Platinum In Acid Solution Research Articles

Selection of low-toxic and highly efficient ionic liquids for the separation of palladium and platinum in acidic solution, and prediction of the metal affinity of ionic liquids

Separation studies were conducted to separate palladium (Pd) or platinum (Pt) from acidic solutions using 15 hydrophobic ionic liquids (ILs) capable of forming a binary phase with acidic aqueous solution. This study considered two cases. The first was to find high-efficient ILs capable of separating metals from a single solution, and the second was to find ILs capable of selectively separating Pt from an acidic mixture solution of Pd and Pt. In addition, we also considered toxic effect of ILs for greener separation process. For this studies, we experimentally examined the separation efficiencies of the ILs, and discuss the properties with their toxicities, theoretically predicted by previously presented toxicity prediction model, were compared. The results revealed that the extraction efficiency of ILs for Pd and Pt from acidic solutions depends on the types of IL and metal concentration. In the separation study on Pd in a single metal solution, trihexyltetradecylammonium bromide [P6,6,6,14]Br and trioctylmethylammonium choride [N8,8,8,1]Cl efficiently separates Pd from acidic solutions, while the separation on Pt in its single metal solution, 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[IM1-1Bz][(CF3SO2)2N], 1-methyl-3-dodecylimidazolim bis(trifluoromethylsulfonyl)imide[IM12][(CF3SO2)2N], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[IM16][(CF3SO2)2N], [P6,6,6,14]Br and [N8,8,8,1]Cl led to high efficiencies. Moreover, in the study to selectively separate Pt from a mixed solution of Pd and Pt, [IM1-1Bz][(CF3SO2)2N] and [IM16][(CF3SO2)2N] have significant separation efficiency. When comparing the efficiencies and toxicities of the two ILs, [IM1-1Bz][(CF3SO2)2N] is more efficient, but it is more toxic than [IM16][(CF3SO2)2N]. Thus, for sustainable development, [IM16][(CF3SO2)2N] can be considered as a better option. Moreover, in order to understand the partitioning mechanisms of ILs we developed quantitative structure–activity relationship models with R2 values greater than 0.91.

Formic Acid Electrooxidation on Platinum, Resolution of the Kinetic Mechanism in Steady State and Evaluation of the Kinetic Constants

A kinetic mechanism recently proposed for the formic acid oxidation (FAO) on platinum in acid solutions was analytically solved in order to verify its capability to correlate experimental measurements in steady state conditions. It involves three adsorbed reaction intermediates (COad, HCOOad, OHad) and two reaction pathways. The derived kinetic equations enabled the fitting of the experimental current-potential curves at three different formic acid concentrations, as well as the evaluation of the corresponding kinetic constants of the elementary steps involved in the mechanism. An expression for the variation of the COad surface coverage on potential was included in the calculations, obtained from measurements of the oxidation charges by voltammetric stripping. Finally, the dependences of the surface coverages of OHad and HCOOad on potential could be simulated, which are consistent with results obtained by other authors through spectroscopic techniques.

