Bisulfate Adsorption Research Articles

Anion-Catalyzed Active Dissolution Model for the Electrochemical Adsorption of Bisulfate, Sulfate, and Oxygen on Gold in H2SO4 Solution

Several plausible mechanisms for the potential-dependent adsorption of bisulfate, sulfate, and oxygen on polycrystalline gold in H2SO4 (0.5 M) were evaluated, based on their fit to the experimental potentio dynamic polarization data using the anion-catalyzed active dissolution model (dc portion). The mechanism giving the best preliminary fit was further analyzed by modelling the electrochemical impedance spectroscopy data collected at various dc potentials in the electric double-layer and pre-oxide formation regions. Electrochemical quartz-crystal microgravimetry and optical ellipsometry provided additional evidence verifying the electrochemical adsorption of solution species on the gold surface. Surprisingly, dissolution was observed throughout the whole potential range under study, despite the electrode surfaces developing significant oxygen surface coverages. Key features of the experimental fractional surface coverages and current density were reproduced by the anion-catalyzed active dissolution model.

Anion-catalyzed active dissolution model for the electrochemical adsorption of bisulfate, sulfate, and oxygen on gold in H2SO4 solution

The mechanism of the electrochemical adsorption of bisulfate, sulfate, and oxygen on polycrystalline gold in H2SO4 (0.5 M) was modeled using a mechanistic analysis of potentiodynamic polarization and electrochemical impedance spectroscopy data acquired at multiple dc potentials in the electric double-layer and pre-oxide formation region. Electrochemical quartz-crystal microgravimetry and optical ellipsometry provided evidence for the potential-dependent adsorption of solution species on the gold surface. Also, despite the high surface coverages of oxygen on the electrode surfaces, dissolution was observed over the entire the potential region studied. The anion-catalyzed active dissolution model accounts well for the experimentally observed current density and fractional surface coverages.

Modifying Interface Solvation and Oxygen Reduction Electrocatalysis with Hydrophobic Species

Ionic liquid interlayers improve the oxygen reduction reaction (ORR) kinetics on bulk and nanostructured catalysts for both Pt and alloyed-Pt materials. Despite the demonstrated performance enhancement at the half-cell and membrane electrode assembly level, the mechanism of the improvement is not fully understood. In this work, we combine single-crystal experiments with microkinetic modeling to uncover the origin of the ORR kinetic improvement on Pt(111) in the presence of ionic liquids and hydrophobic cations. With the incorporation of a modified Frumkin isotherm, our model accurately simulates the disorder–order transition observed in hydroxyl and bisulfate adsorption on Pt(111) under acidic conditions. Voltametric analysis shows that ionic liquids impact solvation to break so-called scaling relations between the adsorption strength of OHad and Oad, but these effects have little impact on ORR activity. Instead, destabilized OHad reduces the overall hydroxyl (spectator) coverage, resulting in higher availability of active sites.

Adsorption and Electrochemical Polymerization of Pyrrole on Au(100) Electrodes as Examined by In Situ Scanning Tunneling Microscopy

Despite many efforts studying polypyrrole (PPy) with scanning probe techniques in the past several decades, its molecular structure and conformation have been elusive. This study presents the first atomic and molecular-resolution scanning tunneling microscopy (STM) images of pyrrole (Py) and PPy electrochemically produced on bare and iodine-modified Au(100) electrodes in pH 1 and 4 sulfate media. First, STM reveals a highly ordered Au(100)—(4√2 × 4√2) R45°—Py between 0.15 and 0.5 V (vs saturated calomel electrode) in 0.1 M H2SO4 containing 30 mM Py. By contrast, a disordered Py adlayer is seen in 0.1 M HClO4 medium, indicating the coadsorption of Py and an acid. This anion effect is manifested in voltammetric results obtained with Au(100), showing a pair of sharp peaks at 0.12 V in 0.1 M H2SO4 + 30 mM Py and a featureless profile seen with Au(111) or Au(100) in HClO4. Py and bisulfate (and perchloric acid) attract each other and are coadsorbed on Au(100) electrodes. The favorable adsorption of bisulfate a...

