Gaseous Photocatalytic Oxidation of Formic Acid over TiO₂: A Comparison between the Charge Carrier Transfer and Light-Assisted Mars–van Krevelen Pathways

Abstract

Under light illumination, it is usually considered that photocatalytic oxidations of organics such as volatile organic compounds over semiconductors are driven by the transfer of photogenerated carriers. Some studies also proposed that the photocatalytic oxidations might take place according to the lightassisted Mars−van Krevelen (MvK) pathway that involved the participation of lattice oxygens under aerobic conditions. Based on the concept of the MvK mechanism, the current work first gives an elaboration on the light-assisted MvK pathway and its intrinsic difference from the charge carrier transfer pathway. We then examined which one of these two mechanisms is responsible for the the photocatalytic oxidation of formic acid over TiO 2. Comprehensive experiments, including apparent kinetics, online electric conductances, vacuum electric conductances, online optical transmittances, and first principle calculations, were carried out to discuss this problem. The results showed that the photocatalytic oxidations of formic acid over TiO 2 dominantly follow the charge carrier transfer pathway at both low and elevated temperatures, the light-assisted MvK mechanism could not play a major role, and there was also no transition from the charge carrier transfer to the light-assisted MvK mechanism with an increase of reaction temperature.

Process Intensification in Photocatalytic Decomposition of Formic Acid over a TiO2 Catalyst by Forced Periodic Modulation of Concentration, Temperature, Flowrate and Light Intensity

“…Their study demonstrated that higher levels of radicals adsorbed on the anatase surface gives rise to significantly higher photoactivity than rutile. Liu et al [54] showed that the mechanism of formic acid photocatalysis over commercial P25 TiO 2 (primary particle size of >25 nm, surface area of ~50 m 2 g −1 , anatase-to-rutile ratio of around 3:1) is the same in the low (30-100 • C) and elevated (100-150 • C) temperature range. The rate of thermal decomposition at 125 • C (4.0•10 −6 mol m −2 s −1 ) was almost two orders of magnitude below the rate of photochemical decomposition, therefore, the former can be neglected in the kinetic analysis.…”

“…The rate of this process is therefore a property of the photocatalyst and independent of the reaction studied. Liu et al [54] studied the photocatalytic decomposition of formic acid at 125 • C by varying the light intensity in the 1-5 mW m −2 range and found out that the index N was 0.3 in the whole range studied. This justified that photocatalysis is driven by the charge carrier transfer mechanism.…”

“…For example, for a reference light intensity of 1.6 mW cm −2 at 365 nm, I 0 = 1.0 × 10 16 cm −2 . As the intensity reduces with the layer thickness, the magnitude of constant k 2 in Equation ( 12) would also depend on the configuration of the photocatalyst layer, and it may vary in a rather wide range between 0.001 and 0.24 s −1 [54].…”

“…By using Equation (54) with either Equation ( 55) or (56), the mean value of the acid outlet molar flowrate can approximately be calculated for any steady-state point and any set of forcing parameters (frequency, amplitude(s) and, for Cases 5-10, phase difference).…”

Silver-Copper Oxide Heteronanostructures for the Plasmonic-Enhanced Photocatalytic Oxidation of N-Hexane in the Visible-NIR Range

“…Transition metal oxides and complex metal oxides (i.e., rare earth element-based perovskites) are also excellent VOCs oxidation candidates that operate at relatively mild temperatures without incurring in the burdening costs of noble metals [3,8,16,37,38,39,40,41,42,43,44,45]. Alternatively, the use of inexpensive arrays of photocatalysts based on titania (TiO 2 ) has become one of the most important research fields towards the sustainable remediation of VOCs [5,20,46,47,48,49,50,51]. The advantages of using solar light or low consume artificial lights to promote VOCs oxidation at room temperature is being actively pursued.…”