TiO2 catalysts by suspension spray reaction method and their catalytic property for CO oxidation

Applied Catalysis A: General 246 (2003) 87–95

Preparation of Au/TiO2 catalysts by suspension spray reaction method and their catalytic property for CO oxidation Lin Fan a , Nobuyuki Ichikuni b , Shogo Shimazu b , Takayoshi Uematsu b,∗ a

b

Center for Frontier Electronics and Photonics, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Department of Materials Technology, Faculty of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Received 14 March 2002; received in revised form 16 November 2002; accepted 13 December 2002

Abstract Gold catalysts supported on titanium dioxide were prepared by the suspension spray reaction (SSP) method that was developed as a modification of the previous solution spray reaction (SPR) method. The effects of suspension spray reaction temperature on the surface structure and the characteristic properties were investigated by means of XRD, TEM, XPS and CO chemisorption measurements. The particle sizes of gold prepared at higher spray temperatures turned out to be smaller and the surface distribution of gold increased. These effects are interpreted in terms of the strong interaction between gold and TiO2 which prevents the coagulation and crystal growth of gold nanoparticles during the spray reaction. The deconvolution of XPS spectra for as-sprayed SSP-Au/TiO2 suggests that Au species existed in three different states: i.e. metallic gold (Au0 ), non-metallic gold (Auδ+ ), and Au2 O3 species. Further, the metallic Au0 was exposed more effectively over the surface of the catalysts prepared by higher temperature spraying. The amount of CO chemisorption over SSP-Au/TiO2 1073 prepared at 1073 K was about five times larger than that over SSP-Au/TiO2 673; this difference is consistent with the above result for XPS. The catalytic activity for CO oxidation expressed by the initial rate and TOF increased with the spray reaction temperature. The high catalytic activity of SSP-Au/TiO2 1073 is attributed to the highly dispersed gold particles being modified by a strong interaction with TiO2 that induced a synergy effect in the catalysis. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Gold catalyst; Au/TiO2 ; Suspension spray reaction; Synergy effects; CO oxidation

1. Introduction The catalytic performance of gold has received considerable attention in recent years [1–11], since Haruta and his co-workers [12–19] demonstrated that gold nanoparticles dispersed on metal oxides can exhibit high catalytic activities for various types of reactions, e.g. CO oxidation [13,14,20], selective oxidation [17,18], NO–CO reduction [19,21], CO ∗ Corresponding author. Tel.: +81-43-290-3378; fax: +81-43-290-3378. E-mail address: [email protected] (T. Uematsu).

and CO2 hydrogenation [15,19] and catalytic combustion of hydrocarbons [12]. These studies revealed that the activity of supported gold catalyst depends significantly on the size of gold particles [14,20]. Moreover, many researchers have pointed out that the catalytic properties are also markedly influenced by the interaction between gold and the support, because the perimeters of gold–metal oxide interfaces are assumed to act as the important active sites [22–25]. For the purpose of obtaining structures which could facilitate high performance catalysis, new effective preparation methods are required. A conventional impregnation method, which is unlikely to lead to high

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dispersion of gold, has been reported to fail to produce active catalysts. In contrast, co-precipitation method [13] and deposition–precipitation method [14,26] were demonstrated to be effective methods to deposit gold nanoparticles on a variety of metal oxides. Au–phosphine complexes [27] and size-controlled gold colloid [28] were also applied as the starting material for the preparation to obtain such supported gold catalysts. Although many efforts were made to control the sizes of gold particles during the preparation process, few studies have paid attention to the improvement of the interaction between gold and support. It seems necessary for us to develop new preparation methods that can deposit gold as fine particles with improved interaction between the metal and the support. We have applied the spray techniques successfully to the preparation of fine composite catalysts and have developed a solution spray reaction method (SPR) [29–31], a suspension spray reaction method (SSP) [32], and hybrid methods of spray impregnation [29]. As for metal-supported catalysts, we have prepared Ni/TiO2 , Ni/Al2 O3 , Pd/ZrO2 , Ru/Al2 O3 , and Ru/TiO2 , and have demonstrated that the spray reaction techniques are excellent methods to prepare multi-component composites with strong interaction among the components, which lead to high catalytic activities due to the improved support effects and/or promoter effects [31]. The enhancement of strong interaction is related to the characteristic nanostructure of fine composites being formed from droplets of a homogeneous solution in a quick-heating-and-quick-quenching process. As for supported gold catalysts, we have prepared SPR-Au/TiO2 and SPR-Au/Al2 O3 by the solution spray method using mixed aqueous solutions of HAuCl4 –TiCl4 and HAuCl4 –Al(NO3 )3 , respectively [33,34]. The results demonstrated again that the interaction between gold and the support was more intensified compared with those for analogous impregnation catalysts. Similar results were previously reported for SPR-Ru/Al2 O3 [30] and SPR-Ru/TiO2 [31]. However, the coagulation and sintering of gold particles tended to occur when a high spray reaction temperature was applied. Moreover, as one of disadvantages of the solution spray method, some of the metal particles were partially buried or entrapped in the support particles, which led to decrease in the