Synthesis and catalytic properties of Pd nanoparticles loaded nanoporous TiO2 material

In the present work, Pd nanoparticles were loaded on the nanoporous TiO2 material by a simple chemical deposition. The amount of Pd nanoparticles was determined by the loading times. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperometry (CA) methods were used to characterize the surface morphology, composition and electro-catalyst activity of the nanoporous Pd/TiO2 material. CV and CA results exhibited the excellent methanol electro-oxidation performance of 6 times loaded sample. The effects of methanol concentration, H2SO4 concentration and upper scan limits on the electro-oxidation performance of six times loaded sample were investigated. In the solution with low methanol concentration, electro-oxidation of methanol was influenced by the amount of methanol molecules on Pd active sites. In the solution with high methanol concentration, electro-oxidation of methanol was controlled by intermediates diffusion. With increasing H2SO4 concentration, driving force of methanol oxidation decreased. However, the increase of bisulfate adsorption, the reduction of thermodynamic tendency and solution conductivity would result in the suppression of peak current density. The upper scanning limit had obvious influence on the active sites for the methanol oxidation reaction in the backward scanning stage. The generation of PdO at high potential was undesired for methanol electro–oxidation.

The adsorption of bisulfate and sulfate anions over a Pt(1 1 1) electrode: A first principle study of adsorption configurations, vibrational frequencies and linear sweep voltammogram simulations

Specific anion adsorption of sulfate and bisulfate anions to the Pt(111) surface was studied using periodic density functional theory (DFT) methods. The origins of experimentally observed voltammetric features have been assigned based on comparison of calculated vibrational frequencies and simulated linear sweep voltammograms (LSVs) to experimental data available in the literature. The inclusion of solvation in the electrochemical model well-predicts the bisulfate and sulfate adsorption potentials. Combined with vibrational frequency analysis, sulfate is identified as the dominant adsorption species over the (bi)sulfate adsorption potential region. Rapid increase of coverage with electrode potential causes a sharp peak in the simulated voltammogram. We have assigned the experimental characteristic vibrational mode of adsorbed (bi)sulfate in the 1200–1300cm−1 range as an S–Ouncoordinated stretch. The Stark tuning slope of the S–Ouncoordinated frequency for sulfate containing (√3×√7) superstructures was calculated for different co-adsorbed species to resolve possible molecular structures for this adlayer. The oxidation of co-adsorbed water to surface hydroxyl at (√3×√7) adlayer causes a decrease of the Stark tuning slope and a high potential feature in the voltammogram. The adsorption of a series of sulfonic acids to Pt(111) is considered, and the calculated adsorption equilibrium potential is observed to correlate linearly with the sulfonate proton affinity.

Elucidation of the Chemical Nature of Adsorbed Species for Pt(111) in H2SO4 Solutions by Thermodynamic Analysis

The nature of the adsorbed species for Pt(111) in sulfuric acid solutions has been elucidated by a careful thermodynamic analysis of the effect of pH on charge density data. This analysis takes advantage of the fact that, for solutions of constant total sulfate + bisulfate concentration, an increase of pH would increase the sulfate concentration, at the expense of decreasing the bisulfate concentration. As a result, sulfate adsorption would be shifted toward lower potentials, while bisulfate adsorption would follow the opposite trend. In the present work, coulostatic data for Pt(111) in (0.2 - x) M Me(2)SO(4) + x M H(2)SO(4) (Me: Li, Na; x: 10(-4) - 0.2) and (0.1 - x) M KClO(4) + x M HClO(4) + 10(-3) M K(2)SO(4) (x: 10(-4) - 0.1) solutions are carefully analyzed. It is concluded that sulfate rather than bisulfate adsorption takes place at potentials higher than the potential of zero charge. This result agrees with the fact that similar FTIRRAS bands for adsorbed sulfate species are observed for pH 0.8-3.5 in (0.2 - x) M K(2)SO(4) + x M H(2)SO(4) solutions.

Investigation into the Competitive and Site-Specific Nature of Anion Adsorption on Pt Using In Situ X-ray Absorption Spectroscopy

In situ X-ray absorption spectroscopy along with electrochemical measurements (CV and RDE) and previously published EQCN data provide further understanding of the nature of chloride poisoning on different faces/sites of carbon supported platinum clusters (1−2 nm) in acidic medium (HClO4). Chloride is shown to adsorb in 3-fold sites on the Pt(111) faces at the investigated Cl− concentrations (10−3 and 10−2 M). Atop chloride was found to be present within a narrow potential range (0.4−0.7 V RHE) when compressed adlayers of Cl− are formed on the Pt(111) faces forcing some Cl− to exist in atop/bridged sites. The interplay of anionic (Cl−, Br−, OH−, and HSO4−) adsorption on the different surfaces of Pt are also considered. For example O/OH can easily displace atop chloride on the edges/corners but not the Cl− at the Pt(111) sites, and therefore Cl− dramatically raises the overpotential for water activation at the Pt(111) sites. Chloride also drastically alters the ORR causing an increase of the overpotential by ∼85 mV for every 10-fold increase in chloride concentration with a total 150−200 mV increase in the overpotential at large concentrations at the Pt(111) sites. Finally Cl− ions cannot displace the bisulfate overlayer on the Pt(111) faces after it is formed at lower potentials; however, once the bisulfate adsorption is disturbed at higher potentials, the bisulfate cannot displace the Cl− adsorption. These relative anion adsorption preferences can help to explain the different dependencies of the important ORR on anion adsorption, and suggests that the effect of Cl− poisoning might be quite dependent on the Pt particle size.

