Photoinduced reactivity of titanium dioxide

Equivalent weight (EW) is the number of grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form. This is an average EW in the sense that the comonomer sequence distribution (that is usually unknown to the investigator and largely unreported) gives a distribution in m in this formula. EW can be ascertained by acid-base titration, by analysis of atomic sulfur, and by FT-IR spectroscopy. The relationship between EW and m is EW ) 100m + 446 so that, for example, the side chains are separated by around 14 CF2 units in a membrane of 1100 EW. Common at the time of this writing are Nafion 117 films. The designation “117” refers to a film having 1100 EW and a nominal thickness of 0.007 in., although 115 and 112 films have also been available. Early-reported studies involved 1200 EW samples as well as special experimental varieties, some being rather thin. The equivalent weight is related to the property more often seen in the field of conventional ion exchange resins, namely the ion exchange capacity (IEC), by the equation IEC ) 1000/EW. The mention of the molecular weight of high equivalent weight (EW > 1000 g‚mol-1) Nafion is almost absent in the literature, although the range 105-106 Da has been mentioned. As this polymer does not form true solutions, the common methods of light scattering and gel permeation chromatography cannot be used to determine molecular weight as well as the size and shape of isolated, truly dissolved molecules. Studies of the structure of this polymer in solvent (albeit not a true solution) will be mentioned in the scattering section of this review. It should be noted that Curtin et al. performed size exclusion chromatography determinations of the molecular weight distribution in Nafion aqueous dispersions after they were heated to high temperatures (230, 250, and 270 °C).1 Before heating, there was a high molecular weight shoulder on a bimodal distribution, due to molecular aggregates, but this shoulder disappeared upon heating, which indicated that the aggregates were disrupted. The peaks for the monomodal distribution for the heated samples were all located at molecular weights slightly higher than 105 g‚mol-1. Also, light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure. Nafion ionomers are usually derived from the thermoplastic -SO2F precursor form that can be extruded into sheets of required thickness. Strong interactions between the ionic groups are an obstacle to melt processing. This precursor does not possess the clustered morphology that will be of great concern in this article but does possess Teflon-like crystallinity which persists when the sulfonyl fluoride form is converted to, for example, the K+ form by reacting it with KOH in water and DMSO. Thereafter, the -SO3H form is achieved by soaking the film in a sufficiently concentrated aqueous acid solution. Extrusion of the sulfonyl fluoride precursor can cause microstructural orientation in the machine direction, * Address correspondence to either author. Phone: 601-266-5595/ 4480. Fax: 601-266-5635. E-mail: Kenneth.Mauritz@usm.edu; RBMoore@usm.edu. 4535 Chem. Rev. 2004, 104, 4535−4585

State of Understanding of Nafion

Overview of membrane science and technology membrane transport theory membrane and modules concentration polarization reverse osmosis ultrafiltration microfiltration gas separation pervaporation ion exchange membrane processes - electrodialysis carrier facilitated transport medical applications of membranes other membranes processed.

Membrane Technology and Applications

The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

where A represents the magnetic vector potential, is an integral of the hydromagnetic equations. This -integral made it possible to formulate a variational principle for the force-free magnetic fields. The integral expresses the fact that motions cannot transform a given field in an entirely arbitrary different field, if the conductivity of the medium isconsidered infinite. In this paper we shall show that the full set of hydromagnetic equations admit five more integrals, besides the energy integral, if dissipative processes are absent. These integrals, as we shall presently verify, are I2 =fbHvdV, (2)

Proceedings of the National Academy of Sciences

Electrochemistry is an increasingly popular technique for the characterization of new compounds. The basic thermodynamic quantity that is assigned to an electrode process is the standard or formal reduction potential (E^o or E^f). In aqueous solution the measurement of reduction potentials is facilitated by the use of reliable and universally accepted reference electrodes such as the normal hydrogen electrode (NHE) or the saturated calomel electrode (SCE). In many instances electrochemical measurements in water are impossible due to insolubility or instability of the compound. Unfortunately, no universal reference electrode exists for nonaqueous solvents.

