Photocatalytic air purification: Effect of HNO3 accumulation on NOx and VOC removal

The effect of photocatalytically generated surface HNO3 on NOx and VOC removal activity is investigated using three very different photocatalytic films, namely, P25 TiO2, a commercial exterior paint (produced by STO Gmb) and a commercial air-purifying curtain fabric (IKEA). The paint required accelerated weathering and the cloth UV-preconditioning before exhibiting significant activity. When tested using the removal of nitric oxide ISO 22197-1:2016, the NO ISO, all films exhibited decreasing NOx removal activity and decreasing photocatalytic activity for NO removal with increasing irradiation time, so much so that both the STO paint and P25 films showed little NOx removal activity after the 5 h irradiation period. In all cases the loss in activity was attributed to the generation of a surface layer of HNO3, which was found to persist for at least 15 h and up to 7 days on a P25 film but could be readily rinsed away. The CaCO3 in the STO film, and the fibrous nature of the IKEA cloth appeared to largely eliminate the negative effect on NOx removal activity after 7 days. The activities exhibited by these films for the photocatalysed oxidation of acetaldehyde appeared unaffected by the presence of a photocatalytically generated HNO3 film. The relevance of the results to current, commercial photocatalytic products for air purification is discussed briefly.