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SiO2 catalysts in CO oxidation
Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Influences of CeO2 microstructures on the structure and activity of Au/CeO2 /SiO2 catalysts in CO oxidation Kun Qian a , Shanshan Lv a , Xiaoyan Xiao b , Huaxing Sun a , Jiqing Lu b , Mengfei Luo b , Weixin Huang a,∗ a
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
b
a r t i c l e
i n f o
Article history: Received 26 December 2008 Received in revised form 10 February 2009 Accepted 11 February 2009 Available online 21 February 2009 Keywords: Au/CeO2 /SiO2 catalysts CO oxidation Structure–activity relation
a b s t r a c t The influences of CeO2 microstructures on the structure and catalytic activity of supported Au nanoparticles in CO oxidation have been investigated in Au/CeO2 /SiO2 catalysts. CeO2 /SiO2 supports with various CeO2 microstructures were prepared and used to prepare Au/CeO2 /SiO2 catalysts by deposition–precipitation using HAuCl4 as the precursor. Au(I) species and Au nanoparticles compete for the surface oxygen vacancies on CeO2 and highly dispersive CeO2 on SiO2 facilitates the formation of Au(I) species. Meanwhile, the presence of Au also facilitates the creation and stabilization of surface oxygen vacancies on CeO2 . The Au nanoparticle–CeO2 interface plays an important role in the activity of Au/CeO2 /SiO2 catalysts in CO oxidation. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Supported Au catalysts have been demonstrated to exhibit unique catalytic activities in several important oxidation reactions, such as preferential and total oxidation of CO, direction oxidation of hydrogen to hydrogen peroxide, epoxidation of propylene, water gas shift reaction, oxidation of alcohols, and total oxidation of volatile organic compounds [1–4]. Among these reactions, CO oxidation catalyzed by supported Au catalysts has been most extensively investigated not only because this reaction has potential applications but also because it is anticipated that the structure–activity relation of supported Au catalysts could be elucidated in the simple CO oxidation. The size of Au nanoparticles and the oxide support play key roles in the activity of supported Au catalysts in CO oxidation. The catalytic activity is higher when the particle size of Au nanoparticles is finer [5]. Oxide supports can be classified as inert or active according to their redox properties, in which Al2 O3 , SiO2 , and MgO fit with the inert supports, while reducible transition metal oxides such as TiO2 , Fe2 O3 , MnO2 , and CeO2 belong to the active supports [6]. The classification is based on the observation that Au nanoparticles supported on inert oxides exhibited lower intrinsic activities than those supported on active oxides that could contribute to the activation and supply of oxygen for the reaction [6]. However, the nature of the active site of sup-
∗ Corresponding author. Tel.: +86 551 3600435; fax: +86 551 3600437. E-mail address: [email protected] (W. Huang). 1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2009.02.014
ported Au catalysts in CO oxidation is still a matter of discussion [7–13]. CeO2 is widely employed as the support or additive in catalysts for oxidation reactions because of its high oxygen storage capacity and redox activity [14]. CeO2 supported Au catalysts have been prepared by various methods [15–22], and some of them are active in low-temperature CO oxidation [15,19–22]. CeO2 as an additive was reported to enhance the activity of inert oxide supported Au catalysts (Au/SiO2 and Au/Al2 O3 ) in CO oxidation [23]. Pillai and Deevi assigned the high activity of Au/CeO2 catalysts for CO oxidation to Au+ –OH− and highly dispersed metallic Au species strongly interacting with defects in the ceria surface [21]. Corma and co-workers observed a direct correlation between the concentration of Au3+ species and catalytic activity in Au/CeO2 for CO oxidation [24] and showed that the cationic Au species were related with the perimeter interface between the Au particle and the support [25] and that the cationic Au species were stabilized during the course of catalytic CO oxidation [26]. Venezia et al. studied the catalytic activities of Au/CeO2 catalysts prepared by various methods in CO oxidation and proposed that the presence of small Au particles was not the main requisite for the achievement of the highest CO conversion, but the strong interaction between ionic Au and ceria might determine the particularly high activity by enhancing the ceria surface oxygen reducibility [20]. The structures of CeO2 supports have been found to greatly affect the activities of Au/CeO2 catalysts in CO oxidation. Carrettin et al. reported that Au deposited on nanocrystalline particles of CeO2 showed two orders of magnitude in the catalytic activity relative to
K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
the Au/CeO2 catalysts prepared by coprecipitation and by Au deposition on a regular CeO2 support [19]. Arena et al. demonstrated that the reducibility of the active CeO2 phase affected the catalytic activity of Au/CeO2 catalysts [27]. Recently Widmann et al. quantitatively reported that a freshly calcined Au/CeO2 catalyst became significantly more active on removal of about 7% of the surface oxygen content [28]. However, a comprehensive investigation of the influence of CeO2 microstructures on the structures and activities of supported Au particles in CO oxidation still lacks because it is difficult to systematically control the microstructures of CeO2 . In this paper, we employed silica supported CeO2 (CeO2 /SiO2 ) with various CeO2 microstructures as the support for Au nanoparticles. Because of the inertness of SiO2 , the structure of Au nanoparticles in the Au/CeO2 /SiO2 catalysts is mainly influenced by the structure of CeO2, therefore, by investigating the structures of various Au/CeO2 /SiO2 catalysts and their catalytic activities in CO oxidation, we find some novel results on the relation between the microstructure of CeO2 and the structure and catalytic activity of Au nanoparticles supported on CeO2 .
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2. Experimental
30 min in the nitrogen atmosphere before the measurement. XRD measurements were performed on a Philips Xpert PRO SUPER Xray diffractometer with a Ni-filtered Cu K␣ X-ray source operating at 40 kV and 50 mA. High resolution X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high performance electron spectrometer using monochromatized Al K␣ excitation source (h = 1486.6 eV)). The binding energy of Si 2p in SiO2 , which was assumed to be 103.3 eV, was employed as the reference to correct the likely charging effect during the XPS measurements. The symmetric Si 2p XPS spectrum excludes the existence of differential charge on the samples. Transmission electron microscopy (TEM) experiments were preformed on a JEOL 2010 high resolution transmission electron microscope with an energy dispersive spectrum (EDS) analysis facility. Temperatureprogrammed reduction (H2 -TPR) experiments were carried out using a 5% H2 –N2 mixture (40 mL/min flow) at a heating rate of 10 ◦ C/min. 50 mg catalyst was used. The catalyst was heated at 200 ◦ C for 0.5 h and then cooled to room temperature in Ar (30 mL/min) prior to the TPR experiment. The consumption of H2 during the TPR experiment was measured by a thermal conductivity detector (TCD).
2.1. Catalyst preparation
2.3. Catalytic activity measurement
The CeO2 /SiO2 supports with a 6% CeO2 /SiO2 weight ratio were prepared in two methods. The first method is incipient wetness impregnation. Ce(NO3 )3 ·6H2 O (Sinopharm Chemical Reagent Co., Ltd., ≥99.0%) was dissolved in an appropriate volume of triply distilled water and then added to SiO2 (20–50 mesh, Qingdao Haiyang Chemicals Co.) under stirring. The powder was then dried at 60 ◦ C overnight and eventually calcinated at desired temperatures (200 and 400 ◦ C) for 4 h. The resulting CeO2 /SiO2 supports were denoted as IWI200 and IWI400. In the second method, the Ce(NO3 )3 ·6H2 O aqueous solution was added into a three-neck bottle containing SiO2 , then the pH of the solution was adjusted between 9 and 10 by adding ammonia water. The mixture was stirred at 60 ◦ C for 24 h, and the solid was filtered and washed several times. The resulting powder was dried at 60 ◦ C overnight, followed by calcination at desired temperatures (200, 400, and 600 ◦ C) for 4 h. These CeO2 /SiO2 supports were denoted as DP200, DP400 and DP600. The CeO2 /SiO2 supports were then used to prepare 2%-Au/CeO2 / SiO2 (Au/support weight ratio) catalysts by deposition– precipitation (DP) using HAuCl4 ·4H2 O (Sinopharm Chemical Reagent Co., Ltd., Au content ≥47.8%) as the Au precursor. The HAuCl4 ·4H2 O aqueous solution and ammonia water were slowly co-added into a three-neck bottle containing CeO2 /SiO2 , whose pH was controlled between 9 and 10. The mixture was stirred at 60 ◦ C for 24 h. Then the solid was filtered and washed several times. The resulting powder was dried at 60 ◦ C overnight, followed by calcination at 200 ◦ C for 4 h. The supported Au catalysts were denoted by “C + support name”, for example, the catalyst using IWI200 as the support was denoted as CIWI200. The Au/SiO2 catalyst was also prepared by the same DP method for the comparison purpose.
