Platinum Single Crystal Electrodes Research Articles

Unraveling the Origin of the Repulsive Interaction between Hydrogen Adsorbates on Platinum Single-Crystal Electrodes.

Hydrogen adsorption on platinum (Pt) single-crystal electrodes has been studied intensively in both experiments and computations. Yet, the precise origin and nature of the repulsive interactions observed between hydrogen adsorbates (Hads) have remained elusive. Here, we use first-principles density functional theory calculations to investigate in detail the interactions between Hads on Pt(111), Pt(100), and Pt(110) surfaces. The repulsive interaction between Hads on Pt(111) is deconvoluted into three different physical contributions, namely, (i) electrostatic interactions, (ii) surface distortion effect, and (iii) surface coordination effect. The long-range electrostatic interaction, which is generally considered the most important source of repulsive interactions in surface adsorption, was found to contribute less than 30% of the overall repulsive interaction. The remaining >70% arises from the other two contributions, underscoring the critical influence of surface-mediated interactions on the adsorption process. Surface distortion and coordination effects are found to strongly depend on the coverage and adsorption geometry: the effect of surface distortion dominates when adsorbates reside two or more Pt atoms apart; the effect of surface coordination dominates if hydrogen is adsorbed on neighboring adsorption sites. The above effects are considerably less pronounced on Pt(100) and Pt(110), therefore resulting in weaker interactions between Hads on these two surfaces. Overall, the study highlights the relevance of surface-mediated effects on adsorbate-adsorbate interactions, such as the often-overlooked surface distortion. The effect of these interactions on the hotly debated adsorption site for the adsorbed hydrogen intermediate in the hydrogen evolution reaction is also discussed.

Enhanced electrocatalysis of oxygen reduction on Se-Modified platinum single crystal electrodes

Se-modified platinum single-crystal surfaces in the [011¯] zone have been used as model electrocatalysts for the oxygen reduction reaction (ORR). The results show that: (1) the Pt(hkl)-[n(111)×(100)] electrodes display enhanced ORR electrocatalytic activity with Se modification. (2) Se adatoms have a double effect on the ORR kinetics at model Pt electrodes: they enhance the activity on the neighboring sites and inhibit the reaction on the sites in direct contact with the Se adatoms. (3) The hydrophobicity and the altered electronic properties are probably the reasons for the enhanced electrocatalysis for ORR with Se modification. The positive shift of the potential of zero free charge of Pt(hkl)@Se electrodes compared to that of un-modified Pt(hkl) are in agreement with these interpretations. (4) At very low coverages, the promotion effect of Se modification on ORR activity at (100) sites is stronger than that at (111) sites, which is probably due to the more pronounced hydrophobicity of the former.

Understanding the electrochemical hydrogenation of acetone on Pt single crystal electrodes

• Pt(1 1 0) electrode is the only basal plane active for the acetone electroreduction. • The inclusion of (1 1 1) steps on (1 1 0) terraces slightly alters the behavior of Pt(1 1 0). • Insertion of (1 0 0) steps on (1 1 0) terraces decreases the activity. • Sulfate competes with acetone for the surface sites. The heterogeneous upgrading of biomass by means of electrocatalytic hydrogenation is an attractive way to refine products for industrial and pharmaceutical purposes. Also, the efficient electrochemical reduction of carbonyl compounds can act as hydrogen vectors, and therefore energy vectors. In this manuscript, we render further fundamental insights into the electrochemical reduction of acetone as a model molecule of carbonyl compounds. The structural sensitivity of the reaction is demonstrated by using platinum single crystal electrodes with low Miller indices and stepped electrodes with (1 1 0) terraces and either (1 1 1) or (1 0 0) monoatomic steps. Among the basal planes, Pt(1 1 0) is the only one active for the electroreduction of acetone. The inclusion of (1 1 1) steps on the (1 1 0) terraces does not significantly alter the behavior of Pt(1 1 0), but increasing the (1 0 0) step density has been observed to decrease the activity. We attribute this different performance to a geometrical effect of the active sites. By using different supporting electrolytes, we have found that sulfate competes with acetone for the surface sites, thus modifying the adlayer interfacial structure and hampering acetone reactivity.

