J. Cent. South Univ. Technol. (2011) 18: 56-62

DOI: 10.1007/s11771-011-0658-x

Photocatalytic H₂ evolution activity of CuO/ZrO₂ composite catalyst under simulated sunlight irradiation

YAN Jian-hui(阎建辉)^{1, 2}, YAO Mao-hai(姚茂海)^{1, 2}, ZHANG Li(张丽)^{1, 2},

TANG You-gen(唐有根)^{1, 2}, YANG Hai-hua(杨海华)¹

1. Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology,

Yueyang 414000, China;

2. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: Zirconia-supported CuO (CuO/ZrO₂) composite photocatalysts were successfully synthesized via citric acid-assisted sol-gel technique. For comparison, CuO/ZrO₂ materials were also prepared by solid state reaction and co-precipitation method. The as-prepared powders were characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM), and thermogravimetric-differential thermal analysis (TG-DTA). The photocatalytic activity of CuO/ZrO₂ catalyst was investigated based on the H₂ evolution from oxalic acid solution under simulated sunlight irradiation. The effects of molar ratio of CuO to ZrO₂, preparation method, phase change with the calcination temperature and the durability on the photocatalytic activity of the photocatalyst were investigated in detail. It is found that the optimal activity of photocatalytic H₂ evolution (2.41 mmol·h^-1·g^-1) can be obtained when CuO/ZrO₂ composite photocatalyst is synthesized by sol-gel technique and the mole ratio of CuO to ZrO₂ is 40%. The activity of copper oxide supported on monoclinic ZrO₂ calcined at higher temperature is much higher than that on tetragonal ZrO₂ calcined at lower temperature, and the best calcination temperature is 900 ?C.

Key words: CuO/ZrO₂; photocatalytic activity; hydrogen evolution; simulated sunlight

1 Introduction

Because of the prominent advantages, such as clean, renewable and environment friendly, hydrogen has been considered to be an effective energy carrier in future. Owing to the fact that water and light are naturally abundant and inexhaustible, photocatalytic H₂ production from water splitting using semiconductor photocatalysts under UV and/or visible light irradiation has become a hot topic after the report by HONDA and FUJISHIMA for photoelectrochemical water splitting using a TiO₂ semiconductor electrode in 1972 [1]. Although TiO₂ is stable, innocuous, environmentally friendly and abundant, and has low cost [2], a great obstacle still exists that the band gap of TiO₂ is 3.2 eV and TiO₂ only responses to UV light irradiation. However, the energy of the UV light just occupies about 5% of the total solar energy [3]. Hence, in the past three decades, a series of photocatalysts have been developed for water splitting under visible light irradiation, such as tantalite [4-5] and niobate photocatalysts [6-7], metal sulfide [8], solid- solution photocatalysts [9-10], and doped photocatalysts [11-12]. Nevertheless, the variety of visible light- induced photocatalysts is still limited. Under such a background, searching new photocatalysts with the ability to absorb visible light is a key factor to photocatalytic H₂ production from water splitting.

Zirconia-supported CuO system has been demonstrated to have interesting catalytic properties in many reactions, such as water gas shift reaction [13], NO abatement reactions [14], CO/CO₂ hydrogenation [15- 16], and generation of H₂ from steam methane reforming [17-18]. However, there are few reports that CuO/ZrO₂ catalyst can be applied for photocatalytic H₂ production from water splitting under simulated sunlight irradiation. At present, in the aspect of the CuO composite photocatalysts, most of the investigations have been still focused on the composite of CuO/TiO₂. BANDARA et al [19] investigated the photocatalytic H₂ production activity of CuO/TiO₂ from methanol solution under UV light irradiation. Also, JIN et al [20] reported CuO/TiO₂ photocatalytic H₂ generation eosin-sensitized under visible light irradiation. Nevertheless, the photocatalytic H₂ evolution activity of CuO/TiO₂ is still limited.    For CuO/TiO₂ composite photocatalyst, TiO₂ plays an essential role for reaction, while CuO has a supportive role and H₂ production reaction starts with the excitation of TiO₂ particles. Compared with the CuO/TiO₂ composite photocatalyst, in the system of CuO/ZrO₂ composite photocatalyst, CuO plays an essential role for the reaction because the band gap of CuO is 1.7 eV [21]. Therefore, it not only is suitable for visible light-induced photocatalysis, but also generates more photo-excited electrons and holes than CuO/TiO₂; while ZrO₂ has a supportive role. ARDIZZONE and ORLANDI [22] indicated that ZrO₂ exhibited excellent adsorption ability to organic compounds. In the present study, oxalic acid, a kind of organism, acted as sacrificial reagent and was adsorbed by ZrO₂ in order to increase the separation of the photo-excited electrons and holes and improve the photocatalytic activity.

