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Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle
Catalysis Today 201 (2013) 62–67
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Oxidation of CO over Ru/Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle Atsushi Satsuma a,∗ , Masatoshi Yanagihara a , Junya Ohyama a , Kenichi Shimizu a,b a b
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
a r t i c l e
i n f o
Article history: Received 13 January 2012 Received in revised form 21 March 2012 Accepted 25 March 2012 Available online 12 April 2012 Keywords: Ru CeO2 Self-dispersion Physical mixing CO oxidation
a b s t r a c t The oxidation of CO was performed using physical mixture of Ru metal powder and various metal oxide supports after calcination in air and reduction in H2 . Among various supports (CeO2 , ZrO2 , MgO, Al2 O3 , TiO2 , SnO2 , and SiO2 ), only Ru + CeO2 showed very high catalytic activity for CO oxidation with the light-off temperature below 100 ◦ C, which was comparable to Ru/CeO2 prepared by a conventional impregnation method. The optimum conditions for the preparation were loading amount of 2 wt%, calcination at 700 ◦ C in air followed by reduction at 400 ◦ C in 5%H2 . It was revealed that the high catalytic activity for Ru + CeO2 is attributed to the high dispersion of Ru on CeO2 . The original Ru metal powder having diameter of 36 nm was dispersed during the calcination leading into small Ru particles having diameter around 2 nm. Raman spectra indicated that the formation of Ru O Ce bond after the calcination is one of the key for the Ru dispersion. The self-dispersion of Ru was not achieved by the calcination in N2 . It was proposed that the self-dispersion of Ru into nano-particles was caused by oxidation of Ru metal into Ru oxide in air, then dispersion of nano-sized Ru oxide by forming Ru O Ce bond, and finally conversion of Ru oxide particles into nano-sized Ru metal particles in a flow of H2 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction One of the serious problems of three-way catalyst (TWC) for automobile exhausts is the sintering of precious metals during the use of catalysts at higher temperatures, e.g., 800–1000 ◦ C. For the minimum use of precious metals, the design of a longlife three-way catalyst is strongly desired. One of the effective ways to suppress the sintering is self-dispersion of precious metals at higher temperatures caused by strong metal-support interaction. The “intelligent” catalyst is a typical example. Nishihata et al. demonstrated that LaFe0.57 Co0.38 Pd0.05 O3 showed excellent thermal-durability [1]. Pd reversibly moved into and out of the B-site of perovskite lattice under exhaust gas cycles between an oxidative and a reductive atmosphere, which results in the self-regeneration of Pd particles. Similar strong metalsupport interaction under high temperature oxidative atmosphere is observed on CeO2 -based supports [2–5]. Nagai et al. clarified that Pt particles on CZY (Ce–Zr–Y mixed oxide) do not sinter even after the treatment at 800 ◦ C in air [2,3]. The EXAFS studies revealed that the high thermal durability was caused by the formation of Pd O Ce bond as an anchor. In the case of supported Pd catalysts, the interaction between Pd and CeO2 -based supports with the formation of Pd O Ce bond is suggested to be a reason for
self-dispersion of Pd and high catalytic activity on NO + CH4 reaction [4] and methane oxidation [5]. CeO2 -based supports usually show very interesting behavior on metal-support interaction. As we have reported, a calcination of physical mixture of Ag metal powder (ca. 66 nm) and CeO2 at 500 ◦ C results in self-dispersion of Ag into nano-particles (ca. 8 nm) [6]. Oxygen vacancy on CeO2 surface is suggested to play an important role in the self-dispersion of Ag. The obtained catalyst showed high activity for soot oxidation, which was comparable to Ag/CeO2 prepared by impregnation method. Further investigation of the metal–CeO2 interaction under an oxidative atmosphere may open up effective strategy to design a long-life catalyst. In the present study, we report unique behavior of Ru on CeO2 . The self-dispersion of Ru large particles into highly dispersed Ru nano-particles results in the highly active catalyst for CO oxidation. Although Ru is not used as an active element of TWC because of volatile property and very low melting point of RuO4 , it will be a good model for a mechanistic study on the self-dispersion of precious metals on CeO2 -based supports. The key-role of the oxidation of metal particles and the formation of metal O Ce bond are discussed. 2. Experimental 2.1. Catalyst preparation
∗ Corresponding author. Tel.: +81 52 789 4608; fax: +81 52 789 3193. E-mail address: [email protected] (A. Satsuma). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.03.048
Ru(2) + MOx catalysts (Ru loading, 2 wt%) were prepared by a mechanical mixing of Ru powder (purchased from Mitsuwa, 99.9%)
A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
and metal oxides in a mortar for 10 min, followed by calcination at 700 ◦ C for 1 h in air. The catalyst name is designated as Ru(x) + CeO2 , where x is the Ru loading in wt%. The Ru content of the obtained catalysts was confirmed using Rigaku SEA2120 X-ray fluorescence (XRF) analyzer. The measured Ru content was 0.99 wt% for Ru(1) + CeO2 , 2.2 wt% for Ru(2) + CeO2 , and 4.1 wt% for Ru(4) + CeO2 , respectively. For a comparison, Ru(2)/CeO2 catalyst was also prepared by a conventional impregnation method using aqueous solution of Ru(C5 H7 O2 )3 (purchased from Alfa Aesar), followed by dryness at 80 ◦ C in a rotary evaporator and calcination in air at 700 ◦ C for 1 h. Pt(2)/Al2 O3 and Pt(2)/CeO2 were also prepared by impregnation of a nitric acid solution of Pt(NH3 )2 (NO2 )2 to Al2 O3 or CeO2 , followed by calcination in air at 700 ◦ C for 1 h. Metal oxide supports of CeO2 (JRC-CEO-1, 157 m2 g−1 ), ZrO2 (JRC-ZRO-1), TiO2 (JRC-TIO-4), and MgO (JRC-MGO-4) were supplied from Catalysis Society of Japan. SiO2 (Fuji Silysia Q-15) was purchased. Al2 O3 was obtained by calcination of ␥-AlOOH (Catapal B Alumina purchased from Sasol). 2.2. Characterization X-ray diffraction (XRD) patterns of the powdered catalysts were recorded with a Rigaku MiniFlex II/AP diffractometer with Cu K␣ radiation at 30 kV, 15 mA. Average metal particle size was estimated from a half-width of the diffraction at 44.0◦ in a XRD pattern using the Scherrer equation. The number of surface Ru atoms and the average particle size were estimated by the CO pulse adsorption method at −78 ◦ C [7] in a flow of He, on an assumption that CO was adsorbed on the surface of semi-spherical Ru particles at the CO/(surface Ru atom) ratio of 1/1. Raman spectra were measured by a JASCO RMP-330 spectrophotometer equipped with a 532 nm green semiconductor laser as an excitation source. The samples were filled in an in situ diffuse reflectance cell having a quartz window (1.0 mm) and a heating system inside connected with a gas flow system [8,9]. A ×20 objective optical lens was used to focus the depolarized laser beam on the sample surface and to collect the backscattered light. The backscattered light was dispersed by a single-stage spectrometer with an 1800-groove mm−1 grating and acquired by an air-cooled 1024 × 256 pixels CCD array detector. The Raman scattering was collected in the spectral region of 89.28–1324.29 cm−1 with a resolution of 1 cm−1 . The exposure time was 60 s for RuO2 and 10 s for the other samples, and 5 scans were accumulated for each spectrum. Before loading of the sample, a physical mixture of Ru + CeO2 was at first calcined in a furnace at 700 ◦ C. Then the sample was filled in the cell and measured at room temperature. The reduction of the sample was carried out in the in situ cell in a flow of 5%H2 /He
a
b Ru
200
Intensity
1000 Ru+CeO2 Ru+ZrO2
Ru+CeO2 Ru+ZrO2
Ru+MgO
Ru+MgO Ru+Al 2O3
Ru+Al 2O3 Ru+TiO 2 Ru+SnO2
Ru+TiO 2 Ru+SnO2 Ru+SiO2
30
35
2θ / degree
40
Ru+SiO2
45 43
44
45
2θ / degree
Fig. 1. XRD patterns of Ru(2) + MOx catalysts prepared by physical mixing followed by calcination in air at 700 ◦ C and reduction in 5%H2 at 400 ◦ C.
