iron oxide aerogel catalysts

Journal of Non-Crystalline Solids 352 (2006) 35–43 www.elsevier.com/locate/jnoncrysol

Surface nature of nanoparticle gold/iron oxide aerogel catalysts Chien-Tsung Wang *, Shih-Hung Ro Department of Chemical Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin 640, Taiwan, ROC Received 13 May 2005; received in revised form 6 November 2005

Abstract The aerogel catalysts investigated are constituted by two chemically different nanoparticle systems consisting of the gold phase and the iron oxide support. High-resolution transmission electronic microscopy (HRTEM) showed an increased surface roughness for aerogel particles with higher gold loading. X-ray photoelectron spectroscopy (XPS) revealed increases in the surface coverage of hydroxyl groups and the Fe2+/Fe3+ ratio due to the addition of gold, and showed the transition of gold from oxidized to metallic states due to calcination. In the presence of gold species, the Fe3+ satellite structure in XPS was not produced. The crystallinity of maghemite as the support was found quite stable with respect to gold addition and thermal treatment. The aerogels were evaluated for methanol oxidation carried out in an ambient flow reactor. The oxidation activity enhanced with decreasing catalyst pretreatment temperatures and with increasing gold loadings up to 5 wt%. A wide selectivity pattern formed between dimethyl ether and carbon dioxide products. The size of gold particles and the status of surface gold species played a crucial role in the catalytic conversion of methanol. The oxidized gold was more active than the metallic gold towards the total combustion to carbon dioxide. The surface nature has been proven to transform from strong Lewis acidic to high basic characters due to the formation of reactive hydroxyl groups near by the gold sites.
2005 Elsevier B.V. All rights reserved. PACS: 82.33.Ln; 82.65.+r Keywords: Catalysis; Nanoparticles; Colloids and quantum structures; Aerogels

1. Introduction Gold catalysts supported on reducible transition metal oxides exhibit excellent catalytic performance in several green applications, such as the water–gas shift (WGS) reaction [1,2], low-temperature oxidation of carbon monoxide [3–5], and combustion of organic compounds [6,7]. Iron oxide is one of the most suitable supports ever found because of its inherent reducibility for electron transfer and high crystallinity for good catalytic stability [8]. The known examples mostly provide results of reactor efficiencies for iron oxide supported gold catalysts towards the destruction to carbon dioxide. The investigations to probe

the synergism between gold and support by using reactor data need more attention. Methanol oxidation is a well-recognized test reaction for use to characterize structural and chemical properties of oxidation catalysts [9]. A series of successive surface reactions produce characteristic intermediates that are associated with the type of active sites. For example, strong Lewis or Bro¨nsted acidity helps to promote the dehydration of methanol to dimethyl ether, whereas high basicity favors the partial oxidation to formate species followed by further destruction to carbon oxides. If redox sites are present, then formaldehyde is produced. Below are the primary reaction pathways under consideration. Dehydration of methanol to dimethyl ether:

*

Corresponding author. Tel.: +886 5 5342601x4623; fax: +886 5 5312071. E-mail address: [email protected] (C.-T. Wang). 0022-3093/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.018

2CH3 OH ! CH3 OCH3 + H2 O

ð1Þ

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C.-T. Wang, S.-H. Ro / Journal of Non-Crystalline Solids 352 (2006) 35–43

