Abstract: For many years, photocatalysis has been proposed as one of the promising techniques to abate environmental pollutants. To improve these reactions it is vital to know the reaction mechanisms of the photocatalytic degradation. This new reactor will make it possible to study the catalytic surface at the moment the reactions occur. By the means of UV LED illumination there is no need of an external UV lamp and thus lowers the cost. The validation of this newly developed reactor is done by investigating the photocatalytic reaction mechanism of nitric oxide (NO) and comparing these findings with those already discussed in literature. From these results, it became clear that the newly developed FTIR in situ reactor allows real time study of photocatalytic degradations.

Abstract: In this study, a new fouling sensor was validated under different conditions of hydraulic (HRT) and sludge retention time (SRT). The MBR-VFM (membrane bioreactor-VITO Fouling Measurement) allows the simultaneous determination of the physically reversible and irreversible fouling potential of a mixed liquor during a single crossflow filtration test. In accordance with the on-line filtration behavior, the measured reversible and irreversible fouling propensities differed significantly between MBRs operated at different combinations of HRT-SRT. Moreover, a significant negative correlation was found between the on-line permeability and the reversible and irreversible fouling propensity measured by the MBR-VFM. This corresponded to observations made on membrane recovery after physical or chemical cleaning actions. Higher reversible and irreversible fouling were observed at lower HRT, presumably as a consequence of increased concentrations of foulants present. The effect of SRT was much smaller and restricted to the reversible fouling component. Possible explanations for the increased fouling at prolonged SRT are the higher sludge and colloid concentration and the smaller floc size. It can be concluded that the MBR-VFM is a useful tool to monitor fluctuations in a mixed liquors (ir)reversible fouling potential and can contribute to a deeper understanding of the occurring fouling phenomena.

Abstract: Photocatalytic reactors for the degradation of gaseous organic pollutants often suffer from major limitations such as small reaction area, sub-optimal irradiation conditions and thus limited reaction rate. In this work, an alternative solution is presented that involves a glass tube coated on the inside with (silvermodified) TiO2 and spiraled around a UVA lamp. First, the spiral reactor is coated from the inside with TiO2 using an experimentally verified procedure that is optimized toward UV light transmission. This procedure is kept as simple as possible and involves a single casting step of a 1 wt% suspension of TiO2 in ethanol through the spiral. This results in a coated tube that absorbs nearly all incident UV light under the experimental conditions used. The optimized coated spiral reactor is then benchmarked to a conventional annular photoreactor of the same outer dimensions and total catalyst loading over a broad range of experimental conditions. Although residence time distribution experiments indicate slightly longer dwelling of molecules in the spiral reactor, no significant difference in by-passing of gas between the spiral reactor and the annular reactor can be claimed. Acetaldehyde degradation efficiency of 100% is obtained with the spiral reactor for a residence time as low as 60 s, whereas the annular reactor could not achieve full degradation even at 1000 s residence time. In a final case study, addition of long-term stable silver nanoparticles, protected by an ultra-thin polymer shell applied via the layer-by-layer (LbL) method, to the spiral reactor coating is shown to double the degradation efficiency and provides an interesting strategy to cope with higher pollutant concentrations without changing the overall dimensions.

Abstract: When studying photocatalysis it is important to consider, beside the chemical approach, the engineering part related to process optimisation. To achieve this a fixed bed photocatalytic set-up consisting of different catalyst placings, in order to vary catalyst distribution, is studied. The use of a fixed quantity of catalyst placed packed or randomly distributed in the reactor, results in an almost double degradation for the distributed catalyst. Applying this knowledge leads to an improved performance with limited use of catalyst. A reactor only half filled with catalyst leads to higher degradation performance compared to a completely filled reactor. Taking into account this simple process optimisation by better distributing the catalyst a more sustainable photocatalytic air purification process is achieved. (C) 2012 Elsevier B.V. All rights reserved.

