Clean Platinum Research Articles

On the mechanism and energetics of electrochemical alloy formation between mercury and platinum for mercury removal from aqueous solutions

Mercury pollution is an acute global concern threatening the health of humans and wildlife. Thus, there is a need for new and improved techniques to reduce emissions and remove toxic mercury from aqueous environments. Electrochemical alloy formation, specifically between mercury ions in aqueous solution and a platinum electrode, emerges as a promising solution. Through analysis of reaction mechanisms and energetics related to the formation and dissolution of the PtHg4 alloy, this study aims to deepen our understanding of the underlying processes of effective electrochemical mercury removal. Potentiodynamic measurements indicate rapid alloy formation at a clean platinum surface, proceeding with only trace amounts of metallic mercury on the surface. However, once the electrode surface has sufficient mercury coverage with an alloy thickness of around 1.5 nm, clear evidence of metallic mercury on the surface is observed. Furthermore, the amount of absorbed mercury on the cathode increases linearly in time. The onset potential for the alloy formation is experimentally determined using electrochemical quartz crystal microbalance with dissipation monitoring to be approximately 0.64 V vs. SHE, and the alloy dissolution (oxidation) onset potential is found to be approximately 0.98 V vs. SHE, at a mercury ion concentration of 10 mg/L. Density functional theory calculations are used to provide a theoretical value for the reversible potential from a thermodynamics perspective, yielding a value of 0.78 V vs. SHE at a mercury ion concentration of 10 mg/L. This value is in excellent agreement with the experimental results, suggesting an overpotential of about 0.14 and 0.20 V for the alloy formation and oxidation, respectively. These findings provide important information on the reaction mechanisms and overpotentials of the PtHg4 alloy formation and dissolution, which are key factors for development of large-scale mercury removal based on this technique, as well as fundamental understanding of the electrochemical alloy formation between mercury ions and platinum.

Anion-Modulated Platinum for High-Performance Multifunctional Electrocatalysis toward HER, HOR, and ORR.

SummaryEfficient electrocatalyst toward hydrogen evolution/oxidation reactions (HER/HOR) and oxygen reduction reaction (ORR) is desirable for water splitting, fuel cells, etc. Herein, we report an advanced platinum phosphide (PtP2) material with only 3.5 wt % Pt loading embedded in phosphorus and nitrogen dual-doped carbon (PNC) layer (PtP2@PNC). The obtained catalyst exhibits robust HER, HOR, and ORR performance. For the HER, a much low overpotential of 8 mV is required to achieve the current density of 10 mA cm−2 compared with Pt/C (22 mV). For the HOR, its mass activity (MA) at an overpotential of 40 mV is 2.3-fold over that of the Pt/C catalyst. Interestingly, PtP2@PNC also shows exceptional ORR MA which is 2.6 times higher than that of Pt/C and has robust stability in alkaline solutions. Undoubtedly, this work reveals that PtP2@PNC can be employed as nanocatalysts with an impressive catalytic activity and stability for broad applications in electrocatalysis.

Impedance of thin film cathodes: Thickness and current collector dependence

The influence of the layer thickness of mixed ionic–electronic conducting (MIEC) cathodes and the type of noble metal current collector on the apparent surface exchange resistance is studied with impedance spectroscopy. The impedance data is analyzed with the ‘General Finite Length Diffusion’ transfer function. Also a simple relation is derived for the influence of the electronic sheet resistance of a thin layer on the apparent exchange resistance. These analysis methods are applied to a set of thin, dense layer electrodes of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) deposited on a Ce0.9Gd0.1O1.95 electrolyte by pulsed laser deposition (PLD). The apparent chemical capacitance is used to check the reliability of obtaining the oxygen exchange rate from the layer resistance. Use of a fresh, clean platinum gauze current collector significantly enhances the surface oxygen exchange rate on LSCF. But both ‘noble metal’ current collectors, Pt and Au, cannot be considered to be inert on LSCF.

X-ray photoemission analysis of clean and carbon monoxide-chemisorbed platinum(111) stepped surfaces using a curved crystal.