The catalytic activity was evaluated on a fixed-bed flow reactor. The used catalyst weight was 100 mg and the reaction gas consisting of 1% CO and 99% dry air was fed at a rate of 20 mL/min. The steady-state composition of the effluent gas was analyzed with an online GC-14C gas chromatograph equipped with a TDX-01 column (T = 80 ◦ C, H2 as the carrier gas at 30 mL/min) after the desired reaction temperature had been kept for 30 min. The conversion of CO was calculated from the change in CO concentrations in the inlet and outlet gases.
2.2. Catalyst characterization The elemental composition of supports and catalysts were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). Two methods were employed to dissolve the catalysts. One was the dissolution of catalyst in aqua regia and the other was the dissolution of catalysts in the mixture of H2 O2 and aqua regia. The ICP-AES working curves for Au and Ce were plotted employing HAuCl4 ·4H2 O and Ce(NO3 )3 ·6H2 O standard solutions, respectively. BET surface areas were acquired on a Beckman Coulter SA3100 surface area analyzer, in which the sample was degassed at 120 ◦ C for
3. Results 3.1. Macroproperties of catalysts The BET surface areas and the compositions of catalysts are summarized in Table 1. The BET surface area of bare SiO2 is 390 m2 /g. DP200, IWI200, and IWI400 are with similar BET surface areas but DP400 and DP600 are with smaller BET surface areas. All Au/CeO2 /SiO2 catalysts are with similar BET surface areas. By dissolving the catalysts in aqua regia, we measured the compositions of catalysts by ICP-AES. Au loadings in all Au/CeO2 /SiO2 catalysts are similar, much higher than the Au loading in Au/SiO2 [29] prepared by the same procedure. This indicates that the DP efficiency of Au is higher on CeO2 /SiO2 than on bare SiO2 . Similar results have also been observed in Au/CoOx /SiO2 and Au/ZnO/SiO2 catalysts [30,31]. During the course of DP, the pH value of HAuCl4 solution was adjusted to 9–10, forming the surface-reactive anionic gold hydroxide species AuCl(OH)3 − and Au(OH)4 − [32] that would efficiently deposit onto a positively charged oxide surface. Accordingly, oxides with a higher isoelectric point (IEP) will facilitate the deposition and precipitation of the Au precursor. The IEP value of CeO2 is much higher than that of SiO2 . Therefore, employing the same DP procedure, the loading of Au is higher in Au/CeO2 /SiO2 than in Au/SiO2 . The Ce concentrations were also measured but differed very much in various catalysts. The weight ratio of CeO2 in DP200 and DP400 are 5.75% and 5.42%, respectively, whereas that in DP600, IWI200, IWI400 decreased to as low as 1.61%, 0.31% and 0.18%, respectively. The calculated weight ratios of CeO2 are 5.66% and 5.55% in CeO2 /SiO2 and Au/CeO2 /SiO2 , respectively. However, when dissolving the catalysts in the mixture of aqua regia and H2 O2 , the obtained ICP-AES results give similar weight ratios of CeO2 in the
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Table 1 Macroproperties of catalysts. Catalyst
Weight ratios in the catalysts measured by ICP-AES a
DP200 DP400 DP600 IWI200 IWI400 CDP200 CDP400 CDP600 CIWI200 CIWI400 Au/SiO2 c
a
Au
CeO2
N.A. N.A. N.A. N.A. N.A. 2.2% 2.2% 2.2% 1.9% 2.1% 1.0%
5.75% 5.42% 1.61% 0.31% 0.18% 5.61% 5.77% 2.20% 0.28% 0.17% N.A.
CeO2
b
5.52% N.M. 5.47% 5.68% N.M. N.M. N.M. N.M. N.M. N.M. N.A.
Average crystalline size dXRD (nm) Au
CeO2
N.A N.A N.A N.A N.A 15 14 11 10 13 12
N.D. N.D. N.D. 5.6 7.3 N.D. N.D. 2.3 5.6 6.8 N.A.
BET surface area (m2 /g)
366 278 293 355 345 267 278 259 261 291 316
N.A.: not applicable; N.M.: not measured; N.D.: not detectable. a Dissolution in aqua regia. b Dissolution in H2 O2 and aqua regia. c From Ref. [29].
catalysts approaching the calculated values, proving the successful loading of Ce on SiO2 . Therefore, the different solubility of CeO2 supported on SiO2 in aqua regia should arise from their different microstructures. It has been reported that Ce(IV) could be reduced to Ce(III) by H2 O2 in the lower pH range, which could enhances the dissolution rate of CeO2 in aqua regia [33]. Therefore, the dissolution rate of supported CeO2 in aqua regia could be taken as an indication of the oxidation state of Ce. The faster the dissolution rate, the larger the fraction of Ce(III) and thus the concentration of oxygen vacancy in CeO2 . Since the Ce(III) fraction and the oxygen vacancy concentration in CeO2 play an important role in CO oxidation catalyzed by CeO2 , their characterizations are important. With this respect, our results provide a fast and convenient method. Fig. 1 shows XRD patterns of various catalysts. It could be seen that the preparation method greatly affects the dispersion of CeO2 on SiO2 . Strong diffraction peaks arising from the cubic fluoride CeO2 are present in the XRD patterns of IWI200 and IWI400. However, no diffraction peaks associated with CeO2 appears in the XRD patterns of DP200 and DP400, and very weak and diffuse diffraction peaks corresponding to CeO2 appear in the XRD pattern of DP600 due to the elevated calcination temperature. The XRD patterns of CeO2 in Au/CeO2 /SiO2 catalysts are similar to those in the corresponding CeO2 /SiO2 supports. The XRD peaks arising from metallic Au are clearly visible in the XRD patterns of all Au/CeO2 /SiO2 catalysts. The average crystalline sizes of Au and CeO2 in various catalysts were calculated based on the Scherrer equation, whose results are summarized in Table 1. The average crystalline size of Au nanoparticles in Au/CeO2 /SiO2 varies between 10 and 15 nm, similar to that of Au in Au/SiO2 prepared by the same procedure. 3.2. Catalytic activity of catalysts in CO oxidation Fig. 2 displays the CO conversion as a function of the reaction temperature over various catalysts. The CeO2 /SiO2 catalysts exhibit very poor catalytic performances. DP200, DP400, and DP600 do not show any activity in CO oxidation under the investigated temperatures. IWI400 and IWI200 become active above 300 and 270 ◦ C, respectively. The Au/CeO2 /SiO2 catalysts exhibit better catalytic performances than the Au/SiO2 catalyst, and it could be seen that the more active the CeO2 /SiO2 , the more active the corresponding Au/CeO2 /SiO2 . This demonstrates that addition of CeO2 to Au/SiO2 benefits CO oxidation. However, none of our Au/CeO2 /SiO2 catalysts is as active in low-temperature CO oxidation as previously reported Au/CeO2 catalysts [15,19–22]. CIWI200 with the best catalytic performance achieves a 50% CO conversion at 110 ◦ C (T50% )
Fig. 1. XRD patterns of (A) CeO2 /SiO2 and (B) Au/CeO2 /SiO2 . The peaks marked by the solid and dash lines arise from CeO2 and Au, respectively.
