Activation of Dodecanethiol-Capped Gold Catalysts for ... - Springer Link

Catal Lett (2010) 136:209–221 DOI 10.1007/s10562-010-0316-1

Activation of Dodecanethiol-Capped Gold Catalysts for CO Oxidation by Treatment with KMnO4 or K2MnO4 Hongfeng Yin • Zhen Ma • Miaofang Chi Sheng Dai

•

Received: 4 February 2010 / Accepted: 19 February 2010 / Published online: 20 March 2010 Ó Springer Science+Business Media, LLC 2010

Abstract Dodecanethiol-protected gold nanoparticles were deposited onto fumed SiO2 (Cab-O-Sil) via colloidal deposition. The catalyst was treated with a strongly oxidative KMnO4 or K2MnO4 solution. Low-temperature conversion in catalytic CO oxidation increased dramatically following the oxidative treatment and subsequent thermal activation at 300 °C to burn off residual organic species. On the other hand, the treatment with Fenton’s reagent did not lead to any positive effect. The influences of the average sizes of pre-synthesized gold particles (1.8, 2.1, 3.9, 9.9 nm) and the choice of different supports (SiO2, TiO2, C) were investigated, and relevant characterization using TG/DTG, XRD, TEM, EDX, and HAADF was conducted. The catalyst stability as a function of time on stream was also surveyed. This work establishes the beneficial effect of treating dodecanethiol-capped gold catalysts by KMnO4 or K2MnO4. Keywords Gold nanoparticles Au/SiO2 Au/TiO2 Au/C KMnO4 K2MnO4 Fenton’s reagent

Electronic supplementary material The online version of this article (doi:10.1007/s10562-010-0316-1) contains supplementary material, which is available to authorized users. H. Yin S. Dai (&) Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA e-mail: [email protected] Z. Ma Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, People’s Republic of China M. Chi Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

1 Introduction Catalysis by supported gold nanoparticles has attracted much attention recently [1–5]. The most popular method for loading gold is based on deposition–precipitation (DP) using aqueous chloroauric acid solutions under basic conditions. This DP method works fine for supports with high isoelectric points, such as TiO2 and Al2O3, but does not work well with supports with low isoelectric points, such as SiO2. Another way to make supported gold nanoparticles is to deposit organic ligand-capped gold colloids onto a support [6–14]. This method is not constrained by the isoelectric point of the support. Therefore, the influences of the size of gold nanoparticles and the nature of different supports can be studied systematically. However, the organic ligands left behind on catalyst surfaces can affect the development of metal-support interactions and block the active sites for CO oxidation. Although organic ligands may be removed by high-temperature calcination, such a pretreatment can also cause the severe sintering of gold nanoparticles [15, 16]. Recently, we reported a new method to make Au/SiO2 more active in CO oxidation by treating Au(en)2Cl3derived Au/SiO2 in strongly oxidative KMnO4 solutions followed by mild calcination in O2–He [17]. The KMnO4 solution can partially oxidize residual organic ligands at room temperature [18–20], thereby lowering the temperature required for thermal pretreatment and most importantly leaving behind MnOx species strategically near gold nanoparticles. The newly established interfaces between MnOx species and gold nanoparticles enhance the catalytic activity in low-temperature CO oxidation, because the Au–MnOx interface is known to be active for CO oxidation [21–26]. In view of the fact that Au/SiO2 catalysts can be prepared using alkyl thiol-capped gold colloids as the

123

210 Scheme 1 Deposition of presynthesized thiolated Au nanoparticles on supports and oxidation-calcination treatment for the preparation of gold catalysts

H. Yin et al.

HAuCl4

tetraoctylammonium bromide toluene, H2O

SH HS

SH NaBH 4

SH SH SH

Au SH HS

HS HS HS

Murray, R.W et al.Langmuir , 1998 , 14, 17-30 support

SH HS SH

300 oC or 500 oC pretreatment

SH SH

Au SH HS

catalyst for CO oxidation

precursor, herein we demonstrate that the preparation method based on KMnO4 treatment can also be used with these gold colloids (Scheme 1). In addition, we demonstrate that another oxidant, K2MnO4, also works for making the catalysts more active in low-temperature CO oxidation.

2 Experimental Dodecanethiol-capped gold nanoparticles was synthesized according to a classic method [27, 28]. HAuCl4 3H2O (0.63 g) was added into 50 mL deionized water, and the aqueous solution was then added, with vigorous stirring, into a solution composed of 3.0 g tetraoctylammonium bromide and 160 mL toluene. The yellow aqueous HAuCl4 solution became clear quickly and the toluene phase became orange-brown as the gold precursor was transferred into it. The organic phase was isolated, 80 mg dodecanethiol was added, and the resulting solution was stirred for 10 min at room temperature. NaBH4 (0.72 g) in 50 mL deionized water was added in 10 s, with vigorous stirring. The dark organic phase was further stirred at room temperature for 3 h. The organic phase was collected, and the solvent was removed using a rotary evaporator (not exceeding 50 °C to avoid product decomposition). The black product was dispersed into 30 mL acetone, sonicated to ensure complete dissolution, and precipitated by adding 80 mL ethanol. The product was washed with hexane/ ethanol for 3 times and dried in a vacuum oven at room temperature for 10 h, yielding sticky black solid. The resulting gold nanoparticles have an average size of 2.1 nm, as estimated by XRD measurement of gold nanoparticles supported on SiO2. Gold nanoparticles with

