1,721,057 research outputs found
Separation of temperature effects on double-layer and charge-transfer processes for platinum|solution interphases. Entropy of formation of the double layer and absolute molar entropy of adsorbed hydrogen and OH on Pt(111)
No full textThe present work combines results from pulsed-laser experiments and a thermodynamic analysis, to separate temperature effects on double-layer and charge-transfer processes. The entropy of formation of the double layer for Pt(111) in 0.1 M HClO4 is evaluated and compared with the corresponding entropy of formation of the overall interphase. The difference between the two quantities is due to the entropy change associated with charge-transfer processes, thus allowing for the evaluation of the absolute molar entropy of adsorbed hydrogen and OH on Pt(111). The results indicate that adsorbed hydrogen on Pt(111) is very mobile, whereas adsorbed OH species are rather immobile. Finally, values of the entropy of formation of the double layer are compared to data for mercury electrodes. It is concluded that the entropy of the interfacial water network on Pt(111) exhibits a nearly symmetrical dependence on charge density. This result contrasts with the behavior observed for mercury electrodes, for which the state of maximum entropy is achieved at clearly negative charge densitie
Potential-dependent water orientation on Pt(111), Pt(100), and Pt(110), as inferred from laser-pulsed experiments. Electrostatic and chemical effects
No full textThe laser-induced temperature jump method is used to characterize the net orientation of interfacial water on well-defined platinum surfaces, Pt(111), Pt(100), and Pt(110), as a function of the applied potential. A clear effect of the surface structure on the potential of water reorientation is observed, being 0.37 for Pt(111), 0.33 for Pt(100), and 0.14 V vs RHE for Pt(110) in 0.1 M HClO4 solution. The potential of water reorientation also exhibits a different pH dependency for the three basal planes, shifting 0.060 for Pt(111), 0.030 for Pt(100), and 0.015 V/dec for Pt(110). Comparison with charge density data provides a deeper understanding of these results. A quantitative analysis of the electrostatic and chemical effects governing the potential-dependent reorientation of the interfacial water network is addressed. It is concluded that water on Pt(111) exhibits a small net orientation in the absence of electric field at the interphase. On the other hand, the agreement between the relative position of values of the potential of water reorientation and work functions, for the three basal planes, suggests that the same situation holds for Pt(100) and Pt(110)
Influence of alkali cations on the infrared spectra of adsorbed (bi)sulphate on Pt(1 1 1) electrodes
No full textThe role of alkali cations potassium and caesium on the structure of the interfacial region between a Pt(1 1 1) electrode and a sulphuric acid solution is investigated by cyclic voltammetry and infrared spectroscopy techniques. A strong influence of the nature of the cation on the voltammetric profile is observed at potentials higher than the main (bi)sulphate adsorption step. Although the infrared spectra show the same adsorbate bands, regardless of the cation, a different dependence between the peak frequency and the electrode potential is observed depending on the nature of the cation, with a significantly smaller slope d?/dE measured in the solutions containing potassium and caesium. This observation can be explained in terms of the Stark effect as a consequence of the decrease of the electric field in the interphase due to the increase in the size of the catio
Analysis of temperature effects on hydrogen and OH adsorption on Pt(111), Pt(100) and Pt(110) by means of Gibbs thermodynamics
No full textGibbs thermodynamics in the presence of charge-transfer processes have been applied to characterize temperatureeffects on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) electrodes in 0.1 M HClO4 solutions. The present analysis allows the evaluation of the entropy of formation of the interphase for the three platinum basal planes, which is found to be structure-sensitive. In addition, these results have been used to test the suitability of the more common approach to characterize temperatureeffects, based on the application of a generalized isotherm. Besides, the comparison of both approaches allows the characterization of the different components involved in the entropy of hydrogen and OHadsorption, namely, the standard entropy of adsorption, the entropy due to lateral interactions, and the configurational entropy. Finally, the comparison with statistical mechanical calculations demonstrates that adsorbed hydrogen on Pt(1 1 1) and Pt(1 0 0) is rather mobile, while adsorbed OH is rather immobile. Adsorbed hydrogen on Pt(1 1 0) exhibits an unexpected behavior, since it appears to be rather immobile at low coverages, but it becomes very mobile at near saturation coverage
