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The existing literature survey reports rare and conflicting studies on the effect of the preparation method of metal-based semiconductor photocatalysts on structural/morphological features, electronic properties, and kinetics regulating the photocatalytic H2 generation reaction. In this investigation, we compare the different copper/titania-based photocatalysts for H2 generation synthesized via distinct methods (i.e., photodeposition and impregnation). Our study aims to establish a stringent correlation between physicochemical/electronic properties and photocatalytic performances for H2 generation based on material characterization and kinetic modeling of the experimental outcomes. Estimating unknown kinetic parameters, such as charge recombination rate and quantum yield, suggests a mechanism regulating charge carrier lifetime depending on copper distribution on the TiO2 surface. We demonstrate that H2 generation photoefficiency recorded over impregnated CuxOy/TiO2 is related to an even distribution of Cu(0)/Cu(I) on TiO2, and the formation of an Ohmic junction concertedly extended charge carrier lifetime and separation. The outcomes of the kinetic analysis and the related modeling investigation underpin photocatalyst physicochemical and electronic properties. Overall, the present study lays the groundwork for the future design of metal-based semiconductor photocatalysts with high photoefficiencies for H2 evolution.

Currently, palladium represents an expensive and scarce element in the Earth's crust, with the mineral resources of platinum group metals (PGMs) predominately localized in South Africa and Russia. The growing demand for this material necessitates the development of suitable strategies to recover it from end-of-life devices. The recovery of palladium by combining leaching and sacrificial photocatalytic deposition in a low-waste process was proposed here as a possible alternative to the current technologies. The effect of the species involved in the single processes in the recirculated stream was evaluated, where a negligible influence of the sacrificial species (ethanol) in the leaching unit was noticed. Different home-prepared ZnO photocatalysts were tested, and the best performance in terms of recovery of palladium was obtained in the presence of ZnO nanoparticles prepared through sol–gel synthesis. The proposed procedure is a viable route for the recovery of Pd from spent catalysts: by combining the processes, the complete oxidation of palladium was observed in about 15 minutes, while the recovery of Pd through photodeposition on ZnO occurred within 120 minutes of irradiation.

Photocatalytic hydrogen production from alkanol-amines as sacrificial agents by using Cu-modified TiO2–P25 prepared via in situ photodeposition of cupric ions under UV-A light irradiation was investigated. A direct comparison among different sacrificial agents (monoethanolamine, diethanolamine, and triethanolamine) was preliminarily performed. Diethanolamine was selected for further investigation, due to its higher hydrogen production rate with respect to other alkanol-amines. Effect of pH, starting concentration of sacrificial agent, and catalyst/co-catalyst loads were studied. Solution pH exerts major impact on the photoefficiency for hydrogen generation: reaction mechanisms at different pH values were extensively examined. For the first time, a validated kinetic model estimated the unknown rate constants of (i) reaction between diethanolamine and photogenerated holes, (ii) proton reduction, and (iii) diethanolamine adsorption on the photocatalyst surface.

Pd, Rh, Pt are employed in a wide range of applications, such as catalytic converters, fuel cells and electronic devices. In the last years, an increasing pressure on their market was recorded due to a growing demand and limited resources. Therefore, the recovery of these materials from wastes represents an interesting goal to be achieved. The most widely proposed techniques for recovering the palladium from wastes are leaching and ion exchange. Strong oxidizers, acids and high temperature (343–363 K) are used for leaching, leading problems for the environment and the safety. In this work the attention was focused on a system containing zero-valent palladium nanoparticles in which the leaching is performed in mild acidic conditions, by using chloride solutions containing cupric ions (NaCl/CuCl2). The process was studied at varying temperature, pH, chloride and cupric ion concentrations. Good results were obtained at pH 5.0 and temperatures between 288 K and 333 K. The process is more acceptable than the traditional ones from a safety point of view being characterized by less severe conditions (pH and temperatures). A shrinking spherical particles model was adopted to analyse the experimental data from which a development under a kinetic control was demonstrated.