Total oxidation of methanol to carbon dioxide: 3 CH3 OH þ O2 ! CO2 þ 2H2 O 2

ð2Þ

Partial oxidation of methanol to formaldehyde: 1 CH3 OH þ O2 ! HCHO þ H2 O 2

ð3Þ

Partial oxidation of methanol to methyl formate: 2CH3 OH + O2 ! HCOOCH3 + 2H2 O

ð4Þ

Most currently developed gold/iron oxide materials have been made by the wet co-precipitation method and an ambient evaporation drying for solvent. The as-prepared ferric support is generally an amorphous ferrihydrite, and needs another high-temperature treatment to form the oxide with a high degree of crystallinity. The thermal calcination usually leads to a re-distribution effect on the gold and support particles, causing a decrease of the catalytic activity, for instance, in the WGS reaction [2] and CO oxidation [5]. Aerogel materials are typically prepared via sol–gel synthesis and subsequent supercritical drying. High thermal stability has been explored as an advantage of aerogels in the catalytic application, such as Ni–alumina used in the high-temperature methane reforming reaction [10]. Some recent papers that have appeared about fine particle aerogels as catalysts include the work of Wang and Willey [11,12] and Suh [13]. The present work is one of the few known studies that may have worked on gold-oxide aerogels. We attempted to prepare finely nanosized particles of gold/iron oxide aerogels, examine surface compositions of reactive species by X-ray photoelectron spectroscopy, and investigate chemical properties based on the catalytic reaction of methanol oxidation. The reactor study has revealed that both catalyst pretreatment temperature and gold loading exert a decisive influence on the catalytic activity. The obtained findings quite encourage, as the work below will show. 2. Experimental 2.1. Aerogel preparation The aerogels prepared for use in this work are listed in Table 1. In a sol-to-aerogel process, the gold/iron oxide sol was formed by combining ferric(III) acetylacetonate (Fluka, 97%) with gold acetate (Alfa Aesar, 99.9%) in

methanol (J.T. Baker, 99.9%) and adding triply deionized water for hydrolysis (20 mol% excess with respect to the stoichiometric amount required for hydrolyzing both precursors to their corresponding hydroxides). After stirred for 12 h, the reddish brown solution was then placed into a 316 stainless steel autoclave. Heating commenced until the temperature and pressure reached supercritical points with respect to methanol (265 C and 125 bar is typical). After the dense fluid inside the reactor was released out through volume expansion, heating was disconnected, and the autoclave was allowed to cool overnight with a purge flow of nitrogen. The overall yield was above 95 wt% per batch production. Pure and gold-containing iron oxide powders, upon calcined in air for 2 h at 265 or 500 C, appeared reddish brown and orange brown in color, respectively. 2.2. Reactor test The aerogels were evaluated for methanol oxidation in an ambient fixed-bed flow apparatus. The reactor system provided a feed stream precisely controlled at 0.15 g/min, containing 2.0 mol% methanol, 12.7 mol% oxygen, and nitrogen for the balance. About 0.1 g of aerogel was packed inside a tubular Pyrex glass tube (10 mm i.d.). Packing densities were measured from 0.374 (Fe2O3) to 0.307 g/cm3 (5 wt% Au/Fe2O3), decreasing with increasing gold loadings. Before each run, catalyst pretreatment was done for 1 h by passing a 50 vol.% O2/N2 stream across the aerogel bed. The hydrocarbon species in the reactor inlet and effluent (e.g., methanol, dimethyl ether, formaldehyde, and methyl formate) were nicely separated with a Cowax 10 capillary column (Supelco, 60 m · 0.53 mm · 2.0 lm film thickness) and analyzed on a gas chromatograph (China Chromatography, Model 9800) equipped with a flame ionization detector (FID, Varian) in series with a thermal conductivity detector (TCD, Varian). Carbon dioxide was another analyzed with a Carboxen 1000 packed column (Supelco, 15 ft · 1/8 in) and by a TCD. Fractional conversion of methanol (%) is computed based on methanol concentrations in both the reactor inlet and outlet. Selectivity (%) is defined as: moles of product formed/moles of methanol converted · 100 · SR, where SR is the stoichiometric ratio of methanol to product in the reaction as shown in Eqs. (1)–(4). The light-off temperature of methanol is defined as the temperature where the conversion begins to abruptly jump up.

Table 1 BET surface areas of aerogels Calcination temperature (C)

Fe2O3 (m2/g)

1.2 wt% Au/Fe2O3 (m2/g)

2.4 wt% Au/Fe2O3 (m2/g)

5 wt% Au/Fe2O3 (m2/g)

265 350 500

37.8 – 28.0

27.4 – –

29.2 28.2 27.3

29.7 – –

Note: The samples were calcined in air for 2 h at a constant temperature selected.

C.-T. Wang, S.-H. Ro / Journal of Non-Crystalline Solids 352 (2006) 35–43

2.3. Structural characterization

3. Results and discussion

γ

(c)

γ

Au (111)

γ

γ

γ

α

γ

Intensity (a.u.)

