Preparation of Gold Nanoparticles, Au/Fe 2 O 3 by Using a Co-Precipitation Method and their Catalytic Activity

Transcription

1 Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008, pp Preparation of Gold Nanoparticles, Au/Fe 2 O 3 by Using a Co-Precipitation Method and their Catalytic Activity Tran Thi Minh Nguyet, Nguyen Cong Trang, Nguyen Quang Huan and Nguyen Xuan Institute of Material Sciences, VAST, Hanoi, Vietnam Luu Tien Hung Department of Physics, Vinh University, Nghean, Vietnam Masakazu Date Institute of Innovation in Sustainable Chemistry, AIST, Japan (Received 15 July 2007) Au=Fe 2O 3(Au : Fe = 1 : 50) catalyst was prepared by the co-precipitation method. The coprecipitate was studied by using dierential thermal analysis (DTA) and dierential thermal gravimetric analysis (DTGA). The structure of the sample was investigated by using X-ray diraction (XRD) and transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The particle size was determined to be within the range of 1.5 and 8 nm. Results of the study for catalytic properties showed that T 1=2 for CO and H 2 oxidation were 317 and 405 K respectively. Au=Fe 2O 3(Au : Fe = 1 : 50) catalyst was prepared by the co-precipitation method. The co-precipitate was studied by using DTA and DTGA. The structure of the sample was investigated by XRD, TEM and HRTEM. The particle size was determined to be within the range of 1.5 and 8 nm. Results of the study for catalytic properties showed that T 1=2 for CO and H 2 oxidation were 317 and 405 K respectively. PACS numbers: J Keywords: Co-precipitation, Gold-based catalyst, Nano particle, Oxidation I. INTRODUCTION For a long time, gold has been believed to be an inert, but useful, catalyst [1]. The paradigm was overturned in the mid-1980s by the work of Haruta et al. at the Osaka National Research Institute in Japan [2,3] and by Graham Hutchings, then at the University of the Witwatersrand in South Africa [4]. Their work showed that gold was a surprisingly active catalyst provided that it was prepared accordingly to specic recipes. Since then, many studies in the world have focused on this new gold catalyst. In general, special catalytic activity of gold was observed to appear in the chemical environment as discrete particles and these nano-scale particles could exhibit a very particular size range, which might vary according to the relationship/dependency among dierent catalytic reactions. Gold metal becomes catalytically active in several chemical reactions when it is nely dispersed and supported on metal oxides [6, 7] such as Au=TiO 2, Au=Co 3 O 4, Au=Fe 2 O 3, Au=SiO 2 and Au=MnO x. Gold-based catalysts exhibit good activity Tranminhnguyet@hotmail.com for oxidation of carbon monoxide, alkanes and alkenes; for the water gas shift and for reactions involving NO x. The opportunities to use Au-based catalysts appear to arise from three factors. The rst is the extraordinary activity of these catalysts at room temperature and below. The second property is that the activity of some conventional catalyst systems is signicantly prolonged by the incorporation of gold nanoparticles. Finally, goldbased catalysts appear not to be deleteriously eected by humidity [5]. However, the catalytic activity of gold-based catalyst depends on dierent factors, such as the nature of the oxide support, the gold particle size, the structure of the gold surface area and the electronic conguration of its special surface sites [9]. Hence, the preparation method plays an important role in determining the high catalytic activity of the material. For the depositionprecipitation method, Haruta et al. descrebed how the catalytic activity was sensitive to the gold concentration, the ph, the temperature of the solution, the calcination temperature, etc. [13]. The development of low-temperature carbonmonoxide oxidation catalysts has become an important research

