Identification of the Active Sites for CO and C3H8 Total Oxidation over

May 16, 2008 - Nanostructured Co3O4−CeO2 and CuO−CeO2 catalysts with the specific surface areas exceeding 100 m2 g−1 were synthesized by a ...
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8694

J. Phys. Chem. C 2008, 112, 8694–8701

Identification of the Active Sites for CO and C3H8 Total Oxidation over Nanostructured CuO-CeO2 and Co3O4-CeO2 Catalysts Jin-Yong Luo, Ming Meng,* Yu-Qing Zha, and Li-Hong Guo Tianjin Key Laboratory of Catalysis Science and Engineering, Department of Catalysis Science and Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed: January 23, 2008; ReVised Manuscript ReceiVed: March 20, 2008

Nanostructured Co3O4-CeO2 and CuO-CeO2 catalysts with the specific surface areas exceeding 100 m2 g-1 were synthesized by a surfactant-templated method. The catalytic performance of these catalysts was investigated using the total oxidation of CO and C3H8 as model reactions. The results show that the Co3O4-CeO2 catalysts are less active for CO oxidation but are more active for C3H8 oxidation as compared with the CuO-CeO2 catalysts. Moreover, the Co3O4-CeO2 catalysts exhibit a volcano-type performance for CO oxidation with the cobalt content increasing. The in situ diffuse reflectance infrared spectroscopy (DRIFTS) study shows that CO is adsorbed mainly as carbonyl (2106 cm-1) and bidentate carbonate (1568 and 1281 cm-1) on CuO-CeO2, and only as bidentate carbonate (1591 and 1268 cm-1) on Co3O4-CeO2. On the basis of the results of structural characterization, redox properties, and in situ DRIFTS study, the active sites for CO and C3H8 oxidation are identified, respectively. Carbon monoxide oxidation preferentially occurs at the interface between CeO2 and CuO or Co3O4, whereas propane oxidation takes place on the neighboring surface lattice oxygen sites in CuO or Co3O4 crystallites. The different requirements of the active sites are determined by the different reaction mechanisms and the rate-determining steps. It is also found that the introduction of a small amount of Pd to Co3O4-CeO2 can remarkably promote the CO oxidation activity, but it hardly enhanced the C3H8 oxidation activity of the catalyst. The different reaction mechanisms, on molecular level, are identified and discussed in detail. 1. Introduction Recently, stringent regulations for super low-emission vehicles (SULEV) and zero-emission vehicles (ZEV) have presented great demand to reduce carbon monoxide (CO) and hydrocarbons (HC) in the cold-start exhaust, because a considerable fraction (50∼80%) of the total emission is released into the air within the first 200 s after engine ignition.1 Therefore, it is highly recommended to explore a low-temperature oxidation catalyst for the removal of CO and hydrocarbons. For CO oxidation, great attention has recently been paid to nanostructured gold catalysts because of their unique low-temperature oxidation activity and water-resistance ability; however, the very low thermal stability of these catalysts has limited their practical application in exhaust purification. 2–4 In the last several years, some base-metal oxides, CuO and Co3O4 in particular, have been the focus of many researches because of their high CO oxidation activity and low costs. It has been reported that the CuO-CeO2 catalysts are comparable to some noble metal catalysts for CO oxidation, and that some Co-based catalysts exhibits ultralow light-off temperature T50 (temperature at 50% conversion) only at -63 °C for CO oxidation. 5,6 In addition, cobalt oxides are reported to also be very active for hydrocarbon combustion.7,8 Concerning cerium oxide, it has been widely regarded as a key additive to three-way catalysts, due to its large oxygen storage capacity and dispersion enhancement ability. Therefore, it is feasible to develop nanostructured CuO-CeO2 and Co3O4-CeO2 as novel catalysts used for the total oxidation of CO and C3H8. * Corresponding author phone/fax: +86-22-2789-2275; e-mail: mengm@ tju.edu.cn.

