1,721,011 research outputs found
Water dissociation on well-defined platinum surfaces: The electrochemical perspective
No full textThis paper discusses three important discrepancies in the current interpretation of the role of water dissociation on the blank cyclic voltammetry of well-defined single-crystalline stepped platinum surfaces. First, for H adsorption both H-terrace and H-step contributions have been identified, whereas for OH adsorption only OH-terrace has been identified. Second, different shapes (broad vs. sharp) of the H-terrace and H-step voltammetric peaks imply different lateral interactions between hydrogen adatoms at terraces and steps, i.e. repulsive vs. attractive interactions. Third, the H-step peak exhibits an unusual pH-dependent shift of 50 mVNHE/pH unit. We propose here a model that can explain all these observations. In the model, the H-step peak is not due to only ad- and desorption of hydrogen, but to the replacement of H with O and/or OH. The O:OH ratio in the step varies with step geometry, step density and medium. In alkaline media relatively more OH is adsorbed in (or on) the step than in acidic media, under which conditions more O is adsorbed in (or on) the step. This would explain the anomalous pH dependence and would provide a possible explanation for the higher catalytic activity of alkaline media for electro-oxidation reactions. Although the model certainly still contains speculative elements, we believe it provides the most consistent interpretation of platinum single-crystal electrochemistry currently available, and presents an important and significant improvement over previous interpretations. In situ spectroscopic data are ultimately needed to confirm or disprove some of the assumptions of the mode
Effect of the surface structure of gold electrodes on the coadsorption of water and anions
No full textThe potential-dependent water adsorption on gold surfaces in perchloric and sulfuric acid solutions has been studied by surface-enhanced infrared absorption spectroscopy (SEIRAS). It is found that the surface structure of the gold electrodes has a major impact on the SEIRAS spectra. When the gold films are composed of nanoparticles of 47 ± 11 nm, the SEIRAS spectra are in agreement with previous reports. However, when the size of the gold nanoparticles is decreased to 27 ± 8 nm, by depositing the gold at 1 Å/s instead of 0.1 Å/s, it is found that the SEIRAS bands associated with water molecules coordinated to coadsorbed anions are absent. The combination of both types of gold electrodes allows a detailed study of the properties of the adsorbed water molecules. It is found that water molecules coadsorbed with sulfate and perchlorate anions appear to belong to the hydration shell of the anions because (i) the intensity of the SEIRAS bands of these water molecules increase with potential in the same way as the SEIRAS bands of the adsorbed anions and (ii) the frequencies of the O–H stretch resemble those of the water molecules in the hydration shell of the anions in solutio
Theory of multiple proton-electron transfer reactions and its implications for electrocatalysis
Full text linkThis Perspective article outlines a simple but general theoretical analysis for multiple proton-electron transfer reactions, based on the microscopic theory of proton-coupled electron transfer reactions, recent developments in the thermodynamic theory of multi-step electron transfer reactions, and the experimental realization that many multiple proton-coupled electron transfer reactions feature decoupled proton-electron steps in their mechanism. It is shown that decoupling of proton and electron transfer leads to a strong pH dependence of the overall catalytic reaction, implying an optimal pH for high catalytic turnover, and an associated optimal catalyst at the optimal pH. When more than one catalytic intermediate is involved, scaling relationships between intermediates may dictate the optimal catalyst and limit the extent of reversibility that may be achievable for a multiple proton-electron-transfer reaction. The theory is discussed in relation to the experimental results for a number of redox reactions that are of importance for sustainable energy conversion, primarily focusing on their pH dependence
CO electroxidation on gold in alkaline media: a combined electrochemical, spectroscopic, and DFT study
No full textThe aim of the present work is to provide a deeper understanding of gold catalysis for CO electrooxidation in alkaline media, through a combined electrochemical, spectroscopic, and DFT study. Voltammetric and spectroscopic measurements evidence that the amount of CO irreversibly adsorbed on gold increases as the adsorption potential becomes more negative (vs SHE). This explains why higher CO coverages can be achieved in more alkaline solutions, since the value of adsorption potential vs RHE becomes more negative vs SHE with increasing pH. On the other hand, the combination of FTIRRAS experiments and DFT calculations shows that the adsorption site of irreversibly adsorbed CO on Au(111) depends on the value of the adsorption potential. It is concluded that CO adsorption on top sites takes place at all studied potentials, and hollow and bridge sites also become occupied for adsorption potentials lower and higher than 0 V vs RHE, respectively. However, it should be noted that our DFT calculations give values of the CO binding energies that are not strong enough to explain CO irreversible adsorption. This may be partly attributed to the fact that OH coadsorption is not included in the calculations. Indeed, this work presents two experimental facts that suggest that CO adsorption on gold promotes the coadsorption of OH species: (i) CO irreversibly adsorbed on Au(111) and Au(100) leads to an unusual voltammetric feature, whose charge indicates the stabilization of one OH species per adsorbed CO species; (ii) the apparent transfer coefficient of this unusual state is close to unity, suggesting that it is due to a presumed structural transformation coupled to OH adsorption. Finally, the effect of the adsorption potential on the bulk CO electrooxidation is also studied. It is found that, on Au(111), an increased occupation of CO on multifold (hollow) sites seems to result in a less efficient catalysis. However, on Au(110), an increased coverage of CO on top sites does not produce any significant change in catalysi
Competitive adsorption of hydrogen and bromide on Pt(100): Mean-field approximation vs. Monte Carlo simulations
No full textThe 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 (θH,max = 1, θBr,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 interaction
Structural effects on water adsorption on gold electrodes
No full textWe study the molecular properties of the interface formed between aqueous sulfuric acid solutions and gold electrodes by means of surface-enhanced infrared absorption spectroscopy (SEIRAS). The shape of the SEIRAS spectra is observed to be strongly dependent on the deposition rate with which the gold electrodes are prepared. We find that the water molecules coordinating to coadsorbed sulfate anions become invisible in the SEIRAS spectra when the gold films are deposited at 1 Å/s, instead of the customary deposition rate of 0.1 Å/s employed in previous studies. Atomic force microscopy images of the gold deposits demonstrate that the increase of the gold deposition rate produces a decrease in the size of the nanoparticles composing the gold films. This suggests that water molecules coadsorbed with sulfate anions on small gold nanoparticles are oriented parallel to the surface. On the other hand, the fact that these water molecules are not detected by SEIRAS facilitates the study of the adsorption of hydronium cations, since these SEIRAS bands overlap. It is concluded that the adsorption of sulfate anions does not involve the coadsorption of any hydronium–water complex, since the SEIRAS band of the latter species exhibits a steady decrease with increasing potentia
Theory of multiple proton-electron transfer reactions and its implications for electrocatalysis
No full text
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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