Decoupling Adsorption and Photocatalysis: Addressing the Dark Adsorption Pitfall in Catalyst Ranking

Heterogeneous photocatalysis relies on the adsorption of the target molecule to the surface of the catalyst and the photocatalytic action of the material to degrade target molecules. In many reports, a 30 min dark adsorption time is used prior to exposure of the system to light and initiation of photocatalysis with no discussion of differences in adsorption rate or capacity when comparing catalystswhen adsorption rates or capacities are very different, misleading catalyst rankings may result. Even closely related materials, such as the ZnO-based photocatalysts discussed here, adsorb target molecules at different rates, have different adsorption capacities, and have apparently different photocatalytic rates. When typical methods for evaluating photocatalysts are used, the adsorption effects are not accounted for. Here, we illustrate potential concerns while evaluating photocatalysts and present a simple, low-concentration method to possibly addressand avoidadsorption effects. Interestingly, catalyst rankings change when adsorption effects are minimized by circumventing dark adsorption using the low concentration approach presented.