Kinetics of photocatalytic, self-cleaning surfaces: A decision tree approach for determination of reaction order

Abstract

Self-cleaning photocatalytic surfaces have several decades of application, yet satisfactory rate equations for analyzing the kinetics of reactions on such solid surfaces are lacking, due in large part to the many configurations of the catalyst and the deposited contaminants.We analyze the existing literature and show that nearly all studies can be described by application of the power law for rate of reaction:Rate = kcat [C] nwhere n = apparent reaction order, and kcat is a fundamental constant of the catalytic material. The value of reaction order, n, we show requires answers to the following six questions. In each case, the observed apparent kinetic order depends upon interplay among the distributions of photocatalyst, reactant, and irradiance.1. Is the photocatalyst porous or non-porous?Example: Stearic acid on/within non-porous/porous photocatalyst layer.2. Is the photocatalytically active layer optically thin or thick?Example: Dye conversion in TiO2 layers vs. 10% TiO2/SiO2?3. Is the probe reactant deposit a submonolayer or multilayer?Examples: Dye sub/multilayers with TiO24. Is probe reactant light absorption negligible or important?Example: Stearic acid vs. soot5. Is the probe reactant present as a continuous film or as a distribution of discrete islands?Example: Long chain carboxylic acids on TiO26. If distributed, what is breadth of distribution?Example: Stearic acid on TiO2For contaminant removal we demonstrate apparent reaction orders of 0, ½, 1, and 2! Simple analysis is used to explain this diversity of apparent reaction orders. We use the six questions posed to construct a decision tree for determination of the apparent reaction order, n, as a function of responses to the six questions.