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Fig. 2. Catalytic performances of various catalysts in CO oxidation.
and a complete CO conversion at 210 ◦ C, and other catalysts achieve a complete CO oxidation at 270 ◦ C or above. Dekkers et al. [23] previously reported the promotion effect of CeO2 in both the reduction of NO with H2 and the oxidation of CO with O2 catalyzed by Au/SiO2 and Au/Al2 O3 catalysts, and the T50% of their Au/CeO2 /SiO2 catalyst was 115 ◦ C. This value is close to the T50% of our CIWI200 catalyst (T50% = 110 ◦ C). 3.3. Structures of catalysts Fig. 3 shows Ce 3d XPS spectra of various catalysts. The intensity of the Ce 3d XPS peak in IWI and CIWI catalysts is much weaker than that in DP and CDP catalysts. XPS is a surface sensitive technique. At a fixed total volume of particles, particles with a higher dispersion are of a larger surface-to-bulk atomic ratio and thus exhibit a stronger peak in the XPS spectrum. Therefore, the Ce 3d XPS results imply that CeO2 in DP and CDP catalysts is much more dispersive than that in IWI and CIWI catalysts, in consistence with XRD results. The Ce 3d XPS spectra show multiple states arising from different Ce 4f level occupancies in the final state [34]. We performed peakfitting of the Ce 3d XPS spectra in DP and CDP (Fig. 3A). The labels in Fig. 3A follow the convention established by Burroughs et al. [35], in which V and U refer to the 3d5/2 and 3d3/2 spin-orbital components, respectively. The V/U, V /U , and V /U components are features of
Ce(IV) and the V /U component is the feature of Ce(III) [36]. The Ce(III)/Ce(IV) atomic ratio in various catalysts that indicates the concentration of surface oxygen vacancies was also calculated by the peak area of Ce(III)/the total area of Ce(IV) peaks. These results are summarized in Table 2. The Ce(III)/Ce(IV) decreases in the DP catalysts with the increasing calcination temperature, implying that calcination at high temperatures in air annihilates surface oxygen vacancies in CeO2 supported on SiO2 . This is in consistence with the ICP-AES results. Interestingly, the Ce(III)/Ce(IV) atomic ratio in CDP catalysts is nearly as twice as that in the corresponding DP catalysts. Although the low intensity of Ce 3d XPS peaks of IWI and CIWI catalysts does not allow the peak-fitting, it could be also seen in Fig. 3B that the peak V in CIWI catalysts is stronger than that in the corresponding IWI catalysts, indicating a larger Ce(III)/Ce(IV) atomic ratio in CIWI catalysts. The Au 4f XPS spectra are displayed in Fig. 4. The Au 4f XPS spectra in all catalysts could be well fitted with two components with the Au 4f7/2 binding energy at ∼83.8 and ∼85.6 eV, which could be assigned to the metallic Au and Au(I) species. The Au(I)/Au atomic ratio in the catalysts was calculated by the peak area of Au(I)/the peak area of metallic Au. The peak-fitting results are summarized in Table 2. An interesting observation is that the Au(I)/Au ratio varies proportionally with the Ce(III)/Ce(IV) atomic ratio in the CDPseries Au/CeO2 /SiO2 catalysts, indicating that the formation of Au(I)
Fig. 3. Ce 3d XPS spectra of DP200, DP400, DP600, CDP200, CDP400, and CDP600 catalysts (A) and IWI200, IWI400, CIWI200, and CIWI400 catalysts (B).
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Table 2 Peak-fitting results of Ce 3d and Au 4f XPS spectra in various catalysts. Catalyst
Ce 3d5/2 (eV)
Ce(III)/Ce(IV) atomic ratio
Ce(IV)
DP200 DP400 DP600 CDP200 CDP400 CDP600 CIWI200 CIWI400
Ce(III)
U
U
U
U
883.4 883.5 883.5 883.2 883.4 883.4 N.A. N.A.
888.5 888.9 888.8 888.8 888.8 888.7 N.A. N.A.
898.8 898.9 898.9 899.1 899.0 899.0 N.A. N.A.
886.3 886.5 886.4 886.3 886.4 886.4 N.A. N.A.
0.16 0.11 0.05 0.31 0.21 0.10 N.A. N.A.
Au 4f7/2 (eV)
Au(I)/atomic ratio
Metallic Au
Au(I)
N.A. N.A. N.A. 83.9 83.8 83.7 83.8 83.8
N.A. N.A. N.A. 85.7 85.6 85.5 85.5 85.5
N.A. N.A. N.A. 0.30 0.23 0.14 0.14 0.12
N.A.: not applicable.
Fig. 4. Au 4f XPS spectra of Au/CeO2 /SiO2 catalysts.
species is associated with the density of surface oxygen vacancies in CeO2 . Above results demonstrate that the Au loading, the average Au crystalline size, and the Au 4f binding energy are similar in CIWI200, CIWI400, and CDP600, but CIWI200 shows a much higher activity in CO oxidation than CIWI400 and CDP600. We thus investigated the morphology of CIWI200, CIWI400, and CDP600 in detail by TEM. Fig. 5 displays the representative TEM images. The TEM results (not shown) show that large CeO2 aggregates are present in CIWI200 and CIWI400, but absent in CDP600. This agrees with the XRD results that CeO2 in CDP600 are more dispersive than that in CIWI200 and CIWI400. We analyzed the size distribution of observed Au nanoparticles in TEM images (Fig. 6). The average size of Au nanoparticles is 9.7, 11.4 and 14.3 nm in CIWI200, CDP600and CIWI400, respectively. It can also be seen that among these catalysts CIWI200 has the highest density of Au nanoparticles with 5–7 nm sizes. Interestingly, the distribution of Au nanoparticles on CeO2 /SiO2 differs much in CIWI200, CIWI400, and CDP600. We analyzed the composition of the areas indicated by the circle in the TEM images by EDS, whose results is summarized in Table 3. In CIWI200, both TEM results and EDS results demonstrate that Au nanoparticles are dispersed on the CeO2 aggregates (area a in Fig. 5A) and the
Fig. 5. TEM images of CIWI200 (A and B), CIWI400 (C and D), and CDP600 (E and F).
K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
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Fig. 6. Size distribution of Au nanoparticles in CIWI200, CIWI400, and CDP600 estimated from the TEM results. Table 3 The elemental compositions in atomic ratio of each area calculated from EDS results. Element
CIWI200
CIWI400
Fig. 5A
O Si Ce Au
Area a
Area b
66.94 25.65 6.18 1.22
68.45 30.97 0.00 0.58
with their activity in CO oxidation, indicating that the reduction of CeO2 is involved in the catalytic CO oxidation.
CDP600
Fig. 5B
Fig. 5C
Fig. 5D
Fig. 5E
Fig. 5F
67.44 31.16 0.55 0.85
59.73 36.35 3.62 0.30
61.30 34.74 3.96 0.00
59.93 29.73 9.60 0.73
60.74 37.49 0.56 1.21
4. Discussion 4.1. Microstructures of CeO2
bare SiO2 surface (area b in Fig. 5A). However, in CIWI400, few Au nanoparticles are observed on the CeO2 aggregates by TEM (Fig. 5C and D), the weak Au signal on the CeO2 aggregate marked in Fig. 5C detected by EDS might arise from the Au(I) species. There is no CeO2 aggregate in CDP600, but few Au nanoparticles are observed on the area (Fig. 5E) with a high Ce concentration whereas the Ce concentration in the area (Fig. 5F) with abundant Au nanoparticles is low. These results indicate that the density of Au nanoparticles supported on CeO2 follows: CIWI200 > CDP600 > CIWI400. Fig. 7 shows the TPR results of selective catalysts. CeO2 in IWI200 is much more facile to be reduced than that in DP600. The addition of gold significantly improves the reducibility of CeO2 supported on SiO2 , in consistence with previous observations in Au/CeO2 catalysts [21]. Meanwhile, the reducibility of catalysts correlates well
Fig. 7. H2 -TPR profiles of indicated catalysts.