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Table 1 CO conversion at 100 ◦ C and Ru diameter on Ru(2) + MOx catalysts prepared by physical mixing followed by calcination at 700 ◦ C and reduction in 5%H2 at 400 ◦ C. Support (MOx)
CeO2 ZrO2 MgO Al2 O3 TiO2 SnO2 SiO2
Conversion/%
42 3.0 0.3 0.3 0.3 0.3 0.0
Ru diameter (nm) CO adsorption
XRD
2.0 8.2 – – – – –
– 17 31 32 29 21 24
at 400 ◦ C, then the spectrum was measured after cooling to room temperature in a flow of 5%H2 /He. Ru K-edge XAS measurement was carried out in a florescence mode with a Si(1 1 1) monochromator at BL01B1 of SPring-8 (Hyogo, Japan) operated at 8 GeV. The analyses of the extended Xray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were performed using the REX version 2.5 program (RIGAKU). The Fourier transformation of the k3 -weighted EXAFS from k space to R space was carried out over the k range 40–160 nm−1 . A part of the Fourier-transformed EXAFS in the R range of 1.1–3.0 A˚ were inversely Fourier-transformed, followed by the analysis with a usual curve fitting method in the k range of 40–160 nm−1 using empirical parameters for the phase shift and the amplitude functions for the Ru–Ru and Ru–O shells extracted from the data for Ru foil and RuO2 , respectively. 2.3. CO oxidation CO oxidation was carried out using a conventional fixed-bed flow reactor (inner diameter = 4 mm) at atmospheric pressure. A 10 mg of catalyst was at first reduced in a flow of 5%H2 /He at 400 ◦ C for 10 min. After the pretreatment, the catalytic run was carried out under a flow of 0.4%CO/10%O2 /He at a rate of 100 cm3 min−1 . The effluent gas was analyzed by nondispersive infrared (NDIR) CO2 analyzer (Horiba VIA510). The concentrations of CO and CO2 were analyzed during stepwise increase in the reaction temperature, and the steady state CO conversion was measured after 30 min for each temperature. 3. Results and discussion 3.1. Effect of supports on physically mixed catalysts Various metal oxide supports were physically mixed with Ru powder. After calcination in air at 700 ◦ C for 1 h and reduction in 5%H2 at 400 ◦ C for 10 min, the obtained Ru(2) + MOx catalysts were characterized by XRD and the catalytic tests for CO oxidation were carried out. Fig. 1 shows XRD patterns of the Ru(2) + MOx catalysts. Except the diffraction lines of metal oxide supports, the diffraction line attributed to (1 0 1) of hexagonal Ru was observed at 2 = 44.0◦ . In Fig. 1b, the patterns in the range of 43–45◦ were magnified. Although the diffraction line of Ru metal is clear on MgO, Al2 O3 , TiO2 , SnO2 , and SiO2 supports, that is weak on ZrO2 and not clear on CeO2 . From the width of the line at 2 = 44◦ , the particle size of Ru was estimated using the Scherrer equation and listed in Table 1. Since ZrO2 and CeO2 supported catalysts showed weak or negligible diffraction lines of Ru, the particle sizes of Ru were estimated from CO adsorption. On MgO, Al2 O3 , TiO2 , SnO2 , and SiO2 supports, the particle sizes of Ru were in the range of 24–32 nm, which was rather smaller than that of original Ru metal powder (36 nm from XRD pattern). On the other hand, Ru particle size became significantly smaller on CeO2 . The difference in the particle sizes of Ru on ZrO2 estimated by XRD and CO adsorption may be due to wide
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A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
60
50
80
Conversion of CO into CO2 / %
Conversion of CO into CO 2 / %
100
60
40
20
Reduction: variable o Calcination at 700 C
Calcination: variable o Reduction at 400 C
40
30
20
10
0 0
1
2
3
4
0
distribution of the particle sizes, i.e., CO adsorption mainly reflected the contribution of small Ru particles while the line width in XRD pattern mainly dependent on the contribution of larger Ru particles. Table 1 also shows the CO conversion at 100 ◦ C over various Ru(2) + MOx catalysts. When CeO2 was used as a support, very high CO conversion (42%) was obtained. Ru(2) + ZrO2 showed low activity. On the other hand, other Ru + MOx catalysts were almost inactive. Comparing with the size of Ru and the CO conversion in the table, the high activity of Ru + CeO2 can be attributed to the high dispersion of Ru. Before the physical mixing, the average particle diameter of Ru was 36 nm. The sizes of Ru were not significantly changed on most of the supports, however, the diameter of Ru decreased significantly to 2.0 nm on CeO2 and slightly to 8.2–17 nm on ZrO2 . As the decrease in the Ru diameter, the CO conversion increased to 42% over CeO2 and to 3% over ZrO2 . The results suggest very high mobility of Ru particles on CeO2 during the calcination, which resulted in the high dispersion of Ru. 3.2. Effect of preparation conditions for Ru + CeO2 The optimum preparation conditions for the preparation of Ru + CeO2 were examined. As shown in Fig. 2, the CO conversion significantly increased with loading amount of Ru up to 1 wt%, the maximum CO conversion was observed at 2 wt% of Ru loading, and then the conversion slightly decreased with further loading of Ru. The effects of calcination and reduction temperatures are shown in Fig. 3. The activity for CO oxidation was strongly dependent on the calcination temperature. The CO conversion steeply