(Keynote) Effect of the Surface Charge on the Oxygen and Hydrogen Peroxide Reduction Reactions

The electrochemical behavior of hydrogen peroxide, which is a possible intermediate of the oxygen reduction reaction (ORR), has been investigated on platinum single crystal electrodes containing terraces with (111) symmetry separated by monoatomic steps with either (110) or (100) symmetry in different pH solutions [1]. On these electrodes, a decrease, in absolute value, of the reduction current at low potentials is observed. The potential at which the decrease is observed, can be correlated with the potential of maximum entropy (pme) of the double layer formation on the terraces. In addition, it is possible to relate the peak appearing at potentials below 0.3 V (RHE) to the local pme in the steps.By performing experiments at different pH values, up to pH ≈ 5, it is observed that the inhibition potential on the terraces shifts 59 mV per pH unit towards more positive potentials, in line with what was observed for the pme in laser-induced temperature jump experiments. For the possible explanation of this phenomenon, it must be taken into account that OH adsorption may influence this trend when the pme values are close to the potentials corresponding to OH adsorption. From results obtained in alkaline medium, it is clear that the surface charge plays an important role in this inhibition of the limiting diffusion current.Finally, the comparison of the results obtained under similar conditions for the hydrogen peroxide and oxygen reduction reactions suggests that the formation of hydrogen peroxide as an ORR intermediate is less favored with increasing pH. As a consequence, there must be a bifurcation in the ORR mechanism prior to peroxide formation. These results agree with previous results [2], which indicate that ORR involves different steps in which several bifurcation points may exist. Depending on the experimental conditions, the reaction may follow a different reaction pathway [3]. References. (1) Briega-Martos, V.; Herrero, E.; Feliu, J.M. Electrochimica Acta 2020 , 334, 135452.(2) Gomez-Marin, A.M.; Feliu, J.M. ChemSusChem 2013, 6, 172-176.(3) Briega-Martos, V.; Herrero, E.; Feliu, J.M. Chinese J. Catal. 2020, 41, 732-738.

Study of M(hkl)| Ionic Liquid Interfaces in Well-Defined Surroundings

Ionic liquids have very interesting properties for their use in electrochemistry: they have a wide potential window of stability, high electrical conductivity and can replace water in processes in which water is not an appropriate solvent. Because of that, the number of studies has increased exponentially in recent years. When used in electrocatalytic applications, the structure of the interphase alters the electrocatalytic response of the electrode. For this reason, the characterization of the interphase, and especially the nature of the interaction of the ionic liquid with the metal surface becomes an essential element in the study of these reactions. In most cases, the interaction between the ions and the metal surface is specific, that is, a chemical interaction between the ionic liquid and the surface is established. Then, the interaction will depend on the nature of the ionic liquid and the electronic properties of the surface. This implies that this interaction is structure-sensitive. Practical surfaces are composed of several types of sites, which have different geometries and different interactions with the ionic liquid so that the observed responses are difficult to analyze. The best way to simplify the problem is the use of single crystal electrodes, which have a well-defined surface atomic structure, which allows establishing correlations between surface structure and observed behavior.One of the main disadvantages of using ionic liquids in electrochemistry is their high viscosity, which hinders the diffusion of the electroactive species towards the electrode surface and thus, lowering the reaction rates. This has triggered the study of low viscosity ionic liquids, which are usually those based on the imidazole cation ([Im+]) and/or the bis(trifluoromethyl)sulfonyl imide anion ([NTf2-]). It should be noted that the ionic liquid-electrode interphase is more complex than one in an aqueous medium. Ionic liquids have a higher electrostatic charge and interaction between the ions, which complicates the application of simple double-layer models. It is also important to highlight that, being organic compounds, they are prone to have impurities related to their synthesis processes, which can affect their physicochemical properties. In electrochemical systems in which ionic liquids are used, these liquids play the role of both solvent and electrolyte, which complicates the study of interactions, since the concentration of the ions is very high. To simplify the problem and to have a system with which to compare, ionic liquids can be dissolved in water. In this case, the concentration of the ions is reduced, facilitating their study. In addition, models are available to study and analyze their behavior. Thus, it is possible to analyze the effect of the concentration of the ionic liquid on its behavior and to extrapolate it to concentrations in which water is absent. In addition, due to the high hygroscopicity of ionic liquids, it is very difficult to completely eliminate water in practical applications. Furthermore, the presence of ionic liquids in aqueous solutions has been shown to catalyze some electrochemical reactions where small amounts of these have been shown to significantly increase the reaction rate. Thus, the aim of this work is to characterize the electrochemical behavior in water of the different ions that form the ionic liquids based in [Im]+ cation and ([NTf2]-) anion separately on platinum single crystal electrodes. To determine the role of each ion, different salts composed with at least one of the ions of interest ([Im]+ or ([NTf2]-)) will be used. The interactions of the ions and the electrode will be characterized using electrochemical and spectroelectrochemical techniques.

