Horst Kisch

Semiconductor Photocatalysis—Mechanistic and Synthetic Aspects

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

Modified, Amorphous Titania—A Hybrid Semiconductor for Detoxification and Current Generation by Visible Light

Amorphous, microporous TiO2 hybrid semiconductors modified with transition metals induce generation of a photocurrent and photocatalytic degradation of the water contaminant 4-chlorophenol through photoinduced charge separation (the postulated mechanism is shown in the picture, Ar=4-ClC6 H4 ). In contrast to the previously known crystalline titania photocatalysts, which are active only when excited with UV light, the amorphous semiconductors modified with platinum, rhodium, and gold chloride enable both processes also with visible light.

Visible light inactivation of bacteria and fungi by modified titanium dioxide.

Visible light induced photocatalytic inactivation of bacteria (Escherichia coli, Staphylococcus aureus, Enterococcus faecalis) and fungi (Candida albicans, Aspergillus niger) was tested. Carbon-doped titanium dioxide and TiO2 modified with platinum(IV) chloride complexes were used as suspension or immobilised at the surface of plastic plates. A biocidal effect was observed under visible light irradiation in the case of E. coli in the presence of both photocatalysts. The platinum(IV) modified titania exhibited a higher inactivation effect, also in the absence of light. The mechanism of visible light induced photoinactivation is briefly discussed. The observed detrimental effect of photocatalysts on various microorganism groups decreases in the order: E. coli > S. aureus approximately E. faecalis>>C. albicans approximately A. niger. This sequence results most probably from differences in cell wall or cell membrane structures in these microorganisms and is not related to the ability of catalase production.

Visible‐Light Detoxification and Charge Generation by Transition Metal Chloride Modified Titania

Amorphous microporous metal oxides of titanium (AMM-Ti) modified with chlorides of PtIV, IrIV, RhIII, AuIII, PdII, CoII, and NiII have been prepared by the sol-gel method and characterized by various surface analytical methods. These hybrid AMM-Ti powders are catalysts for the photodegradation of 4-chlorophenol (4-CP) in aqueous solution when illuminated with visible (lambda > or = 400 or 455 nm) or UV (lambda > or = 335 nm) light. The initial rate depends on the dopant level and is highest at 3.0% Pt in the case of PtIV/AMM-Ti. When employed in a photoelectrochemical cell, the activity spectrum of the photocurrent extends downward to about 600 nm, as does the photodegradation of 4-CP. It is suggested that the metal salt acts as a redox-active chromophore, transmitting the photogenerated charges to the amorphous matrix.

Photosensitization of Crystalline and Amorphous Titanium Dioxide by Platinum(IV) Chloride Surface Complexes

Anatase, rutile, and amorphous titania powders were surface-modified by grinding with PtCl4 and H2[PtCl6]. Only the anatase modification afforded hybrid photocatalysts capable of degradation of 4-chlorophenol (4-CP) with visible light, with sufficient stability towards decomplexation. Grinding with K2[PtCl4] produced materials of only low photocatalytic activity. Most efficient photocatalysts contained up to 2 wt% of PtIV. At higher surface loading the excess fraction of the complex is desorbed into the aqueous solution. Scavenging experiments with benzoic acid and tetranitromethane revealed that hydroxyl radicals are produced by the primary reduction of oxygen by conduction band electrons generated through electron injection from a postulated surface platinum(III) complex. It is proposed that the latter is formed from a charge-transfer ligand-to-metal (CTLM) excited state through homolysis of the Pt-Cl bond. Accordingly. the primary oxidation of 4-CP may occur by adsorbed chlorine atoms, the intermediary existence of which was demonstrated by scavenging experiments with phenol.

Photocatalytic and photoelectrochemical properties of titania–chloroplatinate(IV)

Abstract Rutile (Ald), anatase (TH), and the mixed anatase/rutile powder (P25) were surface modified by chemisorption of H 2 [PtCl 6 ] from aqueous solution. The resulting materials photocatalyzed the degradation and mineralization of 4-chlorophenol with visible light. Ald adsorbed only traces and was inactive, P25 adsorbed 1.1 wt.% and exhibited medium activity, whereas TH adsorbed 4.0% and was six times more active than P25. In neutral water 4.0%Pt(IV)/TH is stable towards thermal and photochemical desorption of platinate, even in the presence of strongly adsorbing fluoride ions. Contrary to this, complete photodesorption occurred in 0.1 M hydrogen chloride solution. It is postulated that adsorption affords a surface tetrachloroplatinate(IV) complex covalently linked to the titania surface through a [Ti]OPt bond. The flatband potential of 4.0%Pt(IV)/TH at pH 7 is determined as −0.28±0.02 V (vs. NHE), which is more anodic by 260 mV as compared with unmodified TH. Solar experiments revealed that 4.0%Pt(IV)/TH is a much better photocatalyst than 1.1%Pt(IV)/P25, P25, and TH. It catalyzed the photodegradation also in diffuse indoor daylight, conditions under which all other tested materials were inactive. Upon UV excitation 4.0%Pt(IV)/TH is even more active than P25.

