Surface metal modifiers for methanol electrooxidation on platinum; silver and mercury

The catalytic activity of platinum surfaces towards methanol electrooxidation can be modified by the deposition of a second metal using different methodologies. There is little information about the catalytic performance of polycrystalline platinum modified by silver and mercury adatoms using spontaneous and electrochemical deposition methods. Cyclic voltammetrics have been performed to compare the current vs potential profiles of modified platinum surfaces in acid solution at room temperature. The inhibition of the hydrogen adatom voltammetric profile by foreign metal adatoms on platinum was used to calculate the degree of surface coverage by the metal. Poisoning effects were checked by anodic stripping experiments of methanol residues on the modified platinum surfaces at adsorption potentials in the hydrogen electrosorption region using a micro flux cell. Methanol solution oxidation was also evaluated at slow scan rates of up to 0.8 vs reversible hydrogen electrode (RHE) on the platinum-modified surfaces. The comparison between the amounts of carbon-monoxide-type residues and the solution oxidation of methanol was analysed to check for their utility as catalytic surfaces for direct methanol fuel cells. Dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday.     

An electrochemical stripping method for the selective determination of traces of silver

A method has been developed for the determination of traces of silver in the presence of large concentrations of interfering metals, particularly copper, involving reduction of the silver on a glassy-carbon electrode modified by an adsorbed layer of previously deposited hydrogen.     

Individual Detection and Electrochemically Assisted Identification of Adsorbed Nanoparticles by Using Surface Plasmon Microscopy

The increasing production and application of nanoparticles necessitates a highly sensitive analytical method for the quantification and identification of these potentially hazardous materials. We describe here an application of surface plasmon microscopy for the individual detection of each adsorbed nanoparticle and for visualization of its electrochemical conversion. Whereas the adsorption rate characterizes the number concentration of nanoparticles, the potential at which the adsorbed nanoparticles disappear during an anodic potential sweep characterizes the type of material. All the adsorbed nanoparticles are subjected to the potential sweep simultaneously; nevertheless, each of the up to a million adsorbed nanoparticles is identified individually by its electrochemical dissolution potential. The technique has been tested with silver and copper nanoparticles, but can be extended to many other electrochemically active nanomaterials.     

Dependence of electrochemical dissolution of aluminium on the ionic radii of halide ions

The UPD of silver on platinum was studied as a function of the initial oxidation state of the surface. Twin-electrode thin-layer and semi-infinite voltammetric measurements show a strong interaction between silver and oxygenated species. The suerimposed processes are described in terms of the mixed electrosorption valency. However, the irreversibility of both processes restricts exact thermodynamic analysis.     

Potential-dependent sum frequency generation study of 5-methylbenzotriazole on polycrystalline copper, platinum, and gold

In situ sum frequency generation vibrational spectroscopy, at varied potentials and polarization combinations, was performed on polycrystalline copper, polycrystalline platinum, and polycrystalline gold samples in 0.5 M HClO4 with 50 mM 5-methylbenzotriazole (5-methylBTAH) added. These studies were performed to determine the orientation of 5-methylBTAH on the surface at different potentials. For copper surfaces, orientation of the molecule on the surface is not affected by potential within the potential window studied (-500 to -100 mV vs saturated calomel electrode (SCE)). Sum frequency generation spectra of 5-methylBTAH on platinum show a change in orientation over the potential range studied (-250 to 750 mV vs SCE). The orientation of the methyl group tilts more toward the plane of the interface as the potential is scanned in the positive direction. This orientation change is correlated to hydrogen coadsorption on the platinum surface at low potentials. 5-Methylbenzotriazole lies in the surface plane or does not orient on gold at lower potentials but the orientation is tilted toward normal at more positive potentials over the potential range studied (-500 to 900 mV vs SCE). To compliment these results, cyclic voltammetry and electrochemical impedance spectroscopy measurements were performed. Cyclic voltammograms of copper show that addition of 5-methylBTAH protects the surface from copper dissolution, increasing the electrochemical window by 450 mV. Cyclic voltammetry of 5-methylBTAH on platinum showed a partial blockage of adsorbed hydrogen and also prevented the adsorption of oxygenated species at 450-600 mV. Cyclic voltammetry on gold shows that 5-methylBTAH blocks oxide formation for 400 mV thus increasing the electrochemical window. Electrochemical impedance spectroscopy has been performed to determine the potential of zero charge of 5-methylBTAH on copper.     

