High Area Carbon Research Articles

Electrochemical and XAFS investigation of nitrite reduction by heat-treated μ-oxo derivative of iron phthalocyanine supported on high area carbon

The electrochemical nitrite reduction by heat-treated (FePc) 2O/KB (KB=Ketjen Black) in the range of 500–1000 °C has been done in 0.5 M sulfuric acid and the effect of heat-treatment on the structural aspects has been investigated by X-ray absorption fine structure. (FePc) 2O before heat-treatment displayed exactly the same voltammetric behavior and catalytic activity toward nitrite reduction as FePc. Nitric oxide that was produced by disproportionation of nitrite formed a stable adduct with FePc before being irreversibly reduced. The catalytic activity, which was highest for a nonheat-treated sample, decreased monotonically as the heat-treatment temperature increased, which followed approximately the nitrogen-to-carbon ratio in the sample. XANES and EXAFS analyses showed that (i) the μ-oxo structure was still maintained upon adsorption and (ii) (FePc) 2O was converted to FePc in the range of 500–600 °C and to an Fe 2O 3-like structure above 900 °C. The loss of catalytic activity has been attributed to an alteration from the phthalocyanine structure. It is concluded that the FeN 4 structure is essential for maintaining the catalytic activity.

In situ Fe K-edge X-ray absorption spectroscopy of a nitrosyl iron(II) porphyrin adduct adsorbed on a high-area carbon electrode in aqueous electrolytes.

The first in situ Fe K-edge X-ray absorption fine structure study is reported on the electronic and structural properties of a nitrosyl [meso-tetrakis(p-methoxyphenyl) porphyrinato]iron(II) adduct, [Fe(NO)(TMPP)], adsorbed onto a high-area carbon electrode. This adsorbed complex displayed a remarkable stability over a wide potential range, and multiple-scattering XAFS analysis yielded bond lengths of Fe−Np = 1.99, Fe−NNO = 1.74, and Fe−OH2 = 2.02 Å, and an Fe−N−O bond angle of 155°, characteristic of FeIINO porphyrin adducts.

Catalytic decomposition of hydrogen peroxide in alkaline solutions

Catalytic activity of carbon, platinum-supported on high-area carbon, platinum, lead ruthenate, and ruthenium oxide towards hydrogen peroxide decomposition in alkaline solution is investigated using the rotating disk electrode technique. The heterogeneous rate constant for peroxide decomposition on these catalysts is determined from the slope of log( i L) versus time, where i L is the diffusion-limiting current corresponding to the concentration of peroxide at a given time. The order of catalytic activity is found to be platinum>lead ruthenate>ruthenium oxide>carbon. A general reaction mechanism for the peroxide decomposition on these catalysts is also proposed.

Heat-treated iron(III) tetramethoxyphenyl porphyrin chloride supported on high-area carbon as an electrocatalyst for oxygen reduction:

Oxygen reduction was investigated on heat-treated FeTMPP-Cl/BP catalysts in 0.1 M H2SO4. The amount of H2O2 generated in the reaction was detected using a rotating ring-disk electrode. In order to detect H2O2 quantitatively, a method based on cyclization of the ring electrode potential had to be applied. The number of electrons exchanged per O2 molecule was estimated to be between 3.45 and 4, depending on the electrode potential, but with no significant influence of the heat-treatment temperature. It was possible to oxidize and reduce H2O2 electrochemically. The oxidation reaction was found to be faster than the reduction. Based on the data reported here, it is postulated that O2 reduction on this catalyst comprises both direct and series reaction paths. The increase in the catalyst's activity with the increase in the heat-treatment temperature is more likely caused by the decrease in the activation energy of the reaction than by more efficient H2O2 decomposition. The influence of H2O2 on the decay in the activity of the catalyst heat-treated at low temperature was discussed.

