Cooperativity in Ion Hydration Tielrooij, K.J; Garcia-Araez, N; Bonn, M ...
Science (American Association for the Advancement of Science),
05/2010, Letnik:
328, Številka:
5981
Journal Article
Recenzirano
Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect ...(i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.
We report, for the first time, the electrochemical behavior of thallium irreversibly adsorbed on Pt (111) and platinum stepped surfaces composed of (111) terraces and monoatomic steps. Similar to the ...case of thallium UPD, the voltammograms obtained after thallium irreversible adsorption present three characteristic features. After a careful analysis of the effect of the thallium concentration, the concentration and nature of the anion of the supporting electrolyte and the pH of the solution on these voltammetric features, we have been able to ascribe these processes to Tl/Tl+ oxidation and anion adsorption on the Tl-modified surface. In addition, the results obtained with stepped surfaces, indicate that some of the features are clearly associated to the presence of (111) surface domains, and thus they could be used for the quantification of these sites.
The electrochemical behavior of platinum single-crystal electrodes is revisited, with special emphasis on the determination of the potential of zero charge. We show that the measure of the charge ...displaced during CO adsorption allows the determination of the potential of zero total charge (PZTC). The estimation of the potential of zero free charge (PZFC) is discussed, with different degrees of approximation. The application of this methodology to the study of the PZTC of platinum stepped surfaces vicinal to Pt(111) reveals a marked decrease of the PZTC due to the introduction of surface steps. This effect is interpreted as the result of the existence of markedly smaller surface potentials localized on step sites. The importance of considering local aspects of the interface is emphasized with the use of N2O reduction as a sensitive probe to the local structure of the surface. It is proposed that the different local maxima observed in the absolute value of the reduction current correspond to the local values of PZTC. It is shown that there is, in general, good agreement between the overall PZTC, obtained from the CO displacement, and that calculated from the local values inferred from the N2O reduction. Further insight is obtained with the application of the laser-induced temperature jump method. This technique is useful to calculate the potential of maximum entropy of the double-layer formation. The resulting value of this potential for Pt(111) is discussed in the light of the PZFC value obtained from different approaches. For stepped surfaces vicinal to Pt(111), two local maxima in the entropy of the double layer are observed that are close to the local PZTC values estimated from the N2O reduction. This result suggests the existence of cooperative effects in the organization of the water dipoles close to the electrode surface.
The oxidation of ammonia on platinum surfaces is a sensitive-structure reaction that takes place almost exclusively on Pt containing (1
0
0) sites. However it is strongly inhibited on Pt(1
1
1) and ...Pt(1
1
0) surfaces. A study using stepped electrodes with Pt(1
0
0) terraces and monoatomic Pt(1
1
1) steps,
Pt(
S)n(1
0
0)×(1
1
1)
, Miller indices Pt(2
n−1,1,1), shows that the activity of the electrode is strongly dependent on the terrace width, i.e., the wider the terrace the less positive the peak potential for ammonia oxidation. The change in the peak potential from the widest (1
0
0) terraces to terraces 6 atoms wide, i.e., from Pt(1
0
0) to Pt(11,1,1), is approximately 60 mV. This result is unexpected if we bear in mind the size and geometry of the ammonia molecule.
The competitive adsorption of hydrogen and bromide on Pt(1
0
0) has been studied by modeling the experimental data by means of the mean-field approximation (Frumkin isotherm) and Monte Carlo ...simulations. This system is characterized by two kinds of species that adsorb at the same surface sites but whose saturation coverages are different (
θ
H,max
=
1,
θ
Br,max
=
0.5), as a result of different short range interactions. In a first step, hydrogen adsorption on Pt(1
0
0) has been modeled and it is found that the Frumkin isotherm and Monte Carlo simulations agree satisfactorily. The parameters obtained from fitting hydrogen adsorption on Pt(1
0
0) are then used to model the competitive adsorption of hydrogen and bromide. The competitive adsorption of H and Br causes a surface that is completely covered in the whole potential range. This leads to the existence of an extra degree of freedom in the fitting process that was checked by choosing two limiting values for the Br–Br interaction parameter. By doing so, the remaining interaction parameters and the energies of adsorption can be calculated. Both the approximate mean-field approach and the exact Monte Carlo simulations are able to give good fits of the experimental curve. However, comparison of the results given by the Frumkin isotherm and the Monte Carlo simulations demonstrates that the former is not adequate to model this system. The limitations of the mean-field approach are related to its inability to correctly deal with the presence of strong short range interactions.
The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a Pt(1
1
1) electrode in a series of solutions (0.1
−
x) M HClO
4
+
x M KClO
...4
+
2.5
×
10
−3 M NaCl with a constant ionic strength and variable pH. The total charge densities were calculated by integration of cyclic voltammetry curves. The adsorption of chloride present in the electrolyte blocked hydrogen and OH adsorption at potentials 0.3
<
E
<
0.7
V (SHE). Consequently, the differential capacity, charge density and surface energy become pH independent in that potential range providing pH independent integration constants for the integration of CVs. For
E
<
0.3
V (SHE), a complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen were determined without a need to introduce any arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin–Markov coefficient) for adsorbed hydrogen were calculated using the Gibbs excess data. The charge numbers are equal to ∼1, indicating that the adsorbed species is a totally discharged hydrogen atom. For 0.1
M HClO
4
+
2.5
×
10
−3
M NaCl solution Gibbs excesses of both hydrogen and chloride were determined. In a narrow potential range 0.2
<
E
<
0.3
V (SHE) chloride and hydrogen atoms are simultaneously adsorbed at the electrode surface. We have demonstrated that their adsorption has a competitive character.
The electrochemical behavior of the stepped electrodes vicinal to the Pt(1
0
0) surface in bromide containing solutions has been studied by cyclic voltammetry, STM and the CO displacement method. The ...studied electrodes belong to the 0
1
1̄ and 0
0
1 zones, which nominally have
n atom-wide terraces with (1
0
0) symmetry and (1
1
1) or (1
0
0) monoatomic steps, respectively. The voltammetric behavior of these electrodes indicates that the processes on the terrace sites, namely hydrogen and bromide competitive adsorption/desorption take place in a peak centered at 0.19 V, whereas the processes on the step sites occur at potentials below 0.15 V. The STM images obtained after the flame annealing treatment indicate that the electrodes with (1
1
1) steps have a surface topography that corresponds to the nominal structure. On the other hand, the electrodes with (1
0
0) steps appear to be reconstructed, with terrace edges having a zigzag shape. The adsorbed bromide arranges in a c(2
×
2) structure on the (1
0
0) terraces. The CO displacement method has allowed estimating the pztc (potential of zero total charge) for the surfaces. The variation of the pztc is much smaller than that found in other electrolyte solutions, due to the high pseudocapacity of the hydrogen/bromide adsorption processes on the terrace. The total charge at different potentials has been compared to the values predicted from the hard-sphere model. For the electrodes with (1
1
1) steps, the experimental values agree with the model, which facilitates the understanding of the general behavior of these surfaces.
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