M = Cu, Ti, Zr, and Tb - American Chemical Society

J. Phys. Chem. C 2010, 114, 9889–9897

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Modulated CO Oxidation Activity of M-Doped Ceria (M ) Cu, Ti, Zr, and Tb): Role of the Pauling Electronegativity of M Yi Liu, Cun Wen, Yun Guo,* Guanzhong Lu, and Yanqin Wang* Key Lab for AdVanced Materials, Research Institute of Industrial Catalysis, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China ReceiVed: March 3, 2010; ReVised Manuscript ReceiVed: April 29, 2010

Many properties of dopants have been investigated to explore the key factor that influenced the CO oxidation activity of M-doped ceria (CeM). Nevertheless, these reports were controversial. Herein, the Pauling electronegativity (χP) of the M was presented as a convenient guide to screen a proper dopant for ceria. Kinetics results indicated that the T10 (the temperature when CO conversion reached 10%) of CeM73 catalysts (Ce/M molar ratio was 7/3; M ) Cu, Ti, Zr, and Tb) was linearly dependent on the χP of the M, which could adjust the amount of active lattice oxygen (AOL). The AOL was important to catalyst activity because lattice oxygen extraction was the rate-determining step in the overall reaction. 1. Introduction CO catalytic oxidation is indispensable in many processes, such as automobile exhaust purification and CO preferential oxidation.1-5 Generally, ceria is added into CO oxidation catalysts to improve their catalytic performance because ceria has unique oxygen storage properties. Nevertheless, pure ceria cannot satisfactorily fulfill the elevated activity demands for the CO oxidation catalyst owing to its limited oxygen storage capacity and its insufficient thermal stability. One of the effective tactics to tackle these problems is to dope ceria with other elements.6-11 For example, Fornasiero et al.9 studied the redox properties of Ce0.5Zr0.5O2 solid solution and indicated that the redox properties of ceria were greatly improved by Zr doping. Kasˇpar et al.10 also indicated that the reduction behavior of Ce0.6Zr0.4O2 was significantly enhanced compared with that of pure ceria. The effects of the dopant properties on the catalytic performance of doped ceria have been widely studied.12-18 Many groups reported that the ionic radius and redox potential of the doped element were responsible for the enhanced CO catalytic activity of the doped ceria.15,19-24 Balducci et al.23 used a computer simulation method to study the reduction energy of M-doped ceria (CeM, M ) Mn2+, Ni2+, Zn2+, Ca2+, Mn3+, Sc3+, Y3+, Gd3+, and La3+) and indicated that the lower reduction energy of the catalyst was related to the larger ionic radius of M when the valences of M were identical. However, the results of Reddy et al.7 showed that the redox properties of CeM (M ) Si, Ti, and Zr) did not depend on the M4+ ionic radius. Furthermore, in the CeM (M ) Zr and Hf), the smaller ionic radius of M was reported to be responsible for the better CO oxidation activity.15 On the other hand, Andersson et al.22 calculated the reducibility of CeM (M ) Si, Ge, Sn, and Pb) and indicated that the reducibility was enhanced with the ionic radius decreasing. However, their results showed that the reducibility of CePb did not follow the tendency of the ionic radius, and they attributed this extraordinary reducibility of CePb to the low redox potential of Pb4+/Pb2+. The role of the redox * To whom correspondence should be addressed. Tel: +86(21)64253824. Fax: +86(21)64253824. E-mail: [email protected] (Y.W.), yunguo@ ecust.edu.cn (Y.G.).

potential was also considered in CeCu. Some researchers indicated that the low redox potential of Cu species was responsible for the extraordinary CO oxidation activity of these CeCu catalysts.20,21 Those results elucidate that both the ionic radius and the redox potential should be considered when screening a suitable dopant for ceria. Nevertheless, both the ionic radius and the redox potential can be related to the electron structure of the dopant. The ability of the dopant to attract or lose electrons can be related to its ionic radius. On the other hand, if the dopant tends to loss its electrons, its redox potential will be small. These results indicate that the ability of the dopant to attract or lose the electrons, which is evaluated by electronegativity, can correlate with the ionic radius or with the redox potential. However, the ionic radius and redox potential would be influenced by other factors, such as valence.25,26 Therefore, the electronegativity may comprehensively reflect the ability of the dopant to attract/lose electrons, compared to the ionic radius and the redox potential of the dopant. The effect of electronegativity on the catalytic activity of corresponding metal oxides has been widely studied. Zhu et al.27 prepared a solid strong base derived from SiO2 and TiO2. Their research results showed that the cation electronegativity of the oxide was responsible for the basicity via influencing the M-O bond intensity. Wachs et al.28,29 investigated the turnover frequency (TOF) of MoO2 and V2O5 catalysts that were supported on the oxide of Zr, Ce, Ti, Al, and Si. Their research indicated that the TOF was associated with the cation electronegativity of the support because the cation electronegativity of the support would influence the M-O-support bonds. Kobayashi et al.30 studied the effect of additives (Ba, Sr, and La) on the catalytic performance of a Pd/Al2O3-based threeway catalyst and indicated that CO and NOx conversion was advanced because the additives with a lower cation electronegativity stabilized the existence of PdO. Though the application of electronegativity to explain the catalyst properties is successful in these catalyst systems, as far as we know, the effect of electronegativity has not been studied in the ceria-based oxides. Herein, we studied the influence of the Pauling electronegativity (χP) of the dopant on the CO catalytic activity of the M-doped ceria. CeM catalysts (M ) Cu, Ti, Zr, and Tb) were

