Dimitris I. Kondarides

Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions

Abstract The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO 2 suspensions has been investigated with the use of a solar light simulating source. The photoreaction was followed by monitoring the degradation of the dye and the formation of intermediates and final products, as functions of time of irradiation, both in solution and on the photocatalyst surface. It has been found that the dye adsorbs on TiO 2 and undergoes a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids. These molecules are further oxidized toward compounds of progressively lower molecular weight and, eventually, to CO 2 and inorganic ions, such as sulfate, nitrate and ammonium ions. A TiO 2 -mediated photodegradation mechanism for Acid Orange 7 is proposed on the basis of quantitative and qualitative detection of intermediate compounds.

Production of hydrogen for fuel cells by reformation of biomass-derived ethanol

The reformation of biomass-derived ethanol to a hydrogen-rich gas stream suitable for feeding fuel cells is investigated as an efficient and environmentally friendly process for the production of electricity for mobile and stationary applications. Steam reforming of ethanol is investigated over Ni catalysts supported on La2O3, Al2O3, YSZ and MgO. The influence of several parameters on the catalytic activity and selectivity is examined including reaction temperature, water-to-ethanol ratio and space velocity. Results reveal that the Ni/La2O3 catalyst exhibits high activity and selectivity toward hydrogen production and, most important, long term stability for steam reforming of ethanol. The enhanced stability of this catalyst may be due to scavenging of coke deposition on the Ni surface by lanthanum oxycarbonate species which exist on top of the Ni particles under reaction conditions.

Solar light-responsive Pt/CdS/TiO2 photocatalysts for hydrogen production and simultaneous degradation of inorganic or organic sacrificial agents in wastewater.

Photocatalytic degradation of waste material in aqueous solutions and simultaneous production of hydrogen was studied with the double purpose of environmental remediation and renewable energy production. Both powdered and immobilized Pt/CdS/TiO(2) photocatalysts were used to oxidize model inorganic (S(2-)/SO(3)(2-)) and organic (ethanol) sacrificial agents/pollutants in water. Powdered Pt/CdS/TiO(2) photocatalysts of variable CdS content (0-100%) were synthesized by precipitation of CdS nanoparticles on TiO(2) (Degussa P25) followed by deposition of Pt (0.5 wt %) and were characterized with BET, XRD, and DRS. Immobilized photocatalysts were deposited either on plain glass slides or on transparent conductive fluorine-doped SnO(2) electrodes. The results show that it is possible to produce hydrogen efficiently (20% quantum efficiency at 470 nm) by using simulated solar light and by photocatalytically consuming either inorganic or organic substances. CdS-rich photocatalysts are more efficient for the photodegradation of inorganics, while TiO(2)-rich materials are more effective for the photodegradation of organic substances.

The effect of operational parameters and TiO2-doping on the photocatalytic degradation of azo-dyes

Abstract The photocatalytic degradation of Acid Orange 7 (AO7), a non-biodegradable azo-dye, has been investigated over TiO2 photocatalysts irradiated with a light source simulating solar light. The effect of operational parameters, i.e., dye concentration, photocatalyst content, pH of the solution and incident light energy on the degradation rate of aqueous solutions of AO7 has been examined. The effect of incorporating cations with valence higher (W6+) and lower (Ca2+) than the parent cation (Ti4+) in the TiO2 matrix has also been investigated. Results show that the employment of efficient photocatalysts and the selection of optimal operational parameters may lead to complete decolorization and to substantial decrease of the Chemical Oxygen Demand (COD) of the dye solutions.

Mechanistic Aspects of the Ethanol Steam Reforming Reaction for Hydrogen Production on Pt, Ni, and PtNi Catalysts Supported on γ-Al2O3†

Mechanistic aspects of ethanol steam reforming on Pt, Ni, and PtNi catalysts supported on gamma-Al(2)O(3) are investigated from the analysis of adsorbed species and gas phase products formed on catalysts during temperature-programmed desorption of ethanol and during ethanol steam reforming reaction. DRIFTS-MS analyses of ethanol decomposition and ethanol steam reforming reactions show that PtNi and Ni catalysts are more stable than the Pt monometallic counterpart. Ethanol TPD results on Ni, Pt, and NiPt catalysts point to ethanol dehydrogenation and acetaldehyde decomposition as the first reaction pathways of ethanol steam reforming over the studied catalysts. The active sites responsible for the acetaldehyde decomposition are easily deactivated in the first minutes on-stream by carbon deposits. For Ni and PtNi catalysts, a second reaction pathway, consisting in the decomposition of acetate intermediates formed over the surface of alumina support, becomes the main reaction pathway operating in steam reforming of ethanol once the acetaldehyde decomposition pathway is deactivated. Taking into account the differences observed in the mechanism of ethanol decomposition, the better stability observed for PtNi catalyst is proposed to be related with a cooperative effect between Pt and Ni activities together with the enhanced ability of Ni to gasify the methyl groups formed by decomposition of acetate species. On the contrary, monometallic catalysts are believed to dehydrogenate these methyl groups forming coke that leads to deactivation of metal particles.

