Assessing a Photocatalytic Activity Index for TiO₂ Colloids by Controlled Periodic Illumination

Abstract

We demonstrated that controlled periodic illumination (CPI) is unable to increase the quantum yield (or photoefficiency) of the photocatalytic process. Nevertheless, choosing the appropriate experimental conditions and the use of CPI coupled with a simplified kinetic model allowed us to extract the lifetime of the species involved in substrate oxidation. These figures are usually elusive, even though there is a wide collection of hypotheses, guesses, and speculations in the literature. From our analysis, we defined a surface photocatalytic activity index. The latter consists of the ratio between the surface-defined kinetic constant for substrate oxidation and surface recombination and is independent of other experimental conditions, such as irradiance and catalyst concentration.

Controlled Periodic Illumination Enhances Hydrogen Production by over 50% on Pt/TiO2

“…Our research group recently published a paper in which it demonstrated that CPI is unable to increase the quantum yield of the photocatalytic process for pollutant abatement on a bare TiO 2 suspension (oxidation of formic acid by P25 aqueous slurry). 18 This result is coherent with the previous observations of Cornu et al 19 , 20 and with the theory of intermittent illumination developed by Melville and Burnett for homogeneous photopolymerization processes. 21 Conversely, in 2004, Wan et al.…”

“…Conversely, without cocatalysts (Figure 2b), the CPI technique does not lead to an increase in the HER rate (i.e., the dark period is not contributing to the overall reaction as already observed). 18 Pt-loaded materials produce H 2 with greater efficiency (Figure 2c), coherently with the literature. CPI induces larger H 2 production, with a significant increase from 0.8 to 80 Hz, as also witnessed by the more negative photopotential attained during CPI compared with continuous irradiation at the same average power (Figure 3a).…”

Dynamic Control of Elementary Step Energetics via Pulsed Illumination Enhances Photocatalysis on Metal Nanoparticles

“…22−24 We hypothesize that this frequency is related to the time scale for populating the catalyst surface with adsorbed CO, based on the barrier for activating methanol on a clean Pt surface or the time scale for clearing adsorbed CO. 34 Previous studies have examined the influence of pulsed illumination on semiconductor photocatalysts. 43 For semiconductors, QYs for photocatalytic processes generally decrease with increasing light intensity due to increased rates of carrier recombination. 44 Pulsed illumination with a low duty cycle can promote QY to values obtained under steady state illumination at lower light intensity.…”

“…Previous studies have examined the influence of pulsed illumination on semiconductor photocatalysts . For semiconductors, QYs for photocatalytic processes generally decrease with increasing light intensity due to increased rates of carrier recombination .…”

Imposed dynamic irradiation to intensify photocatalytic reactions

“…It is known that the quantum yield under periodic irradiation increases to a value equal to that observed for the same (lower) average incident photon flux under continuous irradiation. [70,71] In the complex reaction network of the nitrobenzene reduction, it is further possible that mass transport effects play a role. If charge separation is induced by excitation but the product blocks the access to the catalyst surface, recombination will occur, leading to a lower photonic efficiency.…”