Crystal phase-dependent generation of mobile OH radicals on TiO2: Revisiting the photocatalytic oxidation mechanism of anatase and rutile

Ji Young Hwang, Gun hee Moon, Bupmo Kim, Takashi Tachikawa, Tetsuro Majima, Seungwoo Hong, Kangwoo Cho, Wooyul Kim, Wonyong Choi

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57 Scopus citations

Abstract

Titanium dioxide has been the most popular environmental photocatalyst of which role critically depends on the generation of OH radicals. In particular, the mobile free OH racial ([rad]OHf) generation and the subsequent diffusion from the surface are critical in achieving the mineralization of non-adsorbing substrates by extending the reaction zone from the surface to the solution bulk. Here the origin of the crystalline phase-dependent generation of [rad]OHf was investigated using tetramethylammonium (TMA) cation as a main probe compound for [rad]OHf in a UV/TiO2 photocatalytic system. We found a clear evidence that the mobile free OH radical is generated through a reductive conversion of dissolved O2 on anatase only (O2 → H2O2[rad]OHf). The surface trapped holes are not involved in [rad]OHf formation, but lead to the generation of surface-bound OH radical ([rad]OHs) on both anatase and rutile. The generation of [rad]OHf is favorable on anatase because more H2O2 are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of [rad]OHf formation. The generation of 18O-labelled p-hydroxybenzoic acid on anatase only (not rutile) from benzoic acid oxidation under 18O2-saturated condition provides a solid evidence that the [rad]OHf generation mechanism on anatase involves the reductive pathway. Better understanding of [rad]OHf production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of [rad]OHf can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.

Original languageEnglish
Article number119905
JournalApplied Catalysis B: Environmental
Volume286
DOIs
StatePublished - 5 Jun 2021

Bibliographical note

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • Anatase
  • Hydroxyl radicals
  • Photocatalytic oxidation mechanism
  • Rutile
  • TiO

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