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J. Phys. Chem. C 2009, 113, 3212–3221
Pd/Support Interface-Promoted Pd-Ce0.7Zr0.3O2-Al2O3 Automobile Three-Way Catalysts: Studying the Dynamic Oxygen Storage Capacity and CO, C3H8, and NO Conversion Meiqing Shen,*,†,‡ Ming Yang,† Jun Wang,† Jing Wen,† Minwei Zhao,† and Wulin Wang§ Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, State Key Laboratory of Engines, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and China AutomotiVe Technology & Research Center, Tianjin 300162, People’s Republic of China ReceiVed: June 11, 2008; ReVised Manuscript ReceiVed: NoVember 20, 2008
The effect of the Pd-support interface on the dynamic oxygen storage capacity (DOSC) and three-way catalytic activities were investigated using Pd chromatographically distributed between Ce0.7Zr0.3O2 and Al2O3. CO-He pulse, H2-TPR, and XPS show that the Pd-(Ce, Zr) Ox interface promotes a higher degree of oxygen releasing than the Pd-(Al2O3) interface while maintaining the oxidized states of Pd. Pd promotion at the Pd-(Ce, Zr) Ox interface depends on the oxygen species of ceria-zirconia, and promotion takes place on surface oxygen and subsurface oxygen species. Dynamic CO-O2 transient measurement shows that DOSC is greatly promoted by the Pd-(Ce, Zr) Ox interface. The transient results indicate that the Pd-(Ce, Zr) Ox interface accelerates the oxygen spillover and back-spillover between Pd and the support oxide, but this phenomenon is not obvious for the Pd-(Al2O3) interface. Higher CO oxidation activity was obtained over a catalyst with a higher degree of Pd-(Ce, Zr) Ox interface in proportion to its DOSC performance. A two-step CO oxidation mechanism at the Pd-(Ce, Zr) Ox interface includes oxygen migration and its reaction with the adsorbed CO. Higher NO reduction and C3H8 oxidation rates were obtained with a catalyst with more Pd-(Al2O3) interface, showing negative correlation with the DOSC performance. 1. Introduction Because of the adoption of more stringent regulations on automotive emissions around the world, three-way catalysts (TWCs) have been widely used to reduce CO, HC, and NOx emissions from gasoline engines.1 A typical three-way catalyst is composed of precious metals as active components, γ-Al2O3 as a metal support, and ceria--zirconia as an oxygen storage material. Due to the lower cost and superior oxidation properties to Pt and Rh, Pd has been widely used in TWC formulations.2 One role of ceria-zirconia is to prevent metal aggregation during thermal treatment.3-5 Furthermore, the surface area and acidity of γ-Al2O3 can be stabilized with ceria-zirconia doping presumably due to the interaction between ceria-zirconia and γ-Al2O3. Ceria-zirconia can also improve the adhesion of washcoat on FeCrAl monolith.6 The most important role of ceria-zirconia is to store and release oxygen in minimizing the fluctuation of the air-to-fuel (A/F) ratio under transient conditions, and this is accomplished by the transformation of cerium chemical states between Ce4+ and Ce3+. Interactions among Pd, ceria-zirconia, and Al2O3 can greatly affect the metal-support interaction in a three-way catalyst.7 Pd supported on binary ceria-zirconia-alumina support is a common approach to study this interaction. Using Pd /CexZr1-xO2/Al2O3 model catalyst, Ranga Rao et al.8,9 showed that ceria promotes NO and CO conversion through Pd-Ce interaction; oxygen vacancies on the Pd-Ce interaction sites favor NO dissociation and decrease its the activation. Hu et al.2 showed that Pd-Ce interaction enhances NO conversion at high * To whom correspondence should be addressed. Phone/Fax: +86-2227892301. E-mail:
[email protected]. † School of Chemical Engineering & Technology, Tianjin University. ‡ State Key Laboratory of Engines, Tianjin University. § China Automotive Technology & Research Center, Tianjin University.