Ferrocene as an internal standard for electrochemical measurements

A series of dimeric metalloporphyrin molecules has been synthesized in which the two porphyrin rings are constrained to lie parallel to one another by two amide bridges of varying length that link the rings together. These cofacial metalloporphyrins have been applied to the surface of graphite electrodes and tested for catalytic activity toward the electroreduction of dioxygen to water in aqueous acidic electrolytes. All molecules tested exhibited some catalytic activity, but hydrogen peroxide rather than water was the chief reduction product. However, the dicobalt cofacial porphyrin linked by four-atom bridges produced a catalyzed reduction almost exclusively to water and at exceptionally positive potentials. Rotating ring-disk voltametric measurements were employed to diagnose the electrode reaction pathway and a possible mechanism for the observed catalysis is suggested. The results seem to demonstrate the participation of two metal centers in controlling the course of a multiple-electron process.

Electrode catalysis of the four-electron reduction of oxygen to water by dicobalt face-to-face porphyrins

Gas separations using non-hexafluorophosphate [PF6]− anion supported ionic liquid membranes

Potent catalysis of the electroreduction of oxygen to water by dicobalt porphyrin dimers adsorbed on graphite electrodes

Electrochemical techniques were employed to study electron transfer properties of a binuclear copper--macrocyclic ligand complex and to prepare several reduced derivatives. The complex, Cu/sup II/Cu/sup II/L(ClO/sub 4/)/sub 2/.2H/sub 2/O, was prepared by condensing 1,3-diaminopropane with 5-methyl-2-hydroxyisophthalaldehyde and Cu(ClO/sub 4/)/sub 2/.6H/sub 2/O. The Cu/sup II/Cu/sup II/ complex was reduced in successive, quasi-reversible, one-electron steps at - 0.52 and - 0.91 V vs. NHE. Constant-potential electrolysis was employed to prepare the reduced species, Cu/sup II/Cu/sup I/L(ClO/sub 4/) and Cu/sup I/Cu/sup I/L, both of which were isolated and fully characterized. The mixed-valence species, Cu/sup II/Cu/sup I/L/sup +/, is stable in oxygen-free solutions (conproportionation constant: 3.97 x 10/sup 6/) and reacts with carbon monoxide to form an adduct, Cu/sup II/Cu/sup I/L(CO)ClO/sub 4/. The diamagnetic Cu/sup I/Cu/sup I/L species was obtained as essentially insoluble black needles which, however, formed a soluble dicarbonyl adduct, Cu/sup I/Cu/sup I/L(CO)/sub 2/, on exposure to CO. The latter complex was not isolated. At the ambient temperature Cu/sup II/Cu/sup I/L/sup +/ exhibits an isotropic seven-line solution EPR spectrum while an anisotropic pattern was observed in frozen solutions (77/sup 0/K), with four lines for g parallel to and unresolved g perpendicular to. Variable-temperature experiments indicated coalescence at about 200/sup 0/K suggesting anmore » intramolecular electron transfer rate of about 1.7 x 10/sup 10/ s/sup -1/ at 298/sup 0/K. Electronic absorption spectral measurements revealed at least two absorptions for Cu/sup II/Cu/sup I/L(ClO/sub 4/)/sub 2/ at 1700 and 1200 nm (CH/sub 2/Cl/sub 2/), which are not present in Cu/sup II/Cu/sup II/L(ClO/sub 4/)/sub 2/.2H/sub 2/O, Cu/sup I/Cu/sup I/L, or the carbonyl derivatives. The new spectral bands may be attributable to intramolecular electron transfer processes. 11 figures, 5 tables.« less

Binuclear complexes of macrocyclic ligands: electrochemical and spectral properties of homobinuclear Cu/sup II/Cu/sup II/, Cu/sup II/Cu/sup I/, and Cu/sup I/Cu/sup I/ species including an estimated intramolecular electron transfer rate

Carl A. Koval

Papers

Ferrocene as an internal standard for electrochemical measurements

Electrode catalysis of the four-electron reduction of oxygen to water by dicobalt face-to-face porphyrins

Gas separations using non-hexafluorophosphate [PF6]− anion supported ionic liquid membranes

Potent catalysis of the electroreduction of oxygen to water by dicobalt porphyrin dimers adsorbed on graphite electrodes

Binuclear complexes of macrocyclic ligands: electrochemical and spectral properties of homobinuclear Cu/sup II/Cu/sup II/, Cu/sup II/Cu/sup I/, and Cu/sup I/Cu/sup I/ species including an estimated intramolecular electron transfer rate