Surface areas of the aerogels were determined from 77 K nitrogen gas adsorption isotherm data (Micromeritics, Model ASAP 2010) by applying the BET method. Highresolution transmission electron micrographs were taken on a JEOL 2000SX microscope operating at 200 kV. Field emission scanning electron micrographs (FESEM) were obtained on a JEOL microscopy (JSM-6340F). X-ray photoelectron spectra, used to determine surface compositions and element interaction, were obtained on an ESCA instrument (Thermo VG Scientific, Model Sigma Probe) supplying an Al-Ka radiation source (1486.6 eV) to excite photoelectrons in an ultra vacuum atmosphere around 10
9 Torr. The binding energy scale was precisely calibrated with taking the adventitious C 1s peak at 285.0 eV as a reference, and experimental data points were resolved by a curve-fitting procedure.

37

γ

γ

Au (200)

(b)

(a)

20

25

γ

30

35

40

45

50

55

60

65

70

2θ (degree) Fig. 2. XRD patterns of calcined aerogels, 2.4 wt% Au/Fe2O3 at (a) 265 C, (b) 500 C, and (c) Fe2O3 at 500 C.

3.1. Structural properties Table 1 lists BET surface areas of the prepared aerogels as a function of calcination temperature and gold loading (wt% Au). It is seen that the surface area reduces with increasing calcination temperatures from 265 to 500 C. Pure Fe2O3 has a large drop by 26% due to thermal sintering of grains, and the Au/Fe2O3 samples possess a smaller surface than the Fe2O3. The result seems to imply that the presence of gold leads to shorten the distance between gold and iron oxide, due to a high solubility of Au in the oxide matrix [2]. Surface areas of 23–53 m2/g [2] and 38–40 m2/g [5] have been reported for co-precipitated Au/iron oxide materials after thermally treated. Fig. 1 is an FESEM photograph of the 2.4 wt% Au/ Fe2O3 aerogel after calcined at 500 C for 2 h. The particle

Fig. 1. FESEM photograph of 2.4 wt% Au/Fe2O3 aerogel calcined at 500 C.

size histogram was determined with a maximum frequency between 30 and 50 nm on the basis of 200 random counts, arithmetically averaged at 38.2 nm. Fig. 2 shows X-ray diffraction patterns of Fe2O3 and 2.4 wt% Au/Fe2O3 aerogels after calcined for 2 h in air at 265 and 500 C, respectively. Maghemite (c-Fe2O3, JCPDS 13-0458) appeared as the prevailing ferric phase commonly in the Fe2O3 and Au/Fe2O3 samples, and the crystallinity remained quite stable after they were heated from 265 to 500 C, except for a small hematite peak at 2h = 33.22 (a-Fe2O3, JCPDS 33-0664). By employing the method of full width at half maximum (FWHM) based on XRD peaks and the Scherrer equation, the Fe-phase crystallite size was estimated to be 22.3–32.6 nm. In the literature, however, hematite is always reported as the major Fe crystalline phase of wet co-precipitated gold/iron oxide materials by many authors [1–7], and it is formed through thermal calcination of amorphous ferrihydrite. Two XRD reflections of gold at 2h = 38.23 and 44.51 were detected on the Au/Fe2O3 aerogel, and identified as metallic Au(1 1 1) and Au(2 0 0) (JCPDS 04-0784), similarly reported in Ref. [14]. After the sample was calcined to 500 C, the two Au peaks became more intense with an increase from 9.1 to 16.8 in the intensity ratio of Au(1 1 1)/c-Fe2O3 (2h = 35.72), indicating the growth of gold particles [15]. The Au crystallite size estimated by FWHM is 18–20 nm. The morphological properties of the aerogel particles, after being calcined at 500 C for 2 h, were studied by HRTEM, as displayed in Fig. 3. The grains of pure Fe2O3 appear smooth visibly (Photo a) and show distinct lattice planes (Photo b). For 2.4 wt% Au/Fe2O3 (Photo c), the particle surface is Ôwavy and roughÕ. For 5 wt% Au/Fe2O3 (Photo d), some 3–5 nm small gold particles are shown distributing randomly in the oxide matrix, and

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C.-T. Wang, S.-H. Ro / Journal of Non-Crystalline Solids 352 (2006) 35–43

Fig. 3. HRTEM images of aerogels calcined at 500 C, (a) and (b) pure Fe2O3, (c) 2.4 wt% Au/Fe2O3, (d) 5 wt% Au/Fe2O3.

