The Bromine Electrode. Part I: Adsorption Phenomena at Polycrystalline Platinum Electrodes

A cyclic voltammetric study of the behaviour of Br^– and Br^– ₃ at Pt electrodes, in the potential range between hydrogen and oxygen evolution, is described. Different experiments were carried out, in the presence of Br^– and Br^– ₃, in which the ratio between the species has been kept constant and equal to 1. The halide concentration was varied between 4 × 10^–6 and 1 × 10^–3 and mol dm^–3, at constant ionic strength, in 1 M HclO₄ as well as in 1 M NaClO₄ adjusted to a pH of 2. Underpotential deposition of Br is observed at potentials as low as –0.125 V vs SCE. The adsorption parameters of Br species were determined from the adsorption/desorption peak pair in the hydrogen adsorption/desorption region, and from the oxide reduction peak data. In the absence of oxygen adsorption, a relatively high coverage of the electrode surface is attained. A Langmuir-type adsorption is observed under the different experimental conditions.     

Influence of adsorbed carbon dioxide on hydrogen electrosorption in palladium-platinum-rhodium alloys

Carbon dioxide electroreduction was applied to examine the processes of hydrogen electrosorption (adsorption, absorption and desorption) by thin electrodeposits of Pd-Pt-Rh alloys under conditions of cyclic voltammetric (CV) experiments. Due to different adsorption characteristics towards the adsorption product of the electroreduction of CO₂ (reduced CO₂) exhibited by the alloy components hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. Reduced CO₂ causes partial blocking of hydrogen adsorbed on surface Pt and Rh atoms, without any significant effect on hydrogen absorption into alloy. It reflects the fact that adsorbed hydrogen bonded to Pd atoms does not participate in CO₂ reduction, while hydrogen adsorbed on Pt and Rh surface sites is inactive in the absorption reaction. In contrast, CO is adsorbed on all alloy components and causes a marked inhibition of hydrogen sorption (both adsorption and absorption)/desorption reactions.     

Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate ion reduction on Pt(1 1 1) and Cu-modified Pt(1 1 1) electrodes have been studied by means of cyclic voltammetry, potentiostatic current transient technique and in situ FTIRS in solutions of perchloric and sulphuric acids to elucidate the role of the background anion. Modification of platinum surface with copper adatoms or small amount of 3D-Cu crystallites was performed using potential cycling between 0.05 and 0.3 V in solutions with low concentration of copper ions, this allowed us to vary coverage θ_Cu smoothly. Following desorption of copper during the potential sweep from 0.3 to 1.0 V allowed us to estimate actual coverage of Pt surface with Cu adatoms. Another manner of the modification was also applied: copper was electrochemically deposited at several constant potentials in solutions containing 10⁻⁵ or 10⁻⁴ M Cu²⁺ and 5 mM NaNO₃ with registration of current transients of copper deposition and nitrate reduction.It has been found that nitrate reduction at the Pt(1 1 1) surface modified by copper adatoms in sulphuric acid solutions is hindered as compared to pure platinum due to induced sulphate adsorption at E < 0.3 V. Sulphate blocks the adsorption sites on the platinum surface and/or islands of epitaxial Cu(1 × 1) monolayer thus hindering the adsorption of nitrate anions and their reduction. The extent of inhibition weakly depends on the copper adatom coverage. Deposition of a small amount of bulk copper does not affect noticeably the rate of nitrate reduction.Nitrate reduction on copper-modified Pt(1 1 1) electrodes in perchloric acid solutions occurs much faster as compared to pure platinum. The steady-state currents are higher by 4 and 2 orders of magnitude at the potentials of 0.12 and 0.3 V, respectively. The catalytic effect of copper adatoms is largely caused by the facilitation of nitrate adsorption on the platinum surface near Cu_ad and/or on the islands of the Cu(1 × 1) monolayer (induced nitrate adsorption).Hydrogen adatoms block the adsorption sites on platinum for NO₃⁻ anion adsorption and inhibit reactions of nitrate reduction even at moderate surface coverage.The products of nitrate reduction in sulphuric and perchloric acids are essentially the same (NO and ammonia) irrespective of the presence or absence of Cu on the platinum surface.     