Heat-treated iron(III) tetramethoxyphenyl porphyrin chloride supported on high-area carbon as an electrocatalyst for oxygen reduction

Oxygen reduction was investigated at FeTMPP–Cl adsorbed on the Black Pearls carbon and heat-treated at temperatures from 200 to 1000°C. Kinetic parameters of the reaction and the dependence of the reaction rate on the heat-treatment temperature were established in acid and alkaline media. It was found that the O2 reduction rate increases with increase in the heat-treatment temperature of the electrocatalyst, reaching a plateau at 700≤T≤1000°C. The reaction rate was higher in alkaline solution on all electrocatalysts examined, but its dependence on the heat-treatment temperature was the same as in acid media. Like similar other macrocycle catalysts, FeTMPP–Cl/BP was found to be methanol-tolerant. The effect of sulfate, perchlorate, and phosphate anions was also investigated and no influence on the reaction rate was found. Activities of FeTMPP–Cl/BP and Pt/BP electrocatalysts were compared. FeTMPP–Cl/BP heat-treated at 700≤T≤1000°C showed similar activity to Pt/BP if they are compared in alkaline solution, but in acid solution, the reaction rate on the Pt electrocatalyst was higher. The controversy in the literature on the role of metallic Fe particles in O2 reduction catalysis is discussed. Comparison of present results with previous cyclic voltammetry and TEM investigations of the same electrocatalyst gave further evidence that some other products of the heat-treatment are responsible for the high activity of the catalyst.

Effect of Heat Treatment on the Redox Properties of Iron Porphyrins Adsorbed on High Area Carbon in Acid Electrolytes: An in Situ Fe K-Edge X-ray Absorption Near-Edge Structure Study

Electronic aspects of an iron porphyrin, 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeTMPPCl) molecularly dispersed on a high area carbon black (Black Pearls 2000, BP) and then heat-treated (or thermally activated) at 800 °C in a flowing inert atmosphere, were investigated in situ in 1.0 M H3PO4 by Fe K-edge X-ray near-edge structure (XANES). Profound differences were observed between the ex situ (dry) XANES of electrodes incorporating FeTMPPCl/BP before and after heat treatment and, likewise, in the case of the thermally treated FeTMPPCl/BP before and after immersion of the electrodes in the electrolyte. Monotonic shifts in the absorption edge toward higher energies were observed for heat-treated FeTMPPCl/BP as the potential was increased over a range of over 1 V. The overall magnitude of the shift, ca. 2.5 eV, was virtually the same as that obtained with non-heat-treated (or intact) FeTMPPCl/BP, which occurred in a much narrower potential range, ca. 0.2 V. In contrast to the behavior observed for the intact adsorbed macrocycle, its heat-treated counterpart displayed no affinity for CO, indicating that, to the level of sensitivity of this technique, iron sites in the thermally activated material are quite different from those in the N4 environment of the intact macrocycle.

In Situ X-ray Absorption of a Carbon Monoxide−Iron Porphyrin Adduct Adsorbed on High-Area Carbon in an Aqueous Electrolyte

The electronic and electrochemical properties of a carbon monoxide−FeTMPP adduct (FeTMPP = (5,10,15,20-tetrakis[4-methoxyphenyl]-21H,23H-porphine)iron(II)) adsorbed on Black Pearls (BP) high-area carbon have been examined in situ in 0.05 M H2SO4 by Fe K-edge X-ray absorption near-edge structure (XANES). This adduct was prepared by first polarizing a Teflon bonded electrode, incorporating the high-area FeTMPP/BP material, at a potential at which the adsorbed macrocycle is present in the reduced state, followed by bubbling CO into the electrolyte. The resulting in situ XANES displayed two new preedge features centered at 7112 and 7115 eV, as found for other well-characterized iron macrocycle−CO adducts, as well as a shift of the main absorption edge toward higher values (ca. 1.3 V) indicative of charge transfer from the iron to CO. Dissociation of the adduct could be achieved by polarizing the electrode at more positive potentials to yield the CO-free form of the macrocycle in the ferric state, as has been reported for solution-phase CO−Fe porphyrin adducts in nonaqueous media.

Methanol-tolerant electrocatalysts for oxygen reduction in a polymer electrolyte membrane fuel cell

Heat-treated μ-oxo-iron(iii) tetramethoxy phenyl porphyrin (Fe-TMPP)2O and iron(iii) tetramethoxy phenyl porphyrin (FeTMPP-Cl) as well as iron(iii) octaethyl porphyrin (FeOEP-Cl) adsorbed on high-area carbons such as deashed and un-deashed RB carbon (Calgon) and Black Pearls-2000 (Cabot) have been found to exhibit stable and very high oxygen reduction rates. Experiments done over a period of 24h showed no performance degradation. Measured performances were very similar to supported platinum (E-Tek), when tested in 85% H3PO4-equilibrated Nafion 117 membrane at 125°C and hydrated-Nafion membrane at 60°C in a minifuel cell. The macrocycle cathodes are insensitive to the presence of methanol whereas the platinum cathodes are very sensitive and show degradation in the oxygen reduction performance.