123

300 oC pretreatment

HS HS HS

KMnO 4

support

or K2MnO 4 or Fenton’s reagent

Au(SR)/support

estimated average sizes of 1.8, 3.9, and 9.9 nm were synthesized with the same procedure except that 640, 52, and 20 mg dodecanethiol was used, respectively. Au/SiO2 samples were prepared via direct colloidal deposition. Gold nanoparticles (60 mg) were dispersed into 30 mL hexane and stirred for 10 min, followed by the addition of 2.0 g SiO2 (Cab-O-Sil, surface area 175 m2/g), TiO2 (Degussa P25, surface area 48 m2/g), or carbon (carbopack, surface area 165 m2/g). The suspension was stirred vigorously for 1 h and stirred slowly in open air to evaporate solvent. The remaining solid was dried in a vacuum oven at 60 °C for 10 h. For KMnO4 or K2MnO4 treatment, 0.10 g Au/support was suspended in 20 mL H2O, and 1 mL 0.05 M KMnO4 or K2MnO4 solution was added. The suspension was stirred at room temperature for 20 h, filtered, and thoroughly washed with H2O. The product was dried at 70 °C for 10 h. For Fenton’s reagent treatment [29], 0.10 g Au/support was suspended in 20 mL of ice-cooled H2O, and 10 mg FeSO47H2O was added. The suspension was stirred at 0 °C for 10 min, then 10 mL 30% H2O2 was added dropwise. The mixture was kept stirring for 30 min at 0 °C and 1 h at room temperature, then filtered, and thoroughly washed with H2O. The product was dried at 70 °C for 10 h. Unless otherwise specified in the text, a sample (50 mg) was packed into a U-type quartz tube (i.d. = 4 mm) sealed by quartz wool and pretreated in flowing 8% O2 (balance He) at a specified temperature for 1 h (heating rate, 30 °C/ min; flow rate, 37 cm3/min). After cooling, a gas stream of 1% CO (balance air, \4 ppm water) flowed through the catalyst at a rate of 37 cm3/min, and the exiting stream was analyzed by a gas chromatograph equipped with a dual molecular sieve/porous polymer column and a thermal conductivity detector. The reaction temperature was varied

Activation of Dodecanethiol-Capped Gold Catalysts

K2MnO4/Au/SiO2, 300 oC 100 Au/SiO2, Fenton's reagent,

CO Conversion / %

using a furnace or by immersing the U-type tube in ice– water or liquid N2 cooled acetone. TGA/DTG experiments were conducted on a TGA 2950 instrument using a heating rate of 10 °C/min under N2 or air. XRD data were collected on a Siemens D5005 diffractometer with Cu Ka radiation, in the range of 2h = 20– 90° at the rate of 0.01°/s. TEM-EDX experiments were carried out on a Hitachi HD-2000 STEM operated at 200 kV. High Angle Annular Dark Field (HAADF) images were obtained using a Cs-corrected FEI Titan 80/300-kV TEM/STEM with a convergence angle of 16.8 mrad and a large inner collection angle of 55 mrad. The images were acquired at ‘‘fresh’’ sample area without pre-electron beam irradiation. SEM-EDX experiments were conducted on a JEOL JSM-6060 scanning electron microscope coupled with an EDX detector.

211

300 oC

50 KMnO4/Au/SiO2, 300 oC Au/SiO2, 500 oC Au/SiO2, 300 oC

0

-150 -100

-50

0

50

100

150

200

250

300

350

Reaction Temperature / oC Fig. 1 CO conversions on SiO2-based gold catalysts as a function of reaction temperature. The catalysts were pretreated in O2–He at a specified temperature for 1 h prior to reaction testing. The starting gold particle size was estimated by XRD as 2.1 nm

3 Results 3.1 General Consideration In this work, three oxidative reagents (KMnO4, K2MnO4, Fenton’s reagent), four starting sizes of gold nanoparticles (1.8, 2.1, 3.9, 9.9 nm), and three supports (SiO2, TiO2, C) were selected. The sizes refer to the average gold particle sizes estimated by XRD. To streamline the presentation, the Results section is structured as follows. In Sect. 3.2, we chose SiO2 as a support and 2.1 nm gold nanoparticles as the gold precursor. The effect of treatment by different oxidative reagents on the catalytic activity was studied. The requirement for the thermal pretreatment to remove residual organic species was addressed, and relevant characterization was conducted. In Sect. 3.3, we again chose SiO2 as the support and KMnO4, K2MnO4, or Fenton’s reagent as the oxidative reagent, but we tuned the starting size of gold nanoparticles (1.8, 3.9, 9.9 nm). We intended to demonstrate that the beneficial effect of KMnO4 or K2MnO4 treatment is general, regardless of the size of gold nanoparticles. In Sect. 3.4, we extended our study to TiO2- and C-based gold catalysts. Different oxidative treatments were adopted, and different gold particles sizes used. We found that KMnO4 or K2MnO4 treatment can also make the catalysts more active under certain conditions, regardless of the support chosen. In Sect. 3.5, the stability of typical gold catalysts as a function of time on stream was studied. 3.2 Effect of Different Oxidative Reagents Figure 1 shows the CO conversions as a function of reaction temperature. Gold nanoparticles with an average size of 2.1 nm were used as the precursor for loading gold. The Au/SiO2 catalyst pretreated at 300 °C is not active when

the reaction temperature is below 100 °C. It reaches 50% conversion at 290 °C. A significant enhancement in activity is achieved when Au/SiO2 is pretreated at 500 °C. The catalyst reaches 50% conversion at -18 °C. Such an effect (enhancement in catalytic activity due to pretreatment at 500 °C) is possibly due to the more sufficient removal of organic ligands, as demonstrated by the TG/ DTG data in Supplementary Fig. S1. There is a dip in the conversion curve, as often seen with other Au/SiO2 catalysts [30–32], but the reason for this dip is still not clear. On the other hand, both KMnO4/Au/SiO2 and K2MnO4/ Au/SiO2 samples do not need pretreatment at 500 °C, because of the enhanced desorption or combustion of organic species at relative low temperatures (Supplementary Fig. S1). When pretreated at 300 °C, these two catalysts show much higher CO conversions below 0 °C. KMnO4/Au/SiO2 shows 50% conversion at -84 °C, whereas K2MnO4/Au/SiO2 shows close to 100% conversion at -84 °C. Such a dramatic promotional effect is also seen with Au(en)2Cl3-derived Au/SiO2 treated by KMnO4 [17]. There are sharp dips in the CO conversion curves as well, due to the deactivation of the catalysts when measuring the conversion curves. Because we initially warm up the catalyst by immersing the U-type tube in a Dewar filled with liquid N2 cooled acetone, the catalyst warms up more slowly near room temperature than at low temperatures. The extended lingering in between -10 °C and room temperature causes more obvious deactivation in that region, unless the Dewar is switched to a beaker with ice water. That is why we see sudden increases in activity when the conversion drops to the lowest. For comparison, Au/SiO2 treated by Fenton’s reagent shows a low catalytic activity (50% conversion at 149 °C). Therefore, the treatment of Au/SiO2 with KMnO4 or K2MnO4 can help