Oxygen Reduction Reaction Catalytic Sites on Carbon-Coated Fe3C Catalyst

Proton-exchange membrane fuel cells (PEMFC) are promising chemical-to-electrical energy conversion devices which can potentially replace combustion engine in automobiles. Their commercialization is currently suppressed primarily due to the price of the platinum catalyst for the cathodic oxygen reduction reaction (ORR). Required platinum loading per fuel-cell car is about 10 times higher than for the conventional one [1], which results in a significant contribution to the car price. Therefore, either the platinum loading needs to be drastically decreased, or a stable and active non-precious metal catalyst has to be developed. Recently, a new type of ORR catalyst composed of carbon-coated iron carbide nanoparticles has been synthesized [2]. It exhibits only slightly lower activity than, and comparable durability to platinum in acidic solutions. It has been suggested that a new ORR catalytic site may have been discovered [3]. In this work we study the catalyst by means of the Density Functional Theory to identify its active sites. Spin-polarized DFT calculations are performed using the Vienna Ab Initio Simulation Package (VASP) with PAW potentials, BEEF-vdW exchange-correlation functional [4], plane wave cutoff of 400 eV and vacuum layer of 14 Å. The model system consists of Fe3C/carbon interface and a few explicit water molecules to model the liquid environment. The following factors influencing the activity are taken into account: (i) thickness of the carbon layer (single- or multilayer graphene), (ii) defects in the carbon structure, (iii) nitrogen doping, (iv) adsorbate coverage, (v) presence of spectator adsorbates. A few other catalyst structures, which are independent of the presence of iron carbide, but still may exist in the real system, are also studied. These include catalysts with porphyrin-iron moiety embedded in the graphene structure and defective graphene with or without nitrogen doping. As a result, a few possible structures of catalytic sites in the carbon-coated Fe3C catalyst are proposed. [1] H.A. Gasteiger, D.R. Baker, R.N. Carter, W. Gu, Y. Liu, F.T. Wagner, P.T. Yu, in Hydrogen and Fuel Cells: Fundamentals, Technologies and Applications (D. Stolten, Ed.) Ch. 1 p. 3, Wiley-VCH, Weinheim, 2010. [2] Y. Hu, J. O. Jensen, W. Zhang, L. N. Cleemann, W. Xing, N. J. Bjerrum, Q. Li, Angew. Chem. Int. Ed., 2014, 53, 3675. [3] J.-P. Dodelet, R. Chenitz, L. Yang, M. Lefèvre, ChemCatChem, 2014, 6, 1866. [4] J. Wellendorff, K.T. Lundgaard, A. Møgelhøj, V. Petzold, D.D. Landis, J.K. Nørskov, T. Bligaard, K.W. Jacobsen, Phys. Rev. B, 2012, 85, 235149.

Kinetic Study of the Formic Acid Oxidation on Steady State Using a Flow Cell

The formic acid oxidation (FAO) reaction is studied on platinum in acid solutions using a specially designed flow cell and different experimental techniques, such as open circuit potential decay, chronoamperometry, voltammetric stripping, HCOOH concentration pulses and rotating disc electrode. It allows for the first time the evaluation of the surface coverage of irreversibly adsorbed reaction intermediates on steady state conditions. It is found that the unique species adsorbed irreversibly is CO (E < 0.5 V). It is demonstrated that at higher potentials the adsorption of the intermediate species involved in the FAO is highly reversible and their decomposition is the rate determining step. From the analysis of the results and taken into account available spectroscopic measurements, a new reaction mechanism is proposed, which is compatible with all the known experimental evidences and allows the interpretations of previous unexplained results.

Kinetics of hydrogen underpotential deposition at polycrystalline platinum in acidic solutions

Kinetics of the hydrogen underpotential deposition (HUPD) reaction at smooth polycrystalline platinum has been studied in 0.1M H2SO4 and HClO4 using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Impedance was measured at Pt electrodes (i) at lower frequencies (LF) up to 10kHz in a classical set up with the potentiostat and the frequency response analyzer at a single Pt disk electrode in a hanging meniscus configuration or Pt wire and in a large frequency range up to 1MHz with the FRA in a stand-alone mode and two arrangements of the electrodes, (ii) at small twin Pt electrodes, and (iii) at one hanging meniscus electrode and another of similar geometric size but of large surface area. Real surface area of the Pt electrodes was determined by the hydrogen adsorption method. All impedance spectra exhibited a CPE behavior. High frequency measurements permitted determination of the double layer capacitance, hydrogen adsorption pseudocapacitance and charge transfer resistance of the HUPD reaction. Deviations from the ideal capacitive response were larger at high than at low frequencies. The possible origins of this dispersion and the electrical equivalent circuits have been discussed.

