The adsorption of Sn on Pt(1 1 1) and its influence on CO adsorption as studied by XPS and FTIR

The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5×10⁻⁵ M Sn²⁺ in 0.5 M H₂SO₄ has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of ?_max=0.35 (Sn to surface Pt atoms).Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm⁻¹ band associated with bridge-bonded CO and the 2070 cm⁻¹ band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm⁻¹ band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2×2)-3CO saturation structure on Pt(1 1 1). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV.     

Electrooxidation of ethanol at polycrystalline and platinum stepped single crystals: A study by differential electrochemical mass spectrometry

The electrooxidation of adsorbed and bulk solution of 10⁻² M ethanol and D₆-ethanol at polycrystalline platinum, smooth, roughened and Ru modified Pt(3 3 2), Pt(3 3 1) and Pt(1 1 1) electrodes was studied by on-line differential electrochemical mass spectroscopy (DEMS) using a dual thin layer flow through cell.On polycrystalline Pt, the main (or even single) product is acetaldehyde; due to the flow through conditions the amount of acetaldehyde further oxidized to acetic acid is negligible. At stepped single crystals with (1 1 1) terraces (Pt(s)[n(1 1 1) × (1 1 1)], acetic acid is produced at a lower potential than acetaldehyde. This demonstrates that in addition to the reaction path involving C-C bond splitting leading to CO₂ (via adsorbed CO and CH_x) and the reaction path leading to acetaldehyde there is a third, direct reaction path leading to the formation of acetic acid.Step decoration by Ru does not lead to an increased reactivity. This is different from the strong cocatalytic effect of Ru at step sites on the oxidation of CO. Furthermore, Ru does not influence the relative amount of acetaldehyde formed.     

In situ FTIR spectroscopic studies of (bi)sulfate adsorption on electrodes of Pt nanoparticles supported on different substrates

Nanostructured Pt electrodes were prepared by electrodeposition of Pt nanoparticles on different substrates (GC, Pt and Au) under cyclic voltammetric conditions and with various number (n) of potential cycling, and were denoted as nm-Pt/S(n) (S = GC, Pt and Au). Adsorption of (bi)sulfate on the nm-Pt/S(n) was studied by in situ FTIR reflection spectroscopy. It has been revealed that the nanostructured Pt electrodes exhibit anomalous IR properties for (bi)sulfate adsorption regardless of the different reflectivity of substrate, i.e. the IR absorption of (bi)sulfate species adsorbed on all the nm-Pt/S(n) electrodes is significantly enhanced and the IR band direction is completely inverted in comparison with the same species adsorbed on a bulk Pt electrode. The two IR bands around 1200 and 1110 cm⁻¹ attributed to adsorbed (bi)sulfate species are shifted linearly with increasing electrode potential, yielding Stark tuning rates () of 152.1 and 21.1 cm⁻¹ V⁻¹ on nm-Pt/GC(20), respectively. Along with increasing n, the Stark tuning rate of the IR band around 1200 cm⁻¹ decreases quickly and declined to 7.6 cm⁻¹ V⁻¹ on nm-Pt/GC(80), while the Stark tuning rate of the IR band near 1100 cm⁻¹ is fluctuated between 23.0 and 16.2 cm⁻¹ V⁻¹. It has determined that the enhancement of IR absorption of (bi)sulfate adsorbed on nanostructured Pt electrode is varied with substrate material and n, and a maximal 16-fold enhancement of the IR band near 1200 cm⁻¹ has been measured on the nm-Pt/GC(30) electrode. The in situ FTIR studies illustrated that the adsorption of (bi)sulfate occurs mainly in the double layer potential region, and reaches a maximum around 0.80 V. The results demonstrated also that the competitive adsorption of CO and oxygen species can inhibit completely (bi)sulfate adsorption, which has evidenced a weak interaction of (bi)sulfate with nm-Pt/S(n) electrode surface.     

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.     