With respect to total oxidation of carbon monoxide and alkanes, it seems that different mechanisms and different active sites are often involved. For example, over the nanostructured gold catalysts supported on metal oxides, CO oxidation is support-dependent, and no clear correlation between the nature of the supporting oxide and the activity for C3H8 oxidation can be found.9 However, the performance for the total oxidation of CO and C3H8 of these catalysts was affected differently by SO2 exposure. Over the Au/TiO2 catalysts, the low-temperature CO oxidation is severely inhibited by exposure to SO2, whereas the effect on propane oxidation is quite small. However, an enhanced propane oxidation activity can be observed over the Pt/TiO2 catalyst by exposure to SO2.10 It is even reported by Luo et al. that over PdO/Ce0.8Y0.2O1.9 catalysts, CO oxidation takes place on both the highly dispersed and the crystalline PdO sites, whereas CH4 oxidation takes place only on the crystalline PdO sites.11 Similarly, different mechanisms are also revealed by Royer et al. on LaCo1-xFexO3 catalysts.12 They have proposed that the CO oxidation involves the adsorbed oxygen via the suprafacial mechanism, whereas the CH4 oxidation involves lattice oxygen via the intrafacial mechanism. Concerning the catalysts for the total oxidation of C3H8, most of the research has been focused on noble metals (Pt, Pd, and Au) and perovskites catalysts; investigations concerning these ceria-based mixed oxides are relatively rare, and few comparisons have been made between CO and C3H8 oxidation over these mixed metal oxide catalysts. It is well-known that the preparation methods often play a crucial role in determining the component dispersion and interaction mode, which will influence the catalytic performance of the resultant catalysts to a great extent. Recently,

10.1021/jp800651k CCC: $40.75  2008 American Chemical Society Published on Web 05/16/2008

Active Sites for CO and C3H8 Total Oxidation

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using surfactant cetyltrimethyl ammonium bromide (CTAB) as a template, mesoporous mixed oxides La-Co-Ce-O, La-Co-Zr-O and CuO-CeO2 catalysts with large surface areas have been successfully synthesized, which show high catalytic activity for CO oxidation due to the existence of large amounts of active sites.13–15 Consequently, in the present work, a series of mesoporous Co3O4-CeO2 and CuO-CeO2 catalysts with high surface area were prepared, and their activities for CO and C3H8 oxidation were evaluated and compared. The active site requirements, the reaction mechanisms, and the rate-determining steps are determined, based upon the results of structural characterization, the redox properties, in situ DRIFTS studies, and the catalytic performance. 2. Experimental 2.1. Catalyst Preparation. CuO-CeO2 and Co3O4-CeO2 catalysts with various compositions were prepared by a surfactant-templated method using CTAB as the template. Appropriate amounts of CuCl2 · 2H2O or CoCl2 · 6H2O, CeCl3 · 7H2O (Shanghai Chemical Reagent), and CTAB (Fuchen Chemical Reagents Factory) were dissolved in distilled water; then, an aqueous solution of NaOH (2 M) was added dropwise until the pH arrived at ca. 11. After stirring for 2 h, the obtained suspension was transferred to a Teflon-sealed autoclave and hydrothermally aged at 120 °C for 48 h. After filtration, washing, and drying, the resulting powder was calcined in air at 500 °C for 4 h. The catalysts were denoted as CeMx, where M stands for the transition metal Co or Cu, and x stands for the M/(M + Ce) atomic ratio. A part of the catalyst CeCo30 was impregnated in the aqueous solution of Pd(NO3)2 to prepare the supported Pd catalyst with the loading of 0.5 wt%. The catalyst was calcined at 500 °C for 1 h and is denoted as Pd/CeCo30. For comparison, 0.5 wt% Pd/CeO2 catalyst was also prepared by the same impregnation. The support CeO2 was prepared by thermal decomposition of Ce(NO3)3 · 6H2O at 600 °C for 4 h (SBET ) 50.4 m2 g-1). 2.2. Catalytic Activity Measurement. The catalytic activity measurement was carried out in a continuous fixed-bed quartz tubular reactor (i.d. ) 8 mm) mounted in a tube furnace. The feed and product mixtures were analyzed by a gas chromatograph (BFS SP-3430) equipped with thermal conductivity and flame ionization detectors. The feed gas mixture, consisting of 1% CO and 5% O2, or 1% C3H8 and 10% O2, balanced by N2, was led over the catalyst (600 mg) at a flow rate of 100 mL min-1, equivalent to a weight hourly space velocity (WHSV) of 10 000 mL g-1 h-1. 2.3. Catalyst Characterization. Surface area, pore volume, and pore size distribution were measured by nitrogen adsorption/ desorption at 77 K using a Quantachrome NOVA-2000 instrument. The surface area (SBET) was determined by the BET method in the 0∼0.3 partial pressure range, and the pore size distribution was calculated, using the Barrett-Joyner-Halenda (BJH) formula, from the desorption branch of the isotherm. X-ray diffraction measurement (XRD) was performed on an X’pert Pro rotatory diffractometer operating at 40 mA and 40 kV using Co KR radiation (λ ) 0.1790 nm). The average crystallite size of CeO2 was calculated, using the Scherrer equation, from the line broadening corresponding to crystal plane (111) without incorporating microstrain effects. Transmission electron microscopy (TEM) images were obtained by a Philips Tecnai G2F20 system operating at 200 kV. The sample for the TEM observation was prepared by dipping a copper-gridsupported transparent carbon foil in an ethanol solution in which the samples were suspended by sonication, and the grid was dried in open air.

