7634
J. Phys. Chem. B 2002, 106, 7634-7642
Alternative Methods for the Preparation of Gold Nanoparticles Supported on TiO2 Rodolfo Zanella,† Suzanne Giorgio,‡ Claude R. Henry,‡ and Catherine Louis*,† Laboratoire de Re´ actiVite´ de Surface, UMR 7609 CNRS, UniVersite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France, and CRMC2 CNRS,§ Campus de Luminy, case 913, 13288 Marseille Cedex, France ReceiVed: December 10, 2001; In Final Form: May 23, 2002
The best current way to prepare Au/TiO2 catalysts is the method of deposition-precipitation with NaOH (DP NaOH) developed by Haruta and co-workers. With this method, it is possible to obtain small gold metal particles (2-3 nm), but the corresponding gold loading remains rather low (∼3 wt %). The main goal of this work is to investigate other methods of preparation of Au/TiO2 catalysts to obtain small gold metal particles (2-3 nm) and a higher Au loading. It is shown that anion adsorption with AuCl4- (AA) does not produce Au loading higher than 1.5 wt % and the average particle size is not very small (∼4 nm). Cation adsorption with Au(en)23+ (CA) leads to small particles (2 nm) when the solution/support contact time is moderate (1 h), but the Au loading does not exceed 2 wt %. The most promising method of preparation appears to be depositionprecipitation with urea (DP urea). Indeed, samples with gold particles as small as those obtained with DP NaOH (∼2 nm) can be prepared, and all gold in solution is deposited on TiO2 in contrast to DP NaOH. The DP urea samples reported in this paper can reach a Au loading as high as 8 wt % using a TiO2 support with a surface area of 45 m2 g-1. The possible mechanisms of deposition of gold on the TiO2 support by the different methods of preparation are discussed.
I. Introduction Gold metal becomes catalytically active in several chemical reactions when it is finely divided and supported on metal oxides.1-3 The most remarkable catalytic properties of supported gold were observed by Haruta et al. in 19874-6 in CO oxidation at subambient temperature. Since then, the most studied catalyst has been gold supported on TiO2. Taken separately, Au and the TiO2 support are catalytically inactive for this reaction, but Au/TiO2 shows a drastic synergetic effect for the reaction of CO oxidation. The optimum size of gold particles is smaller than 5 nm for catalytic applications and about 2-3 nm for CO oxidation.2,7,8 Such particle sizes can be achieved by a careful control of the conditions of preparation. The catalytic activity of Au/TiO2 for CO oxidation depends on the preparation method; for example, Au/TiO2 catalysts prepared by coprecipitation are less active than catalysts prepared by deposition-precipitation.9,10 The parameters used in the preparations are also important. For instance, for the deposition-precipitation method, Haruta et al.3 described how the catalytic activity is sensitive to gold concentration, pH and temperature of the solution, calcination temperature, and addition of magnesium citrate. Haruta developed a preparation method of Au/TiO2 catalysts by deposition-precipitation with NaOH as precipitating agent (DP NaOH).11,12 With a nominal amount of 13 wt % of Au in solution and within a pH range between 7 and 10, this method permits the deposition of up to 3 wt % of Au and the formation of small metal particles with an average size of about 3 nm. Higher Au loading of 8 wt % could be achieved at pH 5.5, but * To whom correspondence should be addressed. E-mail: louisc@ ccr.jussieu.fr. † Universite ´ Pierre et Marie Curie. ‡ CRMC2 CNRS. § Associated with the Universities of Aix-Marseille II and III.