Infrared and UV-vis spectra of phenoxonium cations produced during the oxidation of phenols with structures similar to vitamin E

Several phenols with structures similar to vitamin E were oxidised and the intermediate species produced were characterised by in situ infrared and UV-vis spectroscopies. The Fourier transform infrared (FTIR) measurements were performed by chemically oxidising the phenols with 2 mol equiv. of NO⁺SbF₆⁻ in CH₃CN and recording the spectra between 1900 and 1300 cm⁻¹ with an attenuated total reflectance (ATR) probe utilising a fiber conduit and a diamond composite sensor. The compounds that formed long-lived phenoxonium cations displayed two IR absorbances at 1665 (±15) cm⁻¹ and one at 1600 (±10) cm⁻¹ associated with the carbonyl, symmetric ring stretch and asymmetric ring stretch modes. The para-quinones are one of the long-term products of oxidation of the phenols, and displayed solution phase IR absorbances at 1650 (±10) cm⁻¹. In situ electrochemical UV-vis experiments performed during the oxidation of the phenols led to the detection of bands due to the phenoxonium cations at 295 (±5) and 440 (±15) nm and due to the para-quinones at 260 (±10) nm. The concentration of the substrate and the water content of the solvent had a major effect on the yields of the intermediates and products that were produced during the oxidation reactions.     

Activated carbon nanotubes-supported catalyst in fuel cells

The electrochemical property of platinum loaded on activated carbon nanotubes (Pt/ACNTs) was investigated by cyclic voltammograms (CVs) recorded in H₂SO₄ and H₂SO₄/CH₃OH aqueous solutions, respectively. Compared to 0.0046 A/cm² of Pt-loaded on pristine carbon nanotubes (Pt/CNTs) with a S_BET of 164 m²/g and 0.0042 A/cm² of conventional carbon black (Pt/C, Vulcan XC-72) with a S_BET of ∼250 m²/g, a better electrochemical activity (a high current density of 0.0070 A/cm² for weak-H₂ adsorption/desorption) of the Pt/ACNTs with high specific surface area (S_BET) of 830-960 m²/g was obtained. Furthermore, the highest current density of 0.079 A/cm² at 0.65 V in anodic sweep was observed during the methanol oxidation. On the basis of Pt size, utility ratio, and electro-active specific surface area (EAS), the Pt/ACNTs with a high Pt-loading of 50 wt.% exhibited the best electrochemical activity. The present ACNTs may be an excellent support material for electrochemical catalyst in proton exchange membrane and direct methanol fuel cells.     

Adsorption/oxidation of CO on highly dispersed Pt catalyst studied by combined electrochemical and ATR-FTIRAS methods: Part 1. ATR-FTIRAS spectra of CO adsorbed on highly dispersed Pt catalyst on carbon black and carbon un-supported Pt black

Elecrochemical ATR-FTIRAS measurements were conducted for the first time to investigate nature of CO adsorbed under potential control on a highly dispersed Pt catalyst with average particle size of 2.6 nm supported on carbon black (Pt/C) and carbon un-supported Pt black catalyst (Pt-B). Each catalyst was uniformly dispersed by 10 μg Pt/cm² and fixed by Nafion^® film of 0.05 μm thick on a gold film chemically deposited on a Si ATR prism window. Adsorption of CO was conducted at 0.05 V on the catalysts in 1 and 100% CO atmospheres, for which CO coverage, θ_CO, was 0.69 and 1, respectively. Two well-defined ν(CO) bands free from band anomalies assigned to atop CO (CO(L)) and symmetrically bridge bonded CO (CO(B)_sym.) were observed. It was newly found that the CO(L) band was spitted into two well-defined peaks, particularly in 1% CO, from very early stage of adsorption, which was interpreted in terms of simultaneous occupation of terrace and step-edge sites, denoted as CO(L)_terrace and CO(L)_edge, respectively. This simultaneous occupation was commonly observed in our work both on Pt/C and Pt-B. A new band was also observed around 1950 cm⁻¹ in addition to the bands of CO(L) and CO(B)_sym., which was assigned to asymmetric bridge CO, CO(B)_asym., adsorbed on (1 0 0) terraces, based on our previous ECSTM observation of CO adsorption structures on (1 0 0) facet. The CO(B)_asym. on the Pt/C, particularly in 100% CO atmosphere, results in growth of a sharp band at 3650 cm⁻¹ accompanied by a concomitant development of a band around 3500 cm⁻¹. The former and the latter are assigned to ν(OH) vibrations of non-hydrogen bonded and hydrogen bonded water molecules adsorbed on Pt, respectively, interpreted in term of results from a bond scission of the existing hydrogen bonded networks by CO(L)s and from a promotion of new hydrogen bonding among water molecules presumably by CO(B)_asym..It was found that the frequency ν(CO) of CO(L) both on Pt/C and Pt-B is lower than that on bulky polycrystalline electrode Pt(poly) or different crystal planes of Pt single-crystal electrodes by 30-40 cm⁻¹ at corresponding potentials, which implies a stronger electronic interaction between CO and Pt nano-particles and/or an increased contribution of step-edge sites on the particles. Determination of the band intensities of CO(L), CO(B)_asym. and CO(B)_sym. has led us to conclude a much higher bridged occupation of sites at Pt nano-particles than Pt(poly) electrodes.     