Mechanism of adsorption,electroreduction and hydrogenation of compounds with ethylenic bonds on platinum and rhodium: Part II. Comparison of the electroreduction and catalytic hydrogenation processes

The catalytic liquid-phase hydrogenation of maleic acid on platinum and rhodium has been investigated. It is shown that the rate-determining step of this process as well as of the electroreduction process of maleic acid is the interaction of the chemisorbed particle of maleic acid with the adsorbed hydrogen which is formed at the preceding rapid stage of either the dissociative adsorption of molecular hydrogen, or the electrochemical stage of hydrogen ion discharge. The rate of the process with the same degree of surface coverage with hydrogen and chemisorbed particles of maleic acid does not depend on whether the process is carried out catalytically or electrochemically, on whether maleic acid and hydrogen were preliminarily adsorbed on the surface of the electrode-catalyst or not. With due regard for the mutual influence of chemisorbed particles participating in the rate-determining stage, the main kinetic equations for the electroreduction and catalytic hydrogenation processes have been derived. The difference in the rates of electroreduction of maleic acid on platinum and rhodium, with the same degree of electrode surface coverage with reactants, is shown to be the result of differences in the adsorption heats (or bonding strength with the surface) of hydrogen and maleic acid on these two metals. Experimental procedures are described in Part I [1].     

Dissociative adsorption of molecular hydrogen onto Pt<Subscript>6</Subscript> and Pt<Subscript>19</Subscript> platinum clusters located on the tin dioxide surface: Quantum-chemical modeling

It is suggested that, for the operation of platinum catalysts based on tin dioxide in air hydrogen fuel cells, hydrogen spillover (migration) leading to a change in the electron and proton contributions of the catalyst conductivity is of crucial importance. The hydrogen adsorption, dissociation, and migration in the platinum-tin dioxide-hydrogen system surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that the adsorption energy of a hydrogen molecule onto a platinum cluster increases from 1.6 to 2.4 eV as the distance to the SnO₂ substrate decreases. The calculated Pt-H bond length for adsorbed structures is 1.58–1.78 Å. The computer modeling has demonstrated that: (1) the hydrogen adsorption energy on clusters is higher than on the perfect platinum surface; (2) dissociative chemisorption onto Pt _n clusters can occur without a barrier and depends on the adsorption site and the cluster structure; (3) the adsorption energy of hydrogen onto the SnO₂ surface is higher than the adsorption energy onto the platinum cluster surface: (4) multiple H₂ dissociation on the tin dioxide surface occurs with a barrier; (5) the dissociation adsorption of hydrogen molecules onto the platinum cluster surface followed by atom migration (spillover) is energetically favorable.     

银表面甲醇氧化反应机理的ab initio计算研究 1: 银表面静态吸附物种的几何构型和吸附性质

本文用原子簇模型(CM)的从头计算方法, 计算了银表面甲醇氧化反应中的静态吸附物种的优化几何构型及吸附性质。计算表明在清洁银表面甲醇、甲醛只存在物理吸附; 当表面存在吸附氧原子时, 甲醇可在银表面形成两种分子态吸附;甲醛与表面羟基OH(a)或氢原子H(a)共存时在银表面能够形成化学吸附, 且CH2O(a)极易与O(a)反应生成深度氧化中间体η^2-甲二氧基; 中间产物甲氧基在无氧的银表面能够形成稳定吸附, 在富氧银表面极易进一步氧化脱氢生成产物甲醛。通过计算与实验结果的对照, 我们对反应机理作了初步讨论。     

Surface enhanced Raman spectroscopic studies on 1H-1,2,4-triazole adsorbed on silver colloidal nanoparticles

1H-1,2,4-triazole is a very effective corrosion inhibitor for copper. The adsorption of this compound on silver colloidal nanoparticles has been studied by means of surface enhanced Raman scattering (SERS). SERS data are interpreted with the help of DFT calculations of models of the surface complex formed by 1H-1,2,4-triazole on the silver colloidal nanoparticles surface. It was found that this compound is adsorbed on metal surface in its anionic form and that it interacts with silver through the N₁ and N₂ atoms. The molecular plane assumes a tilted orientation with respect to the silver surface.     

Investigations of copper adsorption in hydrochloric acid media,via the ionization of hydrogen adsorbed on platinized platinum

Copper deposition in hydrochloric acid media, via the ionization of hydrogen adsorbed on platinized platinum, results in an adsorbed metal layer similar to that formed potentiostatically. However, the adsorbed hydrogen is replaced by adsorbed copper only to a certain extent, owing to the formation of Cu⁺ ions. This discrepancy depends on the chloride ion activity and the structure of the platinum black.     

