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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.

In voltammetric CO stripping experiments in acid media, CO pre-oxidation usually takes place on catalysts with a high density of surface defects, obeyed the conditions of full COads layer formation at suitable low potentials into the “hydrogen region” and, importantly, CO traces in the bulk of the solution is required. This study aims to interrogate that. The electro-oxidation of CO was voltammetrically conducted applying different scan rates, employing Pt(1 1 1) terraced surfaces and full CO coverage equilibrated with different content of dissolved CO in the bulk of an acid quiescent solution; also experiments were performed in which COads was present only on the (1 1 0)-type steps in a CO-free solution. It was found that: (1) slopes of peak potentials of CO pre-oxidation versus the logarithm of scan rates were smaller for higher CO contents in the solution; (2) charge density of CO pre-oxidation was higher for slower scan rate; (3) CO pre-oxidation takes place at low potentials where the (1 1 0)-type step sites were inactive in the catalysis of CO electro-oxidation. We conclude that, in such a quiescent solution, the content of dissolved CO in the bulk of the solution determines the magnitude and the shape of the curve of the CO pre-oxidation. This can be understandable in terms of prompt provision of CO molecules from the solution (diffusion layer) to surface active sites, that is, the most active sites, released during the CO pre-oxidation. In the CO pre-oxidation on Pt(1 1 1) terraced surfaces, the most active sites lie at the bottom of (1 1 1) terraces adjacent to the steps. The results provide evidence that, in presence of CO traces in the bulk of the solution, on one hand, the pre-oxidation of CO involves the diffusion-limited electro-oxidation of bulk CO at most active sites released during the CO pre-oxidation . On the other hand, the main CO electro-oxidation peak is a typical behavior of a surface-confined process. Our proposition applies to Pt electrodes in acidic medium. Manuel J.S. Farias is grateful to CAPES (Brazil), Financial support from Ministerio de Ciencia e Innovación (Spain) (Project no. PID2019-105653GB-100 and FJC2018-038607-I) and Generalitat Valenciana (Spain) (Project PROMETEO/2020/063).

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.

The knowledge about how CO occupies and detaches from specific surface sites on well-structured Pt surfaces provides outstanding information on both dynamics/mobility of COads and oxidation of this molecule under electrochemical conditions. This work reports how the potentiostatic growth of different coverage CO adlayers evolves with time on both cubic and octahedral Pt nanoparticles in acidic medium. Data suggest that during the growth of the CO adlayer, COads molecules slightly shift toward low coordination sites only on octahedral Pt nanoparticles, so that these undercoordinated sites are the first filled on octahedral Pt nanoparticles. Conversely, on cubic Pt nanoparticles, adsorbed CO behaves as an immobile species, and low coordinated sites as well as (100) terraces are apparently filled uniformly and simultaneously. However, once the adlayer is complete, irrespectively of whether the CO is oxidized in a single step or in a sequence of different potential steps, results suggest that COads behaves as an immobile species during its oxidation on both octahedral and cubic Pt nanoparticles. M. J. S. Farias is grateful to the CAPES/PNPD (Brazil) for the financial support. G. A. Camara acknowledges CNPq (grants no. 405695/2013-6 and 309176/2015-8) and FUNDECT (grants no. 23/200.583/2012). J. M. Feliu thanks the MINECO (Spain) project CTQ2016-76221-P. J.S.G. acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante.

The identification of active sites in electrocatalytic reactions is part of the elucidation of mechanisms of catalyzed reactions on solid surfaces. However, this is not an easy task, even for apparently simple reactions, as we sometimes think the oxidation of adsorbed CO is. For surfaces consisting of non-equivalent sites, the recognition of specific active sites must consider the influence that facets, as is the steps/defect on the surface of the catalyst, cause in its neighbors; one has to consider the electrochemical environment under which the “active sites” lie on the surface, meaning that defects/steps on the surface do not partake in chemistry by themselves. In this paper, we outline the recent efforts in understanding the close relationships between site-specific and the overall rate and/or selectivity of electrocatalytic reactions. We analyze hydrogen adsorption/desorption, and electro-oxidation of CO, methanol, and ammonia. The classical topic of asymmetric electrocatalysis on kinked surfaces is also addressed for glucose electro-oxidation. The article takes into account selected existing data combined with our original works. M.J.S.F. is grateful to PNPD/CAPES (Brazil). J.M.F. thanks the MCINN (FEDER, Spain) project-CTQ-2016-76221-P.

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

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.