2 Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008 Fig. 1. DTA, TGA and DTGA curves for a Au : Fe = 1 : 50 precipitate. topic during the last twenty years due to many potential areas of application. Gold-based catalysts could be good candidates for removing low concentrations of CO, e.g., from the air conditioning systems of buildings or vehicles, with no need to heat the catalyst bed using external means. Gold catalysts might be employed to remove the rst CO produced from the instant the engine is started. Our previous work [10] focused on studies of the preparation method of nanoparticle perovskites, which exhibits a catalytic activity at somewhat high temperature ( C) for oxidation of carbon monoxide and hydrocarbons. These catalysts could be applied for the decomposition of CO and C x H y of exhauset gas from an engine when car is running. We then need to study the catalysts to decrease the temperature conversion of NO x =C x H y =CO into CO 2 ; N 2 and H 2 O to a temperature as low as possible. In the literature, Haruta et al. [11{13], several research groups have reported that nanoparticles of Au=Fe 2 O 3 exhibit high activity for CO and H 2 oxidations at low temperature (below 100 C and even below 0 C). In the present work, we announce on our rst results from a study of the gold-based catalyst Au=Fe 2 O 3. II. EXPERIMENT The reagents such as HAuCl 4, Fe(NO 3 ) 3 :9H 2 O and Na 2 CO 3, used were of analytical grade and the solution was twice distilled. The catalyst was prepared by using a co-precipitation method. The synthesis procedures were as follows: The aqueous solution of components of Fe(NO 3 ) 3 and HAuCl 4 (with atom ratio Au : Fe = 1 : 50) was added dropwise into a sodiumcarbonate Na 2 CO 3 solution at room temperature with vigorous stirring. The solution was adjusted to a ph = 7 7:5: The ne orange suspension formed was aged for 4 h at room temperature. The precipitate was ltered and washed several times by using hot distilled water to remove the chloride ions (tested by a solution AgNO 3 ). The washed precipitates were dried for 24 h in air at 50 C and then calcined (in air) at dierent temperatures (120, 200, 300, 400 and 500 C) for analysis. The thermal eect of the calcining process was determined by using a TA50 Shimadzu system at a rise rate of 10 C min 1 in the range 25 C 800 C. The X-ray diraction analysis was carried out using a D5000 diractometer with Cu K ( = 1:5406 A) radiation. The morphology and the size of the crystalline grains were examined in a TEM1010 transmission electron microscope and a high-resolution transmission electron microscope (HRTEM, Philip CM20-FEG). The catalytic activity of the obtained sample was also tested. III. RESULTS AND DISCUSSION 1. Thermal Analysis: DTA, TGA and DTGA The dried precipitates were investigated by using thermal analysis. The analytical DTA, thermal gravimetric analysis (TGA) and DTGA results are shown in Figure 1 and indicate that there was only one endothermic at 67 C, which was prolonged to 120 C, with a nearly global weight loss 68:987 %). This eect is possibly

3 Preparation of Gold Nanoparticles, Au/Fe2 O3 { Tran Thi Minh Nguyet et al Fig. 2. (a,b,c,d) X-ray patterns of Au=Fe2 O3 calcined at 120(a), 200(b), 300(c), 400 and 500 C(e) respectively. related to water evaporation and to the decomposition of the carbonate ion, CO3 and hydrogen bonds. In the range C, a weak monotonous exotherm and a weak lost weight were observed; after 255 C the weight was nearly constant and the sample translated into the stable state. 2. X-ray Di raction Analysis The dried precipitates were heated at 120 C, 200 C; 300 C; 400 C and 500 C for 4 h and the powders were studied by using X-ray di raction. Figure 2(a) refers to the calcined sample at 120 C, which shows an amorphous phase of both gold and ferrium compounds. According to Wang et al. [14], below 200 C, the sample contained an amorphous of ferrium and gold species Au2 O3. In the Figure 2(b) (XRD pattern of the sample calcined at 200 C), some weak peaks showing the beginning of crystalline formation of gold (2 = 38:2 ) and of hematite Fe2 O3 (2 = 33.2, 35.5, 40, 49.3, 54, 62.3 and 64 ) are observed. Thus, at this temperature, in the sample included an amorphous phase and a little of the crystalline phases of both Au and hematite Fe2 O3. Because simple gold oxide is readily decomposed to metallic gold even at 80 C, this result shows that gold oxide is thermally stabilized below 200 C on mixing with ferric hydroxide. The samples calcined at higher temperatures (300; 400 and 500 C) were studied by using XRD and the results are shown in Figures 2(c), 2(d) and 2(e). This indicates the real formation of crystallite fractions: the sample contained crystals of Fe2 O3 (2 = 33.2, 35.5, 40, 49.3, 54, 57.6, 62.3 and 64 ) and Au (2 = 38:2; 44:4 and 64:5 ); the intensity of these peaks increased with increasing temperature. XRD did not detect the peaks of Au2 O3, so we can see that the change in the amorphous ferric hydroxide into crystalline hematite ( Fe2 O3 ) was accompanied by a transformation of gold oxide to metallic gold. These results are similar to those in the paper Fig. 3. TEM photograpghs of Au=Fe2 O3 nanocrystals calcined at 120(a), 200(b), 300 C(c). of Wang et al. [14], in which the authors concluded that at 300 C, the gold species were mostly decomposed into a metallic species. However, Haruta et al. [13] thought that at this temperature, both metallic and oxide species existed in the sample, but at 400 C and 500 C, almost all gold oxide species were decomposed into the metallic species. 3. Analysis by TEM and HRTEM Figures 3(a), 3(b) and 3(c) are the TEM photographs of samples calcined at various temperature (120; 200 and 300 C) for 4 h. The photographs show that the samples calcined at 120 and 200 C contained an amorphous phase Figure 3(a), 3(b) because since the 120 C and 200 C were not su cient tempertures for crystalline formation. When the sample was calcined at 300 C, the TEM (Figure 3(c)) and the HRTEM (Figure 4(a)) photographs showed two phases of crystals: The gold particles were

