Smooth Polycrystalline Platinum Research Articles

Design and Investigation of Molybdenum Modified Platinum Surfaces for Modeling of CO Tolerant Electrocatalysts

Mo overlayers were prepared on smooth polycrystalline platinum and platinized platinum electrode surfaces by in situ electrochemical deposition of molybdenum oxide at potential below 500 mV for modeling Mo–Pt electrocatalysts. Correlations were found between the applied potential and the amount of deposited Mo, which never exceeded a monolayer, thus Pt–Mo bonds stabilize the deposited Mo oxide. Electrochemical measurements suggested that Mo deposited from a Mo(VI) solution was reduced to the 4+ oxidation state. In line with the ex situ XPS findings a certain part (20–25%) of the initial Mo layer remained irreversibly adsorbed on the Pt/Pt electrode even after oxidation into the 6+ state at high potentials; this fractional monolayer cannot be dissolved even by prolonged cyclic polarization up to 1000 mV. It has been demonstrated that the irreversibly bound Mo partial monolayer is enough to change significantly the CO poisoning properties of the Pt surface. On this Mo:Pt (1:4) surface CO oxidation is initiated at extremely low potentials (ca. 100 mV). Moreover, only Pt modified by Mo(IV) species is active in low-potential CO oxidation reaction as after oxidizing the irreversibly adsorbed Mo to the 6+ state, CO oxidation is no longer observable. Nevertheless, the catalyst can be reactivated by reduction of molybdenum into the 4+ oxidation state. However, this reduction requires clean, CO-free Pt surface. If Pt is largely covered by CO, reduction of Mo(VI) into Mo(IV) does not occur and thus the low potential CO oxidation remains hindered.

Electroreduction of peroxodisulfate anion at platinum rotating disc electrode in the cyclic voltammetry mode

Electroreduction of peroxodisulfate anion at smooth polycrystalline and platinized (at different deposition potentials) platinum in perchloric acid and sulfuric acid solutions is studied by rotating disc electrode and cyclic voltammetry techniques. Dependences of the process rate on the electrode rotating velocity, the potential scan rate, the anodic limit of the scanning, the peroxodisulfate anion concentration in the solution and the platinizing conditions are found. The suggestion on the complications in the peroxodisulfate anion reduction caused by adsorbate formation is corroborated, at least, for certain potential region. The reaction structure sensitivity is evidenced, which makes it possible to use the reaction for characterization of the platinized Pt surface structure. The comparing of obtained results with literature data concerning smooth platinum and the single-crystal platinum basis faces allows concluding that the peroxodisulfate anion reduction maximal rate in sulfuric-acid solutions occurs at the potentials close to those observed for the (110) face. When the platinized Pt surface roughness factor exceeds ~30, the peroxodisulfate anion reduction reaction proceeds under the inner-diffusion limitation control. The platinized-Pt rotating disc electrode can serve as model tool in the studying of properties of disperse material microdeposits.

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.

Kinetic study of CO oxidation on clean and oxidized Pt

Bulk CO oxidation on smooth polycrystalline platinum was studied with a rotating disk electrode and cyclic voltammetry in CO-saturated 0.5M sulfuric acid. The reaction rate was modelled with differential equations. The modelling goal was to build a model that reproduces the experimental current–potential characteristics, with special attention to the platinum oxide region. Two different models were tested: one containing conventional chemical and electrochemical reaction steps, and one built around an empirical equation describing the rate of platinum oxide formation and reduction. The latter model produced good fits to the experimental results. Both models showed that CO oxidation in the platinum oxide range proceeds via direct reaction between an adsorbed species such as PtOH and dissolved CO.

A Study of Methanol Oxidation by Dynamic Electrochemical Impedance Spectroscopy

Methanol oxidation on a smooth polycrystalline platinum electrode is studied by dynamic electrochemical impedance spectroscopy at elevated temperatures employing an autoclave setup. By utilizing custom written software, impedance spectra for all applied electrode potentials are calculated. By increasing the temperature and/or sweep rate, it is shown that the region with three adsorbed species disappears, which is interpreted as less contribution of the parallel pathway at higher temperatures.