Here we discuss the most recent advances in the comprehension of how CO electrooxidizes on oriented Pt surfaces, that is, shape-controlled Pt nanoparticles and well-defined Pt(111) stepped surfaces. The discussion is focused on the role that the surface structure of the catalyst plays in the emergence of its overall catalytic activity, and we investigate the effect of extrinsic changes in the catalyst environment (by altering the electrolyte and pH of the solutions) and describe how these perturbations affect the catalytic activity of Pt nonequivalent sites. Furthermore, by observing the dynamics/mobility of adsorbed CO (in a compressed CO layer) we demonstrate that its oxidation preferentially takes place over those (111) terraces adjacent to (110) steps, which explains the electrochemical behavior.

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.

In order to deepen the knowledge about the origin of the CO preoxidation process and the intrinsic catalytic activity of Pt superficial steps toward CO oxidation, a series of CO stripping experiments were performed on stepped Pt electrodes in acidic medium. For the occurrence of CO preoxidation, it was found that it arises (reproducibly) whenever four interconnected conditions are simultaneously fulfilled: (1) CO adsorption at potentials lower than about 0.2 V; (2) on surfaces saturated with COads; (3) in the presence of traces of CO in solution; (4) in the presence of surface steps. If any of these four conditions is not satisfied, the CO preoxidation pathway does not appear, even though the steps on the electrode surface are completely covered by CO. By controlling the removal of the CO adlayer (voltammetrically), we show that once the CO adlayer has been partially oxidized, the (111) terrace sites of stepped surfaces are released earlier than the (110) step sites. Moreover, if (110) steps are selectively decorated with CO, its oxidation occurs only at potentials ∼150 mV higher than the CO preoxidation peak. Our results systematically demonstrate that step sites are less active to oxidize CO than those ones responsible for the CO preoxidation process. Once the sites responsible for the CO preoxidation are made free, there is no apparent motion of the remaining adsorbed CO layer, suggesting that the activation of the surface controls the whole process, rather than the diffusion of COads toward hypothetically “most active sites”. Voltammetric and chronoamperometric experiments performed on partially covered CO adlayers suggest that adsorbed CO behave as a motionless species during its oxidation, in which the CO adlayer is removed piece by piece. By means of in situ FTIR experiments, the stretching frequency of CO selectively adsorbed on (110) step sites was examined. Band frequency results confirm that those molecules adsorbed on steps are fully coupled with the adsorbed CO on (111) terraces when the surface reaches full coverage. M.J.S. Farias is grateful to the CNPq (Brazil) (grants 200390/2011-2 and 313402/2013-2) for the financial support. G. A. Camara acknowledges financial assistance from CNPq (grants 305494/2012-0 and 405695/2013-6) and FUNDECT (grant 23/200.583/2012). J. M. Feliu thanks the MINECO (Spain) project CTQ2013-44083-P.

To understand how the CO oxidation reaction proceeds on nanoparticles, which have complex surface structures, the behavior of the nanoparticles has to be related to that of single crystal electrodes with a well-defined surface structure. However, the direct extrapolation of the results is not possible because significant differences in the behavior between both type of surfaces are observed. In single crystal electrodes in both acidic and alkaline media, the reaction initiates on defects on the CO adlayer. These defects can be already present on the surface, as surface defects or steps, or generated during the formation of the CO adlayer. In the case of steps, the oxidation starts on the lower part of the step. The only difference between the reaction behavior between acidic and alkaline solutions is the lower mobility of the CO in alkaline solutions, which generates adlayers with higher number of defects and give rise to multiple stripping peaks in stepped surfaces. Using these results, the differences of the behavior between single crystal electrodes and nanoparticles can be rationalized. In spite of the fact that nanoparticles have small ordered domains, the presence of sites in which the reaction is initiated, equivalent to the site in the lower part of the step, is almost negligible, and thus, the oxidation reaction takes place at higher potential values than in stepped surfaces with similar domain size. Also the effect of nanoparticle agglomeration in the oxidation has been rationalized. This work has been financially supported by the MINECO-FEDER (projects CTQ 2013-44083-P and CTQ2013-48280-C3-3-R) and Generalitat Valenciana (project PROMETEO/2014/013). M.J.S. Farias acknowledges financial support from the CAPES – Brasil (PNPD).

