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A study on the catalytic synergy effect between noble metals and cobalt phases in Ce-Al-O supported catalysts
Applied Catalysis A: General 301 (2006) 145–151 www.elsevier.com/locate/apcata
A study on the catalytic synergy effect between noble metals and cobalt phases in Ce-Al-O supported catalysts Ming Meng a,*, Yu-qing Zha a, Jin-yong Luo a, Tian-dou Hu b, Ya-ning Xie b, Tao Liu b, Jing Zhang b a
Department of Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China b Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China Received 1 April 2005; received in revised form 27 May 2005; accepted 2 June 2005 Available online 18 January 2006
Abstract A series of Co-Pt(Pd, Rh)/Ce-Al-O catalysts were prepared by successive wetness impregnation method. The catalytic activities of the samples for CO oxidation were evaluated in a flow fixed-bed micro-reactor. The techniques of XRD, XPS, EXAFS, H2-TPR and TPO-MS were employed to characterize the catalysts. The results of activity measurement show that for CO oxidation the presence of a small amount of noble metals greatly enhances the activity of Co/Ce-Al-O catalyst, the existence of cerium has increased the activity of the samples because of its capacity for oxygen storage and the interaction between cerium oxide and cobalt phase. The structural characterization results of XRD, XPS and EXAFS indicate that the cobalt in Co-Pt(Pd, Rh)/Ce-Al-O catalysts exists as metallic phase, while in Co/Ce-Al-O catalyst only part of the cobalt has been reduced to zero valence, the rest exists as Co-Al spinel which cannot be reduced at 450 8C by H2. H2-TPR results suggest that the hydrogen spillover may occur during the reduction pretreatment, and therefore increasing the reduction deepness of cobalt phases. The results of TPO-MS show that the oxygen spillover from noble metals to cobalt phase during CO oxidation is very potential. It decreases the activation energy of CO oxidation reaction and results in a prominent increase of the activity. The spilled over species, such as atomic H and O, generated on noble metal sites are mobile on the surface of the catalysts, which makes both of the close and remote cobalt phases involve in reactions at lower temperatures. Additionally, the coordination numbers from EXAFS indicate that the presence of noble metals greatly enhances the dispersion of cobalt phases. The order of this enhancement effect is Pt > Pd > Rh. As a result, the oxygen spillover effect and the enhancement effects on the reduction deepness and the dispersion of cobalt phases should be the main contributions to the catalytic synergy effect between cobalt phases and noble metals. # 2005 Elsevier B.V. All rights reserved. Keywords: Co-Pt(Pd, Rh)/Ce-Al-O catalysts; CO oxidation; Hydrogen spillover; Oxygen spillover; Catalytic synergy effect; Structural characterization
1. Introduction Carbon monoxide (CO) and hydrocarbons (HCs) are the main pollutants produced in the course of combustion of fossil fuels and biomass. During the cold start of vehicles, the tailgases contain a lot of CO (1–7%) and HCs (0.1–0.7%) [1–3]. At this time, since the temperatures of tail-gases and catalyst bed are relatively low (<150 8C), the conventional three-way catalysts (TWC) show very low catalytic efficiency, therefore, most of the tail-gases are released into the air without effective
* Corresponding author. Tel.: +86 22 27892275; fax: +86 22 27405243. E-mail address: [email protected] (M. Meng). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.06.035
purification [1–5]. So, research work on the low-temperature oxidation catalysts is still necessary. Noble metals are the main active components in three-way catalysts (TWC), which show very good activities for the complete oxidation of CO and HCs [4,6]. However, the resources of noble metals are very limited, which makes them expensive. In the past several years, much attention has been paid to base metal catalysts [7–9]. Although some transition metal oxides, such as cobalt, copper and manganese, have shown high catalytic activity for the oxidation of CO, alkenes and aromatics [4,7,8], these catalysts exhibits less specific activities than noble metals, especially for the oxidation of HCs. Meanwhile, they normally show low thermal stability and poor activity for the reduction of NOx [9]. Many studies have indicated that if a small amount of noble
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metals is added to some transition metal oxides, their activities can be greatly enhanced, and their dispersion and thermal stability can be improved, too [10–15]. It is reported that the complex oxides La0.45Sr0.15Ce0.35Zr0.05M1.0 (M = Cu or Co) promoted by a small amount of Pt and Rh show a three-way activity quite similar to that of a commercial three-way catalyst containing four times as much Pt-Rh [14]. The study by Skoglundh et al. [15] has shown that the starting temperature for the conversion of CO and C3H6 is about 100 8C lower over Pd/Co/La/Al2O3 than over Pd/La/Al2O3. Torncrona et al. [16] found that the addition of 1–1.25 wt% noble metals to 25 wt% cobalt oxide on g-Al2O3 gave rise to pronounced catalytic synergy effect after prereduction treatment, it is explained by the potential hydrogen spillover effect, however, no work upon this aspect was reported. The similar phenomena for g-Al2O3 supported Cu, Co and Mn catalysts promoted by noble metals for CO and NO removal are also found in our previous work [17,18]. Up to now, the essential of the synergy effect between noble metals and transition metal oxides is not totally clarified, and few investigations are performed systematically on this aspect from the view of not only the H2 spillover effect, but also the O2 spillover effect. In the present work, the Ce-Al-O supported samples with very low loadings of noble metals (0.1 wt%) and cobalt oxide (8 wt%) are prepared, we aim to elucidate the synergism between cobalt phases and noble metals in Co-Pt(Pd, Rh)/Ce-Al-O catalysts from the cobalt valence, cobalt phase dispersion and spillover effects of hydrogen and oxygen. The correlation between the catalyst structures and catalytic properties is demonstrated. How the so small amount of noble metal cooperates with much larger amount of cobalt phase during the spillover process is discussed as well. 2. Experimental 2.1. Catalyst preparation The support g-Al2O3 (BET surface area: 152 m2/g) was supplied by the Third Petroleum Manufacturer of Fushun of China from a dispersible boehmite calcined at 750 8C for 16 h. The g-Al2O3 powder was pelletized, ground and sieved to 40– 60 mesh particle size. The samples were prepared by the incipient wetness impregnation method, by impregnating gAl2O3 to a given aqueous solution of cerium nitrate. After drying at 120 8C and calcination at 500 8C for 4 h, the Ce-Al-O support was impregnated in aqueous solution of cobalt nitrate, and was dried and calcined at the same condition to obtain Co/ Ce-Al-O precursor. This precursor was then impregnated in the aqueous solution of H2PtCl66H2O, PdCl2 and RhCl33H2O, respectively. The wet Co-Pt(Pd, Rh)/Ce-Al-O precursors were dried at 120 8C, and calcined in air at 500 8C for 2 h. Before use, all samples were reduced with pure hydrogen (30 ml/min) at 450 8C for 1 h. The contents of the components in the catalysts are calculated according to the equations: Co/ Al = 5 mol%, M/Ce-Al-O = 0.1 wt% (M = Pt, Pd or Rh), CeO2/Al2O3 = 20 wt%. To investigate the function of cerium, the samples without cerium were also prepared in the same
condition, so that they can be compared with the cerium promoted samples. 2.2. Evaluation of catalytic activity The activities for CO oxidation of the samples were determined in a flow fixed-bed micro-reactor with a quartz tube (i.d. 8 mm). A given amount of the sample (1.0 ml, 40– 60 mesh) was used each time. The gas mixture was analyzed by means of a chromatograph (model GC 102, supplied by Shanghai Analyzing Instruments Factory, China) equipped with a thermal conductivity detector (TCD). The reaction gas mixture contains 0.50 vol.% CO and 5.0 vol.% O2 balanced with pure nitrogen to yield a space velocity (GHSV) of 4500 h 1. 2.3. X-ray diffraction (XRD) X-ray diffraction measurement was carried out on a D/ MAX-RA rotatory diffractometer, using Cu Ka as radiation source (l = 0.15418 nm). The data were collected under the same conditions (40 kV and 100 mA). 2.4. X-ray photoelectron spectroscopy (XPS) XPS spectra were recorded on ESCA-LAB MK-II type spectrometer using Mg Ka as radiation source (1253.6 eV). The binding energies of the elements were corrected, using C1s (284.5 eV) peak as standard. 2.5. Extended X-ray absorption fine structure (EXAFS) The EXAFS data of Co K-edge and Pt L-III-edge were collected on the XAFS station, 1W1B beamline of Beijing Synchrotron Radiation Facility of National Laboratory (BSRF NL), using transition mode for Co K-edge and fluorescence mode for Pt L-III-edge. The critical beam energy was 2.2 GeV with an average storage ring current of 80 mA. Two Si(1 1 1) single crystals were used as monochromators. The structural parameters of the samples were obtained by curve-fitting method, using CoAl2O4 or Co metal as model compound for Co K-edge and PtO2 or Pt metal for Pt L-III-edge. The details of EXAFS data treatment were described elsewhere [17]. 2.6. Temperature-programmed reduction by H2 (H2-TPR) TPR experiment was carried out in a micro-reactor with a quartz tube (i.d. 4 mm). A given amount of the sample (100 mg, 40–60 mesh) was used each time. The reduced sample was initially pretreated at 500 8C in air for 1 h, then cooled to room temperature and finally reduced with hydrogen (5 vol.% H2 in N2) at a flow rate of 30 ml/min. The sample was heated up to 700 8C at a rate of 10 8C/min. The resulting gas mixture was analyzed by a gas chromatorgraph (model SP-2305, supplied by Beijing Analyzing Instruments Factory, China) equipped with a thermal conductivity detector (TCD) with the bridge current of 100 mA.