much larger particles were obtained (∼10 nm). It may be noted that the amount of Au deposited on TiO2 by DP NaOH is always lower than the amount of Au contained in solution, that is, the yield of DP is lower than 100%. It may be also noted that this method of preparation does not exactly correspond to the principle of the method of DP, largely developed by Geus13,14 and then extensively studied by our group for the preparation of Ni/SiO2 samples.15-18 In the DP method, the metal precursor is added to an aqueous suspension of the support and subsequently precipitated as a hydroxide by raising the pH. The surface of the support acts as a nucleating agent, and this method, if it is properly performed, leads to the greater part of the active precursor being attached to the support. The key factor of this preparation is the prevention of precipitation away from the support surface. The method of deposition-precipitation developed by Geus et al.13,14 using urea (CO(NH2)2) as the precipitating base permits the gradual and homogeneous addition of hydroxide ions throughout the whole solution, CO(NH2)2 + 3H2O f 2NH4+ + CO2 + 2OH-, and avoids local increase in pH and the precipitation of metal hydroxide in solution. In the preparation of Au/TiO2 catalysts, one can also take into account the fact that TiO2 is an amphoteric oxide (isoelectric point, IEPTiO2 ) 6 19). Therefore, this oxide can be used for preparation (i) by cation adsorption when the solution pH is higher than IEPTiO2 (the main surface species is O-, so the TiO2 surface is negatively charged) and (ii) by anion adsorption when the pH is lower than IEPTiO2 (the main surface species is OH2+, so the TiO2 surface is positively charged). Following these principles, we decided to prepare Au/TiO2 catalysts by three methods: (1) deposition-precipitation with urea (DP urea) (Bond and Thompson,1 had suggested this method, and Dekkers et al.20 had prepared Au/TiO2 samples by this method. However, they obtained rather large gold
10.1021/jp0144810 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/13/2002
Small Metal Particles in Au/TiO2 particles with an average size of 7.5 nm for a gold loading of 4.5 wt %); (2) anion adsorption (AA) with AuCl4- complex; (3) cation adsorption (CA) with Au(en)23+ complex (en ) ethanediamine) (this method was first successfully developed by Guillemot et al.21,22 for the introduction of gold into Y zeolites). The goal of the study is to investigate whether it is possible by these three methods to prepare Au/TiO2 catalysts with small metal particles, in the same range of size as samples prepared by DP NaOH (2-3 nm),3,11,23 but with a better control of the Au loading, especially with higher Au loading. In this paper, we show for the first time that deposition-precipitation with urea (DP urea) can be successfully applied to the preparation of Au/TiO2 catalysts with high metal loading and small Au particle sizes. For comparison, other Au/TiO2 samples were also prepared by (i) deposition-precipitation with NaOH (DP NaOH) in the presence or in the absence of magnesium citrate and (ii) incipient wetness impregnation (Imp) with HAuCl4 (this is the very first method reported in the literature for the preparation of supported gold catalysts.28,29 However, the gold particle sizes are large even at low metal loading. In addition, the samples contain large amounts of chlorides, which are known to poison catalysis for many reactions). II. Experimental Section 1. Au/TiO2 Preparations. Titania Degussa P25 was used as the support (BET surface area ) 45 m2 g-1, nonporous, 70% anatase and 30% rutile, purity > 99.5%) and solid HAuCl4‚ 3H2O (Acros) as the gold precursor. Before preparation, TiO2 was previously dried in air at 100 °C for at least 24 h. All of the preparations were performed in the absence of light, which is known to decompose the gold precursors. For most of the preparations, 1 g of TiO2 was added to 100 mL of an aqueous solution of gold precursor (4.2 × 10-3 M). The amount of gold in solution corresponds to a maximum gold loading of 8 wt % on TiO2. After deposition of gold onto TiO2 according to the various methods described below, all the solids were submitted to the same procedure: (i) separation from the precursor solution by centrifugation (12 000 rpm for 10 min); (ii) washing (the solids were suspended in water (100 mL g-1), stirred for 10 min at RT, and centrifuged again. This washing procedure was repeated four times to remove residual Cl- and Na+ ions as well as Au species not interacting with the support); (iii) drying under vacuum at 100 °C for 2 h; (iv) calcination at 300 °C (300 mg of sample was heated in a flow (30 mL min-1) of industrial air (Air Liquide) from room temperature to 300 °C with a rate of 2 °C min-1 then maintained at 300 °C for 4 h. Calcination treatment leads to the decomposition of the Au(III) complexes into gold metal particles); (v) storage of the samples away from light and under vacuum in a desiccator at RT. Indeed, a strong increase in the average particle size of the calcined samples was observed when samples are stored in air even for a short period (for instance, from 1.8 to 3 nm for a sample left in air for about 10 days). After several months of storage away from light in a desiccator, the average particle size also slightly increases (for instance, from 1.7 to 2.1 nm after 10 months of storage). To avoid this effect, the samples have been stored after drying, and calcination is performed when needed. a. Deposition-Precipitation with NaOH. In the standard preparation conditions (similar to Haruta’s preparations24), 100 mL of an aqueous solution of HAuCl4 (4.2 × 10-3 M) was heated to 80 °C. The gold concentration in solution corresponds
J. Phys. Chem. B, Vol. 106, No. 31, 2002 7635 to a theoretical Au loading of 8 wt % in the case of a complete deposition-precipitation (DP yield ) 100%). This gold loading was chosen because Haruta23 reported that the most active catalyst for CO oxidation contained 8 wt % of Au. The pH was adjusted to 8 by dropwise addition of NaOH (1 M), and then 1 g of TiO2 was dispersed in the solution, and the pH was readjusted to 8 with NaOH. The suspension thermostated at 80 °C was vigorously stirred for 1 h and then centrifuged, and the solid was washed, dried, and calcined following the previously reported procedure. The main parameters studied were the DP time (1, 2, 4, or 16 h), the addition of magnesium citrate (Mg3(C6H5O7)2, 6.9 × 10-3 M) in the suspension after the first adjustment of pH, and the HAuCl4 concentration. b. Incipient Wetness Impregnation. Titania was impregnated with aqueous solutions of HAuCl4 (1.3 mL per g of TiO2) of various concentrations (0.04, 0.08, and 0.16 M) to obtain samples with 1, 2, and 4 wt % of Au, respectively. In all cases, the solution pH was less than 1. The samples were aged at room temperature (RT) for 1 h. Samples (2 and 4 wt %) were then divided into two parts. One part was directly dried (Imp samples), whereas the other one was washed before drying (ImpW samples) to determine whether some Au species interact with the TiO2 support. All of the samples were calcined at 300 °C. c. Anion Adsorption. A total of 1 g of TiO2 was added to 100 mL of an aqueous solution of HAuCl4 (4.2 × 10-3 M). Under such conditions, the solution pH (∼2) was lower than the IEPTiO2, that is, in adequate conditions of pH for anion adsorption. The suspension thermostated at 25 or 80 °C was vigorously stirred for 15 min, 1 h, or 15 h and finally centrifuged. The solids were washed, dried, and calcined at 300 °C. d. Cation Adsorption. Gold was also deposited on TiO2 by cation adsorption of the Au(en)23+ complex (en ) ethanediamine) the synthesis of which was described by Block and Bailar.25 Au(en)2Cl3 was dissolved in 100 mL of water (4.2 × 10-3 M). By dropwise addition of an ethanediamine solution (1 M), the pH was adjusted to a value of 9.4 or 10.3, that is, at a pH higher than the IEPTiO2. Hence, the adsorption of the Au(en)23+ complex is theoretically possible. The suspension was vigorously stirred for 1 or 16 h at 80 °C in a thermostated vessel. After the adsorption, the samples were centrifuged, washed, dried under vacuum, and calcined at 300 °C. e. Deposition-Precipitation with Urea. In the so-called standard preparation conditions, 1 g of TiO2 was added to 100 mL of an aqueous solution of HAuCl4 (4.2 × 10-3 M) and of urea (0.42 M). The initial pH was ∼2. The suspension thermostated at 80 °C was vigorously stirred for 4 h (pH increases) and then centrifuged, washed, dried, and calcined at 300 °C. The following parameters were studied: the DP time (1, 2, 4, 16, and 90 h), the temperature of DP (80 and 90 °C), the gold concentration (1.1 × 10-3, 1.6 × 10-3, and 4.2 × 10-3 M), the urea concentration (0.42 and 0.84 M), and the addition of magnesium citrate (6.9 × 10-3 M). 2. Techniques of Characterization. Chemical analysis of Au, Cl, Mg, Na, C, and N in the samples was performed by inductively coupled plasma atom emission spectroscopy at the CNRS Center of Chemical Analysis (Vernaison, France). The detection limit is 300 ppm for Cl and 1000 ppm for C and N. Chemical analysis was performed after sample calcination. The Au weight loading of the samples is expressed in grams of Au per grams of sample calcined at 1000 °C: wt % Au ) [mAu/ (mAu + mTiO2)] × 100. Calcined Au/TiO2 samples were examined by transmission electron microscopy (TEM) with a JEOL 2000FX electron
7636 J. Phys. Chem. B, Vol. 106, No. 31, 2002
Zanella et al.