Dynamic impedance study of ethanol and acetaldehyde oxidation at platinum in acid solutions

Ethanol and acetaldehyde oxidation at polycrystalline platinum electrode in 0.5M sulfuric acid were studied using cyclic voltammetry and dynamic electrochemical impedance spectroscopy and different concentrations of electroactive species. It was found that sharp peaks on cyclic voltammograms of ethanol observed by various authors were related to the appearance of negative resistances. The simplest electrical equivalent circuits explaining the impedance were found. Double layer capacitance measurements revealed that hydrogen underpotential deposition exists at the least positive potentials. In the potential range 0.3–0.7V low capacitance of ∼10μFcm−2 was observed due to strong adsorption of organic fragments and increased upon platinum oxidation. These results were reproducible upon cycling. In the potential zone of the formation of the first anodic peak the logarithmic slopes of dE/dRct were larger for the positive sweep than for the negative sweep. The zones of the negative resistances were identified.

Identification of the Species Produced During of the Adsorption/Desorption Processes of Polyethyleneglycol 20,000 on Platinum in Acid Solutions

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.

Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H(12)COOH and H(13)COOH) reveals that formic acid is oxidized to CO(2) via adsorbed formate and the decomposition (oxidation) of formate to CO(2) is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account.

Effect of cadmium and lead adatoms on the reduction kinetics of peroxodisulfate anions at platinized platinum in acid solutions

The reduction of peroxodisulfate anions on a rotating Pt/Pt disk electrode in solutions of sulfuric and perchloric acids with or without cadmium and lead salts is studied. At E r > 0.2V (RHE) the reduction rate in HClO4 exceeds that in H2SO4, but at E r < 0.2V, the process in HClO4 is strongly inhibited upon approaching E r = 0. Lead adatoms catalyze the process, while cadmium adatoms inhibit it in H2SO4 and accelerate it at E r < 0.2V in HClO4. The results are interpreted by taking account of the specific adsorption of sulfuric-acid anions and their co-adsorption with cadmium cations, as well as the adsorption of peroxodisulfate anions on the Pt surface modified by lead and cadmium ions in perchlorate solutions.

Surface-enhanced infrared study of catalytic electrooxidation of formaldehyde, methyl formate, and dimethoxymethane on platinum electrodes in acidic solution

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry has been employed to investigate the electrooxidation of formaldehyde, methyl formate, and dimethoxymethane (byproducts in methanol oxidation) on platinum in acidic solution. Owing to the sensitivity of SEIRAS being much higher than that of infrared reflection–absorption spectroscopy (IR-RAS), the reactions could be studied during cyclic voltammetric measurements at relatively fast potential sweep rates (50 mV s −1 or faster). In the oxidation of formaldehyde, a weak band assigned to the symmetric O–C–O stretching mode of formate adsorbed on the Pt surface in a bridging configuration was observed for the first time in addition to those for linearly and bridge-bonded CO known to be poisons. The oxidation current was found to increase as the band intensity of formate increases. On the other hand, none of the other reactive intermediates claimed in earlier IR-RAS studies was detected. These results led us to propose that adsorbed formate is a reactive intermediate en route to CO 2 production and the oxidation of formate to CO 2 is the rate-determining step. Methyl formate and dimethoxymethane are also oxidized to CO 2 via formate.

XPS and ARXPS study of silver underpotential deposition on platinum in acid solution

The underpotential deposition (upd) of silver on polycrystalline platinum electrodes has been studied using electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Under the electrochemical experimental conditions used for deposition, the cathodic ( q c) voltammetric charge density is around 200 μC m −2 which is equivalent to ∼1.2 ML of deposited silver. XPS and angle-resolved X-ray photoelectron spectroscopy (ARXPS) were used to determine the chemical composition of the deposited films. Two different approaches have been used to extract the concentration depth profiles from ARXPS measurements: (a) a simple parametric model with some hypotheses about the form of the depth profile and (b) a general algorithm that uses regularization and singular value decomposition (SVD) techniques. The structure and composition of the films is rather complex. The upd deposition of silver yields films with concentration gradients that can be modeled by a multilayer structure involving silver islands. Starting from the outer surface, the predominant species are C, O+S+Ag, and the platinum substrate.

