Preferential oxidation of CO in hydrogen stream over nano-gold catalysts prepared by photodeposition method

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Preferential oxidation of CO in hydrogen stream over nano-gold catalysts prepared by photodeposition method Li-Hsin Chang, Yi-Lin Yeh, Yu-Wen Chen Department of Chemical and Materials Engineering, National Central University, Chung-Li 320, Taiwan

art i cle info

ab st rac t

Article history:

A series of gold catalysts supported on TiO2 were prepared by photodeposition method. The

Received 26 October 2007

effects of preparation parameters, such as power of UV light, irradiation time, and initial

Received in revised form

gold concentration, on the characteristics of the catalysts were studied. The catalysts were

18 January 2008

characterized by inductively coupled plasma-mass spectrometry, X-ray diffraction, X-ray

Accepted 19 January 2008

photoelectron spectroscopy, transmission electron microscopy, ultraviolet-visible diffuse

Available online 14 March 2008

reflectance spectroscopy, and high-resolution transmission electron microscopy. The

Keywords: Au/TiO2

catalytic activity of the catalyst for preferential oxidation of CO in hydrogen stream (PROX) was measured in a fixed-bed plug-flow reactor. The reactant gas containing 1.33% CO, 1.33% O2, 65.33% H2 and He for balance was fed into the reactor with a space velocity of

PROX

30,000 ml/g h. The lower power source lamp can deposit small gold particles on the support,

CO Gold supported catalysts

which in turn was responsible for the enhanced catalytic activity. The photodeposition method facilitates to prepare gold particles as small as 1.5 nm on the support. These catalysts were very active and selective in PROX reaction. However, the small gold particles were not stable as long as the reaction temperature was 450  C. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Supported nano-gold catalysts have been regarded as extremely active catalyst to oxidize CO at low temperatures [1]. The generation of hydrogen energy from fuel cell to directly convert chemical energy into electrical energy has gained importance due to the exhaustion of natural fuel reserves and the need for cleaner and sustainable energy. When hydrogen-rich fuel is produced from methanol or gasoline on board by partial oxidation and/or steam reforming combined with water–gas shift reaction, the Pt anodes in fuel cell at these low temperatures are often poisoned by incomplete combustion products, mainly CO, reducing the overall fuel cell performance. Comparing with the platinum and copper series catalysts, gold catalysts also exhibit higher activity in selective CO oxidation in hydrogen-rich stream at temperatures below 100 1C [2–4].

Several techniques, such as co-precipitation [5], deposition–precipitation (DP) [6], chemical vapor deposition [7], laser vaporization [8], modified impregnation [9], and photodeposition (PD) [10] methods, have been used for the synthesis of nano-gold particles on metal oxides. The activity of gold catalyst depends on the type of support, particle size of gold, and preparation method. DP method has been regarded as the most simple and cost-effective method to prepare nano-gold catalysts supported on several metal oxides. Moreover, the preparation parameters have great influence on the particle size and the catalytic activity of the gold catalysts. However, DP method is very tedious and only a part of gold could be deposited [11] on the support. The gold metal has exhibited catalytic activity only with a particle size of o5 nm and the bilayer structure is more active than monolayer as evidenced by previous reports [12–14]. To select a suitable preparation

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E-mail address: [email protected] (Y.-W. Chen). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.01.014