In this work, CuO/ZrO₂ catalyst was synthesized via three approaches: solid state reaction, co-precipitation method, as well as sol-gel technique. More speci?cally, the photocatalytic activity of the as-prepared CuO/ZrO₂ composite photocatalysts was investigated for photocatalytic H₂ generation with oxalic acid as sacrificial reagent under simulated sunlight irradiation.

2 Experimental

2.1 Photocatalysts preparation

All the chemicals used in experiments were of analytical grade with no further treatment. The photocatalyst CuO/ZrO₂ was synthesized by the following three approaches.

1) Solid state reaction. Mixture of CuO and ZrO₂ with different molar ratios was milled for 30 min in an agate mortar, then calcined at 900 ?C for 12 h in air at a heating rate of 10 ?C/min and slowly cooled to room temperature. In the end, it was milled for 10 min again to get the final product.

2) Sol-gel technique. A stoichiometric quantity of Cu(NO₃)₂?3H₂O, Zr(NO₃)₄?5H₂O and anhydrous citric acid were dissolved together in 100 mL anhydrous ethanol to get a mixed solution. Then, the solution was stirred vigorously and heated on a hot plate set at 80 °C by using a water bath, until a transparent and polymeric gel was obtained. The as-prepared gel was subsequently transferred into an oven and kept at 120 °C to dry to form a powder precursor of CuO/ZrO₂. The dried powder precursor was finally calcined at various temperatures (673-1 273 K) for 6 h in air.

3) Co-precipitation method. A stoichiometric quantity of Cu(NO₃)₂?3H₂O and ZrOCl₂?8H₂O were dissolved together in 100 mL distilled water to produce a blue solution. Then, 1 mol/L NaHCO₃ was added into the solution drop by drop under vigorous stirring until the pH became around 9-10, and viscous precipitates were produced. The precipitates were filtrated and washed by distilled water several times until no Cl^- was detected, then dried at 120 °C overnight, and subsequently calcined at 900 °C for 6 h in air.

2.2 Photocatalysts characterization

The crystal structure of the as-prepared photocatalyst was identi?ed by powder X-ray diffraction method (XRD, Bruker D8) using Cu K_α radiation (λ=1.541 8 ?) with a scanning angle (2θ) of 10?-85? at a scan speed of 4 (?)/min, a voltage of 40 kV and a current of 300 mA. The grain size and morphology of the photocatalyst were characterized with a Hitachi H-800 transmission electron microscope (TEM) operated at 200 kV. The temperatures of possible decomposition and phase changes were determined with a thermogravimetric-differential thermal analyzer at a rate of 10 ?C/min from room temperature to 600 ?C.

2.3 Photocatalytic activity measurements

The photocatalytic activity of the as-prepared photocatalyst was evaluated by measuring the amount of H₂ evolution from 0.05 mol/L oxalic acid solution under simulated sunlight irradiation. The H₂ production experiment was carried out in an inner irradiation-type reaction vessel made of pyrex, and a Xe (250 W) lamp was used as the simulated sunlight source to offer photons. In all experiments, to keep the reaction temperature at room temperature, water was used as the external circulation cooling, electric fan and vacuum pump as the internal cooling. Prior to irradiation, 0.6 g of the as-prepared photocatalyst was dispersed with 100 mL distilled water in an ultrasonic bath and put in the reactor, subsequently the reactor was left in dark and nitrogen flux was introduced into the system for 30 min to remove O₂. Afterwards, the photocatalytic reaction system was closed and exposed to the light irradiation. During the irradiation, the mixture was suspended by using a magnetic stirrer within the quartz cell. The as-produced H₂ was collected in a water manometer and was analyzed by gas chromatography (TCD, N₂ as gas carrier, zeolite NaX column). The volume of H₂ was taken as the volume of the displaced water at different time intervals of irradiation. Some experiments were repeated three times and the results were reproducible within the experimental errors (±4%).

