Influence of Pd Morphology and Support Surface Area on Redox

Ind. Eng. Chem. Res. 2007, 46, 7883-7890

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Influence of Pd Morphology and Support Surface Area on Redox Ability of Pd/Ce0.67Zr0.33O2 under CO-He Pulse and Transient CO-O2 Measurements Minwei Zhao,† Meiqing Shen,†,‡ Jun Wang,*,† 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, and 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

The influences of Pd sites and specific surface area on oxygen storage capacity (OSC) were investigated over a series of Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 samples by CO-He pulse and dynamic CO-O2 measurements at 300-550 °C. The results show that the rate and quantity of oxygen storage/release capacity are greatly enhanced by Pd support, showing a more prominent CO2 slope gradient and CO2 reduction peak than that of Ce0.67Zr0.33O2. The existence of Pd-(Ce,Zr)Ox (interface between Pd and ceria-zirconia mixed oxides) interaction is confirmed to be important for oxygen reduction. During the CO-He pulse, acceleration of oxygen reduction by Pd promotion is limited by the reducibility of the whole system. At low reducibility, Pd-(Ce,Zr)Ox interaction is evident for OSC. Conversely, when the reducibility reaches above 12%, Pd(Ce,Zr)Ox interaction is less effective, ascribed to the bulk oxygen migration in ceria-zirconia becoming the rate-determining step of the reduction process. The experiments with dynamic pulses of CO-O2 reveal that dynamic oxygen storage capacity (DOSC) is closely affected by the temperature range; DOSC is greatly affected by Pd-(Ce,Zr)Ox at low temperature. Considering the effect of Pd deterioration and ceria-zirconia sintering on OSC, after hydrothermal aging at high temperature, no significant difference is related to support sintering. Additionally, deteriorated Pd sites are more effective in decreasing OSC. Pd site evolution under hydrothermal aging may be dominated by Pd sintering, rather than by Pd encapsulation. By calculating the CO2 production rate, Arrhenius plots are suggested to show that the apparent activation energy increased by Pd site deterioration, rather than by sintering of ceria-zirconia. 1. Introduction Pd and ceria-zirconia are the key components for typical three-way catalysts (TWCs). The role of ceria-zirconia in a TWC is to attenuate the fluctuation of air to fuel, to store and release the oxygen during oscillation, and to minimize the negative effects of rich/lean oscillation in exhaust gas composition on catalytic activity.1-3 It is generally accepted that loss of oxygen storage capacity (OSC) induces the decrease of TWC efficiency. High thermal stability of ceria-zirconia is pursued because catalytic converters usually deteriorate during severe continuous hydrothermal long running at high temperatures. Due to surface collapse and particle sintering, the ceria-zirconia based catalysts have an inferior oxygen storage/release rate at high working temperatures. Pd-only catalysts usually receive much attention in automobile exhaust elimination for their superior high CO conversion at low temperature and comparable low cost to Pt and Rh.4 Pd/ceria catalyst was also considered in the typical water gas shift (WGS) system.5 Contributed by the interaction between ceria and Pd sites, the reduction behavior of ceria-zirconia was improved by the Pd support; the reduction peak shifts to a lower temperature in the H2-TPR (hydrogen temperature-programmed reduction) profile, ascribing to the mechanism of H2 spillover from the Pd sites to the support.6 Moreover, when CO is employed as reactant, the presence of the Pd/ceria interface was found to facilitate the CO and oxygen reaction by supporting Pd on ceria.7,8 The Pd/ceria interface * To whom correspondence should be addressed: Tel./fax.: +8622-27892301. E-mail: [email protected]. † School of Chemical Engineering & Technology. ‡ State Key Laboratory of Engines, Tianjin University. § China Automotive Technology & Research Center.