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conditions to restrict the particle size of gold within 5 nm is a key issue in preparing active gold catalysts by the available preparation methods. The advantages of utilizing PD method to prepare noble catalysts supported on metal oxide semiconductor have been reported [15–22]. According to the properties of semiconductor metal oxides, the carrier charge can be excited by the energy of incident light greater than or equal to the band gap of the metal oxide, and subsequently electrons can move from valence band to the conduction band, creating holes in the valence band. The excited electrons and holes may recombine in the materials or react with the adsorbed redox groups on the surface. Researchers employed this method to modify the photocatalytic properties of TiO2 by depositing noble metals [18,20,22]. Bamwenda et al. [10] reported that the activity of gold catalysts prepared by DP method were of the order of four times higher than the catalysts prepared by PD method [10]. However, the particle size of gold prepared by PD method was larger by two times than DP method in that study [10]. Under UV light irradiation with a wavelength less than 387 nm, the illuminated TiO2 (anatase type; band gap energy, Eg ¼ 3:2 eV) could generate photo-excited electrons and positive holes in aqueous medium. AuCl 4 ions adsorbed on the surface of TiO2 particles can react with the photogenerated e to form Au3þ , Auþ , and Au0 [19]. The size distribution of metal particles is influenced by the concentration of precursor, pH value, irradiation time, and wavelength of the light source [16,17,19]. Chan and Barteau [16] reported that the size distribution of gold particles decreased significantly with decreasing precursor concentration from 103 to 105 M and the highest Au loading obtained was 10:75 wt% with an average Au particle diameter of 1 nm. Similar to titania-supported platinum and silver catalysts, the morphology of gold supported on titania was mainly controlled by adjusting the pH value of precursor solution during PD [17]. Iliev and co-workers [20] obtained TiO2 with Au-modified particles by PD method. The activity significantly increased towards destruction of oxalic acid. In comparison with DP method [23] and PD method to prepare nano-gold supported on titania, it is an undeniable fact that these methods could successfully produce narrow gold particle size distribution with a few nanometers. The advantages of PD method over DP method are: (1) the extent of gold deposition is almost quantitative, and (2) the heat treatment is unnecessary since gold is reduced by UV irradiation. In this study, a series of Au/TiO2 catalysts were prepared by PD method. By regulating the preparation parameters, it was expected to have the gold particle size less than 3 nm. The catalysts were characterized by inductively coupled plasmamass spectrometry (ICP-MS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectroscopy (UV–vis DRS), and high-resolution transmission electron microscopy (HRTEM). The catalysts were tested for PROX reaction at various temperatures with O2/CO ratio of 1. The objective of this study was to elucidate the effect of preparation parameters on the catalytic properties of Au/TiO2 catalysts.

33 (2008) 1965 – 1974

2.

Experimental

2.1.

Catalyst preparation

The preparation of gold catalysts was accomplished by PD method using chloroauric acid as the gold precursor. Commercially available TiO2 (50 m2 =g) from Degussa (P25) was used. In a typical synthesis for the preparation of Au/TiO2 by PD method, the precursor solutions viz., HAuCl4 containing 8:5  104 M of gold (except a sample with 1:5  103 M) and TiO2 were adjusted to pH 6 using 0.1 M NH4OH before illumination. The light sources used were either 10 W UV lamps (245 nm; germicidal lamp from Sankyo Denki Co., Ltd.) or a 400 W highpressure mercury vapor lamp (SEN, HL400EH-5). The photoreactor contained two 10 W UV light tubes paralleled above the reactor. The irradiation time ranged from 30 to 240 min with a stirring speed of 770 rpm. During irradiation, the pH of the solution was monitored at an interval of 15 min and the temperature of the solution was maintained at 25 1C. After irradiation, the solution was filtered, washed, and dried in vacuum overnight. The filtrate was analyzed using AgNO3 solution to confirm the absence of residual Cl on the catalyst. In the photoreactor containing 400 W high-pressure mercury vapor lamp, the lamp was placed above the reactor and the irradiation time was shortened to 3, 6, and 10 min. The pH value was controlled to be 6. Two electric fans were kept on the side of the reactor and the temperature of solutions was maintained at 25 1C during the process. The slurry was vigorously stirred throughout the process. After irradiation, the solution was filtered, washed, and dried in vacuum overnight.

2.2.

Catalysts characterization

The samples were characterized by XRD, XPS, TEM, HRTEM, ICP-MS, and UV–vis DRS. The exact gold content was analyzed with ICP-MS (PE-SCIEX ELAN 6100 DRC). The solid powder was dissolved by a mixture of HF and aqua regia. XRD experiments were performed using a Siemens D500 ˚ ) at a powder diffractometer using CuKa1 radiation (1.5405 A voltage and current of 40 kV and 30 mA, respectively. The sample was scanned over the range 2y ¼ 20270 at a rate of 0.051/min to identify the crystalline structure. Samples for XRD were prepared as thin layers on a sample holder. The morphologies and particle sizes of the samples were determined by TEM on a JEM-2000 FX II operated at 160 kV and HRTEM on a JEOL JEM-2010 operated at 200 kV. Initially, a small amount of sample was placed into the sample tube filled with a 95% ethanol solution and after agitating under ultrasonic environment for 10 min, one drop of the dispersed slurry was dipped onto a carbon-coated copper mesh (300#) (Ted Pella Inc., CA, USA), and dried in an oven at 100 1C for 1 h. Images were recorded digitally with a Gatan slow scan camera (GIF). The XPS spectra were recorded with a Thermo VG Scientific Sigma Prob spectrometer. The XPS spectra were collected using AlKa radiation at a voltage and current of 20 kV and