3 Results and discussion

3.1 Characterization

Fig.1 depicts the XRD patterns of CuO/ZrO₂ composite photocatalyst prepared via solid state reaction, co-precipitation method, as well as sol-gel technique. All the samples were calcined at 900 ?C in air. It is obvious that the crystal structures of ZrO₂-supporter prepared via different approaches all are composed of monoclinic zirconia (M-ZrO₂). The diffraction intensities of sample prepared by sol-gel technique are lower than those of the sample prepared via solid state reaction. LI et al [23] consider this is probably due to the small particle size and/or the discrepancies in crystallinities.

Fig.1 XRD patterns of CuO/ZrO₂ composite photocatalyst prepared via three different routes

Fig.2 presents the XRD patterns of the CuO/ZrO₂ photocatalysts by sol-gel method calcined at different temperatures. At 400 ?C, the tetragonal zirconia (T-ZrO₂) structure is dominant with a small amount of the monoclinic zirconia (M-ZrO₂) and some amorphous phase. At 500 ?C and 600 ?C, the peak intensities of M-ZrO₂, T-ZrO₂ and copper oxide all get stronger, and the full-width at half-maximum (FWHM) of T-ZrO₂ and copper oxide all become narrower. Whereas, after being calcined at 700 ?C, drastic crystallization of M-ZrO₂ has occurred and M-ZrO₂ is in a dominant position. When the calcination temperature is raised from 800 ?C to     1 000 ?C, the diffraction peaks of T-ZrO₂ disappear, only M-ZrO₂ structure exists, and stronger integral peaks intensity and narrower FWHM are observed, indicating

Fig.2 XRD patterns of CuO/ZrO₂ composite photocatalysts calcined at different temperatures for 12 h

good crystallinity. Other studies have also indicated that the calcination temperature changes the crystal structure of ZrO₂ [24]

Fig.3 presents TEM images of the CuO/ZrO₂ photocatalyst fabricated via solid state reaction, co-precipitation method, as well as sol-gel technique. It can be seen that CuO and ZrO₂ particles are successfully synthesized by any of three methods. The particle size of ZrO₂-supporter synthesized by sol-gel technique is about 90 nm which is smaller than that prepared by co-precipitation method or solid state reaction. In addition, some particles prepared by solid state reaction

Fig.3 TEM images of CuO/ZrO₂ composite photocatalysts synthesized via three different approaches: (a) Sol-gel technique; (b) Co-precipitation method; (c) Solid state reaction

are found to be aggregated badly.

Fig.4 shows the TG-DTA curves of the precursor sample of CuO/ZrO₂ dried gel. From the thermo- gravimetric curve, there is three obvious mass losses in the temperature range of 30-350 ?C. The first mass loss before 200 ?C may be attributed to the release of H₂O in the existence of physical absorption and the volatilization of organic solvent such as ethanol. The second mass loss at 200-300 ?C can be ascribed to the decomposition of the citric acid. The third mass loss at 300-350 ?C is due to the decomposition of nitrate and synthesis of the CuO/ZrO₂ compounds. Meanwhile, the corresponding strong endothermic peak at about 340 ?C is observed from differential thermal curves.

