Methodology for Simultaneous Analysis of Photocatalytic deNOx Products

The ISO standard 22197-1:2016 used for the evaluation of the photocatalytic nitric oxide removal has a main drawback, which allows only the decrease of nitric oxide to be determined specifically. The remaining amount, expressed as “NO2”, is considered as a sum of HNO3, HONO NO2, and other nitrogen-containing species, which can be potentially formed during the photocatalytic reaction. Therefore, we developed a new methodology combining our custom-made analyzers, which can accurately determine the true NO2 and HONO species, with the conventional NO one. Their function was validated via a photocatalytic experiment in which 100 ppbv of either NO or NO2 dispersed in air passed over (3 L min−1) an Aeroxide© TiO2 P25 surface. The gas-phase analysis was complemented with the spectrophotometric determination of nitrates (NO3−) and/or nitrites (NO2−) deposited on the P25 layer. Importantly, an almost perfect mass balance (94%) of the photocatalytic NOx abatement was achieved. The use of custom-made analyzers enables to obtain (i) no interference, (ii) high sensitivity, (iii) good linearity in the relevant concentration range, (iv) rapid response, and (v) long-term stability. Therefore, our approach enables to reveal the reaction complexity and is highly recommended for the photocatalytic NOx testing.