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30 mA, respectively. The base pressure in the analyzing chamber was maintained in the order of 109 torr. The spectrometer was operated at 23.5 eV pass energy and the binding energy was corrected by contaminant carbon (C1s ¼ 285:0 eV) in order to facilitate the comparisons of the values among the catalysts and the standard compounds. Peak fitting was done using XPSPEAK 4.1 with Shirley background and 30:70 Lorentzian/Gaussian convolution product shapes. The UV–vis DRS was measured with a Cary 300 Bio UV–visible Spectrophotometer. Powder samples were loaded in a quartz cell, and spectra were collected in the range of 200–800 nm against barium sulfate standard.

2.3.

1967

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as follows: CO conversion ðXCO %Þ ¼

O2 conversion ðXO2 %Þ ¼

Selectivity ðS%Þ ¼

½COin  ½COout  100%, ½COin

½O2 in  ½O2 out  100%, ½O2 in

0:5XCO  100%, XO2

(1)

(2)

(3)

O2 consumption by CO ðCCO %Þ ¼ 12XCO ,

(4)

O2 consumption by H2 ðCH2 %Þ ¼ XO2  CCO .

(5)

PROX reaction testing

The catalytic activity was measured in a downward, pyrexglass fixed-bed continuous-flow reactor loaded, with 0.1 g catalyst. No pretreatment of catalyst was carried out before the catalytic test. The mass flow controller was used to control the feed. The reactant gas containing 1.33% CO, 1.33% O2, 65.33% H2 and He for balance was fed into the reactor with a total flow rate of 50 ml/min. The reactor was heated in a temperature regulated furnace (heating rate: 1 1C/min) and the temperature was measured using a thermocouple placed inside the catalyst bed. After reaching an equilibrium temperature at 5 min, the product was analyzed by a gas chromatograph equipped with a thermal conductivity detector using MS-5A column. Calibration was done with a standard gas containing known compositions of the components. The catalytic performance of Au/TiO2 was determined

3.

Results and discussion

3.1.

Catalysts characterization

3.1.1.

XRD

Fig. 1 shows the XRD patterns of various Au/TiO2 catalysts. It can be discerned from Fig. 1 that all catalysts showed strong XRD peaks for anatase and rutile phase of TiO2. The XRD pattern showed the absence of corresponding gold peaks at 2y ¼ 38:2 , 44.5 1, and 64.6 1, because the gold particle was too small to be detected. Since the gold peak at 2y ¼ 38:2 was overlapped with the peak of anatase TiO2, the lower intensity peaks located at 44.5 1 and 64.6 1 were used to locate the peaks corresponding to gold. The catalyst prepared by using a 400 W UV lamp and subjected to irradiation for 10 min showed some feeble peaks corresponding to gold particles. The results are

Au TiO2 (anatase)

Intensity (a.u.)

TiO2 (rutile)

(a)

(b)

(c)

(d)

(e) 20

30

40

50

60

70

2 θ (degree) Fig. 1 – XRD patterns of Au/TiO2 catalysts: (a) PD method (400 W, 10 min); (b) PD method (400 W, 6 min); (c) PD method (400 W, 3 min); (d) PD method (10 W, 120 min, 1:5  103 M of Au); (e) TiO2.

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3.3 nm) during PROX reaction possibly infers that the main reason for the color change was due to the reduction of Au3þ or Auþ to metallic gold under the reaction conditions. The uncalcined Au/TiO2 catalysts prepared by DP method also showed color change after CO oxidation [9].

in accordance with the average particle size of gold particles determined from TEM analysis as shown in Table 1. The particle size of gold on TiO2, prepared by using a 400 W lamp and irradiated for 10 min, was larger than 6 nm, and hence the presence of gold particles was observed from XRD patterns.

3.1.3. 3.1.2.