123

212

H. Yin et al.

6000 Au/SiO2, Fenton's reagent, 300 oC

Au(111)

5000

XRD Intensity / a.u.

promote the activity, whereas the treatment by Fenton’s reagent is not effective. To compare the activity more quantitatively, specific rates were calculated based on the CO concentration (1 mol%), flow rate (37 mL/min), CO conversions at -70 °C read from the conversion curves, the amount of catalyst put into the reactor (50 mg), and gold loadings measured by EDX (Table 1). For instance, the CO conversions of Au/SiO2 (2.0 wt% Au), KMnO4/Au/SiO2 (2.0 wt% Au), and K2MnO4/Au/SiO2 (2.1 wt% Au) at -70 °C are 4%, 64%, and 100%, respectively. Therefore, the specific rates at -70 °C are calculated as 0.04, 0.58, and -1 [0.86 mol g-1 Au h , respectively. For comparison, the KMnO4/Au/SiO2 (1.68 wt% Au) made using Au(en)2Cl3 as -1 the precursor has a specific rate of 1.1 mol g-1 at Au h -70 °C [17]. It is noted that the CO conversion on K2MnO4/Au/SiO2 is so high that a precise measurement of the specific rate at -70 °C is not possible. Therefore, we decreased the amount of catalyst to 20 mg and measured the conversion again. As shown in Supplementary Fig. S2, the CO conversion is 46% at -70 °C, corresponding to a -1 specific rate of 0.99 mol g-1 Au h . Figure 2 shows the corresponding XRD patterns of several samples. In general, amorphous SiO2 has a broad feature at 2h & 22°, and gold peaks appear at 2h & 38, 44, 65, and 78°. The as-prepared Au/SiO2 catalyst shows very broad gold peaks, indicating that these gold nanoparticles are fairly small. Upon pretreatment of the Au/SiO2 catalyst at 300 or 500 °C, the XRD peaks corresponding to metallic gold become sharper, revealing the sintering of gold nanoparticles. For comparison, the gold peaks of the

Au(200) Au(220)

Au(311)

K2MnO4/Au/SiO2, 300 oC

4000

KMnO4/Au/SiO2, 300 oC

3000

Au/SiO2, 500 oC Au/SiO2, 300 oC

2000

as-prepared Au/SiO 2

1000

as-received SiO 2

0 20

30

40

50

60

70

80

90

2 Theta / degree Fig. 2 XRD patterns of SiO2-based gold catalysts collected after thermal pretreatment and reaction testing. The XRD patterns of as-received SiO2 and as-prepared Au/SiO2 (bottom traces) are listed for comparison. The starting gold particle size was estimated by XRD as 2.1 nm

KMnO4/Au/SiO2 and K2MnO4/Au/SiO2 catalysts treated at 300 °C are broader than those of the Au/SiO2 catalyst treated at 500 °C, whereas those of the Au/SiO2 catalyst treated by Fenton’s reagent are clearly the sharpest. The shaper the gold peaks, the bigger the size of gold nanoparticles. Therefore, the treatment with Fenton’s reagent leads to a negative effect. Figure 3 shows the corresponding TEM images of several samples. The as-prepared Au/SiO2 catalyst shows a uniform size distribution. The gold particle size increases when the catalyst is pretreated at 300 or 500 °C. However, after the treatment of KMnO4/Au/SiO2 and K2MnO4/Au/

Table 1 EDX and specific rate data of Au/SiO2 or oxidants/Au/SiO2 after pretreatment and CO oxidation test Sample

Au (wt%)

Mn (wt%)

K (wt%)

Fe (wt%)

CO conversion at -70 °C/%

Specific rate -1 at -70 °C/mol g-1 Au h

Au(1.8 nm)/SiO2, 500 °C

2.8

–

–

–

21

0.14

KMnO4/Au(1.8 nm)/SiO2, 300 °C

2.1

1.4

0.3

–

100

[0.86

K2MnO4/Au(1.8 nm)/SiO2, 300 °C

2.0

1.5

0.5

–

100

[0.91

Au(1.8 nm)/SiO2, Fenton’s reagent, 300 °C

2.7

–

–

2.2

Au(2.1 nm)/SiO2, 500 °C KMnO4/Au(2.1 nm)/SiO2, 300 °C

2.0 2.0

– 0.7

– 0.5

– –

4 64

0.04 0.58

100

[0.86


2.1

0.7

0.6

–


2.1

–

–

2.0

–

–

_

–

Au(3.9 nm)/SiO2, 500 °C

2.3

–

–

–

4

0.03


2.5

0.8

0.3

–

57

0.41

12


2.4

0.7

0.6

–


2.1

–

–

2.2

–

–

Au(9.9 nm)/SiO2, 500 °C

0.9

–

–

–

0

0 –

0.09


0.8

0.8

0.4

–

–


1.0

0.8

0.3

–

_

_


1.0

–

–

3.8

_

_

123


213

Fig. 3 Representative TEM images of fresh Au/SiO2 (a), 300 °Cpretreated Au/SiO2 (b), 500 °C-pretreated Au/SiO2 (c), 300 °Cpretreated KMnO4/Au/SiO2 (d), 300 °C-pretreated K2MnO4/Au/SiO2

(e), and 300 °C-pretreated Fenton’s reagent-treated Au/SiO2 (f). The starting gold particle size was estimated by XRD as 2.1 nm

SiO2 at 300 °C, there is a surprising formation of large agglomerates composed of tiny gold nanoparticles and MnOx species. This assignment is confirmed by EDX analysis (Fig. 4) showing the presence of Au and Mn, together with a small amount of K, in the composite, whereas no Au or Mn is found in a region without gold nanoparticles. These agglomerates already appear after room-temperature treatment by KMnO4 or K2MnO4 and subsequent drying at 70 °C (Supplementary Figs. S3B, S3C). On the other hand, the Au/SiO2 catalyst treated by Fenton’s reagent shows individual gold nanoparticles with bigger sizes (Fig. 3f). Supplementary Fig. S3D shows that the treatment with Fenton’s reagent already leads to such a negative effect even when the treated catalyst is not subjected to further treatment at 300 °C.

