Effect of metal doped and co-doped TiO2 photocatalysts oriented to degrade indoor/outdoor pollutants for air quality improvement. A kinetic and product study using acetaldehyde as probe molecule

This study demonstrates the photocatalytic decomposition of an indoor air pollutant, acetaldehyde (CH3CHO), over 0.04 mol% metal-doped TiO2 (Mn-, Co- and Mn/Co-) nanoparticles activated by ultraviolet and visible irradiation. The photocatalytic activity, the photodegradation kinetics, and the final product analysis were examined using a Static Photochemical Reactor coupled with a FTIR spectrophotometer. CH3CHO undergoes efficient 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 electron/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-TiO2) = (1.9 +/- 0.2) x 10(-1) h(-1), k(d)(UV)(Co-TiO2) = (2.8 +/- 0.3) x 10(-1) h(-1), and k(d)(UV)(Mn/Co-TiO2) = (6.0 +/- 0.7) x 10(-1) h(-1). These degradation kinetics under UV light irradiation are significantly faster than undoped TiO2 revealing that the transition metal doping of TiO2 nanomaterials boosts the photocatalytic degradation of organic pollutants. Substantial decomposition of CH3CHO was achieved over Mn-TiO2 under visible light in oxygen presence k(d)(Vis)(SA) = (0.44 +/- 0.04) x 10(-1) h(-1) while for other samples no visible light photocatalysis was observed. CO2, CO, and H2O were the main oxidation products, with CO2 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-TiO2 (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.