Cation Size Effect on Oxygen Reduction Reaction in Aprotic Electrolytes

The most important cathodic process in alkali metal-air (M-O2) batteries is the electrocatalytic oxygen reduction reaction (ORR) typically forming superoxide (MO2), and peroxide (M2O2).1 Cation size in aqueous solvents have been demonstrated to affect the ORR activity due to the cation hydration energy.2 However, within aprotic electrolytes, the reaction mechanisms and the activity of ORR with respect to cation size remains undetermined.Traditionally, electrochemistry has been involved in the analysis of the reaction mechanisms of ORR. Nonetheless, this technique by itself is not suitable to identify intermediaries and products of the redox reactions taking place at the electrochemical interface. The combination of cyclic voltammetry with in situ Raman spectroscopy provide the opportunity to study and develop a fundamental understanding of the ORR under potential control, accessing reaction pathways and relating them directly to surface structure.3 In this sense, the breakthrough of Shell-Isolated Nanoparticles for Enhanced Raman Spectroscopy (SHINERS)4,5 is noteworthy. SHINERS is a non-destructive, powerful technique for surface analysis that has been demonstrated to provide SERS effect even in the absence of intrinsic roughness and without requiring Au, Ag or Cu to be used as working electrodes.3,5 In this work, electrochemical measurements and SHINERS are utilised to study the effect of cation upon electrified interfaces during ORR. Zhang, J., Zhang, X.-G., Dong, J.-C., Radjenovic, P. M., Young, D. J., Yao, J.-L., Yuan, Y.-X., Tian, Z.-Q. & Li, J.-F. Real-Time Monitoring of Surface Effects on the Oxygen Reduction Reaction Mechanism for Aprotic Na–O2 Batteries. J. Am. Chem. Soc. 143, 20049–20054 (2021).Kumeda, T., Tajiri, H., Sakata, O., Hoshi, N. & Nakamura, M. Effect of hydrophobic cations on the oxygen reduction reaction on single‒crystal platinum electrodes. Nat. Commun. 9, 1–7 (2018).Guan, S., Attard, G. A. & Wain, A. J. Observation of Substituent Effects in the Electrochemical Adsorption and Hydrogenation of Alkynes on Pt{hkl} Using SHINERS. ACS Catal. 10, 10999–11010 (2020).Li, J. F., Huang, Y. F., Ding, Y., Yang, Z. L., Li, S. B., Zhou, X. S., Fan, F. R., Zhang, W., Zhou, Z. Y., Wu, D. Y., Ren, B., Wang, Z. L. & Tian, Z. Q. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392–395 (2010).Galloway, T. A., Dong, J.-C., Li, J.-F., Attard, G. & Hardwick, L. J. Oxygen reactions on Pt{hkl} in a non-aqueous Na+ electrolyte: site selective stabilisation of a sodium peroxy species. Chem. Sci. 10, 2956–2964 (2019).