10.1021/jp101939v  2010 American Chemical Society Published on Web 05/10/2010

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TABLE 1: The SSA, TD, ACO2, TR, AOL, T10, and χP over the CeM73 (M ) Cu, Ti, Zr, and Tb) catalysts

SSA (m2 g-1)

TD (°C)a

ACO2 (E-11)b

TR (°C)c

AOL (µL g-1)d

T10 (°C)e

χPf

CeCu73 CeTi73 CeZr73 CeTb73

62.9 83.1 109.9 112.6

48 sg 45 46

1.5 s 1.6 1.4

42 46 40 49

481 281 153 53

76 174 197 237

1.90 1.54 1.33 1.20

a TD: onset temperature when CO2 signals were detected by the MS in CO-TPD. b ACO2: the data were obtained from the integration of the CO2 signal during CO-TPD from 25 to 550 °C. c TR: onset temperature when CO2 signals were detected by the MS in CO-TPR. d AOL: the amount of active lattice oxygen that was detected by the CO pulse. e T10: the temperature when CO conversion reached 10% in CO oxidation. f χP: Pauling electronegativity of the M. g “s” indicates that the CO2 signal was not observed on the CeTi73 during CO-TPD.

prepared by doping ceria with the M. The influence of the χP was understood via investigating the reaction mechanism of the CO oxidation on these catalysts, and the quasi-elementary steps in the mechanism were simulated by temperature-programmed experiments, CO pulse experiments, and isotopic oxygen exchange (IOE) experiments. Furthermore, the relation between the catalyst structure and activity was investigated. 2. Experimental Section 2.1. Synthesis of Catalysts. CeM73 catalysts (molar ratio of Ce/M is 7/3; M ) Cu, Ti, Zr, and Tb) were synthesized by a hydrothermal method.31 Raw materials were analytical grade reagents of (NH4)2Ce(NO3)6, Cu(NO3)2, ZrO(NO3)2 · 5H2O, Ti(OC4H9)4, Tb(NO3)3, and urea. The Ce concentration was 0.228 mol L-1. In a typical synthesis process, certain amounts of the precursor of M and stoichiometric urea were dissolved in deionized water with the Ce/M molar ratio of 7:3. The solution was then transferred into a Teflon-lined container and hydrothermally treated at 130 °C for 4 h with stirring. After it cooled to room temperature, the precipitate was filtered, washed with deionized water, and then dried at 110 °C overnight. After calcination in the air at 550 °C for 4 h, the obtained powder was defined as CeM. CeCuxy catalysts (x/y is the molar ratio of Ce/Cu), CuO, and CeO2 were synthesized by the hydrothermal method mentioned above. 2.2. Characterizations. The BET specific surface areas (SSAs) of the samples were measured by adsorption of N2 at 77 K using a Quantachrome NOVA 4000e apparatus. Powder X-ray diffraction (XRD) patterns of catalysts were collected on a Rigaku D/max 2550 VB/PC diffractometer using Cu KR radiation (λ ) 1.54056 Å) in the range of 10° e 2θ e 80°. The X-ray tube was operated at 40 kV and 100 mA. Raman spectra were obtained at room temperature by using a Raman laser spectrometer (inVia+Reflex, Renishaw) with a He+-Ne+ ion laser with an excitation wavelength of 514 nm. CO oxidation was conducted in a quartz tube reactor at the atmospheric pressure. The typical feed was composed of 2% CO and 20% O2 in Ar, with a total flow of 100 mL min-1. The catalyst (100 mg) was mounted at the middle of the reactor, and a K-type thermocouple was used to monitor the catalyst bed temperature. The reaction temperature was increased from room temperature at a heating rate of 5 °C min-1. The CO conversion was analyzed by an online gas chromatography equipped with a TCD detector. An isotopic oxygen tracer (IOT) experiment was performed on the catalyst at 150 °C. The carrier gas was He (purity g 99.999%, BOC Gases) with a flow rate of 50 mL min-1, and the 18O2 gas was purchased from Shanghai Research Institute of Chemical Industry with a purity g 97%. The reactants (CO/ 18 O2/Ar ) 1:10:39) were pulsed into the system until the equilibrium was reached, and the loop volume was 1 mL. The

