Modelling the kinetics of stearic acid destruction on TiO2 'self-cleaning' photocatalytic films

The kinetics of stearic acid, SA, removal by photocatalytic films are modelled using a log-normal distribution in surface reactivity, which shows that as the distribution in reactivity, rho, increases, so too does the apparent order of reaction, m, of the observed variation in SA concentration, [SA], versus irradiation time, t. The model is used to fit the observed [SA] vs. t profiles exhibited by three very different TiO2 films, namely ActivTM self-cleaning glass, and prepared sol-gel TiO2 and P25 TiO2 films. These films show decay kinetics, with m = 0.3, 0.44 and 0.6, respectively, which fit the model predicted decay profiles with distribution width, rho, values, 0.4, 0.54 and 0.7, respectively. A Ag photocatalyst indicator ink is used on these films to highlight the non-uniform distribution in surface reactivity on the micron scale. The use of the log-normal model to better understand the kinetics exhibited by photocatalytic films is discussed briefly.