3.3 Extension to Gold Catalysts with Different Gold Particle Sizes To see whether the trend is general, we also loaded gold nanoparticles of different sizes (1.8, 3.9, and 9.9 nm) onto SiO2. It can be seen from Fig. 5 that both Au(1.8 nm)/SiO2 and Au(3.9 nm)/SiO2 show high CO conversions below room temperature, although there is a dip in the conversion curves at higher temperatures, similar to the case with Au(2.1 nm)/SiO2 in Fig. 1. The sizes in the parentheses mean the starting sizes of gold nanoparticles estimated by XRD, and do not necessarily represent the final values after thermal pretreatment. On the other hand, Au(9.9 nm)/SiO2 shows virtually no conversion when the reaction temperature is below 300 °C.

123

214

H. Yin et al.

Fig. 4 A TEM and regional EDX analysis data of KMnO4/ Au/SiO2 sample, showing the coexistance of Au nanoparticle and Mn species on the sample’s surface. The starting gold particle size was estimated by XRD as 2.1 nm 2 1

Si

200

Si

Cu

Cu

150 150

1

2

100 100 Au

50

Cu

50

Cu Mn

K

0

0 0

2

4

6

8

keV

Upon treating the catalysts with oxidative agents, the same trend seen in Fig. 1 was observed: i.e., the treatment with KMnO4 or K2MnO4 leads to higher CO conversions at low temperature, whereas the treatment with Fenton’s reagent does not have such a beneficial effect (Fig. 5). By comparing Figs. 1 and 5, it is straightforward to see that low-temperature CO conversions on KMnO4/Au(1.8 nm)/ SiO2 and K2MnO4/Au(1.8 nm)/SiO2 are the highest, and the CO conversions of KMnO4- or K2MnO4-treated Au/ SiO2 decrease with the size of gold nanoparticles. Indeed, Table 1 shows that the specific rates generally decrease with the size of gold nanoparticles. Although the KMnO4/ Au(9.9 nm)/SiO2 and K2MnO4/Au(9.9 nm)/SiO2 catalysts are not particularly active, the CO conversions on these catalysts are significantly higher than those on Au(9.9 nm)/ SiO2. This observation again demonstrates the beneficial effect of KMnO4 or K2MnO4 treatment. On the other hand, the reason for the inferior effect of the treatment by Fenton’s reagent is at least partly due to the sintering of gold nanoparticles, as seen from the XRD patterns in Supplementary Fig. S4. Our TEM experiments indicate that the agglomerates composed of gold nanoparticles and MnOx are visible when the original size of the gold precursor is 1.9 or 3.9 nm (Supplementary Figs. S5, S6), whereas no formation of similar agglomerates is observed when the original size of the gold precursor is 9.9 nm (Supplementary Fig. S7), possibly because these big gold nanoparticles are difficult to move around. HAADF images show that the agglomerates

123

Au

10

12

0

2

4

6

8

10

12

keV

are indeed composed of gold nanoparticles embedded in amorphous MnOx (Fig. 6). To the best of our knowledge, such a feature was not reported in previous papers on gold catalysts prepared by colloidal deposition. 3.4 Extension of Gold Catalysts with Different Supports We also used TiO2 as a support to study the support effect. The TiO2-based gold catalysts were all pretreated at 300 °C because such a pretreatment is enough to obtain high activities. As shown in Fig. 7, Au/TiO2 catalysts are less active than Au/SiO2 prepared using the current preparation method. The former catalysts show virtually no CO conversion at room temperature. It is also clear from Fig. 7 that KMnO4/Au/TiO2 and K2MnO4/Au/TiO2 catalysts are more active than the Au/TiO2 catalyst, whereas the treatment of Au/TiO2 by Fenton’s reagent leads to a negative effect. This conclusion is still valid when the activities are expressed by specific rates (Table 2). However, in general, TiO2-based gold catalysts are less active than SiO2-based gold catalysts at -70 °C, as seen by a comparison of Tables 1 and 2. In addition, Table 2 also shows that the specific rates generally decrease with the size of gold nanoparticles. Because some of the XRD peaks of TiO2 overlap with those of gold (Supplementary Fig. S8), we then used TEM to characterize the corresponding samples. According to Fig. 8, gold nanoparticles tend to agglomerate on TiO2


A

215


CO Conversion / %

100

50


Au/SiO2, Fenton's reagent, 300 oC

Au/SiO2, 500 oC

0

B


CO Conversion / %

100

Au/SiO2, 500 oC

50 K2MnO4/Au/SiO2, 300 oC

Au/SiO2, Fenton's reagent, 300 oC

0

C 100

CO Conversion / %



50 Au/SiO2, Fenton's reagent, 300 oC

0 -150 -100

Au/SiO2, 500 oC

-50

0

50

100

150

200

250

300

350

Reaction Temperature / oC Fig. 5 CO conversions on SiO2-based gold catalysts as a function of reaction temperature. The catalysts were pretreated in O2–He at a specified temperature for 1 h prior to reaction testing. From top to bottom, the starting gold particle sizes were estimated by XRD as 1.8 (a), 3.9 (b), and 9.9 nm (c), respectively

Fig. 6 HAADF images of KMnO4/Au/SiO2 catalyst collected after pretreatment at 300 °C and subsequent reaction testing. The starting gold particle size was estimated by XRD as 1.8 nm

surfaces, consistent with the trend seen in our previous publications dealing with Au/TiO2 catalysts prepared by another method (deposition–precipitation) [33, 34]. On the other hand, agglomerates composed of small gold nanoparticles and amorphous MnOx are still observed with the KMnO4/Au/TiO2 and K2MnO4/Au/TiO2 catalysts, consistent with the trend seen with the KMnO4/Au/SiO2 and K2MnO4/Au/SiO2 catalysts described above. The Au/TiO2 catalyst treated by Fenton’s reagent has relatively big gold particles.