number of exposed active sites [33]. To solve this problem, the SSP method was developed to prepare SSP-Au/TiO2 , in which a suspension of TiO2 powder was used instead of an aqueous solution of TiCl4 . We presumed that a quick decomposition of HAuCl4 followed by a quick deposition of Au on TiO2 would be effective to prevent the coagulation of gold particles, leading to the higher dispersion of gold on the outer surface of support particles. In the present paper, we have tried to study how the spray temperature will influence the characteristics of SSP-Au/TiO2 . The surface structure of SSP-Au/TiO2 was characterized by XRD, TEM, XPS and CO chemisorption measurements. Catalytic behavior in CO oxidation was also studied to clarify the relation between catalytic property and surface structure. 2. Experimental 2.1. Catalyst preparation The Au/TiO2 catalysts were prepared by the suspension spray reaction method as follows: TiO2 powder (Nippon Aerosil, P-25, specific surface area 50 m2 /g, mainly composed of anatase) was suspended in distilled water by ultrasonic treatment; then the aqueous solution of HAuCl4 was added to obtain a spray suspension with the total concentration of 0.05 M. The suspension was atomized by an ultrasonic device to produce a mist without separation of components. This process was followed by calcination in an air flow under the suction of an aspirator. The product fine particles were collected on a glass filter at the outlet. Three Au/TiO2 samples with the same gold loading (1 mol%) were prepared at different spray reaction temperatures at 673, 873 and 1073 K. These samples were symbolized hereafter as SSP673, SSP873 and SSP1073, respectively. 2.2. Catalyst characterization The properties of SSP-Au/TiO2 were characterized by XRD, TEM, XPS and CO chemisorption measurements. Powder X-ray diffraction (XRD) profiles were obtained on a Mac Science MXP3 X-ray diffract meter with Cu K␣ radiation, operated at 40 kV and 20 mA. The crystallite size of gold was determined

L. Fan et al. / Applied Catalysis A: General 246 (2003) 87–95

from the broadening of Au(2 0 0) reflection using Scherrer’s equation. The mean particle size of gold and the size distribution were also determined from transmission electron micrographs (TEM) obtained on a JEOL JEM-4000FXII. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Shimazu ESCA-850 spectrometer with Mg K␣ radiation. The binding energies were corrected for the surface charge by referencing to C 1s peak of contaminant carbon at 285.0 eV. The surface composition of the catalysts was determined from the peak areas of the corresponding lines using a Shirley-type background. The curve fitting of the XPS spectrum was performed using a Gaussian peak shape after the background subtraction. The CO chemisorption measurements were carried out at 298 K using a static system. The amount of irreversible adsorption was determined by the differences among repeated adsorptions. The samples used for CO chemisorption measurements were pretreated with O2 and then H2 at 573 K for 1 h and subjected to evacuation for 30 min at the same temperature. 2.3. Catalytic reaction CO oxidation was carried out in a closed circulation system with a reaction mixture of CO (20 Torr) and O2 (10 Torr). The reaction products were analyzed on a gas chromatograph. The initial rate of CO2 formation was taken to stand for the catalytic activity. The catalyst samples were pretreated before use under the same conditions as described for CO chemisorption.