temperature rather than low temperature. On the other hand, by comparing with Pd-Ce interaction, Martı´nez-Arias et al.10,11 reported that Pd-Al2O3 interaction promotes NO reduction with a minor impact on CO oxidation. The geometrical/electronic state of Pd supported on Al2O3 plays a role in NO conversion. The catalytic activities can be correlated with the degree of metal support interaction. With regard to the OSC performance, by exposing catalysts to sequential pulses of CO and O2,12-14 Descorme et al.15 reported that the Pd-(Ce, Zr) Ox interface increased the OSC more compared to the Pd-(Al2O3) interface under 1 Hz CO-O2 measurement. Efstathiou et al.16 compared the OSC behavior between fresh and aged Pd-Rh/CexZr1-xO2/ Al2O3 model catalysts and found that the loss of oxygen storage capacity is a consequence of the loss of metal surface area due to reduced interfacial contact between metal and ceria-zirconia. When supporting Pd on a binary CexZr1-xO2/Al2O3 support several factors, such as thermal stability of support, surface ceria sites, and interaction with ceria-alumina, can affect the catalytic performance.17 It is difficult to distinguish the effects of Pd-Ce interaction and Pd-Al2O3 interaction from the synthesis effect of the catalyst. To study the metal-support interface Pd sites are distributed between ceria-zirconia and Al2O3. Without interaction between ceria-alumina, the effect of Pd-Ce interaction and Pd-Al2O3 interaction on catalytic performance should be easy to identify. Moreover, the relationship between dynamic OSC and catalytic activities (simultaneous oxidation of CO, HC, and reduction of NOx) is investigated, and this work correlates the interactions among metal-support, dynamic OSC, and catalytic activities (CO, C3H8, and NO). 2. Experimental Section 2.1. Sample Preparation. Ce0.7Zr0.3O2 was synthesized by coprecipitation of Ce and Zr salts with ammonia. Mixed
10.1021/jp805128u CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
Pd-Ce0.7Zr0.3O2-Al2O3 Automobile Three-Way Catalysts solutions of Ce(NO3)3 and ZrO(NO3)2 were added into ammonia with vigorous stirring at an ending pHof 9. The mixture was then kept at 70 °C for 12 h. Following solid separation the precipitates were rinsed with deionized water, dried, and finally calcined at 500 °C for 5 h to obtain the yellow powder. Similarly, γ-Al2O3 powder was obtained by precipitating Al(NO3)3 with ammonia. Pd catalysts with nominal 1 wt % were prepared by incipient wetness impregnation with an aqueous Pd(NO3)2 solution. It is designed to chromatographically distribute Pd species between Ce0.7Zr0.3O2 and Al2O3 powder. Here, chromatogram distribution is defined as the behavior of metal distribution among the mixed supports. Al2O3 and ceria-zirconia were considered as the stationary phase and mobile phase, respectively, used in chromatography. Pd(NO3)2 is considered as the solute, which is distributed between stationary phase and mobile phase. Fives kinds of Pd catalysts, consisting of Pd/Ce0.7Zr0.3O2, (Pd/ Ce0.7Zr0.3O2) + Al2O3, (Pd/Ce0.7Zr0.3O2) + (Pd/ Al2O3), Ce0.7Zr0.3O2 + (Pd/Al2O3), and Pd/Al2O3, were prepared, where the ratio of Ce0.7Zr0.3O2 to Al2O3 powder was 1:1 by weight. (Pd/Ce0.7Zr0.3O2) + Al2O3 catalyst, which is designated as (Pd/ CZ) + A, was obtained by mechanically mixing Pd/Ce0.7Zr0.3O2 and Al2O3, where Pd is loaded by impregnating Ce0.7Zr0.3O2 with Pd (NO3)2 solutions. Similarly, Ce0.7Zr0.3O2 + (Pd/Al2O3) catalyst, designated as (Pd/A) + CZ, was prepared by mechanically mixing Ce0.7Zr0.3O2 and Pd/Al2O3 powder, where Pd is loaded by impregnating Al2O3 with Pd(NO3)2 solution. (Pd/ Ce0.7Zr0.3O2) + (Pd/Al2O3) catalyst, designated as (Pd/CZ) + (Pd/A), was prepared by equally distributing Pd(NO3)2 on Ce0.7Zr0.3O2 and Al2O3 powder. Pd/Ce0.7Zr0.3O2 and Pd/Al2O3 catalysts with 1 wt % loading were obtained as the reference and designated as Pd/CZ and Pd/A. All resulting solids were dried at 120 °C overnight and calcined at 500 °C for 5 h. Aged samples were obtained by treating the fresh samples with 10% steam/air atmosphere at 1050 °C for 10 h. 2.2. Oxygen Storage Capacity. For OSC measurement 25 mg of catalysts was diluted with 40 mg of quartz beads and placed at the bottom of heat-transfer reactor (10 mm diameter) at a height of 1.5 mm. Concentrations of CO, O2, CO2, Ar, and He were monitored online by a Hiden HPR20 quadrupole spectrometer. The oxygen storage capacity (OSC) of a material was measured in two modes: CO-He pulses and CO-O2 pulses. The CO-He measurement was conducted at 500 °C with alternating CO (4%CO/1%Ar/He at 300 mL/min for 5 s) and He (300 mL/min for 20 s) pulses, and a CO-He test lasts for 10 CO-He cycles. Dynamic CO-O2 measurements were performed from 200 to 500 °C at an interval of 100 °C. CO (4%CO/1% Ar/ He at 300 mL/min for 10 s) and O2 (2% O2/ 1% Ar/ He at 300 mL/ min for 10 s) streams were pulsed into the system alternately with a frequency of 0.05 Hz (the number of CO and O2 pulses per second). A pulsation interval of 5 s was used for 0.1 Hz. A DOSC (dynamic oxygen storage capacity) amount was obtained by integrating the CO2 formed during one CO-He pulse or CO-O2 cycle and can be expressed as micromoles of O per gram of Pd-supported catalyst (µmol (O) g -1). The residence time for CO and O2 through the catalyst bed was calculated to be only 0.02 s (exclude the volume of solid catalysts), which is 2% compared with the 1 s reaction rate. Even estimating that CO and O2 could be well mixed together in the catalysts’ bed (exclude the volume occupied by solid catalysts), the turbulent flow would produce only 0.9 µmol CO2/gcat by 100% CO conversion in 1 s, which is not proportional to the calculated
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3213 TABLE 1: Structural and Textual Characterization of Pd-Supported Catalysts lattice constants (Å)
crystal sizesa (Å)
surface area (m2/g)
samples
fresh
aged
fresh
aged
fresh
aged
Pd/CZ (Pd/CZ) + A (Pd/CZ) + (Pd/A) CZ + (Pd/A) Pd/A
5.4025 5.4075 5.3969 5.3889
5.3530 5.3465 5.3540 5.3546
66 74 62 66
294 292 296 299
74.4 146.2 148.6 141.4 226.4
2.1 23.0 19.2 16.7 19.6
a Lattice constants and crystal sizes are referred to ceria-zirconia solid solution; crystal size is calculated by the (111) plane; lattice constant is based on the Jade program.