We also chose carbon as a support to evaluate support effects. Carbon-based gold catalysts were all pretreated at 300 °C due to the concern that a high pretreatment temperature would lead to the combustion of carbon catalyzed by gold and MnOx [26]. The Au/C catalysts with the starting gold particles sizes of 1.8, 2.1, and 3.9 nm are active for CO oxidation below 0 °C (Fig. 9). At least they show higher CO conversions below 0 °C than Au/TiO2 catalysts reported above. Such a unique feature (high CO conversion below 0 °C) has been seldom reported [35], and

123

216

A

B K2MnO4/Au/TiO 2, 300 oC

Au/TiO 2, 300 oC

50

KMnO4/Au/TiO 2, Au/TiO 2, Fenton's reagent, 300 oC

300 oC

0

Au/TiO 2, 300 oC

Au/TiO 2, Fenton's reagent, 300 oC

Au/TiO 2, 300 oC

K2MnO4/Au/TiO 2, 300 oC

CO Conversion / %

CO Conversion / %

300 oC

D 100

Au/TiO 2, 300 oC

KMnO4/Au/TiO 2, 300 oC

K2MnO4/Au/TiO 2,

50

0

C 100


100

CO Conversion / %

100

CO Conversion / %

Fig. 7 CO conversions on TiO2-based gold catalysts as a function of reaction temperature. The catalysts were pretreated in O2–He at a specified temperature for 1 h prior to reaction testing. The starting gold particle sizes were estimated by XRD as 1.8 nm (a), 2.1 nm (b), 3.9 nm (c), and 9.9 nm (d), respectively

H. Yin et al.

K2MnO4/ Au(SR)/TiO 2, 300 oC

50


50



0

0 -150 -100 -50 0

50 100 150 200 250 300 350

-100 -50

Reaction Temperature / oC

0

50

100 150 200 250 300

350


Table 2 EDX and specific rate data of Au/TiO2 or oxidants/Au/TiO2 after calcination and CO oxidation test Sample

Au (wt%)

Mn (wt%)

K (wt%)

Fe (wt%)

Au(1.8 nm)/TiO2, 300 °C

2.0

–

–

–

0

KMnO4/Au(1.8 nm)/TiO2, 300 °C

2.1

0.9

0.3

–

29

K2MnO4/Au(1.8 nm)/TiO2, 300 °C

1.9

1.0

0.7

–

59

Au(1.8 nm)/TiO2, Fenton’s reagent, 300 °C

2.0

–

–

1.7

Au(2.1 nm)/TiO2, 300 °C KMnO4/Au(2.1 nm)/TiO2, 300 °C

1.7 2.0

– 1.0

– 0.1

– –

– 21 15


1.7

0.8

0.4

–


1.6

–

–

1.7

Au(3.9 nm)/TiO2, 300 °C

1.9

–

–


2.1

0.8

0.1


2.2

0.7


–

Specific rate -1 at -70 °C/mol g-1 Au h 0 0.25 0.56 – – 0.19 0.16

–

–

–

–

–

–

15

0.3

–

–

0.13 –


2.2

–

–

1.4

–

–

Au(9.9 nm)/TiO2, 300 °C

1.1

–

–

–

–

–


1.2

0.7

0.1

–

–

–


1.2

0.9

0.3

–

–

–


1.0

–

–

1.5

–

–

in most cases Au/C catalysts are known to be inactive for CO oxidation [36–40]. However, the CO conversion on Au/C decreases when the reaction temperature is above 0 °C, and increases gradually above 100 °C, similar to the case with Au/SiO2 described above. The reason behind this observation is not clear at the moment. Upon modification with KMnO4 or K2MnO4, the big dip in the light-off curves is essentially removed. These promoted catalysts show 100% conversion above 50 °C. The

123

beneficial effect of KMnO4 or K2MnO4 treatment is also seen from specific rates calculated in Table 3. Such a promotional effect is clearly not due to the size of gold nanoparticles (Fig. 10), but due to the presence of MnOx that creates Au– MnOx interfaces. The Au/C catalyst shows no activity when the starting gold particle size is 9.9 nm, but the modification with KMnO4 or K2MnO4 leads to a significant increase in catalytic activities, similar to the case with the KMnO4/Au/ SiO2 and K2MnO4/Au/SiO2 catalysts described above.


217

Fig. 8 Representative TEM images of spent Au/TiO2 (a), KMnO4/ Au/TiO2 (b), K2MnO4/Au/TiO2 (c), and Au/TiO2 treated by Fenton’s reagent (d) collected after thermal pretreatment at 300 °C under O2–

A

KMnO4/Au/C, 300 oC

K2MnO4/Au/C, 300 oC

He and CO oxidation. The starting gold particle size was estimated by XRD as 2.1 nm

B

50

0 100

CO Conversion / %

CO Conversion / %

Au/C, Fenton's reagent, 300 oC

C

Au/C, Fenton's reagent, 300 C

300 oC

K2MnO4/Au/C, 300 oC KMnO4/Au/C, 300 oC

50 Au/C, 300 oC Au/C, Fenton's reagent, 300 oC

0

KMnO4/Au/C, 300 oC

0

KMnO4/Au/C,

100

o

-150 -100 -50

50

D

K2MnO4/Au/C, 300 oC

Au/C, 300 oC

K2MnO4/Au/C, 300 oC

0

Au/C, 300 oC

50

0

Au/C, Fenton's reagent, 300 oC Au/C, 300 oC

100

CO Conversion / %

CO Conversion / %

100

50 100 150 200 250 300 350


-150 -100 -50

0

50

100 150 200 250 300 350


Fig. 9 CO conversions on carbon-based gold catalysts as a function of reaction temperature. The catalysts were pretreated in O2-He at a specified temperature for 1 h prior to reaction testing. The starting

gold particle sizes were estimated by XRD as 1.8 nm (a), 2.1 nm (b), 3.9 nm (c), and 9.9 nm (d), respectively

3.5 Stability of the Catalysts

amount of catalyst put into the reactor, so as to make sure that the conversion at room temperature is not high enough to reach 100% while not low enough to become 0% all the time. Such a precaution is mentioned in a book [4].