0

Au 4f 7/2

0

Au 4f 5/2

Intensity (a.u.)

others are seen at the oxide surface clustering to form large agglomerates. With increasing Au loadings, the surface roughness follows to increase, likely due to aggregates of gold clusters on top of the oxide [16]. For co-precipitated gold/iron oxide materials reported in the literature, the size of gold particles varies widely, such as 7–30 nm (by atomic force microscopy) and 10–30 nm (by SEM) [16], 5.8– 12.8 nm (by XRD) and 5.7–8.5 nm (by TEM) [2], and 3– 5 nm for particles and a few micrometers for agglomerates (by TEM) [5]. The surface compositions of elements such as Au, Fe and O in the aerogels were determined by X-ray photoelectron spectra. Fig. 4 shows XPS Au 4f spectra of 2.4 wt% Au/Fe2O3 aerogel as a function of calcination temperature. The spots correspond to the experimental data. It can be seen that both oxidized and metallic gold species are present on the surface of the aerogel after calcined at 265 C for 2 h in air (Curve a). The small peak at 86.4 eV was assigned to Au3+4f7/2 photoelectron of Au2O3 [3,17], and this reflection disappeared with another heating to 500 C, owing to the reduction reaction of Au3+ + 3e
! Au0. The Au3+ phase in Au(OH)3 was not detected. Gold in the metallic state was detected with two broad intense bands; one ranged at 83.0–85.0 eV for Au0 4f7/2 and the other peaked at 87.5 eV for Au0 4f5/2. Note that the Au0 4f7/2 doublet band in Curve a can be de-convoluted into two peaks; one centered at 83.8 eV indicates the presence of larger bulk gold (typically at BE < 84 eV [18]) and the other centered at 84.3 eV (symbol *) was characteristic of smaller gold parti-

3+

Au 4f 7/2

*

(b) (a)

82

83

84

85

86

87

88

89

Binding energy (eV) Fig. 4. XPS Au 4f spectra of 2.4 wt% Au/Fe2O3 aerogel calcined at (a) 265 C, (b) 500 C.

cles (a few nanometers in size) [19]. After the sample was another calcined at 500 C for 2 h in air (Curve b), the latter almost vanished, whereas the former grew up with some intensity at lower binding energy. This demonstrates the growth of gold particles by agglomeration in the near-surface region. Fig. 5 presents XPS Fe 2p spectra of Fe2O3 and 2.4 wt% Au/Fe2O3 aerogels after being calcined at 265 C for 2 h in air. Two intense bands were measured and assigned to Fe 2p3/2 and Fe 2p1/2 photoelectrons, respectively. Note that

C.-T. Wang, S.-H. Ro / Journal of Non-Crystalline Solids 352 (2006) 35–43

(a)

(a)

OH/O=0.56 O1s (O)

Fe2p 3/2

Intensity (cps)

39

Fe2p1/2

2+

Fe

Fe

Intensity (cps)

Satellites 3+

3+

Fe

2+

Fe

2+

O1s (OH)

3+

Fe /Fe = 0.18 705

710

715

720

725

730

Binding energy (eV)

527

528

529

530

(b) Fe2p3/2

Intensity (cps)

531

532

533

534

535

Binding energy (eV)

OH/O=0.98

(b)

Fe2p1/2

Satellite

O1s (O)

2+

Fe

Intensity (cps)

3+

Fe

2+

Fe

2+

3+

Fe /Fe = 0.23 705

710

715

720

725

O1s (OH)

730

Binding energy (eV) Fig. 5. XPS Fe 2p spectra of aerogels calcined at 265 C, (a) Fe2O3 and (b) 2.4 wt% Au/Fe2O3.