composition of the effluent gas was monitored by an online quadrupole mass spectrometer (MS, IPC400, INFICON Co. Ltd.). A CO temperature-programmed reduction (CO-TPR) experiment was performed on the catalyst that was pretreated under He flow at 550 °C for 2 h. When the system was cooled to room temperature, a 30% CO/Ar flow was introduced into the system, and the TPR experiment was performed at a heating rate of 10 °C min-1. The evolution of CO2 during the experiment was monitored by the MS. A CO pulse experiment was conducted at 150 °C on 50 mg of catalyst that was pretreated under He flow at 550 °C for 2 h. A 35.7% CO/Ar mixture was pulsed into the system with a loop volume of 73.7 µL. The effluent gas was monitored by the MS. The amount of the active lattice oxygen (AOL) was calculated according to the amount of CO consumption that was calibrated using the Ar signal. A CO temperature-programmed desorption (CO-TPD) experiment was conducted on the catalyst after the catalyst was pretreated under He flow (50 mL min-1) at 550 °C for 2 h. The catalyst was then cooled to room temperature under the He flow and exposed to CO for 2 h. Subsequently, when the system reached equilibrium under He purging, the TPD experiment was performed at a heating ramp of 10 °C min-1. The experimental temperature did not exceed 550 °C because CO can be totally converted at this temperature in CO oxidation. The effluent gas was monitored by the MS. An IOE experiment was conducted on the catalyst at different temperatures. Before the experiment, the catalyst was preoxidized in 16O2 at 550 °C for 3 h and then flushed by He flow at this temperature for 0.5 h. After cooling to room temperature under He flow, 10% 18O2/Ar was pulsed into the system. The catalyst was then treated at 550 °C as follows: 16O2 (0.5 h) f He (0.5 h), and the IOE experiment was conducted at the desired temperature. The effluent gas was monitored by the MS, and the 18O2 conversion was calculated after being calibrated with the Ar signal. 3. Results 3.1. CeM73 Catalysts. 3.1.1. Carbon Monoxide Oxidation. CO oxidation was conducted to evaluate the catalytic activities of the CeM73, and the T10 (temperature at which the CO conversion reached 10%) was employed to evaluate the catalytic activities of the oxides. The T10 (Table 1) decreases in the sequence of CeTb73, CeZr73, CeTi73, and CeCu73. 3.1.2. CO Pulse. The amount of active lattice oxygen (AOL) of the CeM73 was measured by the CO pulse experiment. The AOL decreases in the sequence of CeCu73, CeTi73, CeZr73, and CeTb73, and a linear relation (R2 ) 0.958) is found between AOL and T10 (Figure 1). 3.1.3. IOT. Whether the reaction follows the LangmuirHinshelwood mechanism or the Mars-van Krevelen mechanism

Modulated CO Oxidation Activity of M-Doped Ceria

Figure 1. Linear relation between the AOL and the T10 on the CeM73 (M ) Cu, Ti, Zr, and Tb).

Figure 2. Products of the isotopic oxygen tracer experiment at 150 °C on CeCu73. (Signals 44, 46, and 48 represent C16O2, C16O18O, and C18O2, respectively.)

can be identified by analyzing the isotopic oxygen distribution in the products of IOT experiments. In these experiments, signal 44 (m/e) represents the C16O2, which is produced in the reaction between the lattice oxygen (16O) of the catalyst and the pulsed C16O. The signal 46 (m/e) represents the C16O18O, which is the product of the reaction between the C16O and the dissociated 18 O2. The results of the IOT experiment on CeCu73 are shown in Figure 2, and the C16O2 is the main product. This phenomenon is also observed in the IOT experiments on CeTi73, CeZr73, and CeTb73 (Figure S1 in the Supporting Information). On those catalysts, only C16O2 is observed, and C16O18O or the C18O2 is not detected. The results in Figure 2 indicate that the pulsed CO predominantly reacts with the lattice oxygen. One may consider that the product C16O2 is from the reaction between the CO and the adsorbed 16O2 that might have been formed before the experiment when the catalysts were exposed to the atmosphere. If the C16O2 results from the reaction between the CO and the adsorbed 16O2, the C16O2 will be the dominant product in first several pulses. The C16O18O and C18O2 will then become the main products in the following pulses because the adsorbed 18O2 will be formed when the catalyst is exposed to reactants (CO + 18O2). Nevertheless, the results in Figure 2 show that the C16O2 is the main product throughout the experiments. Thus, the pulsed CO predominantly reacts with the lattice oxygen, and the lattice oxygen is more active than the gas-phase oxygen. 3.1.4. CO-TPR. CO-TPR experiments were performed to study the reducibility of the lattice oxygen in the CeM catalysts. The results of the IOT experiment have indicated that the lattice oxygen is more active than the gas-phase oxygen. Therefore, the onset temperature in the CO-TPR (TR) reflects the reduc-