1 s reaction rate (less than 2%). The effect of Pd-promoted CO-O2 combustion in our experiment is insignificant. 2.3. Characterization. BET surface areas were determined by N2 adsorption with a Quanta chrome NOVA 1200 apparatus. Before testing, samples were degassed under flowing N2 at 200 °C for 2 h. Powder X-ray diffraction (XRD) patterns were acquired with a Rigaku D/max 2500 diffractometer operating at 40 kV and 200 mA with nickel-filtered Cu KR radiation and ranging from 10° to 90° with a 0.02° step size. The lattice constants and crystallite sizes calculated from XRD patterns and BET surface areas of Pd-supported catalyst are shown in Table 1. Though Pd is chromatographically distributed between ceria-zirconia and Al2O3 the absence of an appreciable difference in XRD patterns suggests that the analogy among Pd particles in all supported catalysts is similar.18-20 H2-TPR was conducted using a Micromeritics AutoChem 2910 analyzer with a U-tube containing 50 mg of sample. The catalysts were first purged under N2 (30 mL/min) at 200 °C for 1 h and then cooled to room temperature. A flow of 10% H2/ Ar (30 mL/min) was switched into the system with the temperature ramped from room temperature (RT) to 900 °C at a rate of 10 °C /min. X-ray photoelectron spectroscopy (XPS) experiments were carried out with a PHI-1600 ESCA system with Mg KR radiation under UHV (667mPa) at 15 kV and 250 W. The instrument was calibrated internally by the carbon deposit C 1 s binding energy (BE) at 284.6 eV. Catalytic activity tests were conducted in a quartz microreactor using the stoichiometric gas mixture (2% CO + 0.1% C3H8, 0.1% NO + 1.5% O2 + 12% CO2 in volume, N2 balance) at a space velocity of 50 000 h-1. Steam (7%) was injected into the inlet of the reactor system to simulate the real working situation of TWC. The catalyst powder (400 mg) was diluted with quartz pellets to 1.8 mL, and this mixture was placed at the bottom of the reactor. A standard activity test was conducted from 100 to 600 °C at a rate of 15 °C min-1. Oxygen concentrations were determined by an oxygen gas sensor. The effluents of the reactor (CO, CO2, NO, C3H8) were determined by an IR analyzer. 3. Results and Discussion 3.1. Presence of Pd-(Ce, Zr) Ox and Pd-(Al2O3) Interaction. 3.1.1. CO-He Pulse. Figure 1 shows the CO2 response curves of the first and second CO pulse with the CO (5s)-He (20s) cycle. For the first CO pulse the intensity of the CO2 curves increases when more Pd is supported on Ce0.7Zr0.3O2 but decreases when more Pd is supported on Al2O3. This finding suggests that the chromatogram distribution of the Pd solution affects CO2 production. By integrating the CO2 response curves
3214 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Figure 1. CO2 response curves of the first and second CO pulse on fresh catalysts under CO-He pulse measurement at 500 °C.
Figure 2. Representative OSC performance of Pd-supported catalysts under 10 consecutive CO-He pulses measurement.
in Figure 1 we can calculate the OSC of 10 repeated CO-He cycles of the CO-He measurement (Figure 2). For the initial CO pulse remarkable differences between fresh Pd/CZ and Pd/A catalysts are observed. The OSC of Pd/CZ is larger than 600 µmol (O) g-1 for the first CO pulse, which is almost three times that of Pd/A (200 µ mol [O] g-1). OSC follows the order of Pd/CZ > (Pd/CZ) + A > (Pd/CZ) + (Pd/A) > (Pd/A) + CZ > Pd/A. Pd supported on Ce0.7Zr0.3O2 is designated as Pd-(Ce, Zr) Ox, while Pd supported on Al2O3 is designated as Pd-(Al2O3). It can be concluded that the Pd-(Ce, Zr) Ox
Shen et al. interface has a higher OSC than the Pd-(Al2O3) interface. At the second CO pulse the differences in OSC associated to the Pd distribution become smaller. This behavior is consistent with the second CO2 curves in Figure 1, where the CO2 curve is less sensitive to the Pd distribution. The theoretical reducibility of ceria-based material is 25% due to the transformation from eight coordinate (CeO2) to six coordinate (Ce2O3). Therefore, the CO-He pulse is considered an anaerobic environment. Under an anaerobic environment the reduction process of the oxygen storage material starts from surface to bulk. Surface oxygen is first consumed by CO oxidation, which dominates the overall OSC behavior of the material. With the increasing number of CO pulses more oxygen vacancies are exposed to the surface, and thus, bulk oxygen is gradually migrated to the surface, participating in CO oxidation.18 The increment of CO pulses induces the increment of reducibility. Therefore, effects of Pd-(Ce, Zr) Ox on reduction are dependent on the reducibility of ceria-zirconia. During initial CO pulses ceria-zirconia is close to be fully oxidized. Assuming 1 wt % Pd is fully dispersed on the ceria-zirconia surface the surface PdO species is calculated as 94.0 µmol · g -1. In Figure 1 the similar CO2 curves are observed during 0-1.2 s, which can be attributed to reduction of Pd species. For the oxide, 220 µmol · g-1 of available surface oxygen, which is calculated as 25% of the bulk oxygen, come from Ce0.7Zr0.3O2. In this regard, OSC measured by the first CO pulse is far more than the sum of oxygen from both Ce0.7Zr0.3O2 and PdO. It implies that under the present experimental conditions the first CO pulse can reduce not only all of the surface oxygen but also oxygen sites associated to subsurface oxygen. Interestingly, the OSC of Pd/A at the first CO pulse is almost at 200 µmol · g-1, which is more than the oxygen from PdO. Nibbelke et al.14 considered Al2O3 to be the active site under CO-O2 transient measurement. Reaction between CO and limited amount oxygen adsorbed on Al2O3, PdO, and possible CO dissociation could