Effect of silver deposits on the photocatalytic activity of titanium dioxide for the removal of 2-chlorophenol in water

Abstract

The effect of Ag photodeposition on the rate of photocatalytic degradation of 2-chlorophenol (2-CP) in aqueous suspensions of Degussa P 25 titanium dioxide has been studied using HPLC. For Ag contents between 5 × 10⁻⁶ and 1.6 × 10⁻⁴ wt%, the rate was slowest in the absence of Ag and increased with increasing Ag content. For a Ag content of 6 × 10⁻⁴ wt%, the degradation rate was about 50% faster than with bare TiO₂, and decreased for a Ag content of 7.2 × 10⁻⁴ wt% and beyond, eventually becoming slower than unmodified TiO₂. Bare TiO₂ has been compared with the 6 × 10⁻⁴ wt% Ag–TiO₂ sample in terms of the variation of chlorohydroquinone (CHQ) and chloro-1,4-benzoquinone (CBQ) concentrations during 2-CP degradation, and also for the degradation of CHQ and CBQ separately. On the basis of these experiments, it is tentatively suggested that the net effect of Ag on 2-CP removal results from opposite influences, viz. the Ag⁺ cation-induced oxidation of CHQ to CBQ and the Ag⁰ particle-induced recombination of photoproduced charges, both influences being assumed to depend on the Ag particle size, itself related to the Ag content.

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1 Introduction

The presence of harmful organic (especially aromatic) pollutants in wastewater effluents causes serious environmental problems, and purification of contaminated waters is one of the most interesting challenges in catalysis today. Over recent years, much attention has been directed towards heterogeneous photocatalytic detoxification, where water pollutants are degraded by irradiating aqueous suspensions of powdered semiconductor oxides, primarily focusing on TiO₂ as a highly active and durable photocatalyst.^1–5 In many cases, complete photocatalytic mineralisation of organic compounds in aqueous media has been reported. However, the quantum efficiency of photomineralisation on bare titania is relatively low because the photodegradation process, which involves interfacial transfer of photogenerated electrons and holes created within the semiconductor particles, competes with the recombination of these charge carriers. Therefore improvements in the photocatalytic activity of titania are required. Many studies have been devoted to increasing TiO₂ photocatalytic activity by depositing noble metals on the surface. In particular, Ag nanoparticles were found to slightly improve the dehydrogenation and the oxidation of 2-propanol,⁶ as well as the degradation of chloroform,⁷ urea,⁷ 1,4-dichlorobenzene,⁸ phenol⁹ and malic acid.¹⁰ Beneficial effects from Ag metal deposits were observed only at low silver contents.⁶

In this study, the photocatalytic degradation of 2-chlorophenol (2-CP), one of the standard test aromatic pollutants used in TiO₂ photocatalysis, has been investigated over Ag-loaded titania. Particular attention has been paid to the effect of the silver content on the formation and subsequent elimination of some aromatic intermediate products.

2 Experimental

The photochemical reactor was a 100 mL Pyrex cylindrical flask with an optical quartz window of 3.6 cm in diameter at the bottom. The radiation from a high pressure mercury lamp (Philips HPK 125 W) was filtered through a cuvette (thickness = 2.2 cm) full of water and a Corning 0.52 filter transmitting wavelengths above ca. 340 nm. The total radiant flux entering the photoreactor was approximately 40 mW cm⁻². All photocatalytic tests were performed under continuous magnetic stirring. For each irradiation experiment, 40 mL of aerated aqueous 2-CP solution (10⁻³ mol L⁻¹, pH 4.5) containing TiO₂ (2 g L⁻¹) was used. To equilibrate the starting system, the suspension of photocatalyst and 2-CP was first kept in darkness for 1.5 h before starting the irradiation. The percentage of 2-CP adsorbed on the TiO₂ particles was < 4%.

The concentrations of 2-CP and its intermediate products were measured by means of HPLC instrumentation comprising a LDC Constametric 3000 isocratic pump and a LDC Spectro Monitor D UV detector set at 254 nm. Separation of 2-CP and its aromatic intermediate products was achieved with a reverse-phase column (length 25 cm, inner diameter 4.6 mm) packed with Spherisorb 50DS2. The mobile phase was a ternary mixture of methanol (35 vol%), water (55 vol%) and acetonitrile. The intermediate products were identified by comparing their relative retention times with that of external standards.