The effects of the specific adsorption of anion on the reactivity of the Ru(0001) surface towards CO adsorption and oxidation: in situ FTIRS studies

The dynamics of adsorption and oxidation of CO on Ru(0001) electrode in sulfuric acid solution have been studied using in situ FTIR spectroscopy under potential control and at open circuit, the latter at 20 and 55 degrees C. The in situ IR data show clearly that the bisulfate anion adsorbs on the Ru(0001) surface over the potential range from -200 mV to 350 mV (vs. Ag/AgCl) at 20 degrees C in the absence and presence of adsorbed CO; however, increasing the temperature to 55 degrees C and/or increasing the concentration of dissolved O(2) reduces the bisulfate adsorption. The formation of surface (hydro-) oxide at higher potentials replaces the bisulfate adsorbates. Both linear (CO(L)) and three-fold hollow bonded CO (CO(H)) adsorbates were produced following CO adsorption at Ru(0001) in H(2)SO(4), as was observed in our previous studies in HClO(4). However, the amount of adsorbed CO observed in H(2)SO(4) was ca. 10% less than that in HClO(4); in addition, the CO(L) and CO(H) frequencies were higher in H(2)SO(4), and the onset potential for CO(ads) oxidation 25 mV lower. These new results are interpreted in terms of a model in which the adsorbed bisulfate weakens the CO adlayer, allowing the active Ru oxide layer to form at lower potentials. Significantly different results were observed at open circuit in H(2)SO(4) compared both to the data under potential control and to our earlier data in HClO(4), and these observations were rationalized in terms of the adsorbed HSO(4)(-) anions (pre-adsorbed at -200 mV) inhibiting the oxidation of the surface at open circuit (after stepping from the initial potential of -200 mV), as the latter was no longer driven by the imposed electrochemical potential but via chemical oxidation by trace dissolved O(2). Results from experiments at open circuit at 55 degrees C and using oxygen-saturated H(2)SO(4) supported this model. The difference in Ru surface chemistry between imposed electrochemical control and chemical control has potential implications with respect to fuel cell electrocatalysis.

Bisulfate Adsorption on Quasi-Perfect Pt(111) Facets from Acidic Aqueous Electrolytes as Monitored by Optical Techniques

not Available.

Oxidation of Ammonium on Platinum in Acidic Solutions

Oxidation of ammonium on polycrystalline Pt has been studied in perchloric and sulfuric acid solutions at room temperature. Ammonium was electrochemically active, and and NO were detected using differential electrochemical mass spectroscopy. Cyclic voltammetry and electrochemical quartz crystal microbalance data showed that ammonium affects the formation and reduction of Pt oxides, probably due to formation of very stable adsorbed nitrogen or nitrogen-oxygen species stable down to the region. An oxidation peak occurred at 0.8 in positively going scans, decreasing strongly with increasing sweep rate. A corresponding reduction shoulder at about 0.66 was seen, probably caused by formation and reduction of or species. Formation of Pt oxides was shifted to slightly higher potentials in the presence of ammonium, and the reduction charge of the Pt oxide reduction peak at 0.80 was independent of sweep rate, indicating that the amount of Pt oxides formed was limited by other adsorbates. In particular, the most strongly adsorbed hydrogen was shifted to lower potentials by adsorbed species formed at high potentials as well as adsorption of bisulfate mutually stabilized by ammonia. The voltammetric response in the hydrogen desorption region was not affected, showing that all adsorbed species were desorbed at low potentials in the negatively going scan.

In situ FTIR spectroscopic studies of the oxidation of CO adsorbates on Ru(0 0 0 1) electrodes under open circuit conditions

This paper reports the first observation, using in situ FTIR spectroscopy, of the oxidation of CO adsorbates on the Ru(0 0 0 1) electrode to CO 2 under open circuit (oc) conditions in both perchloric acid and sulphuric acid solution at 20 and 55 °C. While the significant oc oxidation of the adsorbed CO on the Ru(0 0 0 1) electrode was observed in perchloric acid solution, much less oc oxidation took place in sulfuric acid solution due to the specific adsorption of bisulfate at the Ru surface which inhibits the surface oxidation and reduces the reactivity of the surface towards the oxidation of CO ads. The oc oxidation of the CO ads depends strongly on the oxygen concentration in the solution and the temperature. The data so obtained are compared to those observed at the gas|solid interface, as well as to those obtained from the electro-oxidation of CO ads, and possible new catalytic oxidation reaction mechanisms are discussed. In addition, it is shown that the C–O frequency of the adsorbed CO may be used as an effective probe of the open circuit potential.