3. Results and discussion 3.1. Characterization of SSP-Au/TiO2 catalysts The XRD patterns for SSP-Au/TiO2 (Fig. 1) show that the anatase phase (unlabeled) dominated the rutile phase of TiO2 irrespective of the spray temperature. The diffraction pattern and the intensities of TiO2 were little influenced by the spray temperature. As for metallic Au0 , clear peaks were detected at 2θ = 38.2, 44.4, 64.6 and 77.6◦ which were assigned to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively. The peak intensity, however, was reduced and some

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Fig. 1. XRD patterns of SSP-Au/TiO2 catalysts: (a) SSP673; (b) SSP873; (c) SSP1073.

broadening occurred when the spray temperature was raised up to 1037 K. Table 1 summarizes the crystallite sizes of Au(2 0 0) which have no overlap with the diffraction peaks of TiO2 . The crystallite sizes of Au particles were in the order: SSP673 (12.4 nm)  SSP873 (9.7 nm) > SSP1073 (8.9 nm). Such results suggest that the crystal growth of Au could be prevented by the suspension spray reaction even at the relatively higher temperatures. Moreover, we found that over SSP673 further growth of Au crystallite took place more by about 2 nm during the catalytic reaction, though SSP873 and SSP1073 showed no significant changes from the corresponding as-prepared samples. The TEM photographs and the size distributions of gold particles (Fig. 2) support these facts. The spherical gold nanoparticles were observed as dark spots that were homogeneously dispersed on the surfaces of TiO2 particles. The mean particle sizes of gold Table 1 Crystallite sizes of Au(2 0 0) for SSP-Au/TiO2 catalysts Sample

SSP673 SSP873 SSP1073 a

Crystallite sizes of Au(2 0 0)a (nm) As-sprayed

After reaction

12.4 9.7 8.9

14.1 9.4 8.4

Calculated from Au(2 0 0) peak of XRD.

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Fig. 2. TEM photographs and the size distribution of Au particles for SSP-Au/TiO2 catalysts: (a, d) SSP673; (b, e) SSP873; (c, f) SSP1073.

L. Fan et al. / Applied Catalysis A: General 246 (2003) 87–95

determined by the TEM image decreased with the increase of spray temperature, i.e. 20.4 nm (SSP673) > 10.8 nm (SSP873) > 8.7 nm (SSP1073). As described above, a similar change was also observed for the crystallite size by XRD. On comparing, the mean particle sizes with XRD crystallite sizes for the corresponding samples, SSP873 and SSP1073 gave close values. However, the mean TEM particle size for SSP673 (20.4 nm) was almost twice of the XRD crystallite size (12.4 nm). This may suggest that twinned particles were formed by the coalescence of two or more smaller Au particles [35]. Additionally, SSP673 gave a much broader size distribution (Fig. 2d), extending from 9 to 40 nm, while SSP873 and SSP1073 showed much narrower distributions. The fraction of particles ≤9 nm apparently increased with the increase of the spray reaction temperature. Based on the results of XRD and TEM, we conclude that the gold particles could be highly dispersed on TiO2 support particles with the smaller size and narrower size distribution by SSP method at higher spray temperatures. It should be noted here that in the case of goldsupported alumina (SPR-Au/Al2 O3 ) prepared by a solution spray method, we found a similar fact: the higher temperature spraying could also produce smaller Au particles with higher catalytic activity for NO– CO reaction [34]. The analysis of XRD profiles and TEM image evidenced the formation of Au nanoparticles with the strong interaction with Al2 O3 support. However, to our knowledge, almost all the reported results concerning the effect of calcination temperature on supported gold catalysts, gave tendencies just contrary to ours [25,36,37]. Boccuzzi et al. [36] found that as for the gold particles on Au/TiO2 prepared by deposition–precipitation method, the size distribution shifted to larger values with the increase of calcination temperature. Tsubota et al. [25] also reported a similar result for a mechanically mixed sample of Au colloids and TiO2 power. Thus, the question is how we do understand the opposite results for SSP-Au/TiO2 . The XRD study described above indicates no marked change in the crystallite sizes of Au on SSP873 and SSP1073 after use for CO oxidation. In contrast, Au crystallite on SSP673 became larger during the catalytic reaction. This implies that the interaction between gold and support particles was enhanced over SSP873 and SSP1073 by the higher temperature preparation.