Controllable‐Nitrogen Doped Carbon Layer Surrounding Carbon Nanotubes as Novel Carbon Support for Oxygen Reduction Reaction

Novel nitrogen‐doped carbon layer surrounding carbon nanotubes composite (NC‐CNT) (N/C ratio 3.3–14.3 wt.%) as catalyst support has been prepared using aniline as a dispersant to carbon nanotubes (CNTs) and as a source for both carbon and nitrogen coated on the surface of the CNTs, where the amount of doped nitrogen is controllable. The NC‐CNT so obtained were characterized with scanning electron microscopy (SEM), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and nitrogen adsorption and desorption isotherms. A uniform dispersion of Pt nanoparticles (ca. 1.5–2.0 nm) was then anchored on the surface of NC‐CNT by using aromatic amine as a stabilizer. For these Pt/NC‐CNTs, cyclic voltammogram measurements show a high electrochemical activity surface area (up to 103.7 m² g^–1) compared to the commercial E‐TEK catalyst (55.3 m² g^–1). In single cell test, Pt/NC‐CNT catalyst has greatly enhanced catalytic activity toward the oxygen reduction reaction, resulting in an enhancement of ca. 37% in mass activity compared with that of E‐TEK.     

Electrocatalytic reduction of nitric oxide on Pt nanocrystals of different shape in sulfuric acid solutions

Pt tetrahexahedral (Pt-THH) nanocrystals enclosed with 24 {h k 0} facets, Pt nanothorns (Pt-Thorn) with a high surface density of atomic steps, and congeries of Pt nanoparticles (Pt-NP) were prepared and served as catalysts to study the electrocatalytic reduction of both adsorbed and solution nitric oxide. The structure sensitivity for the reduction of a saturated NO adlayer on the Pt nanocrystals (NCs) of different shape was studied by cyclic voltammetry (CV) and in situ FTIR spectroscopy in sulphuric acid solutions. The results revealed that two types of NO adsorbates can be reduced independently at separated potentials, i.e. the reduction of linear bonded NO (NO_L) on the Pt-NP electrode gives rise to a current peak at −0.01 V (vs. SCE), while the bridge adsorbed NO (NO_B) yields a current peak at −0.08 V. The in situ SNIFTIRS results confirmed the assignment of NO adsorbates, i.e. the NO_B species yielding a IR absorption bipolar band with its negative-going peak at 1636 cm⁻¹ and positive-going peak around 1610 cm⁻¹, and the NO_L species giving rise to a bipolar band with its negative-going peak at 1809 cm⁻¹ and positive-going peak around 1720 cm⁻¹. It has determined that the NO_L species can be preferentially formed on the Pt NCs with open surface structure, i.e. the more open the surface structure of the Pt NCs, the larger the relative quantity of NO_L versus NO_B. It has also revealed that the Pt NCs with a high surface density of atomic steps exhibit superior electrocatalytic activity for the reduction of solution NO species. The steady-state current density of NO reduction on Pt-THH NCs is 7.5-12 times as large as that on Pt-NP, and that on Pt-Thorn is 2.5-4 times of that on Pt-NP in the reduction potential region of electrochemical dynamic control.     

Characterizing electrocatalytic surfaces: Electrochemical and NMR studies of methanol and carbon monoxide on Pt/C

We use cyclic voltammetry (CV) on fuel cell electrodes to elucidate the important differences between adsorbates resulting from carbon monoxide adsorption and methanol adsorption onto commercial Pt/C electrocatalysts in a sulfuric acid electrolyte. Under open circuit conditions, methanol was found to adsorb preferentially onto the Pt sites associated with “strongly bound” hydrogen. The sites associated with “weakly bound” hydrogen adsorbed methanol more slowly. In the case of CO adsorption, which requires no adsorbate dehydrogenation, all adsorption sites showed similar affinity towards the adsorbate. Electrochemical oxidation of the adsorbates derived from both methanol and CO exposure exhibit slower oxidation when the adsorbate is associated with cubic-packed-like sites than from close-packed-steps and other sites. NMR of a ¹³CO-adlayer prepared by electrochemical adsorption from low concentration ¹³CH₃OH shows a lower NMR shift and smaller linewidth than the previously reported values for electrochemically adsorbed ¹³CO gas. These results are interpreted in terms of adsorbate motion on the electrocatalyst surface.     