the Fe 2p3/2 peak shifted slightly from 711.2 to 710.9 eV due to the addition of gold. This suggests a strong electronic interaction occurring between Au and Fe2O3 [17]. Further, two peaks of interest can be de-convoluted from the broad Fe 2p3/2 band; one near 709.3 eV was attributed to Fe2+ (of FeO) and the other at 711.0 eV was assigned to Fe3+ (of Fe2O3). The two Fe cations co-existed on the surface, and an energy separation by 1.7 eV arose from a charge effect [20]. The Fe2+/Fe3+ ratio increases from 0.18 (of Fe2O3) to 0.23 (of Au/Fe2O3), suggesting an enhanced Fe reducibility. In Fig. 5(a), another two peaks at 713.5 and 719.0 eV were assigned to the high-spin Fe2+2p3/2 and Fe3+2p3/2 shake-up satellite structures of the Fe2O3 phase [21–23], respectively. The satellite structures in the XPS spectra are known to be caused by the Fe 3d–O 2p hybridization [24]. However, the Fe3+ satellite peak was not shown in the XPS spectrum of the 2.4 wt% Au/Fe2O3 (in Fig. 5(b)) due to the formation of Fe2+ ions [24]. Evidently, the iron reduction is affected by the presence of gold. The XPS O 1s spectra of Fe2O3 and 2.4 wt% Au/Fe2O3 aerogels after calcined at 265 C are displayed in Fig. 6. De-convolution of the broad O 1s band leads to generate two distinct peaks; the larger near 530.5 eV was assigned to the O in Fe–O and/or Au–O bonds, and the smaller

527

528

529

530

531

532

533

534

535

Binding energy (eV) Fig. 6. XPS O 1s spectra of aerogels calcined at 265 C, (a) Fe2O3 and (b) 2.4 wt% Au/Fe2O3.

was indicative of the O in O–H groups near iron [17] (Au(OH)3 is absent). Note that the OH/O ratio increases from 0.56 (of Fe2O3) to 0.98 (of Au/Fe2O3), by around 1.75 times. Evidently, the presence of gold gave rise to a significant increase in the surface coverage of hydroxyl groups. In a study of Ilieva et al. [25], the hydroxyl coverage of Au/a-Fe2O3 is about twice higher than that of a-Fe2O3, as analyzed by temperature-programmed desorption of adsorbed water. Moreover, when the Au/Fe2O3 aerogel was heated from 265 to 500 C, a decrease in the OH/O ratio from 0.98 to 0.78 was obtained (data not shown for brevity). Therefore, the presence of gold helps a thermal stability for the OH groups on the surface. We agreed with Andreeva et al. [1] that the additional hydroxyl groups are generated following the dissociative adsorption of water on the gold atoms that are exceptionally reactive. To our Au/iron oxide aerogel system, the reactive interaction may have taken place during the sol hydrolysis process and/or the ambient exposure of the material to water moisture. This helps to explain the favorable formation of OH species on the Au/iron oxide aerogel surface. The hydrophilic OH groups are highly apt to locate at the interface between gold and iron oxide [26], and also can pass

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through spillover onto adjacent sites of the iron oxide [1]. In our postulation, the high mobility favors the OH groups to be chemically reactive, as will be shown in the methanol oxidation work below. 3.2. Catalytic activity The catalytic properties of the gold/iron oxide aerogels were evaluated based on methanol oxidation carried out in an ambient flow reactor. To probe the surface reactivity of supported gold species, the reactor work was conducted for obtaining conversion and product selectivity data as a function of catalyst pretreatment temperature, from 265, 350 to 500 C (Fig. 7). For the 2.4 wt% Au/Fe2O3 aerogel (atomic ratio Au/Fe = 0.01), it is seen that the conversion of methanol enhances catalytically with increasing reactor temperatures from 225 to 325 C. Note that the catalytic activity increases significantly with decreasing the calcination temperature, accompanied with the lowering of the Arrhenius activation energy (Ea) from 106.1 to 94.8 kJ/ mol. Since the crystallinity of maghemite as the support was quite stable to the thermal treatment, the variation

100

(a)

Conversions (%)

80 Ea:94.8kJ/mol 60 40

(b)

100.1kJ/mol 106.1kJ/mol

(c)

20 0 225

250

275

300

325

Reaction temperature (oC) 100

Reaction at 275oC

CO2

Selectivities (%)

