes place at gold electrodes in the double-layer region. As a difference with platinum electrodes, no dissociative adsorption processes leading to the blocking of the electrode surface are detected. ATR-SEIRAS experiments show potential-dependent adsorbate bands at potentials below 1.20 V vs RHE that, according to DFT calculations, can be assigned to adsorbed squarate. For platinum electrodes, the potential-dependent adsorption of squarate anions is coupled with the oxidative stripping of adsorbed carbon monoxide, which is formed upon dissociative SQA adsorption. Bonding of squarate species to the platinum and gold surfaces involves two oxygen atoms in a bidentate configuration, with the molecular plane perpendicular to the metal surface. The ATR-SEIRA spectra obtained for gold electrodes in the SQA oxidation region show bands for adsorbed bicarbonate anions formed from dissolved carbon dioxide molecules. In the case of platinum, distinct bands are observed for adsorbed oxidation products which probably are formed upon opening of the SQA ring.The authors acknowledge the funding by Ministerio de Economía y Competitividad (through projects CTQ2016-76221-P (AIE/FEDER, UE) and CTQ2016-76231-C2-2-R (AEI/FEDER, UE)) and by the University of Alicante (VIGROB-263 project)

Spectroelectrochemical detection of specifically adsorbed cyanurate anions at gold electrodes with (111) orientation in contact with cyanate and cyanuric acid neutral solutions

The adsorption and reactivity of cyanate at Au(111) single crystal and Au(111)-25 nm thin film electrodes is studied spectroelectrochemically in sodium perchlorate solutions and compared to those of cyanuric acid (C3N3O3H3). From the Surface Enhanced Infrared Reflection Absorption spectra obtained under Attenuated Total Reflection conditions (ATR-SEIRAS) it can be concluded that adsorbed cyanate species predominate at the electrode surface for low cyanate concentrations. However, for cyanate concentrations above 1 mM, the similarity of the ATR-SEIRA spectra with those obtained in cyanuric acid containing solutions indicates that some species coming from cyanuric acid is formed and adsorbed at the (111) gold surface sites as the result of an electroless trimerization reaction. Taking into account the results of Density Functional Theory (DFT) calculations, the experimental voltammetric and ATR-SEIRAS results agree with the formation of adlayers of specifically adsorbed triketo-monocyanurate species that adsorb perpendicular to the electrode surface.The authors acknowledge the funding by Ministerio de Economía y Competitividad (projects CTQ2016-76221-P and CTQ2016-76231-C2-2-R) and the University of Alicante (VIGROB-044 and VIGROB-263). William Cheuquepán is grateful for the award of a F.P.I. grant associated to project CTQ2009-13142

Squaric acid adsorption and oxidation at gold and platinum electrodes

The adsorption and oxidation of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione, H2C4O4, SQA) at platinum and gold electrodes were studied spectroelectrochemically in perchloric acid solutions. Voltammetric experiments demonstrate that reversible adsorption takes place at gold electrodes in the double-layer region. As a difference with platinum electrodes, no dissociative adsorption processes leading to the blocking of the electrode surface are detected. ATR-SEIRAS experiments show potential-dependent adsorbate bands at potentials below 1.20 V vs RHE that, according to DFT calculations, can be assigned to adsorbed squarate. For platinum electrodes, the potential-dependent adsorption of squarate anions is coupled with the oxidative stripping of adsorbed carbon monoxide, which is formed upon dissociative SQA adsorption. Bonding of squarate species to the platinum and gold surfaces involves two oxygen atoms in a bidentate configuration, with the molecular plane perpendicular to the metal surface. The ATR-SEIRA spectra obtained for gold electrodes in the SQA oxidation region show bands for adsorbed bicarbonate anions formed from dissolved carbon dioxide molecules. In the case of platinum, distinct bands are observed for adsorbed oxidation products which probably are formed upon opening of the SQA ring.The authors acknowledge the funding by Ministerio de Economía y Competitividad (through projects CTQ2016-76221-P (AIE/FEDER, UE) and CTQ2016-76231-C2-2-R (AEI/FEDER, UE)) and by the University of Alicante (VIGROB-263 project)

Unraveling the Nature of Active Sites in Ethanol Electro-oxidation by Site-Specific Marking of a Pt Catalyst with Isotope-Labeled 13CO

This works deals with the identification of preferential site-specific activation at a model Pt surface during a multiproduct reaction. The (110)-type steps of a Pt(332) surface were selectively marked by attaching isotope-labeled 13CO molecules to them, and ethanol oxidation was probed by in situ Foureir transfrom infrared spectroscopy in order to precisely determine the specific sites at which CO2, acetic acid, and acetaldehyde were preferentially formed. The (110) steps were active for splitting the C–C bond, but unexpectedly, we provide evidence that the pathway of CO2 formation was preferentially activated at (111) terraces, rather than at (110) steps. Acetaldehyde was formed at (111) terraces at potentials comparable to those for CO2 formation also at (111) terraces, while the acetic acid formation pathway became active only when the (110) steps were released by the oxidation of adsorbed 13CO, at potentials higher than for the formation of CO2 at (111) terraces of the stepped surface.M.J.S.F. is grateful to PNPD/CAPES (Brazil). A.A.T. acknowledges CAPES (PROCAD-2013) and CNPq (309066/2013-1). J.M.F. thanks the MINECO (Spain) project-CTQ2013-44083-P