Surface metal modifiers for methanol electrooxidation on platinum, molybdenum and tungsten

New promising results on the electrocatalysis of methanol electro-oxidation on platinum surfaces modified with molybdenum and tungsten deposition were found by combining different methods (spontaneous, potentiostatic and potentiodynamic). A quasi-reversible process involving MoO 2 and MoO 3 inter-conversion characterizes platinum/molybdenum interaction, the MoO 2 formation on platinum being favoured according to the calculated equilibrium constant. The maximum surface excess for molybdenum species was 1.83 × 10 −4 mol cm −2 considering a quasi-reversible charge transfer under langmuirian conditions for the adsorbates. Tungsten preferentially occupies platinum (1 1 1)-stepped sites exhibiting a complex redox process between WO 4 2 − and WO 4 3 − soluble species, being more remarkable with lower values of the upper potential limit. The analysis of methanol oxidation was accomplished by using linear sweep voltammetry in a bulk methanol solution or by stripping voltammetry of adsorbed residues on smooth and columnar polycrystalline platinum after deposition of molybdenum and/or tungsten. The platinum surface modified by potentiodynamic deposition of molybdenum followed by double deposition of tungsten presents the best performance upon methanol electro-oxidation, recording at 0.7 V, a 50% higher current density than on bare platinum. It was also found that platinum/tungsten surfaces prepared by potentiostatic or potentiodynamic deposition exhibit the lowest amount of carbon monoxide residues, i.e. 43% of coverage on platinum/tungsten against 81% on bare platinum. Tafel slopes for methanol oxidation on pure platinum and double molybdenum and tungsten depositions are ca. 120 mV dec −1 suggesting a first monoelectronic charge transfer as rate-determining step. For the simultaneous deposition of molybdenum and tungsten, higher Tafel slopes (as 200 mV dec −1) were calculated, but for platinum surfaces modified by single metal deposition, Tafel slopes slightly higher than 2 RT/ F were observed.

Activating and deactivating mass transport effects in methanol and formic acid oxidation on platinum electrodes

Transient increases in rotation rate at an RDE increase the methanol oxidation current, even though in slower experiments the current decreases with increasing rotation rate as usually reported. Methanol oxidation on smooth polycrystalline platinum rotating disk electrodes in sulfuric acid electrolyte was studied by RDE voltammetry and hydrodynamic impedance spectroscopy combined with cyclic voltammetry. A positive low-frequency real part in the hydrodynamic admittance spectra for the main oxidation peak was used to predict that a transient increase in rotation rate would increase the current, as was observed. In contrast, slow scan rate voltammograms showed a decrease in current with increasing rotation rate. The transient current increase was explained by enhanced production of soluble intermediates, while increased production of adsorbed CO poisoning explained the slower inhibition. Comparative experiments for formic acid oxidation showed increasing current with rotation rate in both hydrodynamic admittance spectra and slow-scan voltammograms.

Preliminary study of ethanol electrooxidation in the presence of sulfate on polycrystalline platinum

The electrooxidation of ethanol and its inhibition by the presence of adsorbed sulfate have been investigated by cyclic voltammetry and chronoamperometry using several concentrations of sulfuric acid on smooth polycrystalline platinum. The results show that the concentration of sulfuric acid influences the current in both potentiostatic and potentiodynamic experiments. The results are interpreted in terms of the competitive adsorption of sulfate and ethanol on the same Pt sites and suggest that, when the sulfuric acid concentration is increased, there is a reduction of Pt free sites able to adsorb and oxidize ethanol. The voltammetric data reveal that the peak currents observed during ethanol oxidation are not affected in the same way by the presence of H 2SO 4, which, based on previously obtained FTIR results, suggests that the sulfate adsorption is able to inhibit the oxidation of ethanol in a selective way.