Due to the different preparation methods, CeO2 in various CeO2 /SiO2 exhibits different microstructures, such as the particle size and the surface oxygen vacancy concentration. CeO2 in DP catalysts is finer in the average particle size than that in IWI catalysts, and the particle size of CeO2 increases with the increasing calcination temperature. Meanwhile, the surface oxygen vacancy concentration of CeO2 in DP catalysts, as indicated by the Ce(III)/Ce(IV) atomic ratio, decreases with the increasing calcination temperature. CeO2 in IWI200 is with a better crystallinity and a larger average particle size than that in DP600, but CeO2 in IWI200 is much more facile to be reduced than that in DP600. All these results imply that the CeO2 –SiO2 interaction is stronger in DP catalysts than in IWI catalysts. This could be attributed to the different cerium species prior to the calcination, which is cerium hydroxide and Ce(NO3 )3 in DP and IWI catalysts, respectively. Calcination of cerium hydroxide supported on SiO2 leads to the formation of ultrafine CeO2 particles whereas calcination of Ce(NO3 )3 supported on SiO2 leads to the formation of large CeO2 aggregates. Very interestingly we observed the different solubility of CeO2 in various catalysts in aqua regia, which could be associated with the microstructures of supported CeO2 . Due to the disordered structure, CeO2 with a poor crystallinity might dissolve facilely in aqua regia. A dramatic change of the solubility of CeO2 was observed between CDP400 and CDP600 although CeO2 in CDP600 only displays very weak and diffuse diffraction peaks in the XRD spectrum. CeO2 in DP200 and DP400 dissolves in aqua regia much faster than CeO2 in DP600, IWI200 and IWI400. The complete dissolution of catalysts in H2 O2 and aqua regia shows that Ce contents in catalysts prepared by different methods are almost the same. These phenomena together with XPS results indicate more Ce(III) and oxygen vacancies exist in DP200 and DP400 than other catalysts since H2 O2 enhances the dissolution of CeO2 by reducing Ce(IV) to Ce(III). Various CeO2 /SiO2 catalysts exhibit different catalytic performances in CO oxidation. It is generally accepted that CO oxidation under stationary conditions over ceria follows the Marsvan Krevelen-type mechanism, where reaction involves alternate reduction and oxidation of the oxide surface with the formation of surface oxygen vacancies (as the key step) and their replenishment by gas-phase oxygen [14]. We believe that the strong CeO2 –SiO2 interaction in DP-series CeO2 /SiO2 catalysts blocks the reduction–oxidation cycle on CeO2 so that DP-series CeO2 /SiO2 catalysts do not show any activity in CO oxidation although they exhibit large densities of surface oxygen vacancies. It is most likely that the replenishment of surface oxygen vacancies by gas-phase oxygen is blocked because the surface oxygen vacancies in DP-series CeO2 /SiO2 are stabilized by the strong CeO2 –SiO2 interaction. With
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the reduced CeO2 –SiO2 interaction, CeO2 in IWI200 and IWI400 exhibits certain activities in CO oxidation, and IWI200 is more active that IWI400 due to the finer particle size of CeO2 . 4.2. Au–CeO2 interaction in Au/CeO2 /SiO2 catalysts An interesting observation is that the Ce(III)/Ce(IV) atomic ratio in supported CeO2 increases greatly when CeO2 /SiO2 catalysts are loaded with Au, which is independent of the microstructures of CeO2 . Since CeO2 /SiO2 was calcined prior to the loading of Au, this observation clearly demonstrates that the loading of gold on CeO2 /SiO2 facilitates the formation of surface oxygen vacancies in CeO2 , which could then be stabilized. TPR results also demonstrate that the addition of gold significantly improves the reducibility of CeO2 supported on SiO2 . Fu et al. firstly reported that the addition of gold improve the reducibility and the OSC of cerium oxide [16]. Pillai and Deevi also observed a similar result [21]. Shapovalov and Metiu performed a theoretical study of CO oxidation catalyzed by Aux Ce1−x O2 and found that vacancy formation in Aux Ce1−x O2 is exothermic, which suggests the oxygen atoms nearest and nextnearest the Au dopant are chemically active [37]. Our results evidence that the formation of Au(I) species is closely associated with Ce(III) sites, i.e., surface oxygen vacancy sites on CeO2 in CeO2 supported gold catalysts. The Au(I)/Au atomic ratio varies proportionally with the Ce(III)/Ce(IV) atomic ratio in Au/CeO2 /SiO2 whereas no Au(I) species was observed by XPS on Au/SiO2 prepared by the similar method [29]. Furthermore, the TEM and EDS results show that there is still considerable gold signals on CeO2 areas without Au nanoparticles, which could only arise from Au(I) species. These results suggest that surface oxygen vacancies and Au(I) species in Au/CeO2 /SiO2 interact so as to stabilize each other. The microstructure of CeO2 in CeO2 /SiO2 affects the distribution of Au nanoparticles. A surprising result is that Au nanoparticles supported on CeO2 are evident only in CIWI200, but not in CIWI400 and CDP600. This indicates that both Au(I) species and Au nanoparticles are formed on CeO2 in CIWI200 whereas Au(I) species dominates on CeO2 in CIWI400 and CDP600. The Ce(III)/Ce(IV) atomic ratio in DP catalysts demonstrates that increasing the calcination temperature reduces the surface oxygen vacancy concentrations on CeO2 , but the XPS results show that the Au(I)/Au atomic ratios are almost the same in CIWI200 and CIWI400. Therefore, the surface oxygen vacancies on CeO2 in IWI400 are mostly occupied by Au(I) species, and subsequently few Au nanoparticles are formed. We propose that Au(I) species and Au nanoparticles compete as products from the Au precursor on CeO2 . During the course of calcination, some of AuCl(OH)3 − and Au(OH)4 − is transformed into Au(I) stabilized on CeO2 , other forms Au clusters which migrate and aggregate on the surface. Au(I) species is stabilized by the surface oxygen vacancy site on CeO2 and Au nanoparticles also nucleate on the surface oxygen vacancy site on CeO2 . The available surface oxygen vacancy sites and their distribution on CeO2 , the surface oxygen vacanc–Au(I) interaction, and the surface oxygen vacancy–Au cluster interaction might cooperatively determine resulted Au species in the catalysts. Inferred from the results of the catalysts prepared by DP method, highly dispersed CeO2 in CeO2 /SiO2 facilitates the formation of Au(I) species. 4.3. Structure–activity relation of Au/CeO2 /SiO2 catalysts in CO oxidation No matter where Au nanoparticles distribute in the catalyst, all Au/CeO2 /SiO2 catalysts exhibit better catalytic performances in CO oxidation than Au/SiO2 with similar Au particle size. The promotion effect of CeO2 could be attributed to the contribution of CeO2 to the activation and supply of oxygen for the reaction, which has
been previously proposed [6] and spectroscopically observed [24]. The microstructure of Au/CeO2 /SiO2 catalysts strongly affects their catalytic activities, which offers an opportunity to understand their structure–activity relation in CO oxidation. CIWI200 with both Au(I) species and Au nanoparticles on CeO2 is much more active in CO oxidation than CIWI400 and CDP600 with Au(I) species dominating the CeO2 surface. This indicates that Au(I) species on CeO2 alone is not active in CO oxidation and the presence of Au nanoparticles on CeO2 is important, i.e., the Au nanoparticles–CeO2 interface plays an important role in the activity of Au/CeO2 /SiO2 catalysts in CO oxidation. Meanwhile, the activity of Au/CeO2 /SiO2 catalyst in CO oxidation could be correlated well the reducibility of CeO2 in Au/CeO2 /SiO2 and with the activity of the corresponding CeO2 /SiO2 catalyst in CO oxidation, indicating the important role of the ability of CeO2 to activate and supply oxygen. However, CIWI200 with the best performance does not show any activity in CO oxidation at room temperature, indicating the lack of the active sites for low-temperature CO oxidation. The ensemble in Au/CeO2 catalysts active for low-temperature CO oxidation has been proposed to consist of coexisting highly dispersed Au particles (2–4 nm) and Au(I)–OH species in intimate contact with the ceria surface defects [19–21], which lacks in CIWI200 because Au(I) species exists on CeO2 but 2–4 nm Au nanoparticles do not. Meanwhile, comparing unsupported CeO2 catalysts, the ability of CeO2 supported on SiO2 in CIWI200 for the activation and supply of oxygen is poor due to the CeO2 –SiO2 interaction as reflected by the catalytic performance of IWI200 in CO oxidation. It is under investigation in our lab how to engineer the microstructure of supported CeO2 for the fabrication of Au/CeO2 /SiO2 catalysts active in low-temperature CO oxidation. 5. Conclusions The influences of CeO2 microstructures on the structure and catalytic activity of supported Au nanoparticles in CO oxidation have been manifested in Au/CeO2 /SiO2 catalysts with various CeO2 microstructures. Au(I) species and Au nanoparticles compete for the surface oxygen vacancy sites on CeO2 . Highly dispersive CeO2 on SiO2 facilitates the formation of Au(I) species, and the presence of Au species also facilitates the creation and stabilization of surface oxygen vacancies on CeO2 . Au(I) species on CeO2 alone is not active in CO oxidation and Au nanoparticles in contact with CeO2 in Au/CeO2 /SiO2 catalysts are important in catalyzing CO oxidation reaction. Acknowledgements This work was financially supported by National Natural Science Foundation of China (grant 20773113), the “Hundred Talent Program” of Chinese Academy of Sciences, the MOE program for PCSIRT (IRT0756), and the MPG-CAS partner group program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Influences of CeO2 microstructures on the structure and activity of Au/CeO2 /SiO2 catalysts in CO oxidation Kun Qian a , Shanshan Lv a , Xiaoyan Xiao b , Huaxing Sun a , Jiqing Lu b , Mengfei Luo b , Weixin Huang a,∗ a