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Figure 3. MS signal of CO2 during CO-TPD on CeM73 (M ) Cu, Zr, and Tb).

Figure 4. 18O2 conversion on the CeM73 (M ) Cu, Zr, and Tb) during the IOE experiment.

ibility of the catalyst lattice oxygen. The TR (Table 1) is much lower than the T10, which indicates that the lattice oxygen is active to react with CO at low temperatures. All the TR are similar, which indicates that all catalysts have similar reducibilities. Furthermore, the variance trend of the TR is inconsistent with that of the T10. 3.1.5. CO-TPD. CO-TPD was conducted to study CO adsorption and CO2 desorption properties of the catalyst. The CO2 desorption curves are shown in Figure 3. The CO signal is not shown because this signal will be influenced by the CO2 fragment that is produced by ionization in the MS. The CO2 signals in Figure 3 show the oscillation phenomena, which may be produced by the dynamic self-organization of the molecular system.32-35 The amount of desorbed CO2 (ACO2) on CeZr73 is the largest (Table 1), and no CO2 desorption signal is detected on CeTi73 during the experiment. Furthermore, the variance trend of ACO2 on the catalysts is different from that of the T10. On the other hand, the onset temperature of CO2 desorption (TD) is listed in Table 1. The TD of these catalysts is similar, and no direct relation is found between the TD and the T10. 3.1.6. IOE Experiment. The IOE experiment has been widely adopted to study the ability of a catalyst to activate the gasphase oxygen.36,37 The results of the IOE experiment on CeCu73, CeZr73, and CeTb73 are shown in Figure 4. Because the catalyst was preoxidized by 16O2 at 550 °C before the experiment, the 18 O2 consumption precludes the possibility of 18O2 adsorption without exchange. In Figure 4, CeCu73, which shows the best CO oxidation activity, has the best ability to activate 18O2 below 300 °C, but the ability of CeCu73 to activate 18O2 at the higher temperature is worse than that of CeTb73. At high temperatures (above 320 °C), the ability to activate the gas-phase oxygen of these catalysts is in the sequence of CeTb73 > CeCu73 >

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Figure 5. XRD patterns of CeM73 (M ) Cu, Ti, Zr, and Tb).

Figure 6. Raman spectra of CeM73 (M ) Cu, Ti, Zr, and Tb).

Figure 7. XRD patterns of CeCuxy.

CeZr73. Furthermore, at low temperatures (below 320 °C), the 18 O2 conversion on CeTb73 is higher than that on CeZr73, which indicates that the CeTb73 has the better ability to activate the gas-phase oxygen than CeZr73. These results indicate that the sequence of the oxygen activation ability of the catalysts is CeCu73 > CeTb73 > CeZr73 at low temperatures. The results of IOE at both high and low temperatures show that the sequence of oxygen activation ability is not consistent with the trend of the CO oxidation activity (CeCu73 > CeZr73 > CeTb73), which indicates that the ability to activate the gasphase oxygen cannot be linearly related to the T10. 3.1.7. Surface Area and Structure. The SSA data of these catalysts are shown in Table 1. The SSAs increase in the sequence of CeCu73, CeTi73, CeZr73, and CeTb73. The lattice structures of prepared CeM73 (M ) Cu, Ti, Zr, and Tb) are exhibited in Figure 5. XRD patterns of all catalysts show that the structures are related to the cubic fluorite structure of CeO2, and the diffraction peaks show slight shifts from the corresponding diffraction peak of the CeO2. These phenomena elucidate that the M elements are doped into the ceria lattice, and ceria-based solid solutions are formed. However, in Figure 5, the CuO diffraction peaks appear in CeCu73. These results show that Cu was partially doped into the ceria lattice, and some of them existed as the bulk structure of CuO in CeCu73. 3.1.8. Raman Spectroscopy. Raman spectroscopy was used to study the lattice structure, and the corresponding spectra of CeM73 (M ) Cu, Ti, Zr, and Tb) are shown in Figure 6. The Raman active mode of the CeO2 cubic fluorite structure is at 462 cm-1 (result not shown here), which is consistent with the reported results.38,39 For all these CeM73 oxides, the strong bands in the range of 400-520 cm-1 are observed, which was attributed to the Raman vibration of the CeO2 fluorite structure

