It has been reported that hydrothermally prepared flake ball (FB)-shaped bismuth tungstate (Bi₂WO₆: BWO) particles exhibit photocatalytic activity for oxidative decomposition of organic compounds, which is comparable to that of a commercial titania photocatalyst, while the activity for methanol dehydrogenation is almost negligible even in the presence of loaded with platinum [1]. These facts suggest that two (or four)-electron transfer to the surface-adsorbed oxygen (O₂), requiring relatively lower, i.e., more anodic potential of electrons in photocatalyst particles, proceeds on photoirradiated FB-BWO, but there has been reported no evidence for this multielectron-transfer mechanism. In the present study, light-intensity dependence of the rate of acetic-acid decomposition was studied to obtain kinetic evidence for the multielectron transfer.

FB-BWO, spherical assembly of BWO flakes, was prepared following the previous report [1]. As-prepared FB, its ball-milled samples (L and H) and their 773 K-calcined samples (500FB, 500L and 500H) were used for decomposition of acetic acid in aerobic aqueous suspensions under monochromatic photoirradiation by (A) a diffraction grating-type illuminator (Jasco CRM-FD; max. 10 mW) or (B) a 365-nm UV-LED (max. 320 mW).

It was found that order (n) of light-intensity dependences at four wavelengths (irradiation A) calculated by assuming; R = a × I ⁿ (R: rate, a: constant and I: light intensity). Except for the samples L and H at 380 and 410 nm, BWO showed almost first-order light-intensity dependences at wavelengths between 320 and 410 nm. With the higher intensity irradiation (irradiation B), the order (n) was changed (decreased) to ca. 0.5 for FB and L at a folding point (Fig. 1). As has been reported previously, photocatalytic acetic-acid decomposition proceeds through radical-chain mechanism with an alkyl peroxy radical as a chain carrier when titania was used as a photocatalyst, and the observed order of light-intensity dependence was ca. 0.5 [2]. The above-mentioned first-order light-intensity dependence for the most BWO samples can be interpreted by combination of second-order dependence for the accumulation of two electrons to reduce O₂ and 0.5th-order dependence owing to the radical-chain mechanism. Difference in folding points of plots between FB and L was observed; a folding point for FB was appreciably lower than that of L. One of the possible reasons for this difference is that the probability of the second-photon absorption by one photon-absorbed FB particle within its lifetime is higher than that of L particle owing to larger volume of FB particles [3].

Fig. 1 Double-logarithmic plots of the reaction rate (R) and light intensity (I) observed in the FB or ML-photocatalyzed acetic-acid decomposition under high-intense 365-nm UV-LED photoirradiation (open circles). Data obtained by the diffraction grating type illuminator were also plotted; closed triangles, circles and squares correspond to irradiation wavelength of 320, 350 and 380 nm.

References

[1] (a) H. Hori, M. Takase, F. Amano and B. Ohtani, Chem. Lett. 44 (2015) 1723-1725. (b) H. Hori, M. Takase, M. Takashima, F. Amano, T. Shibayama and B. Ohtani, Catal. Today in press.

[2] T. Torimoto, Y. Aburakawa, Y. Kawahara, S. Ikeda, and B. Ohtani Chem. Phys. Lett., 392 (2004) 220.

[3] H. Hori, M. Takashima, M. Takase, F. Amano and B. Ohtani, Catal. Today in revision.