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
b
a r t i c l e
i n f o
Article history: Received 26 December 2008 Received in revised form 10 February 2009 Accepted 11 February 2009 Available online 21 February 2009 Keywords: Au/CeO2 /SiO2 catalysts CO oxidation Structure–activity relation
a b s t r a c t The influences of CeO2 microstructures on the structure and catalytic activity of supported Au nanoparticles in CO oxidation have been investigated in Au/CeO2 /SiO2 catalysts. CeO2 /SiO2 supports with various CeO2 microstructures were prepared and used to prepare Au/CeO2 /SiO2 catalysts by deposition–precipitation using HAuCl4 as the precursor. Au(I) species and Au nanoparticles compete for the surface oxygen vacancies on CeO2 and highly dispersive CeO2 on SiO2 facilitates the formation of Au(I) species. Meanwhile, the presence of Au also facilitates the creation and stabilization of surface oxygen vacancies on CeO2 . The Au nanoparticle–CeO2 interface plays an important role in the activity of Au/CeO2 /SiO2 catalysts in CO oxidation. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Supported Au catalysts have been demonstrated to exhibit unique catalytic activities in several important oxidation reactions, such as preferential and total oxidation of CO, direction oxidation of hydrogen to hydrogen peroxide, epoxidation of propylene, water gas shift reaction, oxidation of alcohols, and total oxidation of volatile organic compounds [1–4]. Among these reactions, CO oxidation catalyzed by supported Au catalysts has been most extensively investigated not only because this reaction has potential applications but also because it is anticipated that the structure–activity relation of supported Au catalysts could be elucidated in the simple CO oxidation. The size of Au nanoparticles and the oxide support play key roles in the activity of supported Au catalysts in CO oxidation. The catalytic activity is higher when the particle size of Au nanoparticles is finer [5]. Oxide supports can be classified as inert or active according to their redox properties, in which Al2 O3 , SiO2 , and MgO fit with the inert supports, while reducible transition metal oxides such as TiO2 , Fe2 O3 , MnO2 , and CeO2 belong to the active supports [6]. The classification is based on the observation that Au nanoparticles supported on inert oxides exhibited lower intrinsic activities than those supported on active oxides that could contribute to the activation and supply of oxygen for the reaction [6]. However, the nature of the active site of sup-
∗ Corresponding author. Tel.: +86 551 3600435; fax: +86 551 3600437. E-mail address: [email protected] (W. Huang). 1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2009.02.014
ported Au catalysts in CO oxidation is still a matter of discussion [7–13]. CeO2 is widely employed as the support or additive in catalysts for oxidation reactions because of its high oxygen storage capacity and redox activity [14]. CeO2 supported Au catalysts have been prepared by various methods [15–22], and some of them are active in low-temperature CO oxidation [15,19–22]. CeO2 as an additive was reported to enhance the activity of inert oxide supported Au catalysts (Au/SiO2 and Au/Al2 O3 ) in CO oxidation [23]. Pillai and Deevi assigned the high activity of Au/CeO2 catalysts for CO oxidation to Au+ –OH− and highly dispersed metallic Au species strongly interacting with defects in the ceria surface [21]. Corma and co-workers observed a direct correlation between the concentration of Au3+ species and catalytic activity in Au/CeO2 for CO oxidation [24] and showed that the cationic Au species were related with the perimeter interface between the Au particle and the support [25] and that the cationic Au species were stabilized during the course of catalytic CO oxidation [26]. Venezia et al. studied the catalytic activities of Au/CeO2 catalysts prepared by various methods in CO oxidation and proposed that the presence of small Au particles was not the main requisite for the achievement of the highest CO conversion, but the strong interaction between ionic Au and ceria might determine the particularly high activity by enhancing the ceria surface oxygen reducibility [20]. The structures of CeO2 supports have been found to greatly affect the activities of Au/CeO2 catalysts in CO oxidation. Carrettin et al. reported that Au deposited on nanocrystalline particles of CeO2 showed two orders of magnitude in the catalytic activity relative to
K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
the Au/CeO2 catalysts prepared by coprecipitation and by Au deposition on a regular CeO2 support [19]. Arena et al. demonstrated that the reducibility of the active CeO2 phase affected the catalytic activity of Au/CeO2 catalysts [27]. Recently Widmann et al. quantitatively reported that a freshly calcined Au/CeO2 catalyst became significantly more active on removal of about 7% of the surface oxygen content [28]. However, a comprehensive investigation of the influence of CeO2 microstructures on the structures and activities of supported Au particles in CO oxidation still lacks because it is difficult to systematically control the microstructures of CeO2 . In this paper, we employed silica supported CeO2 (CeO2 /SiO2 ) with various CeO2 microstructures as the support for Au nanoparticles. Because of the inertness of SiO2 , the structure of Au nanoparticles in the Au/CeO2 /SiO2 catalysts is mainly influenced by the structure of CeO2, therefore, by investigating the structures of various Au/CeO2 /SiO2 catalysts and their catalytic activities in CO oxidation, we find some novel results on the relation between the microstructure of CeO2 and the structure and catalytic activity of Au nanoparticles supported on CeO2 .
41
2. Experimental
30 min in the nitrogen atmosphere before the measurement. XRD measurements were performed on a Philips Xpert PRO SUPER Xray diffractometer with a Ni-filtered Cu K␣ X-ray source operating at 40 kV and 50 mA. High resolution X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high performance electron spectrometer using monochromatized Al K␣ excitation source (h = 1486.6 eV)). The binding energy of Si 2p in SiO2 , which was assumed to be 103.3 eV, was employed as the reference to correct the likely charging effect during the XPS measurements. The symmetric Si 2p XPS spectrum excludes the existence of differential charge on the samples. Transmission electron microscopy (TEM) experiments were preformed on a JEOL 2010 high resolution transmission electron microscope with an energy dispersive spectrum (EDS) analysis facility. Temperatureprogrammed reduction (H2 -TPR) experiments were carried out using a 5% H2 –N2 mixture (40 mL/min flow) at a heating rate of 10 ◦ C/min. 50 mg catalyst was used. The catalyst was heated at 200 ◦ C for 0.5 h and then cooled to room temperature in Ar (30 mL/min) prior to the TPR experiment. The consumption of H2 during the TPR experiment was measured by a thermal conductivity detector (TCD).
2.1. Catalyst preparation
2.3. Catalytic activity measurement
The CeO2 /SiO2 supports with a 6% CeO2 /SiO2 weight ratio were prepared in two methods. The first method is incipient wetness impregnation. Ce(NO3 )3 ·6H2 O (Sinopharm Chemical Reagent Co., Ltd., ≥99.0%) was dissolved in an appropriate volume of triply distilled water and then added to SiO2 (20–50 mesh, Qingdao Haiyang Chemicals Co.) under stirring. The powder was then dried at 60 ◦ C overnight and eventually calcinated at desired temperatures (200 and 400 ◦ C) for 4 h. The resulting CeO2 /SiO2 supports were denoted as IWI200 and IWI400. In the second method, the Ce(NO3 )3 ·6H2 O aqueous solution was added into a three-neck bottle containing SiO2 , then the pH of the solution was adjusted between 9 and 10 by adding ammonia water. The mixture was stirred at 60 ◦ C for 24 h, and the solid was filtered and washed several times. The resulting powder was dried at 60 ◦ C overnight, followed by calcination at desired temperatures (200, 400, and 600 ◦ C) for 4 h. These CeO2 /SiO2 supports were denoted as DP200, DP400 and DP600. The CeO2 /SiO2 supports were then used to prepare 2%-Au/CeO2 / SiO2 (Au/support weight ratio) catalysts by deposition– precipitation (DP) using HAuCl4 ·4H2 O (Sinopharm Chemical Reagent Co., Ltd., Au content ≥47.8%) as the Au precursor. The HAuCl4 ·4H2 O aqueous solution and ammonia water were slowly co-added into a three-neck bottle containing CeO2 /SiO2 , whose pH was controlled between 9 and 10. The mixture was stirred at 60 ◦ C for 24 h. Then the solid was filtered and washed several times. The resulting powder was dried at 60 ◦ C overnight, followed by calcination at 200 ◦ C for 4 h. The supported Au catalysts were denoted by “C + support name”, for example, the catalyst using IWI200 as the support was denoted as CIWI200. The Au/SiO2 catalyst was also prepared by the same DP method for the comparison purpose.