Surface chemistry and catalysis studies could significantly gain from the systematic variation of surface active sites, tested under the very same conditions. Curved crystals are excellent platforms to perform such systematics, which may in turn allow to better resolve fundamental properties and reveal new phenomena. This is demonstrated here for the carbon monoxide/platinum system. We curve a platinum crystal around the high-symmetry (111) direction and carry out photoemission scans on top. This renders the spatial core-level imaging of carbon monoxide adsorbed on a ‘tunable' vicinal surface, allowing a straightforward visualization of the rich chemisorption phenomenology at steps and terraces. Through such photoemission images we probe a characteristic elastic strain variation at stepped surfaces, and unveil subtle stress-release effects on clean and covered vicinal surfaces. These results offer the prospect of applying the curved surface approach to rationally investigate the chemical activity of surfaces under real pressure conditions.

Promotion effect of manganese oxide on the electrocatalytic activity of Pt/C for methanol oxidation in acid medium

The modification of Pt/C by incorporating metal oxides for electrocatalytic oxidation of methanol has gained major attention because of the efficiency loss during the course of long-time operation. This work describes the preparation of Pt–MnO2/C electrocatalysts through a chemical route using ethylene glycol or a mixture of ethylene glycol and sodium borohydride as a reducing agent. The crystallite structure and particle size of synthesized electrocatalysts are determined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The addition of MnO2 improves the dispersion of Pt nanoparticles. The electrocatalytic activity of Pt–MnO2/C towards methanol oxidation in H2SO4 solution is investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The onset potential value of methanol oxidation peak is negatively shifted by 169mV when MnO2 is introduced to Pt/C. Moreover, the charge transfer resistance value at Pt–MnO2/C is about 10 times as low as that at Pt/C. Chronoamperometry and chronopotentiometry show that CO tolerance is greatly improved at Pt–MnO2/C. The increased electrocatalytic activity and enhanced ability to clean platinum surface elect manganese oxide as a suitable promoter for the anode performance in direct methanol fuel cells (DMFCs).

Properties of a Model Aryl Boronic Acid and Its Boroxine

Many boronic acid-containing molecules are currently under investigation as possible therapeutics. An increase in the knowledge of the physical and chemical properties of these compounds will lead to their improved formulation into usable dosage forms. The current study describes the formation and characterization of a boronic acid anhydride, called a boroxine. The model boronic acid, 4-methoxybenzeneboronic acid is used. The current study demonstrates the utility of thermal (differential scanning calorimetry and thermogravimetric analysis) and spectral (solid-state nuclear magnetic resonance and ultraviolet spectroscopy) methods to quantitate and characterize the boronic acid and its boroxine, validated against definitive structures solved by single-crystal X-ray diffraction. The present work also demonstrates that the hydrolysis of the boroxine in the presence of water is very rapid in a time scale relative to drug substance manufacture and product formulation.

Molecular dynamics characterization of the contact between clean metallic surfaces with nanoscale asperities

We use molecular dynamics (MD) simulations to characterize the tensile strength of contacts formed between various clean platinum surfaces with nanoscale asperities. Both commensurate contacts between (001) and (111) surfaces and incommensurate (001) ones are considered over a wide range of asperity sizes. In cyclic closing and opening, fresh asperities that form contacts for the first time show significant plastic deformation; this leads to a reduction in the effective contact area during the first few cycles, after which steady state is achieved both in terms of contact size and the pull-out force necessary to open the contacts. As is the case for commensurate surfaces [H. Kim and A. Strachan, Phys. Rev. Lett. 104, 215504 (2010)], the strength of the metallic bridges that form in incommensurate contacts exhibit strong size effects; their strength increases with decreasing size until a length of \ensuremath{\sim}5 nm, below which weakening is observed. Commensurate contacts lead to stronger bridges than incommensurate ones but only during the initial closing events; after steady state is achieved, commensurate and incommensurate (001) surfaces lead to bridges of similar strengths.

Adsorption of cyclohexene and its dehydrogenation intermediates on nAu/Pt(100)( n = 0, 1, 2) surfaces: A DFT study

Abstract Adsorption of cyclohexene and its dehydrogenation intermediates on the nAu/Pt(100) (n = 0, 1, 2 means clean Pt, one monolayer and two layers of Au covered Pt surfaces, respectively.) has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, i.e., hollow sites and bridge sites, and the adsorption energy at the hollow site is larger than that at the bridge site. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike at both the bridge site and the top site. The magnitude order of the adsorption energies is as follows: clean Pt > Au/Pt > 2Au/Pt. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C-M (M = Pt or Au, on the top layer of the slab) bonds are closely related to the corresponding adsorption energies.