80 60 40 20

HCOOCH3 HCHO

CH3OCH3 0 250

300

350

400

450

500

Pretreatment temperature (oC) Fig. 7. Effect of catalyst pretreatment temperature on methanol oxidation over 2.4 wt% Au/Fe2O3 aerogel, (a) 265 C, (b) 350 C and (c) 500 C. Conditions: feed composition CH3OH/O2/N2 = 2.0/12.7/85.3 mol%; catalyst contact time 0.1 s. Ea: Arrhenius activation energy estimated based on a first-order rate model with respect to methanol.

between the three conversion curves can be attributed to singularly a catalytic effect of the supported gold species. The effect of the hematite phase generated during the 265–500 C calcination of Au/Fe2O3 (XRD in Fig. 2) on the activity difference was almost negligible with our reactor observations. As we recalled from the XPS Au 4f spectra in Fig. 4, the calcination from 265 to 500 C caused the phase transformation of gold from Au2O3 to metallic Au0 and simultaneously the growth of gold particles. Thus, we are convinced that the Au3+ species are at least as important as the small Au particles in the catalytic activity for methanol oxidation. The conclusion well agrees with the Bond and Thompson model [27], in which the catalysts showing good conversion for CO oxidation definitely contain mixtures of Aux+ and Au0 species. Further, the Au3+ was proven to be more active than the Au0. In the experiment, the Au/Fe2O3 catalysts calcined at 350 (Curve 7b) and 500 C (Curve 7c) eventually reached a nearly same activity, and those at 265 (Curve 7a) and 350 C (Curve 7b) exhibited a similar light-off behavior. Thus, the reduction of Au3+ to less active Au0 is directly related to the latter. This is in good agreement with the result of Minico` et al. [28] that the cationic gold is more active but less stable than metallic gold, as characterized by FT-IR for CO adsorption. In addition, the former is considered to be associated with the agglomeration of smaller gold particles into larger bulk gold (18–20 nm by XRD FWHM). Therefore, we can safely conclude that the gold state and the size of gold particles play a crucial role for the catalytic oxidation of methanol. Several authors ever reported that gold clusters smaller than 10 nm were highly active for the catalytic oxidation of carbon monoxide [29], and that gold particles having a size of 7–30 nm were shown active in the oxidative reactions of carbon monoxide and methanol [16]. A wide product selectivity pattern as a function of catalyst pretreatment temperature is revealed in Fig. 7. At the same reactor temperature of 275 C, the catalyst calcined at 265 C produced carbon dioxide as the main product, whereas dimethyl ether prevailed over the surface calcined at 500 C. Remember that with increasing pretreatment temperatures from 265 to 500 C a state transition of gold from Au3+ to Au0 occurs. One possible explanation of this selectivity distribution involves with the less exposure of Au3+ species and OH groups due to the restructuring of surface and the re-distribution of elements by heat (see XPS data). This further elucidates that the Au3+ is more active towards the total combustion to CO2 than the Au0. Besides, the hydroxyl groups generated by the Au3+ species play a role of base sites. It is plausible to suggest that the facile formation of the interface between gold and support is critical for achieving a high catalytic activity. However, the obtained product distribution is different from the result of Minico` et al. [7]. They found that the oxidation of methanol over 8.2 wt% Au/Fe2O3, after pretreated at 200–450 C, produces CO2 with no formation of intermediate products.

C.-T. Wang, S.-H. Ro / Journal of Non-Crystalline Solids 352 (2006) 35–43 100

CH 3OCH3

(b)

60

(d)

80

Selectivities (%)

80

Conversions (%)

100

(c)

CH 3OH

41

(a)

40

(b)

60 40 20

20

(a)

(c) (d)

0 150

175

200

225

250

275

300

0 150

325

175

200

Temperature (oC) 100

275

300

325

300

325

HCHO

Selectivities (%)

(c)

80 60

(b)

(a)

40

5

(b)

175

200

225

250

275

300

0 150

325

(a)

(c)

20 0 150

250

10

CO2 (d)

Selectivities (%)

225

Temperature (oC)

(d) 175

200

Temperature (oC)

225

250

275

Temperature (oC)

20

Selectivities (%)

HCOOCH3 15

(c) (b)

10

(d) (a)

5

0 150

175

200

225

250

275

300

325

Temperature (oC) Fig. 8. Effect of gold loading on methanol oxidation over aerogels, (a) pure Fe2O3, (b) 1.2 wt% Au/Fe2O3 (dashed), (c) 2.4 wt% Au/Fe2O3 and (d) 5 wt% Au/Fe2O3. Conditions: pretreatment temperature 265 C; feed composition CH3OH/O2/N2 = 2.0/12.7/85.3 mol%; catalyst contact time 0.1 s.