Changes in the surface of platinum in hot concentrated phosphoric acid at low potentials

Evidence is presented which suggests that, at high temperature and phosphoric acid concentration, reductive adsorption of the acid occurs on Pt in the vicinity of the hydrogen potential. This adsorption, presumably in the form of surface phosphide, does not noticeably inhibit the anodic hydrogen oxidation. It is, however, a source for an anodic peak in the voltammogram for Pt in such acids which has been observed by other workers and which commonly had been attributed to acid-born impurities such as phosphorous acid.     

Direct deposition of nanostructured Pt particles onto a Ni foam from lyotropic liquid crystalline phase by displacement plating

Nanostructured Pt particles are directly deposited onto a Ni foam by utilizing displacement plating after a lyotropic liquid crystalline phase including Pt species was prepared within macropores of the Ni foam. The EDS mapping of Pt after deposition corresponds to the macroscopic framework of the Ni foam, indicating the uniform displacement plating of Pt on the surface of the Ni foam. The Pt particles of 150-250 nm in size are formed over the entire area of the surface of the Ni foam. The TEM images prove that the nanoscale rods (width: about 3 nm) are aggregated with each other to form nanoscale porosity. The active area of Pt can be estimated to be ca. 12 m²/g by using the cyclic voltammogram in sulfuric acid. Our method realizes one-step production of hierarchical macro-meso type porous electrodes.     

Electrochemical Characteristics and Performance of Platinum Nanoparticles Supported by Vulcan/Polyaniline for Oxygen Reduction in PEMFC

We report a Pt/Vulcan carbon–polyaniline (VC–PANI) catalyst for the oxygen reduction reaction (ORR). This electrocatalyst was prepared from Pt nanoparticles supported by a VC–PANI composite substrate. Electrochemical performance was measured using potentiostat/galvanostats technique and a proton exchange membrane fuel cell (PEMFC) test station. The electrochemical properties of the electrodes were characterized using linear sweep voltammetry, AC impedance spectroscopy and chronoamperometry. Electrochemical characterization by hydrogen adsorption/desorption cyclic voltammetry and CO stripping voltammetry indicates that the electrochemical active surface areas of the Pt/VC–PANI are comparable to the commercial catalyst. The performance of the Pt/VC–PANI and Pt/C(E‐TEK) + PANI electrocatalysts were found to be 1.82 and 1.33 times higher than of the Pt/C(E‐TEK) electrode. The surface morphologies of the electrodes were characterized by using scanning electron microscopy (SEM). PANI has a fibrous structure and the improved performance was attributed to the PANI effect and synergistic effects between the carbon Vulcan and the PANI fiber. These results indicate that Pt/VC–PANI is a promising catalyst for the ORR in PEMFCs using an H₂/O₂ feed.     

Effects of Ga doping on Pt/CeO2‐Al2O3 catalysts for propane dehydrogenation

This paper describes catalytic consequencesThis paper describes catalytic consequences of Pt/CeO₂‐Al₂O₃ catalysts promoted with Ga species for propane dehydrogenation. A series of PtGa/CeO₂‐Al₂O₃ catalysts were prepared by a sequential impregnation method. The as‐prepared catalysts were characterized employing N₂ adsorption‐desorption, X‐ray diffrtaction, temperature programmed reduction, O₂ volumetric chemisorption, H₂‐O₂ titration, and transmission electron microscopy. We have shown that Ga³⁺ cations are incorporated into the cubic fluorite structure of CeO₂, enhancing both lattice oxygen storage capacity and surface oxygen mobility. The enhanced reducibility of CeO₂ is indicative of higher capability to eliminate the coke deposition and thus is beneficial to the improvement of catalytic stability. Density functional theory calculations confirm that the addition of Ga is prone to improve propylene desorption and greatly suppress deep dehydrogenation and the following coke formation. The catalytic performance shows a strong dependence on the content of Ga addition. The optimal loading content of Ga is 3 wt %, which results in the maximal propylene selectivity together with the best catalytic stability against coke accumulation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4365–4376, 2016     