Reduction of hexavalent chromium by polypyrrole deposited on different carbon substrates

Pollution due to toxic hexavalent chromium has reached dangerous levels in some parts of the world, especially in Third World countries. This work focuses on the reduction of Cr(vi) ions in aqueous media by polypyrrole deposits on porous, high area three-dimensional carbon electrodes. Since many properties of deposited polymers are known to depend on the method of synthesis we performed the electropolymerization of pyrrole either at varying potential scan rates or at constant potential on different high area carbon materials. The deposits thus obtained were then put in contact with acidic hexavalent chromium solutions; high reduction yields were obtained (up to 80%). In addition, the possibility of repeating this cycle was studied as well as the wear resistance of these deposits. Finally, the morphology of the deposits was monitored at different stages of the process by scanning electron microscopy.

Novel Multinuclear Catalysts for the Electroreduction of Dioxygen Directly to Water

The electroreduction of O_2 to H_2O in aqueous acid at potentials close to the thermodynamically permitted value remains a daunting challenge for designers of superior fuel cells and batteries that utilize dioxygen as the reducible reactant. The four-electron reduction of O_2, which involves the rupture of the O-O bond and the formation of four O-H bonds, requires the use of catalysts to obtain useful rates at cathode potentials of interest in practical applications. The standard potential of the O_2/H_2O couple in solutions containing 1 M H^+ and saturated with O_2 at 1 atm is ca. 1.0 V (vs the saturated calomel electrode, SCE), but the highest cathode potentials achievable with currently available catalysts are closer to 0.55 V. (Molecules, functional groups, or metallic deposits that accelerate the rates of electrode reactions when they are confined to the surfaces of electrodes are often called electrocatalysts, a terminology that will be adopted in this Account.) Finely divided platinum supported on high area carbon is the electrocatalyst employed most frequently to achieve the electroreduction of O_2 to H_2O in presently available fuel cells. However, this type of electrocatalyst suffers from the disadvantages of high cost and gradual loss in catalytic activity as the surface area of the active platinum particles decreases because of sintering, dissolution, physical dislodgment, and/or adsorption of impurities. Searches for superior electrocatalysts for the reduction of O2 have often focused on cobalt porphyrins which are well-known to exhibit electrocatalytic activity toward the reduction of O_2, although H_2O_2 instead of H_2O is the usual product. However, it was discovered in recent years that a variety of molecular catalysts consisting of dimeric cofacial cobalt porphyrins adsorbed on the surface of graphite electrodes are able to catalyze the direct four-electron electroreduction of O_2 without passing through H_2O_2 as an intermediate. Both dimeric and monomeric iridium por phyrins have also been found to accomplish the electroreduction of O_2 to H_2O at unusually positive potentials. The mechanisms through which dimeric electrocatalysts are believed to operate involve the simultaneous interaction of both metal centers with the two oxygen atoms of the O_2 molecule as the O-O bond is severed. The ideas and strategies that underlay the development of these electrocatalysts have been described.

Thermo-osmosis of sorbable gases in porous media. Part IV. Mixture separation by two procedures

High internal area carbon compacts have been used to measure thermo-osmotic separation factors for the mixtures C 3H 8+CO 2, n-C 4H 10+CO 2 and n-C 4H 10+CH 4 over ranges of initial pressures and compositions. Two procedures were employed: in the first, two vessels were connected by the thermo-osmosis membrane only; in the second, in addition to the membrane, a return capillary was provided in which isothermal flow was at least partly viscous in character. In all three gas mixtures, using either method, the more strongly adsorbed components ( n-C 4H 10 or C 3H 8) were enriched at the hot side and the more weakly sorbed (CO 2 or CH 4) at the cold side of the membrane. In the second method the separation factors obtained with a long narrow return capillary were comparable with or larger than those measured without the return capillary. Separation factors increased the richer the mixture in the more strongly adsorbed component and the higher the initial pressure. For both methods separation factors were often large enough to indicate good potential for thermo-osmosis as a separation strategy. Heats of transport of the components in mixtures differed significantly, sometimes even in sign, from these heats for the pure components. Thus separation factors cannot be predicted from heats of transport of each pure gas or vapour. Rates of approach of separation factors to their steady state values were sensitive to the cross-sectional area of the membrane and also to stirring the gas mixtures during their un-mixing.