Interactions between interfacial water and CO adsorbed on Pt and Pt-Ru alloy surfaces under electrochemical conditions: Density-functional theory study

The structural and electronic properties of interfacial water and adsorbed CO on platinum and platinum/ruthenium alloy have been studied via density-functional theory calculations to gain insight into the water-adsorbate interaction under electrochemical conditions. The computational simulations reveal a new interpretation for the interaction of adsorbed CO and water at the electrochemical interfaces. The new interaction model rationalizes the observed quantitative relationship between infrared intensities for adsorbed bridging CO and water molecules that impart a high-frequency O-H stretch, ca. 3630-3660 cm⁻¹ on pure Pt and 3600-3620 cm⁻¹ on PtRu alloy. The theoretical modeling indicates that the observed feature common to both pure Pt and PtRu alloy surfaces is due to interfacial water molecules firmly hydrogen-bonded to bridging CO.     

Ionic liquid electrolytes based on multi-methoxyethyl substituted ammoniums and perfluorinated sulfonimides: Preparation, characterization, and properties

New functionalized ionic liquids (ILs), comprised of multi-methoxyethyl substituted quaternary ammonium cations (i.e. [N(CH₂CH₂OCH₃)_4−n(R)_n]⁺; n = 1, R = CH₃OCH₂CH₂; n = 1, R = CH₃, CH₂CH₃; n = 2, R = CH₃CH₂), and two representative perfluorinated sulfonimide anions (i.e. bis(fluorosulfonyl)imide (FSI⁻) and bis(trifluoromethanesulfonyl)imide (TFSI⁻)), were prepared. Their fundamental properties, including phase transition, thermal stability, viscosity, density, specific conductivity and electrochemical window, were extensively characterized. These multi-ether functionalized ionic liquids exhibit good capability of dissolving lithium salts. Their binary electrolytes containing high concentration of the corresponding lithium salt ([Li⁺] >1.6 mol kg⁻¹) show Li⁺ ion transference number (tLi⁺) as high as 0.6-0.7. Their electrochemical stability allows Li deposition/stripping realized at room temperature. The desired properties of these multi-ether functionalized ionic liquids make them potential electrolytes for Li (or Li-ion) batteries.     

Infrared spectroscopic and voltammetric study of adsorbed CO on stepped surfaces of copper monocrystalline electrodes

Voltammetric and infrared (IR) spectroscopic measurements were carried out to study adsorbed CO on two series of copper single crystal electrodes n(1 1 1)-(1 1 1) and n(1 1 1)-(1 0 0) in 0.1 M KH₂PO₄ + 0.1 M K₂HPO₄ at 0 °C. Reversible voltammetric waves were observed below −0.55 V versus SHE for adsorption of CO which displaces preadsorbed phosphate anions. The electric charge of the redox waves is proportional to the step atom density for both single crystal series. This fact indicates that phosphate anions are specifically adsorbed on the step sites below −0.55 V versus SHE. Voltammetric measurements indicated that (1 1 1) terrace of Cu is covered with adsorbed CO below −0.5 V versus SHE. Nevertheless, no IR absorption band of adsorbed CO is detected from (1 1 1) terrace. Presence of adsorbed CO on (1 1 1) terrace is presumed which is not visible by the potential difference spectroscopy used in the present work. IR spectroscopic measurements showed that CO is reversibly adsorbed with an on-top manner on copper single crystal electrodes of n(1 1 1)-(1 1 1) and n(1 1 1)-(1 0 0) with approximately same wavenumber of CO stretching vibration of 2070 cm⁻¹. The IR band intensity is proportional to the step atom density. Thus CO is adsorbed on (1 1 1) or (1 0 0) steps on the single crystal surfaces. An analysis of the IR band intensity suggested that one CO molecule is adsorbed on every two or more Cu step atom of the monocrystalline surface. The spectroscopic data were compared with those reported for uhv system. The CO stretching wavenumber of adsorbed CO in the electrode-electrolyte system is 30-40 cm⁻¹ lower than those in uhv system.     