Surface-enhanced Raman scattering of pyridine using different metals: differences and explanation based on the selective formation of alpha-pyridyl on metal surfaces

A simple method has been developed to produce SERS-active metal surfaces. Six metal surfaces (cadmium, nickel, gold, iron, copper, and silver) have been prepared on an aluminum foil underlayment by chemical reduction and strong surface-enhanced Raman signals have been observed for pyridine species on these surfaces. This permits the direct comparison of pyridine spectra on different metal surfaces prepared by the same chemically clean method. The differences among the SER spectra of the aqueous pyridine species using different metals generally follow the trend of silver, cadmium, nickel, iron, gold, and copper, which can be explained by the selective formation of alpha-pyridyl species and the equilibria between end-on adsorbed pyridines and edge-on adsorbed alpha-pyridyl species on the different metal surfaces.     

Electrochemical nanogravimetric studies of platinum in acid media

Electrochemical quartz crystal nanobalance (EQCN) is one of the most powerful tools to obtain information on the events occurring at the electrode surface. This method has been exploited to monitor the surface mass changes and hence to draw conclusions in respect of the formation and removal of adsorbed species and oxides as well as changes in the electrochemical double layer also in the case of platinum electrodes. However, the results that had been obtained so far are somewhat contradictory, and consequently diverse interpretations can be found in the literature. Therefore, it is worth to review the knowledge accumulated and to carry out systematic study in this respect. In this work smooth and platinized platinum electrodes in contact with acidic solutions were studied using EQCN technique. The effects of temperature, the nature of cations and anions, pH, concentrations, potential range were investigated on the electrochemical, and the simultaneously detected nanogravimetric responses. It is shown that in the underpotential deposition (upd) of hydrogen the adsorption/desorption of species from the solution phase is governed by the oxidative desorption/reductive adsorption of hydrogen; however, unambiguos conclusions cannot be drawn regarding the actual participation of anions and water molecules in the surface coverage. In the hydrogen evolution region a weak cation adsorption can be assumed and the potential of zero charge can be estimated. Cs⁺ cations affect the EQCN response in the hydrogen upd region. In some cases, e.g., in the case of upd of zinc the mass change can be explained by an induced anion adsorption. Two types of dissolution processes have been observed. A platinum loss was detected during the reduction of platinum oxide, the extent of which depends on the positive potential limit and the scan rate, and to a lesser extent on the temperature. The platinum dissolution during the electroreduction of oxide is related to the interfacial place exchange of the oxygen and platinum atoms in the oxide region. At elevated temperatures two competitive processes take place at high positive potentials: a dissolution of platinum and platinum oxide formation.     

超微量铂丶铑连续催化极谱测定及其催化波机理的探讨

For simultaneous determination of ultramicro amounts of platinum and rhodium the optimum condition has been described as 1.5N H2SO4-1.2% NH4Cl-0.0012M (CH2)6N4-0.003% N2H4XH2SO4. Both platinum and rhodium produce hydrogen catalytic waves with peak potential at-1.03 V and - 1.27 V (vs. S. C. E.) and the peak height of differential wave in single-sweep polarograph is directly proportional to the concentration of the metals in the range from 0.05 ppb to 1.0 ppb for platinum and from 0.0025 ppb to 0.1ppb for rhodium, respectively. The influence of other platinum group metals and some base metals on the height of catalytic waves has been examined. It has been shown that the method is very selective. It is applied even at 200:1 or at 1:10 (Pt:Rh). The mechanism of the catalytic waves has been discussed. The catalytic waves of both platinum and rhodium are due to "surface catalytic wave of hydrogen". The wave of rhodium can be attributed to catalytic discharge of hydrogen ion by the complex (Rh(CH2O)2Cl4)-. One of the ligands, formaldehyde, is the product of hydrolysis of hexamethylenetetramine. The wave of plainum can be attributed to catalytic discharge of hydrogen ion by the complex (PtACl5)-, where a denotes intermediate product (a substance containing CH2=N group) formed during the hydrolysis of hexamethylenetetramine. The role of hydrazine sulfate in catalytic system has been shown. Hydzazine can react with formaldehyde to from (CH2=N)2 which promotes the growth of platinum catalytic wave and in this way the concentration of formaldehyde in the system can be controlled.     