The Use of Fluorocarbon Surfactants to Improve the Manufacture of PEM Fuel Cell Electrodes

Fluorocarbon surfactants are used to improve surface wetting during the screen printing of carbon black inks onto PEM fuel cell electrodes. The fluorosurfactants were tested in inks that comprised a Nafion® ionomer solution with platinum‐loaded carbon black. Four commercially available fluorosurfactants (Zonyl FSO, Zonyl 1033D, Forafac 1098 and Novec FC 4430) were screened and assessed for electrochemical activity (via cyclic voltammetry), leaching and the ability to form ink layers. Good wetting characteristics were observed and the inks showed a similar specific electrochemical active area (200–430 cm² mg^–1 Pt) to a standard reference ink (370 cm² mg^–1 Pt), indicating that the surfactants did not adversely adsorb on the platinum catalyst surface or block the adsorption/desorption of hydrogen. Additionally, the fluorosurfactants in the cured inks were shown to be electrochemically inactive in the potential region relevant to fuel cell operation.     

Electrochemical Oxidation of Ferulic Acid in Aqueous Solutions at Gold Oxide and Lead Dioxide Electrodes

A kinetic study of the electrochemical oxidation of ferulic acid (3-methoxy-4-hydroxycinnamic acid) by direct electron transfer at treated gold disk was combined with results of electrolyses in order to produce total degradation into CO₂ and H₂O at Ta/PbO₂ anode. The oxidation of ferulic acid at gold electrode was studied by cyclic voltammetry. At low concentration, ferulic acid shows one irreversible anodic peak. The peak current shows adsorption characteristics. For ferulic acid concentrations higher than 0.02 mmol dm⁻³, the voltammogram shows two anodic peaks. The effect of experimental conditions on the ratio of these two peaks was examined. The proposed mechanism is based on the hypothesis of two-electron oxidation of ferulic acid molecule involving a three intermediate cation mesomers. Hydrolysis of these mesomers leads to the formation of caffeic acid, methoxyhydroquinone and 3,4-dihydroxy-5-methoxycinnamic acid. Then ferulic acid was quantitatively oxidised by electrolysis on lead dioxide to produce, via intermediate aromatic compounds, maleic acid, oxalic acid and formic acid whose oxidation leads to carbon dioxide.     

Adsorption of cationic platinum complex onto carbon support

Adsorption of cationic Pt complex, Pt(NH₃)₄²⁺, in aqueous solution onto carbon was carried out by varying the initial pH of the solution. The Pt uptake showed a maximum as a function of the uptake time and afterward decreased gradually, showing that the adsorbed species is desorbed from the carbon surface. It was found that the degree of desorption was strongly dependent on the solution pH, as well as on the IEP (Isoelectric point) of the carbon support. In the solution pH lower than the IEP of the carbon, the desorption was almost complete. The desorption was found to be caused by a gradual decomposition of the ion-exchanged and/or electrostatically adsorbed Pt amine complex after giving the ligand ammonia to the acidic sites of the carbon surface, most probably, positive sites like protonated carboxyl group. The desorption could be prevented to a considerable extent by using a carbon support treated by nitric acid, because lowering the IEP of the carbon by acid treatment must result in a decrease in the interaction of basic ligand ammonia and positive surface sites, which eventually leads to the desorption of Pt complex. Measurements of the Pt dispersion of the catalysts that have undergone desorption indicate that the desorption of the adsorbed Pt complex occurs on relatively weak adsorption sites like carboxyl group, rather than strong adsorption sites (e.g., phenolic hydroxyl or lactonic groups.).     