contribute to CO2 formation. The intensity of CO2 curves correlates well with the Pd distribution and is closely related to the Pd-(Ce, Zr) Ox interface. High CO2 intensity means more oxygen participating in CO oxidation. The Pd-(Ce, Zr) Ox interface facilitates the oxygen back-spillover, which is associated to the OSC promotion.21 Conversely, the absence of oxygen back-spillover at the Pd-(Al2O3) interface results in the lower amount of oxygen participating in CO oxidation. After 1050 °C hydrothermal aging, as shown in Figure 2, promotion of OSC by the Pd chromatogram distribution is weakened, and the decrease in OSC is due to the decreased Pd-(Ce, Zr) Ox interaction. Under severe aging conditions sintering of Pd and ceria-zirconia particles contribute to the declining Pd-(Ce, Zr) Ox interface.22 Compared with ceria-zirconia, sintering of Pd exerts more influence on the deterioration of the Pd-(Ce, Zr) Ox interface and OSC decrease. In this regard, the contrast behavior between fresh and aged catalysts underlines the importance of the interface between Pd and support. With increasing number of CO pulses, OSC gradually decreased and reached a plateau from the 2nd to the 10th CO pulse. After depletion of PdO and oxygen on the surface and in the subsurface region the ceria-zirconia oxide became more reduced. Unlike the first CO pulse, the differences in OSC for pulses from the 2nd to the 10th are gradually diminished. (Pd/ CZ) + A, Pd/CZ + Pd/A, and CZ + (Pd/A) catalysts show the similar OSC behavior. Generally speaking, OSC available at this stage comes from the bulk structure of ceria-based materials.18 It is generally accepted that oxygen vacancies play an
Pd-Ce0.7Zr0.3O2-Al2O3 Automobile Three-Way Catalysts important role in oxygen migration. Diffusion of O2- driven by the oxygen concentration gradient from the bulk to the surface determines the OSC behavior.23 Hori et al.24 suggested that the solubility of the anion vacancies in the lattice determines the “bulk” reducibility of ceria-zirconia. By combining the Figure 1 and Figure 2 we find that the Pd-(Ce, Zr) Ox interface facilitates the OSC at the first CO pulse, and more oxygen vacancies in the ceria-zirconia structure are created when more Pd-(Ce, Zr) Ox interface exists. Therefore, there are more oxygen vacancies in (Pd/CZ) +A than (Pd/A) + CZ. However, the difference in OSC between these two catalysts is not significant. More oxygen vacancies do not seem to be facilitating subsequent bulk oxygen migration. It is estimated that the distribution of oxygen vacancies in this continuous reduction from surface to bulk and the excess in vacancies’ amount are not positive for oxygen release. The degree of bulk reduction is thermodynamically controlled by the theoretical reducibility from CeO2 to Ce2O3. A higher enthalpy is theoretically required when the stoichiometry of CeOx is closer to Ce2O3. Therefore, compared with surface Ce reduction at initial CO pulses, bulk reduction is less efficient even after hydrothermal aging.22 The OSC behavior on aged samples for pulses from 2 to 10 implies that bulk oxygen migration has little correlation with the Pd distribution. 3.1.2. H2-TPR. H2-TPR profiles as a function of temperature are shown in Figure 3. Pd-supported catalysts display three peaks with maxima at 90-120, 370-550, and 720-770 °C. The intensity of the reduction peak between 90 and 120 °C is greatly affected by the Pd distribution. Compared with Pd/A, Pd/CZ presents a higher intensity of the H2 consumption peak. Further, the peak intensity between 90 and 120 °C increases with the Pd-(Ce, Zr) Ox interface with the following order: Pd/CZ > (Pd/CZ) + A > (Pd/CZ) + (Pd/A) > CZ+ (Pd/A) > Pd/A. Pd/CZ presents the peak at even lower temperature than the others due to higher Pd dispersion on more abundant ceria-zirconia. Table 2 summarizes the calculated oxygen storage data of H2TPR measurement. The sequence of oxygen storage capacity below 200 °C is in a good agreement with the amount of Pd-(Ce, Zr) Ox interface. The total OSC of a Pd catalyst is composed of three parts: OSC of Pd sites, OSC of the Pd-(Ce, Zr) Ox interface, and OSC of ceria-zirconia. The reduction feature below 200 °C is ascribed to reduction of the part of Pd interacting with ceria-zirconia and associated surface oxygen.25,26 Oxygen storage capacity of Pd/CZ below 200 °C is 42 µmol · g-1. The oxygen storage capacity of Pd/CZ between 200 and 500 °C is 97 µmol · g-1. This value is lower than the oxygen available in Ce0.7Zr0.3O2 (220 µmol · g-1) and PdO (94 µmol · g-1). It implies that part of the oxygen species in PdO should have been reacted immediately upon contacting H2. Reduction of PdO could be detected even as low as RT in IR analysis,10 while generally no significant consumption peak can be observed for PdO reduction alone under the present experiments. Therefore, reduction peaks at 90-120 °C are dominated by consumption of surface oxygen species associated with Pd sites. It is obvious that the oxygen associated with the Pd-(Ce, Zr) Ox interface is easier to be consumed compared to the Pd-(Al2O3) interface. Similar to the results of CO-H, H2-TPR results show that oxygen back-spillover plays an important role in accelerating oxygen consumption. On the other hand, because of the close interaction between precious metal and H2, hydrogen spillover may also play a role in accelerating oxygen consumption. Contrary to the high H2 consumption peaks on fresh sample, H2 consumption peaks on aged catalysts are not observed. The absence of a H2 consumption peak can be ascribed
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3215
Figure 3. H2-TPR profiles of fresh and aged Pd-supported catalysts.