Catalyst stability is an important factor for practical applications. We measured the stability by adjusting the

123

218

H. Yin et al.

Table 3 EDX and specific rate data of Au/C or oxidants/Au/C after calcination and CO oxidation test Sample

Au (wt%)

Mn (wt%)

K (wt%)

Fe (wt%)


Specific rate -1 at -70 °C/mol g-1 Au h

Au(1.8 nm)/C, 300 °C

2.6

–

–

–

0

KMnO4/Au(1.8 nm)/C, 300 °C

2.1

2.6

0.5

–

50

K2MnO4/Au(1.8 nm)/C, 300 °C

2.2

–

–

–

30

0.25

Au(1.8 nm)/C, Fenton’s reagent, 300 °C

2.1

–

–

0.9

20

0.17

– 0.43

Au(2.1 nm)/C, 300 °C

2.2

–

–

–

4

0.03

KMnO4/Au(2.1 nm)/C, 300 °C

1.7

2.1

0.3

–

5

0.05

K2MnO4/Au(2.1 nm)/C, 300 °C Au(2.1 nm)/C, Fenton’s reagent, 300 °C

2.1 1.7

2.6 –

0.8 –

– 1.6

22 0

0.19 0

Au(3.9 nm)/C, 300 °C

2.4

–

–

–

11

0.08

KMnO4/Au(3.9 nm)/C, 300 °C

1.7

1.2

0.2

–

14

0.15

K2MnO4/Au(3.9 nm)/C, 300 °C

1.6

2.0

0.5

–

67

0.76


2.2

–

–

0.8

–

Au(9.9 nm)/C, 300 °C

1.9

–

–

–

–

–

KMnO4/Au(9.9 nm)/C, 300 °C

1.7

2.7

0.5

–

–

–

–

K2MnO4/Au(9.9 nm)/C, 300 °C

1.2

2.9

0.7

–

–

–


1.0

–

–

2.6

–

–

Fig. 10 Representative TEM images of spent Au/C (a), KMnO4/Au/ C (b), K2MnO4/Au/C (c), and Au/C treated by Fenton’s reagent (d) collected after thermal pretreatment under O2-He at 300 °C and

subsequent CO oxidation. The starting gold particle size was estimated by XRD as 2.1 nm

As shown in Fig. 11, there is some initial deactivation when using the SiO2-based gold catalysts, but they reach steady state after operation for 30 h. It is straightforward to see from the figure that the TiO2- and carbon-based gold

catalysts are more stable than the SiO2-based gold catalysts. For instance, the initial conversion on 36 mg KMnO4/Au(2.1 nm)/SiO2 is 50%, and the CO conversion decreases to 5% at 40 h. The initial conversion on 20 mg

123


CO Conversion / %

A

4 Discussion and Conclusions 100

CO Conversion / %

42 mg Au(2.1 nm)/SiO 2

40 mg Au(3.9 nm)/SiO 2

50 32 mg Au(1.8 nm)/SiO 2 36 mg KMnO4/Au(2.1 nm)/SiO2

0

B

26 mg K2MnO4/Au(2.1 nm)/SiO2

100

59 mg Au(1.8 nm)/TiO 2

20 mg KMnO4/Au(2.1 nm)/TiO2

50

20 mg K2MnO4/Au(2.1 nm)/TiO2

72 mg Au(3.9 nm)/TiO 2 64 mg Au(2.1 nm)/TiO 2

0

CO Conversion / %

C

219

50 mg Au(1.8 nm)/C

100

31 mg K2MnO4/Au(2.1 nm)/C

50

49 mg Au(2.1 nm)/C

26 mg KMnO4/Au(2.1 nm)/C

60 mg Au(3.9 nm)/C

0

0

10

20

30

40

Reaction Time on Stream / h Fig. 11 CO conversions on SiO2-, TiO2-, and carbon-based gold catalysts (a, b, and c, respectively) as a function of reaction time on stream. Au/SiO2 catalysts were pretreated at 500 °C, and KMnO4 or K2MnO4 treated Au/SiO2 was pretreated at 300 °C. TiO2- and carbon-based gold catalysts were pretreated at 300 °C. The reaction testing was conducted at room temperature

KMnO4/Au(2.1 nm)/TiO2 is 71%, and the CO conversion decreases to 61% at 40 h. The initial conversion on 26 mg KMnO4/Au(2.1 nm)/C is 69%, and the CO conversion decreases to 62% at 40 h. The difference seems to be dependant on the nature of supports, since Au/SiO2 catalysts deactivate more quickly than Au/TiO2 and Au/C catalysts, as seen clearly from Fig. 11. The reason for the catalyst deactivation should be studied more carefully in the future with the aid of in situ infrared spectroscopy.

Catalysis by finely divided supported gold nanoparticles has been a hot topic recently. Most gold catalysts are prepared by loading gold on a support, such as TiO2, Al2O3, SiO2, and C [1–5]. In these cases, a simple metalsupport interface (such as Au–TiO2) is formed. It would be interesting to build up more complex interfaces involving an additional modifier [41]. Through rational interfacial engineering, more active and stable catalysts may be developed by interplaying the interactions among a metal, a support, and a modifier. For instance, we soaked Au/C prepared by deposition–precipitation in an aqueous KMnO4, and found that the activity in CO oxidation was significantly enhanced, even though small gold nanoparticles were leached away and big gold nanoparticles were left behind [26]. The enhancement in activity is explained by the creation of Au–MnOx interface active for CO oxidation [21–26]. Aqueous KMnO4 is known to react with carbon surfaces to form MnOx on carbon surfaces (4KMnO4 ? 3C ? 2H2O ? 4MnO2; ? 3CO2 ? 4KOH) [42]. In another work, we treated Au(en)2Cl3-derived Au/ SiO2 by an aqueous KMnO4 and pretreated the dried catalyst at mild temperatures, and found an obvious enhancement in activities in low-temperature CO oxidation [17]. TEM-EDX data reported therein clearly show the presence of Au–MnOx interface. Because Au/MnOx is known to be active for CO oxidation [21–26], the enhancement in activity due to the presence of additional Au–MnOx interfaces is understandable. Here we demonstrate that the treatment of Au/SiO2, Au/ TiO2, and Au/C catalysts by an aqueous KMnO4 or K2MnO4 followed by mild thermal pretreatment could make the above catalysts more active in certain reactiontemperature regions. We believe that the presence of Au–MnOx interface is important for this enhancement (Scheme 2). Both KMnO4 and K2MnO4 can partially oxidize some residual organic groups, thus leaving some MnOx species near gold nanoparticles. Some remaining KMnO4 and K2MnO4 may also react with residual organic species during the thermal treatment, leading to the creation of Au– MnOx interface. Our TG/DTG data (Supplementary Fig. S1) indicate that KMnO4 and K2MnO4 can not completely oxidize these residual organic species to CO2 at room temperature, and therefore the thermal treatment prior to the reaction testing is necessary to remove organic species and achieve high activities. As for the reason why Fenton’s reagent often leads to a negative effect, we have shown clearly that the treatment by Fenton’s reagent can cause the sintering of gold nanoparticles, as seen from the XRD patterns in Fig. 2 and Supplementary Fig. S4. Alternatively, since the treatment involves the use of FeSO4 solution, the residual sulfate ions