Temperature effects on platinum single-crystal electrodes
No full textThis work reviews three different approaches for the study of temperature effects on the platinum single-crystal | solution interphase. First, the method of analysis of temperature-dependent voltammetric data with the help of a generalized isotherm is described and illustrated for the case of adsorbed hydrogen and OH species on Pt(111) in 0.1 M HClO4. This method of analysis allows a detailed evaluation of the thermodynamic data of charge-transfer adsorbed species, namely, the standard molar Gibbs energy, enthalpy and entropy of adsorption and the magnitude of the lateral interaction between adsorbed species. However, a number of assumptions are involved in the application of a generalized isotherm, namely, the assumption of a Langmuirian configurational term and an arbitrary separation of capacitive and faradaic processes. The second approach described here overcomes these assumptions, since it employs Gibbs thermodynamic equations for interphaces to describe temperature effects on electrosorption processes including those in which charge-transfer is allowed. This approach allows evaluation of the entropy of formation of the interphase, being defined as the difference in entropy of the components of the interphase when they are forming it and when they are in the bulk phases. This method of analysis is illustrated through the evaluation of the entropy of formation of the interphase for Pt(111) in 0.1 M HClO4. Finally, it is shown that temperature effects on interfacial processes can also be studied by means of the application of a fast temperature perturbation. This approach opens the possibility to separate the effect of temperature on the different components of the interphase. The fast temperature perturbation is usually achieved with short laser pulses, and hence the method is called the laser-induced temperature jump method. This approach is illustrated here for the case of Pt(111) in 1 mM HClO4 + 0.1 M KClO4, and it is shown that the corresponding laser-induced transients provide direct evidence on the reorientation of the interfacial water networ
Thermodynamic analysis of (bi)sulphate adsorption on a Pt(111) electrode as a function of pH
No full textA complete thermodynamic study of (bi)sulphate adsorption on Pt(1 1 1) electrodes from solutions at four different pHs (pH 0.43, 2.1, 3.1 and 4.1) is reported. The effect of pH on the sum of the Gibbs excesses of sulphate and bisulphate species, standard Gibbs energies of adsorption and formal partial charge numbers is analyzed. The results provide relevant information on the nature of species involved in the different voltammetric features. The experiments at pH 0.43 were performed in a higher base electrolyte concentration (0.5 M), that allows the study of (bi)sulphate adsorption in a broader range of concentrations. Under these conditions, two adsorption steps are clearly defined, associated to two different voltammetric features, between 0.30 and 0.60 V and between 0.65 and 0.90 V (standard hydrogen scale, SHE). Once the pH is increased, a marked decrease in absolute value of the (bi)sulphate adsorption Gibbs energy is observed, concomitant with an increasing amount of OH co-adsorptio
Determination of the entropy of formation of the Pt(111)? perchloric acid solution interface. Estimation of the entropy of adsorbed hydrogen and OH species
No full textThe entropy of formation of the interface between a Pt(111) electrode and a 0.1-M HClO4 solution is calculated here for the first time from the temperature dependence of total charge vs potential curves following a thermodynamic analysis based on the electrocapillary equation. From this quantity, the absolute entropies of specifically adsorbed species (hydrogen and OH) can be estimated. The present method is an alternative treatment of data that overcomes some of the limitations involved in the approach that uses a generalized isotherm. However, it requires additional experimental data: the temperature coefficient of the potential of zero total charge of the working electrode and the temperature coefficient of the reference electrode. Comparison of the results obtained by both approaches shows that, for hydrogen adsorption, the agreement is reasonable, but the differences are larger for OH adsorption, thus showing the limitations inherent in the treatment based on the generalized isother
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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