CuO nano-composites via a new method

Available online at www.sciencedirect.com

Materials Chemistry and Physics 108 (2008) 165–169

Materials science communication

Preparation and catalytic properties of Ag/CuO nano-composites via a new method Long-Shuo Wang, Jian-Cheng Deng ∗ , Fan Yang, Ting Chen College of Chemistry, Xiangtan University, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan 411105, Hunan, China Received 29 April 2007; received in revised form 15 September 2007; accepted 22 September 2007

Abstract Ag/CuO composite nanopowder was successfully prepared using metal copper, silver carbonate, ammonia, and ammonium bicarbonate as starting materials via coordination oxidation homogeneous precipitation method. Field-emission scanning electron microscopy (FESEM), thermogravimetry/differential thermal analysis (TG–DTA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared absorption spectra (FT-IR) were used to characterize the microstructure and morphology of the products. Furthermore, catalytic activities of the products to H2 O2 decomposition were investigated. The results show that the obtained Ag/CuO nano-composites possess high catalytic activity to H2 O2 decomposition. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite materials; Chemical synthesis; Copper oxide; Silver

1. Introduction Copper oxide with a narrow band gap (Eg = 1.2 eV) is of considerable interest owing to its roles in catalysis, metallurgy, electrode materials, solar energy transformation, semiconductors, gas sensors and high-Tc superconductors [1–5]. With the decrease in the crystal size, nanosized copper oxides may exhibit unique properties which can be significantly different from those of their bulk counterparts, for example, the large interfacial areas, highly reactive surfaces, unusual optical, electrical, and catalytic properties, etc. Hydrogen peroxide can be used as a green fuel/propellant instead of carcinogenic hydrazine in spaceflight [6]. The key technique is catalytic decomposition of hydrogen peroxide. Nanosized CuO has shown to be one of suitable catalyst to the decomposition of hydrogen peroxide in recent researches [7]. But the catalytic activity of simplex nano CuO is not very high. CuO/Fe2 O3 and Cu2 (OH)3 Cl/CuO nano-composites with high catalytic activities have been synthe-

∗

Corresponding author. Tel.: +86 732 8292229; fax: +86 732 8292477. E-mail address: [email protected] (J.-C. Deng).

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.09.029

sized by sonochemical decomposition method [8,9], however the synthesis process is complicated, and the agglomeration of nano-particles cannot be easily controlled. Studying a new type nanosized copper oxide composites in order to improve its catalytic activity remains challenge. The addition of Ag in metal oxides can improve their catalytic activities [10–12], and only a few reports on the preparation of Ag/CuO composites with nanostructures have been published, so we have researched about this aspect. Synthesis of nano CuO and related copper compounds were commonly accomplished in solution using copper salts and other metal salts as starting materials. The drawbacks of these synthesis approaches are that the anion should be removed through abstersion, which results in the longer production process and the higher cost. Moreover, during the several washing processes, the particle size increases because of the agglomeration of products, which decrease the catalytic ability at last. In this paper, Ag/CuO nano-composites were prepared using metal copper as Cu2+ source, ammonia as coordination agent and air as oxidizer via coordination oxidation homogeneous precipitation method. During the synthesis process, anion that could not be easily removed was not introduced into reactant solution, so washing step was not needed.

166

L.-S. Wang et al. / Materials Chemistry and Physics 108 (2008) 165–169

Ag/CuO nano-composites with an average diameter of about 10 nm displayed high catalytic activity to H2 O2 decomposition. 2. Experimental 2.1. Coordination oxidation homogeneous precipitation principle The standard electrode potential of copper ϕθ (Cu2+ /Cu) is 0.337 V, which indicates that copper is a very stable metal. Copper cannot be dissolved by strong nonoxidation acid such as sulfuric or hydrochloric acid unless in the presence of a very strong oxidant. But when coordination agent is in the presence, metal copper can been dissolved by the oxidation of air. It is because Cu2+ can form copper complex to result in the decrease of electrode potential [13]. The decrease degree of electrode potential increases with the increase of stabilization constant. If choosing low-cost ammonia as coordination agent, the standard electrode potential of Cu(NH3 )4 2+ /Cu is proposed as follows: ϕθ (Cu(NH3 )4 2+ /Cu) = ϕθ (Cu2+ /Cu) + (0.0591/2) lg Kunstable = 0.337 + (0.0591/2) lg(9.33 × 10−18 ) = −0.166 (V) Due to the characteristic that copper and silver cations are prone to form complexes with ammonia, we chose low-cost ammonia as the coordination agent, air as oxidant, metal copper, silver carbonate and ammonium bicarbonate as starting materials, [Cu(NH3 )4 ]2+ and [Ag(NH3 )2 ]+ complexes solution was synthesized firstly, the complexes moved in dissociation direction by diluting the complexes solution with water, heating the complexes solution to remove ammonia from aqueous solution. The ammonia concentration in the solution was decreasing, and the free copper and silver ions were increasing, when the cations and precipitator anions reached a certain amount, precipitates of basic copper carbonate and silver carbonate mixed precursors were yielded homogeneously in the solution. Because the reaction system did not introduce any other metal cation and anion that was uneasily removed, the very pure Ag/CuO nano-composite powder without containing any other impurity components could be obtained through centrifuging, desiccation and calcination. The reactions are as follows: Cu + 3NH3 ·H2 O + (1/2)O2 + NH4 HCO3 → [Cu(NH3 )4 ]2+ + CO3 2− + 4H2 O