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TEM and HRTEM

The influence of irradiation time on the particle size of gold can be observed in Fig. 2 where the particle size of gold varied from 1 to 5 nm. Increasing the irradiation time (from 3 to 10 min by 400 W lamp) lead to the agglomeration of gold particles (45 nm), and the particles were widely distributed from 1 to 5 nm as shown in Fig. 2. Herrmann et al. [15] also reported similar relationship between the amount of silver deposited and the illumination time, 400 nm of silver particles was obtained with longer illumination time. However, large particles of gold were not observed on the samples prepared by the irradiation of 10 W UV lamps. In the images of gold catalysts prepared by 10 W lamps, it can be observed that the small gold particles were highly dispersed on the rough surface of TiO2 (Fig. 3), but it was too small to recognize each particle in the TEM image as shown in Fig. 3a. HRTEM was therefore used to investigate the particle size of Au. Based on the several images of HRTEM, more than 150 particles were counted and the size distribution graph was made. Au particles in Au/TiO2 catalyst prepared by low-power source (10 W) were mainly in the range of 1.5 nm. The gold particles prepared with a 400 W lamp were greater than those prepared with 10 W lamps. The results demonstrated that the power of UV lamp should be low to have small gold particles. If the power of UV light is too high, the photoreduction rate would be too fast to have large gold particles. By optimizing suitable power of UV light and irradiation time, it can be possible to prepare a catalyst with a gold particle size of 1.5 nm and uniform particle size distribution. The PD method was proved to produce uniform 1 nm gold particles on the support [16]; however, its catalytic activity has not been explored.

Color change and gold loading

Table 1 shows average gold particle size, exact gold loading, preparation conditions, and the color of the catalysts before and after PROX reaction. The exact gold loading on the catalysts prepared by a 400 W lamp increased as the irradiation time increased from 3 to 10 min. The particle size also increased with the irradiation time. The effect of precursor (HAuCl4) concentration on the gold loading on TiO2 support was also investigated. By comparing different precursor concentrations of gold (Table 1) irradiated by 10 W, it is shown that the higher concentration of solution results in more gold deposition on the support. In irradiation process by low-power lamps, the color of catalysts was changed from baby blue to light purple during 120 min irradiation and after 180 min the color immediately changed to purple. Since the color of the catalyst changed from white to purple only after 10 min, an irradiation time of o10 min was maintained while 400 W UV lamp was used for irradiation to prepare the catalyst. After PROX reaction, the color of the catalysts changed obviously. When the temperature of the PROX reaction was increased, the color of the catalysts changed obviously to purple at 50 1C. The results inferred that there is an obvious alteration in the gold particles size or surface chemical state of gold during PROX reaction at the temperature 450  C. In the preparation process of the catalyst by high-power lamp, the distinct color change was observed due to an increase in particles size from 2.7 to 6.1 nm (Table 1). Wang et al. [19] reported that hybrid colloidal including gold shell and TiO2 particles irradiated by UV light changed color from yellow to violet-red with a change in the average diameter to 10 nm, which is in agreement with our observation. However, comparing with color change before and after reaction, the obvious change in color was observed as shown in Table 1. The insignificant change in the particles size (from 1.7 to

3.1.4.

XPS

In order to study the change in surface states of gold before and after PROX reaction, XPS analysis was carried out in Au 4f

Table 1 – Characteristics of Au/TiO2 catalysts Catalysts (light power)

Precursor concentration of gold (103  M)

Irradiation time (min)

Average particle size (nm)a

Exact gold loading (%)b

Color of the as-prepared sample

Color of the sample after reaction

Au/TiO2 10 W

1.5

120

1.7c

0.478

Light blue

0.85 0.85 0.85 0.85

120 3 6 10

1.9 2.7 4.2 6.1

0.272 0.187 0.211 0.526

Light purple Baby blue Light blue Light purple

Medium Purple Purple Purple Purple Purple

Au/TiO2 Au/TiO2 Au/TiO2 Au/TiO2 a b c

10 W 400 W 400 W 400 W

Average particle size of gold obtained by TEM. Exact gold loading obtained by ICP-MS. After reaction the average particle size of gold was 3.3 nm by HRTEM.