Distribution profile of water and suppression of methanol crossover in sulfonated polyimide electrolyte membrane for direct methanol fuel cells

We have developed novel cross-linked sulfonated polyimide (c-SPI) membrane as an electrolyte for direct methanol fuel cells (DMFCs). When the DMFC using the c-SPI membrane (thickness = 155 μm), Pt-Ru dispersed on carbon black (Pt-Ru/CB) anode and Pt/CB cathode with a Nafion^® ionomer was operated at 80 °C and 0.1 A cm⁻² with 1 M CH₃OH and oxygen (oxidant), the methanol crossover rate, j(CH₃OH), was suppressed to about 1/2 compared with that of the Nafion^® 117 membrane (thickness = 180 μm) with the same electrodes. It was found for both cells that the j(CH₃OH) was not so small as expected from the membrane thickness. In order to obtain a clue for the suppression of j(CH₃OH), the distribution profiles of water (containing CH₃OH) in thickness direction were investigated by measuring the specific resistances (ρ) between Pt probes inserted into the electrolyte membrane. Values of ρ at the anode side were low irrespective of the discharge current density, because such a part of the membrane was humidified thoroughly by liquid water (1 M CH₃OH) allowing free penetration of CH₃OH into the swollen polymer. In contrast, the values of ρ at the cathode side were high at the low current density due to drying of the membrane contacting with oxidant gas (O₂ or air) in low humidity. We have succeeded to suppress the j(CH₃OH) further (about 1/2 at 0.2 A cm⁻²) by using bilayer c-SPI, having a low ion exchanging (low swelling) barrier layer at the anode side without increasing the ohmic resistance, compared with that of the single c-SPI.     

Electrochemical investigations of the oxidation-reduction of furfural in aqueous medium: Application to electrosynthesis

The present study concerns the electrochemical properties of furfural in aqueous medium on noble (Au and Pt) and non-noble (Pb, Cu and Ni) metal electrodes. The anodic and cathodic reactions are investigated by cyclic voltammetry on Au, Pt and Ni electrodes and during prolonged electrolyses on Pt, Pb and Cu in order to find the optimum conditions for a paired electrosynthesis. Anodic reactions are controlled by diffusion in the range of the stability of the solvent (water). Beside these limits, the gas evolution competes with the conversion of furfural. The best conditions for preparative electrooxidation (Ni anode, 0.5 M NaOH, j=0.8 mA cm⁻¹) gave furoic acid in a 80% yield and furfuryl alcohol was obtained by electroreduction in a 55% yield on Cu cathodes at pH 10 and 30 mA cm⁻².     

Electrogravimetric investigation of formaldehyde oxidation at Pt electrodes in acidic media

The electrochemical and adsorptive behavior of formaldehyde at Pt electrodes in acidic media was investigated using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques. All chemical and electrochemical steps related to formaldehyde oxidation (e.g. bulk adsorption and oxidation, CO (sub)monolayer adsorption and oxidation and electrons per Pt site) were analyzed. All the mass and charge density data in this paper are referred to the real surface area. The charge density associated with formaldehyde oxidation was close to 420 μC cm⁻², which is related to the oxidation of approximately one CO monolayer with two electrons transferred. For CO adsorption the experimental mass value was 50 ng cm⁻². In the region of CO oxidation the analysis of mass and charge variations indicates simultaneous CO oxidation, anion and water adsorption and CO readsorption. The mechanism was confirmed by CO and CO₂ flux calculations. From the analysis of the mass-charge ratio and species flux it was concluded that CO, an intermediate produced during formaldehyde oxidation, is adsorbed at the Pt surface and the main contribution to the mass increase during formaldehyde oxidation is CO readsorption, and water adsorption.     