Figure 1. Schematic of preparation of Pd/Ce0.67Zr0.33O2 samples.

exhibited a low reduction peak from CO-TPR profiles, different from that of H2-TPR, where the mechanism was described as the CO adsorbing on the precious metal (PM) sites and reacting with the migrated oxygen from bulk ceria.9 The control step was estimated to be the process of oxygen migration to the PM/ ceria interface.4 Most importantly, CO oxidation is a key step to overall pollutant eliminations. The interface between precious metals and ceria-zirconia is of great importance for CO oxidation and oxygen release in TWCs. Under real working conditions, automobile catalysts experience an atrocious hydrothermal aging process at high temperature which would cause the sintering of both PM sites and

10.1021/ie061590y CCC: $37.00 © 2007 American Chemical Society Published on Web 10/31/2007

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Table 1. SBET and Treatment of Pd/Ce0.67Zr0.33O2 Samples XRD sample

sample designation

surface area (m2‚g-1)

aging procedure [gas, time (h), T (°C)]

crystalline phase

Ce0.67Zr0.33O2 crystalline size (Å)

Ce0.67Zr0.33O2 lattice constant (Å)

Pd/Ce0.67Zr0.33O2 (HSA) Pd/Ce0.67Zr0.33O2 (SSA) Pd/Ce0.67Zr0.33O2 (HSA aged) Pd/Ce0.67Zr0.33O2 (SSA aged)

Pd/HA Pd/SA Pd/HAA Pd/SAA

56 9 7 7

10% steam/air, 5, 800 10% steam/air, 5, 800 10% steam/air, 5, 800

Ce0.67Zr0.33O2 Ce0.67Zr0.33O2 Ce0.67Zr0.33O2 Ce0.67Zr0.33O2

75 140 142 146

5.3565 5.3608 5.3695 5.3697

Table 2. DOSC Behavior under Transient CO-O2 Measurements

sample

surface area (m2‚g-1)

Pd/HA Pd/SA Pd/HAA Pd/SAA HA SA

56 9 7 7 56 9

CO DOSC at various temperatures (µmol‚g-1) pretreatment hydrothermal hydrothermal hydrothermal hydrothermal

300 °C

350 °C

400 °C

450 °C

500 °C

550 °C

730.5 549.6 390.5 328.7 32.5 12.9

910.5 782.9 587.0 565.4 143.4 21.4

975.4 900.9 769.5 698.2 280.5 48.0

1011.8 960.5 850.9 782.3 469.8 104.3

1119.8 1076.3 879.1 821.4 628.0 209.8

1134.8 1098.1 879.8 825.2 873.2 444.1

support, leading to the loss of oxygen storage capacity and catalyst deactivation. With respect to the effect of PM, the rate of CO2 formation under a CO-He pulse was found to be linearly related to the Pt surface area. However, when the surface area is lower than a threshold value of 0.15 m2/g, this factor may also have a significant weight.10 The rate of the water gas shift reaction was also confirmed to be proportional to the Pd surface area in the Pd/ceria system.5 Therefore, to avoid PM particle growth is a necessity for catalyst stabilization under high temperature aging. With respect to the ceria-zirconia sintering, after Rh supported on ceria-zirconia with low surface area, a reduction peak at low temperature observed on the H2-TPR profile contributed to the interaction between the support and PM.11 It is indicated that the specific surface area of support may exhibit less relationship with OSC. In this regard, it is of profound significance to study the effect of PM sites and ceriazirconia on OSC behavior related to the Pd-(Ce,Zr)Ox (interface between Pd and ceria-zirconia mixed oxides) interface with the oxygen migration process. Recent great research12,13 has been devoted to dynamic oxygen storage capacity (DOSC) measurement. DOSC measurement, which provides a TWC working condition with a more similar fluctuation to a real working process, is employed in the present work. It is aimed at studying the effects of PM and support on OSC performance under transient CO/O2 conditions. The presence of Pd-(Ce,Zr)Ox interface is confirmed to facilitate the oxygen migration. Moreover, PM deterioration and support sintering are observed to exert different effects on Pd(Ce,Zr)Ox interface evolution, resulting in differing DOSC performance. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. The original Ce0.67Zr0.33O2 sample was prepared by a citric sol-gel method.14,15 Citric acid and glycol, as complex reagents, were added to the mixed solution of Ce(NO3)3 and ZrO(NO3)2 in a stoichiometric ratio. After continuous stirring for 2 h at room temperature, the mixed solution was kept at 80 °C overnight to obtain the wet transparent yellow gel. The gel was dried at 100 °C for 3 h to obtain a sponge dry yellow gel. After calcination of the dried gel at 300 °C for 30 min and then at 500 °C for 5 h, the as-prepared Ce0.67Zr0.33O2 material was obtained, which was indicated as the high surface area sample (HA). The SA (severe aging) sample was obtained by heating