increased above 500 ◦ C, but the calcination above 800 ◦ C suppressed the catalytic activity. After the calcination of Ru(2) + CeO2 at 700 ◦ C, the conversion of CO was 17% without reduction, while the reduction in 5%H2 /He at 400 ◦ C increased the CO conversion above 40%. Hereafter, the catalysts were prepared according to the following optimum conditions: Ru loading 2 wt%, the calcination temperature 700 ◦ C in air for 1 h, and the reduction in 5%H2 /He at 400 ◦ C for 10 min. In Fig. 4, the conversions of CO over various catalysts were compared as a function of reaction temperature. The light-off temperature of Ru(2) + CeO2 was below 100 ◦ C, and the CO conversion reached to 100% around 180 ◦ C, which was almost comparable to Pt(2)/CeO2 and Ru(2)/CeO2 prepared by conventional impregnation method. Comparing with Pt(2)/Al2 O3 as a typical oxidation catalyst, Ru(2) + CeO2 catalyst converted CO around 50 ◦ C lower temperatures. The slightly lower activity of Ru(2) + CeO2 than Ru(2)/CeO2
400
600
800
1000
Temperature / C Fig. 3. Effects of (䊉) calcination temperature in air () and reduction temperature in 5%H2 on CO conversion over Ru + CeO2 at 100 ◦ C.
should be due to the difference in the dispersion of Ru particles. The dispersion of Ru was 85% for Ru/CeO2 prepared by impregnation method and that was 56% for Ru(2) + CeO2 prepared by physical mixing. However, it is surprising that the physical mixing and calcination of Ru and CeO2 resulted in an active catalyst comparable to Ru(2)/CeO2 prepared by impregnation method. It was suggested that the high activity of Ru(2) + CeO2 is due to the relatively high dispersion of Ru particles on CeO2 surface. In order to understand the mechanism of the self-dispersion of Ru into small particles during physical mixing, calcination, or reduction, the structure of Ru(2) + CeO2 and its precursors are characterized in the next section. 3.3. Mechanism of self-dispersion of Ru on CeO2 Fig. 5 shows the influence of oxygen concentration in the calcination atmosphere on the CO oxidation activity over the obtained catalysts. The effect of atmosphere during calcination was examined. The calcination of Ru(2) + CeO2 in air resulted in the very active catalyst for the CO oxidation, however, the catalytic activity was far
100
Conversion of CO into CO2 / %
Fig. 2. Effect of Ru loading of Ru + CeO2 on CO conversion at 100 ◦ C.
200
o
Loading amount of Ru / %
80
60
40
20
0 0
100
200
300
o
Temperature / C Fig. 4. Conversion of CO into CO2 over (䊉) Ru(2) + CeO2 , () Ru(2)/CeO2 , () Pt(2)/CeO2 , and () Pt(2)/Al2 O3 as a function of temperature.
A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
65
100
60
in N2
Ru+CeO2 in air at 700oC
40
FT [k3χ(k)]
Conversion of CO into CO 2 / %
Ru+CeO 2 o in H 2 400 C
10
in air
80
20
0 0
100
200
300
Ru
400
o
Temperature / C Fig. 5. Conversion of CO into CO2 over Ru(2) + CeO2 obtained by calcination at 700 ◦ C for 1 h (䊉) in air and () in N2 . The catalysts were reduced in H2 at 400 ◦ C for 10 min before the activity measurement.
RuO2 lower after the calcination in pure N2 than that calcined in air. The particle size of Ru after the calcination in pure N2 was 18 nm, as estimated by the XRD diffraction line (figure not shown). The results clearly indicate that the calcination under the presence of oxygen is essential for the self-dispersion Ru into nano-sized particles. That means that the oxidation of Ru particles plays an important role in the self-dispersion process. Then, the structure of the Ru(2) + CeO2 catalyst after the calcination and the reduction was characterized by means of XAFS and Raman spectroscopy. Fig. 6 shows the Fourier transforms of the k3 -weighted Ru Kedge EXAFS of Ru(2) + CeO2 after the calcination at 700 ◦ C and reduction at 400 ◦ C. The FTs are not corrected for phase shift. The spectra of Ru(2) + CeO2 showed the scattering from the nearest O and Ru atoms. Although we tried the curve-fitting analysis on the inverse Fourier-transform, the presence of Ru O Ce or Ru O Ru was unclear. The data of curve-fitting analysis of Ru K-edge EXAFS are summarized in Table 2. The coordination numbers of the Ru–Ru shell were 4.79 after the calcination and 7.23 after the reduction. The increase in the coordination number of Ru–Ru shell indicates the increase in the contribution of Ru metal after the reduction. Furthermore, the significant contribution of the Ru–O shell even after the reduction in H2 means that the Ru particle on CeO2 is still partly oxidized. Fig. 7 shows Ru K-edge XANES spectra of Ru(2) + CeO2 after calcination at 700 ◦ C and reduction at 400 ◦ C. The spectra of Ru metal powder and RuO2 were also shown as reference materials. As shown in the bold lines, the obtained spectra of Ru(2) + CeO2 can be represented by the combination of the spectra of Ru metal and RuO2 . After the calcination, the obtained spectrum can be represented by the combination of 53% of Ru metal and 47% of RuO2 . Table 2 Curve-fitting analysis of Ru K-edge EXAFS data for Ru(2) + CeO2 . Shell
CNa
˚ Rb (A)
˚ c (A)
Rf d (%)
◦
Calcination at 700 C
Ru O
4.79 2.45
2.68 1.96
0.071 0.076
2.57
Reduction at 400 ◦ C
Ru O
7.23 2.00
2.67 2.00
0.073 0.100
1.53
Treatment
a b c d
Coordination numbers. Bond distance. Debye–Waller factor. Residual factor.