Figure 1. N2 adsorption/desorption isotherms of the catalysts (inset: BJH pore diameter distribution).

Temperature programmed reduction by H2 (H2-TPR) was performed on a Thermo-Finnigan TPDRO 1100 instrument with a thermal conductivity detector (TCD). The quartz tube reactor was loaded with a 50 mg sample in powder form and was heated from room temperature to 900 °C in a 5% H2/N2 mixture (v/v, the following is the same) with a flow rate of 20 mL/min. Before detection by TCD, the gas was purified by a trap containing CaO + NaOH materials in order to remove H2O and CO2. By replacing the 5% H2/N2 with 5% CO/He, the CO-TPR tests were also carried out. After the TPR was tested to 600 °C and held for 10 min, temperature programmed oxidation (TPO) tests were subsequently performed from room temperature to 900 °C in 6% O2/He with a flow rate of 20 mL/min. Prior to temperature programmed desorption of O2 (O2-TPD), 200 mg sample was preoxidized in pure O2 from room temperature to 500 °C and held for 30 min. After cooling to room temperature, the sample was heated from room temperature to 900 °C in pure He. The amount of desorbed oxygen was quantified by pulsing a known amount of pure oxygen at the end of the test. In situ diffuse reflectance infrared spectroscopy (DRIFTS) measurement was performed on a Nicolet Nexus spectrometer equipped with a MCT detector cooled by liquid nitrogen. First, the powdered sample (30 mg) was treated in situ at 300 °C in 6% O2/He with a flow rate of 50 mL min-1 to eliminate water traces. After the system was cooled to room temperature, a background spectrum was collected for spectra correction. Then, 3% CO was introduced to the in situ chamber. The spectra were collected at each temperature, accumulating 32 scans at a resolution of 2 cm-1 and displayed in Kubelka-Munk units. 3. Results and Discussion 3.1. Structural Properties. Figure 1 shows the typical nitrogen adsorption/desorption profiles and the pore size distribution (insert) of the samples CeCu20, CeCo30, and Pd/ CeCo30. All these catalysts exhibit typical isotherms of mesoporous structure, with uniform mesopores centered at 4.9 nm. The texture data of the catalysts are listed in Table 1. All these samples exhibit high surface area exceeding 100 m2 g-1 after calcination at 500 °C. Among them, the catalyst CeCu20 possesses the highest surface area of 139 m2 g-1. Regarding

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TABLE 1: Texture Properties, Crystallite Size, Oxygen Desorption Amount and Catalytic Performance of the Catalysts sample

SBET (m2 g-1)

pore diameter (nm)

crystallite size (nm)a

O2R (µmol/g)

O2β (µmol/g)

T50b

T50c

CeCo15 CeCo20 CeCo30 CeCo40 Pd/CeCo30 CeCu20

107 100 121 106 85 139

3.6 3.8 4.9 6.1 4.9 4.9

7.0 7.1 6.0 6.2 6.7 5.5

23.4 25.3 37.5 29.4 23.8 0

48.2 40.7 58.0 59.5 27.5 9.7

138 120 94 101 58 61

253 254 223 223 223 279

a Calculated from the line broadening of the (111) reflection of CeO2 using the Scherrer equation. b Temperature of 50% conversion for CO oxidation. c Temperature of 50% conversion for C3H8 oxidation.

Figure 2. TEM or HR-TEM images of the catalysts (a) CeCo30, (b) Pd/CeCo30, (c) and CeCu20; (d) HR-TEM image of CeCo30.

the cobalt catalysts, CeCo30 shows the largest surface area of 121 m2 g-1. After impregnation with Pd, the surface area of the catalyst CeCo30 decreases to 85 m2 g-1 without appreciable change in the pore diameter. The XRD patterns of these catalysts exhibit typical diffraction peaks of fluorite CeO2, as well as evident peaks of Co3O4 for Co3O4-CeO2 catalysts and very weak peaks of CuO for the CeCu20 catalyst (see Supporting Information Figure S1). The CeO2 crystallite sizes are calculated from the line broadening of the (111) reflection of CeO2, using the Scherrer equation, and the results are listed in Table 1. In accordance with the surface area measurement, the CeCu20 catalyst has the smallest CeO2 crystallite size of only 5.5 nm. Among the Co3O4-CeO2 catalysts, the sample CeCo30 possesses the smallest CeO2 crystallite size of 6.0 nm. The TEM images of CeCo30, Pd/CeCo30, and CeCu20 catalysts are shown in Figure 2, panels a-c, respectively, and the HR-TEM image of CeCo30 is displayed in Figure 2d. Small grape-like nanoparticles with uniform size can be identified in these samples without the formation of the well-defined structure such as that in the Si-containing molecular sieves, suggesting that the mesopores with uniform size determined by the N2 physisorption are randomly distributed among these particles. From the HR-TEM image of CeCo30 shown in Figure 2d, it can be deduced that all these observed particles are mainly composed of CeO2 crystallites, preferentially exposing the crystal planes of (111) with the interplanar spacing of 0.31 nm.