Electrochemical Hydrogenation of Aromatic Hydrocarbons

Electrochemical hydrogenation (ECH) of bio-mass derived compounds is an attractive alternative to traditional thermochemical hydrogenation (TCH) methods that are used in the U.S. chemical and petroleum industries to produce value-added fuels and chemicals. TCH uses high pressures and temperatures, along with an external source of hydrogen gas typically produced via methane reformation, in order to achieve hydrogenation; these requirements make it an energy intensive process. ECH has the advantages of operating near ambient conditions and sourcing the participating hydrogen from the aqueous electrolyte solution, resulting in reduced energy costs and CO2 emissions compared to TCH. Furthermore, the applied potential provides an additional parameter for controlling selectivity, making ECH more suitable to handle the wide chemical variability of biomass-derived feedstocks.Aromatic hydrocarbons are of interest as model compounds for lignocellulosic bio-oil [1]. The ECH of aromatic hydrocarbons proceeds with high faradaic efficiency (FE) at low overpotentials, however, the ECH turnover rate remains low at these potentials. There is no simple fix to this, as any increase in overpotential in an attempt to increase the degree hydrogenation will reduce the reaction FE, due to competition with the hydrogen evolution reaction (HER). In this work we address the need for a more fundamental understanding of ECH systems to promote high turnover rates at low overpotentials. In phenol TCH, it is well known that the hydrogenation is limited by the formation/addition of surface adsorbed hydrogen. The current ECH literature draws on these TCH studies and assumes that phenol ECH is also limited by surface adsorbed hydrogen [2]. Thus, most ECH work is done is acidic media, where the kinetics of surface adsorbed hydrogen formation are fastest on platinum [3]. HER, however, is also faster on platinum in acidic media [4], making it more difficult to achieve high faradaic efficiency.We have investigated the ECH of phenol in aqueous electrolyte at varying pH (1, 3, 11, and 13) on low-index single crystal platinum electrodes using rotating disk electrode voltammetry. We have also included the ECH of acetophenone and benzaldehyde to compare the effect of different functionalities on the phenyl ring. The conversion and faradaic efficiency have been determined for these molecules at pH 1 and pH 11 at selected potentials. In this poster we will present new insights on the ECH reaction mechanism of aromatic hydrocarbons along with methods to promote higher ECH rates at low overpotentials.[1] J. Yan et al., "Characterizing Variability in Lignocellulosic Biomass: A Review," ACS Sustainable Chemistry & Engineering, vol. 8, no. 22, pp. 8059-8085, 2020, doi: 10.1021/acssuschemeng.9b06263.[2] N. Singh et al., "Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals," Journal of Catalysis, vol. 382, pp. 372-384, 2020, doi: 10.1016/j.jcat.2019.12.034.[3] S. Intikhab, J. D. Snyder, and M. H. Tang, "Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis," ACS Catalysis, vol. 7, no. 12, pp. 8314-8319, 2017, doi: 10.1021/acscatal.7b02787.[4] I. Ledezma-Yanez, W. D. Z. Wallace, P. Sebastián-Pascual, V. Climent, J. M. Feliu, and M. T. M. Koper, "Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes," Nature Energy, vol. 2, no. 4, 2017, doi: 10.1038/nenergy.2017.31.

Oxygen reduction reaction on platinum single-crystal electrodes modified with protonic ionic liquid

The oxygen reduction reaction (ORR) on the low- and high-index planes of Pt modified with a protonic ionic liquid ([7-methyl-1,5,7-triazabicyclo(4.4.0)dec-5-ene][bis(pentafluoroethylsulfonyl)imide], referred to here as [MTBD][beti]) was studied in 0.1 M perchloric acid solution. The specific activity of the ORR on Pt(111) modified with [MTBD][beti] was 3.1 times higher than that on unmodified Pt(111), while the ORR was deactivated on modified Pt(100). [MTBD][beti] slightly enhanced the ORR activity of the Pt n(111)–(111) planes (where n = 2–9). These results show that a larger (111) terrace is essential for a marked increase in the ORR activity on Pt electrodes modified with [MTBD][beti]. Although the ratio of the increase in the activity of n(111)–(111) was lower than that of Pt(111), Pt(331) = 3(111)–(111) showed the highest ORR activity after [MTBD][beti] modification. The effects of MTBD and beti were also studied separately. MTBD provided the main contribution to the increase in ORR activity, giving the highest activity at a concentration of 2.0–3.5 × 10–5 M, whereas beti affected the ORR activity only slightly.