0
1
2
3
4
5
R/ Å Fig. 6. Fourier transforms of Ru K-edge EXAFS spectra of Ru(2) + CeO2 and reference samples (Ru foil and RuO2 ).
After the reduction in H2 , the contribution of Ru metal increased, but 21% of Ru is remained oxidized. The result is qualitatively in harmony with the EXAFS spectra because the both of the samples showed both Ru–O and Ru–Ru shell, and the coordination number of Ru–O shell was reduced after the reduction in H2 . From the ratio of Ru metal and RuO2 estimated from the XANES spectra, images of the Ru particles can be drawn. The contribution of oxidized Ru should be mainly on the interface of Ru and CeO2 support. On the assumption of hemispherical Ru particle with diameter of 2 nm, the contribution of interface layer between Ru and CeO2 is 40%. The ratio of RuO2 in XANES spectrum of Ru(2) + CeO2 after the reduction (21%) suggests that a half of the interface layer is oxidized. In the case of Ru(2) + CeO2 after the calcination, the ratio of RuO2 in XANES spectrum was 47%, which exceeded the contribution of interface layer. The difference amount of RuO2 before and after the reduction (25%) corresponds well with the external surface of Ru hemisphere (26%), suggesting that the reduction at 400 ◦ C results in the reduction of Ru oxide on the external surface of Ru particle but not the interface between Ru and CeO2 support. Taking the composition of Ru metal into account, the particle size of metallic Ru can be estimated as follows. After the reduction of Ru(2) + CeO2 , the coordination number of Ru–Ru shell was 7.23. Taking 78.9% contribution of metallic Ru in this sample, coordination number of Ru–Ru shell in Ru metal particles can be estimated as 7.23/0.789 = 9.16. On the assumption of cubic Ru particles, Ru particle is composed of ca. 250 Ru atoms with side Ru number of 6.30 [10]. Based on the Ru atom diameter of 0.268 nm, the diameter of Ru particle can be estimated as 1.69 nm. Although the Ru diameter was roughly estimated from the XAFS data (1.7 nm), this value fairly corresponds with that measured by CO adsorption (2.0 nm). The detailed change of the Ru structure during calcination was also evaluated by Raman spectra, as shown in Fig. 8. CeO2 shows the bands at 258, 460, and 595 cm−1 , which are assignable to
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A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
Measured (solid) Simulated (broken)
a
b Measured (solid) Simulated (broken)
Ru x 0.527
RuO 2 x 0.473
Normalized absorption
0.1
Normalized absorption
0.1
Ru x 0.789
RuO 2 x 0.211
22100
22150
22200
Photon energy (eV)
22100
22150
22200
Photon energy (eV)
230
690
500
963
Intensity / arb.
450
Fig. 7. Ru K-edge XANES spectra of Ru(2) + CeO2 (a) after calcination at 700 ◦ C in air and (b) followed by reduction in 5%H2 at 400 ◦ C. Simulated spectra were synthesized by the sum of the spectra of pure Ru metal and pure RuO2 with appropriate ratios.
(a) air at 700oC (b) N 2 at 700oC x 1/40 (c) (a) followed by 5%H2 at 400oC (d) CeO2 x 1/20 (e) RuO2 x3
1000
500
Raman shift / cm -1 Fig. 8. Raman spectra of Ru(2) + CeO2 after various treatments and pure CeO2 and RuO2 as reference samples.
2nd-order transverse acoustic mode, F2g mode of CeO2 fluorite phase, and defect-induced mode, respectively [11–16]. As for pure RuO2 , weak bands were observed at 522, 635, and 703 cm−1 assignable to Eg , A1g , and B2g modes of RuO2 respectively [17]. However, since the band intensities are very low compared to the bands assignable to CeO2 , these RuO2 bands are not detectable in the spectra of Ru(2) + CeO2 . Only after the calcination in air, two new bands were observed at 690 and 963 cm−1 . Since the Ru(2) + CeO2 after the reduction did not give any bands above 600 cm−1 , the contribution of reduced cerium oxides to these new bands can be ruled out. The new bands cannot be attributed to other ruthenium oxides having various oxidation states (800 cm−1 for RuO3 , 822–881 cm−1 for RuO4 , 808 cm−1 for RuO4 2− , 380–440 and 590 cm−1 for hydrated RuO2 ) [18–20]. Therefore, the new bands are due to the interaction between Ru and CeO2 in highly oxidized state, and tentatively assigned to Ru O Ce bond, though any mixed oxide phase of Ru and Ce have not been reported. As a similar phenomenon, Lin et al. detected the formation of Pt O Ce bond by Raman spectroscopy [21]. The formation of Ru O Ce bond and its detection is probable. The formation of the Ru O Ce bond under O2 indicates that a
Fig. 9. Schematic model of self-dispersion of Ru on CeO2 support.