Electrooxidation of primary alcohols on smooth and electrodeposited platinum in acidic solution

The electrooxidation of primary alcohols with four to eight carbon atoms on black Pt electrodeposits on gold/quartz, and on commercial bright platinum/quartz electrodes in perchloric acid solution, was studied by cyclic voltammetry (CV) and with the electrochemical quartz crystal microbalance (EQCM). Alcohol physisorption and Pt oxide reduction by chemical reaction with the alcohols were observed, both processes increasing with increasing chain length of the alcohol. Hexanol addition at 0.4 V versus SCE, the potential at which Pt oxidation starts, or higher potentials produced both an anodic current of hexanol electrooxidation and a mass decrease of bright Pt/Q electrodes, which shows unequivocally that the alcohol reduces the Pt oxide chemically.

Overlap of the oxide and hydrogen regions of platinum electrodes in aqueous acid solution

Two unusual features of noble metal electrode surfaces, active states of the metal and their anodic oxidation products (hydrous oxides), are of increasing interest at the present time owing to the important role of such species in electrocatalysis. The extent to which the hydrous oxide reduction process overlaps with the hydrogen adsorption region was investigated for platinum in acid solution. At least three distinct hydrous oxide reduction peaks (or regions) were observed and in some cases one of these peaks commenced at ca. 0.0 V, i.e. it was almost totally within the hydrogen gas evolution region. Following repeated hydrous oxide growth and reduction, which disrupted and thus activated the metal surface, a sequence of four low-level premonolayer oxidation peaks (each of which has been noted earlier by other authors) appeared in the positive sweep. As discussed earlier for copper in base, the transitions giving rise to such peaks are assumed to be mediator generation reactions, which strongly influence electrocatalytic processes occurring on platinum at low potentials.

Role of ethylamines on the electrochemical behaviour of Fe-Ni alloy films

Cyclic voltammetric experiments were carried out on platinum in acidic solution (pH 3) containing ferrous sulfate, nickel sulfate and ethylamines (EtNH2, Et2NH, Et3N). Spectral ultraviolet absorption studies indicate the complexation of both Fe2+ and Ni2+ ions with ethylamines. The results under transient polarisation conditions indicate the reduction of Fe2+ ions through the intermediate species FeOH+, with second electron transfer as a slow step. The higher charge transfer rate of FeOH+ over NiOH+ reduction causes the anomalous codeposition of Fe–Ni alloy film. Among the ethylamines, Et3N considerably assists the alloy deposition process. A gradual variation in free energy of alloy formation with Fe2+:Ni2+ (mol:mol) in the bath suggests the formation of an alloy intermediate phase rich in iron. Stripping voltammetric curves indicate the preferential dissolution of iron from iron rich alloy intermediate phase. X-ray diffraction studies further confirm the phase to be b.c.c. Fe–Ni alloy. The extent of corrosion of the Fe–Ni alloy film in the presence of ethylamines is in the following order: Et3N > Et2NH > EtNH2.

New insights into the influence of upd Sn on the oxidation of formic acid on platinum in acidic solution

The influence of a stabilized submonolayer of upd tin adatoms on the oxidation of adsorbed and bulk formic acid on sputtered platinum electrodes in 0.1 M HClO 4 has been investigated by measuring simultaneously the rate of CO 2 production from both adsorbed and bulk formic acid. The use of on-line MS technique (DEMS) enables us to distinguish the volatile products (CO 2) formed in the oxidation of adsorbed and bulk species, respectively, with the help of isotope-labeled 13C-formic acid as a probe. A considerable promoting effect of upd tin adatoms on the oxidation of the bulk formic acid was observed. This catalytic activity depends strongly on the coverage of the tin. On the other hand, the oxidation of the formic acid adsorbate is accelerated significantly in presence of Sn adatoms on the surface or tin ions in solution. Based on our results, a discrimination between the electrocatalytic models of tin adatoms postulated in literature was given. We suggested that a combination of a bifunctional mechanism and an electronic interaction between the upd Sn adatoms and Pt substrate could account for the pronounced catalytic influence on the oxidation of formic acid.