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Development of reversible and stable catalysts for the electrochemical reduction of CO2 is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO2 reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

Evidence for the Lack of Caffeine Specific Adsorption and Its Impact on Water Structure to Increase HOR/HER Activity on Pt

As the cornerstone for much of our electrochemical knowledge, the inexplicable 2 orders of magnitude decrease in the hydrogen oxidation and evolution reactions (HOR/HER) in moving from pH 1 to 14 on precious metals continues to undermine our understanding of and ability to better design electrochemical systems.1 Several theories have been proposed for this anomalous change, from electric field effects, the influence of cations, co-adsorption of H and OH, and the orientation of interfacial water, driving innovations in in situ electrochemical techniques, such as potential changes with Fourier Transform Infrared Spectroscopy (FTIR) or X-ray Absorption Spectroscopy (XAS).2–6 Recently, it was found that including small concentrations of caffeine in pH 13 was able to drastically increase the HOR/HER catalytic activity of platinum through what was postulated as surface specific adsorption.7 In this work, we challenge this notion through an in situ surface sensitive technique, termed Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) while simultaneously analyzing the underlying theory of water reorganization. Due to SEIRAS’ s surface sensitivity, we can distinguish between specific electrochemical surface adsorption and molecules merely located in the outer Helmholtz plane (OHP) through stark tuning, where species located in the inner Helmholtz plane (IHP) experience the strongest electric fields, and demonstrate a potential dependence of their vibrational frequency.8 Further, trends in caffeine induced water structure changes support our hypothesis that surface bound water layer changes observed in our in situ spectra are responsible for the “apparent pH dependence” of HOR/HER.The electrochemical interface is traditionally viewed as a double layer model, with specific electrochemical adsorbates existing in the Inner Helmholtz Plane and the first layer of non-adsorbates at the Outer Helmholtz Plane. ATR-SEIRAS enhanced surface sensitivity allows for strong signals to be obtained within 5 nm of the surface, selectively capturing both the IHP and OHP with decreasing strength into the diffuse layer and bulk electrolyte. In situ surface sensitive IR shows strong peaks corresponding to caffeine on addition of 10-4 M to basic electrolyte, but a lack of stark tuning suggests no specific adsorption of caffeine on the Pt surface. Furthermore, SEIRAS spectra reveal subtle changes to the interfacial water with addition of caffeine, namely losses of certain types of interfacial water and changes in the water-water hydrogen bonding network. These spectra compared with similar samples collected in traditional 0.1 M HClO4 electrolyte point toward the “apparent pH dependence” being a function of pH induced changes to the interfacial water structure. 1. Durst, J., Siebel, A., Simon, C., Hasche, f., Herranz, J., & Gasteiger, H. A. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014). Chen, X., McCrum, I. T., Schwarz, K. A., Janik, M. J. & Koper, M. T. M. Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 56, 15025–15029 (2017).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).Liu, E. Jiao, L., Li, J., Stracensky, T., Sun, Q., Mukerjee, S., & Jia, Q. Interfacial water shuffling the intermediates of hydrogen oxidation and evolution reactions in aqueous media. Energy Environ. Sci. 13, 3064–3074 (2020).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Intikhab, S. et al. Caffeinated Interfaces Enhance Alkaline Hydrogen Electrocatalysis. ACS Catal. 10, 6798–6802 (2020).Lambert, D. K. Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models. J. Chem. Phys. 89, 3847–3860 (1988). Figure 1

Platinum single crystal electrodes: Prediction of the surface structures of low and high Miller indexes faces

Platinum is widely used as electrode in electrocatalysis studies; however, the use of polycrystalline surfaces introduces a series of variables in the electrochemical system, due to the random contribution of all crystallographic orientations with different superficial packing of atoms. On the other hand, low Miller index platinum single crystal electrodes present surface structure with high regularity and serve as models for establishing the correlation between atomic arrangements and macroscopic properties, among which their electrochemical behavior. Here we predict the different contributions of each low Miller indexes planes in the size of terraces and number of steps of the high Miller indexes faces of platinum (called stepped surfaces), establish​ the correlation between the structural properties of these surfaces and their electrochemical behavior, and discuss the surface reconstruction process of each platinum low Miller index face.

Understanding Electric and Non-Electric Field Effects on Electrochemical Double Layer Restructuring for the Model Platinum HOR

Recent progresses in hydroxide exchange membrane fuel cells (HEMFCs) have led to renewed interest in the strong pH effect on hydrogen oxidation reaction (HOR) rate on precious metals: two orders of magnitude decrease when pH is changed from 0 to 14.1 Various theories have been proposed to explain this, ranging from adsorbed cation effects, changes in the electric field strength, to pH induced changes to the apparent hydrogen binding energy.2-4 One leading argument is the increase of pH leads to stronger electric field and thus more rigid ordering of water at the electrode/electrolyte interface, resulting in greater resistance to proton/hydroxide transfer through the double layer. This increased energetic barrier due to reorganization results in the decreased activity.5 In this work, we examine this theory by using a combination of traditional electochemical and spectroscopic techniques to highlight the importance of both electric field and non-electric field effects on reaction rates. Further we use this distinction to understand how each factor contributes to water structure re-arrangement on the surface and in the double layer, which we hypothesize to be the pivotal factor in the aforementioned activity decrease from acid to base.The electrochemical double layer is traditionally described by specifically adsorbed species (inner helmholtz plane, IHP), the next layer of hydrated ions (outer Helmholtz plane, OHP) and further layers (diffuse region) protruding and mixing into the bulk electrolyte. It has been shown that the stark tuning rate, which is a function of electric field strength, is dictated by hydrated cation size, and provides a metric to measure the thickness of the electrochemical double layer.6-8 By introducing crown ether to chelate cations, the stark tuning rate, and thus the electric field, can be decreased up to a 0.5:1 ratio of crown ether to cation. Beyond this point, no change in stark tuning rate and electric field were observed. However, further decreases in HOR activity are observed beyond this threshold. This decreasing activity at a constant field strength provides strong evidence for mixed electric and non-electric field effects. We hypothesize that cations and ether molecules impact the electric field strength to a degree, but also interact with surface water species. Spectroscopic and reactivity data demonstrates that both chelating and linear ethers, while having drastically different impacts on the strength of the electric field, result in similar activity decreases. Additionally, both types of ether demonstrate removal of the same types of interfacial water, suggesting that their influence on the surface water structure rather than their effect on the electric field strength determines the HOR activity.(1) Durst, J.; Siebel, A.; Simon, C.; Hasché, F.; Herranz, J.; Gasteiger, H. A. New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7 (7), 2255–2260. https://doi.org/10.1039/c4ee00440j.(2) Chen, X.; McCrum, I. T.; Schwarz, K. A.; Janik, M. J.; Koper, M. T. M. Co-Adsorption of Cations as the Cause of the Apparent PH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 2017, 56 (47), 15025–15029. https://doi.org/10.1002/anie.201709455.(3) Yang, X.; Nash, J.; Oliveira, N. J.; Yan, Y.; Xu, B. Understanding the PH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 2019, 58, 17718–17723. https://doi.org/10.1002/ange.201909697.(4) Zheng, J.; Nash, J.; Xu, B.; Yan, Y. Perspective—Towards Establishing Apparent Hydrogen Binding Energy as the Descriptor for Hydrogen Oxidation/Evolution Reactions. J. Electrochem. Soc. 2018, 165 (2), H27–H29. https://doi.org/10.1149/2.0881802jes.(5) Ledezma-Yanez, I.; Wallace, W. D. Z.; Sebastián-Pascual, P.; Climent, V.; Feliu, J. M.; Koper, M. T. M. Interfacial Water Reorganization as a PH-Dependent Descriptor of the Hydrogen Evolution Rate on Platinum Electrodes. Nat. Energy 2017, 2 (4), 1–7. https://doi.org/10.1038/nenergy.2017.31.(6) Ringe, S.; Clark, E. L.; Resasco, J.; Walton, A.; Seger, B.; Bell, A. T.; Chan, K. Understanding Cation Effects in Electrochemical CO 2 Reduction. Energy Environ. Sci. 2019. https://doi.org/10.1039/C9EE01341E.(7) Dunwell, M.; Wang, J.; Yan, Y.; Xu, B. Surface Enhanced Spectroscopic Investigations of Adsorption of Cations on Electrochemical Interfaces †. Phys. Chem. Chem. Phys 2017, 19, 971. https://doi.org/10.1039/c6cp07207k.(8) Waegele, M. M.; Gunathunge, C. M.; Li, J.; Li, X. How Cations Affect the Electric Double Layer and the Rates and Selectivity of Electrocatalytic Processes. J. Chem. Phys. 2019, 151 (16), 160902. https://doi.org/10.1063/1.5124878. Figure 1