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The strong interaction could be suggested also by the change of TEM profiles: the shape for some of the gold particles converted from spherical into semispherical over SSP873 and SSP1073. Therefore, we suppose that for SSP-Au/TiO2 catalyst prepared at much higher temperature than the decomposition temperature of HAuCl4 (ca. 463 K), the nucleation of gold particles took place instantaneously at the surface of TiO2 particles. The induced interaction between Au particles and the support was strong enough to prevent the coagulation and crystal growth. 3.2. Surface state of gold over SSP-Au/TiO2 catalysts XPS analysis was carried out to obtain information about the surface states of gold on SSP-Au/TiO2 . The spectra of Au 4f region are shown in Fig. 3. According to the literature in this region [38–40], each gold species shows two peaks, due to Au 4f7/2 and Au 4f5/2 transitions. The peaks for bulk metallic gold were centered at 84.0 and 87.7 eV, respectively, with 1.1 eV of FWHM [29–31], while the peaks for Au2 O3 were observed at 86.3 and 89.6 eV, respectively, with slight increase in FWHM. The rather broad peaks for SSP673 were centered at 84.2 and 87.8 eV, respectively. Thus, the peaks were shifted considerably to the higher binding energies together with an increase in FWHM relative to the corresponding values for bulk metallic gold. These features, however, were different from those for Au2 O3 species. Such results were suggestive of the existence of some different gold species as discussed below. Moreover, by comparing the XPS spectra in detail, we found that the Au 4f peaks became stronger and shaper according to the increase of spray temperature, although no appreciable difference appeared in the binding energies. We suppose that the increase in the peak intensity with the spray temperature is due to the increased distribution of gold particles over the catalyst surface [27]. The surface composition and the surface atomic ratios of Au/Ti and O/Ti were derived from the corresponding peak areas (Table 2). The Au/Ti ratio increased with raising spray temperature, i.e. the value for SSP1073 was almost two times higher than that for SSP673. An increase of Au/Ti ratio combined with decrease of O/Ti ratio was also revealed with the rise of spray temperature. Thus, the higher SSP temperature

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Fig. 3. XPS spectra of Au 4f region for SSP-Au/TiO2 catalysts: (a) SSP673; (b) SSP873; (c) SSP1073. Experimental spectra (䊏), deconvoluted peaks and convoluted spectra (—).

Table 2 Surface composition and surface atomic ratios of Au/Ti and O/Ti for SSP-Au/TiO2 catalysts determined by XPS spectra

is favorable for the gold particles to distribute and stabilize in metallic state over the oxide support. Further, we analyzed the XPS spectra of Au 4f region for SSP-Au/TiO2 using a curve-fitting method. The deconvolution results in Fig. 3 indicate that at least three components may coexist in Au 4f7/2 and Au 4f5/2 signals, giving peaks at 84.0, 84.7 and 86.3 eV for Au 4f7/2 signal as well as at 87.7, 88.3 and 89.6 eV for Au 4f5/2 signal. These three components can be attributed to metallic Au0 , non-metallic gold (Auδ+ ) and Au2 O3 states, respectively, as referred to the literature results [14,38,39,41,42]. Minicò et al. [41] also found that the gold particles on Au/Fe2 O3 were present in three different states when they investigated the influence of catalyst pretreatments on the oxidation of organic compounds. Minicò et al. [43] and other researchers [14,44] suggested that both metallic Au and non-metallic Au are active for CO oxidation, although the latter is less stable than the former. On the other hand, Au2 O3 species show no direct correlation with the catalytic performance. In addition, in our previous work [34] on SPR-Au/Al2 O3 prepared by the solution spray method, the existence of different gold species has been also confirmed by XANES analysis. The ratio of the three components for these samples was calculated from the corresponding area of Au 4f7/2 region. The results are shown in Table 3. When the spray temperature was raised, the presence of Au2 O3 decreased with corresponding increase of the metallic Au. Such tendencies suggest that Au2 O3 species remaining on SSP673 were transformed at higher temperatures into non-metallic Au and metallic Au. Most of the gold particles on SSP1073 exist in metallic Au state though about 10% Au2 O3 species still remained. We thought this was due to the incomplete decomposition of HAuCl4 caused by the quick process of suspension spray method, within a few seconds. Table 3 The surface states of gold in SSP-Au/TiO2 catalysts obtained by deconvolution of Au 4f peaks

Sample

Au (atom%)

Ti (atom%)

O (atom%)

Au/Ti ratio

O/Ti ratio

Sample

Metallic Au

Non-metallic Au

Au2 O3

SSP673 SSP873 SSP1073

0.21 0.28 0.39

22.54 23.21 26.17

77.25 76.51 73.44

0.009 0.012 0.015

3.43 3.30 2.81

SSP673 SSP873 SSP1073

0.56 0.58 0.68

0.26 0.29 0.22

0.17 0.13 0.10

L. Fan et al. / Applied Catalysis A: General 246 (2003) 87–95

Fig. 4. CO chemisorption over SSP-Au/TiO2 catalysts at 298 K.

