Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod Selective removal of organic and inorganic air pollutants by adjusting the g- C 3 N 4 /TiO 2 ratio Ilias Papailias a,c , Nadia Todorova a , Tatiana Giannakopoulou a , Dana Dvoranová b, *, Vlasta Brezová b , Dimitra Dimotikali c , Christos Trapalis a, * a Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Athens, 15341, Greece b Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Bratislava, SK-812 37, Slovakia c Department of Chemical Engineering, National Technical University of Athens, Athens, 15780, Greece ARTICLEINFO Keywords: g-C 3 N 4 TiO 2 EPR spin trapping NO x Acetaldehyde ABSTRACT During the recent years, g-C 3 N 4 /TiO 2 composites have been regarded as efficient photocatalysts for the removal of air pollutants. The combination of these materials gives excellent photocatalytic performances, as each ones advantage covers the disadvantages of the other. In the present study, g-C 3 N 4 /TiO 2 heterostructures were synthesized by mixing chemically exfoliated g-C 3 N 4 with commercial TiO 2 P25 in different ratios and were examined for the removal of NO x and acetaldehyde under visible-light irradiation. The results of photocatalytic tests in correlation with the EPR experiments revealed that heterostructures containing higher amount of TiO 2 are efficient in systems where the attack of hydroxyl radicals prevails (acetaldehyde), while dominant g-C 3 N 4 content provides effectiveness in systems where superoxide radical anions play the key role (NO x ). This outcome is directly coupled with the energy positions of conduction- and valence-band edges of each material. 1. Introduction The severe effect of air pollution on human health and environment has been a matter of increasing importance during the last decades [1]. The presence of gaseous pollutants in the atmosphere has been identi- fied as a significant air pollution factor and is considered to be the cause of numerous health and environmental issues. Among the pollutants, nitrogen oxides (NO x ) and volatile organic compounds (VOCs) like acetaldehyde are recognized as two of the most dangerous groups of outdoor and indoor contaminants. The term NO x normally refers to the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO 2 )[2]. The accumulation of NO x , especially in urban areas, contributes to the creation of tropospheric ozone and is responsible for various human health damage [3,4]. Volcanic activity and decomposition of organic matter in nature and human activities such as combustion processes, industrial furnaces and vehicle emissions represent some of the main sources of NO x [5]. Respectively, VOCs pose a significant threat to human health as most people spend a great amount of time in closed spaces [6]. Air quality measurements have shown that there is a considerable amount of indoor VOC sources, such as building materials, hardwood, plywood, laminate floorings, ad- hesives, paints and varnishes [7,8]. One of the main and most common indoor pollutants is acetaldehyde, which is considered as a toxic, irri- tant and possibly carcinogenic organic compound. Both source control and air cleaning have been demonstrated to be effective strategies for dealing with air pollutants. However, traditional air cleaning technologies are limited to physical methods such as ven- tilation, adsorption or filtration, yet an effective strategy for the re- moval of such pollutants has to be established [9]. Photocatalysis offers an economic and effective solution, as the photocatalysts can be added into paints, coatings or building materials during their production. To date, numerous photocatalysts have been used to remove gas- eous pollutants. Titania (TiO 2 ) is recognized as one of the most versatile photocatalysts due to its strong oxidation ability, thermal and chemical stability, and low cost [10,11]. However, its photocatalytic efficiency is compromised by the large band gap energy (∼3.2 eV). The polymeric photocatalyst g-C 3 N 4 also possesses a high thermal and chemical sta- bility, while its appropriate band gap energy (∼2.7 eV) allows the ab- sorption of light up to 450 nm [12,13]. Still, its performance is limited by the fast recombination rate of the photo-induced charge carriers. In order to eliminate the disadvantages of each individual photocatalyst, their combination into g-C 3 N 4 /TiO 2 composites has been intensively investigated [14–16]. The formation of successful heterojunctions be- tween these materials can promote the transfer of photo-induced charge https://doi.org/10.1016/j.cattod.2019.12.021 Received 19 September 2019; Received in revised form 19 November 2019; Accepted 14 December 2019 ⁎ Corresponding authors. E-mail addresses: dana.dvoranova@stuba.sk (D. Dvoranová), c.trapalis@inn.demokritos.gr (C. Trapalis). Catalysis Today xxx (xxxx) xxx–xxx 0920-5861/ © 2019 Published by Elsevier B.V. Please cite this article as: Ilias Papailias, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.12.021