Search

In the field of catalysis on solid surfaces, discrimination of the site-specific chemistry taking place on surfaces consisting of multiple types of sites is challenging, especially at electrified solid/liquid interfaces. In this study, site-specific chemical labeling on Pt stepped surfaces was achieved by attaching the 13CO species exclusively on the top side of the (110) steps, while making all of the (111) sites available. The catalysts were then employed in a spectro-electrochemical study of the electro-oxidation of methanol and ethanol. The onset potentials for the formation of 13CO2 and 12CO2 revealed the existence of two channels of carbon dioxide formation, separated by about 0.22 V, on the same Pt surface. The active sites with lower overpotential requirement (or lower energy input) for activation of the reaction pathway of CO2 formation resided on the (111) terraces. On the other hand, the active sites with higher overpotential requirement (or higher energy input) for activation of the reaction pathway for electro-oxidation of 13COads species to 13CO2 were at the top of the (110) steps. The findings revealed the identities of the most active sites and least active sites involved in the formation of CO2 during the electro-oxidation of alcohols. On Pt surfaces, the complex interplay involving the steps on the surface favors activation of the pathways for COads oxidation on the (111) terraces, rather than promoting reaction steps directly on the steps themselves. 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 CTQ2016-762213-P.

One of the main current goals of humanity is the CO2 conversion into high energy compounds for facilitating both a diminution of the CO2 atmospheric levels and the development of energy storage strategies. In many studies, TiO2 has been successfully used as a photocatalyst for CO2 reduction, but there is still a lack of understanding of its catalytic behavior. In this context, CO2 reduction has been studied on nanoporous TiO2 electrodes in acetonitrile media by means of (spectro) electrochemical methods (ATR-IR and UV–vis). Importantly, the onset of the cathodic Faradaic processes related with CO2 reduction on TiO2 electrodes is located at −0.81 V versus SHE, which is less negative than that observed for metal electrodes under similar conditions. UV–vis spectroelectrochemical results indicate that the electrocatalytic behavior of TiO2 is related to the generation of oxygen vacancies and Ti3+ sites at its surface and promoted by electrolytes with nonintercalating cations in agreement with recent results on WO3 electrodes. ATR-IR spectroelectrochemical measurements allow for monitoring of the TiO2/solution interfacial state as reduction proceeds. Specifically, IR bands for carbon monoxide and carbonyl groups related with carbonate and oxalate are observed. Additionally, a chromatographic analysis shows CO and oxalate as main products. With controlled water addition (0.5 M), methanol and CO were found to be the main products. Based on these results, a mechanism for CO2 reduction on TiO2 electrodes is presented in which the regeneration of the TiO2 surface by oxide electrodissolution/deposition is a critical step. We are grateful for the financial support of the Ministerio de Ciencia, Innovación y Universidades through projects MAT2015-71727-R and RTI2018-102061-B-I00 (FONDOS FEDER).

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.

[EN] Advanced applications of colloidal nanoparticles (NPs) become to depend on their specific shape, which is controlled by the adsorption behavior of the capping agent involved in their synthesis. To understand the way in which citric acid determines the shape of gold NPs, the adsorption behavior of citrate on gold under the synthesis conditions is here investigated from electrochemical experiments on well-defined surfaces. Gibb excesses and charge numbers for the citrate adlayers deposited on the Au (111), Au(100) and Au(110) electrodes when a potential is applied were estimated at pHs 1 and 3. From these results, FTIR spectra and DFT calculations, it is concluded that solvated citrate can become simultaneously adsorbed through three dehydrogenated carboxylic groups in bidentate configuration on Au(111), but only through two on Au(100) and Au(110). As a result of this behavior, citrate can become more strongly adsorbed on Au(111) than on the other two basal planes of gold under the synthesis conditions, which would explain why tetrahedral and octahedral colloidal gold NPs are preferentially shaped when citric acid is used as the capping agent in water. This conclusion coincides with the previously one obtained on platinum, suggesting that the mechanism here described would operate also on other metals having fcc structure., This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P.