Flux‐Free Fusion of Silicate Rock Preceding Acid Digestion for ICP‐MS Bulk Analysis

We present a new method for the decomposition of silicate rocks by flux‐free fusion in preparation for whole‐rock trace element determination (Sc, Rb, Sr, Y, Zr, Nb, Cs, Ba, rare earth elements and Hf) that is especially applicable to zircon‐bearing felsic rocks. The method was verified by analyses of RMs of mafic (JB‐1a, JB‐2, JGb‐1) and felsic rocks (JG‐3, JR‐3, JSd‐1, GSP‐2, G‐2). Pellets of powdered sample (up to 500 mg) without flux were weighed and placed in a clean platinum crucible. The samples were then fused in a Siliconit® tube furnace and quenched to room temperature. The optimum condition for the fusion of granitic rock was determined to be heating for 2–3 min at 1600 °C. The fused glass in the platinum crucible after heating was decomposed using HF and HClO4 in a Teflon® beaker. Decomposed and diluted sample solutions were analysed using a quadrupole inductively coupled plasma‐mass spectrometer. Replicate analyses (n = 4 or 5) of the RMs revealed that analytical uncertainties were generally < 3% for all elements except Zr and Hf (∼ 6%) in JG‐3. These higher uncertainties may be attributed to sample heterogeneity. Our analytical results for the RMs agreed well with recommended concentrations and recently published concentrations, indicating complete decomposition of our rock samples during fusion.

Underpotential deposition of cobalt onto polycrystalline platinum

An electrochemical study of cobalt electrodeposition onto a polycrystalline platinum electrode from an aqueous solution (10−2 M CoCl2 + 1 M NH4Cl (pH 9.5)) was carried out through cyclic voltammetry and potential step techniques. Analysis of the voltammetric data clearly showed that a cobalt adlayer is formed during the application of potential in the underpotential deposition (upd) region. Formation of this cobalt adlayer involved the simultaneous presence of both adsorption and 2D nucleation processes. Cobalt adlayers obtained by linear voltammetry in upd region were analyzed employing diffuse reflectance spectroscopy (DRS). By using theoretical quantum studies at PM6//HF/LANL1MB level, it was possible to assign the peaks obtained by DRS at 328 and 337 nm to the cobalt adsorption on Pt(111) and Pt(100), respectively, while the signals recorded at 355 and 362 nm were related with the clean platinum surfaces Pt(100) and Pt(111). Also, quantum calculations at the PM6 level indicated that the energy formation order is Co-Pt(100) > Co-Pt(111) > Co-Pt(110) > Co-Co(surface).

Nanoscale Metal-Metal Contact Physics from Molecular Dynamics: The Strongest Contact Size

Using molecular dynamics we find that the tensile strength of the contacts between two clean platinum surfaces with nanoscale asperities is strongly size dependent with a maximum strength for contact lengths of approximately 5 nm. This is the first time a strongest size is observed in single crystals. The strengthening with decreasing size down to 5 nm results from a decrease in the initial density of mobile dislocations available for plastic deformation and the subsequent weakening originates from a reduction in the constraint to mechanical deformation inside the contact by the bulk.

Sum Frequency Generation and Catalytic Reaction Studies of the Removal of Organic Capping Agents from Pt Nanoparticles by UV−Ozone Treatment

We report the structure of the organic capping layers of platinum colloid nanoparticles and their removal by UV-ozone exposure. Sum frequency generation vibrational spectroscopy (SFGVS) studies identify the carbon-hydrogen stretching modes on poly(vinylpyrrolidone) (PVP) and tetradecyl tributylammonium bromide (TTAB)-capped platinum nanoparticles. We found that the UV-ozone treatment technique effectively removes the capping layer on the basis of several analytical measurements including SFGVS, X-ray photoelectron spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The overall shape of the nanoparticles was preserved after the removal of capping layers, as confirmed by transmission electron microscopy (TEM). SFGVS of ethylene hydrogenation on the clean platinum nanoparticles demonstrates the existence of ethylidyne and di-σ-bonded species, indicating the similarity between single-crystal and nanoparticle systems.