to deterioration of the Pd-(Ce, Zr) Ox interface. A negative peak at 75 °C is observed on an aged catalyst. In line with the literature,27,28 a low-temperature negative peak is due to decomposition of Pd hydride. It is important to note that metallic palladium adsorbs hydrogen to form Pd hydride even at low hydrogen pressure and room temperature.29 It is interesting to note that the metal-support interface also plays a role in the intensity of the negative peak. The negative peak over Pd-(Ce, Zr) Ox is lower than that of Pd-(Al2O3), indicating the inferior inclination to form metallic Pd particles and stronger Pd-support oxidative interactions. Similar reduction profiles are observed between 370 and 550 °C on all fresh samples except Pd/A. In this temperature range OSC from the Pd sites and the Pd-(Ce, Zr) Ox interface have already been consumed. The reduction profiles at 370-550 °C show that the effect of the Pd distribution is weak. On the basis of the calculated oxygen storage in Table 2 the reduction profiles at 370-550 °C are expected to be related to oxygen consumption at the surface and subsurface region. The reduction features at 720-770 °C are associated with oxygen consumption in bulk ceria-zirconia. With increasing temperature to 720-770 °C, H2-TPR profiles display similar reduction profiles on fresh samples. It is clear that the Pd-(Ce, Zr) Ox interface has a minimal impact on bulk oxygen reduction, which is consistent with the behavior of the CO-He pulse. Similar reduction behavior was observed on hydrothermal aged catalysts from 370
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TABLE 2: Oxygen Storage Data under H2-TPR Measurements OSC(µmol · g-1)Fresh
OSC(µmol · g-1)Aged
samples
(Pd/CZ) + (Pd/A) > CZ + (Pd/A) > Pd/A, which is consistent with the results of the first CO-He pulse. The significant difference between (Pd/CZ) + A and CZ + (Pd/A) in DOSC highlights the importance of the metal-support interface. CO oxidation is greatly enhanced by oxygen backspillover under transient CO-O2 pulses, and these pulses enable oxygen to participate in CO oxidation at higher frequency and low temperature.18 In three OSC parts OSC from PdO sites (94.0 µmol · g-1) represents a small proportion of the total DOSC. Most of DOSC comes from the ceria-zirconia structure induced by the metal-support interaction. However, the contribution of the ceria-zirconia structure to DOSC is temperature dependent. The DOSC difference between Pd/A and CZ + (Pd/ A) indicates that OSC from the ceria-zirconia structure becomes significant above 300 °C, which is the threshold temperature of Ce0.7Zr0.3O2 reduction.18,33 Therefore, the DOSC gap between (Pd/CZ) + A and CZ + (Pd/A) should be more related to the OSC of the metal-support. For Pd-(Ce, Zr) Ox containing catalysts (including (Pd/CZ) + A, (Pd/CZ) + (Pd/A), and Pd/CZ) DOSC usually reaches a plateau above 300 °C. Conversely, for Pd-(Al2O3) containing catalysts (including CZ + (Pd/A) and Pd/A) DOSC increases linearly with temperature.18 The sensitivity of DOSC to temperature is varied by the metal-support interface. Pd-(Ce, Zr)
Figure 6. Integrated DOSC amount of Pd-supported catalysts under 0.05 Hz transient CO(10s)-O2(10s) measurement.
Ox greatly contributes to DOSC below 300 °C. Addition of Pd shits the threshold reduction temperature of ceria-zirconia from 300 to 120 °C. Compared to pure ceria-zirconia, Pt/ ceria-zirconia accelerates oxygen migration via oxygen backspillover.34 After hydrothermal aging DOSCs of Pd catalysts decrease sharply and approach a similar value regardless of their Pd-support interfaces. Because the OSC from Pd represents a small proportion of the total OSC, deterioration of the Pd-(Ce, Zr) Ox interface by aging must cause a decrease of OSC. The fact that the DOSC gap between CZ + (Pd/A) and Pd/A samples is small after aging implies that the contribution of ceria-zirconia to OSC is also small. Under transient CO-O2 measurement sintering of ceria-zirconia should be responsible for the OSC decrease on CZ + (Pd/A) catalyst. 3.3. CO2 Response of Dynamic OSC. To quantify the oxygen storage/release process CO2 profiles of the CO-O2 measurement at temperatures of 200, 400, and 500 °C are shown in Figure 7. It is clear that the initial slopes for all CO2 profiles are the same. At 200 °C similar slopes were found between t ) 0 and 0.5 s, and at 400 and 500 °C this period is between t ) 0 and 1.1 s. It implies that at the beginning of the CO-O2 cycle the metal-support interaction does not play an important role. CO oxidation during the pulse experiment takes place most likely via the Eley-Rideal mechanism, where CO is pulsed into the catalyst with little adsorbed on the surface.14 At the start of the CO-O2 experiment (1) CO reacts with the oxygen mainly from the PdOx species, amounting to 94.0 µmol · g-1. After the initial stage the metal-support interaction becomes more important, which increases the number of surface oxygen species (Os) available for CO oxidation with rapid oxygen back-
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Figure 7. CO2 response curves under 0.1 (top) and 0.05 Hz (bottom) transient CO-O2 measurements.
