On the nature of adsorbed species on Platinum Single Crystal electrodes

The dependence of voltammetric currents on multiple parameters (concentration, pH, temperature, etc) has become a primary source of information in interfacial studies of noble metal electrodes. Peak potential, charge, and width are intimately related to surface structure and reactivity. However, this interpretation usually neglects the complexity of the redox processes involved. For the so-called hydrogen adsorption region in platinum, anion competitive adsorption plays an important role that is usually overlooked. While charge displacement already demonstrated decades ago the existence of anion adsorption contributions, only recently a combination of several surface-sensitive techniques has proven unambiguously the presence of OH adsorbed on step site at potentials much lower than usually considered. This information must not be neglected when analysing the properties of complex catalysts such as those composed of nanoparticles since it is of great importance for understanding its overall reactivity, for comparing with computational results, and for performing coulometric analysis.This research was funded by Ministerio de Ciencia e Innovación (Spain) grant number PID2019-105653GB-I00) and Generalitat Valenciana (Spain) grant number PROMETEO/2020/063

Exploring the interfacial neutral pH region of Pt(111) electrodes

The interfacial properties of Pt(111) single crystal electrodes have been investigated in the pH range 3 < pH < 5 in order to obtain information about the acidity of electrosorbed water. Proper experimental conditions are defined to avoid local pH changes while maintaining the absence of specifically adsorbed anions and preserving the cleanliness of the solution. For this purpose, buffer solutions resulting from mixtures of NaF and HClO4 are used. Total charge curves are obtained at different pHs from the integration of the voltammetric currents in combination with CO charge displacement experiments. Analysis of the composition of the interphase as a function of the pH provides information for the understanding of the notion of interfacial pH.Support from MINECO (Spain) through project CTQ2013-44083-P is greatly acknowledged. RMH thankfully acknowledges support from Generalitat Valenciana under the Santiago Grisolia Program (GRISOLIA/2013/008). PS thankfully acknowledges to the Generalitat Valenciana the award of a valid grand

Sobre las anomalías de precipitaciones en las Islas Baleares durante 2008

Se presenta un estudio de las anomalías de la precipitación en las islas Baleares durante el año 2008 respecto al período 1951-2007. Se analizan las variables ‘número de días de precipitación’ y ‘cantidad de precipitación’. Se comprueba que en 2008 ambas variables representan un extremo en las respectivas distribuciones de frecuencia para el bimestre mayo-junio y el trimestre octubre-noviembre-diciembre. Destaca especialmente el mes de mayo, en el que se registraron valores sin precedentes en ambas variables

Study of the interface Pt(111)/ [Emmim][NTf2] using laser-induced temperature jump experiments

The interface between a Pt(111) electrode and a room temperature ionic liquid, 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, was investigated with the laser-induced temperature jump method. In this technique, the temperature of the interface is suddenly increased by applying short laser pulses. The change of the electrode potential caused by the thermal perturbation is measured under coulostatic conditions during the subsequent temperature relaxation. This change is mainly related to the reorganization of the solvent components near the electrode surface. The sign of the potential transient depends on the potential of the experiment. At high potential values, positive transients indicate a higher density of anions than cations close the surface, contributing negatively to the potential of the electrode. Decreasing the applied potential to sufficiently low values, the transient becomes negative, meaning that the density of cations becomes then higher at the surface of the electrode. The potential dependence of the interfacial response shows a marked hysteresis depending on the direction in which the applied potential is changed.Support from MINECO (Spain) through project CTQ2013-44083-P is greatly acknowledged. Paula Sebastian acknowledges to the Generalitat valenciana for the award of vali + d gran

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.This work has been financially supported by the MINECO (Spain) project no. CTQ2016-76221-P

Investigating the M(hkl)| ionic liquid interface by using laser induced temperature jump technique