Surface-enhanced Raman scattering measurements on silver nanoparticles covered with differently formed platinum films

Gold and silver electromagnetic nanoresonators covered by a thin layer of platinum are often used to study adsorption of various molecules on “model platinum surfaces” with surface-enhanced Raman scattering (SERS) spectroscopy. In this contribution spectra of pyridine adsorbed on films formed from core–shell Ag@Pt and Ag@Ag–Pt nanoparticles and pure Pt or Ag nanoparticles were measured using a confocal Raman microscope. The SERS spectra of pyridine adsorbed on alloy Ag@Ag–Pt nanoparticles could not be obtained as a linear combination of spectra measured on pure Ag and Pt surfaces. In other words, for silver electromagnetic nanoresonators covered by platinum there is no simple correlation between the “quality” of the deposited Pt layer and the relative intensity of SERS bands characteristic for adsorbate interacting with silver. The SERS spectra accumulated from various places of a film formed from Ag@Pt or Ag@Ag–Pt nanoclusters may differ significantly. Using Ag@Pt nanoparticles with practically negligible amount of Ag on the surface (as per the stripping measurement), it is possible to record SERS spectrum in which the contribution characteristic for pyridine adsorbed on the Ag surface is well visible. It means that, even for macroscopic samples of core–shell Ag–Pt nanoparticles, averaging of many spectra measured at various locations of the sample should be carried out to characterize reliably their properties.     

Radiotracer study of the reduction of noble metals by amalgamated copper powder

A systematic study has been made on the reducing power of amalgamated copper powder in hydrochloric acid solution for palladium, platinum, rhodium, iridium, gold and silver. In order to apply this method to the activation analysis of palladium, platinum and rhodium in industrial concentrates which contain a large amount of ‘base elements’, the behaviour of palladium, platinum and rhodium in the presence of the ‘base elements’ has also to be considered. Research associate of I.I.K.W., Belgium     

Response to high acidity and basicity at a platinum electrode in chronopotentiometry

A simple and rapid method is developed to determine the high acidity and the basicity of solutions by chronopotentiometry with a platinum working electrode. The acidity range from 5.0 mol/l H⁺ to 1.0 mol/l OH⁻ can be measured by the adjustment of deposition potential and time. The response mechanism to acidity and basicity has been explored. The transition potential plateau in chronopotentiograms is caused from the oxidation of hydrogen adsorbed on electrode surface.     

平面铂电极上吸附氢原子表面扩散系数的测定

This Paper describes an experimental method that established a local Pt/Nafion interface on the Platinum Plane, so that the boundary conditions and the initial condition of the (diffusion equation about the diffusion of adsorbed hydrogen atom the platinum plane can be controlled. The average diffusion coefficient of underpotential deposition of hydrogen on a surface of platinum was obained for the first time and D=1.50 ×10-4cm2•s-1.The accuracy of the datum was discussed.     

Determination of some noble metals and copper by destructive neutron activation analysis of different matrices

Neutron activation analysis of palladium, iridium, platinum, gold, silver and copper in different matrices was carried out by using inorganic adsorbers such as molybdenum dibromide, zinc ferrocyanide and cadmium metal for their radiochemical separation. The matrices chosen to show the versatility of the method were: high-purity copper and nickel metals, NBS standard reference material SRM 1571 (Orchard leaves), Pultusk meteorite and the U. S. Geological Survey standard rock W-1. The method elaborated is very simple as it involves only a few steps and it has been shown to be accurate and capable of yielding reliable results.     

Specular reflectance studies of bromide adsorption on gold

Specular reflectance changes have been used to examine the specific adsorption of bromide on gold in the presence of a large excess of supporting electrolyte (NaF) which is not specifically adsorbed. A linear relation has been demonstrated between the reflectance changes and the surface excess of bromide through the examination of the time dependence of the reflectance under conditions where the rate of adsorption of the bromide is diffusion controlled and hence known. The adsorption isotherms have been found to follow Temkin behavior. The electrosorption valency has been evaluated from the charge and surface excess at constant potential and found to be ?0.49 to ?0.59, depending on the potential. Various mechanisms for the subtantial changes in reflectance attending the specific adsorption of anions are discussed. The observed effects cannot be explained on the basis of changes in the charge on the electrode and corresponding changes in the contribution of the conduction band to the surface optical properties. The principal mechanism is proposed to be modifications in the surface electronic states of the metal electrode through direct orbital interactions between the adsorbed anions and the metal.     

Substoichiometric isotope dilution analysis of copper traces in zinc by adsorptive sampling

The spontaneous deposition of Cu²⁺ from aqueous solutions on a hydrogen-saturated platinum surface as a possibility for the substoichiometric sampling of copper is shown. A sensitive isotope dilution method has been developed for the determination of traces of copper in high purity zinc making use of the spontaneous deposition of microquantities of Cu²⁺ from aqueous solutions of zinc salts on a cathodically polarized platinum surface. If the same platinum surface is used in each run, the saturation values of the deposited copper are practically independent of the concentrations and contents of copper and zinc in the solution. The copper traces were labelled with the product of the⁶⁴Zn(n,p)⁶⁴Cu nuclear reaction. A copper content of 0.18±0.03 ppm in zinc was determined by this method.