the HA samples to 800 °C at 10 °C/min in 250 mL/min flowing 10% steam/air atmosphere and then kept at 800 °C for 5 h. Pd/Ce0.67Zr0.33O2 samples, including Pd/HA and Pd/SA, were prepared by incipient wetness, impregnating Ce0.67Zr0.33O2 (HA and SA) with Pd(NO3)2 aqueous solutions with 0.5 wt % nominal Pd loading. Then the mixtures were placed at room temperature for 2 h, dried at 100 °C for 2 h, and calcined at 500 °C for 3 h subsequently. As shown by Figure 1, by further heating the Pd/HA and Pd/SA samples to 800 °C at 10 °C/min in 250 mL/min flowing 10% steam/air atmosphere and holding at 800 °C for 5 h, Pd/HA aged and Pd/SA aged samples were obtained and designated as Pd/HAA and Pd/SAA. A Quantachrome NOVA 2000 apparatus was used to measure the BET specific surface area of the samples by N2 adsorption at 77 K. X-ray powder diffraction (XRD) patterns were acquired with an X’Pert Pro diffractometer operating at 40 kV and 40 mA with Ferrum-filtered Co KR radiation by 0.03 step size. The lattice constants were calculated based on Bragg’s law 2d sin θ ) kλ with the (111), (200), (220), and (311) planes, where d is the distance of the crystal plane, θ is the angle of the diffraction peak, and λ is the wavelength of Co KR radiation. Crystal sizes were determined by the Scherrer equation calculated from the (111) plane. The specific surface area and calculated crystal information derived from XRD are shown in Table 1. 2.2. OSC Measurement. The measurements of OSC were carried out with the OSC equipment.16-18 The system dead volume was 3.5 mL. In the experiments, 25 mg of catalyst samples diluted with 40 mg of quartz beads was placed at the bottom of a heat transfer reactor. The concentration of the five components in the outlet (CO, O2, CO2, Ar, and He) was monitored on-line by a Balzers QMS200 quadrupole spectrometer. All the experiments of CO-He pulses for OSC measurement were operated at 500 °C.14,15 The experiment consists of three steps: (1) preoxidizing the sample under 2% O2/1% Ar/He atmosphere, where 1% Ar was used as a reference to monitor the state of the mass spectrometer and system; (2) outgassing under pure He to clean the sample; (3) carrying out OSC measurement under transient CO-He condition. Two flows of 4% CO/1% Ar/He and pure He were alternately pulsed 10 times respectively in the sequence where first was the 4% CO/1% Ar/He pulse for 5 s at a flow of 300 mL/min and next was the

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Figure 2. OSC of Ce0.67Zr0.33O2 and Pd/Ce0.67Zr0.33O2 samples under COHe pulse measurement.

