Enrico Borgarello

Visible light induced generation of hydrogen from H2S in mixed semiconductor dispersions; improved efficiency through inter-particle electron transfer

Electron transfer from the conduction band of CdS to that of TiO2 particles occurs in alkaline suspensions containing SH– ions and is exploited to improve the performance of a system that decomposes H2S with visible light.

Photocatalytic degradation of polychlorinated dioxins and polychlorinated biphenyls in aqueous suspensions of semiconductors irradiated with simulated solar light

Abstract The complete photocatalytic degradation of two polychlorinated dibenzo-p-dioxins and one polychlorinated biphenyl to CO 2 and HC1 has been investigated in aerated aqueous suspensions of several semiconductors (TiO 2 , ZnO, CdS, TiO 2 Pt 5% by weight, and Fe 2 O 3 ) irradiated by simulated solar light. The particular compounds were the 2-chlorodibenzo-p-dioxin (CDD), the 2,7-dichlorodibenzo-p-dioxin (DCDD), and the 3,3′-dichlorobiphenyl (DCB). TiO 2 shows the highest photo-activity for the degradation process studied; platinization seems to have little effect. The presence of conduction band electron scavengers such as Ag + and Fe 3+ ions increase the activity of TiO 2 .

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.

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!

Electron transfer and dimerization of viologen radicals on colloidal TiO2

The reaction of several viologen radical cations with colloidal TiO2 particles of 70 A radius has been studied in detail. The electron-transfer reaction from methyl viologen radicals (MV+) to the TiO2 particles is controlled by the rate of the heterogeneous electron-transfer step. Protonation of the reduced particle follows the electron-transfer reaction in a temporally well separated reaction. Analysis of the subsequent equilibrium stage, but before protonation occurs, allows an estimate of the charge carrier density in the colloid. The effect of added Pt, either as a separate colloid or by photodeposition on the TiO2 particle, has also been studied. Kinetic analysis indicates that a mixture of Pt and TiO2 colloids results in adsorption of the Pt particle on the TiO2 colloid. However, the adsorbed Pt colloid reacts independently of the TiO2 particle.Heptyl viologen radicals (HV+) catalytically dimerize in the presence of TiO2 particles in a heterogeneous reaction between an absorbed HV+ and a dissolved radical. The competition between dimerization and hydrogen evolution can be directed towards the latter reaction either by reducing the pH or by loading the TiO2 particles with Pt. The unsymmetric C14MV+ viologen radicals aggregate even in the absence of any colloid. At low pH values and high Pt loadings they can, however, be directed towards hydrogen evolution.

Dioxygen evolution from inorganic systems. Reactions and catalytic properties of loaded TiO2 particles in photochemical dioxygen generation

Abstract The efficiency of differently prepared TiO 2 particles in photochemical water splitting through band gap irradiation of aqueous suspensions has been investigated. The effect of pH and loading with noble metals and RuO 2 has been examined. Particular attention has been devoted to dioxygen evolution and photoadsorption.

A photoconductivity study of electron transfer between CdS and TiO2 powders in vacuum and in an O2 atmosphere

Photoconductance measurements have been carried out in the visible and/or ultraviolet spectral region, under vacuum or under various O2 pressures, at room temperature for the powders CdS, TiO2, mixtures of CdS with a small amount of TiO2(or SiO2 for comparison) and a CdS–TiO2 double pellet. Under vacuum, regardless of the spectral range, the effects of TiO2 on the CdS photoconductance or conductivity after illumination are interpreted by electron transfer from TiO2 to CdS. In the presence of O2, this transfer is limited by oxygen species adsorbed on TiO2, which deplete this material of its free electrons (pressure dependence) and which can also capture free electrons of CdS to an extent which depends on the number and areas of the interfaces between particles of the two semiconductors.

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.

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).