Stepped Pt surfaces having different width (1 1 1) terraces interrupted by (1 1 0) or (1 0 0) monoatomic steps were employed to evaluate the catalytic activity toward CO oxidation at Pt specific sites in HClO4, H2SO4, and H3PO4 solutions, as well as in phosphate buffer and alkaline solution. The catalytic activity at the (1 1 1) terraces was sensitive to the nature of the anions derived from the electrolyte dissociation, while no effect on activity was detected at the monoatomic steps. A change in solution pH, passing from acid to alkaline solutions, had contrasting effects on catalytic activity at the (1 1 1) terraces and the step sites, with the catalytic activity of the (1 1 1) terraces improving, while activity at the step sites deteriorated. During the CO adlayer oxidation, the release of surface sites occurred preferentially from the (1 1 1) terraces of the Pt(s) − [(n − 1)(1 1 1) × (1 1 0)] surfaces, while from Pt(s) − [(n)(1 1 1) × (1 0 0)] surfaces, (1 1 1) terrace and (1 0 0) step sites were released simultaneously. M.J.S. Farias acknowledges financial support from PNPD/CAPES (Brazil). G.A.B.M. received a post-doctorate fellowship from CNPq (grant no. PDE 233268/2014-6). A.A.T. acknowledges support from CAPES (PROCAD 2013) and CNPq (grant no. 309066/2013-1). J.M.F. thanks MICINN (Project CTQ2016-76221-P) and Generalitat Valenciana (Project PROMETEOII/2014/013) for financial support.

In this paper, we investigate the electro-oxidation of isotopically labeled ethanol on PtSnRh electrodeposits. By monitoring the oxidation products by in situ FTIR we show that the high catalytic activity of PtSnRh catalysts is due to their ability to promote the C–C cleavage, as well as the oxidation of methyl groups. Also, the use of Sn in proper ratio enhances the global performance of PtSnRh catalysts towards the oxidation of ethanol, since it seems to prevent the electro-reduction of –CH3 groups to methane, making them available to produce CO2 in the same potential range where the alcoholic group is oxidized. As a consequence, important amounts of CO2 are produced from both extremities of ethanol. Keywords: Ethanol electro-oxidation, Platinum–tin–rhodium, Isotopically-labeled ethanol, Selectivity towards CO2

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).

We report time evolution studies of low coverage CO adsorption (surface hydrogen site blocking 40%, the lateral interactions might play a role in the preferential CO site occupancy. Keywords: Platinum, CO adsorption, CO occupancy, Stepped surface

In this work, we investigated some important aspects related to the electrooxidation of isotope-labeled ethanol on Pt. The central point was the effect of the adsorption potential on the formation of CO2 produced from both alcohol and methyl groups during the electrooxidation of ethanol in acid media. As a way to follow these contributions, we used isotope-labeled ethanol (13CH3CH2OH) and Fourier transform infrared spectroscopy (FTIRS). The yield of 13CO2 increased with the ethanol adsorption potential. Despite the visualization difficulties due to dipole coupling, 13CO was detected, indicating that the formation of 13CO2 follows a mechanism analogous to the one of 12CO2.

Parallel pathways forming CO2 and acetic acid occur during the electrooxidation of acetaldehyde at Pt in acid medium. The yields of products depend on potential and acetaldehyde concentration. In the whole range of concentrations investigated (2.5 × 10−3 – 0.5 M) and at potentials below 0.6 V, CO2 is the only product of acetaldehyde oxidation. Acetic acid is detected at potentials higher than 0.7 V. According to the analysis of products using FTIR spectroscopy, a maximum yield of CO2 production is obtained for an acetaldehyde concentration of 0.01 M at 0.6 V. The pathway forming CO2 is strongly inhibited for 0.5 M of acetaldehyde. It is suggested that, at high concentrations, a competition with water for active sites occurs, which inhibits the oxidation of adsorbed species, which probably follow a Langmuir–Hinshelwood mechanism.

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. This work has been financially supported by the MICINN (Spain) (project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045 – FEDER).

In this work, the behavior of the CO electro-oxidation reaction on shape-controlled Pt nanoparticles in alkaline medium was examined in order to understand the effect of the surface structure on this reaction. A series of experiments using Pt nanoparticles of different surface structures/shapes was used and the results obtained were compared with the previous knowledge gained from stepped platinum single crystal electrodes. Independently of the preferential orientation of the nanoparticles, the CO oxidation voltammetry exhibits two main peaks: one at ca. 0.56–0.59 V and the second one at 0.66–0.67 V, being the intensity of the peaks dependent on the shape of the nanoparticle. These two peaks have been assigned to the oxidation of CO on the (1 1 1) terraces and on the rest of the sites, respectively. The appearance of two differentiated peaks reveals that these (1 1 1) terraces and the rest of the sites on the nanoparticle surface behave independently of the presence of the other type of sites, that is, they are not connected. The results are discussed considering the effects of the surface mobility of CO and of the OH adsorption properties on the different sites in the oxidation peaks. Farias, M.J.S. would like to thanks CNPq, Brazil, for financial support for his postdoctoral stay at Universidad de Alicante. This work has been financially supported by the MICINN (Spain) (project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045, FEDER).

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. M.J.S.F would like to thanks CNPq, Brazil, for financial support for his stay at Universidad de Alicante. This work has been financially supported by the MICINN (Spain) (project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045, FEDER).

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 (