In Situ Core-Electron Spectroscopy of Carbon Monoxide Adsorbed on High-Area Platinum in an Acid Electrolyte

Modifications in the electronic density of states of small Pt particles (4−5 nm diameter) dispersed in high-area carbon (XC72-R) induced by the presence of adsorbed CO were examined by in situ Pt-LIII-edge X-ray absorption near edge structure (XANES) in a 1.0 M H3PO4 solution. An increase in the intensity of the in situ Pt-LIII-edge XANES was observed at energies a few electronvolts positive of the white line maximum (11 567 eV), after CO was bubbled in the solution, with the electrode polarized at potentials in the range in which CO undergoes irreversible adsorption on Pt. A well-defined peak centered at 11 570 eV could be clearly identified upon subtracting these spectra from those acquired in that same potential region prior to CO adsorption. This feature has been ascribed to transitions from the Pt 2p3/2 orbital to unoccupied states above the Fermi level localized predominantly on CO.

In SituVanadium K-Edge and Tungsten LIII-Edge X-ray Absorption Fine Structure of Vanadium-Substituted Heteropolytungstates Immobilized in a High-Area Carbon Electrode in Acid Aqueous Electrolytes

Electronic and structural aspects of vanadium-substituted heteropolytungstates immobilized in a high-area carbon (XC-72) as a function of oxidation state have been examined by in situ X-ray absorption near-edge structure (XANES) in an acidic electrolyte. The results obtained for K4PVW11O40 revealed a sizable shift in the V K-edge XANES region, which is characterized by a prominent pre-edge peak, following a one-electron reduction. Such behavior has been attributed to the transfer of an electron to an orbital localized mainly on vanadium. Injection of a second electron gives rise to the near disappearance of the pre-edge peak without major shifts in the position of edge jump, a phenomenon ascribed to an increase in the symmetry of the vanadium site upon reduction to yield a nearly octahedral environment. Similar behavior is observed for Cs6PV3W9O40 when the electrode is polarized in the potential regions where the vanadium ions are reduced to VIV and VIII, respectively. No changes in the in situ W LIII-edge XANES could be discerned for these vanadium-substituted heteropolytungstates in their various oxidation states, for which the spectral features were the same as those of H3PW12O40 adsorbed on XC-72 under otherwise identical conditions.

Carbon materials for lithium-ion (shuttlecock) cells

Carbon materials as possible alternatives to metallic lithium were described with emphasis on the electrochemical characters of carbon materials, such as (natural and artificial) graphite, petroleum coke, pitch-based carbon fibers, high-area carbons, and other carbonaceous materials. Well-defined graphite showed the lowest operating voltage (0-0.3 V versus Li) and the highest volumetric capacity (about 0.6 Ah·cm −3 based on the observed density and rechargeable capacity) in addition to excellent rechargeability. The theoretical capacity of graphite was 372 mAh·g −1 or 850 mAh·cm −3 based on the graphite sample for the reaction to form the first-stage compound (LiC 6). Among the materials examined, some of them showed the capacity more than 372 mAh·g −1, while the operating voltage was higher and the density was lower than that of graphite. To understand such an anomalous behavior of carbonaceous materials, we modeled carbon electrodes and explained why some of carbonaceous materials exceed a capacity limit of 372 mAh·g −1. According to our model, high-capacity materials more than 500 mAh·g −1 were possible but traded off volumetric capacity, operating voltage, and consequently energy density. A lithium-ion (shuttlecock) cell consisting of natural graphite and LiNiO 2 was also reported and the specific problems in applying carbon materials to lithium-ion cells were discussed.