A shortage of market supplies is nowadays recorded for precious metals such as palladium, due to a wide range of applications in which they are used. The development of efficient and inexpensive recovery methodologies of precious metals from waste materials is thus necessary and urgent. In the present study a zinc oxide based photocatalytic system under UV/Visible-light radiation was chosen for palladium (II) deposition from solutions containing it. The effect of ethanol as sacrificial agent was investigated at varying catalyst load, irradiation conditions (UV+visible/visible/dark), metal starting concentration as well as the presence of cupric and chloride ions in solution. Moreover, ZnO dissolution at pH under 6.0 was proposed for the recovery of pure metallic palladium. A network of reactions and a kinetic model were proposed.

The photocatalytic reduction of nitrate to nitrogen and the simultaneous production of hydrogen was investigated over copper loaded titania (P25) nanoparticles using glycerol as sacrificial agent and UV-A/Vis radiation. The experiments demonstrated that total nitrogen (i.e., nitrate, nitrite, and ammonia) removal efficiencies over 93% were achieved for initial nitrate concentrations up to 150 mg/I. A parallel hydrogen production up to 14 mu M was recorded. Hydrogen generation was affected by nitrate reduction. The reaction rates were dependent on pH and both glycerol and nitrate starting concentrations in the mixture. However, a linear model can be used to describe the process for low concentration of nitrate and constant pH. The system showed a remarkable photoefficiency in decontamination of nitrate-containing solutions and concomitant hydrogen evolution. This study lays the foundation for the future development of numerical models able to predict the chemical behavior of simultaneous proton and nitrate reduction over solar photocatalytic reforming of organics. (C) 2016 Elsevier B.V. All rights reserved.

The selective oxidation of ethanol to acetaldehyde and acetic acid in aqueous solution, through sacrificial TiO2 photocatalysis in the presence of cupric ions at ambient temperature, under acidic conditions and artificial sunlight, has been investigated. Different commercial TiO2 photocatalyst samples and loads have been tested. The influence of starting cupric ion and ethanol concentrations has been investigated. Ethanol conversion, overall product selectivity, and product yield have been calculated and compared with the aim of maximizing the recovery of valuable commodity chemicals. For the run corresponding to optimal values of ethanol conversion, overall product selectivity and yields, a value of process photoefficiency equal to 33% has been calculated. Based on the experimental results, the main focus of the study has been the development of a photo-oxidation mechanism and a new kinetic model constituted by a set of mass balance equations for the main species involved in the photocatalytic oxidation process. The model has been employed for analyzing data collected at different starting substrates and cupric ion concentrations. A reliable prediction of the concentrations of cupric ions and organic species during selective photo-oxidation runs has been achieved. The modeling investigation has allowed to estimate for the first time the kinetic parameters not available in the literature review, such as the equilibrium adsorption constants and the rate constants for the hole-capture by sacrificial agents. A new alternative strategy to convert ethanol from biorefinery wastewaters into valuable fine chemicals, by using water as solvent, a low-cost catalyst, solar radiation and ambient temperature, has been proposed and kinetically characterized. The model presented can be easily applied to other photocatalytic systems for the oxidation of organic species over metal doped photocatalysts.

The production of hydrogen by photocatalytic reforming of methanol and glycerol was investigated using metal copper-modified TiO2 nanoparticles, prepared “in situ” by reduction of cupric ions. A modeling investigation was performed through the development of a simplified kinetic model taking into account the mass balance equations for the main reactive species involved in the photocatalytic system. The kinetic model was tested to predict hydrogen generation rates for experimental runs carried out at different initial concentrations of sacrificial agent (methanol and glycerol) and at varying photocatalyst load. The modeling investigation allowed to estimate for the first time the equilibrium adsorption constants and the kinetic constant for the hole-capture by sacrificial agents, as well as the quantum yield and the rate constant of electron-hole recombination for the copper modified-TiO2 nano-photocatalyst. This study provide a reliable approach to model photocatalytic reforming of alcohols over metal modified-TiO2 catalyst for hydrogen generation.

