Gessie Brisard

Oxygen reduction and hydrogen evolution–oxidation reactions on Cu(hkl) surfaces

Taking advantage of the ring collection properties of the rotating ring–disk electrode with a single-crystal copper disk, we have shown that the oxygen reduction reaction and the hydrogen evolution–oxidation reactions on Cu(111) and Cu(100) surfaces in sulfuric acid are structure-sensitive processes. An interpretation of the structural sensitivity is presented, based on the premise of the structure-sensitive adsorption of (bi)sulfate anions. We have observed that the ordering kinetics of the (bi)sulfate on Cu(hkl) have an important role in the mechanism of the oxygen reduction reaction.

The reduction mechanism of Cd(II) at mercury in dimethylsulfoxide containing tetraethylammonium perchlorate

Abstract The electrochemical reaction Cd 2+ + 2 e − = CD(Hg) has been studied in dimethylsulfoxide containing tetraethylammonium perchlorate as a supporting electrolyte, using dc polarography, ac admittance measurements, cyclic voltammetry with convolution and chronoamperometry. It has been found that a chemical reaction, probably partial desolvation of the reactant, precedes the electron tranfer and Cd 2+ is reduced according to a CE(E) mechanism, with a heterogenous chemical reaction followed by one two-electron or two one-electron reduction steps. The kinetic parameters of this process were determined.

Double-layer effects in the kinetics of the Cd2+/Cd(Hg) system in dimethylsulfoxide

Abstract The effects of the nature and concentration of the supporting electrolyte cation on the kinetics of the electroreduction of Cd 2+ have been studied at a mercury electrode in dimethylsulfoxide solutions. The standard rate constants, k s , were found to decrease in the order k s (Na + ) > k s (TEA + ) > k s (TBA + ). The apparent transfer coefficients depend also on the nature and concentration of the supporting electrolytes; in the presence of TBAP they are as low as 0.09. Digital simulations of cyclic voltammetric curves were performed and good agreement was found with experimental results. The influence of the double layer structure on the kinetic parameters has also been studied. The results cannot be explained in terms of the simple Frumkin theory. Two possible mechanisms are proposed: (i) slow cation transfer through the double layer, (ii) slow one-electron transfer followed by a fast cation transfer.

Reduced and oxidized forms of the Pt-organometallic version of polyaniline.

This work represents an effort to synthesize all four forms of polyaniline (PANI) in its organometallic versions. Polymers containing substituted 1,4-benzoquinone diimine or 1,4-diaminobenzene units in the backbone exhibiting the general structure (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C-PtL(2))(n) and (C≡CC(6)H(4)NH-C(6)X(4)-NHC(6)H(4)C≡C-PtL(2))(n) along with the corresponding model compounds (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C)(PtL(2)Cl)(2) and (C≡CC(6)H(4)NH-C(6)X(4)-NHC(6)H(4)C≡C)(PtL(2)Cl)(2) (L = PBu(3); X = H, F, Cl) were synthesized. The polymers and corresponding model compounds were characterized (including (1)H and (31)P NMR, IR, mass spectra, elemental analysis, and X-ray structure determinations) and investigated for their redox properties in the absence and in the presence of acid. Their optical properties, including ns transient spectroscopy were also investigated. These properties were interpreted through density functional theory (DFT) and time-dependent DFT (TDDFT) computations. These materials are found to be oligomers (GPC) with thermal stability (TGA) reaching 350 °C. The greatest stabilities were found in the cases with X = F. Using a data bank of 8 X-ray structures of diimine derivatives, a relationship between the C═N bond distance and the dihedral angle between the benzoquinone ring and the flanking phenyl planes is noted. As the size of the substituent X on the benzoquinone center increases, the degree of conjugation decreases as demonstrated by the C═N bond length. The largest dihedral angles are noted for X = Cl. These polymers exhibit drastic chemical differences when X is varied (X = H, F, Cl). The completely reduced polymer (C≡CC(6)H(4)NH-C(6)H(4)-NHC(6)H(4)C≡C-PtL(2))(n) (i.e., X = H) was not chemically accessible whereas in the cases of X = F, Cl, these materials were obtained and represent the first examples of fully reduced organometallic versions of PANI (i.e., leucoemaraldine). For the (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C-PtL(2))(n) polymers, the completely oxidized form for X = H was isolated (pernigraniline), but for X = F and Cl, only the largely reduced mixed-valence form (i.e., emaraldine) was obtained via chemical routes. In acidic solutions, the chemically accessible polymer for X = H, (C≡CC(6)H(4)-N═C(6)H(4)═N-C(6)H(4)C≡C-PtL(2))(n), exhibits two chemically reversible waves indicating that the reduced form (C≡CC(6)H(4)NH-C(6)H(4)-NHC(6)H(4)C≡C-PtL(2))(n) can be generated. The absorption spectra of the highly colored diimine-containing species exhibit a broad charge transfer band (assigned based on DFT calculations (B3LYP); C(6)H(4)C≡C-PtL(2)-C≡CC(6)H(4) → N═C(6)X(4)═N) in the 450-800 nm window red shifting according X = H → Cl → F, consistent with their relative inductive effect. The largest absorptivity is measured for X = H because this polymer is fully oxidized whereas for the cases where X = F and Cl, these polymers exists in the mixed valence form. The ns transient absorption spectra of two polymers (X = F; reduced and mixed-valence polymers) were measured. The triplet excited state in the mixed-valence polymer is dominated by the reduced diamine residue and the T(1)-T(n) absorption of the diimine is entirely quenched.