The catalytic activity was evaluated on a fixed-bed flow reactor. The used catalyst weight was 100 mg and the reaction gas consisting of 1% CO and 99% dry air was fed at a rate of 20 mL/min. The steady-state composition of the effluent gas was analyzed with an online GC-14C gas chromatograph equipped with a TDX-01 column (T = 80 ◦ C, H2 as the carrier gas at 30 mL/min) after the desired reaction temperature had been kept for 30 min. The conversion of CO was calculated from the change in CO concentrations in the inlet and outlet gases.
2.2. Catalyst characterization The elemental composition of supports and catalysts were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). Two methods were employed to dissolve the catalysts. One was the dissolution of catalyst in aqua regia and the other was the dissolution of catalysts in the mixture of H2 O2 and aqua regia. The ICP-AES working curves for Au and Ce were plotted employing HAuCl4 ·4H2 O and Ce(NO3 )3 ·6H2 O standard solutions, respectively. BET surface areas were acquired on a Beckman Coulter SA3100 surface area analyzer, in which the sample was degassed at 120 ◦ C for
3. Results 3.1. Macroproperties of catalysts The BET surface areas and the compositions of catalysts are summarized in Table 1. The BET surface area of bare SiO2 is 390 m2 /g. DP200, IWI200, and IWI400 are with similar BET surface areas but DP400 and DP600 are with smaller BET surface areas. All Au/CeO2 /SiO2 catalysts are with similar BET surface areas. By dissolving the catalysts in aqua regia, we measured the compositions of catalysts by ICP-AES. Au loadings in all Au/CeO2 /SiO2 catalysts are similar, much higher than the Au loading in Au/SiO2 [29] prepared by the same procedure. This indicates that the DP efficiency of Au is higher on CeO2 /SiO2 than on bare SiO2 . Similar results have also been observed in Au/CoOx /SiO2 and Au/ZnO/SiO2 catalysts [30,31]. During the course of DP, the pH value of HAuCl4 solution was adjusted to 9–10, forming the surface-reactive anionic gold hydroxide species AuCl(OH)3 − and Au(OH)4 − [32] that would efficiently deposit onto a positively charged oxide surface. Accordingly, oxides with a higher isoelectric point (IEP) will facilitate the deposition and precipitation of the Au precursor. The IEP value of CeO2 is much higher than that of SiO2 . Therefore, employing the same DP procedure, the loading of Au is higher in Au/CeO2 /SiO2 than in Au/SiO2 . The Ce concentrations were also measured but differed very much in various catalysts. The weight ratio of CeO2 in DP200 and DP400 are 5.75% and 5.42%, respectively, whereas that in DP600, IWI200, IWI400 decreased to as low as 1.61%, 0.31% and 0.18%, respectively. The calculated weight ratios of CeO2 are 5.66% and 5.55% in CeO2 /SiO2 and Au/CeO2 /SiO2 , respectively. However, when dissolving the catalysts in the mixture of aqua regia and H2 O2 , the obtained ICP-AES results give similar weight ratios of CeO2 in the
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Table 1 Macroproperties of catalysts. Catalyst
Weight ratios in the catalysts measured by ICP-AES a
DP200 DP400 DP600 IWI200 IWI400 CDP200 CDP400 CDP600 CIWI200 CIWI400 Au/SiO2 c
a
Au
CeO2
N.A. N.A. N.A. N.A. N.A. 2.2% 2.2% 2.2% 1.9% 2.1% 1.0%
5.75% 5.42% 1.61% 0.31% 0.18% 5.61% 5.77% 2.20% 0.28% 0.17% N.A.
CeO2
b
5.52% N.M. 5.47% 5.68% N.M. N.M. N.M. N.M. N.M. N.M. N.A.
Average crystalline size dXRD (nm) Au
CeO2
N.A N.A N.A N.A N.A 15 14 11 10 13 12
N.D. N.D. N.D. 5.6 7.3 N.D. N.D. 2.3 5.6 6.8 N.A.
BET surface area (m2 /g)
366 278 293 355 345 267 278 259 261 291 316
N.A.: not applicable; N.M.: not measured; N.D.: not detectable. a Dissolution in aqua regia. b Dissolution in H2 O2 and aqua regia. c From Ref. [29].
catalysts approaching the calculated values, proving the successful loading of Ce on SiO2 . Therefore, the different solubility of CeO2 supported on SiO2 in aqua regia should arise from their different microstructures. It has been reported that Ce(IV) could be reduced to Ce(III) by H2 O2 in the lower pH range, which could enhances the dissolution rate of CeO2 in aqua regia [33]. Therefore, the dissolution rate of supported CeO2 in aqua regia could be taken as an indication of the oxidation state of Ce. The faster the dissolution rate, the larger the fraction of Ce(III) and thus the concentration of oxygen vacancy in CeO2 . Since the Ce(III) fraction and the oxygen vacancy concentration in CeO2 play an important role in CO oxidation catalyzed by CeO2 , their characterizations are important. With this respect, our results provide a fast and convenient method. Fig. 1 shows XRD patterns of various catalysts. It could be seen that the preparation method greatly affects the dispersion of CeO2 on SiO2 . Strong diffraction peaks arising from the cubic fluoride CeO2 are present in the XRD patterns of IWI200 and IWI400. However, no diffraction peaks associated with CeO2 appears in the XRD patterns of DP200 and DP400, and very weak and diffuse diffraction peaks corresponding to CeO2 appear in the XRD pattern of DP600 due to the elevated calcination temperature. The XRD patterns of CeO2 in Au/CeO2 /SiO2 catalysts are similar to those in the corresponding CeO2 /SiO2 supports. The XRD peaks arising from metallic Au are clearly visible in the XRD patterns of all Au/CeO2 /SiO2 catalysts. The average crystalline sizes of Au and CeO2 in various catalysts were calculated based on the Scherrer equation, whose results are summarized in Table 1. The average crystalline size of Au nanoparticles in Au/CeO2 /SiO2 varies between 10 and 15 nm, similar to that of Au in Au/SiO2 prepared by the same procedure. 3.2. Catalytic activity of catalysts in CO oxidation Fig. 2 displays the CO conversion as a function of the reaction temperature over various catalysts. The CeO2 /SiO2 catalysts exhibit very poor catalytic performances. DP200, DP400, and DP600 do not show any activity in CO oxidation under the investigated temperatures. IWI400 and IWI200 become active above 300 and 270 ◦ C, respectively. The Au/CeO2 /SiO2 catalysts exhibit better catalytic performances than the Au/SiO2 catalyst, and it could be seen that the more active the CeO2 /SiO2 , the more active the corresponding Au/CeO2 /SiO2 . This demonstrates that addition of CeO2 to Au/SiO2 benefits CO oxidation. However, none of our Au/CeO2 /SiO2 catalysts is as active in low-temperature CO oxidation as previously reported Au/CeO2 catalysts [15,19–22]. CIWI200 with the best catalytic performance achieves a 50% CO conversion at 110 ◦ C (T50% )
Fig. 1. XRD patterns of (A) CeO2 /SiO2 and (B) Au/CeO2 /SiO2 . The peaks marked by the solid and dash lines arise from CeO2 and Au, respectively.
