Elsevier

Journal of Catalysis

Volume 225, Issue 1, 1 July 2004, Pages 223-229
Journal of Catalysis

Quantitative analysis of superoxide ion and hydrogen peroxide produced from molecular oxygen on photoirradiated TiO2 particles

https://doi.org/10.1016/j.jcat.2004.04.001Get rights and content

Abstract

The reduction of molecular oxygen is the counter reaction of most photocatalytic reactions proceeding oxidatively on titanium dioxide particles. We have quantitatively analyzed the reductive production of hydrogen peroxide and superoxide ion from oxygen in an aqueous solution containing 2-propanol as the scavenger of positive holes. The rates for the production of hydrogen peroxide and superoxide ion were determined by colorimetry using iodide and nitroblue tetrazolium, respectively. In addition, the oxidation of 2-propanol to acetone was monitored. Based on a comparison of these production rates, it was concluded that the main product from oxygen is hydrogen peroxide when TiO2 powder consisting mainly of anatase-form particles is used, whereas the main product is superoxide ion when TiO2 powder consisting mainly of rutile-form particles is used. The difference in the photocatalytic activity between these powders can be attributed to the difference between the reduction paths of oxygen on these powders. It was also found that the superoxide ion generated from molecular oxygen spontaneously reacts with 2-propanol to produce acetone and hydrogen peroxide.

Introduction

In recent years, studies of TiO2 photocatalytic reactions have attracted much attention because these reactions are useful for the decomposition and mineralization of pollutants and undesirable compounds in the air and in waste water [1], [2], [3], [4], [5], [6], [7], [8], [9]. The TiO2 photocatalytic reactions are also interesting from the viewpoint of organic syntheses [10], [11], [12], [13], [14], [15]. In addition, the photochemical reactions on TiO2 particles and on TiO2 films are of interest due to their potential application for the conversion of solar energy into chemical energy [16], [17], [18], [19], [20] and electric energy [21], [22]. In photocatalysis, light irradiation of TiO2 powder with a photon energy larger than the band-gap energy produces electrons (e) and holes (h+) in the conduction band and the valence band, respectively. These electrons and holes are thought to have the respective abilities to reduce and oxidize chemical species adsorbed on the surface of TiO2 particles. In most photocatalytic reactions, oxidation processes are utilized for a variety of purposes, and the reduction of molecular oxygen is often used as a counter reaction. Hence, in the field of photocatalysis, most of the research interest has been focused on oxidation processes. However, the reduction of molecular oxygen is important because it is an essential part of the photocatalytic processes taking place on TiO2 particles. In some cases, the reduction of molecular oxygen determines the efficiency of the overall photocatalytic reaction. This process is also essential because the products from molecular oxygen, i.e., superoxide ion (O2) [23], [24] and hydrogen peroxide [25], are considered to participate in subsequent reactions. However, few studies have focused on the quantitative analysis of these products from molecular oxygen.

In this paper, we discuss the reduction products from molecular oxygen in aqueous solutions containing 2-propanol as the hole scavenger. By using 2-propanol, we can neglect the oxidative production of hydrogen peroxide and superoxide ions, because the holes photogenerated in TiO2 particles are efficiently scavenged. In the discussion, the crystal structure effects of TiO2 particles on the reduction of molecular oxygen are highlighted. In addition, we show that the high photocatalytic activity of anatase particles is correlated with the high yield of hydrogen peroxide from molecular oxygen on anatase particles.

Section snippets

Materials

Titanium dioxide powders obtained from the Catalysis Society of Japan (TIO-1, TIO-2, TIO-3), Ishihara Sangyo Kaisha, Ltd. (ST-11, ST-21, CR-EL, PT-101), and Toho Titanium Co., Ltd. (NS-90) were used as the photocatalysts. Table 1 summarizes the physical properties of these TiO2 powders. Nitroblue tetrazolium (NBT), which was used for the analysis of the superoxide ion, was obtained from Wako Pure Chemical as a guaranteed reagent. Potassium superoxide was obtained from Aldrich Chemical Co. All

General properties of the photocatalytic reaction of 2-propanol on TiO2 particles

When molecular oxygen is reduced on the surface of TiO2 particles, the reduction products can be either superoxide ions or hydrogen peroxide. Although no experimental result has been reported, it is theoretically possible that molecular oxygen is reduced to water. The purpose of the present study was to quantitatively examine these reduction products obtained from molecular oxygen in TiO2-photocatalyzed reactions. For the analysis of the reduction products, the oxidation processes must be

Discussion

As shown in Table 3, the rate for the production of acetone agreed with the sum of the rate for the production of hydrogen peroxide and the half-rate for the production of superoxide ion, within the error limits. This result indicated that the reduction of molecular oxygen, as the counter reaction of the oxidation of 2-propanol, leads to the production of both hydrogen peroxide and superoxide ions, and that the production of water is negligibly small.

The results shown in Table 3 also indicate

Conclusions

We have quantitatively studied the reduction products obtained from molecular oxygen in TiO2-photocatalyzed reactions carried out in aqueous solutions containing 2-propanol. It was concluded that the main product is superoxide ion when rutile particles are used, whereas the main product is hydrogen peroxide when anatase particles are used. This difference is important because it is related to the fact that the photocatalytic activity of anatase powders is higher than that of rutile powders in

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  • Cited by (0)

    1

    Present address: Ion Engineering Research Institute Co., 2-8-1 Tsudayamate, Hirakata, Osaka 573-0128, Japan.

    2

    Present address: Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata, Kitakyushu, Fukuoka 804-8550, Japan.

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