Nucleation of Sn and Sn-Cu alloys on Pt during electrodeposition from Sn-citrate and Sn-Cu-citrate solutions

The initial stages of Sn and Sn-Cu electrodeposition from Sn-citrate and Sn-Cu-citrate solutions on Pt were studied using both current-controlled and potential-controlled electrochemical techniques. For both Sn-citrate and Sn-Cu-citrate solutions, when the current density is controlled to lower than 15 mA/cm², potentials remain almost constant which is appropriate to plate dense and uniform films. When the current density is controlled to between 25 and 35 mA/cm², potentials drop quickly initially, followed by a gradual increase to a constant value. When current density is controlled to higher than 50 mA/cm², potential oscillation happens, and significant hydrogen evolution prevents the formation of dense and continuous Sn and Sn-Cu films. A constant transition time constant indicates a diffusion-controlled process. The diffusion coefficient calculated from the Sand equation is about 3.8 × 10⁻⁶ cm²/s for the Sn-citrate solution and 4.1 × 10⁻⁶ cm²/s for the Sn-Cu-citrate solution. The morphology of both Sn and Sn-Cu deposits plated under different potentials was examined by atomic force microscopy (AFM) and the distribution of each element were analyzed using Auger imaging. Analysis of both the electrochemical results at −0.72, −1.1 and −1.5 V and AFM images for both Sn and Sn-Cu deposits at −1.1 and −1.5 V suggested progressive nucleation controlled by diffusion for both Sn and Sn-Cu electrodeposition. Tin reacted with Pt to form PtSn₄, and co-deposited with Cu to form Cu₆Sn₅ during nucleation, with more Sn forming at higher applied potentials.     

2,2,6,6-Tetramethyl piperidine-1-oxyl (TEMPO)-mediated catalytic oxidation of benzyl alcohol in acetonitrile and ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate [BMIm][PF6]: Kinetic analysis

TEMPO (2,2,6,6-tetramethyl piperidine-1-oxyl) is electrochemically oxidized to a stable form of the cation (TEMPO⁺) in acetonitrile (CH₃CN) or 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIm][PF₆]) media. Cyclic voltammograms were characterized by a well-defined one-electron reversible redox couple in both media at low scan rates. The reduced form of TEMPO⁺ is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol (BA) in the presence of 2,6-lutidine. It was observed that in [BMIm][PF₆], the redox currents are largely suppressed compared to that in CH₃CN. The apparent heterogeneous electron-transfer rate constant () of the quasi-reversible redox reaction of TEMPO was determined at a Pt electrode and found to be 1.9 × 10⁻³ cm s⁻¹ and 4.5 × 10⁻² cm s⁻¹ in [BMIm][PF₆] and CH₃CN, respectively. With the aid of chronoamperometry (CA), the homogeneous rate constant for the catalytic oxidation of benzyl alcohol by TEMPO, in the presence of 2,6-lutidine in CH₃CN was estimated to be 5.53 × 10¹ M⁻¹ s⁻¹ which is approximately double, relative to the value of 2.91 × 10¹ M⁻¹ s⁻¹ determined in [BMIm][PF₆].     

CO tolerance of PdPt/C and PdPtRu/C anodes for PEMFC

The performance of H₂/O₂ proton exchange membrane fuel cells (PEMFCs) fed with CO-contaminated hydrogen was investigated for anodes with PdPt/C and PdPtRu/C electrocatalysts. The physicochemical properties of the catalysts were characterized by energy dispersive X-ray (EDX) analyses, X-ray diffraction (XRD) and “in situ” X-ray absorption near edge structure (XANES). Experiments were conducted in electrochemical half and single cells by cyclic voltammetry (CV) and I-V polarization measurements, while DEMS was employed to verify the formation of CO₂ at the PEMFC anode outlet. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with the PdPt/C and PdPtRu/C anodes containing 0.4 mg metal cm⁻², with the cell presenting potential losses below 200 mV at 1 A cm⁻², with respect to the system fed with pure H₂. For the PdPt/C catalysts no CO₂ formation was seen at the PEMFC anode outlet, indicating that the CO tolerance is improved due to the existence of more free surface sites for H₂ electrooxidation, probably due to a lower Pd-CO interaction compared to pure Pd or Pt. For PdPtRu/C the CO tolerance may also have a contribution from the bifunctional mechanism, as shown by the presence of CO₂ in the PEMFC anode outlet.     