Steam reforming of biomass-derived ethanol for the production of hydrogen for fuel cell applications

Ni/La2O3 catalyst exhibits high activity and good long term stability for steam reforming of ethanol to hydrogen production and is a good candidate for ethanol reforming processors for fuel cell applications.

Mechanistic study of the reduction of NO by C3H6 in the presence of oxygen over Rh/TiO2 catalysts

The mechanism of selective reduction of NO by propylene has been examined over Rh/TiO2 catalysts, in the absence and presence of oxygen in the feed, employing FT-IR and transient-MS techniques. It has been found that no direct interaction between NO and propylene is required for the reaction to proceed. Reduction of NO requires the presence of reduced rhodium sites and the role of the hydrocarbon is to remove adsorbed oxygen and restore the catalytically active sites. Oxidation of propylene and removal of atomic oxygen occurs via intermediate formation of acrolein and acrylate species, which eventually decompose to yield CO and carboxylates on the catalyst surface. The mechanism of activation of propylene does not depend on the nature of the oxidant (NO or O2) and is the same in the absence and presence of oxygen in the feed.

Mechanistic and kinetic study of solar-light induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions

The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO2 suspensions has been investigated with the use of a solar light simulating source. It has been found that, when the full range of emitted photons is used, decolorization and complete mineralization of the solution is achieved with satisfactory rates, depending on initial dye concentration. A reaction pathway is proposed according to which degradation of the dye molecules adsorbed on the photocatalyst surface takes place via a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids, which are further oxidized toward compounds of progressively lower molecular weight. Eventually, mineralization is achieved leading to the formation of gas phase CO2 and inorganic ions in solution. Kinetic results show that the initial rate of decolorization depends strongly on surface coverage and on incident photon energy. Visible light photons contribute to decolorization, via the photosensitization mechanism, but with reaction rates which are more than two orders of magnitude lower than the corresponding ones induced by photons of energy higher than that of the band gap of the semiconductor. In both cases, the initial rate of decolorization is significantly reduced for dye coverages close to monolayer.

Photodegradation of ethyl paraben using simulated solar radiation and Ag3PO4 photocatalyst

In this work, the solar light-induced photocatalytic degradation of ethyl paraben (EP), a representative of the parabens family, was studied using silver orthophosphate, a relatively new photocatalytic material. The catalyst was synthesized by a precipitation method and had a primary crystallite size of ca 70nm, specific surface area of 1.4m2/g and a bandgap of 2.4eV. A factorial design methodology was implemented to evaluate the importance of EP concentration (500-1500μg/L), catalyst concentration (100-500mg/L), reaction time (4-30min), water matrix (pure water or 10mg/L humic acid) and initial solution pH (3-9) on EP removal. All individual effects but solution pH were statistically significant and so were the second-order interactions of EP concentration with reaction time or catalyst concentration. The water matrix effect was negative (all other effects were positive) signifying the role of humic acid as scavenger of the oxidant species. Liquid chromatography-time of flight mass spectrometry revealed the formation of methyl paraben, 4-hydroxybenzoic acid, benzoic acid and phenol as primary transformation by-products; these are formed through dealkylation and decarboxylation reactions initiated primarily by the photogenerated holes. Estrogenicity assays showed that methyl paraben was more estrogenic than EP; however, parabens are slightly estrogenic compared to 17β-estradiol.

Photocatalysis and photoelectrocatalysis using nanocrystalline titania alone or combined with Pt, RuO2 or NiO co-catalysts

Photocatalytic mineralization of ethanol in the presence of oxygen has been studied in aqueous photocatalyst suspensions by employing either pure nanocrystalline titania or TiO2 combined with Pt, RuO2 or NiO co-catalysts. Combined photocatalysts demonstrated a diverse behavior. Highest mineralization rates were obtained with Pt/TiO2 and lowest with RuO2/TiO2 and NiO/TiO2. These results were related with the photocatalysts’ behavior when used as photoanodes for the production of electricity in a photoactivated fuel cell running with ethanol as fuel. The highest current was obtained with pure titania. The current dropped in the case of Pt/TiO2 and became much lower in the case of RuO2/TiO2 and NiO/TiO2 photoanodes. Both current and voltage were lower in the presence of oxygen than in its absence. It is concluded that the presence of electron scavengers, like O2, and/or the use of efficient photocatalysts, like titania-supported Pt, yield less electric power but assist ethanol mineralization process.