Photocatalyzed and photosensitized conversion of organic dyes on porous and non-porous air–solid surfaces: Kinetic models reconsidered

• Uniformly illuminated photocatalyst yields intrinsic dye oxidation reaction order.
• Non-uniformly illuminated photocatalyst exhibits order higher by one.
• Use of organic dyes to characterize porous photocatalyst activity is problematic.

The use of organic dyes to assay photocatalyst activity in air–solid systems has been explored frequently in recent years. We earlier proposed a two step kinetic model for photcatalyzed conversion of dyes in sub-monolayer and multi-layer deposits on titania films and powders. The present paper reconsiders both our own work and that of other researchers to propose more fundamental kinetic models which include the influence of illumination profiles within porous films and powder layers. The new models are shown to rationalize results for dye-photocatalyst data from multiple laboratories published over the last two decades. This outcome indicates that different kinetic analyses must be used to obtain rate constants in non-porous, uniformly illuminated systems (e.g., self-cleaning glasses) vs. porous systems such as thick macro- and meso-porous layers and powders, which exhibit illumination intensity variation with depth.

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