Electrocatalytic mechanism and kinetics of SOMs oxidation on ordered PtPb and PtBi intermetallic compounds: DEMS and FTIRS study

The electrocatalytic activities and mechanisms of PtPb and PtBi ordered intermetallic phases towards formic acid, formaldehyde and methanol oxidation have been studied by DEMS and FTIRS, and the results compared to those for a pure polycrystalline platinum electrode. While PtPb exhibits an enhanced electrocatalytic activity for the oxidation of all three organic molecules when compared to a Pt electrode, PtBi exhibits an enhanced catalytic activity towards formic acid and formaldehyde oxidation, but not methanol. FTIRS data indicate that adsorbed CO does not form on PtPb or PtBi intermetallic compounds during the oxidation of formic acid, formaldehyde and methanol, and therefore their oxidation on both PtPb and PtBi intermetallic compounds proceeds via a non-CO(ads) pathway. Quantitative DEMS measurements indicate that only CO(2) was detected as a final product during formic acid oxidation on Pt, PtPb and PtBi electrodes. At a smooth polycrystalline platinum electrode, the oxidation of formaldehyde and methanol produces mainly intermediates (formaldehyde and formic acid), while CO(2) is a minor product. In contrast, CO(2) is the major product for formaldehyde and methanol oxidation at a PtPb electrode. The high current efficiency of CO(2) formation for methanol and formaldehyde oxidation at a PtPb electrode can be ascribed to the complete dehydrogenation of formaldehyde and formic acid due to electronic effects. The low onset potential, high current density and high CO(2) yield make PtPb one of the most promising electrocatalysts for fuel cell applications using small organic molecules as fuels.

Special features of methanol interaction with adsorbed oxygen at platinized platinum electrode: Transients of the open-circuit potential

Open-circuit potential transients are measured under the conditions of methanol interaction with the pre-adsorbed oxygen at platinized platinum electrode. The time necessary for complete removal of the adsorbed oxygen monolayer appeared being shorter by a factor of ∼1.5 as compared with smooth polycrystalline platinum. The dependence of platinum surface coverage with adsorbed oxygen on the potential during its decay is found. It was shown that the reaction of methanol with the adsorbed oxygen is most slow at a high coverage (1–0.8). It is suggested that at these coverages, like the case of polycrystalline platinum, the adsorbed oxygen directly interacts with the methanol molecules from the solution. At moderate coverages (0.8–0.2), the reaction of the adsorbed oxygen with methanol at the platinized platinum is better described by the “conjugated reactions” mechanism. The specific rates of the methanol dissociative adsorption at the platinized platinum turned out to be close to those observed earlier for the polycrystalline platinum.

Electrodeposition of platinum metal on titanium and anodised titanium from P salt: application to electro-oxidation of glycerol

Pt/Ti and Pt/TiO2 electrodes were made by galvanic deposition of platinum metal from an acid plating bath containing cis-dinitro-diammin-platinum(II) complex (P salt) onto titanium and anodic TiO2 layer on titanium substrates respectively. Titanium oxide nanotubes were fabricated by anodising titanium foil in acid solution. Pt/Ti and Pt/TiO2 electrodes were characterised using electrochemical methods and scanning electron microscopy. Cyclic voltammetric analysis of Pt/Ti electrodes in dilute sulphuric acid yields voltammograms with features characteristic of bulk polycrystalline platinum electrodes. In contrast, the voltammograms for Pt/TiO2 electrodes are not quantitatively similar to those for smooth polycrystalline platinum electrodes. Voltammetry in potassium ferricyanide yields reversible voltammograms, indicating the good electrical conductivity of and connectivity between the deposited platinum and substrates and hence the absence of any significant resistive surface oxide film. The electrocatalytic behaviour of this type of electrodes in the glycerol electro-oxidation was studied.

Electrochemical surface reshaping of polycrystalline platinum: Morphology and crystallography

A novel process of morphological evolution was observed on a polycrystalline platinum electrode surface during intensive potential perturbation in sulfuric acid electrolyte. The process was started from a smooth polycrystalline platinum surface, and ended up with a roughened overlayer which is composed of numerous morphological units. According to its appearance, the unit is named “feather-like morphology (FLM)” which contains finer structures at nanometer scale. The active surface area of platinum at different stages during the morphological evolution was estimated by cyclic voltammetry. Electron microscopic measurements provided both morphological details and crystallographic information.