spillover. It should be noted that Pd-(Ce, Zr) Ox containing catalysts show a broader initial period for the similar slope behavior than Pd-(Al2O3) containing catalysts, and Pd/ Ce0.7Zr0.3O2 has 220 µmol · g-1 oxygen available for CO-O2 pulse reaction.34 In this regard, oxygen back-spillover at the metal-support interface greatly affects CO oxidation at this stage. After CO2 peak maxima, oxygen vacancies populate the surface of ceria near Ce3+ sites.10 IR results during a CO pulse experiment confirm that the reduced ceria-based materials favor CO adsorption and carbonates formation.35-37 At this stage CO2 concentrations of Pd catalysts have the following order: Pd/CZ > (Pd/CZ) + A > (Pd/CZ) + (Pd/A) > CZ + (Pd/A) > Pd/A. This sequence is consistent with CO2 profiles of the CO-He measurements shown in Figure 1. Note, CO is strongly adsorbed on Pd0 and Ce3+ sites and reacts with migrated oxygen to form CO2 (Langmuir-Hinshelwood mechanism). The dual pathway of CO oxidation suggests that both bulk oxygen migration (Ob) and surface oxygen mobility (Os) determine the CO oxidation when CO is adsorbed on Pd0 and Ce3+ sites. The Pd-(Ce, Zr) Ox interface plays a decisive role in facilitating bulk oxygen (Ob) back-spillover from ceria-zirconia to Pd sites, as evidenced by the higher CO2 intensities on Pd-(Ce, Zr) Ox containing samples. The role of unsupported Ce0.7Zr0.3O2 is evident by comparing the profiles of CZ + (Pd/A) and Pd/A. Figure 7 shows that the gap between the CO2 profile of CZ + (Pd/A) and that of Pd/A
increases with temperature. At 200 °C, the two CO2 profiles overlap, implying that the OSC contribution from Ce0.7Zr0.3O2 is negligible at this temperature. Temperatures increased to 400 and 500 °C result in an obvious difference of the CO2 response curves. The threshold reduction temperature of ceria-zirconia is estimated to be around 300 °C. Note, the CO2 gap appears after CO2 peak maxima. In the absence of the Pd-(Ce, Zr) Ox interface the reduction of Ce0.7Zr0.3O2 takes place after the reduction of PdOx. Due to weak adsorption of CO on ceria-zirconia CO may react with oxygen from Ce0.7Zr0.3O2 via the E-R pathway. On the other hand, CO adsorbed at Pd-(Al2O3) may react with oxygen species migrated from Ce0.7Zr0.3O2 via the L-H pathway. This kind of oxygen transfer is weaker than that of the Pd-(Ce, Zr) Ox interface. It was also proposed in the literature that CO spillover can increase the reduction activity.38 One may argue that CO spillover from Pd-(Al2O3) to far distant Ce0.7Zr0.3O2 sites can hardly take place, contributing little to the activity enhancement, while Ce0.7Zr0.3O2 catalyst can readily react with gaseous CO above 300 °C. For Pd-(Ce, Zr) Ox containing catalysts, however, the Pd-(Ce, Zr) Ox interface facilitates the oxygen back-spillover and oxygen spillover, and therefore, the effect of O2 back-spillover may overlap the effect of CO spillover. On the basis of the eq 1 suggested by Hori et al.,34 CO2 production rates at the slope can be calculated as follows
Pd-Ce0.7Zr0.3O2-Al2O3 Automobile Three-Way Catalysts
∫t t CO2 Signal
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3219
PdOx(s) + xCO(g) f Pd(s) + xCO2(g)
1
0
t1 - t0
) Rate[µmol of CO2 ⁄ s]
(1)
where ∫tt10CO2 Signal is the integrated peak area from t1 to t0 and Rate (µmol of CO2 · g-1 · s-1) is the initial CO2 production rate from t1 to t0. Figure 8 depicts the Arrhenius plots calculated at t ) 0.5 and 2.5 s. At 0.5 s, at low temperature, similar plots are observed on Pd catalysts regardless of the metal-support interface. It confirms that the CO2 production rate is determined by reduction of Pd oxides. Increasing temperature results in some difference between Pd-(Ce, Zr) Ox and Pd-(Al2O3) containing catalysts. The Pd-(Ce, Zr) Ox interface is more effective in promoting oxygen back-spillover. OSC from bulk ceria-zirconia is transported from ceria-zirconia to the Pd-(Ce, Zr) Ox interface by spillover and participates in CO oxidation. After CO2 peak maxima, Arrhenius plots at 2.5 s are clearly divided into two groups: one is Pd-(Ce, Zr) Ox containing catalyst, and the other is Pd-(Al2O3) containing catalysts. Steeper slopes are observed on Pd-(Al2O3) containing catalysts. On the other hand, the plots for Pd-(Ce, Zr) Ox containing catalysts are changed little with temperature. The CO oxidation process can be depicted as follows, where Vo is the oxygen vacancy