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2.7. Temperature-programmed oxidation-mass spectroscopy (TPO-MS) A given amount of the freshly reduced sample (500 mg, 40– 60 mesh) was fixed in a stainless tube (i.d. 6 mm) of the microreactor, and pretreated in a flow helium (99.99%) at 500 8C for 1 h. After cooling to room temperature, the TPO experiment was performed, using the mixture of 1.0% O2/N2 as oxidation gas (30 ml/min). The sample was heated up to 600 8C at a rate of 10 8C/min. The peak signal of m/z = 32 (O2 mass number) was detected by a quadruple mass spectrometer (model LZL203, supplied by Beijing Analyzing Instruments Factory, China). 3. Results and discussion 3.1. The activity for CO oxidation The results of activity evaluation are shown in Fig. 1. From Fig. 1, it can be seen that the Co/Ce-Al-O catalyst possesses good activity for CO oxidation. At 188 8C, the conversion of CO reaches 100%. When a small amount of Pt, Pd or Rh is added to Co/Ce-Al-O, the activity is greatly enhanced, the temperature for the 100% conversion of CO is decreased by 73, 63 and 44 8C, respectively. The activities of Co-Pt(Pd, Rh)/CeAl-O are also higher than those of Pt(Pd, Rh)/Ce-Al-O. These results reveal that there exists a pronounced catalytic synergy effect between cobalt phases and noble metals, especially between cobalt and platinum or palladium, though the loadings of noble metals are very low. To investigate the function of cerium, the catalytic activity of the samples without cerium was also evaluated for comparison. Among these catalysts, CoPt/g-Al2O3 shows the best activity, which is selected and presented in Fig. 1 (line with diamond symbol). By comparison, it is found that the sample Co-Pt/Ce-Al-O is
Fig. 1. The relationships between CO conversions and reaction temperatures over the samples: (*) Co-Pt/Ce-Al-O; (&) Co-Pd/Ce-Al-O; (&) Co-Rh/CeAl-O; (~) Pt/Ce-Al-O; (~) Pd/Ce-Al-O; () Rh/Ce-Al-O; (*) Co/Ce-Al-O; (^) Co-Pt/g-Al2O3.
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more active than Co-Pt/g-Al2O3, the temperature of full conversion of CO on the former is about 30 8C lower than that on the later, suggesting the cerium has played an important role during CO oxidation. As reported widely [19–22], the main function of CeO2 should be the storage capacity for oxygen, during the oxidation reaction, CeO2 can provide oxygen species to the close active phase, generating corresponding vacancies, which are eliminated by adsorption of oxygen from gas phase. In addition, cerium oxide may contact and interact with other components, such as alumina, noble metals and cobalt species. Since the loading of cerium oxide (CeO2/ Al2O3 = 20 wt%) has exceeded the amount for monolayer dispersion on alumina (in Section 3.2, diffracted peaks of CeO2 are detected), the most surface of alumina should be covered by cerium oxide. The function of alumina is mainly providing big surface area for the dispersion of cerium oxide, which is the real promoter for enhancing the catalytic activity. Considered the preparation procedure and the big difference of the loadings between cobalt phase and noble metals, the cobalt species have much larger opportunities to contact cerium oxide, and may interact with each other. Similar point of view was also proposed by Bruce et al. [23]. During CO oxidation, this kind of interaction makes the phase-to-phase interface between cobalt and cerium oxide geometrically continuous, and therefore facilitates the transferring of active species from cerium oxide to cobalt phase, or vice versa, which is indirectly supported by the H2-TPR and TPO-MS results in the following parts, where the cobalt phases effectively enhanced the reduction and oxidation of cerium oxides. 3.2. XRD The XRD patterns of the fresh Co-Pt(Pd, Rh)/Ce-Al-O catalysts are similar to each other, so, only that of Co-Pt/Ce-Al-O is selected and presented in Fig. 2. Since in other region of the XRD patterns, all peaks correspond to CeO2 and g-Al2O3, and no
Fig. 2. The XRD patterns of the samples: (a) Pt/Ce-Al-O; (b) Co/Ce-Al-O; (c) Co-Pt/Ce-Al-O.
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obvious difference is found for the three samples Pt/Ce-Al-O, Co/Ce-Al-O and Co-Pt/Ce-Al-O, only the signals in the region of 2u = 32–428 are amplified by four times and shown in Fig. 2. From Fig. 2(a), it can be seen that for sample Co/Ce-Al-O, there is a weak peak appearing at 2u = 36.98 (d = 0.2442 nm), which may be contributed by Co3O4 or CoAl2O4 phase because the strongest diffracted peaks of Co3O4 and CoAl2O4 are at the same positions (ICDD, PDF 80-1533 and 82-2248). Considered that Co3O4 is relatively easy to be reduced [24–26], the peak at 2u = 36.98 is assigned to CoAl2O4 or CoAl2O4-like spinel, which cannot be reduced by H2 below 500 8C [27,28]. With the addition of a small amount of platinum to Co/Ce-Al-O, the peak at 2u = 36.98 disappears (Fig. 2(c)), implying the complete reduction of the cobalt phase. Although no any peak of metallic cobalt is detected in the whole region of 2u = 10–708 for the samples containing cobalt, its existence is possible because the reduced cobalt phase may be highly dispersed in the catalysts or its amount is below the detecting limit of XRD.
3.4. EXAFS
To confirm the state of the surface cobalt, the XPS spectra of Co2p3/2 of the samples were recorded. The spectrum of Co2p3/2 for Co/Ce-Al-O is shown in Fig. 3(a). There are two peaks at the position of low binding energy (778.3 and 781.2 eV), and another weak peak at high binding energy position (786.0 eV). According to references [29–31], the former could be assigned to Co0 and Co2+ in spinel, respectively, while the later weak peak is the typical satellite peak of Co2+. Since the similarity of the Co2p3/2 spectra of the samples containing noble metals, only the spectrum of Co-Pt/Ce-Al-O is presented in Fig. 3(b). It is obvious that there is only one peak in Fig. 3(b), which corresponds to metallic cobalt. These results means that the addition of a small amount of noble metals does enhance the reduction deepness of cobalt phase, which is in good agreement with that of XRD.