pure He pulse for 20 s at a flow of 300 mL/min. The OSC of each pulse was expressed as micromoles of oxygen per gram of ceria-zirconia catalyst (µmol of [O]‚g-1). The degree of reduction was determined by calculation the ratio of oxygen consumed to oxygen of CeO2 in solid solution (Ce0.67Zr0.33O2). The experiments of dynamic pulses of CO and O2 for DOSC measurement were operated at temperatures ranging from 300 to 550 °C with intervals of 50 °C.14,15 The experiment consists of three steps: (1) and (2) were the same as that of CO-He pulse measurement; (3) DOSC measurement was carried out under transient CO-O2 conditions. In a specific experiment, two flows of 4% CO/1% Ar/He and 2% O2/1% Ar/He were pulsed alternately in sequence with the 4% CO/1% Ar/He pulse for 5 s at a flow of 300 mL/min and the 2% O2/1% Ar/He pulse for 5 s at a flow of 300 mL/min. The frequency of the former and the latter pulses was 0.1 Hz. A DOSC value was calculated by integrating the CO2 formed during the corresponding stable CO-O2 cycle and was expressed as micromoles of oxygen per gram of ceria-zirconia catalyst as shown in Table 2 (µmol of [O]‚g-1). 3. Results and Discussion 3.1. Presence of Pd-(Ce,Zr)Ox Interaction. 3.1.1. OSC of CO Pulse Measurement. The OSC performance of COHe pulse versus reducibility is plotted in Figure 2, where the OSC is expressed by micromoles of O atom per gram of catalyst. The OSC gradually decreases by the increment of reducibility and attains a plateau at high reducibility. Remarkable differences between Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 samples are observed at the initial CO pulses. Figure 2 shows that OSC higher than 800 µmol‚g-1 appears on Pd/HA and Pd/SA samples relative to 300 µmol‚g-1 of HA and SA samples, suggesting that the interaction between Pd and ceria-zirconia promotes the oxygen reduction. After Pd support, assuming 0.5 wt % Pd loading is in its full oxidation state, the surface PdO species is calculated with 47.0 µmol‚g-1 available oxygen. Similarly, by calculating the number of surface oxygens, 221 and 36 µmol‚g-1 exist on HA and SA samples, respectively.10 Therefore, oxygen consumed during the first CO pulse is far more than the oxygen summation of Ce0.67Zr0.33O2 support and PdO sites. That means that under present experimental conditions the first CO pulse is able to reduce not only surface oxygen but also oxygen related to the near-surface region on the Ce0.67Zr0.33O2 sample. Interaction between Pd and ceria-zirconia causes oxygen back-

Figure 3. CO2 formation peak of Ce0.67Zr0.33O2and Pd/Ce0.67Zr0.33O2 samples under CO-He pulse measurement.

spillover behavior, facilitating the oxygen reduction.19 Assuming a ceria-zirconia particle to be a sphere, the depth of particle reduction by CO should be contributed by Pd support. The further analysis of transient CO2 profiles under CO-He pulse in Figure 3 gives sufficient evidence of Pd promotion, showing as a higher extent and significant inclination peak on Pd/Ce0.67Zr0.33O2 sample ranging from 0 to 2 s, implying that higher reaction rates appear on Pd-supported samples. It is concluded that Pd facilitates oxygen of the ceria-zirconia contributing to the CO oxidation. Upon analysis of the relationship between OSC and reducibility, it is found to be evident that Pd promotion is favored by reducibility preferably lower than 12%. After the reducibility of ceria-zirconia reaches 12%, it does not seem to affect OSC performance by Pd support. It is inferred that rate-determining steps are varied by the reducibility. At low reducibility, there is an indication that Pd promotion for OSC only occurs at the near-surface region, where oxygen back-spillover dominates the OSC process. Increased with the reducibility, bulk oxygen migration becomes the rate-determining step. The point of reducibility related to the shift of the rate-determining step is located at 12% for Pd/HA and Pd/SA samples (seen in Figure 2), whereas it is located at 10% for the Pd/HAA sample. For Ce0.67Zr0.33O2, the theoretical reducibility is 25% due to its coordination number variation from 8 to 6 with transformation from Ce4+ to Ce3+. Thus, it is clear that Pd promotion is effective for OSC lower than the reducibility of 10% related to the oxygen reduction at the surface and near-surface region. According to the Pd/CeO2 catalyst, the schematic mechanism of CO oxidation at the Pd-(Ce,Zr)Ox interface is depicted as a two-steps reaction, where the two steps of oxygen backspillover always occur at reducibility lower than ca. 10%:5