K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
43
Fig. 2. Catalytic performances of various catalysts in CO oxidation.
and a complete CO conversion at 210 ◦ C, and other catalysts achieve a complete CO oxidation at 270 ◦ C or above. Dekkers et al. [23] previously reported the promotion effect of CeO2 in both the reduction of NO with H2 and the oxidation of CO with O2 catalyzed by Au/SiO2 and Au/Al2 O3 catalysts, and the T50% of their Au/CeO2 /SiO2 catalyst was 115 ◦ C. This value is close to the T50% of our CIWI200 catalyst (T50% = 110 ◦ C). 3.3. Structures of catalysts Fig. 3 shows Ce 3d XPS spectra of various catalysts. The intensity of the Ce 3d XPS peak in IWI and CIWI catalysts is much weaker than that in DP and CDP catalysts. XPS is a surface sensitive technique. At a fixed total volume of particles, particles with a higher dispersion are of a larger surface-to-bulk atomic ratio and thus exhibit a stronger peak in the XPS spectrum. Therefore, the Ce 3d XPS results imply that CeO2 in DP and CDP catalysts is much more dispersive than that in IWI and CIWI catalysts, in consistence with XRD results. The Ce 3d XPS spectra show multiple states arising from different Ce 4f level occupancies in the final state [34]. We performed peakfitting of the Ce 3d XPS spectra in DP and CDP (Fig. 3A). The labels in Fig. 3A follow the convention established by Burroughs et al. [35], in which V and U refer to the 3d5/2 and 3d3/2 spin-orbital components, respectively. The V/U, V /U , and V /U components are features of
Ce(IV) and the V /U component is the feature of Ce(III) [36]. The Ce(III)/Ce(IV) atomic ratio in various catalysts that indicates the concentration of surface oxygen vacancies was also calculated by the peak area of Ce(III)/the total area of Ce(IV) peaks. These results are summarized in Table 2. The Ce(III)/Ce(IV) decreases in the DP catalysts with the increasing calcination temperature, implying that calcination at high temperatures in air annihilates surface oxygen vacancies in CeO2 supported on SiO2 . This is in consistence with the ICP-AES results. Interestingly, the Ce(III)/Ce(IV) atomic ratio in CDP catalysts is nearly as twice as that in the corresponding DP catalysts. Although the low intensity of Ce 3d XPS peaks of IWI and CIWI catalysts does not allow the peak-fitting, it could be also seen in Fig. 3B that the peak V in CIWI catalysts is stronger than that in the corresponding IWI catalysts, indicating a larger Ce(III)/Ce(IV) atomic ratio in CIWI catalysts. The Au 4f XPS spectra are displayed in Fig. 4. The Au 4f XPS spectra in all catalysts could be well fitted with two components with the Au 4f7/2 binding energy at ∼83.8 and ∼85.6 eV, which could be assigned to the metallic Au and Au(I) species. The Au(I)/Au atomic ratio in the catalysts was calculated by the peak area of Au(I)/the peak area of metallic Au. The peak-fitting results are summarized in Table 2. An interesting observation is that the Au(I)/Au ratio varies proportionally with the Ce(III)/Ce(IV) atomic ratio in the CDPseries Au/CeO2 /SiO2 catalysts, indicating that the formation of Au(I)
Fig. 3. Ce 3d XPS spectra of DP200, DP400, DP600, CDP200, CDP400, and CDP600 catalysts (A) and IWI200, IWI400, CIWI200, and CIWI400 catalysts (B).
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K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
Table 2 Peak-fitting results of Ce 3d and Au 4f XPS spectra in various catalysts. Catalyst
Ce 3d5/2 (eV)
Ce(III)/Ce(IV) atomic ratio
Ce(IV)
DP200 DP400 DP600 CDP200 CDP400 CDP600 CIWI200 CIWI400
Ce(III)
U
U
U
U
883.4 883.5 883.5 883.2 883.4 883.4 N.A. N.A.
888.5 888.9 888.8 888.8 888.8 888.7 N.A. N.A.
898.8 898.9 898.9 899.1 899.0 899.0 N.A. N.A.
886.3 886.5 886.4 886.3 886.4 886.4 N.A. N.A.
0.16 0.11 0.05 0.31 0.21 0.10 N.A. N.A.
Au 4f7/2 (eV)
Au(I)/atomic ratio
Metallic Au
Au(I)
N.A. N.A. N.A. 83.9 83.8 83.7 83.8 83.8
N.A. N.A. N.A. 85.7 85.6 85.5 85.5 85.5
N.A. N.A. N.A. 0.30 0.23 0.14 0.14 0.12
N.A.: not applicable.
Fig. 4. Au 4f XPS spectra of Au/CeO2 /SiO2 catalysts.
species is associated with the density of surface oxygen vacancies in CeO2 . Above results demonstrate that the Au loading, the average Au crystalline size, and the Au 4f binding energy are similar in CIWI200, CIWI400, and CDP600, but CIWI200 shows a much higher activity in CO oxidation than CIWI400 and CDP600. We thus investigated the morphology of CIWI200, CIWI400, and CDP600 in detail by TEM. Fig. 5 displays the representative TEM images. The TEM results (not shown) show that large CeO2 aggregates are present in CIWI200 and CIWI400, but absent in CDP600. This agrees with the XRD results that CeO2 in CDP600 are more dispersive than that in CIWI200 and CIWI400. We analyzed the size distribution of observed Au nanoparticles in TEM images (Fig. 6). The average size of Au nanoparticles is 9.7, 11.4 and 14.3 nm in CIWI200, CDP600and CIWI400, respectively. It can also be seen that among these catalysts CIWI200 has the highest density of Au nanoparticles with 5–7 nm sizes. Interestingly, the distribution of Au nanoparticles on CeO2 /SiO2 differs much in CIWI200, CIWI400, and CDP600. We analyzed the composition of the areas indicated by the circle in the TEM images by EDS, whose results is summarized in Table 3. In CIWI200, both TEM results and EDS results demonstrate that Au nanoparticles are dispersed on the CeO2 aggregates (area a in Fig. 5A) and the
Fig. 5. TEM images of CIWI200 (A and B), CIWI400 (C and D), and CDP600 (E and F).
K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47
45
Fig. 6. Size distribution of Au nanoparticles in CIWI200, CIWI400, and CDP600 estimated from the TEM results. Table 3 The elemental compositions in atomic ratio of each area calculated from EDS results. Element
CIWI200
CIWI400
Fig. 5A
O Si Ce Au
Area a
Area b
66.94 25.65 6.18 1.22
68.45 30.97 0.00 0.58
with their activity in CO oxidation, indicating that the reduction of CeO2 is involved in the catalytic CO oxidation.
CDP600
Fig. 5B
Fig. 5C
Fig. 5D
Fig. 5E
Fig. 5F
67.44 31.16 0.55 0.85
59.73 36.35 3.62 0.30
61.30 34.74 3.96 0.00
59.93 29.73 9.60 0.73
60.74 37.49 0.56 1.21
4. Discussion 4.1. Microstructures of CeO2
bare SiO2 surface (area b in Fig. 5A). However, in CIWI400, few Au nanoparticles are observed on the CeO2 aggregates by TEM (Fig. 5C and D), the weak Au signal on the CeO2 aggregate marked in Fig. 5C detected by EDS might arise from the Au(I) species. There is no CeO2 aggregate in CDP600, but few Au nanoparticles are observed on the area (Fig. 5E) with a high Ce concentration whereas the Ce concentration in the area (Fig. 5F) with abundant Au nanoparticles is low. These results indicate that the density of Au nanoparticles supported on CeO2 follows: CIWI200 > CDP600 > CIWI400. Fig. 7 shows the TPR results of selective catalysts. CeO2 in IWI200 is much more facile to be reduced than that in DP600. The addition of gold significantly improves the reducibility of CeO2 supported on SiO2 , in consistence with previous observations in Au/CeO2 catalysts [21]. Meanwhile, the reducibility of catalysts correlates well
Fig. 7. H2 -TPR profiles of indicated catalysts.