The possible existence of subsurface H-atom adsorbates and H2 electrochemical evolution reaction intermediates on platinum in acid solutions

The possible existence of H-subsurface states on Pt electrodes in acid solutions was studied using Pt electrodes of different topographies which were characterized by ex-situ scanning tunnelling microscopy (STM). The H-adatom electrodesorption current peak at ca. 0.22 V versus RHE resulting from combined voltammetric and potentiostatic potential routines applied in the H-adatom potential range was assigned to H-subsurface states. Seemingly, the complementary electroreduction occurs at potentials slightly more positive than the hydrogen evolution reaction threshold potential and the reactant, in this case, appears to be related to the hydrogen evolution reaction. The charge density involved in these current contributions is only a small fraction of the H-adatom monolayer charge density. The formation of H-subsurface states appears to be favoured for those Pt surfaces exhibiting a larger contribution of weak H-atom electrosorption. A reasonable correlation between electrochemical and chemisorption data on H on Pt surfaces with different topographies and crystallographic orientations can be established. This fact provides further support for the existence of adsorbed H atom precursor states related to the formation of subsurface H adatoms and to the hydrogen evolution reaction on Pt in acid solutions.

Voltammetric response of hydrogen adsorbates on platinum in acid solutions: a possible H-adatom subsurface state

Combined potential routines involving voltammetric sweeps and potential holdings covering the H-adatom potential range were applied to Pt electrodes in different aqueous acid solutions to produce an enhancement of the "third" H-atom electrodesorption current peak at ca. 0.22 V vs. RHE. This peak can be observed for Pt electrodes of different topographies. Seemingly, the corresponding cathodic current contribution appears in a potential range slightly more positive than the hydrogen evolution reaction threshold potential. The peak potential difference related to the conjugated redox reactions is consistent with an appreciable irreversibility of the process. The small charge involved in this pair of peaks is expected from a subsurface and an interstitial H-adatom state on Pt. This type of structure has been predicted from quantum chemistry calculations.

Components of overvoltage of hydrogen electrode reaction on nickel

For the case of hydrogen evolution reaction (HER) in alkaline solutions on nickel, the overvoltage of the electron transfer step (the first step of HER) was separated from that of the overall reaction on the basis of the results obtained by the galvanostatic transient method, which has been developed by us earlier. The difference of the overall-reaction overvoltage and electron-transfer overvoltage, which is considered as the overvoltage of the step of an adsorbed hydrogen atom recombination (the last step of HER), was also determined. It is known that on nickel, during cathodic polarization in alkaline solutions, the intermetallic compound M(I) of an alkali metal with nickel is formed as practically the main intermediate. In this case, the Tafel line, corresponding to the last step, shows a slope of 70–80 mV, and the observed overvoltage can be explained in terms of the decrease of the work function of the surface component, caused by the M(I) formation. On the other hand, on a cobalt-deposit-modified nickel, the Tafel line shows a slope of 30 mV, even in alkaline solutions. The nickel surface may be completely covered with the cobalt deposit. Therefore, no intermetallic compound seems to be formed on the surface, and the main intermediate is likely to be an adsorbed hydrogen atom, as in the case of HER on platinum in acid solutions.

Electrochemical behavior of nitromethane and its influence on the electro-oxidation of formic acid: an on-line MS study

The electrochemical behavior, including both oxidation and reduction, of nitromethane at a porous platinum electrode in sulfuric acid solution was studied using differential electrochemical mass spectroscopy (DEMS). It was found that under these conditions nitromethane can be oxidized to CO 2 and NO x in reactions involving only adsorbed species. Below 0.4 V/RHE nitromethane is reduced, as shown by the cathodic current density in the cyclic voltammogram and a simultaneous decrease in the ion currents associated with CH 3NO 2. The presence of nitromethane catalyzed the bulk oxidation of formic acid in the low potential range below 0.8 V/RHE. In contrast, at potentials above 0.8 V/RHE, the oxidation of formic acid was inhibited by nitromethane. These effects were studied for various concentrations of nitromethane ranging from 0.01 to 1 M. In contrast with formic acid, the oxidation of methanol on platinum in acid solution was inhibited by the presence of nitromethane. The electrochemical behavior of nitromethane on platinum in acid is compared with that of acetonitrile. Both compounds are catalytically active towards the oxidation of formic acid. Possible explanations for the catalytic action of nitromethane are discussed.