Alessandra Molinari

Influence of the electrode history and effects of the electrolyte composition and temperature on O2 evolution at β-PbO2 anodes in acid media

An investigation is reported on O2 electrogeneration at β-PbO2 electrodes in HClO4, H2SO4 and CF3SO3H (TFMSA) using steady-state and impedance measurements. It is established that desorption of reaction intermediates is rate limiting in all the cases examined, and it is proposed that elementary reactions leading to oxygen evolution take place at a surface consisting of crystal line (PbO2) and hydrous [PbO(OH)2] zones. Contribution of the latter becomes particularly pronounced at high potentials, probably on account of PbO2 dissolution and re-deposition in the form of Pb(OH)4 that subsequently evolves to give PbO(OH)2. This process is indicated as a possible cause of an inflexion point in the Tafel plots between 1.95 and 2.0 V SCE. Different important factors are found to affect the electrochemical process: (i) pre-treatment of the electrode, which makes electrodes more active by reason of an increase of the hydrous zones; (ii) the temperature at which electrodeposition of PbO2 is carried out has an effect on the hydration state of the surface and hence the electrocatalytic activity; (iii) electrolyte anions, particularly SO42− and CF3SO3−, are adsorbed and inhibit both water discharge and desorption of reaction intermediates; besides, they undergo oxidation at the more positive potentials; (iv) fluoride added to the electrolyte is strongly adsorbed and suppresses and/or modifies the structure of the hydrous layer with the consequence that water discharge is inhibited in the lower potential range and, prevalently at the more positive potentials, desorption of oxygen intermediates is strongly retarded; and (v) a low electrolyte temperature also inhibits O2 evolution mainly by inhibition of the desorption of reaction intermediates in the whole potential range examined, in contrast to the selective effect of fluoride.

Phororedox and photocatalytic processes on Fe(III)–porphyrin surface modified nanocrystalline TiO2

Abstract Surface derivatization of titanium dioxide nanoparticles with a Fe(III)–porphyrin has been carried out following a new procedure whereby the complex, rather than the surface, contains the aminopropylsilane functional group. This avoids the problems of surface deactivation by silane groups, reported in earlier investigations, on analogous systems. Characterization of the light-transparent dispersions by laser flash photolysis, UV-vis spectroscopy and photo-electrochemical methods has shown that the nature of the solvent is an important parameter in determining the redox processes involving the grafted porphyrin. In particular, one observes marked effects on the stability of the Fe(II)–porphyrin formed upon capture of the photogenerated electrons. The photocatalytic activity of the composite systems was assessed in the process of monooxygenation of cyclohexane and cyclohexene by molecular oxygen. The bonded porphyrin enhances the yield and the formation of the monooxygenation products with respect to total degradation to CO2 for both the examined substrates. On this basis, we can claim an increase in the efficiency and selectivity with the composite photocatalytic system. In the case of cyclohexane, we observed, in addition, that the iron–porphyrin complex also changes the selectivity of the process, increasing the alcohol to ketone ratio.

Partial oxidation of allylic and primary alcohols with O2 by photoexcited TiO2

Proper reaction conditions have been found for the conversion of geraniol, citronellol, trans-2-penten-1-ol and 1-pentanol to the corresponding aldehydes with good chemo-selectivity (>70%) by photochemical excitation of suspensions of P25-TiO(2). It is demonstrated that adsorption of the alcohol on the surface as an alcoholate is necessary for its oxidation. ESR-spin trapping experiments point out that oxidation of alcohols starts with the formation of alkoxide radicals. Water content in the dispersing medium strongly inhibits alcohol adsorption and subsequent oxidation. In fact, water increases the polarity of the dispersing medium favouring the affinity between the polar alcohol and the CH(3)CN-H(2)O mixture itself; moreover, water competitive adsorption with the alcohol causes the removal of the latter from the photocatalytic surface with consequent difficult oxidation, as evidenced by ESR-spin trapping investigation. The reactivity of the alcohol on the surface of photoexcited P25-TiO(2) is also affected by the nature of its hydrophobic aliphatic chain: geraniol and citronellol are more susceptible to the water content than their short analogues trans-2-penten-1-ol and 1-pentanol. Moreover, in anhydrous CH(3)CN, specific interaction between the surface and the OH group enhances the reactivity of the primary aliphatic alcohols towards their partial oxidation to aldehyde, which can be accumulated in the reaction environment.