The interface between several Room Temperature Ionic Liquids (RTILs) in contact with both Au(hkl) basal planes and Pt(111) was studied by using cyclic voltammetry and Laser Induced Temperature Jump Technique (LITJT). Three RTILs, based on the imidazolium cation and the [Tf2N] anion were investigated: [Emmim][Tf2N], [Emim][Tf2N] and [Bmmim][Tf2N]. These three RTILs were selected with the aim to analyse how the balance between the different ion-ion interactions influences the interfacial properties of the M(hkl)|RTIL interface. It was found that the voltammetric response of the Au(hkl)|[Emmim][Tf2N] was highly sensitive to the geometry of the active surface sites, displaying sharp spikes superimposed to a capacitive voltammetric current. Conversely, these sharp spikes disappeared when [Bmmim][Tf2N] replaced [Emmim][Tf2N], although the capacitive voltammetric current profile was essentially maintained. This result is most likely related to the increase of the van der Waals interactions in the [Bmmim][Tf2N]. When [Emim][Tf2N] was analysed, the increase of the hydrogen bond interactions due to the hydrogenation of C2 (second carbon at the imidazolium ring) resulted also in the disappearance of the voltammetric spikes. The laser measurements showed that the highest values of the potential of maximum entropy (pme) in RTIL media correspond to the atomically closest packet surface structures, following the order: Au(111)>Au(100)>Au(110), in agreement with work function values. The measurement with Pt(111) revealed that the voltammetric profiles for this surface are featureless in all cases. However, the laser experiments revealed that solvent restructuration, as a function of both value and direction of the applied potential, is dependent on the type of cation. Finally, the interface Au(hkl)|Choline chloride:urea Deep Eutectic Solvent (DES) was also investigated by using cyclic voltammetry and LITJT. The voltammetric response of DES was also sensitive to the orientation of the Au single crystal, and the cyclic voltammograms displayed distinct sharp and characteristic features. Nevertheless, the laser response could not provide a value of the pme for the Au(hkl)|DES interface, likely due to the complex chemical structure of the DES which, in addition, strongly adsorbs on Au(hkl)

Towards the understanding of the interfacial pH scale at Pt(1 1 1) electrodes

The determination of the potentials of zero total and free charge, pztc and pzfc respectively, were made in a wide pH range by using the CO displacement method and the same calculation assumptions used previously for Pt(1 1 1) electrodes in contact with non-specifically adsorbing anions. Calculation of the pzfc involves, in occasions, long extrapolations that lead us to the introduction of the concept of potential of zero extrapolated charge (pzec). It was observed that the pztc changes with pH but the pzec is independent of this parameter. It was observed that the pztc > pzec at pH > 3.4 but the opposite is true for pH > 3.4. At the latter pH both pzec and pztc coincide. This defines two different pH regions and means that adsorbed hydrogen has to be corrected in the “acidic” solutions at the pztc while adsorbed OH is the species to be corrected in the “alkaline” range. The comparison of the overall picture suggests that neutral conditions at the interface are attained at significantly acidic solutions than those at the bulk.Support from MINECO (Spain) through project CTQ2013-44083-P is greatly acknowledged. E.S. also thanks CNPq (Brazil) for the scholarship (grant No. 200939/2012-2)

Activation Energy of Hydrogen Adsorption on Pt(111) in Alkaline Media: An Impedance Spectroscopy Study at Variable Temperatures

The hydrogen evolution reaction is one of the most studied processes in electrochemistry, and platinum is by far the best catalyst for this reaction. Despite the importance of this reaction on platinum, detailed and accurate kinetic measurements of the steps that lead to the main reaction are still lacking, particularly because of the fast rate of the reaction. Hydrogen adsorption on Pt(111) has been taken as a benchmark system in a large number of computational studies, but reliable experimental data to compare with the computational studies is very scarce. To gain further knowledge on this matter, a temperature study of the hydrogen adsorption reaction has been carried out to obtain kinetic information for this process on Pt(111) in alkaline solution. This was achieved by measuring electrochemical impedance spectra and cyclic voltammograms in the range of 278 ≤ T ≤ 318 (K) to obtain the corresponding surface coverage by adsorbed species and the faradaic charge transfer resistance. From this data, the standard rate constant has been extracted with a kinetic model assuming a Frumkin-type isotherm, resulting in values of 2.60 × 10–7 ≤ k0 ≤ 1.68 × 10–6 (s–1). The Arrehnius plot gives an activation energy of 32 kJ mol–1. Comparisons are made with values calculated by computational methods and reported values for the overall HER, giving a reference frame to support future studies on hydrogen catalysis.This work was financially supported by the MINECO (Spain) project no. CTQ2016-76221-P. L.E.B. thanks the Generalitat Valenciana for funding from the Santiago Grisolia Program (no. GRISOLIAP/2017/181)