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Fig. 2 – TEM images of Au/TiO2 by a 400 W lamp at pH 6 during (a) 3 min; (b) 6 min; (c) 10 min.

region for the catalysts as shown in Fig. 4 and full-width at half maximum (FWHM) in the entire spectra was 1.3 eV. The deconvolution of XPS spectra for all the catalysts suggests that Au species exists in three different states as metallic gold (Au0 ), non-metallic gold (Audþ ), and Au2O3 species, in accordance with literature [24–28]. The peaks for metallic gold were centered at 84.0 and 87.7 eV [24]. The peaks for Au3þ were located at 86.3 and 89.6 eV [25]. However, in this case, these peaks are most unlikely to be due to Au(III) chloride as gold catalysts prepared by PD method are, in general, free from chloride ions [26]. Auþ were located on 85.1 and 88.9 eV, and the presence of Auþ species on the surface may be ascribed to AuO species in the sample [27]. In addition, the catalysts were washed several times to remove the residual chloride after preparation and AgNO3 was used to confirm that there was no chloride ions left. The spectra of Ti 2p and O 1s before and after reaction confirmed that there is no significant difference in the Ti species after PROX reaction. Table 2 shows the XPS results in terms of the binding energies and the surface states of gold obtained by deconvolution of Au 4f peaks of the gold catalysts before and after the reaction. In general, the concentration of metallic gold in the used catalysts was higher than that of fresh one. The cationic Au is reduced to metallic state by CO reduction during reaction. The XPS results are also in accordance with the color change

due to the reduction of oxidic gold to metallic gold during reaction.

3.1.5.

UV–vis DRS

Fig. 5 shows that the intensities of plasmon bond at 550 nm increased with gold loading (Fig. 5b, c, e). It can be observed that even after PROX reaction, all the catalysts have the similar plasmon at 550 nm and exhibit the same purple color, but with different intensities. The absorbance at 550 nm corresponds to the presence of metallic gold particles. Zanella et al. [29] reported the presence of plasmon bond of metallic gold at 400–600 nm and the intensity of the peak increased with the pretreatment temperature. Tuzovskaya and coworkers [30] also indicated the feature of Au3þ at 190 nm for Au/zeolite samples before reduction and the appearance of bands at 530 nm (Au0 nanoparticles) after reduction. Purple color is the characteristics of small metallic gold particles. The presence of peaks corresponding to metallic gold appeared obviously in UV–vis spectra after reaction were in consistence with XPS results.

3.2.

PROX reaction

A series of Au/TiO2 catalysts has been prepared either by 10 W UV lamps irradiated for 30–240 min or by a 400 W UV lamp irradiated for 3–10 min, respectively. Table 3 clearly depicts

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70

Distribution (%)

60

Davg = 1.74 nm N = 150

50 40 30 20 10 0 0

1

2

3 4 5 6 Au particle size (nm)

7

8

Fig. 3 – TEM images of Au/TiO2 by (a) PD method (10 W, 1:5  103 M); (b) HRTEM image of Au/TiO2 by PD method at pH 6 during 120 min irradiation (10 W); (c) particle size distribution of gold prepared by PD methods.

Intensity (a. u.)

After reaction (x1.5)

Au0 Au3+ Au

80

+

82

Before reaction

84

86 88 Binding energy (eV)

90

92

Fig. 4 – XPS spectra of Au 4f (before and after PROX reaction).

the relationship between irradiation time and catalytic activity. The catalyst prepared by 10 W lamps with 120 min irradiation time showed maximum catalytic activity toward PROX reaction. The catalysts prepared by the 400 W UV lamp irradiation has lower activities than those prepared by 10 W UV lamps in

spite of similar gold content in both the catalysts (Fig. 6). The results also show that the catalyst prepared with high concentration of HAuCl4 has a higher CO conversion since more gold was deposited on TiO2. The optimum illumination time was 6 min to prepare gold catalyst by 400 W UV lamp as shown in Table 3. Comparing two different power sources of UV lamp, the lower power source lamp (10 W) can deposit small gold particles. The catalyst prepared by 10 W UV lamp has the highest CO selectivity of 98% at 25 1C and decreased rapidly from 25 to 65 1C. The catalyst prepared by low-power lamp (10 W) demonstrated a high CO conversion and high selectivity of O2 reacting with CO in PROX reaction. The color of the catalysts changed from light blue to purple when the reaction temperature was increased. It might be caused by the aggregation of gold particles or the change of electronic state of gold species. The hydrogen competes with CO to adsorb on the gold surface and oxidize with oxygen at high temperatures. Since limited oxygen was used in this study, therefore, CO conversion was decreased at high temperature. Fig. 7 shows the oxygen consumption by H2 and CO in PROX reaction. Since O2/CO ratio in reactant was 1, CO was completely consumed when the black bar was 50%. Oxygen was consumed quickly by H2 oxidation when the temperature increased from 35 to 65 1C. The catalyst prepared by the 400 W UV lamp exhibited a lower activity either for CO or H2 oxidation than the catalysts

