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Humidity can have an appreciable influence on the photocatalytic degradation of volatile organic compounds (VOCs) in air. It has been proposed that there are multiple layers of water molecules on the surface of the photocatalyst, and a VOC molecule must penetrate this layer and be able to diffuse to the photocatalytic surface for a reaction to occur. An updated analysis of related literature from the past 12 years indicates that this proposed mechanism remains reasonable for explaining observed effects. According to this mechanism the VOC's hydrophilicity should dominate the influence of humidity on the photocatalytic reaction kinetics, and this effect can be seen in a categorization of literature observations based on the octanol‐water partition coefficient. The frequently cited Langmuir‐Hinshelwood kinetic model does not directly account for humidity effects in this way but can be adapted for this purpose. As an example, the rate of photocatalytic decomposition of chlorobenzene in air under humid conditions can be described by a modified Langmuir‐Hinshelwood kinetic model, where the reaction rate coefficient is dependent on the water concentration. Understanding and considering the effect of humidity is useful for optimizing the efficacy of photocatalytic VOC treatment of air. [ABSTRACT FROM AUTHOR]

Abstract: The destruction of a chlorinated aromatic compound by UV photolysis and advanced oxidation was examined for potential use in air emissions treatment. The fluence rate distribution in a photoreactor was analyzed by a finite model, and this was used to estimate quantum yield and photonic efficiency for the photochemical decomposition of chlorobenzene in air. The analysis indicates that the quantum yield was constant at approximately 115%, but the presence of the common flue gas components sulfur dioxide and water could increase it to approximately 145%. Ozone, which strongly absorbs UV photons and promotes radical chain reactions, enhanced the quantum yield to 1534% when present at 2253ppm. When SO2 or ozone are present, “effective” quantum yield may be more informative for analysis, since it includes the number of photons absorbed by all reacting species. In this work, the effective QY ranged from 3.9% to 108.7%, with low values indicating that photons were wasted in processes that did not degrade the target organic. An analysis of photonic efficiency shows that the photo-reactor is more efficient at high concentrations of the target organic pollutant, and when ozone or sulfur dioxide are present. [Copyright &y& Elsevier]

The influence of humidity on the decomposition of gas phase chlorobenzene (CB) was studied in a batch photocatalytic reactor using TiO2 (anatase). Increasing relative humidity (>7% at 25°C and 1atm) resulted in an increasingly detrimental effect on the adsorption and photocatalytic decomposition rate of CB. Mechanistic analysis indicates that the relative hydrophilic/hydrophobic nature of a given chemical should dictate adsorption onto or penetration through water to the TiO2 surface. It is suggested that multiple layers of water molecules are formed at the TiO2 interface with air, even at low relative humidities. The multiple-layered film of water retards/prevents CB from reaching the reactive TiO2 surface or contacting radical species in the boundary layer. [Copyright &y& Elsevier]

Abstract: Bench-scale laboratory experiments were conducted using broad spectrum UV-A to induce TiO2-based photocatalytic treatment of mono-chlorobenzene under aerobic and anaerobic conditions. Previous photolytic-based studies conducted on chlorinated aromatics support the decomposition pathway of de-halogenation followed by cleavage of the aromatic ring due to attack by OH radicals. However, in this study of anaerobic degradation, GC–MS and HPLC analyses of intermediate by-products accumulated on the catalyst surface indicate that chlorophenols (o-, m-, p-) are the predominant species rather than benzene or phenol, and that dechlorination proceeds after ring cleavage. [Copyright &y& Elsevier]