(1a)
Ce0.67Zr0.33O2 + Pd(s) f PdOx(s) + Ce0.67Zr0.33O2-x+Vo
(2) PdOx(s) + xCO(g) + Ce0.67Zr0.33O2-x+Vo f xCO2(g) + Pd(s) + Ce0.67Zr0.33O2-x+Vo (3) Two sources of CO2 (2) formation are reported in Figure 7. One is the CO adsorbed on the Pd0 and Ce3+ sites forming carbonate-bicarbonate, which is then desorbed as CO2 upon switching from CO to O2.12 According to the literature carbonate decomposition is clearly atmosphere dependent; inert Ar atmosphere leads to the CO and Ce3+ sites with high reducibility of ceria-zirconia. Conversely, oxygen atmosphere leads to CO2 and Ce4+ sites.37 Therefore, at the start of the O2 pulse carbonates are decomposed into CO2 and the amount of CO2 (2) is proportional to the number of surface sites available for carbonate adsorption. Unlike Pd-(Al2O3) containing catalysts, Pd-(Ce, Zr) Ox containing catalysts show higher CO2 (2) peak intensities and larger peak areas. This suggests that the Pd-(Ce, Zr) Ox interface facilitates carbonate adsorption. The other possible pathway involves reaction between O2 and preadsorbed CO via the L-H reaction mechanism.35 The peak area of CO2 (2) is proportional to the amount of CO preadsorbed. The disappearance of CO2 (2) with time has the following order: Pd/A < (Pd/A) +CZ < (Pd/CZ) + (Pd/A) < (Pd/CZ) + A < Pd/CZ. Therefore, Pd-(Ce, Zr) Ox is more favorable for CO adsorption compared with Pd-(Al2O3). In this regard, the enhanced CO adsorption can be ascribed to the Pd0 and reduced Ce3+ sites at Pd-(Ce, Zr) Ox. However, it is possible that adsorption of CO2 on the catalyst surface can also contribute to the CO2 (2) release. The CO2 (2) peaks decrease with increasing temperature. Conversely, CO2 (1) peaks increases somewhat with temperature. The ratio of CO2 (2) to CO2 (1) decreases with increasing temperature. At low temperature, because of favorable conditions for CO adsorption and readily available carbonate on the catalyst surface, formation of CO2 (2) is more favorable. On the contrary, high temperature does not favor carbonates adsorption, and therefore, more CO is oxidized to form gaseous CO2 during a CO pulse. At the same time, Pd is transformed from the Pd0 state to PdOx as follows
Pd(s) + x ⁄ 2O2(g) f PdOx(s)
(4)
Ce0.67Zr0.33O2-x+Vo+PdOx(s) f Pd(s) + Ce0.67Zr0.33O2
(5) Pd(s) + x ⁄ 2O2(g) f PdOx(s)
Figure 8. Arrhenius plot for fresh Pd-supported catalysts with reaction rates calculated with 0.5 (a) and 2.5 s (b) time span for CO injection.
(6)
3.4. Three-Way Catalytic Activity. The light-off tests for CO, NO, and C3H8 were conducted over the Pd-supported catalysts. The conversion profiles are shown in Figure 9, and the light-off temperatures at 50% conversion (T50) and 90% conversion (T90) are compiled in Table 4. For CO oxidation T50 has the following order: Pd/CZ < (Pd/ CZ) + A < (Pd/CZ) + (Pd/A) < CZ + (Pd/A) < Pd/A. It appears that catalysts containing Pd--(Ce, Zr) Ox is more active for CO oxidation, which positively correlates with the OSC performance. According to the literatures,39,40 CO oxidation follows a two-step mechanism: CO is adsorbed on the active sites first and then reacts with the oxygen species from ceria-zirconia. The presence of Pd-(Ce, Zr) Ox facilitates the oxygen back-spillover from the ceria-zirconia to Pd. The rankings of CO conversion and DOSC performance among these catalysts are consistent. It is reasonable to assume that catalyst
3220 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Shen et al. TABLE 4: Temperature Required To Reach 50% and 90% Conversion of Catalysts CO(°C) samples fresh Pd/CZ (Pd/CZ) + A (Pd/CZ) + (Pd/A) CZ + (Pd/A) Pd/A aged Pd/CZ Pd/CZ + A Pd/CZ + Pd/A CZ + Pd/A Pd/A a
HC(°C)
T50 T90 ∆T 120 135 154 174 193 236 250 248 243 253
151 164 187 202 214 239 271 258 251 277
31 29 33 28 21 3 21 10 8 24
a
NO(°C)
T50 T90 ∆T T50 T90 ∆T 267 269 248 245 244 414 400 380 357 330
307 318 288 278 254 486 459 434 413 379
40 49 40 33 10 72 59 54 56 49
253 256 244 236 232 407 387 367 349 325
299 307 266 256 247 475 453 409 391 376
46 51 22 20 15 68 66 42 42 51
∆T is defined as T90 - T50.
Figure 10. Schematic system of Pd-(Al2O3) and Pd-(Ce, Zr) Ox interfaces in Pd catalysts.