The radial structure functions (RSFs) of Co K-edge EXAFS of the samples and model compounds are shown in Fig. 4. For Co metal, there is a strong coordination peak appearing at 0.21 nm (uncorrected). Considering the phase scattering shift, this peak should correspond to the first Co–Co shell in Co metal, whose real coordination distance is 0.250 nm. The RSFs of the samples promoted by a small amount of noble metals are very similar to that of Co metal, only the peak intensities are lower, while the RSF of Co/Ce-Al-O is typically different from that of Co metal, where appearing two coordination peaks at 0.15 and 0.27 nm. The first one can be well fitted by the first Co–O shell in model compound CoAl2O4, however, the second one cannot be fitted by either the Co–Co shell in cobalt metal or the Co–Co shell in CoAl2O4. The position of this peak (0.27 nm) is just between those of Co metal (0.21 nm) and CoAl2O4 (0.32 nm). Combined with XRD and XPS results, it is inferred that the second peak in the RSF of Co/Ce-Al-O sample are contributed by both Co metal and CoAl2O4 or CoAl2O4-like spinel. To obtain the structural parameters, curve-fitting was performed on the first shell, using Co metal and CoAl2O4 as the model compounds for Co-Pt(Pd, Rh)/Ce-Al-O and Co/CeAl-O, respectively. The best fitting values of the structural parameters are listed in Table 1. From Table 1, it can be seen that the coordination distance (R) of the first shell for sample Co/Ce-Al-O is close to that of CoAl2O4, implying that there still exists some Co–O coordination. With the addition of the noble metals to Co/ Ce-Al-O, the distances of the first shells change to be almost the same as that of Co metal, indicating that the Co–O shells have completely disappeared. Combined with the results of XRD and XPS, it is deduced that the Co mainly exists in metallic state in the samples promoted by a small amount of noble metals, while in Co/Ce-Al-O the Co exists as the mixture of Co metal and CoAl2O4 or CoAl2O4-like spinel. The presence of noble
Fig. 3. XPS spectra of Co2p3/2 of the samples: (a) Co/Ce-Al-O and (b) Co-Pt/ Ce-Al-O.
Fig. 4. The radial structure functions of Co K-edge of the samples and model compounds: (a) Co metal; (b) Co-Pt/Ce-Al-O; (c) Co-Pd/Ce-Al-O; (d) Co-Rh/ Ce-Al-O; (e) Co/Ce-Al-O; (f) CoAl2O4.
3.3. XPS
M. Meng et al. / Applied Catalysis A: General 301 (2006) 145–151 Table 1 Best-fitting values of the structural parameters for the first coordination shells in the samples from Co K-edge EXAFS Sample
Co-Pt/Ce-Al-O Co-Pd/Ce-Al-O Co-Rh/Ce-Al-O Co/Ce-Al-O CoAl2O4 a Co metal a
First coordination shell Shell
N
R (nm)
Residual (%)
Co–Co Co–Co Co–Co Co–O Co–O Co–Co
4.1 4.8 5.6 2.6 4.0 12.0
0.249 0.251 0.252 0.194 0.195 0.250
5.47 5.92 6.05 7.69
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coordination. These results indicate that most of platinum has been reduced to metal Pt, except for a small amount of platinum chloride. Similar phenomena have ever been found by Pirault et al. on Pt-Rh/Al2O3-CeO2 system [33]. 3.5. TPR results and hydrogen spillover
metals prominently enhanced the reduction of cobalt phases during the pretreatment. Additionally, the very low values of coordination number (N) indicate that the cobalt in the samples possesses very high dispersion, the order of the enhancement effect on cobalt dispersion is Pt > Pd > Rh, which is in accordance with activity law. Therefore, it is thought that the enhancement effect of noble metals on cobalt dispersion should be one of the main contributions to the catalytic synergy effect between cobalt phases and noble metals. In order to confirm the chemical state of the noble metals, we increased the platinum content to 1.0 wt.% in Co-Pt/Ce-Al-O, and recorded the EXAFS data using fluorescence mode. The radial structure functions of Pt L-III-edge for Co-Pt/Ce-Al-O and the model compounds are shown in Fig. 5. From Fig. 5, it can be seen that the main coordination peaks of the model compounds PtO2, H2PtCl66H2O and Pt metal appear at 0.16, 0.21 and 0.26 nm, respectively. For Pt metal, there is another weak peak at 0.22 nm, which is resulted from the non-linearity of the phase shift function. Compared with those of model compounds, the RSF of Co-Pt/Ce-Al-O is analogous to that of Pt metal. However, the weak peak appearing at 0.22 nm is more obvious and the ratio of this peak to the main peak is much larger than that of Pt metal, which implies that there may be still a little Pt-Cl
H2-TPR characterization is regarded as a mirror to reflect H2 spillover effect. In this work, the reduced samples were first reoxidized in air, then were used for TPR experiments. The TPR profiles of the reoxidized samples are shown in Fig. 6. For the support Ce-Al-O, there is a reduction peak at about 387 8C. Since alumina cannot be reduced by H2, this peak must correspond to the reduction of cerium species. Several studies [19,20,23] have ever indicated that there is a kind of oxygen species (called capping oxygen) in the surface of CeO2, which can be reduced below 500 8C, while the lattice oxygen is normally reduced at 750 8C or above. Therefore, the peak in Fig. 6(a) is assigned to the reduction of the capping oxygen in CeO2. When cobalt is supported on Ce-Al-O, there appear two reduction peaks at 350 and 468 8C. According to the study of literature [23], there exists strong interaction between cobalt and cerium oxides, which makes the reduction of the capping oxygen in CeO2 shift to lower temperature. So, in Fig. 6(b), the peak at low temperature is assigned to the reduction of the capping oxygen in CeO2, while the other at high temperature corresponds to the reduction of cobalt species. With the addition of noble metals to Co/Ce-Al-O catalyst (Fig. 6(c–e)), the two peaks shift remarkably to lower temperature direction. Compared with Fig. 6(b), it is found that the temperature for the first peak is decreased by 62, 75 or 93 8C, and that for the second peak is also decreased by 80, 91 or 99 8C for Rh, Pd or Pt-promoted sample, respectively. These results indicate that the presence of noble metals does make the reduction of cobalt and cerium species easier. Additionally, with increasing of noble metal contents, the reduction peaks shift monotonically
Fig. 5. The radial structure functions of Pt L-III-edge of the sample and model compounds: (a) Pt metal; (b) Co-Pt/Ce-Al-O; (c) H2PtCl66H2O; (d) PtO2.
Fig. 6. The H2-TPR profiles of the samples: (a) Ce-Al-O; (b) Co/Ce-Al-O; (c) Co-Rh/Ce-Al-O; (d) Co-Pd/Ce-Al-O; (e) Co-Pt/Ce-Al-O.
a The structural parameters of the model compounds are taken from reference [32].