of ceria. However, the central locations of these bands more or less deviate from the 462 cm-1, which indicates that the doping influences the lattice structure of ceria. This is because the doping of M into the ceria lattice will induce the lattice distortion, which will influence the crystal symmetry and make the Raman active mode of the ceria structure in CeM73 shift from that in the pure ceria. These results are consistent with the analysis of XRD that the solid solution was formed. From Figure 6, the bands at about 600 cm-1 appear in all catalysts. These bands are related to the Raman active mode of oxygen vacancies that are produced by doping in the ceria fluorite structure.38,39 Nevertheless, Fornasiero et al.40 studied the luminescence signal of CexZr1-xO2 (x ) 0-1) and indicated that the electron transition of trivalent lanthanide impurities will contribute to the Raman signal at about 525-560 cm-1, although the concentrations of these impurities are very low (at the parts per million or sub-parts per million level). However, the Raman signal in this study is closer to the signal of oxygen vacancies, which indicates that the signal at about 600 cm-1 is attributed to the Raman active mode of oxygen vacancies that are formed by doping. These results indicate that the solid solution with the cubic fluorite structure is formed in the CeM73. Furthermore, the amounts of solid solution in CeTi73, CeZr73, and CeTb73 should be very similar because of their uniform structure and the same molar ratios of Ce/M. 3.2. CeCuxy Catalysts. 3.2.1. Structure. The lattice structure of CeCuxy was investigated by XRD, and the corresponding XRD patterns are shown in Figure 7. All CeCuxy catalysts have the crystal structures that are consistent with that of ceria. No diffraction peak of CuO is observed on CeCu91. When the Ce/ Cu ratio is lower than 9/1, the CuO diffraction peak becomes more intensive.


J. Phys. Chem. C, Vol. 114, No. 21, 2010 9893 reported by other researchers.41,42 On the other hand, the AOL in Figure 9 shows that the T10 of CeCuxy linearly depends on the AOL of the catalyst (R2 ) 0.998).

Figure 8. Raman spectra of CeCuxy.

Figure 9. Linear relation between the AOL and the T10 on CeCuxy.

3.2.2. Raman Spectroscopy. The detailed lattice structure of CeCuxy was studied by Raman spectroscopy, and the Raman spectra are shown in Figure 8. The main peaks of CeCuxy appear at about 450 cm-1 and deviate from the Raman active mode of the ceria fluorite structure (462 cm-1). These shifted Raman signals are attributed to the triply degenerate F2g mode of fluorite ceria.38,39 The deviation of the main band on CeCuxy from that on pure ceria indicates that the symmetry of the ceria lattice is changed by formation of solid solution via doping of M into the ceria lattice.38 On the other hand, the Raman main band on CeCu91 locates at 455 cm-1, and the main band on CeCu73 is present at 453 cm-1, which is very close to that on CeCu55 and CeCu37 (452 cm-1). This phenomenon indicates that CeCu91 has the ceria-based solid solution structure. As the Cu amount increases, more solid solution is formed. When the Ce/ Cu molar ratio is lower than 5/5, no more solid solution is produced. The same locations of Raman main bands on CeCu55 and CeCu37 indicate that these catalysts have a similar quantity of solid solution. This result further indicates that only a limited amount of Cu can be doped into ceria to form the solid solution, and the redundant Cu cannot form the solid solution but exists as bulk CuO in the catalyst. 3.2.3. CO Oxidation and CO Pulse Experiment. The CO oxidation activities and the AOL of CeCuxy were studied. The results are shown in Figure 9. The T10 shows that all the CeCuxy catalysts exhibit better catalytic activities than CuO and CeO2. CeCu73, CeCu55, and CeCu37 show the better CO oxidation activities than CeCu91, and the T10 of CeCu73, CeCu55, and CeCu37 is very similar. These results indicate that even the very low ratio of Cu doped into ceria will significantly decrease the T10 of the catalyst. Nevertheless, when the doping ratio of Cu exceeds a certain value, the catalytic activity of CeCuxy is not promoted any more. The similar phenomena are also