123

220

H. Yin et al.

Scheme 2 Formation of AuMnOx composites and MnOxmodified gold nanoparticles upon treatment of supported dodecanethiol-capped gold catalysts by aqueous KMnO4 or K2MnO4

on catalyst surfaces may also poison some active sites [33]. We previously treated Au/TiO2 by an aqueous (NH4)2SO4 followed by filtering and drying, and found that the catalytic activity in CO oxidation was totally lost [33]. To test the validity of this explanation, we subsequently treated the Fenton’s reagent-treated Au/SiO2 catalyst by an aqueous NaOH solution, and found that the activity was not significantly increased, because the gold particle size was still big (data not shown), thus supporting that the ineffectiveness of the Fenton’s regent treatment is probably due to the sintering of gold nanoparticles. It was recently reported that OH radicals generated by Fenton’s reagent may facilitate the movement of gold atoms and cause the re-structuring of gold surfaces [43]. One may notice from Tables 1, 2, and 3 that there are some uptakes of iron on catalyst surfaces when using Fenton’s reagent to treat Au/support catalysts. The adsorption of iron species on these oxide supports could result from a surface ion-exchange process. The surfaces of these oxides are negatively charged under our experimental conditions and expected to absorb metal cations (e.g., Fe2?). The iron species (possibly in the form of FeOx after pretreatment in O2–He at 300 °C) left behind may be far away from the gold nanoparticles and are unable to prevent gold nanoparticles from sintering. This is understandable, because hydroxyl radicals generated from Fe2? and H2O2 are responsible for the oxidation of organic species [44] whereas Fe2? does not react with the capping agent. However, this may not be the case with KMnO4 and K2MnO4. An unexpected interesting observation is a composite structure composed of numerous gold nanoparticles connected by amorphous MnOx (Scheme 2, Figs. 4, 6, 8). To the best of our knowledge, such a feature was not reported in previous publications dealing with supported gold catalysts. We believe that the MnOx behaves as a glue, and the initially deposited gold nanoparticles are movable on support surfaces, so that they can be aggregate together to form the composite structure. Although it is not exactly clear whether the composite formed on SiO2 and TiO2 surfaces is porous, i.e., whether the gold nanoparticles

123

within the composite are accessible to the reactants, we note that such catalysts show high activities in CO oxidation. We think that the removal of residual organic species during thermal pretreatment may open pores for the access to reactants [45–47]. We notice that the formation of large agglomerates composed of gold nanoparticles and MnOx happens on SiO2 and TiO2 supports, but does not happen on carbon supports. This may indicate that the gold nanoparticles move around more easily on SiO2 and TiO2 supports. The surfaces of both SiO2 and TiO2 are hydrophilic while our gold nanoparticles are hydrophobic. Accordingly, the interaction of the support surfaces for SiO2 and TiO2 with the gold nanoparticles is week, leading to a facile mobility of the gold nanoparticles. In contrast, the carbon support used in our investigation is derived from hightemperature graphitization are hydrophobic. Accordingly, the hydrophobic gold nanoparticles can be highly dispersed on the carbon support via the favorable interaction between the hydrophobic capping ligand and the hydrophobic carbon surface. However, another effect is that KMnO4 or K2MnO4 also reacts with the carbon support [42], depositing MnOx on carbon surfaces and making the movement of gold nanoparticles more difficult. Indeed, a comparison of Tables 1, 2, and 3 indicates that the Mn contents are significantly higher on KMnO4- or K2MnO4treated Au/C than the corresponding SiO2- or TiO2-based gold catalysts. It is surprising to see that both Au/SiO2 and Au/C catalysts prepared using the specific thiolated gold nanoparticles as the precursor show high activities in CO oxidation below 0 °C, although Au/SiO2 [48–53] and Au/C [36–40] catalysts are often known to be inactive in this reaction. We believe that the difference is due to the use of different gold precursors. The gold nanoparticles used in colloidal deposition in the literature are often not small enough to reach high activities, whereas gold nanoparticles used in our study can be very small. Further experiments may be conducted to elucidate the reason for the high activity at low temperature and abnormal dips in the conversion curves.

Activation of Dodecanethiol-Capped Gold Catalysts Acknowledgements Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DEAC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. This research was also supported by the appointment for H.F. Yin to the ORNL Research Associates Program, administered by Oak Ridge Associated Universities. The electron microscopy experiments were carried out at the Oak Ridge National Laboratory SHaRE User Facility, which is supported by the Division of Scientific User Facilities, DOE Office of Science, Basic Energy Sciences.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20. 21. 22.