The photoelectrochemical measurements were carried out by irradiating a stirred aqueous suspension of TiO₂ at 365 nm under potentiostatic conditions, employing a standard three-electrode polarisation device. A platinum flag having an area of 1 cm² was used as the collector electrode. Photocurrents were measured at +1 V vs. a Ag/AgCl reference electrode.

Silver nitrate, 2-CP, hydroquinone (HQ), chloro-2,4-dihydroxybenzene (chlorohydroquinone; CHQ), catechol (CT), chloro-1,4-benzoquinone (CBQ), hydroxy-1,4-benzoquinone (HBQ) and 1,2,4-trihydroxybenzene (hydroxyhydroquinone; HHQ) were supplied by Aldrich; all chemicals were of reagent grade and were used as received. The water employed in the experiments was purified using a three-stage Millipore Milli-Q Plus 185 system and had a resistivity higher than 18 MΩ cm.

Degussa P25 (∼80 wt% anatase, ∼20 wt% rutile, average particle size ca. 30 nm) titanium dioxide was used as the photocatalyst. A specific BET surface area of 51 m² g⁻¹ was determined with a Micromeretics Gemini II 2375 surface area analyser. To prepare Ag–TiO₂ samples, 50 mL of a 10⁻³ mol L⁻¹ AgNO₃ solution containing 100 mg of P25 TiO₂ was irradiated with UV light for 2 s to 4 min; the suspension was then filtered off, thoroughly washed and dried at 333 K. The amount of silver deposited was measured by conventional chemical analysis of the solid. The deposition of silver produced a slight increase in the surface area (up to 58 m² g⁻¹)

3 Results and discussion

Fig. 1 shows the influence of silver loading on the photocatalytic removal of 2-CP. The deposition of ∼5 × 10⁻⁶ wt% of silver resulted in a ca. 3-fold decrease in the 2-CP removal rate. At higher silver contents, the rate increased, but remained lower than that observed with bare TiO₂ for a loading of 1.6 × 10⁻⁴ wt% (plot 3 in Fig. 1). It reached a maximum at 6 × 10⁻⁴ wt% (referring to the Ag contents studied here); this maximum corresponds to a ca. 1.5-fold higher 2-CP removal rate with respect to bare TiO₂. The photocatalytic activity diminished by a factor of about five on increasing the Ag content from 6 × 10⁻⁴ to 9 × 10⁻⁴ wt% [Fig. 1(b)].

Fig. 1 (a) Photocatalytic removal of 2-CP as a function of time over bare TiO₂ (1), 5 × 10⁻⁶ wt% Ag–TiO₂ (2), 1.6 × 10⁻⁴ wt% Ag–TiO₂ (3) and 6.0 × 10⁻⁴ wt% Ag–TiO₂ (4). (b) Photocatalytic removal of 2-CP after 90 min irradiation as a function of Ag loading.

It has been shown previously using the HPLC analysis⁹ that the primary aromatic intermediate products of 2-CP in irradiated TiO₂ aqueous suspensions are CHQ and, to a lesser extent, CT.¹¹ CHQ is further oxidised to CBQ and, subsequently, to HBQ.¹¹ These quinones can act as electron scavengers,¹² competing with molecular oxygen for photogenerated electrons. As a result, back reaction to CHQ and HHQ occurs, producing the so-called photocatalytic short-circuit effect,¹³ which, at least in part, might account for the relatively low yields for the TiO₂-photosensitised degradation of chlorophenols and other phenols.^11,13–15

The variations in the concentrations (divided by the concentration of 2-CP that has decomposed at that particular time) of two main aromatic intermediate products (CHQ and CBQ) during 2-CP degradation are shown in Fig. 2. The kinetic curves show that the 6 × 10⁻⁴ wt% Ag loading results in a shift of the quinone–hydroquinone equilibrium towards the more oxidised product. This observation clearly illustrates the greater oxidising power of this Ag-loaded sample, a fact which is further confirmed by the higher 2-CP removal rate (Fig. 1) using this catalyst.

Fig. 2 Normalised CHQ and CBQ concentrations versus irradiation time over bare TiO₂ (1, 1′), 6 × 10⁻⁴ wt% and Ag–TiO₂ (2, 2′). Initial concentration of 2-CP: [2-CP]₀ = 1 m mol L⁻¹.