Glucose is an important biomolecule that participates as a main source of energy in living organism reactions. Considering its biological importance, the electrochemical investigation about glucose electro-oxidation has attracted attention with the aim to develop non-enzymatic catalysts able to achieve the complete oxidation of glucose to be used as fuel in Direct Glucose Fuel Cells (DGFC). However, there are some challenges related to the oxidation of this relatively large, complex molecule, such as product distributions, adsorbed species, surface poisoning and structure sensitivity that must be understood. The use of well-defined surfaces, as single crystals, is a helpful tool to progress in this knowledge. In this context, the goal of this manuscript is the elucidation of the glucose electro-oxidation reaction in phosphate buffered solution (pH 7) on Pt(100) using cyclic voltammetry and in situ FTIR techniques at room temperature. It was observed that the reaction product distribution depends on glucose concentration. The maximum current density was reached with 2 × 10−2 mol dm−3 glucose (~2.04 mA cm−2) and then decreases with the increase of glucose concentration, because the reaction becomes dominated by the formation of strongly adsorbed species. In situ FITR experiments on Pt(100) show that the reaction proceeds through a complex mechanism, involving CO2, COL, COB, cyclic carbonate, γ-lactone, δ-lactone and carboxylic acids as intermediates and/or oxidation products. CO2 is generated mainly from COB and cyclic carbonate. Adsorbed CO seems to be the poisoning species responsible of the decrease of the catalytic activity on Pt(100), especially in the linear configuration, at higher glucose concentration. Gisele A. B. Mello thanks the post doctorate fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), of the Ministery of Science, Technology and Innovation of Brazil (grant no. PDE 233268/2014–6). Juan M. Feliu thanks MICINN-FEDER, Spain (project CTQ2016–76221.P).

Glucose electro-oxidation is a topic of great relevance in electrochemistry, especially due to the possibility of using this carbohydrate as a fuel in implantable DGFC cells (Direct Glucose Fuel Cell). In spite of it has already been reported in the 1990s that this reaction is sensitive to the symmetry of surface surfaces, the understanding of the structure-reactivity relationship is not completely elucidated, especially in a neutral medium, in which the particularities at well defined surface of this sensitivity is not reported yet. In this context, this work studies the electro-oxidation of glucose in phosphate buffer solution (pH 7 ~ physiological) on low-index Pt single crystal surfaces, using cyclic voltammetry and in situ FTIR techniques at room temperature, with the aim to investigate the reactivity of surface catalysts in neutral medium, and thus, understand and establish the relation between the reaction reactivity and the surface structure (structure-sensitivity). Under these conditions it is observed that glucose electro-oxidation is strongly dependent of the electrode structure in terms of catalytic activity and reaction mechanism. Pt(110) is the less active surface, due to the highest formation of COL, which is strongly adsorbed on (110) sites, poisoning them. Pt(100) is the most active surface and the reaction proceeds through a more complex mechanism. Although the highest formation of CO2 has been observed on this surface, the major activity of Pt(100) is mainly associated to the production of cyclic carbonate on (100) sites. Among Pt low-index planes, Pt(111) is the less poisoned surface, and this is because on this surface carbonate ring is absent and COL generated during the oxidative process is completely stripped out from the surface during the potential scan. Gisele A. B. Mello thanks the post doctorate fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), of the Ministery of Science, Technology and Innovation of Brazil (grant no. PDE 233268/2014-6). Juan M. Feliu thanks MICINN-FEDER, Spain (project CTQ2016-76221.P).

The adsorption and reactivity of cyanate at silver electrodes is studied spectroelectrochemically in sodium perchlorate solutions and compared to those of cyanuric acid (1,3,5-triazine-2,4,6-trione, C3H3N3O3). The surface enhanced infrared reflection absorption spectra obtained under attenuated total reflection conditions (ATR-SEIRAS) for low cyanate concentrations show that some amount of adsorbed CO is formed at the silver electrode surface at low electrode potentials. Adsorbed cyanate species are formed at higher potentials and predominate at the electrode surface. However, for cyanate concentrations above 1 mM, the ATR-SEIRA spectra show potential-dependent absorption features between 1800 and 1300 cm–1, which are similar to those obtained after dosing cyanuric acid to the sodium perchlorate solution. This observation suggests that adsorbed cyanuric acid related species are formed from cyanate anions at the silver surface sites as the result of an electroless trimerization reaction. The infrared spectra collected in the cyanuric acid containing solutions also show a higher amount of adsorbed CO at low potential when compared to that observed in cyanate-containing solutions, thus indicating certain instability of cyanuric acid at the silver surfaces. Density functional theory (DFT) calculations have allowed the study of the effect of crystallographic orientation and bonding geometry on the adsorption energy and vibrational frequencies of adsorbates. Also, the effect of adsorbate coverage on the vibrational frequencies has been investigated. In the case of cyanate, DFT calculations show that the most favorable adspecies is N-bonded cyanate adsorbed in 4-fold hollow (100) sites. The experimental spectra in cyanuric acid solutions agree with the formation of adlayers of specifically adsorbed triketo-monocyanurate species that adsorb, in a tridentate configuration, perpendicular to the electrode surface. The polyoriented nature of the silver surface used in the spectroelectrochemical experiments, together with the eventual existence of cyanate in domains with different local coverages, can explain the broadness of the band for the asymmetric OCN stretching. For adsorbed triketo-monocyanurate, high local coverages are at the origin of the band observed experimentally at ca. 1790 cm–1 that can be associated with the in-phase collective CO stretching. 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).