[object OThe photocatalytic removal of nitrate with simultaneous hydrogen generation was demonstrated using zero-valent nano-copper-modified titania (P25) as photocatalyst in the presence of UV-A-Vis radiation. Glycerol, a by-product in biodiesel production, was chosen as a hole scavenger. Under the adopted experimental conditions, a nitrate removal efficiency up to 100% and a simultaneous hydrogen production up to 14 ?mol/L of H2 were achieved (catalyst load = 150 mg/L, initial concentration of nitrate = 50 mg/L, initial concentration of glycerol = 0.8 mol/L). The reaction rates were independent of the starting glycerol concentration. This process allows accomplishing nitrate removal, with the additional benefit of producing hydrogen under artificial UV-A radiation. A kinetic model was also developed and it may represent a benchmark for a detailed understanding of the process kinetics. A set of acute and chronic bioassays (Vibrio fischeri, Raphidocelis subcapitata, and Daphnia magna) was performed to evaluate the potential ecotoxicity of the nitrate/by-product mixture formed during the photocatalytic process. The ecotoxicological assessment indicated an ecotoxic effect of oxidation intermediates and by-products produced during the process. © 2017, Springer-Verlag Berlin Heidelberg.bject]

Hydrogen is the ideal candidate to fulfill the growing energy demand in a sustainable manner becauseof its high energy content and no emission of greenhouse gases from its combustion. Currently most ofhydrogen generation techniques involve the employment of fossil fuels, with consequent production oftoxic greenhouse gases. The possibility to produce hydrogen by means of photocatalytic processes usingthe solar radiation as energy source fits in perfectly with the switch to a more sustainable energy pro-duction. The solar photocatalytic hydrogen generation can be achieved by reforming organic substancescontained in civil or industrial wastewaters. This could allow to combine water decontamination withproduction of an energy carrier starting from a renewable source, the solar radiation.Within this perspective, a novel nano-TiO2photocatalytic system based on the solar reforming of formicacid in presence of cupric ions and chlorides has been investigated. The effect on hydrogen generationrate of the initial concentrations of formic acid, chloride and cupric ion, and pH values has been evaluated.For both formic acid and chloride ions, at least up to a starting concentration of 103mM, the higher theinitial concentration, the higher the rate of hydrogen generation. Hydrogen production has turned outto be noticeably dependent on cupric ion concentration. An almost opposite behavior has been foundvarying the starting cupric ion concentration in the range 2.5–20 mM, with the highest value of hydrogenproduction rate recorded for Cu(II) initial concentration equal to 5.0 mM. The pH value has been identifiedto be a crucial parameter of the system. A decrease in hydrogen production has been also observed risingpH of the solution from 1.0 to 4.0. A characterization of the solid samples recovered at the end of theruns has been performed by X-ray Diffractometry. These experimental outcomes have been rationalizedwithin a consistent reaction mechanism able to predict the system behavior under different operatingconditions. This work opens the way to the development of new competitive processes able to use wasteorganic streams for hydrogen generation through photacatalytic system based on solar energy.© 2015 Elsevier B.V. All rights reserved.

The conversion of pyridinemethanol isomers into their respective aldehydes and carboxylic acids has been attempted in aqueous solution through sacrificial TiO2 photocatalysis in the presence of cupric ions, at ambient temperature, under acidic and deaerated conditions, using artificial sunlight. The presence of a nitrogen atom in the aromatic ring, with respect to benzylic alcohol and its derivatives, affects the photocatalytic behavior of the substrate. The influence of both pH and temperature has also been investigated. The position of the methanolic group on the aromatic ring leads to slight changes in the selectivities, yields and the oxidation rates. Under the adopted conditions, the yields and selectivities to aldehydes are always higher than those of the respective carboxylic acids. The photocatalytic process can be carried out in consecutive copper reduction/oxidation steps in order to reuse the catalyst. In this way greater yields and selectivities to the desired product than 60% can be achieved. The process can be considered interesting as it concerns the eco-green production of valuable fine chemicals, using water as a solvent at ambient conditions, a cheap heterogeneous catalyst and solar radiation.

