Recommendations for improving the experimental protocol for the determination of photocatalytic activity by nitric oxide oxidation measurements

The photocatalytic nitric oxide (NO) oxidation reaction is used as a standard diagnostic tool for photocatalytic activity according to the well-defined protocol described by ISO Standard 22197-1-2007. This protocol identifies the negative peak showing a NOx concentration drop during a gas flow switch from the calibration bypass to the reactor as adsorption of NOx on the surface. Evidence is provided for this first transient to be due to a dilution effect in the gas phase within the reactor. With proper models of residence time distribution analysis, this transient revealed the internal hydrodynamics and it can be used to determine the internal volumes of the system. The second transient occurs immediately after the light is switched on. The conversions strongly depend on the time constant of this transient. Controlled measurements of the effect of illumination intensity revealed that at higher light intensities the transient takes longer to reach steady state. The longer transient was attributed to the time needed to reach a thermal steady state of the hot spots generated by the recombination of excess charge carriers. When the catalyst amount was investigated as a parameter, a saturation effect was observed. This saturation effect was correlated with the gas phase concentrations of NOx and moisture and their ratios to the available specific surface area. Hence, additional constraints with respect to the illumination intensity and catalyst amounts are recommended for accurate measurements of photocatalytic activity by NO oxidation.