Preparation, Characterisation, and Photocatalytic Behaviour of Co-TiO2 with Visible Light Response

The preparation of cobalt-modified T i O 2 (Co- T i O 2 ) was carried out by the incipient impregnation method starting from commercial T i O 2 (Degussa, P-25) and cobalt acetate. XPS data show that cobalt is incorporated as divalent ion, and it is likely present within few subsurface layers. No appreciable change in structural-morphologic properties, such as surface area and anatase/rutile phase ratio, was observed. Conversely, Co addition brings about conspicuous changes in the point of zero charge and in surface polarity. Diffuse reflectance spectra feature a red shift in light absorption that is dependent on the amount of cobalt. The influence of cobalt addition on the performance of T i O 2 as a photocatalyst in the degradation of 4-chlorophenol and Bisphenol A is investigated. The results show that the modified oxide presents a higher photoactivity both for illumination with UV-visible ( 𝜆 > 3 6 0  nm) and visible light ( 𝜆 > 4 2 0  nm; 𝜆 > 4 5 0  nm), and that this enhancement depends on the amount of the added species and on the final thermal treatment in the preparation step. We also show that Co- T i O 2 is a more active catalyst than pure T i O 2 for the reduction of O 2 in the dark, which is an important reaction in the overall photocatalytic processes.

Photocatalytic oxidation of cyclohexane by (nBu4N)4W10O32Fe(III)prophyrins integrated systems

Abstract The oxidation of cyclohexane by photoexcited (nBu4N)4W10O32 has been investigated in the presence of iron meso-tetraarylporphyrins bearing different substituents in the β-pyrrole positions and/or in the meso-aryl groups. Irradiation at 325 nm leads to the reduction of the polyoxotungstate with the simultaneous oxidation of cyclohexane to cyclohexyl radicals which can be detected by the ESR spin trapping technique. In oxygen-free solutions, the photoreduced polyoxotungstate is able to transfer one electron to the Fe(III)porphyrin to give the ferrous complex. The subsequent reaction between this species and cyclohexyl radicals leads to the formation of σ-alkyl-Fe(III)porphyrin complexes, as demonstrated by UV-VIS and NMR spectroscopy. In the presence of oxygen, the photoreduced polyoxotungstate has the role of initiating the activation of O2 through its reduction to O2 and H2O2. As a consequence, cyclohexane is converted to cyclohexanone and cyclohexanol in a cyclic way. The Fe(III)porphyrin complex strongly affects the product distribution probably through its hydrogen peroxide and alkyl-hydroperoxide-dependent oxidation. In comparison with (nBu2N4)W10O32 alone, a higher selectivity is obtained with the formation of cyclohexanol as a major product.

Photocatalyzed Oxidation of Cyclohexene and Cyclooctene with (nBu4N)4W10O32 and (nBu4N)4W10O32/FeIII[meso-Tetrakis(2,6-dichlorophenyl)porphyrin] in Homogeneous and Heterogeneous Systems

Photoexcitation of (nBu4N)4W10O32 is a suitable mean of oxidizing cyclohexene and cyclooctene with O2 at room temperature and pressure. This process can be carried out in homogeneous solution as well as using the decatungstate in a dispersed form after its heterogenisation on silica. Cyclohexene and cyclooctene are mainly oxidized to the corresponding hydroperoxides as a consequence of primary photoprocesses which lead to the formation of allylic radicals. The presence of the FeIII[meso-tetrakis(2,6-dichlorophenyl)porphyrin] chloride as cocatalyst strongly affects the photocatalytic properties of (nBu4N)4W10O32, playing a key role in the allylic-hydroperoxide dependent oxidation of the cycloalkenes. In the photooxidation of cyclohexene, the porphyrin increases the photocatalytic efficiency of the decatungstate in terms of total turnover number and catalyses the decomposition of cyclohexenyl hydroperoxide with the selective formation of cyclohex-2-en-1-ol. On the other hand, its presence during the photoinduced oxidation of cyclooctene favours the formation of cyclooctene epoxide by addition of ROO· and RO· radicals to the double bond. In the case of cyclooctene, the heterogenisation of the decatungstate on the solid support also affects the chemoselectivity of the photocatalytic process in the absence of the iron porphyrin complex.

Photoinduced reactivity of Au–H intermediates in alcohol oxidation by gold nanoparticles supported on ceria

H-Au-np/CeO2 species, that are important intermediates in the alcohol oxidation by gold nanoparticles, show a photoinduced reactivity. UV-visible photoexcitation in deaerated conditions causes a hydride transfer from gold to the spin trap DMPO. The photoassisted breaking of the Au–H bond restores the catalyst in its active form. As a consequence, photochemical excitation has a positive effect on the catalytic oxidation of 2-pentanol under anaerobic conditions, demonstrating that light can be used in place of oxygen. This new photocatalytic activity of H-Au-np/CeO2 species is of general interest in the field of catalysis with gold nanoparticles, since the photoinduced hydride shift from gold surface to organic substrates opens up exciting possibilities to use alcohols for hydrogenation reactions.