A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
driving force of the self-dispersion is the oxidation of Ru and strong interaction between ruthenium oxide and CeO2 . From the obtained results, the self-dispersion process of Ru on CeO2 support can be illustrated as shown in Fig. 9. The calcination of physical mixture of Ru and CeO2 powders results in the oxidation of Ru, which may cause by the effect of CeO2 as an oxidation catalyst. In the case of Ag + CeO2 catalyst, the oxidation of metal particles was already suggested to be an important factor for the self-dispersion, however, a driving force of dispersion of oxidized species was not clarified. In this study, Raman spectra indicated that Ru species is dispersed with the formation of Ru O Ce bond on the surface. After the calcination, both the external surface and interface layer of Ru particle are oxidized. After the reduction in H2 , the external surface is reduced to Ru metal surface and an active catalyst is obtained. 4. Conclusion A physical mixture of Ru metal powder and CeO2 shows high activity for CO oxidation after calcination in air at 700 ◦ C and the reduction in H2 at 400 ◦ C. The activity is comparable to Ru/CeO2 catalyst prepared by a conventional impregnation method. The high activity for Ru + CeO2 is caused by self-dispersion of Ru into nanoparticle during the calcination. The key for the self-dispersion is the oxidation of Ru particle and the formation of Ru O Ce bond. The self-dispersion of Ru is achieved through (1) oxidation of Ru metal into RuO2 during calcination in air, (2) dispersion of nano-sized and oxidized Ru particle onto CeO2 surface with the formation of Ru O Ce bond, (3) the reduction of external surface of Ru particle in a flow of H2 at relatively low temperature results in an active catalysts for CO oxidation. Acknowledgements This study was partly supported by Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Kakenhi) and by a project of the New Energy and Industrial
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Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Oxidation of CO over Ru/Ceria prepared by self-dispersion of Ru metal powder into nano-sized particle Atsushi Satsuma a,∗ , Masatoshi Yanagihara a , Junya Ohyama a , Kenichi Shimizu a,b a b
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
a r t i c l e
i n f o
Article history: Received 13 January 2012 Received in revised form 21 March 2012 Accepted 25 March 2012 Available online 12 April 2012 Keywords: Ru CeO2 Self-dispersion Physical mixing CO oxidation
a b s t r a c t The oxidation of CO was performed using physical mixture of Ru metal powder and various metal oxide supports after calcination in air and reduction in H2 . Among various supports (CeO2 , ZrO2 , MgO, Al2 O3 , TiO2 , SnO2 , and SiO2 ), only Ru + CeO2 showed very high catalytic activity for CO oxidation with the light-off temperature below 100 ◦ C, which was comparable to Ru/CeO2 prepared by a conventional impregnation method. The optimum conditions for the preparation were loading amount of 2 wt%, calcination at 700 ◦ C in air followed by reduction at 400 ◦ C in 5%H2 . It was revealed that the high catalytic activity for Ru + CeO2 is attributed to the high dispersion of Ru on CeO2 . The original Ru metal powder having diameter of 36 nm was dispersed during the calcination leading into small Ru particles having diameter around 2 nm. Raman spectra indicated that the formation of Ru O Ce bond after the calcination is one of the key for the Ru dispersion. The self-dispersion of Ru was not achieved by the calcination in N2 . It was proposed that the self-dispersion of Ru into nano-particles was caused by oxidation of Ru metal into Ru oxide in air, then dispersion of nano-sized Ru oxide by forming Ru O Ce bond, and finally conversion of Ru oxide particles into nano-sized Ru metal particles in a flow of H2 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction One of the serious problems of three-way catalyst (TWC) for automobile exhausts is the sintering of precious metals during the use of catalysts at higher temperatures, e.g., 800–1000 ◦ C. For the minimum use of precious metals, the design of a longlife three-way catalyst is strongly desired. One of the effective ways to suppress the sintering is self-dispersion of precious metals at higher temperatures caused by strong metal-support interaction. The “intelligent” catalyst is a typical example. Nishihata et al. demonstrated that LaFe0.57 Co0.38 Pd0.05 O3 showed excellent thermal-durability [1]. Pd reversibly moved into and out of the B-site of perovskite lattice under exhaust gas cycles between an oxidative and a reductive atmosphere, which results in the self-regeneration of Pd particles. Similar strong metalsupport interaction under high temperature oxidative atmosphere is observed on CeO2 -based supports [2–5]. Nagai et al. clarified that Pt particles on CZY (Ce–Zr–Y mixed oxide) do not sinter even after the treatment at 800 ◦ C in air [2,3]. The EXAFS studies revealed that the high thermal durability was caused by the formation of Pd O Ce bond as an anchor. In the case of supported Pd catalysts, the interaction between Pd and CeO2 -based supports with the formation of Pd O Ce bond is suggested to be a reason for
self-dispersion of Pd and high catalytic activity on NO + CH4 reaction [4] and methane oxidation [5]. CeO2 -based supports usually show very interesting behavior on metal-support interaction. As we have reported, a calcination of physical mixture of Ag metal powder (ca. 66 nm) and CeO2 at 500 ◦ C results in self-dispersion of Ag into nano-particles (ca. 8 nm) [6]. Oxygen vacancy on CeO2 surface is suggested to play an important role in the self-dispersion of Ag. The obtained catalyst showed high activity for soot oxidation, which was comparable to Ag/CeO2 prepared by impregnation method. Further investigation of the metal–CeO2 interaction under an oxidative atmosphere may open up effective strategy to design a long-life catalyst. In the present study, we report unique behavior of Ru on CeO2 . The self-dispersion of Ru large particles into highly dispersed Ru nano-particles results in the highly active catalyst for CO oxidation. Although Ru is not used as an active element of TWC because of volatile property and very low melting point of RuO4 , it will be a good model for a mechanistic study on the self-dispersion of precious metals on CeO2 -based supports. The key-role of the oxidation of metal particles and the formation of metal O Ce bond are discussed. 2. Experimental 2.1. Catalyst preparation
∗ Corresponding author. Tel.: +81 52 789 4608; fax: +81 52 789 3193. E-mail address: [email protected] (A. Satsuma). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.03.048
Ru(2) + MOx catalysts (Ru loading, 2 wt%) were prepared by a mechanical mixing of Ru powder (purchased from Mitsuwa, 99.9%)
A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
and metal oxides in a mortar for 10 min, followed by calcination at 700 ◦ C for 1 h in air. The catalyst name is designated as Ru(x) + CeO2 , where x is the Ru loading in wt%. The Ru content of the obtained catalysts was confirmed using Rigaku SEA2120 X-ray fluorescence (XRF) analyzer. The measured Ru content was 0.99 wt% for Ru(1) + CeO2 , 2.2 wt% for Ru(2) + CeO2 , and 4.1 wt% for Ru(4) + CeO2 , respectively. For a comparison, Ru(2)/CeO2 catalyst was also prepared by a conventional impregnation method using aqueous solution of Ru(C5 H7 O2 )3 (purchased from Alfa Aesar), followed by dryness at 80 ◦ C in a rotary evaporator and calcination in air at 700 ◦ C for 1 h. Pt(2)/Al2 O3 and Pt(2)/CeO2 were also prepared by impregnation of a nitric acid solution of Pt(NH3 )2 (NO2 )2 to Al2 O3 or CeO2 , followed by calcination in air at 700 ◦ C for 1 h. Metal oxide supports of CeO2 (JRC-CEO-1, 157 m2 g−1 ), ZrO2 (JRC-ZRO-1), TiO2 (JRC-TIO-4), and MgO (JRC-MGO-4) were supplied from Catalysis Society of Japan. SiO2 (Fuji Silysia Q-15) was purchased. Al2 O3 was obtained by calcination of ␥-AlOOH (Catapal B Alumina purchased from Sasol). 2.2. Characterization X-ray diffraction (XRD) patterns of the powdered catalysts were recorded with a Rigaku MiniFlex II/AP diffractometer with Cu K␣ radiation at 30 kV, 15 mA. Average metal particle size was estimated from a half-width of the diffraction at 44.0◦ in a XRD pattern using the Scherrer equation. The number of surface Ru atoms and the average particle size were estimated by the CO pulse adsorption method at −78 ◦ C [7] in a flow of He, on an assumption that CO was adsorbed on the surface of semi-spherical Ru particles at the CO/(surface Ru atom) ratio of 1/1. Raman spectra were measured by a JASCO RMP-330 spectrophotometer equipped with a 532 nm green semiconductor laser as an excitation source. The samples were filled in an in situ diffuse reflectance cell having a quartz window (1.0 mm) and a heating system inside connected with a gas flow system [8,9]. A ×20 objective optical lens was used to focus the depolarized laser beam on the sample surface and to collect the backscattered light. The backscattered light was dispersed by a single-stage spectrometer with an 1800-groove mm−1 grating and acquired by an air-cooled 1024 × 256 pixels CCD array detector. The Raman scattering was collected in the spectral region of 89.28–1324.29 cm−1 with a resolution of 1 cm−1 . The exposure time was 60 s for RuO2 and 10 s for the other samples, and 5 scans were accumulated for each spectrum. Before loading of the sample, a physical mixture of Ru + CeO2 was at first calcined in a furnace at 700 ◦ C. Then the sample was filled in the cell and measured at room temperature. The reduction of the sample was carried out in the in situ cell in a flow of 5%H2 /He
a
b Ru
200
Intensity
1000 Ru+CeO2 Ru+ZrO2
Ru+CeO2 Ru+ZrO2
Ru+MgO
Ru+MgO Ru+Al 2O3
Ru+Al 2O3 Ru+TiO 2 Ru+SnO2
Ru+TiO 2 Ru+SnO2 Ru+SiO2
30
35
2θ / degree
40
Ru+SiO2
45 43
44
45
2θ / degree
Fig. 1. XRD patterns of Ru(2) + MOx catalysts prepared by physical mixing followed by calcination in air at 700 ◦ C and reduction in 5%H2 at 400 ◦ C.