This work investigates the mechanisms by which Sn and Ru can improve or inhibit the site-specific catalytic activity of Pt (neighboring or distant from foreign atoms) at bimetallic surfaces. For this purpose, we decorated Pt stepped surfaces (nonequivalent sites) by site-selective electrodeposition of different coverages of either Sn or Ru on (110) Pt steps, forming Snsteps/Pt(hkl) and Rusteps/Pt(hkl) bimetallic surfaces, and we used CO adlayer electro-oxidation as a surface probe reaction, monitored by in situ FTIR and cyclic voltammetry techniques. The results showed that both Sn and Ru selectively accelerated the reaction pathway of CO electro-oxidation only at the (111) Pt terrace sites but importantly played different underlying roles in favoring activity at these active sites. In the case of Snsteps/Pt(hkl) catalysts, the CO adlayer oxidation started at potentials lower than those on Rusteps/Pt(hkl), but Sn only improved the activity at sites on atoms of the first rows of (111) Pt terraces, while the catalytic benefit of Ru seemed to extend further along the (111) Pt terraces. In comparison to unmodified Pt surfaces, Ru did not influence the activity at the line of the (110) Pt steps, while Sn slightly inhibited the activity there, which characterized a slight contrasting effect in catalytic activity at the (111) terraces in comparison to the (110) step sites. In this regard, the chemical modification by irreversible deposition of either Sn or Ru at lines of Pt steps on a stepped Pt surface interestingly resulted in a nonuniform synergistic effect or balancing of energies involving different site-specific catalytic activities at nonequivalent Pt surface sites. Since the electro-oxidation of CO takes place at the (111) Pt terrace sites away from Sn or Ru, and because COads behaves as an immobile species during its oxidation, it is reasonable to assume that the classical bifunctional mechanism completely fails as a model to interpret the enhancement of catalytic activity toward CO electro-oxidation at Snsteps/Pt(hkl) or Rusteps/Pt(hkl) catalysts. The selective alteration in site-specific catalytic activity of Pt was related to (i) the type and coverage of foreign atoms (Sn or Ru) at the lines of Pt steps, (ii) the crystallographic orientation of Pt sites, whether (111) terraces or steps, and (iii) the width of the (111) Pt terraces. M.J.S.F. acknowledges financial support from PNPD/CAPES (Brazil). A.A.T. acknowledges support from CAPES (PROCAD 2013) and CNPq (grant #309066/2013-1). J.M.F. thanks MINECO (Project CTQ2016-76221-P) and GV (Project PROMETEOII/2014/013) for financial support. W.C. is grateful for the award of an F.P.I. grant associated with project CTQ2009-13142.

Herein we investigate the effect of irreversibly adsorbed bismuth on polycrystalline platinum (Ptp) on the electrooxidation of glycerol in alkaline media by combining electrochemical, spectroscopic (in situ FTIR), and analytical (HPLC on line) techniques. We found that the activity of Ptp increases by about 5-fold when the optimal quantity of Bi ions is added to the solution. Besides, the adatom strongly impacts the reaction products by suppressing the pathways with C–C bond breaking, hindering the formation of CO (and other unknown intermediates) and enhancing the production of glycerate. Different from the results in acid media for Ptp-Bi systems where Bi blocks the oxidation pathway through the primary carbon, glycerate is the main product in alkaline media, and dihydroxyacetone is either produced in extremely low quantities or not produced. Besides, comparing our results with those in acid media, the peak current recorded at 1 mV·s–1 in this work was 1 order of magnitude higher. These results show the strong impact of the pH in the reaction rate and selectivity. Financial support from the Brazilian agencies: PSF and JLB thanks FAPESP (grants: 2016/01365-0) and Shell and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. MBCS thanks PRP-FAEPEX and RAV and VYY thanks SAE-Unicamp for their scholarships. J.S-G. acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02).