Structure Sensitivity of Acetophenone Reduction on Palladium-Modified Platinum Single-Crystal Electrodes

The electrochemical reduction of acetophenone has been investigated in acidic conditions on the three low-index [namely (111), (110), and (100)] single-crystal platinum electrodes coated with an at...

Coupling Surface Coverage and Electrostatic Effects on the Interfacial Adlayer–Water Structure of Hydrogenated Single-Crystal Platinum Electrodes

Atomically flat, single-crystal solid–liquid interfaces attract considerable interest through their electrochemical relevance and well-defined structure facilitating controlled atomistic characteri...

Determination of the potential of zero charge of Pt/CO electrodes using an impinging jet system

In this manuscript, the potential of zero charge (pzc) of platinum single-crystal electrodes has been determined using the impinging jet system and recording the transient associated with the interphase formation. Initially, the Pt(111), Pt(110), and Pt(100) electrodes covered with CO were used. To demonstrate the validity of the procedure, the values obtained for the pzc of these electrodes were compared with those previously reported in the literature and also with those expected from work function changes. An excellent agreement has been found. After that, the values for the pzc of the stepped surfaces having (111) terraces and (110) steps were determined. For these surfaces, the pzc initially diminishes as the step density increases, in accordance with the expected diminution of the work function due to the Smoluchowski effect on the step site. However, for the shorter terraces, the pzc value increases with the step density, probably because of step dipole-dipole coupling due to the presence of a CO layer on the electrode.

New insights into the Pt(hkl)-alkaline solution interphases from the laser induced temperature jump method

The interfacial properties of platinum single crystal electrodes in contact with alkaline aqueous solutions (pH = 13) have been investigated using the laser induced temperature jump method. This technique offers insights into the net orientation of water dipoles in contact with the electrode surface by recording the coulostatic potential changes after a sudden increase of the interfacial temperature in the submicrosecond time scale. This information is intimately related with the magnitude and sign of charge separation at the interphase and the resulting electric field. In all cases, water shows a net orientation with the hydrogen towards the metal at the lowest investigated potential value, reflected in negative potential transients. The magnitude of the water orientation decreases as the applied potential increases. Eventually, the sign of the potential transient changes, reflecting a reorientation of the water dipoles. The potential where such inversion takes place follows the order Pt(110) < Pt(100) < Pt(111) in accordance with the observed behavior in acid solution and the trend of the work function. For Pt(111) the change of sign of the laser induced potential transient takes place at the onset of hydroxyl adsorption. For the three surfaces, when the pH is decreased to ca. pH = 11, a slow response is detected at potentials values above the inversion point. This could be due to a fast adsorption process or to a slow reorientation of water. After the introduction of steps on the (111) terrace, the inversion shifts to the double layer region, allowing the unambiguous identification of the inversion with a change on the net orientation of the water molecules. For stepped surfaces, a second inversion of the laser induced potential transient is observed that could be related with an effect of the local charge on steps disrupting the ordering of the water network. Comparison with analogous results in acid solution gives information about the local distribution of charges on the stepped surfaces.

Electrochemical Reduction of the Carbonyl Functional Group: The Importance of Adsorption Geometry, Molecular Structure, and Electrode Surface Structure.

