The 14th International Congress on Catalysis

Development and monitoring of mesoporous titania doped with vanadium oranions for selective ethanol oxidation under visible light
Yasuo Izumia,*, Kazushi Konishib, Dilshad Masihc, Hideaki YoshitakedaGraduate School of Science, Chiba University, Chiba, 263-8522, Japan
Interdisc. Grad. Sch. of Sci. & Eng. and cGlobal Edge Institute, Tokyo Inst. of Tech., Yokohama, 226-8502, Japan
dGraduate School of Engineering, Yokohama National University, Yokohama, 240-8501, Japan*Corresponding author: Phone&Fax +81-43-290-3696, E-mail yizumi@faculty.chiba-u.jpb

Abstract: Mesoporous TiO2 doped with V or anions (C, N, P, S) were synthesized to exhibit narrow mesopore size
distribution and high specific surface area as much as 1000 m2g–1. Under visible light, mesoporous TiO2 was inactive
for ethanol oxidation and dehydrogenation proceeded over [doped] TiO2 crystalline (P25). In contrast, selectiveethanol dehydration proceeded over mesoporous V-TiO2 under visible light. Dynamic equilibrium was demonstrated ofa few percents of V(III) state reduced by photoelectrons with anionic ethoxyl groups bound to partially-reduced V(IV–δ) state by selective XAFS spectroscopy. Tunable, efficient ethanol oxidation proceeded on anion-doped TiO2 due tolower electron-hole recoupling probability.

Keywords: Mesoporous, Visible light responsive, XAFS (X-ray absorption fine structure).
1. Introduction
One possibility for solar technology is photocatalysis over TiO2-based catalysts. Only 3% of solar energy
is used with crystalline TiO2 at the earth surface. The efficiency can be improved by doping with V,1 C, N,or S atoms2,3 and applied to the decomposition of ethanol, methylene blue, and nitric oxide.4
In this work, the framework of TiO2 was ultimately dispersed by mesoporous synthesis technique anddoping atom was added in the first synthetic step. Because the specific surface area was as much as 1000m2g–1, the doped atoms were effectively dispersed to surface. The surface sites will be coordinatively
unsaturated and reactive. The quantum size effect for mesoporous doped TiO2 and the difference of
impurity levels for between doped cation and anions are discussed.
2. Experimental section
Mesoporous V-TiO2 was synthesized from Ti(O-iC3H7)4, V=O(O-iC3H7)3, and dodecylamine.4 The
uniform pore size distributed at 3 nm. The doping of C, N, P, or S was performed similarly by adding NH3,P=O(OH)3, (NH2)2C=O, or (NH2)2C=S. 0.1 g of mesoporous doped TiO2 was homogeneously distributed inquartz reaction cell illuminated with xenon arc lamp operated at 500 W (Ushio). UV-cut filter L42 (Kenko)
was inserted to illuminate visible light only (> 420 nm). Ethanol (1.33 kPa) and O2 (2.67 kPa) wereintroduced in closed glass system (132 ml) at 290 K. Valence state-sensitive XAFS measurements wereperformed at beamline 7C of Photon Factory, KEK. V Kβ5,2 emission from samples associated withelectronic transition from V impurity level (between band gap) to V 1s was analyzed by homemade
fluorescence spectrometer and V K-edge XAFS spectra were measured by detecting the V Kβ5,2 emission.5
3. Results and discussion
Results of ethanol oxidation reactions were depicted in Figure 1. In contrast to ethanol dehydrogenationto acetaldehyde over TiO2 (P25) (A) and poor activity of mesoporous TiO2 (B), mesoporous V-TiO2
produced water, especially remarkable in the first 1h in addition to acetaldehyde formation (C). Synthesis

The 14th International Congress on Catalysis

rates for each compound became steady and almost equivalent. The reaction profile (initial remarkablewater formation followed by steady ethanol dehydration) was repeated by catalyst reactivation under UVvisible light for 3h between reaction batches. Steady reaction rates to acetaldehyde and water were 79 –72% and 65 – 44%, respectively, in the repeated batch tests compared to the first batch (Figure 1C).

Figure 1. Ethanol oxidation reaction as a function of time under visible light (> 420 nm) over crystalline
TiO2 (P25) (A), mesoporous TiO2 (B), and mesoporous V-TiO2 (3.0 wt% V; C) at 290 K.
Because mesoporous TiO2 was essentially inactive for ethanol oxidation, doped V is the key ofphotocatalysis. The V site structure transformation was followed step by step according with the specificdehydration. Based on the V Kβ5,2-selecting XAFS, dissociative adsorption of ethanol was found on V(IV)sites (Figure 2A-b). When visible light was turned on, V(III)sites were discriminated by V(III) state-selective XAFS, but thepopulation of V(III) species was a few percents. Thus, V(III)state reduced by photoelectrons (Figure 2A-c) and anionicethoxyl groups bound to partially-reduced V(IV–δ) species werein dynamic equilibrium. This finding was enabled by V stateselective XAFS to extract and confirm minor V(III) species.
The intermediate state was oxidized back (Figure 2A-d, a) byadding O2 and/or under visible light.
This paper is especially important because selective O–O
bond activation was discovered by creating impurity levels
near conduction band for V and near valence band for C, N, P,S (Figure 2B) for mesoporous TiO2 and catalytic key role undervisible light was clarified by state-selective XAFS.
Figure 2. (A) Proposed reaction mechanism of ethanol
dehydration over the mesoporous V-TiO2 under visible light (a – d). (B) Band gap and impurity levels forcrystalline and mesoporous doped TiO2.

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