Infrared spectroscopy is a widely employed analytical tool in (electrochemical) surface science as the spectra contain a wealth of information about the interaction of interfacial adsorbates with their environment. Separating and quantifying individual contributions, for example, of co-adsorbates, the substrate or electric field effects, on the overall spectral response, however, is often non-trivial as the various interactions manifest themselves in similar spectral behavior. Here, we present an experimental approach to differentiate between and quantify potential-induced coverage dependence and field-related Stark effects observed in a sulfate band shift of 93.5 ± 1.5 cm−1/V in electrochemical infrared spectra of the showcase sulfate/Au(111) interface. In combination with a simple linear model equation used to describe the potential-induced peak shift of the sulfate stretch vibration, we determine the coverage dependence contribution to be 15.6 ± 1.2 cm−1/θSO and the Stark effect to amount to 75.6 ± 2.7 cm−1/V. Our work provides a novel route to gain fundamental insight into interfacial adsorbate interactions in electrochemical surface science. J.H.K.P. and K.F.D. gratefully acknowledge financial support by the Max Planck Graduate Center with the Johannes Gutenberg University Mainz (MPGC). U.E.Z. is grateful for financial support from the Alexander von Humboldt Foundation. J.M.F. thanks MCINN-FEDER (Spain) for support through Project No. CTQ2016-76221-P. K.F.D. acknowledges generous support through the Emmy Noether Program of the Deutsche Forschungsgemeinschaft (No. DO1691/1-1) and through the “Plus 3” Program of the Boehringer Ingelheim Foundation.

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

The adsorption and oxidation of squaric acid (H2C4O4, H2SQ) at gold single crystal and thin-film electrodes with preferential (111) orientation were studied spectroelectrochemically in perchloric acid solutions. The existence of reversible adsorption–desorption processes in the double-layer region is reflected by structure-sensitive voltammetric features. Infrared reflection absorption spectroscopy experiments carried out with Au(111) and Au(100) single-crystal surfaces in 10 mM H2SQ solutions show potential-dependent adsorbate bands at ca. 1780–1785 and 1511–1577 cm–1 for potentials below 1.00 V RHE. The increasing sensitivity of the attenuated total reflection (ATR)–surface-enhanced infrared reflection absorption (SEIRA) experiments allows the detection of similar features for much lower H2SQ concentrations. According to density functional theory (DFT) calculations, these bands can be assigned to adsorbed squarate anions which are bonded to the gold surfaces in a bidentate configuration through two oxygen atoms, with the molecular plane perpendicular to the metal surface. For 10 mM H2SQ solutions, additional bands are detected in the ATR–SEIRA spectra at ca. 1630 cm–1 both in water and deuterium oxide solutions. Even if this frequency fits with one of the vibrational modes of adsorbed bisquarate, DFT calculations provide an alternative explanation for this potential-dependent feature that could be ascribed to collective vibrational modes of adsorbed squarate appearing at high adsorbate coverage. The existence of in-phase and out-of-phase contributions under these conditions would also explain the broadening and/or splitting of the observed bands. DFT calculations also show that squaric acid molecules adsorb very weakly at the gold surfaces and can be discarded as the origin of the observed infrared bands. The external reflection infrared spectra obtained for gold single-crystal electrodes in the H2SQ oxidation region show bands for dissolved carbon dioxide molecules as the main product. Bands for adsorbed bicarbonate anions formed from carbon dioxide are detected in the ATR–SEIRA spectra. 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).

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).

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.

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.

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.

The adsorption of cyanate anions at Au(111) and Au(100) single crystal electrodes has been studied spectroelectrochemically in neutral solutions. Potential-dependent in situ InfraRed Reflection Absorption spectra obtained below the onset of cyanate oxidation were compared with previously published data and analyzed on the basis of periodical Density Functional Theory (DFT) calculations. The calculated adsorption energies for cyanate and related species suggest that cyanic and isocyanic acid adsorb weakly at the studied gold surfaces and, thus, seems not to be at the origin of any of the adsorbate bands in the experimental infrared spectra collected in the cyanate-containing solutions. The latter features can be clearly ascribed to the asymmetric OCN stretching of N-bonded cyanate species. The observation of absorption bands in a wide spectral region, including features above 2200 cm− 1, agrees with the coexistence of N-bonded cyanate species with different adsorption sites and tilting angles. DFT calculations have revealed that although these adspecies can have significantly different frequencies, their adsorption energies are rather close. In addition, the existence of collective in-phase vibrations at relatively high cyanate coverages also contributes to the widening of the absorption bands. The authors acknowledge the funding by the Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and the University of Alicante (VIGROB-263). William Cheuquepán is grateful for the award of a F.P.I. grant associated to project CTQ2009-13142.

