Photo-oxidation of ethanol on mesoporous vanadium–titanium oxide catalysts and the relation to vanadium(IV) and (V) sites
Introduction
Titanium dioxide works as photocatalysts based on the semi-conducting property [1], [2]. The light absorption by TiO2 predominantly lies in UV range. Only 3% of solar energy is utilized using the anatase TiO2 phase at the surface of earth [1]. The modification of TiO2 improved the efficiency of solar radiation utilization up to 20–30% by adding different elements, e.g. chromium, vanadium, platinum, or nitrogen to TiO2 [3], [4], [5], [6], [7], [8], [9]. In contrast, chloride, sulfate, or phosphate exhibited detrimental effects on the photocatalysis [10], [11], [12], [13].
To improve the photocatalysis of TiO2-based materials, another approach is to synthesize mesoporous TiO2 with high specific surface area. The applications of nano-crystalline and mesoporous TiO2 to the photodecomposition of 2,4,6-trichloropheonol and other organic compounds were suggested utilizing the availability of larger number of active sites [14], [15]. Syntheses of mesoporous and nano-crystalline TiO2 were reported via different routes [15], [16], [17], [18], [19], [20], [21], [22]. In this paper, wormhole-like, amorphous mesoporous materials with specific surface area as much as 1200 m2 g−1 were used [23], [24]. The doping effect on the red shift of UV–vis absorption was reported to follow the order V > Cr > Mn > Fe > Ni to TiO2 [3]. Therefore, series of mesoporous V + TiO2 samples were prepared and the performance of ethanol photo-oxidation reaction was compared to conventional V + TiO2(anatase) catalysts with the illumination of UV + visible light or visible light only.
The V site structures on/in TiO2 have been intensively studied by means of Raman, UV–vis, 51V nuclear magnetic resonance, and X-ray absorption fine structure spectroscopies [25], [26], [27], [28] and we recently reported the V structure transformation in on-site conditions and also the V structure for mesoporous V + TiO2 catalysts [24], [29]. The ethanol photo-oxidation reactivity over various mesoporous and conventional V + TiO2 catalysts was compared to the V site structure (geometric and electronic) information.
Section snippets
Syntheses of V + TiO2 catalysts
TiO2 (P-25, Degussa) with a specific surface area of 60 m2 g−1 was impregnated with V triisopropoxide oxide (1) in 2-propanol solution (impregnated V/TiO2). Major phase of TiO2 (P-25) was anatase being the ratio of anatase/rutile 95/5. Mesoporous V–TiO2 samples were prepared from compound 1, Ti tetraisopropoxide (2), and dodecylamine (3). An aqueous solution of the reactants was maintained at 333 K for 6 days and then filtered. The obtained powder was heated at 453 K for 10 days, and then washed
Ethanol photo-oxidation kinetics with UV–vis illumination
The time course of photo-oxidation reaction for ethanol (initial pressure 1.33 kPa) was depicted in Fig. 1 on mesoporous V + TiO2 catalysts. Major products were acetaldehyde, water, carbon dioxide, and acetic acid on mesoporous TiO2 (Fig. 1A) [34]. Because the ratio of formation rates for acetaldehyde and water was 2.2 (Table 1A), dehydration and dehydrogenation reactions for ethanol proceeded with comparable rates. Further photo-oxidized products acetic acid and carbon dioxide were minor. The
Discussion
Under the illumination of UV + visible light, total formation rates on carbon basis were in the order (Table 1)As a general trend, mesoporous TiO2-based catalysts were superior to anatase TiO2-based catalysts (Table 1A). Various kinds of specific photocatalysis under the illumination of light > 320 nm was reported using mesoporous TiO2 [35]. Acetic acid that was further oxidized from acetaldehyde was exclusively found in the mesoporous
Conclusions
- (1)
Under the illumination of UV + visible light, mesoporous V + TiO2 catalysts generally showed faster ethanol oxidation reaction than anatase V + TiO2 catalysts did. Major products were acetaldehyde, water, acetic acid, and carbon dioxide. Deeply oxidized acetic acid and carbon dioxide were preferably formed over the mesoporous V + TiO2 catalysts.
- (2)
Under the illumination of visible light only, mesoporous V–TiO2 catalyst was best and superior to anatase V + TiO2 catalysts. The phase of TiO2 controlled the
Acknowledgements
The authors are thankful for financial support from the Grant-in-Aid for Scientific Research (YI, C-17550073, B-13555230) and that on the Priority Area “Molecular Nano-dynamics” (YI, 432-17034013) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The XAFS experiments were performed under the approval of the SPring-8 Program Review Committee (2003A0146-NX-np and 2002B0739-NX-np) and that of the Photon Factory Proposal Review Committee (2001G308 and 2002G285).
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