Christopher O'Rourke

Titania-Promoted Carboxylic Acid Alkylations of Alkenes and Cascade Addition–Cyclizations

Photochemical reactions employing TiO2 and carboxylic acids under dry anaerobic conditions led to several types of C–C bond-forming processes with electron-deficient alkenes. The efficiency of alkylation varied appreciably with substituents in the carboxylic acids. The reactions of aryloxyacetic acids with maleimides resulted in a cascade process in which a pyrrolochromene derivative accompanied the alkylated succinimide. The selectivity for one or other of these products could be tuned to some extent by employing the photoredox catalyst under different conditions. Aryloxyacetic acids adapted for intramolecular ring closures by inclusion of 2-alkenyl, 2-aryl, or 2-oximinyl functionality reacted rather poorly. Profiles of reactant consumption and product formation for these systems were obtained by an in situ NMR monitoring technique. An array of different catalyst forms were tested for efficiency and ease of use. The proposed mechanism, involving hole capture at the TiO2 surface by the carboxylates followed by CO2 loss, was supported by EPR spectroscopic evidence of the intermediates. Deuterium labeling indicated that the titania likely donates protons from surface hydroxyl groups as well as supplying electrons and holes, thus acting as both a catalyst and a reaction partner.

Assessment of the Activity of Photocatalytic Paint Using a Simple Smart Ink Designed for High Activity Surfaces

The use of an acid violet 7 (AV7) smart ink to assess the activity of photocatalytic paint is demonstrated. A linear correlation is established between the change in oxidized dye concentration, as measured by diffuse reflectance, and the change in the green component of the RGB color values, obtained using a portable hand-held scanner, suggesting that such tests can be monitored easily using an inexpensive piece of hand-held office equipment, as opposed to an expensive lab-based instrument, such as a diffuse reflectance UV/vis spectrophotometer. The bleaching of the AV7 follows first order kinetics, at a rate that is linearly dependent upon the UVA irradiance (0.30-3.26 mW cm(-2)). A comparison of relative rate of bleaching of the AV7 ink with the relative rate of removal of NOx, as determined using the ISO test (ISO 22197-1:2007), established a linear relationship between the two sets of results and the relevance of this correlation is discussed briefly.

Absorption and Destruction of Methylene Blue by Semiconductor Photocatalysis

Abstract The observed adsorption of methylene blue, MB+, on P25 titania over a range of pH values (pH 2–11) is reported and the results fitted to a charge distribution, multisite complexation, i.e. CD-MUSIC, model, modified for both monomer and aggregated dye adsorption and (for aggregates) co-adsorption of hydroxyl ions. The rate of bleaching of MB+, photocatalysed by titania using UV light, increases over the pH range 4–11 in much the same way the amount of adsorbed MB+, [MB+]ads, increases, indicating that the rate-determining step involves direct hole oxidation of adsorbed MB+. In contrast, at pH 2, the rate of UVA dye photobleaching appears unduly high but is due to an additional reductive, as well as the usual oxidative, bleaching processes. The initial rate of bleaching of MB+ via a visible light-driven, dye-sensitised process, measured at different pHs correlates with the associated CD-MUSIC-calculated values of [MB+]ads, indicating that only dye molecules that are in direct contact with the surface can undergo photodegradation via a dye-sensitised route.

Periodate – an alternative oxidant for testing potential water oxidation catalysts

The redox catalyst ruthenium dioxide, prepared via the Adams technique, i.e. Ru(Adams), is used as a water oxidation catalyst using the oxidants (i) Ce(IV) in 0.5 M H2SO4 and (ii) periodate in 0.5 M H2SO4, water and 0.1 M KOH. Like Ce(IV), periodate is a very strong oxidant that is able to oxidise water to oxygen and can be readily monitored spectrophotometrically at 280 nm, compared with 430 nm for Ce(IV). More importantly, unlike Ce(IV), which is unstable towards hydrolysis above pH 1, periodate is stable in acid, water and strong alkali. A spectrophotometric study of the kinetics of periodate reduction, and concomitant oxidation of water to O2, reveals that in the presence of a suitable redox catalyst, Ru(Adams) in this work, periodate is able to effect the stoichiometric oxidation of water, with a turnover number > 48. In just water, the kinetics of the latter reaction appear diffusion-controlled, due to the large thermodynamic driving force, a measure of which is the difference in redox potential, i.e. ΔE = 423 mV. As this difference is decreased, ΔE = 396 mV in acid and 290 mV in strong alkali (0.1 M KOH), the kinetics become increasingly activation-controlled and slower. These findings are discussed briefly with regard to the possible use of (i) periodate as an alternative oxidant in the rapid screening of new potential water oxidation catalyst material powders that are stable only under near neutral and/or alkaline conditions, and (ii) Ru(Adams) as a benchmark catalyst.

Catalyst-Free Photoredox Addition-Cyclisations: Exploitation of Natural Synergy between Aryl Acetic Acids and Maleimide

Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build-up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo-excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical-cation reactive intermediates were also characterised in solution by EPR spectroscopy.

In Situ, Simultaneous Irradiation and Monitoring of a Photocatalyzed Organic Oxidation Reaction in a TiO2-Coated NMR Tube.

The semiconductor photocatalyzed (SPC) oxidation of toluene is performed inside an NMR spectrometer and the reaction is monitored simultaneously in situ, using a fiber optic probe/diffuser to provide the UV light to activate the titania photocatalyst coating on the inside of the NMR tube. Such a system has great potential for the simple rapid screening of a wide range of SPC mediated organic reactions.

Light-Driven Generation of Chlorine and Hydrogen from Brine Using Highly Selective Ru/Ti Oxide Redox Catalysts

Ultrafine ruthenium–titanium oxide catalysts were directly produced using a continuous hydrothermal flow synthesis process and assessed as chloride oxidation catalysts. Selectivity towards chlorine (over oxygen) evolution was shown to generally increase with decreasing ruthenium content. The optimum catalyst was then used to make an anode for a light-driven brine-splitting demonstrator device to produce hydrogen and chlorine gases.