Figure 3. Light-off curves for CO and C3H8 oxidation over different catalysts. Conditions: (a) 1% CO, 5% O2, and balance N2, WHSV ) 10 000 mL g-1 h-1; (b) 1% C3H8, 10% O2 and balance N2, WHSV ) 10 000 mL g-1 h-1.

3.2. Catalytic Performance. The results of the catalytic evaluation for CO and C3H8 total oxidation are shown in Figure 3, panels a and b, respectively. The corresponding light-off temperatures are listed in Table 1. For CO oxidation over Co3O4-CeO2 catalysts, a volcano-type performance can be observed, namely, with the cobalt content increasing gradually to 30%, the activity of the catalysts is increased correspondingly at first, then subsequently decreased with further increase of the cobalt content to 40%. The most active CeCo30 catalyst exhibits a light-off temperature T50 value of only 94 °C. After deposition of 0.5 wt% Pd over this catalyst, greatly enhanced CO oxidation activity can be achieved. The corresponding light-

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Figure 5. CO-TPR profiles of the catalysts. Figure 4. H2-TPR profiles of the catalysts.

off temperature T50 is only 58 °C, lowered by 36 °C as compared with that of the unpromoted one. Additionally, it is surprising that this promoted catalyst exhibits some oxidation activity even at room temperature. By contrast, the Pd/CeO2 catalyst shows the light-off temperature T50 as high as 138 °C, suggesting that there exists strong synergism between Co3O4 and Pd species in Pd/CeCo30. As expected, the CeCu20 catalyst shows excellent CO oxidation activity, with the T50 value at 61 °C, very close to that of Pd/CeCo30, but in the low temperature region below 50 °C, the catalyst Pd/CeCo30 is still more active than CeCu20. The C3H8 oxidation over these catalysts, as shown in Figure 3b, presents a quite different tendency. Over Co3O4-CeO2 catalysts, the volcano-type activity is not observed with the cobalt content increasing, whereas both catalysts CeCo30 and CeCo40 exhibit the best oxidation ability, with the T50 value at 223 °C, lowered by 30 °C as compared with catalysts CeCo15 and CeCo20. At the same time, the deposition of Pd over the CeCo30 catalyst can hardly enhance the C3H8 oxidation activity, which is different from the case for CO oxidation. It is worth noting that although the CeCu20 catalyst shows excellent CO oxidation activity, it exhibits rather poor C3H8 oxidation activity with the light-off temperature T50 at 279 °C, increased by 56 °C as compared with the CeCo30 catalyst. The obvious differences in the activities of CO and C3H8 oxidation over these catalysts strongly suggest different mechanisms for CO and C3H8 oxidation; therefore, careful characterization on these catalysts is highly required. 3.3. Redox Properties from Temperature-programmed Techniques. The H2-TPR profiles of these catalysts are shown in Figure 4. Pure CuO and Co3O4 are characterized by a single reduction peak at 345 and 434 °C, respectively. The CeCu20 catalyst exhibits two major reduction peaks in the low temperature range, one at 154 °C and the other at 182 °C, which can be attributed to the reduction of well-dispersed CuO species that strongly interacted with CeO2 and slightly larger CuO crystals that interacted less with CeO2, respectively;16 At the same time, the catalyst CeCo30 also exhibits two major reduction peaks, one at a relatively low temperature of 290 °C and the other at a temperature as high as 560 °C, which can be attributed to the reduction of Co3O4 interacting with CeO2 to CoO and the subsequent reduction of CoO to metallic Co.13,17 From these reduction patterns, it can be seen that the interaction between the transition metal oxides TMOs (CuO, Co3O4) and