Controlled growth and shape formation of platinum nanoparticles and their electrochemical properties

Cubic Pt nanoparticles were prepared from a solution of K₂PtCl₄ containing sodium polyacrylate as a capping reagent. The effects of the Pt/polymer molar ratio, the average molecular weight (M_w) of the polymer, and reaction temperature on the shape and size were investigated. When the polymer of M_w = 5100 was added at a molar ratio of Pt/polymer = 1/12, cubic platinum nanoparticles of an average size of 10.3 nm were predominantly formed (ca. 50% in number) at 25 °C. The electron diffraction pattern of the cubic nanoparticles revealed that they are single crystals with Pt {1 0 0} faces on the surface.The cubic nanoparticles were electrochemically active, and showed strong features of Pt {1 0 0} faces on cyclic voltammogram under argon atmosphere. After repeated potential cycling in the range 0.05-1.4 V, the features of Pt {1 0 0} were gradually lost, and changed to those of polycrystalline Pt. Rotating ring disk electrode measurements in O₂-saturated H₂SO₄ solution revealed that the cubic nanoparticles had a high catalytic activity for oxygen reduction reaction (ORR). After polycrystallization by repeated potential cycling, the activity for ORR and hydrogen peroxide formation decreased slightly, which were attributed to the surface structural change from Pt {1 0 0} to polycrystalline.     

Heterogeneously assisted oxidation of adsorbates from carbonmonoxide, methanol and ethanol by hydrogen peroxide solutions on platinum electrodes in sulphuric acid

The influence of hydrogen peroxide on the adsorption and oxidation of carbon monoxide, methanol and ethanol adlayers on porous Pt electrodes were studied in 2 M sulphuric acid solution by means of cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). The oxidation of adsorbed species is observed at electrode potentials far less negative than those required for electrochemical adsorbate oxidation. The oxidation by H₂O₂ is dependent on its concentration in solution, as well as on the adsorbates and their coverages. In all cases the isolated adlayers are oxidised by dissolved H₂O₂. However, the presence of H₂O₂ during adsorption partially inhibits adlayer formation from CH₃OH and C₂H₅OH, but avoids almost completely the adsorption of carbon monoxide. The removal of the residues from the surface by dissolved hydrogen peroxide probably occurs through O_ad species formed during the heterogeneous decomposition reaction of H₂O₂ on Pt.     

Hydrogen spillover phenomena on Pt/ZrO2

A Pt/ZrO₂ catalyst has been investigated by temperature-programmed reduction and temperature-programmed desorption of hydrogen. Hydrogen spills over from Pt onto the ZrO₂ surface at about 550°C. One part of spillover hydrogen is consumed by a partial reduction of zirconia. The other part is adsorbed on the surface and is desorbed at about 650°C. This desorption is a reversible one, i.e. it can be followed by a renewed uptake of spillover hydrogen. No connection between dehydroxylable OH groups and spillover hydrogen adsorption has been observed. The adsorption sites for the reversibly bound spillover hydrogen were possibly formed during the reducing hydrogen treatment.     

Towards a determination of the active surface area of polycrystalline and nanoparticle electrodes by Cu upd and CO oxidation