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toward the direction of lower temperature (not shown). Similar phenomena are also found by Lin et al. [14] and Van’t Blik et al. [34] in other systems, where hydrogen spillover from platinum (or rhodium) to cobalt and from ruthenium to copper is proposed. It is well known that the hydrogen is normally chemically adsorbed on the noble metals, and is readily dissociated into atomic species, which are very active spillover species [35]. Therefore, in our case, it is assumed that the hydrogen molecule first adsorbs on Pt, Pd or Rh (donor) and dissociates into atomic species, then these active species spill over to the surrounding oxides, such as cobalt or cerium (acceptor) and react with them. Here, the essential of the spillover process is a catalytic gas (H2)–solid (cobalt or cerium oxides) reaction using noble metals as catalysts, which has prominently decreases the activation energy of this reaction. This point of view is just reflected and supported by the big shifts of the reduction peaks to lower temperatures in Fig. 6. According to the above analysis, it is believed that during the sample pretreatment by H2, the spillover of hydrogen species from noble metals to cobalt oxide may have taken place, leading to the different reduction deepness between the samples with and without noble metals. This point can be quantitatively proved by the analysis results of H2-TPR peak area (listed in Table 2). From Table 2, it can be seen that the difference for the first peak areas of the samples is insignificant, while that for the second peak areas is much more obvious between the samples promoted and non-promoted by noble metals. These results demonstrate that the amount of reducible cobalt species in Co/ Ce-Al-O is much less than those in Co-Pt(Pd, Rh)/Ce-Al-O, meaning that some cobalt phase in the sample without noble metals cannot be reduced below 700 8C by 5 vol% H2 in N2. Combined the structural characterization results of XRD, XPS and EXAFS, the non-reducible cobalt phase should be CoAl2O4 or CoAl2O4-like spinel. The enhancement effect of the noble metals on the reduction of cobalt species may be an important source for the catalytic synergy effect between cobalt phases and noble metals. The sequence for this effect is Pt > Pd > Rh, which is the same as that for the activity enhancement effect. Now, there is still a question remaining, that is, how the so small amount of noble metals enhances the reduction of much larger amount of cobalt oxide during H2-TPR process. On the basis of the loadings of noble metals and cobalt phase, it is not difficult to understand that in the catalysts noble metals can only contact part of the cobalt oxide, and interact with each other, which is called ‘‘Close cobalt phase’’, the other cobalt oxide is called ‘‘Remote cobalt phase’’. Normally, noble metals can dissociate adsorbed H2 molecule into active species, such as
atomic H, at lower temperature than cobalt oxide. Assuming that the lowest temperature for dissociating H2 on one kind of noble metal is TN, while that on cobalt oxide is TC, when temperature is between TN and TC, no dissociated hydrogen species are formed on cobalt oxide, however, such species can be generated on the noble metals, which can spill over to the ‘‘Close cobalt phase’’. If the spillover process stops here, the amount of reduced cobalt oxide will be very small, because the ‘‘Remote cobalt phase’’ will not be reduced. However, the fact in this work is opposite, so, it is inferred that the spilled hydrogen species must be mobile on the surface, which can further transfer from close cobalt oxide to remote oxide sites, including cobalt oxide and cerium oxide, and finally make them be reduced. 3.6. TPO-MS results and oxygen spillover The TPO-MS profile for the support Ce-Al-O is presented in Fig. 7(a). There is a O2-consumption peak at 212 8C. It is well known that the cerium oxide is a non-stoichiometric compound (CeOx, 1.5 < x < 2.0). When reduced, some of the surface oxygen, such as capping oxygen, is removed, and the corresponding vacancies are generated. When exposed to oxygen at appropriate temperature, these vacancies can be eliminated through oxidation or storage. Therefore, the peak in Fig. 7(a) can be assigned to the oxidation of cerium species or the storage of oxygen. For Co/Ce-Al-O, there is another O2consumption peak appearing at 307 8C, which should be resulted from the oxidation of cobalt species. With the addition of noble metals to Co/Ce-Al-O, both the two peaks shift to lower temperature direction. The temperature for the first peak is decreased by 56, 37 or 12 8C, and that for the second peak is also decreased by 65, 48 or 20 8C for Pt, Pd or Rh-promoted sample, respectively. With increasing of noble metal contents, the peaks shift to lower temperatures further (not shown). All these phenomena are very similar to those found in H2-TPR. Therefore, it is thought that the oxygen spillover may have taken place during the TPO process. The Pt, Pd or Rh acts as
Table 2 The relative peak areas of H2-TPR peaks in Fig. 6 after normalization Sample
Relative area a
Co-Pt/Ce-Al-O Co-Pd/Ce-Al-O Co-Rh/Ce-Al-O Co/Ce-Al-O Ce-Al-O
0.98 0.96 0.97 0.96 1.00
a
(257) (275) (288) (350) (387)
The data listed in the parentheses are peak top temperatures.
1.83 1.80 1.78 0.98 –
(369) (377) (388) (468) Fig. 7. The TPO-MS profiles of the samples: (a) Ce-Al-O; (b) Co/Ce-Al-O; (c) Co-Rh/Ce-Al-O; (d) Co-Pd/Ce-Al-O; (e) Co-Pt/Ce-Al-O.
M. Meng et al. / Applied Catalysis A: General 301 (2006) 145–151
donor to provide active oxygen species, while the reduced phases act as acceptor to accept the spilled over oxygen species and react with them. For enhancing the oxidation of cobalt species, the order for the noble metals is still Pt > Pd > Rh. In the course of CO oxidation, it is believed that the noble metals have played a key role for adsorbing, activating and providing oxygen species for cobalt phases through spillover. If the oxygen adsorption and dissociation is the rate-determining step, the oxygen spillover must decrease the activation energy of CO oxidation reaction and therefore prominently enhance the activity of Co/Ce-Al-O. Similar to that in H2-TPR process, it is supposed that the spilled over oxygen species from noble metal sites, such as atomic O, are also mobile on the catalyst surface, so that not only the close reduced cobalt phase, but also the remote reduced cobalt phase are involved in the CO oxidation reaction. As a summary, the oxygen spillover effect during CO oxidation is regarded as another important source for the catalytic synergy effect between cobalt phases and noble metals. 4. Conclusions (1) For CO oxidation, the addition of 0.1 wt% noble metals (Pt, Pd or Rh) to Co/Ce-Al-O catalyst, the activity is greatly enhanced. The catalytic synergy effect between such three kinds of noble metals and cobalt phases is very pronounced. The sequence of the noble metals for the activity enhancement is Pt > Pd > Rh. The interaction between cobalt phases and cerium oxide also play a role for improving the activity of the samples. (2) The existence of 0.1 wt% noble metals prominently decreases the reduction temperature of cobalt oxides and increases their reduction deepness. The order for this function is Pt > Pd > Rh. During reduction pretreatment, a H2 spillover process may take place, and it is supposed that the active spilled over hydrogen species, such as atomic H, be mobile from noble metals to the surrounding cobalt oxides, and further to the remote cobalt oxides. This is a reasonable explanation for the enhancement effect of so small amount of noble metals on the reduction of a much larger amount of cobalt oxides. The increase of reduction deepness produces more active species accessible to reaction gases, which should be one of the main contributions to the catalytic synergy effect between such small amount of noble metals and cobalt phases. (3) The promoting effect of noble metals on cobalt phase dispersion should be another important source for the catalytic synergy effect between noble metals and cobalt phases. The order for this promoting effect is Pt > Pd > Rh, which is totally consistent with that for the catalytic activity. (4) During CO oxidation, the spillover of oxygen species from noble metals to cobalt phases is suggested, and the spilled over oxygen species are believed to be mobile from the close cobalt phase to remote cobalt phase. As a result, the spillover effect of oxygen decreases the activation energy of CO oxidation reaction, and therefore it is regarded as the third main contribution to the catalytic synergy effect between noble metals and cobalt phases.