Abstract: In the field of photocatalytic air purification, the immobilisation of catalyst particles on support surfaces without loss of photon efficiency is an important challenge. Therefore, an immobilisation method involving a one-step suspension coating of pre-synthesised photocatalysts on glass beads was applied. The various benefits are exemplified in the gas phase photodegradation of ethylene. Coating of glass beads is easy, fast, cheap and offers a more efficient alternative to bulk catalyst pellets. Furthermore, this coating procedure allows to use porous, pre-synthesised catalysts to their full potential, as the surface area and morphology of the initial powder is barely altered after coating, in strong contrast to pelletising. With this technique it became possible to study the gas phase photocatalytic activity of commercial titanium dioxide, trititanate nanotubes and mixed phase anatase/trititanate nanotubes in a packed bed reactor towards the degradation of ethylene without changing the catalyst properties. Coating of glass beads with the photocatalyst revealed the superior activity of the as-prepared nanotubes, compared to TiO2 Aerolyst® 7710 in gaseous phase.

Abstract: We present the use of computational fluid dynamics (CFD) for accurately determining the adsorption parameters of acetaldehyde on photocatalytic fiber filter material, integrated in a continuous flow system. Unlike the traditional analytical analysis based on Langmuir adsorption, not only steady-state situations but also transient phenomena can be accounted for. Air displacement effects in the reactor and gas detection cell are investigated and inherently made part of the model. Incorporation of a surface aldol condensation reaction in the CFD analysis further improves the accuracy of the model which enables to extract precise, intrinsic adsorption parameters for situations in which analytical analysis would otherwise fail.

Abstract: Air pollution, especially particulate matter (PM), is an increasingly urgent problem in urban environments, causing both short and long-term health problems, climate interference and aesthetical problems due to building fouling. Photocatalysis has been shown to be a possible solution to that end. In this work two complementary detection methods for photocatalytic soot oxidation are studied and their advantages and disadvantages are discussed. First, a colour-based digital image analysis method is drastically improved towards an accurate, detailed and straightforward detection tool, that enables simultaneous measurement of the degradation of different grades of soot fouling (for instance a shallow soot haze versus condensed soot deposits). In the next part, a second soot oxidation detection method is presented based on in situ FTIR spectroscopy. This method has the additional advantage of providing more insight into the photocatalytic soot degradation process by monitoring both gaseous and adsorbed intermediates as well as reaction products while the reactions are ongoing. As an illustration, the proposed detection strategies were applied on four different commercially available and synthesized photocatalytic materials. The digital image analysis showed that P25 (Evonik) is the fastest photocatalytic soot degrader of all studied materials for both a uniform soot haze as well as concentrated soot spots. Application of the in situ method showed that for all studied materials adsorbed formate-related surface species were formed and that commercially available ZnO nanopowder has the highest specificity towards complete mineralization into CO2. With this we aim to provide a set of complementary experimental tools for the convenient, reliable, realistic and standardised detection of photocatalytic soot degradation.

Abstract: Three upscaled multi-tube photocatalytic reactors designed for integration into HVAC (Heating, Ventilation and Air Conditioning) systems were proposed and evaluated using a CFD modelling approach, with emphasis on the flow, irradiation and concentration distribution in the reactor and hence, photocatalytic performance. Based on the obtained insights, the best reactor design was selected, further characterized and improved by an additional proof of concept study and eventually converted into practice. Subsequently, the scaled-up prototype was experimentally tested according to the CEN-EN-16846-1 standard (2017) for volatile organic compound (VOC) removal by an external scientific research center. The combined modelling and experimental approach used in this work, leads to essential insights into the design and assessment of photocatalytic reactors. Therefore, this study provides an essential step towards the optimization and commercialization of photocatalytic reactors for HVAC applications.

