A Holistic Approach to Model the Kinetics of Photocatalytic Reactions

Understanding and modeling kinetics is an essential part of the optimization and implementation of chemical reactions. In the case of photocatalytic reactions this is mostly done one-dimensionally, i.e., only considering the effect of one parameter at the same time. However, as discussed in this study, many of the relevant reaction parameters have mutual interdependencies that call for a holistic multi-dimensional approach to accurately model and understand their influence. Such an approach is described herein, and all the relevant equations given so that researchers can readily implement it to analyze and model their reactions

Ruthenium-modified zinc oxide, a highly active vis-photocatalyst: the nature and reactivity of photoactive centres

We recently reported a highly active photocatalyst, ruthenium-modified zinc oxide, which was found to be able to utilise the red part of the visible light spectrum for photocatalytic reactions [Bloh et al., Environ. Sci. Pollut. Res., 2012, 19, 3688-3695]. However, the origin and mechanism of the observed activity as well as the nature of the photoactive centres are still unknown. Herein, we expand on that by reporting a series of experiments specifically designed to unravel the mechanism of the visible light induced photocatalytic reactions. The absolute potentials of the valence and the conduction band edge are identified by the combined use of electrochemical impedance and UV-vis diffuse reflectance spectroscopy. The conduction band electron and the valence band hole activity are assessed through a novel approach tracing their signature oxidative species, i.e., hydrogen peroxide and hydroxyl radicals, respectively. Oxygen reduction currents are measured at different potentials to investigate the role of molecular oxygen as an electron scavenger as well as the underlying reduction pathways. Additionally, the photocatalytic activity of the samples is verified using another (ISO standard) degradation test, the gas-phase oxidation of nitric oxide. The experimental results reveal that the employed synthetic route yields a unique mixture of ruthenium(VI)-doped zinc oxide and ruthenium(VI) oxide particles with both forms of the ruthenium playing their own independent role in the enhancement of the photocatalytic activity. The ruthenium ions acting as dopants enable a better charge separation as well as the absorption of red light resulting in the direct promotion of electrons from the Ru(VI)-species to the conduction band. Both, the conduction band electrons and the thus formed Ru(VII) subsequently participate in the degradation of the pollutant molecules. The ruthenium dioxide particles, on the other hand, act as catalysts increasing the efficiency of the reaction by improving the oxygen reduction properties of the material.BMBF/HelioClean/03X0069

Toxicological Issues of Nanoparticles Employed in Photocatalysis

A huge amount of different nanomaterials is nowadays on the market used for various specific applications. Some nanomaterials such as TiO2, ZnO as well as several other semiconductors exhibit photocatalytic activity. Hence these materials are used for many applications, e.g., for self-cleaning and antibacterial coatings on different surfaces and for the purification of wastewater where the cleaning can be induced by simple exposure to sunlight. Because of the frequent use of these nanoparticles it is important to investigate the life cycles of these nanostructured materials as well as their environmental impact and their toxicity to animals and humans. This review first gives a short overview about nanotechnology and nanotechnological products as well as about photocatalysis and semiconductors used in this field. We then discuss the need for a new technology named nanotoxicology and the problems occurring when investigating the toxic potential of nanomaterials as well as the life cycle of nanomaterials. Furthermore, we focus on the environmental impact of TiO2 and ZnO nanoparticles including toxic effects to bacteria, water organisms and plants as well as their toxic effects to humans including in vitro and in vivo studies

Kinetics and Optimization of the Photocatalytic Reduction of Nitrobenzene

The photocatalytic reduction of nitrobenzene to aniline in alcoholic solutions appears as an interesting alternative to the classical hydration. However, little is known about the influence of reaction parameters on the kinetics of the reaction which were therefore studied herein. The effects of light intensity, catalyst concentration, initial concentration, and temperature were systematically investigated under more than 50 different conditions and accurately described with an appropriate kinetic model. The results show that the efficiency of the reaction is extremely high and apparent quantum yields of up to 142 % were observed under optimized conditions. Particularly interesting is the fact high efficiencies were also obtained at high reaction rates of up to 74.3 mM h−1. Overall these results demonstrate that heterogeneous photocatalytic reactions can be very efficient and productive at the same time and may therefore present a powerful tool in synthetic organic chemistry

Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NOx abatement

Semiconductor photocatalysis could be an effective means to combat nitrogen oxides (NO(x)) based air pollution through mineralisation of NO(x) to nitrate. However, most of the typically TiO(2)-based catalysts employed show a much higher reactivity towards NO than NO(2), leading to an accumulation of this unwanted and toxic intermediate. By grafting the photocatalyst with small amounts (≤0.1 at%) of isolated iron(iii) ions, the reactivity towards NO(2) is increased by the factor of 9, bringing it up to par with the NO-reactivity and alleviating the problem with intermediate accumulation. Consequently, the observed selectivity of the reaction is dramatically increased from less than 40% to more than 90%. The paper also discusses possible mechanisms for this very beneficial behavior

Completely integrated wirelessly-powered photocatalyst-coated spheres as a novel means to perform heterogeneous photocatalytic reactions

Heterogeneous photocatalytic reactions can be efficiently driven by completely integrated photocatalyst-light emitter units which are wirelessly powered from outside the reaction vessel using resonant inductive coupling. To demonstrate the universal applicability of the concept, three representative photocatalytic reactions, H2O2 production, methylene blue degradation and nitrobenzene reduction to aniline, were investigated

Robust Light Driven Enzymatic Oxyfunctionalization via Immobilization of Unspecific Peroxygenase

Unspecific peroxygenases have attracted interest in synthetic chemistry, especially for the oxidative activation of C−H bonds, as they only require hydrogen peroxide (H2O2) instead of a cofactor. Due to their instability in even small amounts of H2O2, different strategies like enzyme immobilization or in situ H2O2 production have been developed to improve the stability of these enzymes. While most strategies have been studied separately, a combination of photocatalysis with immobilized enzymes was only recently reported. To show the advantages and limiting factors of immobilized enzyme in a photobiocatalytic reaction, a comparison is made between free and immobilized enzymes. Adjustment of critical parameters such as (i) enzyme and substrate concentration, (ii) illumination wavelength and (iii) light intensity results in significantly increased enzyme stabilities of the immobilized variant. Moreover, under optimized conditions a turnover number of 334,500 was reached

Improving the selectivity of photocatalytic NOx abatement through improved O2 reduction pathways using Ti0.909W0.091O2Nx semiconductor nanoparticles : from characterisation to photocatalytic performance

Peer reviewedPostprin

Fate and Reactivity of Peroxides formed over BiVO4 Anodes in Bicarbonate Electrolytes

For the electrochemical and photoelectrochemical synthesis of hydrogen peroxide, aqueous bicarbonate electrolytes have been reported with much higher efficiency compared to other alternatives. It was proposed that this is due to efficient oxidation of the bicarbonate to peroxymonocarbonate (PMC) with subsequently hydrolyzes to hydrogen peroxide. However, as we show herein, PMC forms stable concentrations and does not hydrolyze completely. Due to its much better oxidation kinetics this may influence the sensitivity of the employed peroxide quantification methods. Particularly commercial test strips are susceptible to this, giving rise to uncertainty about the validity of the high peroxide formation efficiency in bicarbonate electrolytes. On the other side, PMC’s superior oxidation kinetics may mean that the resulting solutions are even more potent oxidants than previously suspected