The photocatalytic destruction of cinnamic acid and cinnamyl alcohol: Mechanism and the effect of aqueous ions

Abstract

Cinnamic acid was chosen as an exemplar molecule to study the effect of potential contaminants on the kinetics and mechanism of the photocatalytic destruction of hydrocarbons in aqueous solutions. We identify the principal intermediates in the photocatalytic reaction of the acid and corresponding alcohol, and propose a mechanism that explains the presence of these species. The impact of two likely contaminants of aqueous systems, sulfate and chloride ions were also studied. Whereas sulfate ions inhibit the degradation reaction at all concentrations, chloride ions, up to a concentration of 0.5 M, accelerate the removal of cinnamic acid from solution by a factor of 1.6. However, although cinnamic acid is removed, the pathway to complete oxidation is blocked by the chloride, with the acid being converted (in the presence of oxygen) into new products including acetophenone, 2-chloroacetophenone, 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane. We speculate that the formation of these products involves chlorine radicals formed from the reaction of chloride ions with the photoinduced holes at the catalyst surface. Interestingly, we have shown that the 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane products form from 2chloroacetophenone when irradiated with 365 nm light in the absence of the catalyst. The formation of potentially dangerous side products in this reaction suggest that the practical implementation of the photocatalytic purification of contaminated water needs to considered very carefully if chlorides are likely to be present.

The effect of dissolved chlorides on the photocatalytic degradation properties of titania in wastewater treatment

“…A very different effect was observed over rutile however, with the photocatalytic activity in brines accelerated by a factor of more than two, with the pseudo-first-order rate constant increasing from 2.3 Â 10 À3 min À1 to 4.8 Â 10 À3 min À1 . In contrast to some other reported systems, 9,[33][34][35][36] the higher phenol oxidation rates observed in brines over rutile suggests that there are alternative reaction pathways offered by the presence of chlorine radicals.…”

“…Nevertheless, these results do not eliminate the possibility of small amounts of chloride being adsorbed over the surface of the photocatalyst and acting as hole scavengers, as previously suggested. 8,9 To investigate such a possibility, in the next stage of this work, we compare the ability of such systems to self-trap holes and electrons in their structure. chloride anions.…”

“…1,7 Although TiO 2 has shown promise in the degradation of organic compounds dissolved in standard water solutions, the performance of such photocatalysts was observed to decrease drastically when investigating wastewater solutions containing a high content of dissolved salts. 8,9 This depletion of titania's photoactivity is often attributed to blockage of surface sites, impeding the adsorption of hydroxyl anions and formation of hydroxyl radicals, or, hole trapping/scavenging, inhibiting the formation of active radicals responsible for the oxidation of organic pollutants. 8 Conversely, the opposite effect has also been reported when halide anions were present in a wide range of experimental conditions.…”

Effect of Alkali Metal Cations on the TiO2 P25 Catalyst for Hydrogen Generation by the Photoreforming of Glycerol

“…3). [21] Bouleghlimat et al, [8] showed that chlorine radicals formed in this way accelerated the rate of removal of cinnamic acid from an aqueous solution although this also generated chlorine sub-Figure 1. XPS of the photocatalysts post-reaction in the presence of 0.05 M of three different alkali metal chlorides.…”

“…One factor that may contribute to the confusion is that, for some systems, although chloride ions open up new reaction pathways and therefore accelerate conversion of the target, the products of these pathways are more resistant to photodegradation and so complete degradation is inhibited. For example, in a recent study [8] related to the photodecomposition of cinnamic acid, we found that the formation of chloride radicals created new pathways resulting in chlorine substituted products. The result was an accelerated rate of decomposition of the cinnamic acid but since the substituted products were resistant to photocatalytic decomposition, there was an overal reduction in the rate of total combustion to CO 2 .…”