In situ Moessbauer spectroscopy of a species irreversibly adsorbed on an electrode surface

The redox properties of {mu}-oxobis(iron meso-tetrakis(methoxyphenyl)porphyrin)(FeTMPP){sub 2}O, adsorbed on high-area carbon in alkaline solutions have been examined in situ by using Moessbauer spectroscopy at cryogenic temperatures. The spectrum obtained for the electrode polarized at 0.0 V vs Hg/HgO, OH{sup {minus}} yielded at 100 K a doublet with parameters resembling those of bulk crystalline (FeTMPP){sub 2}O. Similar measurements with the electrode polarized at {minus}0.9 V at the same temperature displayed in addition to the original feature two new doublets with Moessbauer parameters consistent with the presence of a high-spin ferrous (S = 2) and either a high-spin ferric (S = 5/2) or an intermediate-spin ferrous porphyrin species (S = 1). As the temperature was increased, these new features collapsed into a single doublet with an isomer shift intermediate between those observed at 100 K. Two possible models have been suggested to account for these results: (i) the formation of a mixed-valence {mu}-oxo-type porphyrin, which below the spin trapping temperature would give rise to ferrous and ferric sites, and (ii) the full reduction and dissociation of the dimer following the first electron transfer, to yield an intermediate-spin and a high-spin ferrous porphyrin.

Heat-treated polyacrylonitrile-based catalysts for oxygen electroreduction

Polyacrylonitrile (PAN), mixed with Co(II) or Fe(II) salts and high-area carbon and then heat treated, has been found to yield very promising catalysts for O2 reduction in concentrated alkaline and acid solutions. The catalytic activities are comparable to those for the heat-treated corresponding transition metal macrocycles and polypyrrole black-based catalysts. The addition of the transition metal to the nitrogen-containing polymer, either before or after the heat treatment with carbon, is an important factor for good activity. The nitrile nitrogen of the PAN is probably retained and converted to pyridyl nitrogen during the heat treatment, and this nitrogen is believed to provide binding sites for the transition metal ions, which then act as catalytic sites for oxygen reduction to peroxide and its decomposition.

ChemInform Abstract: In situ Moessbauer Effect Spectroscopy of a Model Iron Perovskite Electrocatalyst.

AbstractIt is found that the fraction of Fe(IV) sites in SrFeO2.91 dispersed on high area carbon gradually and irreversibly decreases upon exposure to an alkaline solution (0.1 M KOH).

In situ mossbauer effect spectroscopy of a model iron perovskite electrocatalyst

The electrochemical properties of SrFeO 2.91 dispersed on a high area carbon in alkaline media have been investigated by cyclic voltammetry and in situ Mosbauer spectroscopy. The results provide evidence that a fraction of Fe(IV) sites, as monitored in situ, undergo a gradual and irreversible decrease upon exposure of the material to an alkaline solution. This has been attributed to the reduction of such oxidized sites by water, generating most likely an hydroxylated ferric species. In addition it was found that the voltammetry peak observed during the first scan in the negative direction is associated with the irreversible destruction of the perovskite lattice. The only iron species formed and detected by Mossbauer spectroscopy is Fe(OH) 2 which then urdergoes reoxidation at much more positive potentials to yield a hydrated ferric oxyhydroxide.

Electrochemistry of iron oxides: an in situ Moessbauer study

The electrochemical properties of Fe(OH)/sub 2/ dispersed on high-area carbon in strongly alkaline media have been studied in situ by Moessbauer spectroscopy. The oxidation of this material was found to yield either an ensemble of small magnetite particles of different sizes or a mixture of superparamagnetic and bulklike magnetite crystals depending on whether the process was carried out by stepping or sweeping the potential to -0.3 V vs Hg/HgO,OH/sup -/, respectively. This provides clear evidence that the morphological characteristics of the resulting material are determined by the rate at which the oxidation reaction is performed. Further cathodic polarization to -1.2 V gave rise to the irreversible reduction of the Fe(OH)/sub 2/ to metallic iron.

Cobalt tetramethoxyphenyl porphyrin—emission Mossbauer spectroscopy and O 2 reduction electrochemical studies

Cobalt tetramethoxyphenyl porphyrin (Co-TMPP) on a high area carbon support after thermal treatment at 450–900°C has been found to be an effective catalyst for O 2 reduction in alkaline electrolytes. The effect of the heat treatment at 850°C on this catalyst has been examine with emission Mössbauer spectroscopy using Co-57 enriched Co-TMPP. The spectra indicate the presence of cobaltous oxides with no evidence of Co-N 4 centers or any other form of cobalt. The metal-free complex H 2TMPP, given similar thermal treatment on a mineral-free carbon support does not show catalytic activity beyond that of the carbon support, thus confirming the importance of the transition metal to the catalysis.