CO(g) + PdOx(s) f Pd(s) + CO2(g)

(1)

Ce0.67Zr0.33O2 + Pd(s) f PdOx(s) + Ce0.67Zr0.33O2-x + Vo (2) Here, the Pd-(Ce,Zr)Ox interface plays a decisive role in helping the oxygen back-spillover from Ce0.67Zr0.33O2 to Pd sites. It can be said that the kinetics of CO oxidation under the COHe pulse process is varied by the reducibility. When the reducibility is low, by the presence of the Pd-(Ce,Zr)Ox interface, migration of surface and near-surface oxygen to Pd

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Figure 4. CO-O2 pulse DOSC profiles for HA and Pd/HA samples at 450 °C.

sites stabilizes the Pd in its oxidation state, which is favorable for CO oxidation. Oxygen vacancies creation should be another force driving bulk oxygen migration to contribute to CO oxidation.20 At this point, Hori supposed that the solubility of anion vacancies in the oxide lattice limits the “bulk” reducibility of ceria-zirconia but not the diffusion of O2- from the bulk to the surface by the oxygen concentration gradient.10 As shown in Figure 2, at low reducibility, more oxygen vacancies are created in Pd/Ce0.67Zr0.33O2 samples. When the Pd enhances the reducing potential of the support, the diffusion of O2- from the bulk to the surface is not the determining step.21 The role of oxygen vacancies may be overlooked due to the interface promotion of Pd-(Ce,Zr)Ox. Increased by the reducibility, it is generally accepted that more oxygen vacancies facilitate the migration of oxygen from the bulk to the surface.20 However, compared with Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 samples, there are no promotion effects by oxygen vacancies exerted on bulk oxygen migration. It seems that the solubility of anion vacancies in the oxide lattice determines the bulk reducibility of ceria-zirconia. In another way, at higher reducibility, i.e., above 12%, the concentration of oxygen vacancies may not determine the OSC absolutely. In this regard, the activation of bulk oxygen is also controlled by the theoretical reducibility of CeO2 (25%), which is associated with the activation energy (Ea) of oxygen migration. 3.1.2. DOSC of Dynamic CO-O2 Measurement. Figure 4 depicts, as an example, typical DOSC profiles under CO-O2 pulse with comparison with HA and Pd/HA samples at 450 °C. Doublet CO2 peaks are observed, which is consistent with the earlier research.12-14,22 CO2(1) appears when CO pulses into the reactor, related to the oxygen releasing process; CO2(2) emerges when CO switches to O2, ascribed to the oxygen storage process. Earlier literature has reported the mechanism of doublet CO2 peaks. CO2(1) arises from CO reacting with the surface and near-surface oxygen. Evident from the peak area under the CO2(1) profile, DOSC under dynamic CO-O2 measurement is greatly promoted by Pd support, showing a larger peak area and stronger intensity of CO2 profile over Pd/HA catalyst. The decreased peak area of CO profiles over Pd/HA catalyst implies more oxygen contributing to CO oxidation than unsupported HA sample. The presence of the Pd-(Ce,Zr)Ox interaction, to some extent, increases the number of active sites on the support

and facilitates oxygen back-spillover from ceria-zirconia to Pd sites, resulting in a quicker oxygen response and higher amount of oxygen consumed. Meanwhile, after the introduction of O2, two sources of CO2(2) formation are reported. One is a twostep mechanism: CO is first adsorbed on the Ce3+ sites, forming carbonate-bicarbonate, which is then desorbed as CO2.13 It is also considered that CO2(2) is a result of reaction between O2 and preadsorbed CO via the Langmuir-Hinshelwood (L-H) reaction mechanism. Compared with CO2(1), Pd-(Ce,Zr)Ox interaction exerts an insignificant influence on CO2(2), indicating that Pd support does not induce more CO and carbonate adsorption on active sites during a CO pulse. Meanwhile, the great difference in O2 profiles between Pd/HA and HA samples suggests that Pd support facilitates the oxygen storage process, interpreted as an oxygen spillover process from Pd sites to ceria-zirconia. Meanwhile, the Pd state is transformed from Pd0 reduction state to PdOx oxidation state as follows:

Pd(s) + O2(g) f PdOx(s)

(3)

Ce0.67Zr0.33O2-x + Vo + PdOx(s) f Pd(s) + Ce0.67Zr0.33O2 (4) Pd(s) + O2(g) f PdOx(s)

(5)

The presence of the Pd-(Ce,Zr)Ox interaction facilitates Pd oxidation under oxidation state and stabilizes the Pd in its oxidation state. Thus, Pd oxidation occurs first; subsequently oxidation of ceria-zirconia takes place, ascribed to the high redox potential of the Ce4+/Ce3+ couple (1.61 eV), which is higher than that of Pd2+/Pd0.23 The transient CO and O2 pulses by Pd promotion are depicted in Figure 5. From this aspect, the oxygen storage process is affected by the surface of the Pd(Ce,Zr)Ox interface, similar to the oxygen release process (i.e., CO oxidation), which provides more active sites for oxygen spillover and back-spillover processes. The CO2 profiles of Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 at 350, 450, and 550 °C under transient CO-O2 pulses are summarized in Figure 6. Differences can be noted between low temperature and high temperature, with the Pd promotion. At 350 °C, Pd promotion on CO2 profiles is evident from the peak area and peak intensity of CO2 profiles. Increased by the


Figure 5. Schematic mechanism of CO-O2 measurement on Pd/Ce0.67Zr0.33O2.

temperature to 550 °C, CO2 profiles between Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 are more similar. The difference between Pd/ Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 is not as obvious as that at low temperature. By further quantifying the Pd promotion, in Figure 7, the definition of the ratio of DOSCPd/Ce0.67Zr0.33O2 to DOSCCe0.67Zr0.33O2 is given in the whole range of temperature according to DOSC in Table 2. Notably, the effect of Pd promotion is varied within the temperature range. At the low temperature of 300 °C, the ratio of DOSCPd/HA to DOSCHA attains a value of 23; meanwhile, the ratio of DOSCPd/SA to DOSCSA is as high as 42. This is consistent with the results of Hori et al. that the rate of CO2 production by Pd support on ceria-zirconia is as high as 60-fold.10 However, at the high temperature of 550 °C, a significant decrease in the ratio of DOSCPd/HA to DOSCHA is observed, which is 1.2 times for HA and 2.5 times for SA sample, respectively. It seems that Pd support favors the oxygen reduction at low temperature; however, it is less efficient at high temperature. Ciuparu24 pointed out the exchange activity of PM particles at low temperature is considerably higher than that of support by temperature programmed isotopic exchange (TPIE); thus the contribution to the overall oxygen exchange activity of support under fully oxidized catalyst was negligible when low temperature was employed. At high temperature, the oxygen exchange ability of the support was significantly improved due to the support exchange ability. Therefore, at higher temperature, the improved DOSC behavior on unsupported Ce0.67Zr0.33O2 catalyst indicates the importance of bulk oxygen activation by the increment of temperature. From this aspect, the importance of Pd support at low temperature is referred to the activation of near-surface oxygen on the Pd-(Ce,Zr)Ox interface contributing to CO oxidation. It is worth discussing that oxygen consumed under transient CO-O2 measurement is limited by the reducibility lower than ca. 12%. Consistent with the results of COHe pulses, Pd promotion is more effective at low reducibility associated with surface oxygen activation. 3.2. Effects of Pd Deterioration on OSC Loss. With respect to the effect of Pd sintering, as shown in Figure 6, a finding is the same slope of the CO2 peak at 0-2 s over Pd/HA, Pd/SA,

Figure 6. CO2 peak of Ce0.67Zr0.33O2and Pd/Ce0.67Zr0.33O2 samples under transient CO-O2 measurement.