Haruta M, Date´ M (2001) Appl Catal A 222:427 Choudhary TV, Goodman DW (2002) Top Catal 21:25 Hashmi ASK, Hutchings GJ (2006) Angew Chem Int Ed 45:7896 Bond GC, Louis C, Thompson DT (2006) Catalysis by Gold. Imperial College Press, London Kung MC, Davis RJ, Kung HH (2007) J Phys Chem C 111:11767 Tsubota S, Nakamura T, Tanaka K, Haruta M (1998) Catal Lett 56:131 Grunwaldt JD, Kiener C, Wogerbauer C, Baiker A (1999) J Catal 181:223 Porta F, Prati L, Rossi M, Coluccia S, Martra G (2000) Catal Today 61:165 Martra G, Prati L, Manfredotti C, Biella S, Rossi M, Coluccia S (2003) J Phys Chem B 107:5453 Chou J, Franklin NR, Baeck S-H, Jaramillo TF, McFarland EW (2004) Catal Lett 95:107 Comotti M, Li WC, Spliethoff B, Schu¨th F (2006) J Am Chem Soc 128:917 Zheng NF, Stucky GD (2006) J Am Chem Soc 128:14278 Hickey N, Larochette PA, Gentilini C, Sordelli L, Olivi L, Polizzi S, Montini T, Fornasiero P, Pasquato L, Graziani M (2007) Chem Mater 19:650 Liu YM, Tsunoyama H, Akita T, Tsukuda T (2009) J Phys Chem C 113:13457 Zhou SH, Yin HF, Schwartz V, Wu ZL, Mullins DR, Eichhorn B, Overbury SH, Dai S (2008) Chem Phys Chem 9:2475 Zhou SH, Ma Z, Yin HF, Wu ZL, Eichhorn B, Overbury SH, Dai S (2009) J Phys Chem C 113:5758 Yin HF, Ma Z, Overbury SH, Dai S (2008) J Phys Chem C 112:8349 Gomez S, Giraldo O, Garces LJ, Villegas J, Suib SL (2004) Chem Mater 16:2411 Dong XP, Shen WH, Zhu YF, Xiong LM, Gu JL, Shi JL (2005) Micropor Mesopor Mater 81:235 Dong XP, Shen WH, Zhu YF, Xiong LM, Shi JL (2005) Adv Funct Mater 15:955 Gardner SD, Hoflund GB, Upchurch BT, Schryer DR, Kielin EJ, Schryer J (1991) J Catal 129:114 Hoflund GB, Gardner SD, Schryer DR, Upchurch BT, Kielin EJ (1995) Appl Catal B 6:117

221 23. Sanchez RMT, Ueda A, Tanaka K, Haruta M (1997) J Catal 168:125 24. Lee S-J, Gavriilidis A, Pankhurst QA, Kyek A, Wagner FE, Wong PCL, Yeung KL (2001) J Catal 200:298 25. Luengnaruemitchai A, Thoa DTK, Osuwan S, Gulari E (2005) Int J Hydrogen Energ 30:981 26. Ma Z, Liang CD, Overbury SH, Dai S (2007) J Catal 252:119 27. Brust M, Walker M, Bethell D, Schiffrin DJ, Whyman R (1994) J Chem Soc-Chem Commun 801 28. Hostetler MJ, Wingate JE, Zhong CJ, Harris JE, Vachet RW, Clark MR, Londono JD, Green SJ, Stokes JJ, Wignall GD, Glish GL, Porter MD, Evans ND, Murray RW (1998) Langmuir 14:17 29. Neyens E, Baeyens J (2003) J Hazard Mater 98:33 30. Date´ M, Okumura M, Tsubota S, Haruta M (2004) Angew Chem Int Ed 43:2129 31. Wang AQ, Hsieh Y-P, Chen Y-F, Mou C-Y (2006) J Catal 237:197 32. Zhu HG, Ma Z, Clark JC, Pan ZW, Overbury SH, Dai S (2007) Appl Catal A 326:89 33. Ma Z, Brown S, Overbury SH, Dai S (2007) Appl Catal A 327:226 34. Ma Z, Yin HF, Dai S (2010) Catal Lett. doi:10.1007/s10562009-0201-y 35. Peng S, Lee YM, Wang C, Yin HF, Dai S, Sun SH (2008) Nano Res 1:229 36. Okumura M, Tsubota S, Haruta M (2003) J Mol Catal A 199:73 37. Bulushev DA, Yuranov I, Suvorova EI, Buffat PA, Kiwi-Minsker L (2004) J Catal 224:8 38. Wang F, Lu GX (2007) Catal Lett 115:46 39. Ketchie WC, Murayama M, Davis RJ (2007) Top Catal 44:307 40. Ketchie WC, Fang Y-L, Wong MS, Murayama M, Davis RJ (2007) J Catal 250:95 41. Ma Z, Overbury SH, Dai S (2009) In: Lukehart CM, Scott RA (eds) Nanomaterials: Inorganic and Bioinorganic Perspectives. John Wiley & Sons, Chichester, p 247 42. Huang XK, Yue HJ, Attia A, Yang Y (2007) J Electrochem Soc 154:A26 43. Nowicka AM, Hasse U, Hermes M, Scholz F (2010) Angew Chem Int Ed 49:1061 44. Gallegos AA, Martinez SS, Velazquez RF (2008) In: Gunther MB (ed) Heterogeneous Catalysis Research Progress. Nova Science Publishers, New York, p 193 45. Zhu HG, Ma Z, Overbury SH, Dai S (2007) Catal Lett 116:128 46. Ma Z, Brown S, Howe JY, Overbury SH, Dai S (2008) J Phys Chem C 112:9448 47. Ma Z, Dai S (2008) Mater Technol 21:81 48. Haruta M, Tsubota S, Kobayashi T, Kageyama H, Genet MJ, Delmon B (1993) J Catal 144:175 49. Lin SD, Bollinger M, Vannice MA (1993) Catal Lett 17:245 50. Wolf A, Schu¨th F (2002) Appl Catal A 226:1 51. Overbury SH, Ortiz-Soto L, Zhu HG, Lee B, Amiridis MD, Dai S (2004) Catal Lett 95:99 52. Yan WF, Chen B, Mahurin SM, Hagaman EW, Dai S, Overbury SH (2004) J Phys Chem B 108:2793 53. Delannoy L, El Hassan N, Musi A, Le To NN, Krafft J-M, Louis C (2006) J Phys Chem B 110:22471

123