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Table 2 – Binding energy and surface compositions of gold species on the catalysts Catalysts (prepared by 10 W lamp, 120 min, 0.0015 M H AuCl4)

Binding energy (eV)

Gold surface composition (%)

Au 4f 5=2

Au 4f 7=2

Au0

Auþ

Au3þ

87.7 87.7

83.9 83.8

67 92

26 4

7 4

Before reaction After reaction

Absorbance

CO conversion & selectivity (%)

100

e

d c

C

b

D

300

400 600 500 Wavelength (nm)

40

20

20

800

700

Fig. 5 – UV–vis spectra of catalysts before (—) and after PROX reaction (y): (a) TiO2; (b) Au/TiO2 (400 W, 3 min); (c, C) Au/TiO2 (400 W, 6 min); (d, D) Au/TiO2 (10 W, 120 min, 1:5  103 M); (e) Au/TiO2 (400 W, 10 min).

Table 3 – Au/TiO2 catalytic performance prepared by PD method with various irradiated time (using 0.00085 M precursor gold solution at a reaction temperature of 80 1C) UV light power

60

0

a

200

80

Irradiation time (min)

CO conversion (%)

CO selectivity (%)

10 W

30 60 120 180 240

49 62 76 55 35

39 37 47 51 50

400 W

3 6 10

7 18 14

25 42 34

prepared by 10 W lamps. When the oxygen consumption is nearly 100%, the CO conversion started to decrease. A limited amount of oxygen was used to oxidize either carbon or hydrogen. With raising temperature, the relative strengths of adsorption may also move in favor of hydrogen. The long-term test results of PROX reaction over the best performance Au/TiO2 catalyst, which was prepared with 1:5 

40

60 Temperature (°C)

80

100

Fig. 6 – PROX reaction on Au/TiO2: the symbol for conversion is solid, selectivity is open mark. () 10 W, 1:5  103 M; (’) 10 W, 8:5  104 M; (m) 400 W, 6 min.

103 M of gold solution and by 10 W UV lamp, are shown in Fig. 8. In the time on stream, it can be observed that after 10 min, CO conversion decreased gradually until 100 min and then level off, and the catalytic performance was stable during 8 h reaction at 30 1C.

3.3.

Effect of light source

The catalytic activity of the gold catalyst depends on the type of support, particle size of gold, and the preparation method. According to the literature reports [13], the particle size of gold has an important effect on the activity of the gold catalyst, irrespective of the support for gold catalysts. In this study, the particle size of gold with 1.5 nm showed excellent activity for PROX. In PD method, the rate of reduction of ionic gold to metallic state was influenced by the power of light source. It was observed that similar gold loading, 0.211% (400 W) and 0.272% (10 W), was uploaded on TiO2 when irradiated by 400 and 10 W lamps. However, the time of irradiation required for 400 and 10 W was different, i.e., 6 and 120 min (Table 1), respectively. At the same time, the small gold particles continued to aggregate to form larger gold particles during irradiation by the 400 W lamp. The catalytic activity of Au/TiO2 prepared by using a 400 W lamp decreased with an increase in gold particle size. Even though Au/TiO2 was irradiated for 3 min by 400 W lamp, the mean gold particle size was still larger (2.7 nm) than those prepared by 10 W lamp (1.7 nm). By comparing the two

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100 O2 consumption (%)

100 O2 consumption (%)

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80 60 40 20

80 60 40 20 0

0 25

35 50 65 80 Temperature (°C)

100

25

35 50 65 80 Temperature (°C)

100

O2 consumption (%)

100 80 60 40 20 0 25

35 50 65 80 100 Temperature (°C)

Fig. 7 – O2 consumption on PROX reaction: (a) PD method (10 W, 8:5  104 M); (b) PD method (10 W, 1:5  103 M); (c) PD method (400 W, 6 min); black and gray bars represents O2 consumed by CO and H2 oxidation. Dark gray bar represents remaining O2.

Conversion & Selectivity (%)

100

controlling the preparation parameters to obtain gold particle size of 1.5 nm.

90 80

3.4.

