Nano and micro-TiO2 for the photodegradation of ethanol: experimental data and kinetic modelling

The pollution abatement is very important nowadays, but the worldwide research needs to develop new “green” technologies [1]. The TiO2-photodegradation of pollutants is an effective alternative to the much more expensive advanced oxidation processes (AOPs), as the VOC‟s degradation is a crucial point in order to improve both air and human health quality [2]. In this study, two different commercial TiO2 samples were tested in the photodegradation of ethanol, chosen as model molecule, but also considering it as an important atmospheric pollutant: ethanol emissions accounted for about 4% of the total VOCs anthropogenic emissions in the UK in 1993 [3]. From a kinetic point of view, the whole process can be simplified considering a two consecutive first order reactions mechanism on the catalyst surface, in which the adsorbed ethanol is converted to acetaldehyde, which is mineralized to carbon dioxide and water (Langmuir-Hinshelwood mechanism). The experimental data were used for the regression of the characteristics kinetic parameters. Photocatalytic degradations were conducted in a cylindrical glass reactor with an ethanol concentration of 400 ppmv; the photon sources were provided by a 500 W UV lamp (Jelosil model HG 500) and the VOC‟s molecules were monitored by a gas chromatography (Agilent 3000 A microGC). The regression of the adsorption and the kinetic constants were made using MATLAB software; the simulated results exhibit a good fit for the test performed using both the micro- and nano-samples. The analysis of the kinetic elaboration gives us important information about the rate of reaction: it is in general increased if catalysed by nanometric sample. Nevertheless, the catalytic properties of micro-samples are confirmed. In particular, considering the degradation of ethanol, the rate of its conversion is not so different for either nanometric or micrometric samples. The micrometric TiO2, less dangerous and less expensive with respect to the nanometric P25, is active as photocatalysts, being able to degrade VOCs into CO2, also if with rate of reaction, and then kinetic constants, lower respect the P25. The good fitting between experimental and simulated results confirms the assumption of a consecutive first order reaction mechanism degradation pathway that is not influenced by the TiO2 crystallites dimension.