Catalytic self-cleaning coatings for thermal oxidation of organic deposits on glass

Self-cleaning catalytic coatings were developed for use in thermal applications, such as domestic oven windows. Based on a modified citric acid complexation method, oxide coatings were synthesized on commercial glass substrates. Their performance was evaluated in the oxidation of a layer of pre-oxidized lipid at 250 degrees C for 1 h, as followed by FTIR or visual/UV-Vis spectroscopic inspection. Based on both evaluation methods, manganese and mixed manganese-ceria oxide coatings with a thickness of around 35 nm were selected as the most promising ones: they combine the excellent activity for oxidative removal of the contaminant with the desired optical characteristics. XPS analysis revealed that during reaction with the contaminant the valence of the manganese oxide decreased from its initial value between + 3 and + 4. A less pronounced reduction in the presence of ceria suggests the transfer of lattice oxygen from ceria to Mn oxide; nevertheless, the reoxidation of the oxide by gas phase oxygen at 250 degrees C seems to be the rate limiting step. Gas phase analysis indicates that ca. 3 times as much CO2 is produced on the catalyst-coated glass than on a blank glass slide in an oxygen atmosphere. The liquid phase products resemble those of a catalyzed liquid phase autoxidation, with too high catalyst concentrations inhibiting the conversion of the lipid. Therefore, the oxidation mechanism on the catalytic coating shows characteristics of a Mars-van Krevelen mechanism (consumption of lattice oxygen) combined with a metal ion catalyzed liquid phase mechanism.