CATALYTIC OXYGENATION OF CYCLOHEXANE BY PHOTOEXCITED (NBU4N)4W10O32: THE ROLE OF RADICALS

Abstract Photochemical excitation of (nBu4N)4W10O32 in the presence of oxygen (760 torr) leads to the formation of C6H11− and OH− radicals which can be revealed by ESR spin-trapping investigations. Cyclohexane is transformed in cyclohexanone and cyclohexanol with an overall photooxidation quantum yield of 0.35 at 325 nm photoexcitation wavelength. A strong influence of oxygen pressure on the concentration ratio of these products is observed; in that, at low oxygen pressure (15 torr), the yield of cyclohexane is negligible and the photocatalytic oxidation of cyclohexane mainly leads to the formation of cyclohexanol. The photocatalyst investigated does not suffer irreversible modifications allowing the oxygenation of cyclohexane by O2 itself under mild conditions with good turnover values.

Photoinitiated Catalysis in Nafion Membranes Containing Palladium(II) meso-Tetrakis(N-methyl-4-pyridyl)porphyrin and Iron(III) meso-Tetrakis(2,6-dichlorophenyl)porphyrin for O2-Mediated Oxidations of Alkenes

Immobilisation of both palladium(II) meso-tetrakis(N-methyl-4-pyridyl)porphyrin (PdTMPyP4+) and iron(III) meso-tetrakis(2,6-dichlorophenyl)porphyrin (FeTDCPP+) in the same membrane of Nafion creates a new composite system, in which the photoexcited palladium complex induces the O2-mediated oxidation of cyclohexene to the corresponding allylic hydroperoxide and the iron porphyrin works as a catalyst for specific oxygenations of cyclohexene and cyclooctene. The role of PdTMPyP4+ is to induce the photoactivation of O2 with visible light (lambda > 500 nm) to generate singlet oxygen (1O2) by means of energy transfer from the excited triplet state. Consequently, the 1O2-mediated oxidation of cyclohexene to cyclohexenyl hydroperoxide can be realised with a selectivity greater than 90%. Spectroscopic and photophysical investigations show that the tetracationic palladium porphyrin is mainly fixed to the external part of the Nafion membrane, it is characterised by a triplet-state lifetime significantly higher than that in the solution phase. The monocationic FeTDCPP+ is able to diffuse into the anionic cavities of Nafion, where it works as a catalyst for O2-mediated autooxidation processes that are initiated by the photogenerated hydroperoxides. These processes continue in the dark for many hours giving cyclohex-2-en-1-ol and trans-cyclohexane-1,2-diol monoethyl ether as main oxidation products. The presence of this ether, indirectly, reveals the formation of cyclohexene epoxide which undergoes a nucleophilic attack by ethanol and epoxide opening because of the strong acidic environment inside Nafion. The good photocatalytic efficiency of the oxidation process is demonstrated by an overall quantum yield of 1.1, as well as by a turnover value of 4.7 x 10(3) with respect to the iron porphyrin. When cyclooctene is present as co-substrate, it also undergoes oxygenation. In contrast to what was observed for cyclohexene, cyclooctene epoxide can be accumulated in a significant amount. As far as the stability of the system is concerned, FeTDCPP+ undergoes about 1% degradation during the process, while the Nafion matrix can be utilised several times without observable modification.

Regio- and Stereoselectivity in the Decatungstate Photocatalyzed Alkylation of Alkenes by Alkylcyclohexanes

Tetrabutylammonium decatungstate (WO) photocatalyzes radical alkylation reactions starting directly from alkanes. When using tert-butylcyclohexane and methylcyclohexane as the radical precursors, the addition to electrophilic alkenes (beta,beta-dialkylmethylenemalononitriles, acrylonitrile, methyl acrylate, methyl vinyl ketone) is regioselective and exclusively gives 3- and 4-substituted cyclohexyl adducts, with no significant functionalization of the other positions. Furthermore, when a beta,beta-disubstituted alkene is used, the reaction is stereoselective (cis stereochemistry for the 1,3-cyclohexane derivatives and trans for the 1,4 isomers). Some of the reactions have also been carried out by using benzophenone as the photocatalyst, giving the same product distribution. However, the decatungstate anion is a superior catalyst from a preparative point of view, because it is efficient at low concentrations (0.002 m, 2 mol %) and allows for a simple work up. From a mechanistic point of view, the role of both the alkyl radicals and the radical adducts has been assessed by trapping experiments in the presence of suitable additives (alpha-phenyl-N-tert-butylnitrone, PBN, and 2-methyl-2-nitrosopropane, MNP) and by EPR spectroscopic detection of the resulting nitroxides in solution. Furthermore, trapping by the nitroxide TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine N-oxide) gives O-(tert-butylcyclohexyl)hydroxylamines, again as only the 3- and the 4-substituted isomers. We conclude that the observed regioselective activation originates from the initial hydrogen abstraction step (the statistically corrected ratio for positions 3 and 4 ranges from 1.1 to 1.35 for all of the trapping products). The selectivity is mainly due to steric hinderence.