CeO2 promotes the reduction of these transition metal oxides. In addition, the small peak at a high temperature of 820 °C is assigned to the bulk reduction of CeO2. After promotion of the CeCo30 catalyst with Pd, both reduction steps of the cobalt species are greatly enhanced, with the reduction temperature as low as 154 and 350 °C. Two important aspects, including hydrogen spillover and weakened Co-O bond strength due to the introduction of Pd, are strongly proposed for interpretation. For figuring out which one is predominant, CO-TPR measurements were performed, and the results are shown in Figure 5. Similar to H2-TPR results, the interaction between Co3O4 and CeO2 contributes to the easier reduction of the cobalt species, as indicated by the comparison between the reduction peak at 256 °C for the CeCo30 catalyst and that at 370 °C for bulk Co3O4. The relatively small peak at 256 °C can be attributed to the reduction of Co3O4 on the surface as a result of the surfaceto-bulk reduction characteristic of the CO-TPR.18 After introduction of Pd, no appreciable change can be observed with respect to the reduction of the cobalt species, suggesting that the Co-O bond is not weakened by the addition of Pd. Therefore, the promotion in the reduction of the cobalt phase during H2-TPR is really resulted from the hydrogen spillover. The key step during this process is actually attributed to the dissociation of H2 molecules into more reactive H atoms on Pd sites, whereas such an activation process will not occur during CO-TPR. In addition, the CeCu20 catalyst gets reduced at very low temperature, which is due to the strong interaction between CuO and CeO2. The TPO profiles of these catalysts are shown in Figure 6. The sample CeCo30 exhibits two peaks, one oxygen consumption peak at 211 °C, due to the oxidation of metallic Co to Co3O4, and another oxygen release peak at 880 °C, due to the thermal decomposition of Co3O4 to CoO.19 After promotion with Pd, the temperature for the oxidation of Co to Co3O4 is decreased from 211 to 165 °C, lowered by 46 °C, indicating that the presence of Pd promotes the oxidation of the cobalt phase. Similar to H2-TPR, the enhanced oxidation properties of the cobalt phase is related to the oxygen spillover effect. As it is well-known, the noble metal Pd possesses intrinsically high potential in dissociating molecules such as H2 and O2. The generated H and O atoms are considered to be rather mobile and reactive, readily reacting with the cobalt phase through spillover on the support, resulting in the enhancement of both reduction and oxidation of the cobalt phase. The TPO profile of Cu in the Cu-CeO2 catalyst consists of two peaks at very low temperatures, one at 130 °C and another

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Figure 6. TPO profiles of the catalysts.

Figure 7. O2-TPD profiles of the catalysts.

at 210 °C, corresponding to two types of Cu species with various particle sizes and different interaction modes with CeO2, which is consistent with the two kinds of CuO species identified in the H2-TPR profile. In addition, an incomplete oxygen release peak appears at the temperature higher than 950 °C, which is attributed to the thermal decomposition of CuO into Cu2O. The O2-TPD profiles of these catalysts are shown in Figure 7. The oxygen desorption peaks in the low temperature region below 700 °C are much smaller than those in the high temperature region above 700 °C. For clarity purpose, the profiles for different regions are displayed separately (ratio of y-axis scale: 1/6). Three oxygen desorption peaks are observed for Co3O4-CeO2 catalysts, denoted as R, β and γ, which can be attributed to the oxygen desorption from the cobalt sites since no oxygen is desorbed on pure CeO2 (figure not shown). The low temperature peak R, centered at ca. 180 °C, is related to oxygen species adsorbed on the surface, such as O2-. In literature, trivalent or even tetravalent cobalt species can be prepared by using a precipitation-oxidation method, suggesting that the cobalt phase possesses O2-adsorbing ability after oxidation pretreatment.20 Meanwhile, Co3+O2- species have also been proposed in the LaCoO3 perovskite.21 In Co3O4-CeO2 catalysts of this work, such adsorbed species have been evidently characterized by the small peak at 132 °C in the H2-TPR profile. However, it is quite interesting that no such oxygen species are desorbed from the surface of CeCu20, which is consistent with the absence of the reduction peak for adsorbed oxygen in its H2-TPR profile.