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Acknowledgements This work is financially supported by Natural Science Foundation of Tianjin (05YFJMJC09700), the Research Foundation for Doctorial Program of the Ministry of Education of China (20040056028), and the 985 Program on Education Development of Tianjin University. References [1] E.S.J. Lox, B.H. Engler, in: G. Ertl, H. Kno¨zinger, J. Weitkamp (Eds.), Environmental Catalysis, Wiley-VCH, New York, 1999. [2] A.L. Boehman, S. Niksa, Appl. Catal. B 8 (1996) 41. [3] R.M. Heck, R. Farrauto, Catalytic Air Pollution Control, Wiley-Interscience, New York, 2002. [4] J.T. Kummer, Prog. Energy Combust. Sci. 6 (1980) 177. [5] M. Ferna´ndez-Garcı´a, A. Iglesias-Juez, A. Martı´nez-Arias, A.B. Hungrı´a, J.A. Anderson, J.C. Conesa, J. Soria, J. Catal. 221 (2004) 594. [6] H. He, H.X. Dai, L.H. Ng, K.W. Wong, C.T. Au, J. Catal. 206 (2002) 1. [7] P. Thormahlen, E. Fridell, N. Cruise, M. Skoglundh, A. Palmqvist, Appl. Catal. 31 (2001) 1. [8] S.A. Yashnik, V.V. Kuznetsov, Z.R. Ismagilov, V.V. Ushakov, N.M. Danchenko, S.P. Denisov, Top. Catal. 30–31 (2004) 293. [9] P.Y. Lin, W.D. Chen, S.M. Yu, Chin. J. Mol. Catal. 3 (1995) 179. [10] M.H. Kim, J.R. Ebner, R.M. Friedman, M.A. Vannice, J. Catal. 208 (2002) 381. [11] M. Ferrandon, J. Carno, S. Jaras, E. Bjornbom, Appl. Catal. A 180 (1999) 153. [12] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilmen, A. Hoff, A. Holmen, J. Catal. 156 (1995) 85. [13] D. Panayotov, M. Khristova, M. Velikova, Appl. Catal. B 9 (1996) 107. [14] P.Y. Lin, M. Skoglundh, L. Lowendahl, J.E. Otterstedt, L. Dahl, K. Jansson, M. Nygren, Appl. Catal. B 6 (1995) 237. [15] M. Skoglundh, H. Johansson, L. Lowendahl, K. Jansson, L. Dahl, B. Hirshauer, Appl. Catal. B 7 (1996) 299. [16] A. Torncrona, M. Skoglundh, P. Thormahlen, E. Fridell, E. Jobson, Appl. Catal. B 14 (1997) 131. [17] M. Meng, P.Y. Lin, Y.L. Fu, Catal. Lett. 48 (1997) 213. [18] P.Y. Lin, M. Meng, J.J. Yuan, S.M. Yu, Y.L. Fu, Stud. Surf. Sci. Catal. 112 (1997) 201. [19] H.C. Yao, Y.F. Yu Yao, J. Catal. 86 (1984) 254. [20] A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J.C. Lavalley, J.El. Fallah, L. Hilaire, F.L. Normand, E. Quemere, G.N. Sanvion, O. Touret, J. Chem. Soc. Faraday Trans. 87 (1991) 1601. [21] M. Sugiura, Catal. Surv. Asia 7 (2003) 77. [22] R. Si, Y.W. Zhang, S.J. Li, B.X. Lin, C.H. Yan, J. Phys. Chem. B 108 (2004) 12481. [23] L.A. Bruce, M. Hoang, A.E. Hughes, T.W. Turney, Appl. Catal. A 100 (1993) 51. [24] P. Arnoldy, J.A. Moulijn, J. Catal. 93 (1985) 38. [25] W.J. Wang, Y.W. Chen, Appl. Catal. 7 (1991) 223. [26] M. Meng, P.Y. Lin, S.M. Yu, Chin. J. Chem. Phys. 8 (1995) 66. [27] K.S. Chung, F.E. Massoth, J. Catal. 64 (1980) 320. [28] M. Meng, P.Y. Lin, S.M. Yu, Chin. J. Chem. Phys. 8 (1995) 176. [29] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.), Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corporation, Physical Electronics Division, USA, 1979. [30] Y. Okamoto, T. Imanaka, S. Teranishi, J. Catal. 65 (1980) 448. [31] Y. Okamoto, T. Adachi, K. Nagata, M. Odawara, T. Imanaka, Appl. Catal. 73 (1991) 249. [32] G. Sankar, S. Vasudevan, C.N.R. Rao, J. Phys. Chem. 91 (1987) 2011. [33] L. Pirault, M. Guerin, F. Maire, P. Marecot, J. Barbier, Appl. Catal. A 172 (1998) 249. [34] H.F.J. van’t Blik, D.C. Koningsberger, R. Prins, J. Catal. 97 (1986) 210. [35] B. Delmon, Catal. Rev. Sci. Eng. 1 (1996) 69.
A study on the catalytic synergy effect between noble metals and cobalt phases in Ce-Al-O supported catalysts Ming Meng a,*, Yu-qing Zha a, Jin-yong Luo a, Tian-dou Hu b, Ya-ning Xie b, Tao Liu b, Jing Zhang b a
Department of Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China b Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China Received 1 April 2005; received in revised form 27 May 2005; accepted 2 June 2005 Available online 18 January 2006
Abstract A series of Co-Pt(Pd, Rh)/Ce-Al-O catalysts were prepared by successive wetness impregnation method. The catalytic activities of the samples for CO oxidation were evaluated in a flow fixed-bed micro-reactor. The techniques of XRD, XPS, EXAFS, H2-TPR and TPO-MS were employed to characterize the catalysts. The results of activity measurement show that for CO oxidation the presence of a small amount of noble metals greatly enhances the activity of Co/Ce-Al-O catalyst, the existence of cerium has increased the activity of the samples because of its capacity for oxygen storage and the interaction between cerium oxide and cobalt phase. The structural characterization results of XRD, XPS and EXAFS indicate that the cobalt in Co-Pt(Pd, Rh)/Ce-Al-O catalysts exists as metallic phase, while in Co/Ce-Al-O catalyst only part of the cobalt has been reduced to zero valence, the rest exists as Co-Al spinel which cannot be reduced at 450 8C by H2. H2-TPR results suggest that the hydrogen spillover may occur during the reduction pretreatment, and therefore increasing the reduction deepness of cobalt phases. The results of TPO-MS show that the oxygen spillover from noble metals to cobalt phase during CO oxidation is very potential. It decreases the activation energy of CO oxidation reaction and results in a prominent increase of the activity. The spilled over species, such as atomic H and O, generated on noble metal sites are mobile on the surface of the catalysts, which makes both of the close and remote cobalt phases involve in reactions at lower temperatures. Additionally, the coordination numbers from EXAFS indicate that the presence of noble metals greatly enhances the dispersion of cobalt phases. The order of this enhancement effect is Pt > Pd > Rh. As a result, the oxygen spillover effect and the enhancement effects on the reduction deepness and the dispersion of cobalt phases should be the main contributions to the catalytic synergy effect between cobalt phases and noble metals. # 2005 Elsevier B.V. All rights reserved. Keywords: Co-Pt(Pd, Rh)/Ce-Al-O catalysts; CO oxidation; Hydrogen spillover; Oxygen spillover; Catalytic synergy effect; Structural characterization
1. Introduction Carbon monoxide (CO) and hydrocarbons (HCs) are the main pollutants produced in the course of combustion of fossil fuels and biomass. During the cold start of vehicles, the tailgases contain a lot of CO (1–7%) and HCs (0.1–0.7%) [1–3]. At this time, since the temperatures of tail-gases and catalyst bed are relatively low (<150 8C), the conventional three-way catalysts (TWC) show very low catalytic efficiency, therefore, most of the tail-gases are released into the air without effective
* Corresponding author. Tel.: +86 22 27892275; fax: +86 22 27405243. E-mail address: [email protected] (M. Meng). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.06.035
purification [1–5]. So, research work on the low-temperature oxidation catalysts is still necessary. Noble metals are the main active components in three-way catalysts (TWC), which show very good activities for the complete oxidation of CO and HCs [4,6]. However, the resources of noble metals are very limited, which makes them expensive. In the past several years, much attention has been paid to base metal catalysts [7–9]. Although some transition metal oxides, such as cobalt, copper and manganese, have shown high catalytic activity for the oxidation of CO, alkenes and aromatics [4,7,8], these catalysts exhibits less specific activities than noble metals, especially for the oxidation of HCs. Meanwhile, they normally show low thermal stability and poor activity for the reduction of NOx [9]. Many studies have indicated that if a small amount of noble