(1)

Ag2 CO3 + 4NH3 ·H2 O → 2[Ag(NH3 )2 ]+ + CO3 2− + 4H2 O

(2)

[Cu(NH3 )4 ]2+ → Cu2+ + 4NH3

(3)

+

+

[Ag(NH3 )2 ] → Ag + 2NH3 2Cu

2+

+ CO3

2−

−

+ 2OH → Cu2 (OH)2 CO3 ↓

2Ag+ + CO3 − → Ag2 CO3 ↓ 

Cu2 (OH)2 CO3 −→2CuO + CO2 + H2 O 

2Ag2 CO3 −→4Ag + 2CO2 + O2

(4) (5)

Fig. 1. FESEM image of Cu2 (OH)2 CO3 and Ag2 CO3 mixed precursors. and microstructure of the precursors were visualized using a (Philips)-XL30 field-emission scanning electron microscope (FESEM). (HITACHI)-600 transmission electron microscope (TEM) was used to characterize the morphology and size of the products. Powder X-ray diffraction (XRD) measurements were carried out using a (RIGAKU)-D/max-3c automatic X-ray diffractometer. Infrared absorption spectroscopy (IR) was recorded on a Fourier transform infrared spectrometer (Perkin-Elmer spectrum one) from 4000 to 400 cm−1 to determine the structure of the products.

2.3. Preparation of products Cu metal (1.5 g) and Ag2 CO3 (0.15 g) were added to a beaker containing 14 ml saturated NH4 HCO3 solution, and 4.5 ml concentrated ammonia (28 wt.%) was gradually dropped into the beaker under magnetic stirring. The reaction was run until the metal Cu and Ag2 CO3 was dissolved completely, then a deep blue complexes solution was obtained. The deep blue complexes solution was dropped to a beaker containing 100 ml distilled water under stirring, the solution was heated at a certain temperature in water bath until a turbid solution was produced, then centrifuging. The precipitate was obtained, and dried in a vacuum oven at 75 ◦ C for 10 h, then a jade-green Cu2 (OH)2 CO3 and Ag2 CO3 mixed precursors were obtained. The precursors were calcined at 400 ◦ C for 1.5 h, and Ag/CuO composite nanopowder was obtained which was denoted as S2 . For comparison, simplex CuO nanopowder (denoted as S1 ) was prepared by above method. Ag/CuO nanopowder (denoted as S3 ) was also produced by the wet impregnation method below. A given amount of AgNO3 solution was

(6) (7) (8)

Because the metal ions and the precipitator are dispersed in the solution homogeneously, thus the precipitation reaction of metal ions and precipitator can reach molecular level, which ensures the sedimentation of desired nanoparticles yielding and separating out homogeneously from the solution. Partial overconcentration of precipitator can be avoided, and the size of product can been easily controlled.

2.2. Reagents and instrument Copper metal (purity 99.999%) was purchased from Smelt Factory of Zhuzhou, Hunan. Silver carbonate was self-produced by the method in the literature [14]. The other reagents used in the experiment were of analytical purity. Thermal behaviors for the precursors were studied through thermogravimetric and differential thermal analysis (TG–DTA), which were collected with a (SHIMADZU)-DT-4 thermoanalyzer under atmosphere flow. The morphology

Fig. 2. TG–DTA curves of Cu2 (OH)2 CO3 and Ag2 CO3 mixed precursors.

L.-S. Wang et al. / Materials Chemistry and Physics 108 (2008) 165–169

167

Fig. 3. TEM images of samples S1 –S3 .

impregnated to nanosized Cu2 (OH)2 CO3 precursor sufficiently, the mixture was dried at 75 ◦ C till constant weight, then calcined at 480 ◦ C for 1.5 h. The content of Ag (6.2 wt.%) in the products was measured by a (HITACHI)Z-8000 Zeeman polarization atomic absorption spectrometer.