This paper studies the electrochemical hydrogenation of the carbonyl functional group of acetophenone and 4-acetylpyridine at platinum single-crystal electrodes. Comparison with results obtained for the hydrogenation of acetone featuring an isolated carbonyl functional group reveals the influence of the phenyl ring and the pyridine ring, respectively. Lack of acetone adsorption at Pt(111) and Pt(100) due to a weak interaction between surface and carbonyl functional group renders these surfaces inactive for the hydrogenation of acetone. Adsorption through a strong interaction with the phenyl ring of acetophenone activates the Pt(111) and Pt(100) surfaces for hydrogenation of the acetyl substituent. In agreement with previous results for acetone reduction, the Pt(100) surface is specifically active for the hydrogenolysis reaction, breaking the C–O bond, whereas the other surfaces only hydrogenate the carbonyl functionality. In contrast to the phenyl ring, the pyridine ring has a very different effect: due to the dominant interaction of the N atom of the pyridine ring with the platinum electrode, a vertical adsorption mode is realized. The resulting large physical distance between the carbonyl functional group and the electrode surface inhibits the hydrogenation at all platinum surfaces. This also holds for the Pt(110) electrode, which is otherwise active for the electrochemical hydrogenation of the isolated carbonyl functional group of aliphatic ketones. Our results show how the combination of molecular structure of the reactant and surface structure of the catalyst determine the selective electroreduction of functionalized ketones.

Peroxodisulfate reduction on platinum stepped surfaces vicinal to the (110) and (100) poles

The reduction of peroxodisulfate anion at stepped platinum single crystal electrodes vicinal to the (110) and (100) poles has been investigated using cyclic voltammetry. Particular emphasis is made in understanding the effect of the cooling atmosphere used after the flame annealing. Two cooling atmospheres have been used: H2/Ar and CO. The cooling procedure has a significant influence on the reactivity obtained with the stepped surfaces belonging to both crystallographic zones. For the surfaces composed of (110) terraces and (111) or (100) steps cooled in H2/Ar, only one peroxodisulfate reduction peak is observed. On the contrary, for the same surfaces cooled in CO, more than one peak is observed for wide terraces. For surfaces vicinal to the pole (100), the cooling in CO atmosphere seems to produce a surface with higher order than those cooled in H2/Ar. The implications of the results on the understanding of the local distribution of charges at heterogeneous surfaces are discussed, identifying local values of the potential of zero charge. Those results are compared, when possible, with results obtained with other techniques.

A mean-field model for the double layer of stepped platinum single-crystal electrodes

Mean-field models are useful in quantitative interpretation of electrochemical measurements and identification of key parameters and processes for further first-principles computations. Herein, we present a mean-field model for the double layer of stepped Pt[n(111)×(110)] single-crystal electrodes. This model accounts for chemisorption-induced surface dipoles from multiple adsorbates (Had and OHad) at multiple adsorption sites (terrace and step sites). It is demonstrated that lateral interactions depend on both the adsorbate and the substrate and structure-dependent chemisorption brings about non-trivial effects to the surface-charging behavior and double-layer capacitance. Moreover, we emphasize the importance of the activity of interfacial water molecules in understanding the pH and cation effect of voltammetric peaks.

Electrocatalytic Oxidation of Glycerol on Platinum Single Crystals in Alkaline Media

AbstractGlycerol adsorption and oxidation reactivity at platinum single crystal electrodes in alkaline media using electrochemical and Fourier transform infrared (FTIR) techniques are reported. The behavior of Pt(100) and Pt(110) for the glycerol electrooxidation reaction (GEOR) were compared and analyzed altogether with those previously reported for Pt(111) ( R. M. L. M. Sandrini, J. R. Sempionatto, E. Herrero, J. M. Feliu, J. Souza‐Garcia, C. A. Angelucci, Electrochem. commun. 2018, 86, 149–152). The voltammetric profiles confirm the structure sensitivity of GEOR as well as the role of the surface atoms orientation on the electrocatalytic activity. The Pt(100) surface has shown to be less prone to poisoning during multiple potential cycles in contrast to Pt(110), which suffers an accentuate deactivation at the first positive‐going scan. Spectroscopic results show that the GEOR on all three surfaces is characterized by the presence of two broad bands, centered at circa 1400 cm−1 and 1600 cm−1. This provides evidence that the GEOR mechanism undergoes glycerol dehydrogenation to alkoxydes followed by the formation of aldehyde intermediates adsorbed as η1(O)‐aldehyde and η1(C)‐acyl geometry, respectively. The surface atomic arrangement induces different selectivity in the oxidation process.