4. Discussion 4.1. CeM Catalysts. The linear relation between the T10 and AOL (Figure 1) indicates that the T10 of the CeM73 is directly influenced by the AOL. To understand these relations, the reaction mechanism of the CO oxidation was studied, and all the experimental data are summarized in Table 1. First, the reaction mechanism is identified by the IOT experiment. The results in Figure 2 show that the lattice oxygen is more active than the gas-phase oxygen, which indicates that the CO oxidation on the CeM73 mainly follows the Mars-van Krevelen mechanism. This result is consistent with the reported studies.3,43,44 Admittedly, some researchers have reported that the CO oxidation on ceria-based catalysts follows the LangmuirHinshelwood mechanism. For example, Liu and FlytzaniStephanopoulos45 investigated the reaction kinetics of CO oxidation on the Cu-Ce-O catalyst and indicated that the CO oxidation on the catalyst followed the Langmuir-Hinshelwood mechanism. Wang et al.46 also investigated the light-off behavior of CO oxidation on samaria-doped ceria supported CuO and indicated that the Langmuir-Hinshelwood mechanism was the main reaction mechanism, although the Mars-van Krevelen mechanism also existed. Although, in this study, the LangmuirHinshelwood mechanism may exist, it is not the main mechanism in the CO oxidation reaction. If it follows the LangmuirHinshelwood mechanism, the C16O can react with oxygen species from the gas-phase 16O2 (produced by oxygen exchange between the 18O2 and the catalyst) and generate C16O2. However, the results of the IOE show that the oxygen exchange extent on the catalysts is very low at the experimental conditions of the IOT (150 °C). Therefore, if the C16O is mainly converted by oxygen species from the gas-phase oxygen, the carbon dioxide containing 18O should be the main product. In the IOT experiment, C16O2 is the main product, which indicates that the possible Langmuir-Hinshelwood mechanism is not the main reaction mechanism for CO oxidation on the catalysts. Therefore, the CO oxidation on the catalysts mainly follows the Mars-van Krevelen mechanism, which is proposed as follows:

CO + / f CO/

(1)

CO/ + OL f COOL/

(2)

COOL/ f / + CO2 + VL

(3)

O2 + VL f 2OL

(4)

In these steps, the symbol “/” represents a surface adsorption site, and the species with this suffix mean the corresponding adsorption species. “OL” and “VL” denote the lattice oxygen and oxygen vacancies in the catalysts, respectively. At first, CO adsorbs on the catalyst surface (eq 1). These adsorbed CO react with lattice oxygen of the catalyst to form reaction intermediates (eq 2). The intermediates’ desorption produces CO2 and oxygen vacancies, and adsorption sites are refreshed (eq 3). After that, gas-phase oxygen is activated on the catalyst to replenish the oxygen vacancies (eq 4). Through these steps, the catalyst is refreshed.

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The effect of lattice oxygen on the catalyst activity was studied by the CO pulse and CO-TPR experiments. As for ceriabased oxides, the lattice oxygen properties usually include the reducibility and amount of lattice oxygen.19,42,47 The linear correlation between T10 and AOL indicates that the lattice oxygen extraction (eq 2) may be the rate-determining step (RDS) in the overall reaction. Meanwhile, the reducibility of the lattice oxygen was investigated by the CO-TPR. The TR (Table 1) of the catalysts are similar, which indicates that those catalysts have similar reducibilities. Furthermore, the trend of the TR cannot be related to that of the T10, which shows that the CO catalytic performance is not mainly influenced by the reducibility of the catalyst lattice oxygen. Next, the proposed RDS was verified by studying the influences of other catalyst properties on the catalytic performance. The IOE and temperatureprogrammed experiments were employed to simulate other quasi-elementary steps (eqs 1, 3, and 4). The CO adsorption properties of the catalysts were studied by the CO-TPD experiment, which corresponds to eq 1. In Figure 3, no CO2 desorption signal is observed on CeTi73, which indicates that the strength of the CO adsorption on this catalyst is lower than that on other catalysts. One may argue that no CO2 desorption signal on CeTi73 is because the lattice oxygen in this catalyst is not active enough to react with the adsorbed CO. However, the TR of CeTi73 is 46 °C (Table 1), which indicates that the lattice oxygen of this catalyst is active to react with the adsorbed CO at the low temperature. This result elucidates that, if the CeTi73 surface has adsorbed CO during the TPD experiment, the lattice oxygen can react with those adsorbed CO at the low temperature. Therefore, these results exclude the possibility that no CO2 signal on CeTi73 is because of the low activity of the lattice oxygen. On the other hand, one may consider that no CO2 signal in the CO-TPD over CeTi73 is due to the weak CO2 adsorption that makes the formed CO2 be removed by He purging before the TPD experiment is started. However, this possibility can be excluded based on the results of the CO-TPR. The TR of CeTi73 is higher than 40 °C, which indicates that the activity of the lattice oxygen is insufficient to react with the CO at room temperature before the TPD experiment is started. Thus, no CO2 signal on CeTi73 indicates that the CO adsorption strength on this catalyst is lower than that on other catalysts. Though the strength of CO adsorption on CeTi73 is the weakest, this catalyst has the medium CO oxidation activity. These phenomena elucidate that the CO adsorption strength is not the factor that predominantly influences the CO catalytic performance of the CeM73. On the other hand, we calculate the ACO2 (Table 1) to study the influence of the amount of adsorbed CO. The ACO2 on CeZr73 is stronger than that on other catalysts, which indicates that CeZr73 can adsorb more CO than other catalysts. Nevertheless, the CO oxidation activity of CeZr73 is penultimate in all catalysts. These results indicate that the amount of adsorbed CO is not the factor that dominantly influences the CO oxidation activity. Thus, COTPD results indicate that the CO adsorption (eq 1) is not the RDS in the CO oxidation. On the other hand, results of the CO-TPD reflect the CO2 desorption (eq 3) properties on the catalyst. During the COTPD, the adsorbed CO can react with the catalyst to form reaction intermediates (such as carbonates), and these intermediates will be desorbed along with the increased temperature. The CO2 desorption process in the CO-TPD will undergo the breakage of the metal-oxygen bond in the catalyst, which is consistent with the process in the CO oxidation. Nevertheless, the CO2 desorption in CO2-TPD does not need the breakage of