Effect of oxygen adsorption on the laser-induced thermal jump of potential on a smooth polycrystalline platinum electrode

It has been shown for the first time that the laser-induced thermal jump of potential on a platinum electrode is sensitive to the form of oxygen adsorption.

Effect of ionic composition of solutions on the methanol reaction with adsorbed oxygen on smooth polycrystalline platinum electrodes: Transients of the open-circuit potential

Transients of the open-circuit potential observed in the reaction of methanol with oxygen (Oads) preliminarily adsorbed on smooth polycrystalline platinum (pcPt) are measured in 0.05 M HClO4, 0.5 M HClO4, 0.05 M H2SO4, 0.05 M H2SO4 + 0.45 M Na2SO4, and 0.05 M H2SO4 + 0.45 M Cs2SO4. It is shown that the solution pH has a weak effect on the transient characteristics (when the reversible hydrogen electrode potential scale is used). This confirms the chemical nature of rate-controlling stages in the reaction mechanism. The changes in the reaction rate, observed upon going from one electrolyte to another, are preferentially associated with the involvement of solution ions in the formation of activated surface complexes that include CH3OH, Oads, and supporting-electrolyte components.

DEMS Study on Methanol Oxidation at Poly- and Monocrystalline Platinum Electrodes: The Effect of Anion, Temperature, Surface Structure, Ru Adatom, and Potential

Methanol oxidation on Ru-unmodified and modified polycrystalline platinum, Pt(111), and Pt(332) was studied by on-line differential electrochemical mass spectrometry (DEMS) in combination with the dual thin-layer flow-through cell. The effects of anion, temperature, surface structure of Pt electrodes, Ru adatom, and potential on methanol oxidation were discussed. Methanol oxidation on smooth polycrystalline platinum, Pt(111), and Pt(332) electrodes proceeds via the parallel pathway mechanism, and mainly forms the soluble intermediates, i.e., formaldehyde and formic acid. Anion, temperature, and surface structure of Pt electrodes markedly influence the apparent rate of methanol oxidation; however, they do not significantly alter the current efficiency of CO2, and therefore do not change the mechanism of methanol oxidation. On pure platinum electrodes current efficiency of CO2 for methanol oxidation increases from ca. 20% to 32% with the increase of potential from 0.6 to 0.8V, accompanied by the increasing ...

Kinetics and mechanism of interaction between methanol and adsorbed oxygen on a smooth polycrystalline platinum electrode: Transients of the open-circuit potential

The method of transients of the open-circuit potential, combined with cathodic potentiodynamic pulses, is used for studying the methanol interaction with preliminarily adsorbed oxygen (Oads) in 0.5 M H2SO4. It is established that, for the larger part of the time period required for a full reduction of a monolayer of Oads in solutions of methanol on polycrystalline platinum, the process occurs at large coverages of the surface by Oads (ϑO). In the region of medium coverages, transients of the open-circuit potential are accurately described by the equation that corresponds to the mechanism of “conjugated reactions.” The mechanism of the methanol interaction with Oads happens to be close to that for formic acid, which is explained by a dissociative character of these HCO compounds. Kinetic parameters characterizing the methanol reaction with Oads in the region of large and medium coverages are determined and analyzed.

Voltammetric Characterization of Micro‐ and Submicrometer‐Electrode Arrays of Conical Shape for Electroanalytical Use

