Investigation on the dual roles of pollutants adsorption on the catalyst surface during photocatalytic process

Adsorption is relevant step in the photocatalytic process and has a significant impact on the photocatalytic degradation rate. This article investigated the influence and mechanism of adsorption on photocatalytic degradation rate with ZnO/activated carbon (AC) as catalyst and methyl orange (MO) as target pollutant. By changing the preadsorption time, the effects of catalyst surface pollutant concentration on the photocatalytic reaction rate were investigated. Moreover, desorption experiments for residual pollutant on the catalyst surface were conducted after the completion of the photocatalytic process to determine the total degradation rate of MO. Furthermore, the main active species generated in the photocatalytic process were tested. The results indicate that the adsorption of pollutants on the catalyst surface has a dual effect on photocatalytic reactions. On the one hand, preadsorption of pollutants can increase the concentration of reactants, thus improving the photocatalytic reaction rate. On the other hand, the light-absorbed intensity decreases with the increased pollutant adsorbed on the catalyst surface, leading to a decrease in the number of photocarriers which is not conducive to the photocatalytic reaction proceeding. Therefore, for a specific photocatalytic reaction system, there exists an optimal value for the preadsorption of pollutants. For the present photocatalytic system, when the dark adsorption time was 30 min, removal rate in solution was 100 %, and including the residual MO on the catalyst surface, the total degradation rate reached 81 % within 160 min, which was 56 % and 28 % higher than that at saturation adsorption (50 min) and at 0 min dark adsorption, respectively.