As for the CO oxidation over supported gold catalysts, there are general agreements that CO is adsorbed on the gold particles and that the irreversible part of CO adsorption is related to CO2 formation [22,45–47]. Fig. 4 reveals that the amount of CO chemisorption increased when a higher spray temperature was applied; the amount of CO chemisorption for SSP1073 was about five times larger than that for SSP673. Boccuzzi et al. [48] have investigated the effect of gold particle size on CO oxidation over Au/TiO2 by FTIR. They found that the CO is competitively adsorbed on gold step sites without dissociation. Moreover, the step density increases as the particle size decreases. Additionally, Tripathi et al. [37] have studied the adsorption and CO oxidation reaction over Au/Fe2 O3 samples by microcalorimetry. They concluded that the promotion effect of gold in augmented CO oxidation activity can be attributed to the CO chemisorption on small gold particles. Therefore, we can explain as follows: the increase in the amount of CO chemisorption over the sample prepared at higher temperature should be attributed to the high dispersion of gold particles and to the existence of more active sites on the surface. These results for CO chemisorption are also consistent with those for XPS results. 3.3. Catalytic activity Fig. 5 shows the initial rates of CO2 formation for CO oxidation at 298 K and the corresponding TOF val-

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Fig. 5. Initial rates of CO2 formation for CO oxidation over SSP-Au/TiO2 catalysts at 298 K and the corresponding TOF values.

ues over SSP-Au/TiO2 . The initial rates and the TOF values increased with rising of spray temperature. The initial rate and TOF value for SSP1073 were much larger than those for SSP673 and SSP873, by approximately two orders and one order of magnitude, respectively. Haruta [16] has proved that the catalytic activity of supported gold catalyst is affected not only by the particle size of gold, but is also influenced strongly by the interface between the gold and the support particles. Thus, the smaller gold particles and the extensive perimeter interface between gold and support creates high catalytic activities. The dramatic increase of catalytic activity, we observed, can be explained in terms of gold particles and the surface structure: the high catalytic activity of SSP1073 is attributed to the highly dispersed gold fine particles that are strongly interacting with TiO2 support to enhance effective synergy effects. The initial rates of CO oxidation over these samples were also obtained in the range of 273–398 K. The rates increased with the increase of spray temperature for all the samples. The dramatic increase was obtained for CO oxidation by high temperature preparation: the order of the catalytic activity was SSP1073  SSP873 > SSP673. The apparent activation energies deduced from the corresponding Arrhenius plots are shown in Fig. 6. The activation energy for SSP673 was very high as 35.3 kJ/mol, but it decreased down to 18.9 and

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Fig. 6. Arrhenius plots for CO oxidation over SSP-Au/TiO2 catalysts: (䉱) SSP673; (䊉) SSP873; (䊏) SSP1073.

18.1 kJ/mol over SSP873 and SSP1073, respectively. The reaction mechanism over SSP873 and SSP1073 might be different from that over SSP673 due to the different types of active sites. We speculate that the reaction over SSP673 may take place mainly on the surface of gold particles [49], because the size of gold particles is too large to enhance effective interaction between gold and TiO2 . On the contrary, for SSP873 and SSP1073, the gold particles are small enough to enhance strong interaction and modify the morphology and the chemistry of gold nanoparticles. Further, the catalytic mechanism over SSP873 and SSP1073 may be postulated by the following pathway: CO adsorbed on the surface of Au particles and the perimeter of Au–TiO2 interface react with the oxygen activated at the perimeter and finally forms the CO2 product [23,50]. Similar-type active sites are generated on SSP873 and SSP1073, which promote the catalysis, and the initial rate differences between them can be attributed to the differences in the number of active sites.

4. Conclusions From the results of characterization and catalytic test over SSP-Au/TiO2 prepared at different suspension spray temperatures, the following conclusions can be deduced:

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