CO-Coverage-Dependent Oxygen Dissociation on Pt(111) Surface

Oxygen dissociation is one of the most critical steps in the CO oxidation reaction on transition metal surfaces. It has been shown both experimentally and theoretically that oxygen dissociation on clean platinum (Pt) surface proceeds via a precursor-mediated reaction path, with negligible activation barrier. On the other hand, the oxygen dissociation pathway under diesel engine operating conditions, where the metal surface is packed with CO molecules, is understood less clearly. In this paper, we report density functional theory calculations for O2 dissociation on Pt(111) in the presence of varying CO coverage. Classical Monte Carlo simulations have been used to get an estimate of coadsorbed CO and O2 configurations. Oxygen molecular precursor states binding energies were found to shift up in energy with increasing CO coverage, with transition state energies and final product energies following the same trend. The dissociated product state becomes endothermic beyond a critical CO coverage of 0.44 monolaye...

Adsorption of cyclohexene on nAu/Pt(1 0 0) ( n = 0, 1, 2): A DFT study

Adsorption of cyclohexene on the nAu/Pt(1 0 0) ( n = 0, clean Pt; n = 1, Pt covered with monolayer of gold (Au/Pt); n = 2, Pt covered with two layers of gold (2Au/Pt), respectively) has been investigated by using self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, hollow sites and bridge sites, and the adsorption energy in the hollow site is higher than that in the bridge site. Two carbon atoms of the C C moiety are reacted with the same platinum atom through ‘π’ interaction, which forms the chemical bond including the back-donating bond. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike in both of the bridge site and the top site. However, the most stable adsorption site is the hollowR45 site on the three surfaces, and the order of the adsorption energies is clean Pt (−1.35 eV) > Au/Pt (−0.63 eV) > 2Au/Pt (−0.59 eV) from the calculations. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C–M (M = Pt or Au, in the top layer of the slab) bond are 2.08, 2.77 and 3.07 Å, respectively, which can accord with the difference in adsorption energies. Furthermore, considering the conditions for the formation of the back-donating bond, the d-band centers of clean Pt, Au/Pt and 2Au/Pt are calculated, they are −2.04, −2.73 and −3.04 eV, respectively, which are in line with the order of the adsorption energies.

Platinum Nanoflowers for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry of Biomolecules

No-template synthesis of surface clean platinum nanomaterials with thin projections (petals) on the Pt nanoparticle surface, termed Pt nanoflowers, has been developed, and Pt nanoflowers have achieved superior performance on surface-assisted laser desorption/ionization mass spectrometry (SALDI−MS) of biomolecules. With this new material, one can obtain high-resolution mass spectra with lower laser energy and smaller sample amounts, and no organic matrix is needed for laser desorption/ionization. The mass spectra also have fewer obstacle peaks at the lower m/z region. Furthermore, this novel substrate is quite stable against aging and showed no sweet-spot problems, that is, equal sensitivity on the whole substrate surface. Various biomolecules, including peptides and phospholipids, were examined, and Pt nanoflowers showed good performance for matrix-free laser desorption/ionization MS of these materials.

Photonic and Electrochemical Properties of Adsorbed [Ru(dpp)2(Qbpy)]2+ Luminophores

Dense monolayers of [Ru(dpp)2Qbpy]2+, where dpp is 4,4'-diphenylphenanthroline and Qbpy is 2,2':4,4' ':4'4' '-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. The cyclic voltammetry of these monolayers is nearly ideal, and three redox states are accessible over the potential range of +/-1.3 V. Chronoamperometry conducted on the microsecond time scale has been used to probe the dynamics of heterogeneous electron transfer and indicates that the standard heterogeneous electron-transfer rate constant, k degrees , is approximately 106 s-1. The metal complex emits at approximately 600 nm in fluid and solid solution as well as when bound to a platinum electrode surface within a dense monolayer. In the case of the monolayers, it appears that the excited states are not completely deactivated by radiationless energy transfer to the metal because electronic coupling between the adsorbates and the electrode is weak. The dynamics of lateral electron transfer between the electronically excited Ru2+* and ground-state Ru3+ species has been explored by measuring the luminescence intensity after defined quantities of Ru3+ have been produced electrochemically within the monolayer. The rate of lateral electron transfer is between 8 x 106 and 3 x 108 M-1 s-1, indicating efficient electron transfer between adsorbates in close-packed assemblies. Voltammetry conducted at megavolt per second scan rates has been used to directly probe the redox properties of the electronically excited species.

