Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod Overview on microfluidic reactors in photocatalysis: Applications of graphene derivatives Ahmed Yusuf, Corrado Garlisi ⁎ , Giovanni Palmisano Department of Chemical Engineering, Khalifa University of Science and Technology, Masdar City, P.O. Box 54224, Abu Dhabi, United Arab Emirates ARTICLE INFO Keywords: Microreactors Photocatalysis Graphene derivatives Microfluidics Scale-up ABSTRACT Microreactor technology represents one of the many methods for process intensification. Microreactors (MRs) truly inherit their tremendous properties from the dimensions of the reaction environment and their advantages cannot be overemphasized. They have remarkable heat and mass transfer rates, short molecular diffusion dis- tance, good laminar flow, and better spatial illumination homogeneity as compared to conventional reactors due to their high surface-to-volume ratio. This review presents state-of-the-art information on the applications of this technology to photocatalytic processes involving the use of graphene (GR) derivatives, either alone or in com- bination with inorganic semiconductor to form nanocomposites. The latter are hybrid photocatalysts leveraging on the remarkable properties of GR, which include high electron mobility, excellent specific surface area, good mechanical and thermal properties. Because of the resulting high photonic efficiency, enhanced interfacial surface area and reduced electron-hole recombination, such catalysts are increasingly studied and tested in photocatalysis. The utilization of GR derivatives to drive advanced oxidation processes within MRs has, thus, the potential to afford a better efficiency and economic feasibility for such devices. However, studies on the ap- plication of MRs to photocatalytic processes involving the use of GR-derived semiconductors are very limited at this stage. It is hoped that this overview will serve as eye-opener for researchers, and create the needed awareness that more work is still needed to be done, in order to be able to actualize and explore the potentials of MRs. Full understanding of this technology would help in going into more details in the modelling of challenging photocatalytic reactions and in gathering enough data that can help increase photocatalytic efficiency and trigger eventual commercialization of the presented technologies. 1. Introduction Multiphase reaction (MPR) platforms constitute liquid-solid, gas-li- quid and gas-solid processes that are the basis of many industrial pro- cesses [1–3]. Heterogeneous photocatalysis (HPC) is a typical example of MPR process with reactant and product in gas and/or liquid phase, being converted in the presence of light (a photon source) over the surface of an appropriate solid photocatalyst, usually an inorganic semiconductor such as titanium oxide (TiO 2 )[4–6]. A recent review by Ozer et al. [7] summarized the applications of heterogeneous photo- catalytic processes based on innovative graphene-inorganic semi- conductor nanocomposites in various areas, which include hydrogen production, pollutant degradation, organic synthesis, CO 2 reduction and electrochemical processes. Such analysis shows, as well as many other studies in this field, how most of the research works carried out hitherto on HPC have focused on development of new and improved photocatalysts with the purpose of making these processes more effi- cient and less expensive, in order to accelerate their commercialization [8–12]. However, most of these processes are not close to being com- mercialized because data on improved and optimized photocatalysts are not enough for their effective scale-up. Major reasoning that was understood from previous studies is that extensive information and data on kinetic modelling, MPR engineering and process development, are the key to successfully scale-up and eventually commercialize hetero- geneous photocatalytic technologies [1]. With conventional reactors, such reactions are substantially mass and/or photon transfer limited: conventional systems like slurry reactors are stymied by low photonic efficiency due to light shading; therefore, immobilization of photo- catalyst is recommended and believed to be more practical. Although the immobilization of photocatalyst is a valid option, limited amount of exposed photocatalyst surface area is a common problem, which mostly leads to mass transfer limitations [13–15]. Process intensification has been one of many ways to control mass transfer limited reactions. In this area, microreactor (MR) technology has been used extensively for studying multiphase reactions; critical reviews of previous work show that MR technology has found https://doi.org/10.1016/j.cattod.2018.05.041 Received 29 November 2017; Received in revised form 19 May 2018; Accepted 21 May 2018 ⁎ Corresponding author. E-mail address: corrado.garlisi@ku.ac.ae (C. Garlisi). Catalysis Today 315 (2018) 79–92 Available online 23 May 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved. T