Abstract: Unlike the profound knowledge of the reaction mechanisms occurring in water phase photocatalysis, still fairly little is known on the reaction mechanisms occurring on the catalyst surface when dealing with gaseous pollutants. Unfortunately, there are some differences between both reactions. For one, there are no abundant hydroxyl radicals present in the gas phase, so that possibly other species prove to be important in abating the pollutant. In order to unravel the mysteries of gas phase photocatalytic reaction pathways, in situ techniques must be used to allow the detection and identification of reaction intermediates on a working catalyst. Several techniques were already used in the past, of which Fourier transform infrared spectroscopy seems to be the most versatile. This review will therefore give a selective overview of different spectroscopic reaction cells constructed for the in situ study of photocatalytic gas phase reactions.

Abstract: We present a novel manufacture route for silicatitania photocatalysts using the diatom microalga Pinnularia sp. Diatoms self-assemble into porous silica cell walls, called frustules, with periodic micro-, meso- and macroscale features. This unique hierarchical porous structure of the diatom frustule is used as a biotemplate to incorporate titania by a solgel methodology. Important material characteristics of the modified diatom frustules under study are morphology, crystallinity, surface area, pore size and optical properties. The produced biosilicatitania material is evaluated towards photocatalytic activity for NOx abatement under UV radiation. This research is the first step to obtain sustainable, well-immobilised silicatitania photocatalysts using diatoms.

Abstract: For the synthesis of gold-silver bimetallic nanoparticles, the Turkevich method has been the state-of-the-art method for several decades. It has been presumed that this procedure results in a homogeneous alloy, although this has been debatable for many years. In this work, it is shown that neither a full alloy, nor a perfect core-shell particle is formed but rather a core-shell-like particle with altering metal composition along the radial direction. In-depth wet-chemical experiments are performed in combination with advanced transmission electron microscopy, including EDX tomography, and Finite Element Method modeling to support the observations. From the electron tomography results, the core-shell structure could be clearly visualized and the spatial distribution of gold and silver atoms could be quantified. Theoretical simulations are performed to demonstrate that even though UV-Vis spectra show only one plasmon band, this still originates from core-shell type structures. The simulations also indicate that the core-shell morphology does not so much affect the location of the plasmon band, but mainly results in significant band broadening. Wet-chemistry experiments provide the evidence that the synthesis pathway starts with gold enriched alloy cores, and later on in the synthesis mainly silver is incorporated to end up with a silver enriched alloy shell.

Abstract: Carbon dioxide conversion and utilization has gained significant interest over the years. A novel gas conversion technique with great potential in this area is plasma technology. A lot of research has already been performed, but mostly on pure gases. In reality, N2 will always be an important impurity in effluent
gases. Therefore, we performed an extensive combined experimental and computational study on the effect of N2 in the range of 1–98% on CO2 splitting in dielectric barrier discharge (DBD) plasma. The presence of up to 50% N2 in the mixture barely influences the effective (or overall) CO2 conversion and energy efficiency, because the N2 metastable molecules enhance the absolute CO2 conversion, and this compensates for the lower CO2 fraction in the mixture. Higher N2 fractions, however, cause a drop in the CO2 conversion and energy efficiency. Moreover, in the entire CO2/N2 mixing ratio, several harmful compounds, i.e., N2O and NOx compounds, are produced in the range of several 100 ppm. The reaction pathways for the formation of these compounds are explained based on a kinetic analysis, which allows proposing solutions on how to prevent the formation of these harmful compounds.

Abstract: Hierarchical MoS2@TiO2 heterojunctions were synthesized through a one-step hydrothermal method by using protonic titanate nanosheets as the precursor. The TiO2 nanosheets prevent the aggregation of MoS2 and promote the carrier transfer efficiency, and thus enhance the photocatalytic and electrocatalytic activity of the nanostructured MoS2. The obtained MoS2@TiO2 has significantly enhanced photocatalytic activity in the degradation of rhodamineB (over 5.2times compared with pure MoS2) and acetone (over 2.8times compared with pure MoS2). MoS2@TiO2 is also beneficial for electrocatalytic hydrogen evolution (26times compared with pure MoS2, based on the cathodic current density). This work offers a promising way to prevent the self-aggregation of MoS2 and provides a new insight for the design of heterojunctions for materials with lattice mismatches.