Fig. 8 presents conversion and product selectivity of methanol oxidation over the aerogels with different gold loadings. Pure Fe2O3 was included for reference. All catalysts were pretreated at 265 C. In the window of reaction temperatures between 150 and 325 C, each of the catalysts showed CH3OH conversion increasing monotonically to 90% and higher. It must be noted that the catalytic activity enhances significantly with increasing gold loadings up to 5 wt%, which is accompanied by a decrease of 100 C in the light-off temperature. The significant enhancement of activity can be attributed to increases of the surface cover-

age of Au3+ species and the population of Au nanocrystallites (small particles plus large agglomerates). Our HRTEM images (Fig. 3) revealed a tendency of increasing surface disturbance with doping more gold species in the oxide matrix and hence a gradually strengthened interaction between gold and iron oxide. Hodge et al. [5] reported that over co-precipitated gold/iron oxide catalysts the Aux+/Au0 ratio increases with higher gold loadings and has a decisive influence on the catalytic activity for CO oxidation. The oxidation of methanol over the Au/Fe2O3 aerogel catalysts results in a wide selectivity pattern of products

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C.-T. Wang, S.-H. Ro / Journal of Non-Crystalline Solids 352 (2006) 35–43

including dimethyl ether, formaldehyde, methyl formate and carbon dioxide. Dimethyl ether was the most dominant product over pure Fe2O3 aerogel (Curve a in CH3OCH3 figure), and its selectivity decreased remarkably on increasing the amount of gold in aerogel. On the other hand, the CO2 production enhanced remarkably with increasing gold levels up to 5 wt%. As we realized the product distribution based on a rule of thumb for methanol oxidation pathways [9], it is suggested that the surface nature has transformed from strong Lewis acidic to high basic characters due to the presence of gold. The catalyst with more gold dispersion and thus higher basicity exhibited a weaker activity to produce formaldehyde (see HCHO figure), but favored the formation of methyl formate (see HCOOCH3 figure). These reactor observations have demonstrated that the competitive reactivity between gold catalysts and the iron oxide support controls a product distribution for methanol oxidation. The chemical properties of acid–base and redox are associated with the surface chemistry of the dispersed gold species. The active hydroxyl groups located at the interface between gold and iron oxide are postulated to diffuse by spillover onto adjacent Lewis iron centers to convert adsorbed methoxy (i.e., Fe3+–OCH3) into surface formate and further to carbon dioxide. To achieve a high oxidation activity in the methanol reactor, gas oxygen in the feed is required for being the supplier of oxygen ions to the ferric oxide surface. Otherwise, in the absence of oxygen, the reaction proceeded only for a very short duration, and even the activity was observed to drop down to zero in the long run. Haruta [30] reported that the gold–metal oxide perimeter interface is to act as a site for activating at least one of the reactants, for example, oxygen. It is proposed that dissociation of the molecularly adsorbed mobile oxygen ðO
2 Þ takes place at the metal–support interface [4]. Inevitably, the oxygen ions can recombine with free protons to form new mobile hydroxyls at the Au–iron oxide interface. We can plausibly conclude that both mobile hydroxyl groups and lattice oxygen ions near by the gold sites are the reactive species for methanol oxidation on the gold/iron oxide surface. 4. Conclusions Surface characteristics and catalytic properties of the nanosized iron oxide–supported gold aerogels are governed by the presence of gold species. The surface area decreases with an addition of gold, implying a strengthened coordination between gold and iron oxide. The dispersion of gold in the oxide matrix leads to the formation of surface hydroxyl groups and the reduction of iron from Fe3+ to Fe2+. Both oxidized and metallic gold species co-exist in the near-surface region. Thermal treatment causes not only the transformation of gold from oxidized to metallic states but also the agglomeration of small gold particles. The crystallinity of maghemite is quite stable with respect to gold addition and calcination.

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