Figure 9. Catalytic activities of CO, NOx, and C3H8 of fresh (hollow mark) and aged (full mark) Pd-supported catalysts at simulated outlet exhaust.
activities rank similarly for CO light-off after hydrothermal aging. The differences between T50 and T90 are hardly distinguished on aged catalysts (Table 4). Recall that the DOSC of aged samples also decrease sharply (Figure 6). For DOSC measurement the oxygen back-spillover plays a key role in CO oxidation. Oxygen spillover is also a key step in the two-step mechanism for catalytic CO oxidation. Therefore, DOSC and CO are consistent. Furthermore, adsorption of gaseous O2 is less important for the overall CO oxidation. It is not crucial to
cover many Pd sites with adsorbed oxygen activation. Rather, the gaseous O2 participates in the reaction through the Pd-(Ce, Zr) Ox interface, which plays an important role for gas-phase O2 adsorption and activation, transforming gaseous O2 into lattice oxygen. The role of Pd-(Ce, Zr) Ox here is similar to that in the transient O2 storage process during CO-O2 measurement. Because of the lower redox potential of Pd2+/Pd0 than that of Ce4+/Ce3+, Pd-(Ce, Zr) Ox interaction facilitates oxygen back-spillover and oxygen spillover, forming a dynamic oxygen balance between lattice oxygen and gaseous O2. Figure 9 shows similar profiles and activity rankings for NO and C3H8 conversions. T50 has the following order: Pd/A < CZ + (Pd/A) < (Pd/CZ) + (Pd/A) < (Pd/CZ) + A ≈ Pd/CZ. This is exactly the opposite ranking to that of CO. For both fresh and aged catalysts T50 and T90 for NO and C3H8 become lower with more Pd-(Al2O3) in the catalysts. Pd-(Al2O3) is more favorable for NO and C3H8 conversion compared with Pd-(Ce, Zr) Ox. It is reported that NO reduction is a structure-sensitive reaction.11 Reduced Pd and large Pd particles favor NO dissociation and N recombination.11 XPS results show the Pd sites supported on Al2O3 shift to the Pd0 state, and Pd sites supported on Al2O3 are easily converted to the Pd0 state during three-way reactions.41 When supported on ceria-zirconia Pd exists in an oxidized state due to the close interaction with ceria-zirconia.2,41 The reduced Pd0 sites supported on Al2O3 are favorable for NO dissociation and leave oxygen on Pd0. It seems that OSC derived from the Pd-(Ce, Zr) Ox interface is detrimental to NO dissociation and N recombination. Furthermore, the effect of Pd particle size on NO conversion should also be taken into consideration. NO conversion preferentially occurs at larger Pd particles. Pd dispersion is improved by interacting with ceria-zirconia, leading to smaller Pd particles at the Pd-(Ce, Zr) Ox interface.41 Although OSC is greatly improved by the enhanced Pd-(Ce, Zr) Ox interface, the more oxidized Pd and the smaller Pd particles are negative factors for NO conversion. On the contrary, the larger Pd particle size
Pd-Ce0.7Zr0.3O2-Al2O3 Automobile Three-Way Catalysts at the Pd-(Al2O3) interface favors NO conversion. Figure 10 depicts the proposed main reaction paths correlating the features of Pd-support interfaces. 4. Conclusion The presence of Pd-(Ce, Zr) Ox facilitates oxygen release from the catalysts. The Pd-(Ce, Zr) Ox interface promotes removal of surface and surface nearby oxygen but not the bulk oxygen. Because of the electron transfer between Pd and ceria-zirconia the Pd particles in Pd-(Ce, Zr) Ox are in an oxidized state. The Pd particles in an Pd-(Al2O3) containing catalyst are more reduced. The oxygen spillover and backspillover between ceria-zirconia and Pd sites plays an important role in DOSC promotion. The dynamic OSC results show that Pd-(Ce, Zr) Ox accelerates the oxygen storage and releasing process, and this acceleration is accomplished via the oxygen back-spillover from ceria-zirconia. The Pd-(Ce, Zr) Ox interface results in high CO oxidation activity, while Pd-(Al2O3) is favored for NO and C3H8 conversion. By analyzing the results of DOSC and TWC we can make a correlation among the Pd-(Ce, Zr) Ox interface, OSC, and CO conversion. On the other hand, the Pd-(Al2O3) interface can be positively correlated with NO and C3H8 conversion. Acknowledgment. The authors are grateful for financial support from the National Basic Research Program (also called 973 Program) of China (No. 2004CB719503), Program of New Century Excellent Talents in University (NCET-06-0243), Key Program of Natural Science foundation of Tianjin (No. 07JCZDJC01600), and Program of Introducing Talents of Discipline to Universities of China (No. B06006). References and Notes (1) Kasˇpar, J.; Fornasiero, P.; Hickey, N. Catal. Today. 2003, 77, 419. (2) Hu, Z.; Wan, C. Z.; Lui, Y. K.; Dettling, J.; Steger, J. J. Catal. Today 1996, 30, 83. (3) Kasˇpar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285. (4) Di Monte, R.; Kasˇpar, J. Catal. Today 2005, 100, 27. (5) Kasˇpar, J.; Fornasiero, P. J. Solid State Chem. 2003, 171, 19. (6) Jia, L. W.; Shen, M. Q.; Wang, J. Surf. Coat. Technol. 2007, 201, 7159. (7) Kolli, T.; Rahkamaa-Tolonen, K.; Lassi, U. Catal. Today 2005, 100, 297–302. (8) Ranga Rao, G.; Fornasiero, P.; Di Monte, R. J. Catal. 1996, 162, 1–9. (9) Fornasiero, P.; Ranga Rao, G.; Kasˇpar, J. J. Catal. 1998, 175, 269. (10) Martı´nez-Arias, A.; Hungrı´a, A. B.; Ferna´ndez-Garcı´a, M. J. Catal. 2004, 221, 85. (11) Iglesias-Juez, A.; Martı´nez-Arias, A.; Ferna´ndez-Garcı´a, M. J. Catal. 2004, 221, 148. (12) Boaro, M.; Giordano, F.; Recchia, S.; Dal Santo, V. Appl. Catal. B: EnViron. 2004, 52, 225.
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