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Table 1 CO conversion at 100 ◦ C and Ru diameter on Ru(2) + MOx catalysts prepared by physical mixing followed by calcination at 700 ◦ C and reduction in 5%H2 at 400 ◦ C. Support (MOx)
CeO2 ZrO2 MgO Al2 O3 TiO2 SnO2 SiO2
Conversion/%
42 3.0 0.3 0.3 0.3 0.3 0.0
Ru diameter (nm) CO adsorption
XRD
2.0 8.2 – – – – –
– 17 31 32 29 21 24
at 400 ◦ C, then the spectrum was measured after cooling to room temperature in a flow of 5%H2 /He. Ru K-edge XAS measurement was carried out in a florescence mode with a Si(1 1 1) monochromator at BL01B1 of SPring-8 (Hyogo, Japan) operated at 8 GeV. The analyses of the extended Xray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were performed using the REX version 2.5 program (RIGAKU). The Fourier transformation of the k3 -weighted EXAFS from k space to R space was carried out over the k range 40–160 nm−1 . A part of the Fourier-transformed EXAFS in the R range of 1.1–3.0 A˚ were inversely Fourier-transformed, followed by the analysis with a usual curve fitting method in the k range of 40–160 nm−1 using empirical parameters for the phase shift and the amplitude functions for the Ru–Ru and Ru–O shells extracted from the data for Ru foil and RuO2 , respectively. 2.3. CO oxidation CO oxidation was carried out using a conventional fixed-bed flow reactor (inner diameter = 4 mm) at atmospheric pressure. A 10 mg of catalyst was at first reduced in a flow of 5%H2 /He at 400 ◦ C for 10 min. After the pretreatment, the catalytic run was carried out under a flow of 0.4%CO/10%O2 /He at a rate of 100 cm3 min−1 . The effluent gas was analyzed by nondispersive infrared (NDIR) CO2 analyzer (Horiba VIA510). The concentrations of CO and CO2 were analyzed during stepwise increase in the reaction temperature, and the steady state CO conversion was measured after 30 min for each temperature. 3. Results and discussion 3.1. Effect of supports on physically mixed catalysts Various metal oxide supports were physically mixed with Ru powder. After calcination in air at 700 ◦ C for 1 h and reduction in 5%H2 at 400 ◦ C for 10 min, the obtained Ru(2) + MOx catalysts were characterized by XRD and the catalytic tests for CO oxidation were carried out. Fig. 1 shows XRD patterns of the Ru(2) + MOx catalysts. Except the diffraction lines of metal oxide supports, the diffraction line attributed to (1 0 1) of hexagonal Ru was observed at 2 = 44.0◦ . In Fig. 1b, the patterns in the range of 43–45◦ were magnified. Although the diffraction line of Ru metal is clear on MgO, Al2 O3 , TiO2 , SnO2 , and SiO2 supports, that is weak on ZrO2 and not clear on CeO2 . From the width of the line at 2 = 44◦ , the particle size of Ru was estimated using the Scherrer equation and listed in Table 1. Since ZrO2 and CeO2 supported catalysts showed weak or negligible diffraction lines of Ru, the particle sizes of Ru were estimated from CO adsorption. On MgO, Al2 O3 , TiO2 , SnO2 , and SiO2 supports, the particle sizes of Ru were in the range of 24–32 nm, which was rather smaller than that of original Ru metal powder (36 nm from XRD pattern). On the other hand, Ru particle size became significantly smaller on CeO2 . The difference in the particle sizes of Ru on ZrO2 estimated by XRD and CO adsorption may be due to wide
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60
50
80
Conversion of CO into CO2 / %
Conversion of CO into CO 2 / %
100
60
40
20
Reduction: variable o Calcination at 700 C
Calcination: variable o Reduction at 400 C
40
30
20
10
0 0
1
2
3
4
0
distribution of the particle sizes, i.e., CO adsorption mainly reflected the contribution of small Ru particles while the line width in XRD pattern mainly dependent on the contribution of larger Ru particles. Table 1 also shows the CO conversion at 100 ◦ C over various Ru(2) + MOx catalysts. When CeO2 was used as a support, very high CO conversion (42%) was obtained. Ru(2) + ZrO2 showed low activity. On the other hand, other Ru + MOx catalysts were almost inactive. Comparing with the size of Ru and the CO conversion in the table, the high activity of Ru + CeO2 can be attributed to the high dispersion of Ru. Before the physical mixing, the average particle diameter of Ru was 36 nm. The sizes of Ru were not significantly changed on most of the supports, however, the diameter of Ru decreased significantly to 2.0 nm on CeO2 and slightly to 8.2–17 nm on ZrO2 . As the decrease in the Ru diameter, the CO conversion increased to 42% over CeO2 and to 3% over ZrO2 . The results suggest very high mobility of Ru particles on CeO2 during the calcination, which resulted in the high dispersion of Ru. 3.2. Effect of preparation conditions for Ru + CeO2 The optimum preparation conditions for the preparation of Ru + CeO2 were examined. As shown in Fig. 2, the CO conversion significantly increased with loading amount of Ru up to 1 wt%, the maximum CO conversion was observed at 2 wt% of Ru loading, and then the conversion slightly decreased with further loading of Ru. The effects of calcination and reduction temperatures are shown in Fig. 3. The activity for CO oxidation was strongly dependent on the calcination temperature. The CO conversion steeply increased above 500 ◦ C, but the