98

Proceedings of the 90th JSCM Anniversary Conference
J. Jpn. Soc. Colour Mater., 91〔3〕，98–102（2018）

Decomposition of 2-Naphthol in Water by Brookite-Type TiO2 Modifiedwith MnOx and CeOy Under Dark Condition
Yirong Qi＊, Mimori Shiohara＊, Daichi Tanaka＊, Toshihiro Isobe＊,
Sachiko M atsushita＊ and Akira Nakajima＊,†＊ Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,

2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan†Corresponding Author, E-mail: anakajim@ceram.titech.ac.jp(Received November 13, 2017; Accepted January 9, 2018)
Abstract
Brookite-type TiO2 was modified with MnOx and CeOy by chemisorption calcination cycle (CCC) process. Then the decomposition
activity on 2-naphthol in water was evaluated for these samples at 50°C in the dark. The decomposition activity increased concomitantlywith the increase of the MnOx modification cycle. Subsequent modification of CeOy onto a MnOx-modified sample slightly increased thedecomposition activity. The modification of CeOy raised the decomposition rate remarkably when the modification order between MnOx andCeOy was converted, suggesting the importance of interaction between the base material and modified MnOx.
Key-words: Mn, Ce, Brookite, 2-naphthol, Dark

1．Introduction
Numerous studies have been conducted of titanium dioxide(TiO2), a well-known semiconductor photocatalyst1), for
its application to environmental purification2-6). Although
TiO2 is safe and abundant in nature, one shortcoming is
the requirement of UV light, only a small fraction of theentire sunlight spectrum, for electron excitation. Combining
materials with decomposition activity against organicsubstances in the dark with TiO 2 enables continuous
environment purification throughout the day.
Recent investigations have revealed that MnOx decomposesorganic compounds in the dark under normal pressure attemperatures less than 100°C7-11). Their activity is attributed
to the Mars-van Krevelen (MvK) mechanism8,10,11). Veryrecently, we modified MnOx onto a rutile-type TiO2 surface
by chemisorption calcination cycle (CCC) processing12).
The decomposition activity of the materials on 2-naphthol
was evaluated in water at 50°C. Both the decomposition
activity in the dark and the visible light photocatalytic
activity were provided to TiO2 by modification of MnOx.
The results revealed that the dark activity originated from
the MvK mechanism. Synergistic effects on decompositionin the dark were inferred for the subsequent loading of
CeOy after MnOx modification. In the MvK mechanism,oxidation of organic compounds proceeds with reduction ofMnOx. The reduced MnOx is expected to be oxidized againby oxygen (or dissolved oxygen). An earlier study revealed
that CeO2 works as oxygen reservoir to oxidize MnOx13).
© 2018 Journal of the Japan Society of Colour Material

Actually, reductions of Mn (Mn(IV)→Mn(III)→Mn(II))
and Ce (Ce(IV)→Ce(III)) via repeated use were confirmed
for this material system12). Therefore, decomposition in thedark gradually decreased via repeated use, but recovered as aresult of heat treatment in ambient air.
In our earlier study, effects of MnOx and CeOy modification
on dark activity are less remarkable for anatase than forrutile12). One plausible explanation is the interaction between
base material and MnOx. Zhang et al. demonstrated thatthe reduction temperatures for both Mn(IV)→Mn(III) andMn(III)→Mn(II) depend on the base material impregnatedwith MnO 2 14 ) . α-MnO 2 possesses a rutile-type crystal
structure15). Therefore, activation energy for reduction ofMnOx might be different between anatase and rutile. ThreeTiO2 polymorphs are rutile, anatase, and brookite. Althoughanatase has been used widely as a photocatalytic material,
several reports have described the high photocatalytic
activity of brookite16,17). For the present study, using theCCC process, we modified brookite-type TiO2 particles withMnOx and CeOy. Then, the activity of the powders on thedecomposition of 2-naphthol in water was investigated in thedark, as in earlier studies8,12).

2．Experimental
2.1 Sample preparation and characterization
All starting chemicals used for this study were reagentgrade. A brookite-type TiO2 powder (Titanium(IV) Oxide,99.99%, Koujundo Chemical Lab. Co. Ltd. ) was fired
at 500°C for 1 h in air. The TiO 2 powder ( 1.2 g ) was－18－