The electroreduction of hydrogen on platinum(111) in acidic media

We propose a new mechanism for the electroreduction of hydrogen in acidic media. The specific model and theoretical calculations are designed for the Pt(111) surface, because this is the only face that exhibits a clear first order phase transition. The basic mechanism however, applies to the other faces of Pt as well. The principal ingredient of our theory is the assumption that the water molecules in the inner Helmholtz layer are strongly oriented by the field. This means that hydrogen bonds have to be bent considerably. A correct water model has to be tetrahedral and flexible. Then, as the polarity of the electrode is changed: • For positive electrodes most of water dipoles point down, and a zig-zag chain of hydrogen bonded water is formed which is responsible for the 3 × 7 (or perhaps 3 ×5/2 ) structure of the sulfate observed by STM. • For negative polarization most of the water dipoles point up. The (bi)sulfate, and the water form a periodic structure by incorporating 1/3 of hydronium, which then is adsorbed with a hydrogen down in the hollow sites of the Pt(111) electrode. This compound is neutral and forms a two dimensional honeycomb arrangement of water with bisulfate ions. • When the electrode becomes more negative, then the bisulfate is desorbed, but the honeycomb structure remains because of the reaction (1) ( H 5 O 2 +) 3+6 e −→3 H 2+( H 3 O 2 −) 3 • This reaction is studied by a high accuracy generalized gradient PW91 calculation for a five layer Pt slab. The HER or intermediate reaction compound (H 3O 2 −) 3 is stable on the Pt(111) face. The model is consistent with all known experiments: it reproduces well the experimental voltammogram and the recent radiotracer measurements of bisulfate adsorption. Furthermore, it gives a robust explanation of the 2/3 hydrogen yield for this surface.

Theoretical Study of the Adsorption of Bisulfate on Small Gold Clusters

The results of density functional theory energy calculations of bisulfate adsorbed to gold atom clusters are presented and discussed. The gold atom clusters are used to model a gold(111) surface. These calculations involve the Becke three-parameter functional to approximate the exchange energy and the Lee, Yang, and Parr approximation to include the electronic correlation contribution. The structure and stability of the bisulfate ion at different adsorption sites on the gold surface are examined. The bisulfate in its tripod orientation is found to be the most stable configuration due to the direct interaction of each oxygen of the tripod with a gold atom of the surface. The binding of the bisulfate is predominantly electrostatic and is strongly dependent on the local charge of the gold up to second-nearest neighbors from the adsorption site. Additionally, the bisulfate prefers the face-centered cubic 3-fold hollow adsorption site directly above a hollow in the second layer and a gold atom in the third layer.

Adsorption of Bisulfate and Sulfate Anions on a Pt(111) Electrode

A modified radioactive labeling method was used to study bisulfate and sulfate anion adsorption on a Pt(111) electrode from aqueous HClO4/H2SO4 media. The highest surface concentration of the anion is 3.2 × 1014 ions/cm2, which corresponds to a coverage (packing density) of 0.21 ± 0.01monolayer. Overall, the coverage data confirm our previous results reported in J. Electroanal. Chem. 1993, 348, 451. We found fast adsorption kinetics and a semilogarithmic anion adsorption isotherm. In the “butterfly” electrode potential range [that is, in the range where anomalous anion adsorption was found on the Pt(111) electrode (Clavilier, J. J. Electroanal. Chem. 1980, 107, 211)], the charge on the anion and, consistently, electrosorption valency vary with the electrode potential, demonstrating progress in bisulfate dissociation to sulfate, with the increase in potential. On the plateau of the surface concentration−electrode potential plot, we conclude that the adsorbate is a partially discharged (or neutralized) sulf...