TABLE 1: Au/TiO2 Samples Prepared by Deposition-Precipitation with NaOH at 80 °C preparation
results
DP theor Au Au Cl average standard particle size time loading loading loading particle deviation distribution sample (h) pH (wt %) (wt %) (wt %) size (nm) (nm) (nm) DPN1 1 DPN2 2 DPN3 4 DPN4 16 DPN5a 1 DPN6 1 DPN7 1 a
8 8 8 8 8 7 8
8 8 8 8 8 8 16
1.8 2.4 2.1 2.4 3.1 3.3 3.6
0.026 0.025 0.035 0.077 0.09 0.031 0.030
1.8 1.5 1.6 1.5 1.4 1.4 3.3
0.62 0.40 0.36 0.30 0.36 0.34 1.92
0.7-4.3 0.7-3.5 1.0-3.1 1.0-2.4 0.7-2.7 0.7-2.4 0.7-16.0
Addition of magnesium citrate (6.9 × 10-3 M).
microscope. Except when especially mentioned, the histograms of the metal particle sizes were established from the measurement of 300 to 1000 particles. The size limit for the detection of gold particles on TiO2 is about 1 nm. The average particle diameter, dh, was calculated from the following formula: dh ) ∑nidi/∑ni, where ni is the number of particles of diameter di. The standard deviation was calculated from the formula σ ) [(∑(di - dh)2)/∑ni]1/2. In some samples, the presence of Au was also determined by energy-dispersive X-ray spectroscopy coupled to TEM observations. III. Results 1. Deposition-Precipitation with NaOH. Table 1 shows that when the pH of the DP NaOH solution is 8 (DPN1 sample), the average Au particle size is 1.8 nm and the Au loading is 1.8 wt % (Figure 1). When the pH is lowered to 7 (DPN6 sample), the average particle size is smaller (1.4 nm) and the Au loading is higher (3.3 wt %). Another parameter studied is the DP time, from 1 to 16 h (samples DPN1 to DPN4). The gold loading slightly increases from 1.8 to 2.4 wt %, while the average particle size seems to slightly decrease from 1.8 to 1.5 nm (Table 1). The particle size distribution becomes narrower. It may be noted that our preparations can provide smaller gold particles than those obtained by Haruta: at pH 8, 1.8 nm for 1.8 wt % of Au (DPN1, Table 1) to be compared to 2.9 nm for 2.2 wt %,12 and at pH 7, 1.4 nm for 3.4 wt % (DPN6) to be compared to 3.3 or 3.6 nm for 3.6 wt %.12 As for Haruta’s samples, the Au loading in our samples is lower than the nominal amount of gold in solution, indicating that the yield of DP is less than 100% and that part of the gold is not deposited on TiO2. When the concentration of gold in solution is higher (8.4 × 10-3 M), the gold loading on TiO2 is higher (3.6 wt % in DPN7) but the average particle size is much larger (3.3 nm) and the size distribution much broader. A sample was also prepared in the presence of magnesium citrate. According to Haruta et al.,3,11,23 magnesium citrate leads to higher Au loadings and small metal particles because it sticks on the TiO2 surface and prevents gold particles from sintering during calcination. Magnesium citrate is also known to reduce the gold(III) precursors into metallic gold.11,26 Table 1 shows that when magnesium citrate is added (DPN5), the Au loading is indeed higher (3.1 compared to 1.8 wt % for DPN1), and the average particle size is smaller (1.4 instead of 1.8 nm) (Figure 2). The gold particles in DPN5 are also smaller than those obtained by Haruta et al. under close experimental conditions: 1.4 nm to be compared to 2.8-3.8 nm.27 However, DPN5 does not reach gold loadings as high as 6 or 8 wt % as reported by Haruta in two papers.23,27 However, in the first one,27 it is not clear whether the Au loadings reported are those in solution or
Figure 1. (a) TEM image of calcined sample DPN1 prepared by deposition precipitation with NaOH (DP time ) 1 h, pH ) 8, 1.8 wt % Au); (b) size histogram of gold particles.