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metals is added to some transition metal oxides, their activities can be greatly enhanced, and their dispersion and thermal stability can be improved, too [10–15]. It is reported that the complex oxides La0.45Sr0.15Ce0.35Zr0.05M1.0 (M = Cu or Co) promoted by a small amount of Pt and Rh show a three-way activity quite similar to that of a commercial three-way catalyst containing four times as much Pt-Rh [14]. The study by Skoglundh et al. [15] has shown that the starting temperature for the conversion of CO and C3H6 is about 100 8C lower over Pd/Co/La/Al2O3 than over Pd/La/Al2O3. Torncrona et al. [16] found that the addition of 1–1.25 wt% noble metals to 25 wt% cobalt oxide on g-Al2O3 gave rise to pronounced catalytic synergy effect after prereduction treatment, it is explained by the potential hydrogen spillover effect, however, no work upon this aspect was reported. The similar phenomena for g-Al2O3 supported Cu, Co and Mn catalysts promoted by noble metals for CO and NO removal are also found in our previous work [17,18]. Up to now, the essential of the synergy effect between noble metals and transition metal oxides is not totally clarified, and few investigations are performed systematically on this aspect from the view of not only the H2 spillover effect, but also the O2 spillover effect. In the present work, the Ce-Al-O supported samples with very low loadings of noble metals (0.1 wt%) and cobalt oxide (8 wt%) are prepared, we aim to elucidate the synergism between cobalt phases and noble metals in Co-Pt(Pd, Rh)/Ce-Al-O catalysts from the cobalt valence, cobalt phase dispersion and spillover effects of hydrogen and oxygen. The correlation between the catalyst structures and catalytic properties is demonstrated. How the so small amount of noble metal cooperates with much larger amount of cobalt phase during the spillover process is discussed as well. 2. Experimental 2.1. Catalyst preparation The support g-Al2O3 (BET surface area: 152 m2/g) was supplied by the Third Petroleum Manufacturer of Fushun of China from a dispersible boehmite calcined at 750 8C for 16 h. The g-Al2O3 powder was pelletized, ground and sieved to 40– 60 mesh particle size. The samples were prepared by the incipient wetness impregnation method, by impregnating gAl2O3 to a given aqueous solution of cerium nitrate. After drying at 120 8C and calcination at 500 8C for 4 h, the Ce-Al-O support was impregnated in aqueous solution of cobalt nitrate, and was dried and calcined at the same condition to obtain Co/ Ce-Al-O precursor. This precursor was then impregnated in the aqueous solution of H2PtCl66H2O, PdCl2 and RhCl33H2O, respectively. The wet Co-Pt(Pd, Rh)/Ce-Al-O precursors were dried at 120 8C, and calcined in air at 500 8C for 2 h. Before use, all samples were reduced with pure hydrogen (30 ml/min) at 450 8C for 1 h. The contents of the components in the catalysts are calculated according to the equations: Co/ Al = 5 mol%, M/Ce-Al-O = 0.1 wt% (M = Pt, Pd or Rh), CeO2/Al2O3 = 20 wt%. To investigate the function of cerium, the samples without cerium were also prepared in the same
condition, so that they can be compared with the cerium promoted samples. 2.2. Evaluation of catalytic activity The activities for CO oxidation of the samples were determined in a flow fixed-bed micro-reactor with a quartz tube (i.d. 8 mm). A given amount of the sample (1.0 ml, 40– 60 mesh) was used each time. The gas mixture was analyzed by means of a chromatograph (model GC 102, supplied by Shanghai Analyzing Instruments Factory, China) equipped with a thermal conductivity detector (TCD). The reaction gas mixture contains 0.50 vol.% CO and 5.0 vol.% O2 balanced with pure nitrogen to yield a space velocity (GHSV) of 4500 h 1. 2.3. X-ray diffraction (XRD) X-ray diffraction measurement was carried out on a D/ MAX-RA rotatory diffractometer, using Cu Ka as radiation source (l = 0.15418 nm). The data were collected under the same conditions (40 kV and 100 mA). 2.4. X-ray photoelectron spectroscopy (XPS) XPS spectra were recorded on ESCA-LAB MK-II type spectrometer using Mg Ka as radiation source (1253.6 eV). The binding energies of the elements were corrected, using C1s (284.5 eV) peak as standard. 2.5. Extended X-ray absorption fine structure (EXAFS) The EXAFS data of Co K-edge and Pt L-III-edge were collected on the XAFS station, 1W1B beamline of Beijing Synchrotron Radiation Facility of National Laboratory (BSRF NL), using transition mode for Co K-edge and fluorescence mode for Pt L-III-edge. The critical beam energy was 2.2 GeV with an average storage ring current of 80 mA. Two Si(1 1 1) single crystals were used as monochromators. The structural parameters of the samples were obtained by curve-fitting method, using CoAl2O4 or Co metal as model compound for Co K-edge and PtO2 or Pt metal for Pt L-III-edge. The details of EXAFS data treatment were described elsewhere [17]. 2.6. Temperature-programmed reduction by H2 (H2-TPR) TPR experiment was carried out in a micro-reactor with a quartz tube (i.d. 4 mm). A given amount of the sample (100 mg, 40–60 mesh) was used each time. The reduced sample was initially pretreated at 500 8C in air for 1 h, then cooled to room temperature and finally reduced with hydrogen (5 vol.% H2 in N2) at a flow rate of 30 ml/min. The sample was heated up to 700 8C at a rate of 10 8C/min. The resulting gas mixture was analyzed by a gas chromatorgraph (model SP-2305, supplied by Beijing Analyzing Instruments Factory, China) equipped with a thermal conductivity detector (TCD) with the bridge current of 100 mA.
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2.7. Temperature-programmed oxidation-mass spectroscopy (TPO-MS) A given amount of the freshly reduced sample (500 mg, 40– 60 mesh) was fixed in a stainless tube (i.d. 6 mm) of the microreactor, and pretreated in a flow helium (99.99%) at 500 8C for 1 h. After cooling to room temperature, the TPO experiment was performed, using the mixture of 1.0% O2/N2 as oxidation gas (30 ml/min). The sample was heated up to 600 8C at a rate of 10 8C/min. The peak signal of m/z = 32 (O2 mass number) was detected by a quadruple mass spectrometer (model LZL203, supplied by Beijing Analyzing Instruments Factory, China). 3. Results and discussion 3.1. The activity for CO oxidation The results of activity evaluation are shown in Fig. 1. From Fig. 1, it can be seen that the Co/Ce-Al-O catalyst possesses good activity for CO oxidation. At 188 8C, the conversion of CO reaches 100%. When a small amount of Pt, Pd or Rh is added to Co/Ce-Al-O, the activity is greatly enhanced, the temperature for the 100% conversion of CO is decreased by 73, 63 and 44 8C, respectively. The activities of Co-Pt(Pd, Rh)/CeAl-O are also higher than those of Pt(Pd, Rh)/Ce-Al-O. These results reveal that there exists a pronounced catalytic synergy effect between cobalt phases and noble metals, especially between cobalt and platinum or palladium, though the loadings of noble metals are very low. To investigate the function of cerium, the catalytic activity of the samples without cerium was also evaluated for comparison. Among these catalysts, CoPt/g-Al2O3 shows the best activity, which is selected and presented in Fig. 1 (line with diamond symbol). By comparison, it is found that the sample Co-Pt/Ce-Al-O is
Fig. 1. The relationships between CO conversions and reaction temperatures over the samples: (*) Co-Pt/Ce-Al-O; (&) Co-Pd/Ce-Al-O; (&) Co-Rh/CeAl-O; (~) Pt/Ce-Al-O; (~) Pd/Ce-Al-O; () Rh/Ce-Al-O; (*) Co/Ce-Al-O; (^) Co-Pt/g-Al2O3.
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more active than Co-Pt/g-Al2O3, the temperature of full conversion of CO on the former is about 30 8C lower than that on the later, suggesting the cerium has played an important role during CO oxidation. As reported widely [19–22], the main function of CeO2 should be the storage capacity for oxygen, during the oxidation reaction, CeO2 can provide oxygen species to the close active phase, generating corresponding vacancies, which are eliminated by adsorption of oxygen from gas phase. In addition, cerium oxide may contact and interact with other components, such as alumina, noble metals and cobalt species. Since the loading of cerium oxide (CeO2/ Al2O3 = 20 wt%) has exceeded the amount for monolayer dispersion on alumina (in Section 3.2, diffracted peaks of CeO2 are detected), the most surface of alumina should be covered by cerium oxide. The function of alumina is mainly providing big surface area for the dispersion of cerium oxide, which is the real promoter for enhancing the catalytic activity. Considered the preparation procedure and the big difference of the loadings between cobalt phase and noble metals, the cobalt species have much larger opportunities to contact cerium oxide, and may interact with each other. Similar point of view was also proposed by Bruce et al. [23]. During CO oxidation, this kind of interaction makes the phase-to-phase interface between cobalt and cerium oxide geometrically continuous, and therefore facilitates the transferring of active species from cerium oxide to cobalt phase, or vice versa, which is indirectly supported by the H2-TPR and TPO-MS results in the following parts, where the cobalt phases effectively enhanced the reduction and oxidation of cerium oxides. 3.2. XRD The XRD patterns of the fresh Co-Pt(Pd, Rh)/Ce-Al-O catalysts are similar to each other, so, only that of Co-Pt/Ce-Al-O is selected and presented in Fig. 2. Since in other region of the XRD patterns, all peaks correspond to CeO2 and g-Al2O3, and no
Fig. 2. The XRD patterns of the samples: (a) Pt/Ce-Al-O; (b) Co/Ce-Al-O; (c) Co-Pt/Ce-Al-O.