calcination above 800 ◦ C suppressed the catalytic activity. After the calcination of Ru(2) + CeO2 at 700 ◦ C, the conversion of CO was 17% without reduction, while the reduction in 5%H2 /He at 400 ◦ C increased the CO conversion above 40%. Hereafter, the catalysts were prepared according to the following optimum conditions: Ru loading 2 wt%, the calcination temperature 700 ◦ C in air for 1 h, and the reduction in 5%H2 /He at 400 ◦ C for 10 min. In Fig. 4, the conversions of CO over various catalysts were compared as a function of reaction temperature. The light-off temperature of Ru(2) + CeO2 was below 100 ◦ C, and the CO conversion reached to 100% around 180 ◦ C, which was almost comparable to Pt(2)/CeO2 and Ru(2)/CeO2 prepared by conventional impregnation method. Comparing with Pt(2)/Al2 O3 as a typical oxidation catalyst, Ru(2) + CeO2 catalyst converted CO around 50 ◦ C lower temperatures. The slightly lower activity of Ru(2) + CeO2 than Ru(2)/CeO2
400
600
800
1000
Temperature / C Fig. 3. Effects of (䊉) calcination temperature in air () and reduction temperature in 5%H2 on CO conversion over Ru + CeO2 at 100 ◦ C.
should be due to the difference in the dispersion of Ru particles. The dispersion of Ru was 85% for Ru/CeO2 prepared by impregnation method and that was 56% for Ru(2) + CeO2 prepared by physical mixing. However, it is surprising that the physical mixing and calcination of Ru and CeO2 resulted in an active catalyst comparable to Ru(2)/CeO2 prepared by impregnation method. It was suggested that the high activity of Ru(2) + CeO2 is due to the relatively high dispersion of Ru particles on CeO2 surface. In order to understand the mechanism of the self-dispersion of Ru into small particles during physical mixing, calcination, or reduction, the structure of Ru(2) + CeO2 and its precursors are characterized in the next section. 3.3. Mechanism of self-dispersion of Ru on CeO2 Fig. 5 shows the influence of oxygen concentration in the calcination atmosphere on the CO oxidation activity over the obtained catalysts. The effect of atmosphere during calcination was examined. The calcination of Ru(2) + CeO2 in air resulted in the very active catalyst for the CO oxidation, however, the catalytic activity was far
100
Conversion of CO into CO2 / %
Fig. 2. Effect of Ru loading of Ru + CeO2 on CO conversion at 100 ◦ C.
200
o
Loading amount of Ru / %
80
60
40
20
0 0
100
200
300
o
Temperature / C Fig. 4. Conversion of CO into CO2 over (䊉) Ru(2) + CeO2 , () Ru(2)/CeO2 , () Pt(2)/CeO2 , and () Pt(2)/Al2 O3 as a function of temperature.
A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
65
100
60
in N2
Ru+CeO2 in air at 700oC
40
FT [k3χ(k)]
Conversion of CO into CO 2 / %
Ru+CeO 2 o in H 2 400 C
10
in air
80
20
0 0
100
200
300
Ru
400
o
Temperature / C Fig. 5. Conversion of CO into CO2 over Ru(2) + CeO2 obtained by calcination at 700 ◦ C for 1 h (䊉) in air and () in N2 . The catalysts were reduced in H2 at 400 ◦ C for 10 min before the activity measurement.
RuO2 lower after the calcination in pure N2 than that calcined in air. The particle size of Ru after the calcination in pure N2 was 18 nm, as estimated by the XRD diffraction line (figure not shown). The results clearly indicate that the calcination under the presence of oxygen is essential for the self-dispersion Ru into nano-sized particles. That means that the oxidation of Ru particles plays an important role in the self-dispersion process. Then, the structure of the Ru(2) + CeO2 catalyst after the calcination and the reduction was characterized by means of XAFS and Raman spectroscopy. Fig. 6 shows the Fourier transforms of the k3 -weighted Ru Kedge EXAFS of Ru(2) + CeO2 after the calcination at 700 ◦ C and reduction at 400 ◦ C. The FTs are not corrected for phase shift. The spectra of Ru(2) + CeO2 showed the scattering from the nearest O and Ru atoms. Although we tried the curve-fitting analysis on the inverse Fourier-transform, the presence of Ru O Ce or Ru O Ru was unclear. The data of curve-fitting analysis of Ru K-edge EXAFS are summarized in Table 2. The coordination numbers of the Ru–Ru shell were 4.79 after the calcination and 7.23 after the reduction. The increase in the coordination number of Ru–Ru shell indicates the increase in the contribution of Ru metal after the reduction. Furthermore, the significant contribution of the Ru–O shell even after the reduction in H2 means that the Ru particle on CeO2 is still partly oxidized. Fig. 7 shows Ru K-edge XANES spectra of Ru(2) + CeO2 after calcination at 700 ◦ C and reduction at 400 ◦ C. The spectra of Ru metal powder and RuO2 were also shown as reference materials. As shown in the bold lines, the obtained spectra of Ru(2) + CeO2 can be represented by the combination of the spectra of Ru metal and RuO2 . After the calcination, the obtained spectrum can be represented by the combination of 53% of Ru metal and 47% of RuO2 . Table 2 Curve-fitting analysis of Ru K-edge EXAFS data for Ru(2) + CeO2 . Shell
CNa
˚ Rb (A)
˚ c (A)
Rf d (%)
◦
Calcination at 700 C
Ru O
4.79 2.45
2.68 1.96
0.071 0.076
2.57
Reduction at 400 ◦ C
Ru O
7.23 2.00
2.67 2.00
0.073 0.100
1.53
Treatment
a b c d
Coordination numbers. Bond distance. Debye–Waller factor. Residual factor.