on TiO2, and in the second one,23 the amount of gold in solution is not reported. 2. Incipient Wetness Impregnation. Table 2 summarizes the results obtained for the Au/TiO2 samples prepared by impregnation and by impregnation followed by washing. All of the Imp samples contain only a few large Au particles (>10 nm) after calcination (Table 2). These results are consistent with published results. Indeed, Haruta et al. 28 obtained gold particles between 10 and 30 nm for 1 wt % Au/TiO2 catalysts, and Vannice et al. 29 obtained gold particles between 25 and 35 nm for 2 wt % Au/TiO2 sample. Although X-ray fluorescence and chemical analyses indicate the presence of Au, only a small number of Au particles could be observed by TEM in the Imp samples, so the values for the particle size distribution are not reliable. This problem had already been reported by Haruta et al.7 Chemical analysis shows that about 0.9 wt % of gold remains on TiO2 after washing whatever the initial Au loading, and the average particle size is between 3 and 5 nm after calcination (Table 2). Hence, part of the gold species deposited during impregnation is in strong interaction with TiO2 because it remains on the support after washing. Because of this strong interaction, sintering of gold particles is prevented during calcination, so the metal particles are smaller than those obtained after impregnation. It may be noted that although the Au loading of Imp3 is twice that of Imp2, both the Au loading and the average particle size of ImpW3 are smaller than those of ImpW2. We did not attempt
Small Metal Particles in Au/TiO2
J. Phys. Chem. B, Vol. 106, No. 31, 2002 7637 TABLE 3: Au/TiO2 Samples Prepared by Anion Adsorption with AuCl4results preparation
Au Cl average standard time of T loading loading particle deviation sample contact (°C) pH (wt %) (wt %) size (nm) (nm) AA1 15 h
25 2.5
1.0
0.16
5.7
1.38
AA2 15 min 80 2
1.3
0.16
3.7
1.30
AA3 1 h
80 2
1.0
0.07
5.6
1.50
AA4 15 h
80 2
1.5
0.15
4.4
1.29
particle size distribution (nm) 5.0-7.0 (4 particles) 1.7-6.8 (67 particles) 2.0-8.3 (23 particles) 2.0-7.5 (60 particles)
TABLE 4: Au/TiO2 Samples Prepared by Cation Adsorption with Au(en)23+ at 80 °C preparation
results
time of Au Cl average standard contact loading loading particle deviation sample (h) pH (wt %) (wt %) size (nm) (nm) CA1
1
9.4
1.1
-
2.1
0.66
CA2
16
9.4
6.1
0.03
4.1
1.01
CA3
1
10.3
1.7
0.04
1.8
0.33
CA4
16
10.3
6.4
0.08
4.6a
2.74
particle size distribution (nm) 1.0-3.5 (134 particles) 1.7-6.4 (140 particles) 1.0-2.5 (88 particles) 1.7-15.8 (157 particles)
a Plus some very large particles (80-150 nm) not taken into account in the calculation of the average size.
Figure 2. (a) TEM image of calcined sample DPN5 prepared by deposition precipitation with NaOH (DP time ) 1 h, pH ) 8, citrate addition, 3.1 wt % Au); (b) size histogram of gold particles.
TABLE 2: Au/TiO2 Samples Prepared by Incipient Wetness Impregnation at RT preparation
sample Imp1 Imp2 Imp3 ImpW2a ImpW3a a
results
theor Au Au Cl average standard loading loading loading particle deviation (wt %) (wt %) (wt %) size (nm) (nm) 1 2 4 2 4
0.9 0.6
0.09 0.08
>10 >10 >10 4.9 2.7
1.36 1.52
particle size distribution (nm)
1.9-8.5 (165 particles) 1.4-5.8 (78 particles)
The ImpW samples are washed after impregnation.
to repeat these experiments to check these unexpected results because the main goal of these experiments was to determine whether some gold remained in interaction with the support after washing. 3. Anion Adsorption. Because some gold species (e0.9 wt %, Table 2) remain adsorbed on the TiO2 surface after washing of the impregnated samples, preparations by anion adsorption with AuCl4- have been attempted. Table 3 shows that under our experimental conditions the Au loading does not exceed 1.5 wt % and the average particle size is about 4-6 nm whatever the samples. As in the case of Imp samples, only a few particles are observed by TEM, so the average size measurements are not very accurate. However, it can be mentioned that (i) the gold loading is higher when anion adsorption is performed at 80 °C (AA4) rather than at RT (AA1) and (ii) the equilibrium adsorption seems to be reached fast (