Synthesis and characterization of La0.8Sr0.2MO3−δ (M=Mn, Fe, or Co) cathode materials by induction plasma technology

In this work, nanopowders of perovskite cathode materials (La0.8Sr0.2MnO3−δ, La0.8Sr0.2FeO3−δ, and La0.8Sr0.2CoO3−δ), for use in solid oxide fuel cells (SOFC), were successfully synthesized, using induction plasma techniques. Their compositions, structures, morphology, particle size distributions, and BET specific surface areas were determined for comparison with their counterparts prepared by the Pechini method and by the glycine-nitrate combustion (GNC) technique. The particle sizes of the plasma-synthesized powders are mostly around 63 nm. These plasma-synthesized powders are generally globular, their BET specific surface areas being about 26 m2g−1, approximately twice those of powders prepared by the GNC and Pechini methods. These plasma-synthesized powders are readily reproducible and are not agglomerated. Their individual particle sizes and distributions are very independent of their composition.

Study of solvent effects on the kinetics of the Cd(II)/Cd(Hg) reaction

Abstract Effects of the nature of the solvent on the kinetics of the electroreduction of Cd(II) have been studied at a mercury electrode in non-aqueous solutions containing 0.1 M tetraethylammonium perchlorate as a supporting electrolyte. The standard rate constants, corrected for the double layer effects, decrease in the following order: k s,s (acetonitrile) > k s,a (propylene carbonate) > k s,a (fornamide) > k s,a (dimethylformamide) > k s,a (dimethylsulfoxide) > k s,a (hexamethylphosphoramide). Two possible reaction mechanisms were considered, namely electron transfer, and cation transfer. Solvent effects were analyzed in terms of the solvation parameters of the cation and dynamical properties of the solvent.

The Pt‐Organometallic Version of Perigraniline: Going Blue

Four conjugated push-pull organometallic polymers ([Pt]-AQ)n ([Pt] = trans-bis(phenylacetylene)bis(tributylphosphine)platinum(II); AQ = 2-bromo-, 2,6-dibromo-, 2,6-diamino-, and unsubstituted anthraquinone diimine) were prepared and characterized by UV-vis spectroscopy and electrochemistry. A low-energy charge transfer, CT, band ([Pt]*→AQ; confirmed by density functional theory calculations), was found in the 445-500 nm window rather than the expected red-shifted range above 630 nm. X-ray structures of four model compounds reveal that steric hindrance induces large dihedral angles between the C6 H4 and NCC2 planes, rendering π-orbital overlap difficult between the [Pt] and AQ units. The position of the CT band is mainly driven the reduction potential of the anthraquinone diimine unit.

Drastic Tuning of the Photonic Properties of «(Push–Pull)n» trans-Bis(ethynyl)bis(Tributylphosphine)Platinum(II)-Containing Polymers

The trans-Pt(PBu3)2 Cl2 complex reacts with 1 equiv. of 2,6-diethynyl-AQ and 2 equiv. of 2-ethynyl-AQ (AQ = anthraquinone) to form the polymer (trans-Pt(2,6-diethynyl-AQ)2 (PBu3)2)n, 1, and the model compounds, 2, trans-Pt(PBu3)2 (2-ethynyl-AQ)2 (in a 20:1 ratio as trans-(2a) and cis-(2b) rotational isomers), respectively. These redox-active and luminescent materials have been characterized by gel permeation chromatography, thermal gravimetric analysis, X-ray crystallography, electrochemistry, photophysics, and DFT computations (B3LYP). The typical π,π* T2 → S0 phosphorescence centered on the trans-Pt(PBu3)2 (aryl)2 chromophore, [Pt], generally encountered for the analogous polymers (trans-Pt(PBu3)2 (aryl)2-acceptor)n (acceptor = quinonediimine, QN2; anthraquinone diimine, AQN2), for which the CT T1 → S0 emission is silent, has been completely annihilated and replaced by a red-shifted T1 → S0 emission in 1 and 2a, which arise from a triplet charge transfer excited state [Pt]→ AQ.

Kinetics of the electroreduction of Co(salen) in DMSO

The main problem with kinetic measurements in cyclic voltammetry is connected with the increase or the ohmic drop IR S , where R S is the uncompensated solution resistance, as the sweep rate increases. Although modern potentiostats allow some compensation or R S , total compensation is not possible. The presence or the uncompensated resistance causes nonlinearity or the potential sweep and leads to an increase in the separation or peak potentials, which may result in erroneous kinetic parameters. The development or ultramicroelectrodes has increased the possibilities or cyclic voltammetry at high sweep rates

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster

9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)]; dppm = Ph2PCH2PPh2 as a PF6(-) salt) with that of 5,10,15,20-tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd3(2+)]x (x = 0-4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV-vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd3(2+)]x assemblies (K14 = 36000 M(-1)) are slightly more stable than those for TCPP···[Pd3(2+)]x (K14 = 27000 M(-1)). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd3(2+)] assembly compared to that for TCPP···[Pd3(2+)]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd3(2+)] → dye(•+)···[Pd3(•+)], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye(•+)···[Pd3(•+)] → dye···[Pd3(2+)]) occurring in the various observed species (dye···[Pd3(2+)]x; x = 0-4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes.