Nick Serpone

Effect of CdS preparation on the photo-catalyzed decomposition of hydrogen sulfide in alkaline aqueous media

Band-gap irradiation of CdS dispersions in alkaline aqueous media (pH 14) containing 0.1 M Na2S produces hydrogen and sulfur. The reaction is photo-decomposition of hydrogen sulfide by two quanta of visible light (λ > 400 nm). Various batches of commercially available cadmium sulfide, as well as CdS precipitated from nitrate, sulfate, and chloride solutions at neutral pH, produce different amounts of hydrogen. Electronically pure CdS (puratronic grade) generates almost no hydrogen. By contrast, CdS precipitates prepared in the presence of excess cadmium yield forty times more hydrogen than CdS prepared in the presence of excess sodium sulfide. Differences are rationalized in terms of possible surface modification and/or changes in the active sites by anions present as ‘impurities’ which could affect separation and recombination of the charge carries, eCB− and hVB+, in CdS.

Photosensitization of semiconductors with transition metal complexes - a route to the photoassisted cleavage of water

Abstract Development of artificial (non-biological) devices that achieve fuel generation by visible light is currently an ever growing area of research. Light driven redox reactions on organized assemblies afforded by semiconductor dispersions or colloidal sols provide the impetus. The various strategies used in achieving generation of dihydrogen, H 2 , from the photodissociation of water are discussed. One important point that is made is dye sensitization of wide bandgap semiconductors to improve their spectral response to visible light. In practical terms, we present three examples from our recent work, in which the semiconductor particle surface (TiO 2 ) has been modified by adsorption of various dye molecules and by surface derivatization with ruthenium(II) complexes. In the latter case, we have demonstrated the feasibility of producing both H 2 and O 2 in stoichiometric amounts from the cleavage of water, and this without the need for a sacrificial electron donor.

Photoreduction and Photodegradation of Inorganic Pollutants: II. Selective Reduction and Recovery of Au, Pt, Pd, Rh, Hg, and Pb

Toxic metals in their various oxidation states can neither be biodegraded, nor can they be “chemically decontaminated”. Environmental pollution by toxic metals is a function of the form of the metals and not necessarily a function of their bulk concentration. In recent years much concern has been expressed about the state of our chemical environment. Contamination seems to have no boundaries. Experiments with semiconductor materials, coupled or uncoupled to suitable redox catalysts, have demonstrated the feasibility of carrying out several reactions induced by light. Amongst these reactions is the attractive potential to utilize and develop a technology based on semiconductors that will mediate either the reduction of metals, toxic to the environment or of strategic and economic importance, or the oxidation of organic products that are harmful to the environment (for example, pesticides, herbicides, etc…). This lecture summarizes, albeit not exhaustively, metals pollution and their toxic nature, the traditional technologies employed to detoxify aqueous effluents. Finally, it discusses at some length recent work on a new developing technology, based principally on semiconductor materials (TiO2, WO3, ZnO, among others) irradiated by light of suitable energy. The results seem encouraging!

Photocatalytic cleavage of hydrogen sulfide and organosulfur compounds

Processes are described which are of potential interest in solar energy conversion devices. In particular, reference is made to the photocatalytic cleavage of H2S in alkaline aqueous media and mediated by irradiation of low band-gap (2.4 eV) semiconductor dispersions of CdS. Hydrogen evolution is sustained for longer periods in the presence of SO32- to produce S2O32-. We review here some of our recent work and indicate a possible cyclic system that might prove useful in converting the suns energy to useful fuels (H2) and chemicals (S2O32-). Moreover, this process has the added advantage of attacking yet another problem, namely that of disposing of two significant environmental pollutants, H2S and SO2.

Photocatalysis and solar energy conversion (chemical aspects)

This review presents some of the studies published in the years 2004–2007 focusing in large part on developments in three major areas: (1) photocatalysis with metal oxides, (2) generation of solar hydrogen through photoinduced (real) water splitting, and (3) progress in dye-sensitized solar cells.

Photoreduction and Photodegradation of Inorganic Pollutants: I. Cyanides

Pollution is the contamination of air, land and water with materials that detract from their ability to support the ecosystem or provide some human need. Human activities, industrial processes and agricultural usage of some of the materials are the big contributors to the pollution problems. Cyanide is ubiquitous in nature; however, the increasing use of cyanide in industrial processes (for example in the cyanadation process to extract noble metals from their ores, or the manufacturing of plastics, among others) finds a parallel in the increasing quantities of cyanide dispersed in the environment. Conventional means to dispose of CN- involve oxidation by chlorine/alkaline sol-solutions, chlorination, ozonation, and electrolysis. We discuss here two additional methods for detoxifying cyanide waste waters: (i) transformation of CN- to the less toxic SCN- form, and (ii) total degradation via oxidation with peroxides to give CO2 and NH3 (or N2).

Microwave Chemical and Materials Processing

Microwave dielectric heating has been used for various purposes for well over five decades. This chapter thus introduces historical trends on the use of these electromagnetic waves in not only domestic microwave ovens but also more importantly applied in such fields as microwave chemistry and materials processing, as well as being envisioned in solar power satellite (SPS) systems. Furthermore, the chapter classifies the industrial fields in which microwave heating is used and shows a summary of the application fields of microwave chemistry and materials. In addition, overviews are given of microwaves being used in organic synthesis, polymer synthesis, catalyzed reactions, sintering of ceramics, heating of metals, extraction, and discharge electrodeless lamps. A coffee break talks about the Raytheon Corporation where the microwave ovens were first created.

CHAPTER 9:Interplay Between Physical and Chemical Events in Photoprocesses in Heterogeneous Systems

The principal objectives of this chapter are to demonstrate those interplay phenomena that take place between physical and chemical events in heterogeneous photochemistry and photocatalysis through an examination of some relevant processes starting from the initial photoexcitation of solids to subsequent events occurring on absorption of photons by semiconductor/insulator photoactive materials, followed by competition and interconnection between physical and chemical relaxation of heterogeneous systems. Particular examples of such interplay are presented as a competition between physical and chemical decay of the active state of surface-active centers through charge recombination and chemical interaction, as an effect of catalytic and non-catalytic surface photochemical processes on the photostimulated formation of defects, and the interconnection between photocatalyst activity and selectivity. It is deduced that the creation of new generations of photoactive materials will lead to successful applications if and only if such interplay between physical and chemical processes is fully taken into account.

Photo-assisted Mineralization of the Agrochemical Pesticides Oxamyl and Methomyl and the Herbicides Diphenamid and Asulam

Pesticides of the oximecarbamate type, such as Oxamyl and Methomyl, and the aromatic-bearing herbicides, Diphenamid and Asulam, undergo photo-assisted mineralization nearly quantitatively (ca. 90–100% within ~4 h) in aerated UV-illuminated aqueous TiO2 dispersions. The complex structure of the agrochemicals that bear carbon, nitrogen, and sulfur functions are easily mineralized to CO2, ( {hbox{NH}}_4^{+} ) and ( {hbox{NO}}_3^{-} ) ions, and ( {hbox{SO}}_4^{2 - } ) ions, respectively. Evolution of carbon dioxide has been monitored by gas chromatography and by loss of total organic carbon (TOC). The site and mode of adsorption of these agrochemicals onto the TiO2 particle surface has been inferred from point charge calculations, whereas the position of attack by reactive oxygen species such as ˙OH radicals has been estimated by frontier electron density calculations.