Luo et al. The large peak γ can be attributed to the thermal decomposition of the bulk oxides Co3O4 and CuO. Such decomposition also occurs in the TPO process as shown in Figure 6, but the corresponding temperature is higher, which is due to the inhabitation effect of O2. With respect to peak β, it possesses similar characteristics to those related to peak γ, for example, these two peaks are very close or even overlapped, and they have the same temperature sequence for oxygen desorption from both CuO-CeO2 and Co3O4-CeO2 catalysts. On this basis, it can be considered that peak β is also related to the desorption of oxygen from the transition metal oxides, most probably from the surface lattice oxygen of CuO and Co3O4, because it is expected that they are more weakly bonded to the surface transition metal ions as compared with the bulk lattice oxygen. The bulk CuO in CeCu20 decomposes at higher temperature (870 °C) than bulk Co3O4 in the Co3O4-CeO2 catalysts (836 °C); accordingly, the surface lattice oxygen of CuO also desorbs at higher temperature (634 °C) than that of Co3O4 (584 °C). It is well-documented that the surface oxygen species are usually relevant to the catalytic activity; the amounts of the surface-desorbed oxygen from peak R and β are, therefore, calculated, and the results are listed in Table 1. Combining with the oxidation activity of the Co3O4-CeO2 catalysts, it is found that the adsorbed oxygen species (peak R) are relevant to CO oxidation, and the surface lattice oxygen species (peak β) are related to C3H8 oxidation. The larger amounts of the desorbed oxygen species correspond to the higher oxidation activity. However, with respect to the CeCu20 catalyst, although it does not desorb any surface-adsorbed oxygen, it shows excellent CO oxidation activity. Meanwhile, it is worth noting that after promotion of the CeCo30 catalyst with Pd, the amounts of both types of desorbed oxygen species are decreased, but no decline in its activity is observed. To answer these questions, the in situ DRIFTS technique is employed to reveal the reaction pathways for CO and C3H8 oxidation over Co3O4-CeO2 and CuO-CeO2 catalysts, as well as the effect of the small amount of Pd. 3.4. In-situ DRIFTS Study. An in situ DRIFTS study was performed for CO oxidation, and the results are shown in Figure 8. Bands at 2172 and 2116 cm-1 are assigned to the gas phase CO, and bands at 2359 and 2342 cm-1 are assigned to gas phase CO2. Figure 8, panels a and b, shows the CO/O2 adsorption over the CeCo30 catalyst at room and different temperatures. Upon exposure to CO/O2, very strong bands at 1591 and 1268 cm-1, due to bidentate carbonate, quickly develop.22–24 Subsequently, the ambient CO oxidation occurs, accompanied with the appearance of the very strong bands of gas-phase CO2, as well as the bands at 1403 and 1216 cm-1 assigned to gas-phase CO2 adsorption on the catalyst as bicarbonate.24 However, the initial reaction is short-lived and stops after 12 min of exposure. The consumption of the adsorbed oxygen species is possibly responsible for the disappearance of ambient CO oxidation, because they cannot be regenerated at room temperature. Upon heating, the species due to CO2 adsorption gradually desorb until 70 °C. Above this temperature, CO oxidation is accelerated again, which can be seen from the increase in the gas phase CO2 and the corresponding carbonate bands due to CO2 adsorption, whereas bands of bidentate carbonate species decrease. The spectra of CO/O2 reaction over Pd/CeCo30 (figures not shown) are similar to those of CeCo30, except that the intensity

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Figure 8. In situ DRIFT spectra of the catalysts after exposure to CO/O2; (a) CeCo30 at 25 °C, (b) CeCo30 at different temperatures, (c) CeCu20 at 25 °C, and (d) CeCu20 at different temperatures.

of the bidentate carbonate bands decreases to some extent, indicating the decreased CO adsorption capacity after Pd deposition. Figure 8, panels c and d, shows the spectra of CO/O2 reaction over CeCu20. Upon exposure to CO/O2 at room and different temperatures, a carbonyl band at 2093 cm-1, due to CO adsorbed over Cu+ sites, quickly appears.25 At longer exposure time, bands at 1568 and 1284 cm-1, due to bidentate carbonate, develop, as well as some polydentate carbonate at 1470, 1395, and 1050 cm-1.23 Because of the electronic effects induced by these carbonates at the Cu+ sites, the carbonyl band shifts to 2106 cm-1. Generally, the CuO-CeO2 catalyst is incapable of oxidizing CO at room temperature (confirmed by the absence of gas phase CO2 here).25 On this basis, the carbonates may be originated from CO adsorption but not from CO2 adsorption. According to our DRIFTS study, pure CeO2 almost does not adsorb CO (figures not shown), so it is very likely that the CO adsorbed on CeO2 as carbonates may be spilled over from Cu+ sites. Actually, such a spillover-assisted CO adsorption from Pt to CeO2 as bidentate carbonate has been evidenced in reduced Pt/CeO2, as indicated by the very strong bands at 1587 and 1294 cm-1.26 Upon heating the CeCu20 catalyst, the carbonyl band gradually decreases in its intensity, and bands of bidentate carbonate shift to lower frequencies with the intensity decreased