2.4. Evaluation of the catalytic activities The catalytic activities of products were measured by decomposition rates of H2 O2 on alkaline condition [15]. According to the principle and equipment in the literature [16], the H2 O2 catalytic decomposition experiments were carried out in a glass volumetric system equipped with a temperature controlled water jacket at 25 ◦ C, using 100 ml magnetically stirred vessel containing 5 ml of 3 wt.% H2 O2 solution, 20 mg of catalyst product, and 10 ml of 1 M NaOH. The catalytic decomposition rate of hydrogen peroxide over the catalyst products was determined by measuring the volume of oxygen liberated as a function of time. The rate constant, k, was calculated by the expression ln V∞ /(V∞ − Vt ) = −kt, where Vt is the released volume of O2 at time t, and V∞ is the released volume of O2 at the end of the reaction, and then a straight line can be obtained by plotting ln V∞ /(V∞ − Vt ) versus t. The slope of the straight line is the reaction rate constant k values of H2 O2 decomposition.

3. Results and discussion Fig. 1 shows the FESEM image of the mixed precursors. As it has shown, the mixed Cu2 (OH)2 CO3 and Ag2 CO3 precursors appear chrysanthemum-like shape by nanometer sheets stacking together. The average thickness of the sheets is about 20 nm. Fig. 2 shows TG–DTA curves of the mixed precursors. According to the DTA curve, a small endothermic peaks can been seen at a minimum of 60 ◦ C. The TG curve also shows

an unconspicuous weight loss step in the range of 50–80 ◦ C. This weight loss is attributed to the absorption of water on the exterior surface of nanopowder, which can be desorbed from the surface during heating the sample by the TG analyzer. The DTA curve shows a strong and sharp endothermic peak located at a minimum of 307 ◦ C corresponding to a pronounced weight loss shown in the TG curve due to the decomposition of carbonate ion and hydroxyl ion, which resembles the thermal decomposition profile of basic copper carbonate [17]. The weight keeps constant when the temperature is higher than 400 ◦ C, indicating the complete decomposition of the precursors. Due to the small weight ratio of Ag in the precursors, no noticeable weight change about Ag2 CO3 is observed. Fig. 3 presents the TEM images of the products S1 –S3 . It can be observed that the as-prepared CuO appears to have a columnlike shape with a diameter of about 20 nm and length of about 60 nm. The morphology of S2 and S3 are both spherical shapes. The diameter of the Ag/CuO particles via coordination oxidation homogeneous precipitation method is much smaller than that of CuO, which is about 10 nm, while the diameter of the particles via wet impregnation method is larger than CuO, which is about 40 nm. The addition of Ag cannot only affect the morphology of product but also the particle diameter. The increase of Ag/CuO composite particle size by wet impregnation method is not only due to the fact that silver nitrate cannot disperse sufficiently with nano CuO but also that evaporation of water results in the growth and aggregation of particles during desiccation process. Furthermore, the higher calcination temperature via wet impregnation

168

L.-S. Wang et al. / Materials Chemistry and Physics 108 (2008) 165–169

Fig. 4. XRD spectra of samples S1 –S3 .

method leads to increase the particle size compared to that via coordination precipitation method. The XRD patterns for samples are shown in Fig. 4. For all of the samples, characteristic diffraction peaks at 35.3◦ , 38.5◦ , and 48.6◦ are evident, and these peaks agree well with the standard diffraction data for monoclinic CuO (JCPDS file No. 45-0937). The characteristic diffraction peaks of Ag for the sample S2 and S3 occur at 38.1◦ , 44.3◦ , 64.4◦ and 77.4◦ , which are well consistent with standard powder diffraction (JCPDS file No. 870597). The XRD patterns show that the sample S1 is CuO, and sample S2 and S3 are Ag/CuO composites. Fig. 5 is the IR spectra. Characteristic absorption peaks of Cu–O vibrations are indexed around 530 cm−1 [18]. Around 3450 cm−1 , a broad absorption band occurs which may be attributed to the stretching vibrations of absorption water, and the bending vibrations of absorption water molecule is observed around 1630 cm−1 . The absorption regions at 1380 and 1130 cm−1 are related to the vibrations of longitudinal

Fig. 6. First-order plots of H2 O2 decomposition conducted at 25 ◦ C over S1 –S3 catalysts.