Methods for the determination of the surface area for Pt, Ru and Se modified Pt and Ru are compared, in view of their possible application for technical and nanoparticle electrodes. The hydrogen adsorption charge can hardly be used as a reliable measure for the surface area for Ru because it is paralleled by anion adsorption. The charge necessary for the oxidation of adsorbed CO also contains a large contribution due to anion or oxygen adsorption, which amounts to approx. 45% of the charge in the case of Ru. The mass spectrometrically determined amount of CO₂ formed gives a more reliable measure for the surface area, provided that the maximum coverages are constant and independent of the particular surface. Values obtained in this way agree to within 20% with surface area values obtained from measuring the charge needed for the desorption of a complete monolayer of Cu upd on Pt(111) and polycrystalline Pt, polycrystalline Ru, submonolayers of Ru on polycrystalline Pt and on Pt(111) and for nanoparticle, carbon supported electrodes. Se modified Ru has recently found attention as a methanol tolerant cathode material for oxygen reduction. CO does not adsorb on Pt or Ru saturated by Se. For surfaces partially covered by Se, a comparison of the charge measured by cyclic voltammetry in the hydrogen region and of the mass spectrometrically determined amount of CO₂ suggests that the latter can be used for a determination of the area not covered by Se. Cu upd, on the other hand, also takes place on surfaces completely covered by Se; the Cu desorption charge is independent of the Se coverage on Pt and Ru modified Pt as long as it does not exceed 70% of full coverage. In the presence of multilayers of Se, Cu _x Se is formed. On Se modified bulk Ru the amount of Cu upd decreases with increasing Se coverage, approaching only 105 μC m⁻² for full Se coverage.     

Infrared Study of Benzene Hydrogenation on Pt/SiO2 Catalyst by Co-adsorption of CO and Benzene

FT-IR spectra of the co-adsorption of benzene and CO have been performed to identify the preferred adsorption sites of hydrogen and benzene on a Pt/SiO₂ catalyst for hydrogenation of benzene. Results of CO adsorbed on atop sites on Pt/SiO₂ includes: an α peak at 2091 cm⁻¹, a β peak at 2080 cm⁻¹ and a γ peak at 2067 cm⁻¹ indicating three kinds of adsorption sites for dissociative hydrogen on Pt/SiO₂. The site of lowest CO stretching frequency offers stronger adsorbates–metal interaction for benzene and hydrogen. Hydrogen binding on the site of lowest CO stretching frequency before benzene adsorption significantly enhances the reaction rate of benzene hydrogenation.     

Hydrogen electrosorption into Pd-Pt-Au ternary alloys

Thin layers of Pd-Pt-Au alloys were prepared by metal codeposition at constant potential from chloride solutions. The process of hydrogen electrosorption into Pd-Pt-Au alloys was investigated in acidic solution (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry, also coupled with the electrochemical quartz crystal microbalance. It was found that Pd alloying with both Pt and Au decreases the maximum hydrogen solubility, but improves the kinetics of absorbed hydrogen oxidation, which is mirrored in a negative shift of the potential of hydrogen desorption peak and shorter hydrogen desorption time. In Pd-Pt-Au alloys the effect of absorption/desorption hysteresis and the stresses connected with hydrogen absorption are reduced in comparison with pure Pd. After prior hydrogen absorption in Pd-Pt-Au alloys, surface oxides are formed and reduced at potentials even by 200 mV lower than before hydrogen treatment.     

Changes in crystallographic orientation of thin foils of palladium and palladium alloys after the absorption of hydrogen

The adsorption/absorption of hydrogen at room temperature by palladium, 16% silver-palladium and 5% ruthenium-palladium foils was studied using thermal desorption spectroscopy. Hydrogen readily diffused in the palladium and desorbed as one broad peak at about 650 K. Hydrogen also diffuses in the 16% silver-palladium foil and in the 5% ruthenium-palladium foil but with a smaller diffusion constant. Two hydrogen desorption peaks are observed for the Ru-Pd and Ag-Pd alloys, at 440 and around 650 K. The first hydrogen desorption peak is regarded as hydrogen desorbing from the surface sites while the second peak is regarded as hydrogen diffusing from the subsurface sites. The desorption order for surface hydrogen corresponds ton = 2 while the diffused hydrogen desorbs with a fractional order ofn = 1.25. The crystallographic orientation of the foils determined by X-ray diffraction shows a preferential (1,1,0) orientation along the direction of rolling of the foils before hydrogen absorption. This preferential orientation is destroyed after hydrogen adsorption for Pd and Pd-Ag but unaltered for the Pd-Ru alloy. This preferential orientation of the foils might have significant implications in membrane fabrication, since the absorption of hydrogen by Pd is very dependent on surface orientation.