Liu et al. the metal-oxygen bond, although the CO2-TPD experiment is usually used to study the CO2 desorption property of the catalyst. Therefore, the onset temperature of CO2 desorption (TD) in Table 1 reflects the CO2 desorption properties of the catalysts. The TD of these catalysts are similar, which indicates that the basic strengths of the catalysts are similar. Furthermore, no direct relation is found between the TD and the T10, which elucidates that the CO oxidation activity of the catalyst is not mainly influenced by the property of CO2 desorption. Thus, the step in eq 3 is not the RDS in the CO oxidation. Furthermore, the influence of the gas-phase oxygen activation (eq 4) on the CO oxidation activity is studied by the IOE experiments. The results in Figure 4 show that CeTb73 has a better ability to activate the gas-phase oxygen than CeZr73. Nevertheless, results of the CO oxidation test indicate that CeTb73 has a worse CO oxidation activity than CeZr73. These results elucidate that the gas-phase oxygen activation ability is not the key factor that affects the CO oxidation activity of the CeM73. The above results of the kinetics investigation indicate that the catalytic activity is primarily influenced by the AOL because the lattice oxygen extraction is the RDS in the overall reaction. The nature of the active site of the catalyst can be evaluated by the difference of the amount of the active sites and the instinct activity of the active sites. The difference between the instinct activities of the active sites, which is evaluated by the TOF, usually has been reported as a key factor that influences the catalyst activity.48,49 Nevertheless, in this study, the amount of the active site is the factor that primarily influences the catalyst activity, according to the TOF we calculated (results not shown here). This result is consistent with some reported studies.5,42,50 For example, Luo et al.42 investigated the CO oxidation activity of the CuO-CeO2 catalyst and indicated that the amount of the highly dispersed CuO greatly influenced the low temperature CO oxidation activity of the catalysts. Furthermore, Kaliaguine et al.50 studied the oxygen mobility of LaCo1-xFexO3 and indicated that the oxygen mobility correlated well with the theoretical surface area that was calculated based on the crystal domain size. In this study, the AOL should be related to the structure of the catalyst. One may consider that the surface area of the catalyst can influence the AOL. However, the SSA in Table 1 showed that the SSA is negatively correlated with the AOL. CeTb73 has the lowest AOL, albeit its SSA is the biggest in all catalysts. Therefore, the AOL is not influenced by the SSA but is influenced by the catalyst structure. The structural analysis indicates that all CeM catalysts, except CeCu73, formed a uniform solid solution structure. The influence of the heterogeneous structure of CeCu73 on the AOL was studied by the experiments conducted on a series of CeCuxy catalysts. The linear relation between the T10 and AOL of CeCuxy catalyst in Figure 9 indicates that the CeCuxy catalyst activity is primarily influenced by the AOL. The CO oxidation activities of CeCuxy are superior to that of CuO or ceria. As the doping amount of Cu increases, the CO oxidation activity of the CeCuxy catalyst increases. When the Ce/Cu molar ratio is lower than 5/5, the catalytic activity does not enhance further. The changing trend of these results corresponds to that of the solid solution structure of CeCuxy. The amount of the solid solution increases with the Cu doping amount, whereas the quantity of the solid solution does not increase when the Ce/Cu molar ratio is lower than 5/5. These phenomena elucidate that the CO oxidation activities of CeCuxy strongly depend on the formed CeCu solid solution structure. The more solid solution that is formed, the


Figure 10. Linear relation between the AOL of CeM73 and the Pauling electronegativity of the M (M ) Cu, Ti, Zr, and Tb) at 150 °C.