to some extent; However, CO oxidation occurs, which can be indicated by the bands of gas phase CO2, as well as the corresponding adsorption bands of bicarbonate at 1394 and 1218 cm-1. 3.5. Mechanism Suggestion. 3.5.1. CO Oxidation. In the former section, both the CO-TPR and the H2-TPR results have shown that CeCu20 can be reduced at very low temperature, lower than Co3O4-CeO2 catalysts. Logically, the CuO-CeO2 catalysts exhibits higher CO oxidation activity than the Co3O4-CeO2 catalysts. It seems that the CO oxidation activity over transition metal oxides strongly depends on their redox properties. According to the DRIFTS study, CO can be effectively adsorbed on the catalyst surface as bidentate carbonate or carbonyl species at low temperature. Therefore, it is deduced that the oxygen extraction assisted by these adsorbed species should be a crucial step for the successive CO oxidation. TPR analysis indicates that the interaction between transition metal oxides (Co3O4, CuO) and CeO2 can contribute to the easier reduction of the oxides. The interaction can be essentially described as lengthening and weakening of the M-O (M)Cu or Co) bond by sharing oxygen at the interface, which has been theoretically confirmed in the Au, Ag, and Cu doped CeO2 systems using density functional theory.27 Therefore, during CO oxidation, it can be expected that the oxygen extraction

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Figure 10. Schematic illustration of the site requirements for CO and C3H8 oxidation over Co3O4-CeO2 or CuO-CeO2 catalysts.

Figure 9. Schematic models for CO oxidation over different catalysts (a) CeCo30, (b) CeCu20, and (c) Pd/CeCo30.

preferentially takes place at the interface between the transition metal oxides and CeO2. In other words, the most active sites for CO oxidation exist at the MO-CeO2 interface, and the most active oxygen species are the shared interfacial oxygen between MO-CeO2. On this basis, the reaction models for CO oxidation over Co3O4-CeO2 and CuO-CeO2 catalysts are proposed, as shown in Figure 9, panels a and b. In these models, ceria is chemically involved in the CO oxidation. With respect to the Co3O4-CeO2 catalysts, the volcano-type CO oxidation performance can be interpreted as follows: with increasing cobalt content to 30%, CO oxidation activity is enhanced due to the increased Co3O4-CeO2 interface area, whereas upon further increasing the cobalt content to 40%, bulklike Co3O4 interacting weakly with CeO2 is formed, leading to the lowered Co3O4 dispersion and possibly the decreased interface area (see Supporting Information Figure S2); as a consequence, the CO oxidation activity decreases. Such a volcano-type behavior has also been observed in the CuO-CeO2 systems.28 3.5.2. C3H8 Oxidation. Although the CeCu20 catalyst exhibits superior redox properties than Co3O4-CeO2 catalysts, as reflected by both TPR and TPO analysis, it is less active for C3H8 oxidation, suggesting that C3H8 oxidation cannot be simply correlated with the redox properties. Generally, during the combustion of different types of saturated hydrocarbons, the light-off temperature is closely related to the strength of the C-H bond; therefore, the C-H bond breaking is a crucial or even the rate-controlling step during the combustion of these compounds over both noble metal and metal oxides catalysts.29,30 The activation of propane over metal oxides is thought to occur by abstraction of hydrogen atoms from the weakest C-H bond, with a simultaneous reduction of the surface sites and the successive formation of the surface hydroxide ions.31 Therefore, propene, as the dehydrogenation intermediate, has been detected during the total oxidation of propane (See Supporting Information Figure S3). With respect to the activation process, it seems that neighboring M-O sites are required.31 For Co3O4-CeO2 catalysts, the combination of the results of O2-TPD and C3H8 oxidation activity indicates that the surface lattice oxygen of the TMOs