In an effort to better understand the net favourable effect of Ag loading at 6 × 10⁻⁴ wt% on the degradation rate of 2-CP, analogous experiments were performed for the photocatalytic degradation of CHQ (Fig. 3) and CBQ (Fig. 4), separately, under the same experimental conditions as for 2-CP decomposition. Fig. 3 confirms the oxidation of CHQ to CBQ.¹³ Any occurrence of the back reaction cannot be inferred from this experiment. Fig. 3 also shows that this Ag–TiO₂ specimen is more active than TiO₂ for CHQ degradation, as for 2-CP decomposition. The photocatalytic degradation of CBQ yielded more informative results. Fig. 4 very clearly indicates that, in spite of the oxidation conditions, CBQ is reduced to CHQ within the first few minutes of irradiation under our conditions. The concentrations measured after 5 min of irradiation indicate that this reduction is even almost quantitative over bare TiO₂. Also, the aforementioned short-circuit effect is evidenced by the nearly constant CBQ concentration between 5 and 30 min when using bare TiO₂ as the photocatalyst. The presence of Ag deposits produces a decrease in the CBQ reduction rate, as is clearly shown by the initially lower rate of CBQ removal, as well as by the lower concentrations of CHQ over the entire duration of the experiment. The greater oxidising power of this Ag-loaded sample is also corroborated by the lower value of the sum of the concentrations of CBQ and CHQ during the whole experiment.

Fig. 3 Photocatalytic degradation of CHQ in bare TiO₂ (a) and 6 × 10⁻⁴ wt% Ag–TiO₂ (b) suspensions. Comparison of CHQ elimination (1, 1′), CBQ concentration (2, 2′) and the sum of the concentrations of CHQ and CBQ (3, 3′). Initial concentration of CHQ: 0.1 mmol L⁻¹; TiO₂ concentration: 2 g L⁻¹.

Fig. 4 Photocatalytic degradation of CBQ in bare TiO₂ (a) and 6 × 10⁻⁴ wt% Ag–TiO₂ (b) suspensions. Comparison of CBQ elimination (1, 1′), CHQ concentration (2, 2′) and the sum of the concentrations of CHQ and CBQ (3, 3′). Initial concentration of CBQ: 0.1 mmol L⁻¹; TiO₂ concentration: 2 g L⁻¹.

The Ag deposits can play at least two roles. As in photographic development,¹⁶ Ag⁺ cations can effect the oxidation of hydroquinones (here CHQ) into quinones (here CBQ) and, accordingly, intervene positively in the degradation pathway of 2-CP (Fig. 5). The Ag metallic particles can attract the charge carriers and act as recombination centres, therefore decreasing the rate of photocatalytic removal of 2-CP and its intermediate degradation products. Fig. 1 shows that the net effect resulting from these opposite influences very much depends on the amount of silver deposited on the TiO₂. When the amount is very small, that is, when the irradiation time of TiO₂ in the AgNO₃ solution is very short, tiny silver particles are formed. It has been proposed¹⁷ that “shallow” surface states are thus created in TiO₂, which increases the rate of recombination of the photoproduced charges.¹⁸ We tentatively suggest that the increase in the Ag particle size for increasing Ag contents¹⁹ leads to deeper surface states related to these particles and thereby diminishes the charge recombination rate. For 6.0 × 10⁻⁴ wt% of silver, the net favourable effect reflects the dominant influence of the Ag-induced oxidation of CHQ (Fig. 3). For [Ag] ≥ 6.0 × 10⁻⁴ wt% the net unfavourable effect principally arise from an increased difficulty for bigger Ag metallic particles to supply Ag⁺ cations.

Fig. 5 Schematic illustration of 2-CP photocatalytic degradation on Ag-modified TiO₂ photocatalyst.

4 Conclusion

This study shows that, at least in the particular case of benzenic pollutants, Ag deposits can enhance the photocatalytic activity of TiO₂ for water purification. The Ag content must be finely adjusted and, presumably, the deposition method also plays a role. The stability over time also needs to be investigated. These results suggest a way of improving photocatalytic water treatment.

Previous conclusions^12,13 on the role of quinones in the photocatalytic degradation of benzenic pollutants are supported by this work. Our interpretations remain tentative, as further work is necessary to characterise the Ag–TiO₂ samples.

Acknowledgement

D. G. S. gratefully acknowledges the support of INTAS (grant YSF00-161) which, in particular, allowed him to carry out the degradation experiments in Lyon.

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