phonon, which are similar to the character commonly possessed by nanoparticles. Due to quite high ratio of surface atoms in nanosized samples, many suspend bonds that are upright to the particle surface may appear, and the stretching vibrations of suspend bonds become more active. So IR spectra band intensity increases [19]. The catalytic decomposition rates of H2 O2 over the samples S1 –S3 are shown in Fig. 6. It reveals that as-prepared CuO and Ag/CuO composites have high catalytic activities to the decomposition of H2 O2 . The catalytic activity of Ag/CuO nanocomposites is improved much more than that of simplex nano CuO, which is attributed to the addition of Ag changing the crystal lattice structure and superficial morphology of nano CuO. The catalytic activity of composites via coordination precipitation method is better than that via impregnating method, because the particles are smaller and well-dispersed via coordination precipitation method. It is also possible that Ag ions can easily substitute Cu ions, and change the structure of CuO crystal lattice. During the substitution process, a portion of Cu2+ turns to Cu3+ , and electrons are released correspondingly. The electrons transfer to chemisorbed surface oxygen. Then the O2 form O2 − , which lead to the improvement of the superficial activity of the powder. During catalytic reaction, the electron can transfer to H2 O2 from O2 − , thus form an activity center to increase the rate of catalytic decomposition [20]. 4. Conclusion

Fig. 5. IR spectra of samples S1 –S3 .

Ag/CuO nano-composites have been synthesized successfully via coordination oxidation homogeneous precipitation method. The method is new, facile, no need of washing process, and no need of expensive equipment. It is also easy for mass production. TEM images show as-prepared Ag/CuO nano-composites have uniform particle diameter (10 nm) with minimal agglomeration. The catalytic experiments reveal that Ag/CuO nano-composites display higher catalytic activity towards H2 O2 decomposition than nanosized CuO.

L.-S. Wang et al. / Materials Chemistry and Physics 108 (2008) 165–169

References [1] G. Uozumi, M. Miyayama, H. Yanagida, J. Mater. Sci. 32 (1997) 2991–2996. [2] U.R. Pillai, S. Deevi, Appl. Catal. B: Environ. 64 (2006) 146–151. [3] S. Anandan, X.G. Wen, S.H. Yang, Mater. Chem. Phys. 93 (2005) 35–40. [4] A.E. Rakhshani, F.K. Barakat, Mater. Lett. 6 (1987) 37–40. [5] A. D´ebart, L. Dupont, P. Poizot, J.B. Leriche, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A1266–A1274. [6] E. Hurlbert, J. Applewhite, T. Nguyen, B. Reed, Z. Baojiong, W. Yue, J. Propul. Power 14 (1998) 676. [7] X. Wang, Y.X. Luo, L.D. Lu, X.J. Yang, Energetic Mater. (China) 12 (2004) 123–127. [8] D.S. Li, Z.M. Shi, W.L. Wang, L.Z. Zhang, X.H. Shun, J. Yanan Univ. (China) 20 (2001) 56–58. [9] D.S. Li, W.L. Wang, W.X. Yang, Y.J. Gong, Z.M. Shi, Chinese J. Inorg. Chem. 15 (1999) 817–820. [10] A.E.M.M. Turky, N.R.E. Radwan, G.A.E. Shobaky, Colloids Surf. A: Physicochem. Eng. Aspects 181 (2001) 57–68.

169

[11] C. He, Y. Yu, C.H. Zhou, X.F. Hu, L. Andre, Chinese J. Inorg. Mater. 18 (2003) 457–464. [12] S.H. Taylor, C. Rhodes, Catal. Today 114 (2006) 357–361. [13] X.H. Zhong, S.L. Xu, Y.Y. Nu, Inorganic Chemistry Series, vol. 6, Science Press, Beijing, 1995, p. 467. [14] Japanese Chemical Society, Synthesis Handbook of Inorganic Compounds, vol. 2, Chemical Industry Press, Beijing, 1986, p. 531. [15] J.R. Goldstein, A.C.C. Tseung, J. Catal. 32 (1974) 452–465. [16] C.Y. Luo, Experimental Physical Chemistry, 4th ed., High Education Press, Beijing, 1992, p. 132. [17] C.L. Zhu, C.N. Chen, L.Y. Hao, Y. Hu, Z.Y. Chen, Solid State Commun. 130 (2004) 681–686. [18] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York and London, Inc., 1971, pp. 220– 221. [19] C.S. Zhang, F. Zhao, J.J. Zhang, X.Y. Wang, G.L. Bao, Acta Chimica Sinica 57 (1999) 275–280. [20] A.M. Turky, Appl. Catal. A: Gen. 247 (2003) 83–93.