Acetaldehyde adsorption on TiO2: Influence of NO2 preliminary adsorption

Abstract

Titanium dioxide (TiO2) is a widespread metal oxide used in catalysis and photocatalysis, or coupled to non-thermal plasma for volatile organic compound (VOC) oxidation. Adsorption is a key step in the advanced heterogeneous oxidation processes; adsorption is partially addressed in that it is mainly investigated on fresh TiO2 surfaces. However, the treatment of real effluents combines various pollutants with VOCs; among them, NOx are characterized by reactive adsorption properties on TiO2 leading to irreversibly adsorbed species on the surface. The aim of this work is to determine the impact of NO2 preliminary adsorption on the subsequent adsorption of a model VOC: acetaldehyde. Breakthrough methods are used to (i) characterize acetaldehyde adsorption on fresh and NO2 covered TiO2 surface, (ii) control the coverage of TiO2 surface by irreversibly adsorbed NOx(ads)- species. In a first step, acetaldehyde adsorption on TiO2 has been characterized. In a second step, acetaldehyde adsorption has been achieved on TiO2 surface after NO2 exposure corresponding to different surface coverages. Then, acetaldehyde adsorption parameters have been determined. Subsequently, it has been possible to plot the evolution of acetaldehyde reversibly and irreversibly adsorbed quantities, and reversible fraction adsorption constant, as a function of TiO2 surface coverage by NOx(ads)- species, so called TiO2 surface NO2 ageing. Interestingly, both adsorbed acetaldehyde fractions are highly impacted by the presence of NOx(ads)- species on TiO2 surface. The irreversibly adsorbed fraction is considerably decreased since the lowest values of TiO2 surface coverage by NOx(ads)- species. Hypotheses related to competitive adsorption are not sufficient to explain the observed impact, suggesting that NOx(ads)- species modify TiO2 surface chemistry and acetaldehyde reactive adsorption. The reversibly adsorbed fraction of acetaldehyde is highly impacted as well; reversibly adsorbed amounts are decreased since the lowest surface coverage by NOx(ads)- species. The corresponding adsorption constants are abruptly increased as soon as TiO2 surface has been exposed to NO2, suggesting the formation of a new adsorption mode for acetaldehyde on NO2 exposed TiO2 surface. This paper evidences the considerable impact of NO2 on TiO2 adsorption properties regarding VOCs. Considering that NOx may accumulate on TiO2 the long term behaviour of such processes should be investigated further taking into account the NOx vs. VOC interaction for adsorption.