Due to the different preparation methods, CeO2 in various CeO2 /SiO2 exhibits different microstructures, such as the particle size and the surface oxygen vacancy concentration. CeO2 in DP catalysts is finer in the average particle size than that in IWI catalysts, and the particle size of CeO2 increases with the increasing calcination temperature. Meanwhile, the surface oxygen vacancy concentration of CeO2 in DP catalysts, as indicated by the Ce(III)/Ce(IV) atomic ratio, decreases with the increasing calcination temperature. CeO2 in IWI200 is with a better crystallinity and a larger average particle size than that in DP600, but CeO2 in IWI200 is much more facile to be reduced than that in DP600. All these results imply that the CeO2 –SiO2 interaction is stronger in DP catalysts than in IWI catalysts. This could be attributed to the different cerium species prior to the calcination, which is cerium hydroxide and Ce(NO3 )3 in DP and IWI catalysts, respectively. Calcination of cerium hydroxide supported on SiO2 leads to the formation of ultrafine CeO2 particles whereas calcination of Ce(NO3 )3 supported on SiO2 leads to the formation of large CeO2 aggregates. Very interestingly we observed the different solubility of CeO2 in various catalysts in aqua regia, which could be associated with the microstructures of supported CeO2 . Due to the disordered structure, CeO2 with a poor crystallinity might dissolve facilely in aqua regia. A dramatic change of the solubility of CeO2 was observed between CDP400 and CDP600 although CeO2 in CDP600 only displays very weak and diffuse diffraction peaks in the XRD spectrum. CeO2 in DP200 and DP400 dissolves in aqua regia much faster than CeO2 in DP600, IWI200 and IWI400. The complete dissolution of catalysts in H2 O2 and aqua regia shows that Ce contents in catalysts prepared by different methods are almost the same. These phenomena together with XPS results indicate more Ce(III) and oxygen vacancies exist in DP200 and DP400 than other catalysts since H2 O2 enhances the dissolution of CeO2 by reducing Ce(IV) to Ce(III). Various CeO2 /SiO2 catalysts exhibit different catalytic performances in CO oxidation. It is generally accepted that CO oxidation under stationary conditions over ceria follows the Marsvan Krevelen-type mechanism, where reaction involves alternate reduction and oxidation of the oxide surface with the formation of surface oxygen vacancies (as the key step) and their replenishment by gas-phase oxygen [14]. We believe that the strong CeO2 –SiO2 interaction in DP-series CeO2 /SiO2 catalysts blocks the reduction–oxidation cycle on CeO2 so that DP-series CeO2 /SiO2 catalysts do not show any activity in CO oxidation although they exhibit large densities of surface oxygen vacancies. It is most likely that the replenishment of surface oxygen vacancies by gas-phase oxygen is blocked because the surface oxygen vacancies in DP-series CeO2 /SiO2 are stabilized by the strong CeO2 –SiO2 interaction. With
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the reduced CeO2 –SiO2 interaction, CeO2 in IWI200 and IWI400 exhibits certain activities in CO oxidation, and IWI200 is more active that IWI400 due to the finer particle size of CeO2 . 4.2. Au–CeO2 interaction in Au/CeO2 /SiO2 catalysts An interesting observation is that the Ce(III)/Ce(IV) atomic ratio in supported CeO2 increases greatly when CeO2 /SiO2 catalysts are loaded with Au, which is independent of the microstructures of CeO2 . Since CeO2 /SiO2 was calcined prior to the loading of Au, this observation clearly demonstrates that the loading of gold on CeO2 /SiO2 facilitates the formation of surface oxygen vacancies in CeO2 , which could then be stabilized. TPR results also demonstrate that the addition of gold significantly improves the reducibility of CeO2 supported on SiO2 . Fu et al. firstly reported that the addition of gold improve the reducibility and the OSC of cerium oxide [16]. Pillai and Deevi also observed a similar result [21]. Shapovalov and Metiu performed a theoretical study of CO oxidation catalyzed by Aux Ce1−x O2 and found that vacancy formation in Aux Ce1−x O2 is exothermic, which suggests the oxygen atoms nearest and nextnearest the Au dopant are chemically active [37]. Our results evidence that the formation of Au(I) species is closely associated with Ce(III) sites, i.e., surface oxygen vacancy sites on CeO2 in CeO2 supported gold catalysts. The Au(I)/Au atomic ratio varies proportionally with the Ce(III)/Ce(IV) atomic ratio in Au/CeO2 /SiO2 whereas no Au(I) species was observed by XPS on Au/SiO2 prepared by the similar method [29]. Furthermore, the TEM and EDS results show that there is still considerable gold signals on CeO2 areas without Au nanoparticles, which could only arise from Au(I) species. These results suggest that surface oxygen vacancies and Au(I) species in Au/CeO2 /SiO2 interact so as to stabilize each other. The microstructure of CeO2 in CeO2 /SiO2 affects the distribution of Au nanoparticles. A surprising result is that Au nanoparticles supported on CeO2 are evident only in CIWI200, but not in CIWI400 and CDP600. This indicates that both Au(I) species and Au nanoparticles are formed on CeO2 in CIWI200 whereas Au(I) species dominates on CeO2 in CIWI400 and CDP600. The Ce(III)/Ce(IV) atomic ratio in DP catalysts demonstrates that increasing the calcination temperature reduces the surface oxygen vacancy concentrations on CeO2 , but the XPS results show that the Au(I)/Au atomic ratios are almost the same in CIWI200 and CIWI400. Therefore, the surface oxygen vacancies on CeO2 in IWI400 are mostly occupied by Au(I) species, and subsequently few Au nanoparticles are formed. We propose that Au(I) species and Au nanoparticles compete as products from the Au precursor on CeO2 . During the course of calcination, some of AuCl(OH)3 − and Au(OH)4 − is transformed into Au(I) stabilized on CeO2 , other forms Au clusters which migrate and aggregate on the surface. Au(I) species is stabilized by the surface oxygen vacancy site on CeO2 and Au nanoparticles also nucleate on the surface oxygen vacancy site on CeO2 . The available surface oxygen vacancy sites and their distribution on CeO2 , the surface oxygen vacanc–Au(I) interaction, and the surface oxygen vacancy–Au cluster interaction might cooperatively determine resulted Au species in the catalysts. Inferred from the results of the catalysts prepared by DP method, highly dispersed CeO2 in CeO2 /SiO2 facilitates the formation of Au(I) species. 4.3. Structure–activity relation of Au/CeO2 /SiO2 catalysts in CO oxidation No matter where Au nanoparticles distribute in the catalyst, all Au/CeO2 /SiO2 catalysts exhibit better catalytic performances in CO oxidation than Au/SiO2 with similar Au particle size. The promotion effect of CeO2 could be attributed to the contribution of CeO2 to the activation and supply of oxygen for the reaction, which has
been previously proposed [6] and spectroscopically observed [24]. The microstructure of Au/CeO2 /SiO2 catalysts strongly affects their catalytic activities, which offers an opportunity to understand their structure–activity relation in CO oxidation. CIWI200 with both Au(I) species and Au nanoparticles on CeO2 is much more active in CO oxidation than CIWI400 and CDP600 with Au(I) species dominating the CeO2 surface. This indicates that Au(I) species on CeO2 alone is not active in CO oxidation and the presence of Au nanoparticles on CeO2 is important, i.e., the Au nanoparticles–CeO2 interface plays an important role in the activity of Au/CeO2 /SiO2 catalysts in CO oxidation. Meanwhile, the activity of Au/CeO2 /SiO2 catalyst in CO oxidation could be correlated well the reducibility of CeO2 in Au/CeO2 /SiO2 and with the activity of the corresponding CeO2 /SiO2 catalyst in CO oxidation, indicating the important role of the ability of CeO2 to activate and supply oxygen. However, CIWI200 with the best performance does not show any activity in CO oxidation at room temperature, indicating the lack of the active sites for low-temperature CO oxidation. The ensemble in Au/CeO2 catalysts active for low-temperature CO oxidation has been proposed to consist of coexisting highly dispersed Au particles (2–4 nm) and Au(I)–OH species in intimate contact with the ceria surface defects [19–21], which lacks in CIWI200 because Au(I) species exists on CeO2 but 2–4 nm Au nanoparticles do not. Meanwhile, comparing unsupported CeO2 catalysts, the ability of CeO2 supported on SiO2 in CIWI200 for the activation and supply of oxygen is poor due to the CeO2 –SiO2 interaction as reflected by the catalytic performance of IWI200 in CO oxidation. It is under investigation in our lab how to engineer the microstructure of supported CeO2 for the fabrication of Au/CeO2 /SiO2 catalysts active in low-temperature CO oxidation. 5. Conclusions The influences of CeO2 microstructures on the structure and catalytic activity of supported Au nanoparticles in CO oxidation have been manifested in Au/CeO2 /SiO2 catalysts with various CeO2 microstructures. Au(I) species and Au nanoparticles compete for the surface oxygen vacancy sites on CeO2 . Highly dispersive CeO2 on SiO2 facilitates the formation of Au(I) species, and the presence of Au species also facilitates the creation and stabilization of surface oxygen vacancies on CeO2 . Au(I) species on CeO2 alone is not active in CO oxidation and Au nanoparticles in contact with CeO2 in Au/CeO2 /SiO2 catalysts are important in catalyzing CO oxidation reaction. Acknowledgements This work was financially supported by National Natural Science Foundation of China (grant 20773113), the “Hundred Talent Program” of Chinese Academy of Sciences, the MOE program for PCSIRT (IRT0756), and the MPG-CAS partner group program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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