The oxidation of hydroxyurea (H2NCONHOH, HU) at Au(111) single crystal and Au(111)-25 nm thin film electrodes is studied spectroelectrochemically in HClO4 solutions using external reflection infrared and Surface Enhanced Infrared Reflection Absorption Spectroscopy under Attenuated Total Reflection conditions (ATR-SEIRAS). The in situ external reflection spectra prove the formation of dissolved carbon dioxide and adsorbed cyanate during the electrooxidation of HU. The enhanced surface sensitivity of the ATR-SEIRAS experiments facilitates the detection of adsorbed cyanate and allows the observation of other adsorbate bands tentatively ascribed to reaction intermediates involving partial oxidation of the HU molecules, some of them corresponding to nitrosyl groups. The authors acknowledge funding by Ministerio de Economía y Competitividad (projects CTQ2016-76221-P and CTQ2016-76231-C2-2-R) and University of Alicante (VIGROB-044 and 263). William Cheuquepán is grateful for the award of a F.P.I. grant associated with project CTQ2009-13142.

We report the formation of cyanuric acid species at Au(111) electrodes in cyanate-containing solutions, due to the electroless trimerization of isocyanic acid. Similar experimental bands in the range between 1300 and 1900 cm− 1 are observed in Surface Enhanced Infrared Reflection Spectroscopy experiments under Attenuated Total Reflection conditions (ATR-SEIRAS) with solutions containing either cyanate anions at high concentration or cyanuric acid. The experimental frequencies agree well with those obtained from Density Functional Theory (DFT) calculations for the adsorbed cyanurate anion, bonded to the metal in a tridentate configuration, with its molecular plane perpendicular to the metal surface. The authors acknowledge funding by Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and University of Alicante (VIGROB-263).William Cheuquepán is grateful for the award of a F.P.I. grant associated to project CTQ2009-13142.

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).

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.

The adsorption of thiourea (TU) at Au(111) and Au(100) single crystal and evaporated gold thin-film electrodes with preferential (111) orientation was studied in perchloric acid solutions with TU concentrations below 0.1 mM. For this purpose, cyclic voltammetry with gold single crystals was combined with external reflection infrared spectroscopy and surface-enhanced infrared reflection–absorption spectroscopy under attenuated total reflection conditions (ATR-SEIRAS) with gold thin film electrodes. In situ surface enhanced Raman spectroscopy (SERS) experiments were also carried out with these latter samples. In addition, optimized geometries and theoretical harmonic vibrational frequencies, obtained from B3LYP/LANL2DZ, 6-31+G(d) calculations for TU and thioureate species adsorbed on Au clusters with (111) orientation, were used for the interpretation of the experimental spectra. ATR-SEIRAS experiments show irreversible adsorption of TU at 0.10 V, whereas the SERS experiments have confirmed the bonding of the TU molecule to the metal surface through the S atom. The optimized geometry obtained from density functional theory (DFT) calculations for adsorbed TU corresponds to unidentate bonding through the sulfur atom, with the Au–S bond slightly tilted (13°) from the surface normal, whereas the C–S bond appears to be tilted by 45°. In the case of adsorbed thioureate, under the application of an external electric field of 0.01 a.u., a bidentate asymmetrical bridge adsorption configuration is obtained with one N(H) and the S atoms bonded to Au in positions close to “top” adsorption sites and with the molecular plane perpendicular to the metal surface. The observation of an adsorbate band for the asymmetric NCN stretching in the experimental infrared spectra confirms the tilting of the S–C bond of the adsorbed TU at low potentials. Changes of the adsorbate bands in the ATR-SEIRA spectra at potentials around 0.60 vs reversible hydrogen electrode (RHE) can be interpreted on the basis of DFT results as due to the deprotonation of adsorbed TU to form adsorbed thioureate. We thank the financial support from Ministerio de Economía y Competitividad (project CTQ2009-13142, Fondos FEDER), Generalitat Valenciana (ACOMP/2011/200 and ACOMP/2012/137, Fondos FEDER), and Universidad de Alicante.