On the Mechanism of Urea-Induced Titania Modification

The mechanism of surface modification of titania by calcination with urea at 400 degrees C was investigated by substituting urea by its thermal decomposition products. It was found that during the urea-induced process titania acts as a thermal catalyst for the conversion of intermediate isocyanic acid to cyanamide. Trimerization of the latter produces melamine followed by polycondensation to melem- and melon-based poly(aminotri-s-triazine) derivatives. Subsequently, amino groups of the latter finish the process by formation of Ti--N bonds through condensation with the OH-terminated titania surface. When the density of these groups is too low, like in substoichiometric titania, no corresponding modification occurs. The mechanistic role of the polytriazine component depends on its concentration. If present in only a small amount, it acts as a molecular photosensitizer. At higher amounts it forms a crystalline semiconducting organic layer, chemically bound to titania. In this case the system represents a unique example of a covalently coupled inorganic-organic semiconductor photocatalyst. Both types of material exhibit the quasi-Fermi level of electrons slightly anodically shifted relative to that of titania. They are all active in the visible-light mineralization of formic acid, whereas nitrogen-modified titania prepared from ammonia is inactive.

Photoreduction of carbon dioxide catalysed by free and supported zinc and cadmium sulphide powders

Abstract Loading of ZnS onto large-surface-area SiO 2 (340 m 2 g −1 ) affords photocatalysts for the reduction of carbon dioxide to formate using 2,5-dihydrofuran (2,5-DHF) as reducing agent. A 13% coverage gives the most active powders, producing 7 mmol of formate on irradiation with UV light, and a slight excess of zinc ions improves the yield to 10 mmol. This acceleration, together with the observation that no oxalate is formed, suggests that CO 2 is reduced in a two-electron process. The activity decreases at higher and lower coverages, and photocorrosion to Zn(0) is observed when the latter is below 7%. When the SiO 2 surface is modified by aminopropyl groups, increased activity is observed due to their reducing properties. Analogous experiments with CdS at 40% coverage on irradiation with visible light again affords higher yields of formate than unsupported CdS. However, when 2,5-DHF is replaced by sodium sulphite, the supported catalyst reaches only half of the activity exhibited by the unsupported sulphide. When CdS is platinized by 0.5 or 4.3 mol.% Pt, a mixture of CO 2 / KHCO 3 affords, in addition to formate, formaldehyde and methanol in amounts of 0.1 and 0.51 mmol.

Electronic Semiconductor–Support Interaction—A Novel Effect in Semiconductor Photocatalysis

The bandgap of CdS increases with decreasing coverage when CdS is supported on silica (see plot of Ebg against CdS concentration c). Likewise the photocatalytic activity of these heterogeneous photocatalysts increases, as illustrated by the relative rate (vrel ) of an organic addition reaction. CdS (A, vrel =1), CdS-50/SiO2 (B), CdS-30/SiO2 (C), and CdS-12/SiO2 (D).

VISIBLE LIGHT PHOTOCATALYSIS BY A TITANIA TRANSITION METAL COMPLEX

Publisher Summary This chapter discusses visible light photocatalysis by a titania transition metal complex. Hexachloroplatinic acid in solution only induces a stoichiometric visible light photooxidation of the ubiquitous water pollutant 4-chlorophenol (4-CP), whereas, the reaction becomes photocatalytic when the complex is supported onto titania. It was found that the most active catalyst was obtained when the complex was covalently attached to titania. Different from a single crystal electrode, it is more difficult to measure the flatband potential of a semiconductor powder, which is necessary to estimate the absolute positions of the valence and conduction band edges. The degradation of atrazine in general affords cyanuric acid as the final product when the photocatalyst is an unmodified titania material. In the photodegradation with sunlight, a significant difference between the photocatalysts prepared by adsorption from solution and by grinding becomes apparent.