4 Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008 Fig. 5. Oxidation eciencies of CO and H 2 as a functions of the catalyst temperature. Fig. 4. (a) HRTEM photograpghs of the Au=Fe 2O 3 sample calcined at 300 C, (b) The grain size distribution of the Au=Fe 2O 3 sample calcined at 300 C. relatively small and spherical, which were well installed on the Fe 2 O 3 particles. The particle size of the sample calcined at 300 C for 4 h was estimated to be within the range of 1.5 and 8 nm; the grain size distribution is presented in Figure 4(b) and shows more than 70 % of the gold particles having grain sizes smaller than 5 nm. 4. Catalytic Activities of Au=Fe 2O 3 According to Haruta et al. [13], in the case of the Au-Fe coprecipitate, the sample calcined at 300 C had the highest catalytic activity for CO oxidation and the best for H 2 oxidation was the sample calcined at C. The authors have indicated that H 2 oxidation can take place over both oxidic and metallic gold; and on the other hand, the presence of metallic gold is indispensable for low-temperature CO oxidation. In our studies, the catalytic activities of the sample Au=Fe 2 O 3 (obtained by calcination at 300 C=4 h) were measured with x-bed ow reactors. 100 mg of the catalyst sample was used for the measurements. The reactant gas, 1 Vol% CO or H 2 in air, was fed at the rate of 33 ml/min. The euent gas was analyzed by using TCD gas chromatographs. The results are presented in Figure 5 which shows the oxidation eciencies of CO and H 2 as functions of the catalyst temperature. The T 1=2 (temperature for 50 % conversion) for CO and H 2 oxidation were 317 and 405 K, respectively. These results are similar to those of the studies studies by the group of Horvaths et al. [9]. IV. CONCLUSIONS Nanoparticles supporting gold, Au=Fe 2 O 3 (Au : Fe = 1 : 50), are synthesized by coprecipitation from Na 2 CO 3 solution. The sizes of the gold-based particles varied from 1.5 to 8 nm. 70 % of the gold particle were found to be in the range of 1.5 { 5 nm. The obtained nanoparticles exhibited a relatively high catalytic activity. The temperature for 50 % conversion of CO and H 2 oxidation (over the sample calcined at 300 C=4 h) were 317 and 405 K respectively. ACKNOWLEDGMENTS This work is nancially supported by the National Fundamental Research Program for the Development of

5 Preparation of Gold Nanoparticles, Au/Fe 2O 3 { Tran Thi Minh Nguyet et al Nanotechnology of Vietnam. REFERENCES [1] G. C. Bond, Gold Bulletin 34, 117 (2001). [2] M. Haruta, H. Sano and T. Kobayashi, US Patent , [3] M. Haruta, T. Kobayashi, H. Sano and M. Yamada, Chem. Lett. 405 (1987). [4] G. J. Hutching, J. Cat. 96, 292 (1985). [5] M. B. Cortie and E. Van Der Lingen, Materials Forum 26, 1 (2002). [6] G. C. Bond and D. T. Thompson, Catal. Rev. Sci. Eng. 41, 319 (1999). [7] F. Cosandey and T. E. Madey, Surf. Rev. Lett (2001). [8] M. Haruta, The Abilities and Potential of Gold as a Catalyst, Report No. 393, [9] D. Horvath, L. Toth and L. Guczi, Catal. Lett. 67, 117 (2000). [10] T. T. M. Nguyet et al., Proc. of the 2nd IWONN' (Hanoi, Vietnam, 2004), p [11] S. W. Epling, B. Gar Hound and F. Jason Weaver, J. Phys. Chem. 100, 929 (1996). [12] D. S. Gardner and B. G. Hound, Langmuir 7, 2135 (1991). [13] M. Haruta and S. Tsubota et al., J. Catalysis 144, 175 (1993). [14] G. Y. Wang. H. L. Lian. W. X. Zhang, D. Z. Jiang and T. H. Wu, Kinet. and Cat. 43, 433 (2002).