Semiconductor Type A Photocatalysis: Role of Substrate Adsorption and the Nature of Photoreactive Surface Sites in Zinc Sulfide Catalyzed C-C Coupling Reactions

Abstract

The mechanism of the photodehydrodimerization of 2,5-dihydrofuran (2,5-DHF) by suspended zinc sulfide powders was investigated through emission, inhibition, adsorption, and quantum yield studies. Zinc and cadmium ions influenced the emission intensity only marginally but strongly inhibited the reaction, so the photoreactive surface sites were not identical with the emitting states, which had lifetimes in the 0.1 ± 24 ns range. Adsorption isotherms for these metal ions and 2,5-DHF in aqueous solution indicated the presence of mono-and multilayer adsorption. Zn 2 and Cd 2 were both adsorbed physically and by metal sulfide precipitatation, but in the case of Cd 2 a lattice exchange mechanism with ZnS afforded CdS and dissolved zinc ions also. 13C NMR spectra and the good agreement between the calculated number of zinc sites and the measured amount of 2,5-DHF in the saturated solvent ± solute surface monolayer indicated that 2,5-DHF was adsorbed perpendicular to the surface at all the available zinc sites. The true adsorption constant (170 AE 30 L mol À1 ) was consistent with the pseudo-constant (260 AE 50 L mol À1) obtained from the concentration dependence of the reaction rate. 13 C NMR signals of the a-carbon atoms of 2,5-DHF and THF adsorbed onto ZnS from the gas phase were downfield shifted by 1.5 and 0.7 ppm as compared to those of the free substrates, respectively. The downfield shift of the olefinic signals was about 0.4 ppm. It is postulated that a dissociative electron transfer from adsorbed 2,5-DHF to the reactive hole afforded a proton and the dihydrofuryl radical. The corresponding C ± H bond dissociation energies were calculated by ab initio methods for various substrates. As expected, the apparent quantum yield of various substrates increased linearly with decreasing C ± H bond dissociation energy. The intermediate dihydrofuryl radical dimerized to the products in the adsorbed state, as indicated by the linear increase of the square root of the reaction rate with increasing 2,5-DHF surface concentration and by competition experiments with THF/2,5-DHF mixtures. The reaction inhibition by Zn 2 and Cd 2 could be analyzed in terms of the Stern ± Volmer model only when the surface concentration was considered. Inhibition by Cd 2 is about three times faster than inhibition by Zn 2 , in which case no zinc metal was observed.

Photocatalytic Carboxylation of Organic Substrates with Carbon Dioxide at Zinc Sulfide with Deposited Ruthenium Nanoparticles

“…[22][23][24] The photocatalytic carboxylation of organic compounds under mild conditions extends this list. [28] A different mechanism of CO 2 photocoupling was reported by Yanagida and co-workers, who suggested that cadmium sulfide would promote the intermolecular addition of CO 2 to a C=O bond. This mechanism is characteristic of type B photocatalysis, which involves the coupling of two radicals, in contrast to type A photocatalysis, which is characterised by the formation of various reduction and oxidation products.…”

“…[28] In contrast, zinc sulfide, which is a wide band gap (3.6 eV) semiconductor, offers particularly strong reduction potentials, as low as À1.8 to À2.0 V versus SHE, and its activity in photocatalytic water splitting and carbon dioxide reduction has been demonstrated. The commonly used TiO 2 photocatalyst does not enable this process because the bottom of its conduction band is located at about À0.3 to 0.6 V versus SHE, depending on the sample and pH conditions.…”

Solar energy utilization in the direct photocarboxylation of 2,3-dihydrofuran using CO2

“…[22][23][24][25] Since the radical is stabilized by its two resonance structures, its lifetime is sufficient to enable bimolecular reactions. This reaction proceeds through the formation of allylic radicals (hydrogen abstraction from 2,3-DHF), identical to that formed in the reaction of 2,5-DHF oxidation by valence band holes generated upon irradiation of ZnS and CdS.…”

Synthesen durch Halbleiter-Photokatalyse: Solare Chemie

“…Dies gilt vor allem auch für kolloidale Halbleiter, die sich zwar auf Grund ihrer pseudohomogenen Natur konventionell untersuchen lassen, aber als Photokatalysatoren für Synthesen zu labil waren [16]. Wie schon erwähnt, verhindert in den meisten Fällen die Photokorrosion des Halbleiters einen Einsatz in der chemischen Synthese.…”

“…Da die Reaktionsgeschwindigkeit mit dem Quadrat der Konzentration an adsorbiertem 2,5-DHF (n eq ) ansteigt, kann man ferner auf eine C-C-Kupplung zwischen adsorbierten Radikalen schließen, weil die Konzentration der Dihydrofurylradikale direkt proportional zu n eq sein sollte [16]. Dies entspricht dem klassischen Fall der heterogenen Dimerisierung nach einem Langmuir-Hinshelwood-Mechanismus.…”