0
1
2
3
4
5
R/ Å Fig. 6. Fourier transforms of Ru K-edge EXAFS spectra of Ru(2) + CeO2 and reference samples (Ru foil and RuO2 ).
After the reduction in H2 , the contribution of Ru metal increased, but 21% of Ru is remained oxidized. The result is qualitatively in harmony with the EXAFS spectra because the both of the samples showed both Ru–O and Ru–Ru shell, and the coordination number of Ru–O shell was reduced after the reduction in H2 . From the ratio of Ru metal and RuO2 estimated from the XANES spectra, images of the Ru particles can be drawn. The contribution of oxidized Ru should be mainly on the interface of Ru and CeO2 support. On the assumption of hemispherical Ru particle with diameter of 2 nm, the contribution of interface layer between Ru and CeO2 is 40%. The ratio of RuO2 in XANES spectrum of Ru(2) + CeO2 after the reduction (21%) suggests that a half of the interface layer is oxidized. In the case of Ru(2) + CeO2 after the calcination, the ratio of RuO2 in XANES spectrum was 47%, which exceeded the contribution of interface layer. The difference amount of RuO2 before and after the reduction (25%) corresponds well with the external surface of Ru hemisphere (26%), suggesting that the reduction at 400 ◦ C results in the reduction of Ru oxide on the external surface of Ru particle but not the interface between Ru and CeO2 support. Taking the composition of Ru metal into account, the particle size of metallic Ru can be estimated as follows. After the reduction of Ru(2) + CeO2 , the coordination number of Ru–Ru shell was 7.23. Taking 78.9% contribution of metallic Ru in this sample, coordination number of Ru–Ru shell in Ru metal particles can be estimated as 7.23/0.789 = 9.16. On the assumption of cubic Ru particles, Ru particle is composed of ca. 250 Ru atoms with side Ru number of 6.30 [10]. Based on the Ru atom diameter of 0.268 nm, the diameter of Ru particle can be estimated as 1.69 nm. Although the Ru diameter was roughly estimated from the XAFS data (1.7 nm), this value fairly corresponds with that measured by CO adsorption (2.0 nm). The detailed change of the Ru structure during calcination was also evaluated by Raman spectra, as shown in Fig. 8. CeO2 shows the bands at 258, 460, and 595 cm−1 , which are assignable to
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A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
Measured (solid) Simulated (broken)
a
b Measured (solid) Simulated (broken)
Ru x 0.527
RuO 2 x 0.473
Normalized absorption
0.1
Normalized absorption
0.1
Ru x 0.789
RuO 2 x 0.211
22100
22150
22200
Photon energy (eV)
22100
22150
22200
Photon energy (eV)
230
690
500
963
Intensity / arb.
450
Fig. 7. Ru K-edge XANES spectra of Ru(2) + CeO2 (a) after calcination at 700 ◦ C in air and (b) followed by reduction in 5%H2 at 400 ◦ C. Simulated spectra were synthesized by the sum of the spectra of pure Ru metal and pure RuO2 with appropriate ratios.
(a) air at 700oC (b) N 2 at 700oC x 1/40 (c) (a) followed by 5%H2 at 400oC (d) CeO2 x 1/20 (e) RuO2 x3
1000
500
Raman shift / cm -1 Fig. 8. Raman spectra of Ru(2) + CeO2 after various treatments and pure CeO2 and RuO2 as reference samples.
2nd-order transverse acoustic mode, F2g mode of CeO2 fluorite phase, and defect-induced mode, respectively [11–16]. As for pure RuO2 , weak bands were observed at 522, 635, and 703 cm−1 assignable to Eg , A1g , and B2g modes of RuO2 respectively [17]. However, since the band intensities are very low compared to the bands assignable to CeO2 , these RuO2 bands are not detectable in the spectra of Ru(2) + CeO2 . Only after the calcination in air, two new bands were observed at 690 and 963 cm−1 . Since the Ru(2) + CeO2 after the reduction did not give any bands above 600 cm−1 , the contribution of reduced cerium oxides to these new bands can be ruled out. The new bands cannot be attributed to other ruthenium oxides having various oxidation states (800 cm−1 for RuO3 , 822–881 cm−1 for RuO4 , 808 cm−1 for RuO4 2− , 380–440 and 590 cm−1 for hydrated RuO2 ) [18–20]. Therefore, the new bands are due to the interaction between Ru and CeO2 in highly oxidized state, and tentatively assigned to Ru O Ce bond, though any mixed oxide phase of Ru and Ce have not been reported. As a similar phenomenon, Lin et al. detected the formation of Pt O Ce bond by Raman spectroscopy [21]. The formation of Ru O Ce bond and its detection is probable. The formation of the Ru O Ce bond under O2 indicates that a
Fig. 9. Schematic model of self-dispersion of Ru on CeO2 support.
A. Satsuma et al. / Catalysis Today 201 (2013) 62–67
driving force of the self-dispersion is the oxidation of Ru and strong interaction between ruthenium oxide and CeO2 . From the obtained results, the self-dispersion process of Ru on CeO2 support can be illustrated as shown in Fig. 9. The calcination of physical mixture of Ru and CeO2 powders results in the oxidation of Ru, which may cause by the effect of CeO2 as an oxidation catalyst. In the case of Ag + CeO2 catalyst, the oxidation of metal particles was already suggested to be an important factor for the self-dispersion, however, a driving force of dispersion of oxidized species was not clarified. In this study, Raman spectra indicated that Ru species is dispersed with the formation of Ru O Ce bond on the surface. After the calcination, both the external surface and interface layer of Ru particle are oxidized. After the reduction in H2 , the external surface is reduced to Ru metal surface and an active catalyst is obtained. 4. Conclusion A physical mixture of Ru metal powder and CeO2 shows high activity for CO oxidation after calcination in air at 700 ◦ C and the reduction in H2 at 400 ◦ C. The activity is comparable to Ru/CeO2 catalyst prepared by a conventional impregnation method. The high activity for Ru + CeO2 is caused by self-dispersion of Ru into nanoparticle during the calcination. The key for the self-dispersion is the oxidation of Ru particle and the formation of Ru O Ce bond. The self-dispersion of Ru is achieved through (1) oxidation of Ru metal into RuO2 during calcination in air, (2) dispersion of nano-sized and oxidized Ru particle onto CeO2 surface with the formation of Ru O Ce bond, (3) the reduction of external surface of Ru particle in a flow of H2 at relatively low temperature results in an active catalysts for CO oxidation. Acknowledgements This study was partly supported by Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Kakenhi) and by a project of the New Energy and Industrial
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