Abstract
A jointed experimental and theoretical investigation pointing out new insights about the microscopic mechanism of the volatile organic compounds (VOCs) photocatalytic elimination by TiO2 was done. Methane, hexane, isooctane, acetone and methanol were photomineralized in a batch reactor. Values of K (adsorption constant on TiO2) and k (mineralization rate constant) of the five VOCs (treating the kinetic data through a Langmuir–Hinshelwood approach) were determined. Recorded K (in the range of 0.74 × 10−2–1.11 × 10−2 ppm−1) and k (in the range of 1.9–9.9 ppm min−1) values and performed theoretical calculations allowed us to suggest the involvement of an electron transfer step between the VOC and the hole, TiO2(h+), as the rate-determining one.
Highlights
IntroductionThe TiO2 photosensitized mineralization of most volatile organic compounds (VOCs) as pollutants has been widely considered [1,2,3,4,5,6,7,8,9] principally due to its biological and chemical inertness, strong oxidizing power, low cost and long-time stability against photo- and chemical corrosion of this semiconductor.The photocatalytic process is known to involve the VOC pre-adsorption at the sensitizer surface followed by the generation of hole/electron pairs into the semiconductor through the absorption of light with energy equal to or higher than the band-gap energy; the electrons reduce the atmospheric oxygen while the holes oxidize the VOC, directly (through an electron transfer process from the organic substrate to the hole) or indirectly (through the intervention of radicals OH derived from the oxidation of adsorbed water by the hole) [10,11].In current literature, many authors have reported the effect of TiO2 properties on the gas-phase photocatalytic efficiency, while a minor number of papers concern the effect of the VOC structure on the preadsorption at TiO2 surface and on the reaction efficiency
The photocatalytic process is known to involve the volatile organic compounds (VOCs) pre-adsorption at the sensitizer surface followed by the generation of hole/electron pairs into the semiconductor through the absorption of light with energy equal to or higher than the band-gap energy; the electrons reduce the atmospheric oxygen while the holes oxidize the VOC, directly or indirectly [10,11]
It is useful to discuss as first the topic relative to the data of photocatalytic efficiency obtained in the presence of different synthetic powders, considering hexane as substrate; this allows to compare the behavior of this substrate with that of a previously studied VOC, acetone [20]
Summary
The TiO2 photosensitized mineralization of most volatile organic compounds (VOCs) as pollutants has been widely considered [1,2,3,4,5,6,7,8,9] principally due to its biological and chemical inertness, strong oxidizing power, low cost and long-time stability against photo- and chemical corrosion of this semiconductor.The photocatalytic process is known to involve the VOC pre-adsorption at the sensitizer surface followed by the generation of hole/electron pairs into the semiconductor through the absorption of light with energy equal to or higher than the band-gap energy; the electrons reduce the atmospheric oxygen while the holes oxidize the VOC, directly (through an electron transfer process from the organic substrate to the hole) or indirectly (through the intervention of radicals OH derived from the oxidation of adsorbed water by the hole) [10,11].In current literature, many authors have reported the effect of TiO2 properties on the gas-phase photocatalytic efficiency, while a minor number of papers concern the effect of the VOC structure on the preadsorption at TiO2 surface and on the reaction efficiency. The TiO2 photosensitized mineralization of most volatile organic compounds (VOCs) as pollutants has been widely considered [1,2,3,4,5,6,7,8,9] principally due to its biological and chemical inertness, strong oxidizing power, low cost and long-time stability against photo- and chemical corrosion of this semiconductor. The considered VOCs have been principally alkanes [12,13,14,15,16,17], chloroalkanes [12,18], chloroalkenes, [10,12,18], carbonyl derivatives [10,12,15,16,19], alcohols [10,12,18,19], ethers [12] and aromatic compounds [10,12]. The experiments were carried out in a batch reactor (irradiation with high-power external lamp through a Pyrex cap)
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