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obvious difference is found for the three samples Pt/Ce-Al-O, Co/Ce-Al-O and Co-Pt/Ce-Al-O, only the signals in the region of 2u = 32–428 are amplified by four times and shown in Fig. 2. From Fig. 2(a), it can be seen that for sample Co/Ce-Al-O, there is a weak peak appearing at 2u = 36.98 (d = 0.2442 nm), which may be contributed by Co3O4 or CoAl2O4 phase because the strongest diffracted peaks of Co3O4 and CoAl2O4 are at the same positions (ICDD, PDF 80-1533 and 82-2248). Considered that Co3O4 is relatively easy to be reduced [24–26], the peak at 2u = 36.98 is assigned to CoAl2O4 or CoAl2O4-like spinel, which cannot be reduced by H2 below 500 8C [27,28]. With the addition of a small amount of platinum to Co/Ce-Al-O, the peak at 2u = 36.98 disappears (Fig. 2(c)), implying the complete reduction of the cobalt phase. Although no any peak of metallic cobalt is detected in the whole region of 2u = 10–708 for the samples containing cobalt, its existence is possible because the reduced cobalt phase may be highly dispersed in the catalysts or its amount is below the detecting limit of XRD.
3.4. EXAFS
To confirm the state of the surface cobalt, the XPS spectra of Co2p3/2 of the samples were recorded. The spectrum of Co2p3/2 for Co/Ce-Al-O is shown in Fig. 3(a). There are two peaks at the position of low binding energy (778.3 and 781.2 eV), and another weak peak at high binding energy position (786.0 eV). According to references [29–31], the former could be assigned to Co0 and Co2+ in spinel, respectively, while the later weak peak is the typical satellite peak of Co2+. Since the similarity of the Co2p3/2 spectra of the samples containing noble metals, only the spectrum of Co-Pt/Ce-Al-O is presented in Fig. 3(b). It is obvious that there is only one peak in Fig. 3(b), which corresponds to metallic cobalt. These results means that the addition of a small amount of noble metals does enhance the reduction deepness of cobalt phase, which is in good agreement with that of XRD.
The radial structure functions (RSFs) of Co K-edge EXAFS of the samples and model compounds are shown in Fig. 4. For Co metal, there is a strong coordination peak appearing at 0.21 nm (uncorrected). Considering the phase scattering shift, this peak should correspond to the first Co–Co shell in Co metal, whose real coordination distance is 0.250 nm. The RSFs of the samples promoted by a small amount of noble metals are very similar to that of Co metal, only the peak intensities are lower, while the RSF of Co/Ce-Al-O is typically different from that of Co metal, where appearing two coordination peaks at 0.15 and 0.27 nm. The first one can be well fitted by the first Co–O shell in model compound CoAl2O4, however, the second one cannot be fitted by either the Co–Co shell in cobalt metal or the Co–Co shell in CoAl2O4. The position of this peak (0.27 nm) is just between those of Co metal (0.21 nm) and CoAl2O4 (0.32 nm). Combined with XRD and XPS results, it is inferred that the second peak in the RSF of Co/Ce-Al-O sample are contributed by both Co metal and CoAl2O4 or CoAl2O4-like spinel. To obtain the structural parameters, curve-fitting was performed on the first shell, using Co metal and CoAl2O4 as the model compounds for Co-Pt(Pd, Rh)/Ce-Al-O and Co/CeAl-O, respectively. The best fitting values of the structural parameters are listed in Table 1. From Table 1, it can be seen that the coordination distance (R) of the first shell for sample Co/Ce-Al-O is close to that of CoAl2O4, implying that there still exists some Co–O coordination. With the addition of the noble metals to Co/ Ce-Al-O, the distances of the first shells change to be almost the same as that of Co metal, indicating that the Co–O shells have completely disappeared. Combined with the results of XRD and XPS, it is deduced that the Co mainly exists in metallic state in the samples promoted by a small amount of noble metals, while in Co/Ce-Al-O the Co exists as the mixture of Co metal and CoAl2O4 or CoAl2O4-like spinel. The presence of noble
Fig. 3. XPS spectra of Co2p3/2 of the samples: (a) Co/Ce-Al-O and (b) Co-Pt/ Ce-Al-O.
Fig. 4. The radial structure functions of Co K-edge of the samples and model compounds: (a) Co metal; (b) Co-Pt/Ce-Al-O; (c) Co-Pd/Ce-Al-O; (d) Co-Rh/ Ce-Al-O; (e) Co/Ce-Al-O; (f) CoAl2O4.
3.3. XPS
M. Meng et al. / Applied Catalysis A: General 301 (2006) 145–151 Table 1 Best-fitting values of the structural parameters for the first coordination shells in the samples from Co K-edge EXAFS Sample
Co-Pt/Ce-Al-O Co-Pd/Ce-Al-O Co-Rh/Ce-Al-O Co/Ce-Al-O CoAl2O4 a Co metal a
First coordination shell Shell
N
R (nm)
Residual (%)
Co–Co Co–Co Co–Co Co–O Co–O Co–Co
4.1 4.8 5.6 2.6 4.0 12.0
0.249 0.251 0.252 0.194 0.195 0.250
5.47 5.92 6.05 7.69
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coordination. These results indicate that most of platinum has been reduced to metal Pt, except for a small amount of platinum chloride. Similar phenomena have ever been found by Pirault et al. on Pt-Rh/Al2O3-CeO2 system [33]. 3.5. TPR results and hydrogen spillover
metals prominently enhanced the reduction of cobalt phases during the pretreatment. Additionally, the very low values of coordination number (N) indicate that the cobalt in the samples possesses very high dispersion, the order of the enhancement effect on cobalt dispersion is Pt > Pd > Rh, which is in accordance with activity law. Therefore, it is thought that the enhancement effect of noble metals on cobalt dispersion should be one of the main contributions to the catalytic synergy effect between cobalt phases and noble metals. In order to confirm the chemical state of the noble metals, we increased the platinum content to 1.0 wt.% in Co-Pt/Ce-Al-O, and recorded the EXAFS data using fluorescence mode. The radial structure functions of Pt L-III-edge for Co-Pt/Ce-Al-O and the model compounds are shown in Fig. 5. From Fig. 5, it can be seen that the main coordination peaks of the model compounds PtO2, H2PtCl66H2O and Pt metal appear at 0.16, 0.21 and 0.26 nm, respectively. For Pt metal, there is another weak peak at 0.22 nm, which is resulted from the non-linearity of the phase shift function. Compared with those of model compounds, the RSF of Co-Pt/Ce-Al-O is analogous to that of Pt metal. However, the weak peak appearing at 0.22 nm is more obvious and the ratio of this peak to the main peak is much larger than that of Pt metal, which implies that there may be still a little Pt-Cl
H2-TPR characterization is regarded as a mirror to reflect H2 spillover effect. In this work, the reduced samples were first reoxidized in air, then were used for TPR experiments. The TPR profiles of the reoxidized samples are shown in Fig. 6. For the support Ce-Al-O, there is a reduction peak at about 387 8C. Since alumina cannot be reduced by H2, this peak must correspond to the reduction of cerium species. Several studies [19,20,23] have ever indicated that there is a kind of oxygen species (called capping oxygen) in the surface of CeO2, which can be reduced below 500 8C, while the lattice oxygen is normally reduced at 750 8C or above. Therefore, the peak in Fig. 6(a) is assigned to the reduction of the capping oxygen in CeO2. When cobalt is supported on Ce-Al-O, there appear two reduction peaks at 350 and 468 8C. According to the study of literature [23], there exists strong interaction between cobalt and cerium oxides, which makes the reduction of the capping oxygen in CeO2 shift to lower temperature. So, in Fig. 6(b), the peak at low temperature is assigned to the reduction of the capping oxygen in CeO2, while the other at high temperature corresponds to the reduction of cobalt species. With the addition of noble metals to Co/Ce-Al-O catalyst (Fig. 6(c–e)), the two peaks shift remarkably to lower temperature direction. Compared with Fig. 6(b), it is found that the temperature for the first peak is decreased by 62, 75 or 93 8C, and that for the second peak is also decreased by 80, 91 or 99 8C for Rh, Pd or Pt-promoted sample, respectively. These results indicate that the presence of noble metals does make the reduction of cobalt and cerium species easier. Additionally, with increasing of noble metal contents, the reduction peaks shift monotonically
Fig. 5. The radial structure functions of Pt L-III-edge of the sample and model compounds: (a) Pt metal; (b) Co-Pt/Ce-Al-O; (c) H2PtCl66H2O; (d) PtO2.