more active lattice oxygen can be supplied by the catalyst. Nevertheless, some researchers have attributed the lattice oxygen with higher reducibility to the highly dispersed CuO.41,42,51 In this study, the influence of highly dispersed CuO on the active lattice oxygen can be excluded. The structural analysis of CeCuxy catalysts indicates that the low-temperature reduction peaks in H2-TPR (Figure S2 in the Supporting Information) of these catalysts correspond to the reduction of the solid solution. These results are consistent with the work of Li et al.38 and elucidate that the solid solution structure in CeCuxy catalysts is responsible for the AOL. Meanwhile, this result further indicates that the solid solution structure in CeM catalysts is the source of the AOL. 4.2. Effect of the χP of the M. The above results show that the amount of solid solution correlates well with the AOL. Nevertheless, it is noticed that the differences between the AOL of CeTi73, CeZr73, and CeTb73 are huge, though the amounts of the formed solid solution in them are very similar. This phenomenon should be attributed to the influence of the properties of M because all the catalysts were prepared similarly in both synthesis methods and the molar ratio of Ce/M. Further investigation revealed a linear relation (R2 ) 0.992) between the AOL and the χP of the M (Figure 10), which indicates that the AOL is primarily influenced by the χP of the M. Nevertheless, it is noticed that the AOL of CeCu73 is lower than the quantity that can be inferred from the linear correlation. This result corresponds to the structure of CeCu73. Both the XRD and the Raman spectra indicate that the solid solution structure is formed in the CeCu73, but the solid solution structure in CeCu73 is less than that in ideal CeCu73 solid solution because a few Cu exist as CuO instead of solid solution. These results explain why the AOL of CeCu73 deviates from the linear relation between the AOL and the χP of the M. Therefore, the above results show that the χP of the M influences the AOL via affecting the solid solution structure. The results mentioned above indicate that the χP of the M influences the CO oxidation activity of the CeM73 via adjusting the AOL. CeCo73 and CeMn73 were prepared by the hydrothermal method to verify the correlation between the T10 and the χP of the M. The T10 of these two catalysts follow the relation between the χP and the T10 (Figure 11), which further indicates that the M that has a higher χP can effectively improve the CO oxidation performance of the doped ceria. The influence of the χP of the M on the active lattice oxygen is through affecting the solid solution structure. The formation of the solid solution structure by doping has been reported to induce the crystal structure distortion in the catalyst.31,52 The

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9895

Figure 11. Linear relation between the T10 in the CO oxidation and the Pauling electronegativity of the M on the CeM73 (M ) Cu, Co, Mn, Ti, Zr, and Tb).

strain produced by such distortion will relax to lattices that are adjacent to the solid solution structure. By this relaxation, the larger strain will make more lattice distortion in the crystal. That is to say, a higher χP of the M will induce more lattice distortion. It is proposed that the χP of the M may modify the AOL via lattice relaxation. The further investigation on the details by which the χP of the M influences the AOL is being conducted in our lab. Meanwhile, the homogeneity of the microstructure of the solid solution needs to be further studied, which is useful to understand the mechanism of the influence of the χP on the structure. For example, Fornasiero et al.53 used Eu(III) as probe to identify the microstructure of CexZr1-xO2 by luminescence studies. Their results indicated that the heterogeneity phenomena exist in the solid solution. Other researchers also studied the structural homogeneity in the solid solution.54-56 These results indicate that the homogeneity in the microstructure of the solid solution should be carefully investigated, and the related studies are being conducted in our lab. 5. Conclusions The kinetics investigations in this study indicate that the CO oxidation on the CeM73 (M ) Cu, Ti, Zr, or Tb) follows the Mars-van Krevelen mechanism, and the lattice oxygen extraction is the RDS in the overall reaction. The catalytic activities of these catalysts show a linear dependence on the χP of the M. Further investigations indicate that the χP of the M influences the catalyst performance via adjusting the quantity of active lattice oxygen that is mainly related to the solid solution structure formed by M doping. Herein, the χP of the dopant may be a convenient parameter to screen an appropriate dopant for ceria to achieve a better catalytic activity. Acknowledgment. This project was supported financially by the 973 Program of China (2010CB732300), the National Natural Science Foundation of China (Nos. 20601008 and 20973058), the Commission of Science and Technology of Shanghai Municipality (08JC1407900 and 10XD1401400), and the “Excellent Scholarship” of East China University of Science and Technology, China. Supporting Information Available: Results of IOT on CeTi73, CeZr73, and CeTb73 and results of H2-TPR on CeCuxy catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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JP101939V

M = Cu, Ti, Zr, and Tb - American Chemical Society

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