Application of Metal Oxide Semiconductors in Photocatalysis

Project Information

MOSPhotocat

Grant agreement ID: 793677

DOI

10.3030/793677

Project closed

EC signature date 14 March 2018

Start date 16 July 2018

End date 15 July 2020

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 177 598,80

EU contribution

€ 177 598,80

Coordinated by

TECHNISCHE UNIVERSITEIT EINDHOVEN
Netherlands

Periodic Reporting for period 1 - MOSPhotocat (Application of Metal Oxide Semiconductors in Photocatalysis)

Reporting period: 2018-07-16 to 2020-07-15

Summary of the context and overall objectives of the project

The sunlight can be transformed into energy to drive numerous high value natural and synthetic processes. Without going into further detail, in photosynthesis, plants can convert the sunlight into chemical energy to deliver fuel for the organism's activities. However, in some synthetic systems, the use of highly energetic ultraviolet light may involve the use of specialized glassware or the formation of non-desirable compounds. On the other hand, visible light is the most abundant energy that you can get from the sunlight, however, some systems cannot directly use this lower energetic light per se. Thus, the primary goal of this proposal aims to study the formation of ligand to metal charge transition (LMCT) complexes between simple adsorbates and metal oxide semiconductors (MOS) to enhance its visible light response as well as to study the mechanisms involved in these photocatalytic systems. In a second step, it aims to combine visible light-responsive MOS with photomicroflow systems to exploit the photocatalytic potential of these materials in continuous flow.
The formation of LMCT with simple adsorbates and MOS, enhanced their visible light response and made them very attractive candidates to replace complexes based on toxic and rare ruthenium and iridium transition metals to drive visible light organic transformations. However, more extensive studies are needed to determine the best combination between adsorbates and MOS to trigger valuable photocatalytic applications in the field of synthetic organic chemistry. Finally, we have demonstrated that the exploitation of sustainable sunlight to boost photo-driven organic reactions can be afforded using luminescent solar concentrators based photomicroreactors using molecular photocatalysts. The combination of theoretical and experimental studies disclosed the truly active species involved in the photocatalytic processes where the MOS are present. Overall, we have provided a better understanding of the real species that could be present in this “heterogeneous” photocatalytic process.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Formation and application of LMCT complexes as visible light photocatalysts. The first part of the project consisted in the enhancement of the visible light response of metal oxide semiconductors (MOS) by the adsorption of organic molecules (adsorbates) on their surface (called ligand to metal charge transfer: LMCT-sensitization). For that, a variety of potential adsorbates were combined with MOS under different reaction conditions. After the modification of the MOS with the selected adsorbates, a perceptible change in color was observed indicating that these ligands had a significant influence in the optical absorption of the MOS. Next, we examined the photocatalytic activity of the sensitized-MOS to carry out organic transformations induced by light. Although any improvement of the organic reactions was observed, these hybrid complex still has a great potential to be used as visible light photocatalysts to initiate valuable organic reactions.
Application of LMCT complexes as visible light photocatalysts for organic reactions in a continuous flow. Taylor flow is a gas-liquid flow pattern that consists of elongated bubbles separated by liquid slugs. The solid photocatalyst is suspended in a liquid slug and is transported through the photomicroreactor. In line with the use of MOS in photocatalysis, we exploited its application in continuous flow chemistry by using Taylor flow technology. The atom transfer radical addition (ATRA) was chosen as a model reaction. This classic reaction is an atom economic and effective methodology for the direct functionalization of olefins. The hybrid complex composed by a MOS and an adsorbate was used as the visible light photocatalyst of the chosen reaction. Although a good dispersion of the hybrid complex (LMCT-sensitized MOS) and no clogging phenomena was observed, no conversion in the desired products was obtained.
Energy-Efficient Solar Photochemistry. Luminescent solar concentrators (LSCs) are inexpensive slabs of luminophore-doped polymeric materials that harvest and concentrate solar photons. LSCs can be used as photon harvesters for photochemical reactions in so-called luminescent solar concentrator-photomicroreactors (LSC-PM). This device based on fluorescent dye‐doped polymer collects sunlight, focuses the energy to a narrow wavelength region, and then transports that energy to embedded microchannels where the flowing reactants are converted. To demonstrate the efficiency of the device, several different photochemical reactions were carried out. A medicinally relevant molecule was synthesized using this technique (artemisinin-drug against malaria). Also, with the adoption of a reaction control system, stable product quality could be obtained even during fluctuating irradiance conditions.
Unveiling the catalytic species involved in photocatalytic processes. Identifying the truly catalytically active species in heterogeneous processes is a key step towards the development of a reproducible, scalable, and efficient heterogeneous photocatalytic approach. In this regard, the second part of the project aimed to investigate the species involved in the photocatalytic processes carried out by a MOS. For that, the behavior of the Bi2O3 was studied using the ATRA reaction. By combining theoretical and experimental studies we disclosed that Bi2O3 is not the true photocatalyst of this reaction, but it is the precursor of a soluble photocatalytic species triggering the photoredox process. Moreover, we observed that the presence of certain amines accelerated the reaction fourfold by stabilization of the species formed in the reaction medium.
The exploitation and dissemination of the works mentioned above were conducted in monthly progressing meetings realized by the host group, and an exhibition for high school students in the Open Day organized by the Eindhoven University of Technology (TU/e), social media (Twitter, LinkedIn), non-specialized magazine (National Geographic-Dutch version) national and international conferences (Flow Chemistry Europe international congress, RSCPoster Twitter, Photo4future), and open access research publications (Catal. Sci. Technol. 2019, 9, 5186; ACIE 2019, 131, 14512).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During the action, new photocatalysts based on MOS were investigated and developed to carry out valuable organic transformations powered by light. Moreover, the solar light was also exploited as fuel to boost oxidations and cross-coupling reactions by merging continuous flow and leaf inspired-LSC. These achievements are of high interest in modern synthetic organic chemistry and may trigger cleaner and safer chemical processes using the naturally abundant and sustainable solar energy. Our finding may have important implications in the pharmaceutical, food, and agrochemical industry reducing costs and generation of waste, which is one of the most important challenges to be solved by authorities worldwide within the next few years. Most of the work realized during the action was performed in collaboration with interdisciplinary national and international research groups, which demonstrates the interest of the research undertaken.

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