Fig. 6. The H2-TPR profiles of the samples: (a) Ce-Al-O; (b) Co/Ce-Al-O; (c) Co-Rh/Ce-Al-O; (d) Co-Pd/Ce-Al-O; (e) Co-Pt/Ce-Al-O.
a The structural parameters of the model compounds are taken from reference [32].
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toward the direction of lower temperature (not shown). Similar phenomena are also found by Lin et al. [14] and Van’t Blik et al. [34] in other systems, where hydrogen spillover from platinum (or rhodium) to cobalt and from ruthenium to copper is proposed. It is well known that the hydrogen is normally chemically adsorbed on the noble metals, and is readily dissociated into atomic species, which are very active spillover species [35]. Therefore, in our case, it is assumed that the hydrogen molecule first adsorbs on Pt, Pd or Rh (donor) and dissociates into atomic species, then these active species spill over to the surrounding oxides, such as cobalt or cerium (acceptor) and react with them. Here, the essential of the spillover process is a catalytic gas (H2)–solid (cobalt or cerium oxides) reaction using noble metals as catalysts, which has prominently decreases the activation energy of this reaction. This point of view is just reflected and supported by the big shifts of the reduction peaks to lower temperatures in Fig. 6. According to the above analysis, it is believed that during the sample pretreatment by H2, the spillover of hydrogen species from noble metals to cobalt oxide may have taken place, leading to the different reduction deepness between the samples with and without noble metals. This point can be quantitatively proved by the analysis results of H2-TPR peak area (listed in Table 2). From Table 2, it can be seen that the difference for the first peak areas of the samples is insignificant, while that for the second peak areas is much more obvious between the samples promoted and non-promoted by noble metals. These results demonstrate that the amount of reducible cobalt species in Co/ Ce-Al-O is much less than those in Co-Pt(Pd, Rh)/Ce-Al-O, meaning that some cobalt phase in the sample without noble metals cannot be reduced below 700 8C by 5 vol% H2 in N2. Combined the structural characterization results of XRD, XPS and EXAFS, the non-reducible cobalt phase should be CoAl2O4 or CoAl2O4-like spinel. The enhancement effect of the noble metals on the reduction of cobalt species may be an important source for the catalytic synergy effect between cobalt phases and noble metals. The sequence for this effect is Pt > Pd > Rh, which is the same as that for the activity enhancement effect. Now, there is still a question remaining, that is, how the so small amount of noble metals enhances the reduction of much larger amount of cobalt oxide during H2-TPR process. On the basis of the loadings of noble metals and cobalt phase, it is not difficult to understand that in the catalysts noble metals can only contact part of the cobalt oxide, and interact with each other, which is called ‘‘Close cobalt phase’’, the other cobalt oxide is called ‘‘Remote cobalt phase’’. Normally, noble metals can dissociate adsorbed H2 molecule into active species, such as
atomic H, at lower temperature than cobalt oxide. Assuming that the lowest temperature for dissociating H2 on one kind of noble metal is TN, while that on cobalt oxide is TC, when temperature is between TN and TC, no dissociated hydrogen species are formed on cobalt oxide, however, such species can be generated on the noble metals, which can spill over to the ‘‘Close cobalt phase’’. If the spillover process stops here, the amount of reduced cobalt oxide will be very small, because the ‘‘Remote cobalt phase’’ will not be reduced. However, the fact in this work is opposite, so, it is inferred that the spilled hydrogen species must be mobile on the surface, which can further transfer from close cobalt oxide to remote oxide sites, including cobalt oxide and cerium oxide, and finally make them be reduced. 3.6. TPO-MS results and oxygen spillover The TPO-MS profile for the support Ce-Al-O is presented in Fig. 7(a). There is a O2-consumption peak at 212 8C. It is well known that the cerium oxide is a non-stoichiometric compound (CeOx, 1.5 < x < 2.0). When reduced, some of the surface oxygen, such as capping oxygen, is removed, and the corresponding vacancies are generated. When exposed to oxygen at appropriate temperature, these vacancies can be eliminated through oxidation or storage. Therefore, the peak in Fig. 7(a) can be assigned to the oxidation of cerium species or the storage of oxygen. For Co/Ce-Al-O, there is another O2consumption peak appearing at 307 8C, which should be resulted from the oxidation of cobalt species. With the addition of noble metals to Co/Ce-Al-O, both the two peaks shift to lower temperature direction. The temperature for the first peak is decreased by 56, 37 or 12 8C, and that for the second peak is also decreased by 65, 48 or 20 8C for Pt, Pd or Rh-promoted sample, respectively. With increasing of noble metal contents, the peaks shift to lower temperatures further (not shown). All these phenomena are very similar to those found in H2-TPR. Therefore, it is thought that the oxygen spillover may have taken place during the TPO process. The Pt, Pd or Rh acts as
Table 2 The relative peak areas of H2-TPR peaks in Fig. 6 after normalization Sample
Relative area a
Co-Pt/Ce-Al-O Co-Pd/Ce-Al-O Co-Rh/Ce-Al-O Co/Ce-Al-O Ce-Al-O
0.98 0.96 0.97 0.96 1.00
a
(257) (275) (288) (350) (387)
The data listed in the parentheses are peak top temperatures.
1.83 1.80 1.78 0.98 –
(369) (377) (388) (468) Fig. 7. The TPO-MS profiles of the samples: (a) Ce-Al-O; (b) Co/Ce-Al-O; (c) Co-Rh/Ce-Al-O; (d) Co-Pd/Ce-Al-O; (e) Co-Pt/Ce-Al-O.
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donor to provide active oxygen species, while the reduced phases act as acceptor to accept the spilled over oxygen species and react with them. For enhancing the oxidation of cobalt species, the order for the noble metals is still Pt > Pd > Rh. In the course of CO oxidation, it is believed that the noble metals have played a key role for adsorbing, activating and providing oxygen species for cobalt phases through spillover. If the oxygen adsorption and dissociation is the rate-determining step, the oxygen spillover must decrease the activation energy of CO oxidation reaction and therefore prominently enhance the activity of Co/Ce-Al-O. Similar to that in H2-TPR process, it is supposed that the spilled over oxygen species from noble metal sites, such as atomic O, are also mobile on the catalyst surface, so that not only the close reduced cobalt phase, but also the remote reduced cobalt phase are involved in the CO oxidation reaction. As a summary, the oxygen spillover effect during CO oxidation is regarded as another important source for the catalytic synergy effect between cobalt phases and noble metals. 4. Conclusions (1) For CO oxidation, the addition of 0.1 wt% noble metals (Pt, Pd or Rh) to Co/Ce-Al-O catalyst, the activity is greatly enhanced. The catalytic synergy effect between such three kinds of noble metals and cobalt phases is very pronounced. The sequence of the noble metals for the activity enhancement is Pt > Pd > Rh. The interaction between cobalt phases and cerium oxide also play a role for improving the activity of the samples. (2) The existence of 0.1 wt% noble metals prominently decreases the reduction temperature of cobalt oxides and increases their reduction deepness. The order for this function is Pt > Pd > Rh. During reduction pretreatment, a H2 spillover process may take place, and it is supposed that the active spilled over hydrogen species, such as atomic H, be mobile from noble metals to the surrounding cobalt oxides, and further to the remote cobalt oxides. This is a reasonable explanation for the enhancement effect of so small amount of noble metals on the reduction of a much larger amount of cobalt oxides. The increase of reduction deepness produces more active species accessible to reaction gases, which should be one of the main contributions to the catalytic synergy effect between such small amount of noble metals and cobalt phases. (3) The promoting effect of noble metals on cobalt phase dispersion should be another important source for the catalytic synergy effect between noble metals and cobalt phases. The order for this promoting effect is Pt > Pd > Rh, which is totally consistent with that for the catalytic activity. (4) During CO oxidation, the spillover of oxygen species from noble metals to cobalt phases is suggested, and the spilled over oxygen species are believed to be mobile from the close cobalt phase to remote cobalt phase. As a result, the spillover effect of oxygen decreases the activation energy of CO oxidation reaction, and therefore it is regarded as the third main contribution to the catalytic synergy effect between noble metals and cobalt phases.
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