Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

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Abstract

The commercially available TiO2-catalyst (Degussa P25) was modified with nanosized platinum and silver particles by the photoreduction method to obtain different noble metal loading (0.5 and 1 wt.%). The characterization of the synthesized catalysts was carried out by the BET method, XPS, TEM and the adsorption of the model pollutant. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO2, modified with nanosized platinum or silver particles. The photocatalytic activity of TiO2, modified with noble metal, is approximately double that of the semiconducting support. The adsorption properties of the catalysts, as well as the noble metal content on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schottky barriers on the metal–semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.

Introduction

TiO2 heterogeneous photocatalysis has been the subject of numerous investigations recently as it is an attractive technique for the complete destruction of undesirable contaminants both in aqueous [1], [2], [3] and gaseous phase [4], [5], [6] by using solar or artificial light illumination. Titania photocatalysis advantages, such as strong resistance to chemical and photocorrosion, low operational temperature, low cost, significantly low energy consumption, have led the relevant applications to the stage of commercialization [1], [7]. Upon band-gap excitation of TiO2, the photoinduced electrons and positively charged holes can respectively reduce and oxidize species adsorbed on the semiconductor particles. The high degree of recombination between photogenerated electrons and holes is a major rate-limiting factor controlling the photocatalytic efficiency [8]. The improvement of the photocatalytic activity of TiO2 is one of the most important aspects of heterogeneous photocatalysis. Attempts to increase the TiO2 efficiency have been made by doping and coating with transition metals or noble metals [9], [10], [11], [12], [13], combining the effect of two semiconductors or covering the surface with dye microcrystallites to increase the optical absorption in the visible range [14], [15], [16], [17], [18], [19], [20], [21]. The usual methods for modification of TiO2 with noble metals are thermal impregnation [11], [22] and photodeposition [9], [10], [13], [18], [23], [24], [25], [26]. The latter technique was reported to yield more active photocatalysts [23]. The photodeposition process involves the reduction of metal ions by the conduction band electrons, the anodic process being the oxidation of water by valence band holes [10], [13], [24], [25]. Oxidizable additives (sacrificial electron donors) such as acetate, formaldehyde, methanol, 2-propanol or oxalic acid are generally added to improve the rate of photodeposition [18], [27].

The photocatalytic destruction of oxalic acid has been studied in the presence of platinum- or silver-modified TiO2. The modifying with the noble metal was accomplished by the photochemical impregnation method upon irradiation of the sample with UV-light. The aim of the study was to investigate the influence of the noble metal deposited on TiO2 upon the quantum yield of the redox process photocatalyzed by the metallized samples.

Section snippets

Materials and preparation of photocatalysts

TiO2 (Degussa, P25) was used as a starting photocatalytic material. Ammonium hexachloroplatinate (IV), (NH4)2PtCl6·6H2O (Aldrich), AgNO3 (Merck) and oxalic acid (Aldrich) were used without any further purification. A modification of the photodeposition method (described earlier [27], [28]) was applied for the synthesis of the Pt and Ag loaded TiO2 photocatalytic materials. The amounts of Pt or Ag loaded were 0.5 and 1.0 wt.% with respect to the TiO2 amount. The procedures of photodeposition are

Results and discussion

The specific surface area of the catalysts, the degree of crystallinity of TiO2 and the size of the metal clusters on the surface of the TiO2 influence substantially the catalyst efficiency in regard to photodestruction of the water contaminants [11], [13]. In spite of the fact that there is no change in the specific surface area of TiO2, after modifying it with Pt or Ag, the adsorption of the oxalic acid on the modified sample is lower (Table 1). This is due to diminishing of the TiO2

Conclusions

Upon modifying TiO2 with metal nanoparticles (Pt or Ag) the rate of photocatalytic destruction of oxalic acid is increased in comparison to the pure TiO2. At 1% noble metal loading the rate is double that on the pure TiO2. The increase of the reaction rate constants kr is registered also upon increasing the Pt or Ag loading on TiO2. The reasons for the changes in the value of kr cannot be interpreted unambiguously as the photocatalytic activity of the samples is influenced by two factors,

Acknowledgement

The authors gratefully acknowledge financial support by NATO, Programme “Science for Peace”, Contract SfP-977986.

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