Photocatalytic Paints for NOx Removal: Influence of Various Weathering Conditions

Three commercial exterior photocatalytic paints, from STO, Boysen and Graphenstone and one In-house paint, are tested using the NO ISO; FT-IR analysis shows that all paints contain an acrylate binder, but the commercial paints also contain siloxane and/or silicate binders. All exhibit little or no photocatalytic activity (for NO removal) without weathering. The Graphenstone and In-house paints are only activated by ‘standard’ weathering, i.e. a continuous cycle of UV irradiation (0.76 mW cm-2, 8 h at 50 oC) plus condensation (4 h at 60 oC). In contrast, the STO and Boysen paints are also activated by ‘condensation-only’ weathering by water droplets at 60 oC for ca. 56 h. This is the first report of the efficacy of the latter type of weathering. The activity vs weathering time profiles for the STO photocatalytic paint show that it is almost exclusively activated by condensation-only weathering, which is fast (60% maximum activity within 8 h and produces a film that is active and durable. In contrast, the In-house paint is only activated by standard weathering, which is relatively slow (only 25% maximum activity after 24 h) and involves the photocatalytic oxidation of the acrylate binder; as a consequence the In-house paint film is not very durable. Irrespective of the method of activation, all the photocatalytic paints exhibit little, or no, NOx removal activity towards the end of the 5 h irradiation period of the NO ISO, due to the accumulation of surface HNO3, which promotes the conversion of NO to NO2.