corresponds very well with the C3H8 oxidation, suggesting that the C3H8 oxidation preferentially takes place on the surface lattice oxygen sites in the transition metal oxides. Although increasing the cobalt content from 30% to 40% decreases the cobalt dispersion, the Co3O4-CeO2 interface area, and the resultant CO oxidation activity, the C3H8 oxidation activity is not decreased since the amounts of surface Co3O4 crystallites are not decreased. As mentioned above, ceria is chemically involved in CO oxidation by lengthening and weakening the M-O bond, but it is only physically involved in C3H8 oxidation by providing the high surface area. This is in agreement with the findings that CO oxidation is support-dependent and that C3H8 oxidation is support-independent over the nanostructured gold catalysts.9 In summary, the active sites for CO and C3H8 oxidation are different. Schematic illustration of the site requirements for CO and C3H8 oxidation over these catalysts are proposed and shown in Figure 10. Although the CeCu20 catalyst exhibits higher surface area than Co3O4-CeO2 catalysts, it is much less active for C3H8 oxidation. This is related to the intrinsically low activity of CuO as compared with Co3O4, which has been confirmed in literature.8 Using methanol as a probe molecule, another kind of redox property, denoted as TOFredox, is defined by Badlani and Wachs for a large number of oxides.32 It has been proposed that TOFredox is dependent on the rate-determining surface decomposition of the surface methoxy intermediate that involves breaking of C-H bond. On this basis, TOFredox mainly reflects the C-H activation ability. It can be seen that at 300 °C the TOFredox of methanol oxidation is 51 s-1 for Co3O4, whereas it is only 5.7 s-1 for CuO, indicating that Co3O4 possesses much higher C-H breaking ability than CuO. Actually, it has been proposed by E. Finocchio et al. 33 that C-H bond breaking on metal oxides first occurs via direct interaction (or couple of electrons) of σ and σ* C-H orbitals with d-type orbitals of transition metal cations. Therefore, the types of transition metals are crucial because of the characters of the d-type orbitals. The d orbital of Cu2+ in CuO is almost filled up (d9), leading to the relatively low C-H activation activity, whereas the d6-d7 electron configuration in Co3O4 will facilitate the electronic interaction with C-H bond. As mentioned above, the C-H bond breaking is a crucial or even the rate-determining step during propane combustion. Therefore, Co3O4-CeO2 catalysts are more active than CuO-CeO2 for propane oxidation. 3.5.3. The Effect of Pd. After promotion of the CeCo30 catalyst with Pd, the CO oxidation activity is greatly enhanced. Previous analysis shows that the Co3O4-CeO2 catalysts are highly capable of adsorbing CO, and the rate-determining step for CO oxidation is oxygen abstraction from the oxides. Therefore, the main function of nobel metal Pd here is possibly oxygen activation. The activated oxygen atom may react with the adsorbed CO as bidentate carbonate via spillover, because the oxygen spillover has been confirmed by the TPO analysis

Active Sites for CO and C3H8 Total Oxidation here, as well as in other systems, such as Co-Pt(Pd, Rh)/ Ce-Al-O and Pd-Co/Al2O3.34,35 Therefore, the dual-sites reaction pathway is proposed and is shown in Figure 9c. According to this pathway, the activation energy is prominently decreased, and the catalytic activity is greatly enhanced. However, the oxidation activity of Pd/CeO2 is still very low, which is possibly due to the weak CO adsorption on Pd oxide surface, according to the findings of Schalow et al.36 As for C3H8 oxidation, the effect of Pd is insignificant since the fast oxygen activation over Pd is no longer a crucial step. Although the small amount of Pd itself may show some C3H8 oxidation activity, its addition has decreased the surface area and therefore leads to a little decrease in the activity. On the whole, the activity remains almost the same as the unpromoted CeCo30 catalyst. It is worth noting that, after promotion with Pd, no propene can be observed during propane oxidation (see Supporting Information Figure S2). The propene may have been generated on the surface sites of Co3O4, but it was quickly oxidized on Pd sites since Pd possesses high activity for the combustion of unsaturated hydrocarbons.37 4. Conclusion Using CTAB as a template, mesoporous Co3O4-CeO2 and CuO-CeO2 catalysts with high surface area were successfully prepared. Compared to the CuO-CeO2 catalyst, Co3O4-CeO2 catalysts are less active for CO oxidation, but are much more active for C3H8 oxidation. It is found that CO oxidation activity is closely related to the reducibility of the catalysts, whereas C3H8 oxidation activity largely depends on their ability for C-H bond activation. The DRIFTS studies show that CO is mainly adsorbed as carbonyl (2106 cm-1) and bidentate carbonate (1568 and 1281 cm-1) on the surface of the CuO-CeO2 catalyst and only as bidentate carbonate (1591 and 1268 cm-1) on the surface of Co3O4-CeO2 catalysts. The rate-determining step for CO oxidation over the mixed oxides is the creation of oxygen vacancies via abstraction of oxygen by the adsorbed CO species. The interaction between transition metal oxides and CeO2 contributes to the weakening of the metal-oxygen bond by sharing oxygen at the interface. Therefore, CO oxidation preferentially takes place at the interface. By contrast, the C3H8 oxidation mainly takes place on neighboring surface lattice oxygen sites in Co3O4 or CuO crystallites where C3H8 molecule is activated. The deposition of Pd over Co3O4-CeO2 hardly enhances the C3H8 oxidation activity, but greatly promotes the CO oxidation activity of the catalyst, via oxygen spillover between Pd and Co3O4 phase. Supporting Information Available: Additional figures mentioned within the text are also provided. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment. This work is financially supported by the “863 Program” of the Ministry of Science & Technology of China (No. 2006AA06Z348), the National Natural Science Foundation of China (No. 20676097), and the Program of New Century Excellent Talents in Universities of China (NCET-07599). The authors are also grateful to the Cheung Kong Scholar Program for Innovative Teams of the Ministry of Education

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