Active site of Au/TiO2 prepared by PD method

70 60

CO conversion at 30°C CO selectivity at 30°C CO conversion at 80°C CO selectivity at 80°C

50 40 30 0

100

200 300 Time (min)

400

500

Fig. 8 – Variation of CO conversion (circle) and of the selectivity with reaction time during PROX reaction at different temperatures.

power sources of UV lamps, it can be concluded that the lower power source lamp can deposit small gold particles on the support than the high-power lamp, which is responsible for its high catalytic activity. The results included PROX reaction and characterization in Fig. 7 and Table 1 showed the catalytic activities of CO and H2 oxidation on Au/TiO2 catalysts. It demonstrated that the particle size of gold is the key effect for CO oxidation in hydrogen-rich stream for Au/TiO2 catalysts prepared by PD method using a 400 W UV lamp. The results conclude that the catalysts prepared by PD method show excellent activity by

The mechanism of CO oxidation in hydrogen-rich stream is similar to Langmuir–Hinshelwood (L–H) model [31]. The theoretical calculation for CO oxidation suggest that CO and oxygen molecule are vicinally co-adsorbed on gold, and the CO oxidation take place over gold clusters through L–H mechanism rather than Eley–Redeal (E–R) mechanism [32]. CO can adsorb either on cationic gold or metallic gold and those having low-coordination number can react with O2 coadsorbed on the same gold sites. There is infrared evidence for CO adsorption both on metallic (2112 cm1 ) and oxidic gold species (2151 cm1 ) at room temperature [33]. However, the band of Auþ –CO was unstable and the oxidic gold species was slowly reduced by CO. According to the results in this study, it can be hypothesized that the adsorption of CO are in favor of cationic gold at low temperatures. It was reported that the most stable model was Auþ –CO and the interaction between Au atom and CO decreased with an increase in the total electrons of Au atom [34]. Oxygen was also adsorbed on the nearby CO, especially in the interface between the support and gold particles. Therefore, there is a weak adsorption between the gold and the CO2 produced as well as the O atom to be released. Molina and Hammer [35] also reported that the adsorption of O2 does not depend on CO and the reaction proceeds via COO2 formation faster than via O2 dissociation over Au/TiO2 catalysts. However, hydrogen atoms adsorb on metallic gold and react with oxygen at high

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temperatures. The hydrogen and CO species co-adsorbed on the Au nanoparticles was demonstrated by FTIR spectroscopy results [36]. According to the density functional theory calculations, the presence of low-coordinated Au sites plays a vital role in CO oxidation. For the preferential oxidation of CO in hydrogen-rich gases, it is found that the barrier for hydrogen dissociation is large, even in gases with a high partial pressure [37]. Moreover, the drift of reaction was in favor of CO oxidation at low temperature. The cationic gold was unstable even at room temperature. Increasing the reaction temperature reduces the cationic gold to metallic gold by CO as evidenced by the deconvolution of Au 4f spectra of gold catalysts before and after reaction. Casaletto et al. [27] also suggest that the presence of Auþ by XPS seems to be the main requisite for the achievement of the highest CO conversion at lower temperatures. In the catalysts prepared by PD method, the fresh catalyst contains more cationic gold and exhibit high CO selectivity and conversion at room temperature, but there is a rapid decrease when reaction temperature arises to 50 1C. After the reaction, there is a disappearance of cationic gold. The decrease in the selectivity of Au/TiO2 may be due to the loss of relatively active sites (cationic gold) for CO adsorption by the influence of CO or the competition of H atom with CO to adsorb on metallic gold, which has high barrier for hydrogen dissociation; the latter effect may result in the aggregation of gold particles.

4.

Conclusion

Au/TiO2 catalysts prepared by PD method have narrow particle size distribution of gold particles within a few nanometers. The preparation conditions such as precursor concentration, light source and irradiation time control the PD process. The catalysts prepared by this method could produce very smaller gold particles (1.5 nm) on the support. The size of gold nanoparticles deposited on TiO2 was dependent on irradiation time and light source. Longer irradiation time and high-power (400 W) light source not only increased the deposition rate and gold loading on TiO2, but also increased the gold particle size. We have successfully prepared very small gold particle with low-power UV source at suitable irradiation time. Moreover, it has good performance in PROX reaction.

Acknowledgment This research was supported by the Ministry of Economic Affairs, Taiwan, Republic of China. R E F E R E N C E S

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