Effects of electrolyte NaCl on photocatalytic hydrogen evolution in the presence of electron donors over Pt/TiO2

https://doi.org/10.1016/j.molcata.2011.03.026Get rights and content

Abstract

The effects of electrolyte NaCl, a major component of natural seawater, on photocatalytic hydrogen evolution in the presence of electron donors over Pt/TiO2 have been investigated. The adsorption performance and surface reaction of electrolyte NaCl and the electron donors on TiO2 have been characterized by electrophoretic analysis and in situ attenuated total reflectance infrared spectroscopy. Under acidic condition Cl ions are adsorbed on TiO2, while under neutral and basic condition Na+ ions are adsorbed on TiO2. In the case of ethanol as an electron donor, the activity of photocatalytic hydrogen evolution over Pt/TiO2 increases when concentration of NaCl < 0.10 mol L−1, while the activity decreases when the concentration > 0.10 mol L−1. At different pH values, the activity decreases in the order: basic > neutral > acidic. In the cases of formic acid and oxalic acid as electron donors, the activities decrease with increase of NaCl concentration. The Na+ ions can affect adsorption performance of ethanol on TiO2. Possible reaction mechanisms were discussed.

Highlights

► We investigate effect of NaCl on photocatalytic H2 evolution with electron donors. ► In ethanol system, Na+ adsorbed at photocatalyst TiO2 increases the H2 evolution. ► In formic acid or oxalic acid system, adsorbed Cl decreases the H2 evolution.

Introduction

Photocatalytic splitting water (PSW) using a heterogeneous photocatalyst has been studied extensively as a potential method to supply hydrogen from sunlight and water [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Although water is abundant on the earth, pure water is scarce because 93% of the earth's water is present in oceans and inland seas and most of the remaining water is isolated as glacier in the Polar Regions [12]. Almost all studies on PWS so far have been performed in pure water (distilled water). From the viewpoint of practical application, producing hydrogen from natural seawater would be highly desirable. Lee et al. reported PWS from seawater over La2Ti2O7 under ultraviolet light, and over CdS/TiO2 under visible light [12]. Domen et al. have investigated effects of some electrolytes including NaCl on the photocatalytic activity of (Ga1−xZnx)(N1−xOx) for overall water splitting under visible light (λ > 400 nm) [13].

In the absence of an electron donor, the efficiency of photocatalytic hydrogen generation is very low because of the recombination of photoinduced electrons and holes on semiconductor surface [14]. In order to achieve higher efficiency for photoinduced hydrogen production, many researches in this field have involved electron donors as sacrificial agents, which can react irreversibly with the photoinduced holes. From the viewpoint of practical application, the electron donors for the hydrogen generation should be cheap and easy to obtain. We found that photocatalytic hydrogen production increases with simultaneous degradation of pollutants (electron donors) [15], [16], [17], [18]. Kondarides et al. have also investigated photocatalytic hydrogen evolution using pollutants as electron donors [19]. Biomass such as glucose, the most versatile renewable resource, has been utilized for photocatalytic hydrogen generation over Pt/TiO2 [20], [21], [22], Pt/CdxZn1−xS [23] and Pt/ZnS–ZnIn2S4 [24]. However, there are few studies on photocatalytic hydrogen production from seawater with pollutants or biomass as electron donors.

The electrophoretic analysis has been widely used to monitor electrolyte adsorption on colloid particles including TiO2 suspension. Attenuated total reflectance (ATR) infrared (IR) spectroscopy is an effective tool to investigate solid/liquid interfacial phenomena, and many literatures have been published recently [16]. In order to understand the surface performances of electrolyte NaCl and organic electron donors on TiO2, electrophoretic analysis and ATR-IR have been used in this work.

Ethanol is a cheap electron donor from biomasses such as starch, and oxalic acid and formic acid are common organic pollutants. In this paper, the effects of electrolyte NaCl on photocatalytic hydrogen evolution in the presence of the electron donors over Pt/TiO2 have been investigated, because NaCl is a major component of natural seawater. Possible reaction mechanisms were discussed.

Section snippets

Experimental

All reagents were of analytical grade and were used without further purification. 0.50 wt% Pt/TiO2 was prepared by the method of photodeposition [15]: 1.00 g anatase TiO2 (Shanghai Kangyu Co. Ltd., particle size 20 nm, BET surface area 124 m2 g−1), 13.34 mL 1.93 × 10−3 mol L−1 H2PtCl6, 1.0 mL anhydrous ethanol and 85.66 mL distilled water were added to a Pyrex cell with a flat window for illumination. Other reaction conditions for Pt deposition were the same as that for photocatalytic hydrogen evolution

Surface performances of electron donors and NaCl on TiO2

Fig. 1 shows effect of NaCl concentration on zeta potential of TiO2 in the presence of formic acid and oxalic acid. With increase of NaCl concentration, zeta potential of TiO2 in the presence of formic acid decreases (from 26.50 mV to 9.38), while the potential in the presence of oxalic acid increases (from −29.13 mV to ca. 0). At high NaCl concentration (above 0.50 mol L−1), their zeta potentials are almost independent of the NaCl concentration.

In water solution, there are many surface hydroxyl

Conclusions

The effects of electrolyte NaCl on photocatalytic hydrogen evolution in the presence of the electron donors over Pt/TiO2 have been investigated. Under acidic condition Cl ions are adsorbed on TiO2, while under neutral and basic condition Na+ ions are adsorbed on TiO2. In the case of ethanol as an electron donor, the activity of photocatalytic hydrogen evolution increases when NaCl < 0.10 mol L−1, while the activity decreases when NaCl > 0.10 mol L−1. At different pH values, the activity decreases in

Acknowledgements

The financial supports of National Basic Research Program of China (2009CB220003), the National Nature Science Foundation of China (20763006), Specialized Research Fund for the Doctoral Program of Higher Education of China (20060403006), and Research Fund of Education Ministry of Jiangxi, China (GJJ09041) are gratefully acknowledged.

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