Contents lists available at ScienceDirect Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Effect of metal doped and co-doped TiO 2 photocatalysts oriented to degrade indoor/outdoor pollutants for air quality improvement. A kinetic and product study using acetaldehyde as probe molecule Emmanouil S. Karafas a , Manolis N. Romanias b , Vassileios Stefanopoulos a , Vassilios Binas c,d, ⁎⁎ , Apostolos Zachopoulos c , George Kiriakidis c,d , Panos Papagiannakopoulos a, ⁎ a Laboratory of Photochemistry and Kinetics, Department of Chemistry, University of Crete, 71003, Heraklion, Crete, Greece b IMT Lille Douai, Univ. Lille, SAGE – Département Sciences de l'Atmosphère et Génie de l'Environnement, F-59000, Lille, France c Institute of Electronic Structure and Laser (IESL), FORTH, P.O. Box 1527, Vasilika Vouton, GR-71110, Heraklion, Greece d Crete Center for Quantum Complexity & Nanotechnology (CCQCN), Department of Physics, University of Crete, GR-71003, Heraklion, Greece ARTICLE INFO Keywords: Metal-doped TiO 2 Visible-ultraviolet photocatalysis Photodegradation kinetics Acetaldehyde Indoor air pollution ABSTRACT This study demonstrates the photocatalytic decomposition of an indoor air pollutant, acetaldehyde (CH 3 CHO), over 0.04 mol% metal-doped TiO 2 (Mn-, Co- and Mn/Co-) nanoparticles activated by ultraviolet and visible irradiation. The photocatalytic activity, the photodegradation kinetics, and the final product analysis were ex- amined using a Static Photochemical Reactor coupled with a FTIR spectrophotometer. CH 3 CHO undergoes ef- ficient decomposition over all photocatalysts under UV irradiation in the presence of one atmosphere N 2 or synthetic air (SA). Metal doping causes substantial influence to photocatalysis by altering the amount of elec- tron/hole pairs generated and/or the electron/hole recombination rates. Simulating the experimental results with pseudo-first order kinetics the corresponding degradation rate coefficients were determined for each photocatalyst under UV irradiation and SA environment: k d UV (Mn-TiO 2 ) = (1.9 ± 0.2)×10 −1 h −1 , k d UV (Co- TiO 2 ) = (2.8 ± 0.3)×10 −1 h −1 , and k d UV (Mn/Co-TiO 2 ) = (6.0 ± 0.7)×10 −1 h −1 . These degradation kinetics under UV light irradiation are significantly faster than undoped TiO 2 revealing that the transition metal doping of TiO 2 nanomaterials boosts the photocatalytic degradation of organic pollutants. Substantial decomposition of CH 3 CHO was achieved over Mn-TiO 2 under visible light in oxygen presence k d Vis (SA) = (0.44 ± 0.04)×10 −1 h −1 while for other samples no visible light photocatalysis was observed. CO 2, CO, and H 2 O were the main oxidation products, with CO 2 yields almost 100% under UV excitation, and CO yields up to 20% under UV and < 1% under visible excitation. Our experimental results suggest that Mn-TiO 2 (0.04 mol%) nanoparticles may be considered as a potentially safe photocatalyst to remove acetaldehyde particularly from indoor atmospheric environments under visible irradiation, without yielding significant toxic by-products. Other possible atmospheric implications are also discussed in the paper. 1. Introduction Indoor air quality is a major issue of concern to human health, since people spend more than 80% of their time in indoor environments [1–4]. Aldehydes are among the most abundant volatile organic com- pounds (VOCs) that have been detected in the troposphere, as well as, in indoor environments. High levels of CH 3 CHO concentrations have been detected in many living and working indoor environments, which may have serious negative implications to human health (e.g. sick building syndrome, headache, nausea, etc.) [1,5–7]. It is therefore important to develop new purification methods that may effectively remove air-toxic pollutants from indoor environments, and improve indoor air quality conditions [8]. Among the most promising re- mediation technologies for the destruction of air-toxic compounds from indoor environments is the use of photocatalytic processes based on transition metal oxide materials such as TiO 2 , over which gas com- pounds may undergo photodegradation in the presence of ultraviolet (UV) or visible (Vis) irradiation [9–12]. However, such TiO 2 based processes should lead to end-products that are non-toxic and en- vironmentally friendly [12–14]. The photocatalytic degradation of https://doi.org/10.1016/j.jphotochem.2018.11.023 Received 27 August 2018; Received in revised form 1 November 2018; Accepted 14 November 2018 ⁎ Corresponding author. ⁎⁎ Corresponding author at: Institute of Electronic Structure and Laser (IESL), FORTH, P.O. Box 1527, Vasilika Vouton, GR-71110, Heraklion, Greece. E-mail addresses: binasbill@iesl.forth.gr (V. Binas), panos.pap@uoc.gr (P. Papagiannakopoulos). Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 255–263 Available online 16 November 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved. T