Electrochemistry of TiCl4 in 1-butyl-2,3-dimethyl imidazolium azide

Electrochemical behaviour of titanium tetrachloride solutions in 1-butyl-2,3-dimethyl imidazolium azide (BMMImN₃) at 65 °C has been examined. Ti(IV) reduction was studied with chronopotentiometry and cyclic voltammetry methods in melts with different concentrations of TiCl₄. According to IR spectra, Ti(IV) exists in form of a hexaazidotitanate complex. The electrochemical reduction of this complex was found to proceed irreversibly to Ti(III) species only. Diffusion coefficients of Ti(IV) in this ionic liquid at temperature 65 °C were calculated based on the chronopotentiometry measurements at different TiCl₄ concentrations (D_Ti(IV) = 1.3 ± 0.6 × 10⁻⁶ cm² s⁻¹).     

Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

The stability of adsorbed quinoline and cinchonine on poly- and monocrystalline platinum surfaces

The adsorption of quinoline and cinchonine on Pt (1 1 1), Pt (3 3 2) and polycrystalline Pt electrode has been studied by differential electrochemical mass spectrometry (DEMS). From the surface-coverage data and from the potential dependence of both the faradaic oxidation current and the rate of CO₂ formation during the oxidation of quinoline and cinchonine on Pt (1 1 1), we conclude that both molecules are bound through the π-system to the electrode surface. The only anodic desorption product found was CO₂. Surface concentrations for both molecules were found to be around 0.1-0.2 nmol cm⁻². It was also found that quinoline completely desorbs from the Pt (1 1 1) electrode around 0 V, provided that the electrolyte in the thin-layer cell is exchanged for fresh electrolyte; in contrast, desorption from Pt (3 3 2) and polycrystalline Pt is incomplete. Cinchonine does desorb in part from polycrystalline Pt, but not notably from Pt (1 1 1) due to an additional binding interaction of the exocyclic vinyl group linked to the quinuclidine moiety. No decomposition products, e.g. alkanes, were detected during such cathodic potential sweeps.Further experiments revealed that coadsorption of CO on polycrystalline Pt notably reduces the amount of carbon dioxide formed during subsequent anodic potential sweeps for pre-adsorbed quinoline and cinchonine, pointing to a partial displacement of the modifiers. In contrast, ethene is coadsorbed without displacing the original adsorbate and can still undergo hydrogenation when a negative potential is applied. Benzene is also coadsorbed to some extent, but its hydrogenation, which usually occurs on an unmodified surface, is largely diminished.     

Hydrogen oxidation kinetics on model Pd/C electrodes: Electrochemical impedance spectroscopy and rotating disk electrode study

This work reports on the kinetics of the hydrogen oxidation reaction (HOR) on model Pd nanoparticles supported on a low surface area carbon substrate. Two Pd/C samples, with the average particle size 2.6 and 4.0 nm were used. The structure of the catalysts was characterized with the ex situ (electron microscopy) and in situ (electrochemical) methods. We utilized the electrochemical impedance spectroscopy (EIS) and the rotating disk electrode (RDE) voltammetry to study the kinetics of the HOR on Pd/C. The relevance of these techniques for elucidating the kinetics and the mechanism of the HOR on Pd/C was explored. The experimental results suggest that the catalytic activity of Pd in the HOR is more than 2 orders of magnitude lower than that of Pt, and does not depend on the particle size in the range from 2.6 to 4.0 nm. Computational modeling of the experimental steady-state (RDE) and non-steady-state (EIS) data shows that the reaction kinetics can be adequately described within Heyrovsky-Volmer mechanism, with the rate constants υ_0H = (8.8 ± 1.5) × 10⁻¹⁰ mol cm⁻² s⁻¹ and υ_0V = (1.0 ± 0.3) × 10⁻⁸ mol cm⁻² s⁻¹. The model suggests that underpotentially deposited hydrogen H_UPD is unlikely to be the active intermediate H_ad of the HOR. It is concluded that the surface coverage of H_ad deviates from that of H_UPD with increasing overpotential, and the lateral interactions within H_ad adlayer are weak.