Adjustable kinetics in heterogeneous photocatalysis demonstrating the relevance of electrostatic interactions

The performance of a photocatalyst depends on a complicated correlation of a multitude of parameters including extrinsic ones. Among, the pH value is probably the most important parameter. Gaining a detailed knowledge of the underlying processes still represents a major challenge but is required to optimize known or to develop efficient novel photocatalysts. Here, we present a favorable model system comprising nano-beta-SnWO4 as the photocatalyst and azo-dyes in different charge states. Disentangling parameters, we demonstrate that the reaction order of our exemplary photocatalytic degradation reaction can be fully tuned from first over zero- to mixed-order by simply adjusting the pH. Thereby we elucidate the importance of electrostatic interactions which are of significance to various aspects including: the decomposition of non-charged persistent pollutants, the inactivation of bacteria as well as the influence of electrolyte containing media, the polarity of the solvent or of specific crystal facets. Based on our results, we suggest taking the electric double layer on the photocatalyst into account. This multi-component view reconciles reported contradictions and allows interpreting results that have been intriguing or misunderstood so far. (C) 2012 Elsevier B.V. All rights reserved.