Fig.4 TG-DTA curves of precursor sample of CuO/ZrO₂ dried gel

3.2 Photocatalytic activity

3.2.1 Effect of molar ratio of CuO to ZrO₂ on photo- catalytic H₂ evolution activity

Fig.5 summarizes the rate of H₂ evolution on CuO/ZrO₂ composite photocatalysts containing different amounts of CuO obtained via solid state reaction. No H₂ is detected when pure ZrO₂ is used as photocatalyst, suggesting that ZrO₂ is not active for photocatalytic H₂ evolution under simulated sunlight irradiation. But after CuO is mixed with ZrO₂, the activity for H₂ evolution is increased greatly. It can be concluded that the composite of CuO with ZrO₂ is necessary for photocatalytic H₂ evolution under simulated sunlight irradiation. In the system of CuO/ZrO₂ composite photocatalysts, H₂ evolves on the CuO particles, and ZrO₂ particles act as supporter to adsorb the reactive species such as sacrificial reagent so as to improve the separation of the photo-excited electrons and holes. Hence, with the increase of the molar ratio of CuO to ZrO₂, the amount of H₂ evolution on CuO/ZrO₂ photocatalyst is further increased and achieves a maximum when the molar ratio of CuO to ZrO₂ is 40%. However, if the molar ratio exceeds 40%, the quantity of ZrO₂ will be decreased and influence the adsorption to the reactive species, which leads to the decrease of the consumption of the photo-generated holes, and further influences the separation of the photo-excited electrons and holes. Consequently, the photocatalytic activity is decreased. The appearance of a maximum activity with an optimum molar ratio of CuO to ZrO₂ has also been observed for other preparation methods.

Fig.5 Maximal amount of H₂ evolution on CuO/ZrO₂ composite photocatalysts obtained via solid state reaction in different molar ratios of CuO to ZrO₂ (Reaction conditions: concentration of photocatalyst 1 g/L; concentration of oxalic acid 0.05 mol/L; volume of distilled water 600 mL)

3.2.2 Effect of preparation method on photocatalytic H₂ evolution activity

Fig.6 illustrates the effect of photocatalyst preparation method on the photocatalytic H₂ evolution activity. It reveals that the CuO/ZrO₂ composite photocatalysts synthesized by the sol-gel technique show the highest photocatalytic activity for H₂ evolution. The TEM and XRD results demonstrate that the synthesis approach has great effects on the size and crystallinity of the product. The particle size and crystallinity strongly affect the photocatalytic activity for H₂ evolution [25]. The higher the degree of crystallinity is, the smaller the amount of defects is. The defects operate as trapping and recombination centers between photoexcited electrons and holes, leading to a decrease in the photocatalytic activity. And the smaller the particle size is, the shorter the distance that photoexcited electrons and holes have to migrate to reaction sites on the surface become, resulting in a decrease in the recombination probability. The resultant photocatalytic activity is dominated by the balance among these factors.

In the present work, the grinding and calcination processes in the solid state reaction result in the inhomogeneous mixture, great mass change and bad dispersion of CuO particles on the surface of ZrO₂-supporter, going against the photocatalytic activity. In the co-precipitation method, the homogeneity and dispersion are better than those of the samples prepared via solid state reaction, but the particle size and crystallinity of photocatalysts are compared unfavorably to the samples prepared via sol-gel technique. The sol-gel technique is superior to the other two preparation methods. Here, the using of citric acid as chelating agent, an important aspect of the sol-gel technique, ensures the homogeneity and dispersion of CuO on the surface of the ZrO₂-supporter, the smaller size and higher crystallinity of the photocatalysts.

Fig.6 Effect of photocatalyst preparation method on H₂ evolution activity (Reaction conditions: concentration of photocatalyst 1 g/L; concentration of oxalic acid 0.05 mol/L; volume of distilled water 600 mL)

3.2.3 Effect of calcination temperature on photocatalytic H₂ evolution activity

Fig.7 reveals the H₂ evolution activity of the CuO/ZrO₂ composite photocatalyst synthesized via the sol-gel technique as a function of the calcination temperature. It is pointed out that, when the calcination temperature is increased from 400 ?C to 600 ?C, the amount of H₂ evolution on the CuO/ZrO₂ composite photocatalyst is not improved obviously. With further increasing the calcination temperature, however, the amount of H₂ evolution is increased greatly, and reaches the maximum (2.41 mmol·h^-1·g^-1) at 900 ?C. As shown in the XRD results (Fig.2), with the change of the crystal structure from tetragonal at low temperature to monoclinic at high temperature, the photocatalytic activity is improved gradually, indicating that the photocatalytic activity of copper oxide supported on M-ZrO₂ is much higher than that supported on T-ZrO₂. The crystal structure of ZrO₂-supporter has great effect on the activity of CuO/ZrO₂ catalyst.