In the future, solar energy, along with other renewable resources, could play a key role in mass production of fine chemicals. It could also potentially solve environmental problems, as demonstrated by recent developments in the use of solar energy, such as solar photocatalysis. The solar photocatalytic technology has been demonstrated to be effective for: • Treating groundwater, drinking water, industrial wastewater, and air and soil pollution, • Water disinfection, and • Industrial production of fine chemicals. This report summarizes the current status of solar photocatalysis and identifies future opportunities for research and industry in this field, including recent relevant bibliography. The main commercial solar photocatalytic applications are described, included the technologies based on sunlight for antifogging and self-cleaning of coating materials, glass, and concrete. An overview of several different solar photoreactors and the main operating process parameters are also provided. For the estimation of capital costs, it is suggested the use of appropriate “figures of merit”. The present review would be of interest for researchers, technologists, engineers, and industrialists.

An intense scientific activity was recorded during the last several years in the field of preparation, characterization and use of copper-based TiO 2 photocatalysts for hydrogen generation through photocatalytic reforming of organics. Different copper species were used dissolved in aqueous solution or incorporated on the TiO 2 surface as single co-catalyst or in the presence of a second catalyst (e.g., graphene, carbon fibers) to (1) effectively separate the electron–hole pairs, thus reducing the occurrence of the recombination reaction, and (2) extend the light absorption to the visible range of the solar spectrum. Many organic species (e.g., methanol, glycerol, formic acid) were proposed as sacrificial agents for hydrogen generation, although the prevailing idea is that of using organic compounds currently found in industrial wastewaters. The pH value was recognized as a fundamental variable in photocatalytic H 2 generation via copper modified-TiO 2 catalysts. A positive effect to promote hydrogen generation was associated to an increase in pH until moderate alkaline values. On the other hand, a release in the solution of cupric ions and a consequent decrease in photocatalytic activity were observed when decreasing pH. A relevant lack of information was recorded about the efficiencies of hydrogen generation which were reported only in few papers. Therefore, this critical literature review has been performed with the aim of providing a complete background to select the most efficient approaches and eventually promote new competitive systems for hydrogen generation via photoreforming for industrial applications.

The photoreduction of Cu(II) in presence of an organic species as sacrificial agent has been investigated by many research groups but some controversial results are reported in literature. In this work, the photoreduction process of Cu(II) in presence of formic acid is investigated aiming at clarifying the reaction mechanism and assess the reaction kinetics. The effect of operating conditions such as TiO 2 load, initial concentration of sacrificial agent, pH and ionic strength was studied. The results of the work are: (1) the system reactivity does not depend neither on pH (in the range 2.0–3.5) nor on ionic strength of the solution; (2) formic acid is adsorbed on TiO 2 surface; (3) the nature of anionic species can influence the Cu(II) photoreduction (e.g. the addition of sulphate or phosphates reduces the reaction rates); and (4) FA can be completely mineralized during this photoxidation process; for example, this is observed for a TiO 2 load of 0.1 g/L after a reaction time of about 240 min. A simplified kinetic model is developed to describe the behavior of the system and validated by analysing the data collected during the experimental work.

The abatement of ferulic acid (FA), a polyphenolic constituent of olive mill wastewater, is studied in the pH range 5.0-7.0 by using hydrogen peroxide and an amorphous iron oxide as catalyst. The effect of pH, catalyst load, hydrogen peroxide and substrate starting concentrations is assessed during the investigation. A suitable reaction scheme is developed and used to build a mathematical model which satisfactorily describes the system's behavior. Kinetic constants for the proposed scheme as well as the total active site concentration of the catalyst in the studied pH range are estimated. The occurrence of internal mass-transfer limitation for the adopted granulometric fraction of the catalyst is demonstrated. © 2007 Elsevier B.V. All rights reserved.