AbstractDensely packed micro‐ and submicrometer electrode arrays of platinum and gold (the nominal number, N, of electrodes in each array varies between 225 and 3600) are fabricated by photolithographic technique and vapor deposition processes of metal films. The electrodes are conical‐shaped and only their apexes are exposed to the electrolytic solution. The electrode arrays are characterized electrochemically in Ru(NH3)6Cl3 aqueous solutions by using cyclic voltammetry at low scan rates, to establish the number of electrochemically active electrodes (Nac) in each array; the geometric characterization is performed by scanning electron microscopy. All the investigated arrays provide steady‐state voltammograms, indicating diffusionally independent behavior of each microelectrode. The number of microelectrodes that are active in the fabricated arrays depends on microelectrode density. In particular, for the arrays with N=3600 and N=225, the fraction of active sites is about 45% and 90%, respectively. The analytical performance of some of the Pt version of the arrays is tested in hydrogen peroxide solutions, allowing verifying that linear calibration plots over the concentration range (0.1–20 mM) are obtained. This dynamic range is larger than that typically recorded at smooth polycrystalline platinum electrodes (0.5–5 mM), and the better performance is attributed to both the higher aspect ratio of the cone geometry and the higher mass transport associated to each microelectrode of the array. Reproducibility (within 3.5%, r.s.d.) and long‐term stability (within 5%, r.s.d., after 8 h continuous use) of the electrode systems are satisfactory. A low detection limit, based on the signal to noise ratio equal to 3, of 0.05 mM is found, which is adequate for a rapid monitoring of H2O2 in real samples and industrial processes.

Specific features of interaction between formic acid and oxygen adsorbed on smooth polycrystalline platinum: Transients of the open-circuit potential

Transients of the open-circuit potential, which are observed when formic acid is interacting wit adsorbed oxygen (Oads) preliminarily accumulated on polycrystalline “smooth” platinum (pcPt), are measure in an aqueous solution of sulfuric acid. It is shown that, as with platinized platinum (Pt/Pt), at large coverage by adsorbed oxygen (θO = 1−0.8), adsorbed oxygen interacts directly with molecules of formic acid from solution. In the region of medium coverages (θO = 0.8−0.2), on the other hand, a mechanism of “conjugated reactions” is realized. It is established that, in the case of pcPt, the direct interaction of Oads with molecules of HCOOH from solution proceeds slower by nearly three times and the interaction via the mechanism of “conjugated reactions,” faster by about three times, as compared with Pt/Pt.

The Use of Thin‐Layer Electroanalysis in the Study of the Chemisorption and Anodic Oxidation of Aromatic Molecules at Smooth Polycrystalline Palladium

AbstractThin‐layer electroanalytical chemistry has been used in the study of the chemisorption and anodic oxidation of hydroquinone and benzoquinone at smooth polycrystalline palladium electrode surfaces in aqueous sulfuric acid solutions. The results were reminiscent of those obtained previously on smooth polycrystalline platinum: i) At low aqueous‐solution concentrations, the diphenol is oxidatively chemisorbed to form surface‐coordinated benzoquinone oriented parallel to the surface. ii) At higher concentrations, the oxidative chemisorption occurs via CH activation to yield an edge‐oriented diphenolic species. iii) Chemisorption from benzoquinone solutions leads to species identical to those from hydroquinone solutions. iv) The extent of anodic oxidation of the chemisorbed organic depends upon the initial adsorbed‐molecule orientation: the flat‐adsorbed species are oxidized completely to carbon dioxide, whereas oxidation of the edge‐chemisorbed species yields other (unidentified) products that are chemisorbed upon regeneration of the oxide‐free surface.

Specific Features of the Adsorption of Carbon Monoxide on a Smooth Polycrystalline Platinum Electrode Modified with Adsorbed Silver Atoms

The adsorption of carbon monoxide at the surface of smooth polycrystalline platinum (smPt) is studied in conditions of a preliminary accumulation of various quantities of silver (θAg) on the surface. A comparison with similar data obtained previously for Pt/Pt is conducted. It is discovered that on smPt, exactly as in the case of Pt/Pt, carbon monoxide undergoes adsorption at sites that are not occupied by adsorbed silver, without forcing the preliminarily adsorbed silver out. At small and intermediate Agad, as opposed to Pt/Pt, a mere two peaks are observed in a voltametric curve in the region of electrodesorption of the mixed layer on smPt. It is shown that, in the region of potentials of the first peak, there occurs practically no transition of silver into solution in the course of oxidation of the mixed layer. Specific features that characterize the behavior of the COads + Agad mixed layer are discussed under the assumption about an “islet” character of the adsorption of silver.