Adsorption of bisulfate on the Ru(0001) single crystal electrode surface

Abstract Adsorption of anions from sulfuric acid solutions has been studied on Ru(0001) single crystal and polycrystalline surfaces by electrochemical techniques and in-situ Fourier transform infrared spectroscopy. Infrared spectroscopy shows that bisulfate is the anion adsorbed on the Ru(0001) surface. The bisulfate adsorption is detected at the H 2 evolution potential and extends into the potential region where the Ru surface is oxidized. A method for extracting unipolar bands from bipolar bands has been presented. The tuning rate of adsorbed bisulfate in the double layer potential region of Ru(0001) was found to be significantly smaller than those observed for other platinum metals. This has been ascribed to a small change in bisulfate coverage on Ru(0001) in this potential range. Bisulfate vibration frequencies are higher on this surface than at any face-centered cubic metal with the (111) orientation. Oxidation of the Ru(0001) surface is limited to one electron per Ru atom, distinctly different from the high degree of oxidation seen in polycrystalline surfaces. For oxidized polycrystalline Ru, only solution phase sulfates and bisulfates are observed in the IR spectra.

Study of copper underpotential deposition on Au(111) surfaces

First-principles total energy calculations are carried out to study the structure of copper underpotential deposition on Au(111) surfaces in sulfuric acid solutions. The norm-conserving method is used to construct the pseudopotentials of all the elements involved. The copper adlayer structure under various copper coverage is investigated. The results show that the proposed honeycomb structure with 2/3 monolayer copper coverage is unstable without the co-adsorption of sulfate. The co-adsorbed sulfate is found to bind to copper. The calculated structural parameters are in general agreement with those obtained from a recent X-ray experiment. In addition, the sulfate adsorption on clean Au(111) surface is studied. The results show that sulfate molecule binds much more weakly with clean Au(111) surfaces. Total energy calculations for bisulfate adsorption suggest that even though it is the dominant species in acidic electrolyte, the adsorbed bisulfate may dissociate thus leave sulfate adsorbed on the surface.

Temperature-dependent oxygen electrochemistry on platinum low-index single crystal surfaces in acid solutions

Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduction reaction (ORR) was studied in sulfuric acid solution over the temperature range 298–333 K At the same temperature, the exchange current density increased in the order, [Formula: see text] which gives the order of absolute kinetic activities of Pt(hkl) in 0.05 mol/L H2SO4: Pt(111) < Pt(100) < Pt(110). We found that at high current densities every crystal face has an ideally temperature-dependent Tafel slope, i.e., −2 × 2.3(RT/F). The activation energy for the ORR is independent of the surface geometry, [Formula: see text]. The insensitivity of the activation energy to surface structure implies that the reaction pathway for the ORR is the same on all three single-crystal faces. A "series" pathway for the ORR was proposed, with the first electron transfer being the rate-determining step. The structure sensitivity in the kinetics of the ORR on Pt(hkl) is attributed to the structure-sensitive adsorption of bisulfate and hydroxyl anions and a simple site-blocking effect of these adsorbed anions on the rate of the ORR. Keywords: platinum single crystals, oxygen reduction, peroxide reduction, temperature effects, activation energy.

Electrochemical quartz crystal microbalance study of copper ad-atoms on gold and platinum electrodes Part I. Adsorption of anions in sulfuric acid

Abstract The deposition and dissolution processes of copper ad-atoms on a gold or a platinum electrode in sulfuric acid electrolyte solution were investigated by using the electrochemical quartz crystal microbalance. It was found that the weight loss in the removal of the Cu-adlayer from the Au substrate was considerably larger than that expected from Faraday's law whereas the deviation for the Pt substrate was very small. The adsorption of bisulfate or sulfate anions both on Cu ad-atoms and on the electrode substrates was discussed quantitatively. It was demonstrated that higher coverage with Cu ad-atoms and lower adsorbability with bisulfate or sulfate anions were obtained on the Pt electrode than on the Au, and these effects could be ascribed to the difference in electronegativity between Pt and Au substrates.

Ex-situ IRAS and LEED studies of underpotentially deposited copper on a Pt(111) electrode in a sulfuric acid solution: layer exchanges of anions with copper

The electrodeposition of copper on a Pt(111) electrode in a 0.5M H 2SO 4 solution was studied by means of voltammetry, low energy electron diffraction and ex-situ infrared reflection absorption spectroscopy. At any potential between 0.25 and 0.85 V versus the standard hydrogen electrode, the same diffraction pattern of (√3 × √3)-R30° was observed due to a top-layer structure of bisulfate or sulfate before and after the copper deposition. The voltammetric peak at 0.59 V was due to copper adsorption while that at 0.49 V originated from a bisulfate to sulfate conversion on copper during a cathodic sweep. At an electrode potential for underpotential deposition of copper, a bisulfate becomes weakly bound to platinum atoms allowing copper deposition; simultaneous adsorption of bisulfate on the copper layer prohibits further copper deposition.