RHODES and BELL [26] demonstrated that the activity of copper supported on M-ZrO₂ is considerably higher than that on T-ZrO₂. This difference in activities has been attributed to the different adsorption capacities of both ZrO₂ structures to adsorb reactive species. In this

Fig.7 Effect of calcination temperature on H₂ evolution activity over CuO/ZrO₂ composite photocatalyst synthesized via sol-gel technique (Reaction conditions: concentration of photocatalyst 1 g/L; concentration of oxalic acid 0.05 mol/L; volume of distilled water 600 mL)

experiment, oxalic acid has been used as sacrificial reagent in order to consume the photo-generated holes, and it exists in the form of HC₂O₄^-. Therefore, when using M-ZrO₂ as supporter, more reactive species of HC₂O₄^- are adsorbed on the surface of CuO/ZrO₂ composite photocatalyst than that of CuO supported on T-ZrO₂, leading to quick consumption of the photo-excited holes. Accordingly, the photo-excited electrons and holes in CuO/M-ZrO₂ can be separated more efficiently than those in CuO/T-ZrO₂, which results in the higher photocatalytic activity of copper oxide supported on M-ZrO₂ than that supported on T-ZrO₂.

3.2.4 Durability of photocatalyst

The durability of the CuO/ZrO₂ composite photocatalyst was studied by using the same catalyst for H₂ evolution several times. Fig.8 depicts the amount of

Fig.8 Durability of as-obtained CuO/ZrO₂ composite photocatalyst prepared by sol-gel technique (Reaction conditions: concentration of photocatalyst 1 g/L; initial concentration of oxalic acid 0.05 mol/L; volume of distilled water 600 mL; irradiation time 12 h)

H₂ evolution with repeated use of the catalyst. After being irradiated for 12 h, the suspensions were kept in dark for 12 h, purged with N₂ for 30 min and added with fresh oxalic acid, then irradiated again. In each run, the continuous consumption of oxalic acid and the adsorption of by-products on the photocatalyst surface result in slight decrease of photocatalytic activity. However, the addition of fresh oxalic acid in the second or third run restores photocatalytic activity. There is just a little decrease of the photocatalytic H₂ evolution activity in repeated runs for 36 h. So, CuO/ZrO₂ composite photocatalyst is considered to be a stable photocatalyst for photocatalytic H₂ evolution under simulated sunlight irradiation.

4 Conclusions

1) The photocatalytic H₂ evolution activity of CuO/ZrO₂ catalyst is affected by the molar ratio of CuO to ZrO₂. With increasing the molar ratio of CuO to ZrO₂, the amount of H₂ evolution on CuO/ZrO₂ photocatalyst is increased and achieves a maximum when the molar ratio is 40%.

2) The CuO/ZrO₂ photocatalysts synthesized by three different methods exhibit different photocatalytic H₂ evolution activity. Sol-gel technique is superior to the other two preparation methods. Because of the introduction of the citric acid as chelating agent, the CuO/ZrO₂ composite photocatalysts own the best homogeneity and dispersion, as well as the smaller size and higher crystallinity.

3) With increasing calcination temperature, the crystal structure of ZrO₂-supporter is changed from T-ZrO₂ at low calcination temperature to M-ZrO₂ at high calcination temperature, and M-ZrO₂ can adsorb more reactive species than T-ZrO₂. Therefore, The photocatalytic H₂ evolution activity of the CuO/ZrO₂ composite photocatalyst is improved with increasing the calcination temperature, and achieves the maximal activity (2.41 mmol·h^-1·g^-1) at 900 ?C.

4) The CuO/ZrO₂ catalyst is a stable photocatalyst for H₂ evolution from aqueous oxalic acid solution under simulated sunlight irradiation. However, the band structure and the photocatalytic mechanism of the as-prepared CuO/ZrO₂ composite photocatalyst are still unclear and further research needs to be performed.

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(Edited by YANG Bing)

Foundation item: Project(20876039) supported by the National Natural Science Foundation of China; Project(09JJ3023) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2009-12-17; Accepted date: 2010-04-30

Corresponding author: YAN Jian-hui, Professor, PhD; Tel: +86-730-8640436; E-mail: yanjh58@163.com