Oxygen Reduction at the Pt/Recast-Nafion Film Interface at Different Temperatures and Relative Humidities

The effects of temperature (20-80{degree sign}C) and relative humidity (~0- 100%) on the oxygen reduction reaction (ORR) on a Nafion®- covered platinum ultramicroelectrode were investigated using cyclic voltammetry and chronoamperometry. The cyclic voltammograms exhibited a significant hysteresis between the forward and reverse scans, which increased with either humidity or temperature increases. No hysteresis was observed for the fully hydrated film equilibrated with liquid water. It is postulated that the oxygen transport may be obstructed by water produced by oxygen reduction, which accumulates at the interface due to its hindered diffusion in the partially hydrated films. The charge transfer kinetics of ORR at both clean and oxide-covered platinum were found to slow down with increases in relative humidity. The effect was stronger for the oxide-covered platinum and was correlated with differences in platinum oxide coverage associated with changes in film hydration.

Structural Analysis of Electrode Surfaces using Electrochemical STM

Until the mid 1980s, there had been only a few in situ methods available for the atomic level structural determination of electrode surfaces in solution. Now several investigations have demonstrated that electrochemical scanning tunneling microscopy (EC-STM) is a powerful technique for in situ characterization, with atomic resolution, of surfaces under potential control. The object of this article is to highlight some of the advances in in situ scanning tunneling microscopy (STM) with atomic resolution. Several selected topics, observation of clean platinum electrode surface, specifically adsorbed anions such as iodine and sulfate on metal surfaces, adsorption of porphyrin on clean and iodine-modified electrodes, and binary adlayer consisting of porphyrin and phthalocyanine on gold electrode are focused on.

電子遷移誘起脱離研究の先駆者，石川義興博士および太田芳雄博士の業績紹介

Dr. Yoshioki Ishikawa is a pioneer who carried out systematic electron stimulated desorption (ESD) investigations for hydrogen adsorbed on a clean platinum plate (H2/Pt) under ultrahigh vacuum (UHV) conditions in 1942. (His name is spelled as Yosioki Isikawa in the original papers.) Although his papers are referred in several papers on desorption induced by electronic transitions (DIET), these seem to be not as well-known as famous ones by Menzel, Gomer, and Redhead in 1964. We describe his pioneering work in this article. He developed an UHV apparatus made of glass with a Pt plate and an oxide cathode pumped by a small mercury pump via a liquid air trap. Clear ESD signals were observed for H2/Pt at excitation energies of 8, 12, 14, 33-46, and 46-51 eV. He assigned the desorbate to atomic hydrogen (H), because it was pumped by a glass wall baked above 350°C. Based on these results and the potential curves of H2 in the gas phase, he proposed a model for the ESD of H from H2/Pt; that is, the first step is an electronic transition of adsorbed H2, and the second is the desorption of H along the repulsive potential surface of the excited state. This was the first introduction of the notion of DIET. He also carried out an ESD study of H2O/Pt in 1943. We also introduce achievements by Dr. Yoshio Ohta, who was another pioneer in the field of DIET. He measured electron-stimulated ion desorption from an oxidized Ni plate. He found that the threshold excitation energy for ion desorption is 22 eV, and that the kinetic energy of the desorbed ion is 1∼2 eV. To explain these results, he also proposed the concept of DIET.

Structural Transformations on a Platinum Surface under Mechanical Tension

The effect of uniaxial tension on the structure of a recrystallized platinum surface is investigated using low-energy electron diffraction (LEED). The initial LEED patterns indicate that the (111) facet emerges on the surface of a platinum foil after a series of heating cycles under vacuum and in oxygen. After loading at ∼80 MPa, the clean platinum surface is characterized by systems of regular and irregular atomic terraces. The regular terraces have a (9(111) × 100) structure. As the load increases to 90–100 MPa, the ordered arrangement of terraces transforms into a disordered arrangement. After the samples are held under these loads for ∼2 h, the surface structure undergoes a transformation into the diffraction-disordered state. Under tensile deformation, the island structure of graphite molecules on the recrystallized platinum surface containing ∼10 at. % C also undergoes a transformation.