Pd/HAA, and Pd/SAA samples. At 350 °C the rates of CO2 formation appear similar from 0 to 1.3 s. Similarly, at 450 and 550 °C CO2 formation rates are similar until 1.5 and 1.4 s, respectively. It is calculated that complete reduction of 0.5 wt % PdO species with 47.0 µmol‚g-1 oxygen needs ca. 1s at the present CO pulse condition. Thus, the reduction process from 0 to 1.3 s can be ascribed to the reduction of all PdOx and part of the surface oxygen species, which is responsible for the same CO2 slope on Pd/HA, Pd/SA, Pd/HAA, and Pd/SAA samples. For quantitatively evaluating the CO2 reduction rates, the CO2

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Figure 7. Pd promotion to Ce0.67Zr0.33O2 samples on OSC (HA and SA) as a function of temperature.

in 10% steam/air atmosphere. The CO2 production rate is independent of the Pd morphology when CO pulses into the reactor and reacts with surface oxygen by the Eley-Rideal (ER) pathway. However, when the reducibility is 5%, a significant difference over the calculated CO2 production rate is present. When the catalyst is in a slightly reduced state, oxygen back-spillover from ceria-zirconia to Pd sites is greatly affected by Pd sintering. This is consistent with the difference of CO2 profiles ranging from 2 to 5 s in Figure 6 (also CO2 profiles of CO-He pulse measurements in Figure 2). In another way, the OSC difference arises from this stage when the catalyst is in a slightly reduced state. It should recall that high CO coverage on the Pd sites from in situ DRIFTS,25 in which the reaction between CO and oxygen migrated from the bulk, follows the L-H pathway when ceria-zirconia is in reduced state. According to the literature, CO oxidation on the Pd-(Ce,Zr)Ox interface provided the evidence that loss of the Pd-(Ce,Zr)Ox interface upon aging should be responsible for the decline of CO oxidation. Relative to a Pd/alumina sample, the Pd-(Ce,Zr)Ox interface is also the reason that CO oxidation is pronouncedly promoted.25 In the absence of gaseous oxygen, the oxygen migration and reaction with nearby CO adsorbed on the Pd sites is the rate-determining step. The Pd-(Ce,Zr)Ox interface to CO oxidation is in the form of oxygen back-spillover process:

Ce0.67Zr0.33O2 + Pd(s) f PdOx(s) + Ce0.67Zr0.33O2-x + Vo

Figure 8. Arrhenius curves of Pd/Ce0.67Zr0.33O2 samples under reducibilities of 1% and 5%.

production rates are calculated by the equation suggested by Hori et al.10 Figure 8 plots the results of rates of when reducibility attains 1%. No significant difference is present over Pd/HA, Pd/SA, Pd/HAA, and Pd/SAA samples (Figure 8, top). This is consistent with the CO2 profiles ranging from 0 to 2 s of CO-He pulse. It is deduced that surface reduction at initial 2 s is not affected by Pd sintering, though after aging at 800 °C

Likewise, Lambrou et al.26,27 attributed the shift CO2 response curve and difference between fresh and aged TWC samples to the change of rate constant (k) associated with the oxygen backspillover process. All the experimental parameters affecting the rate constant (k) can induce the shift of reaction time. Hence, Pd sintering has a significant effect on CO oxidation due to deterioration of the Pd-(Ce,Zr)Ox interaction, ascribed to the oxygen back-spillover process. It is referred to the bulk oxygen reduction process.14 The Pd-(Ce,Zr)Ox interface is inferred to be more important when bulk oxygen back-spillovers to the Pd sites. Upon analysis of the results of the Arrhenius plots, both Pd sintering and ceria-zirconia sintering induce deteriorated Pd(Ce,Zr)Ox interface and the decline of CO2 production rates. However, In Figure 8, by comparison with the slopes of Arrhenius plots of Pd/SA, Pd/SAA, and Pd/HA samples, more apparent activation energy is deduced after Pd sintering in terms of the slopes of the Arrhenius plots. Relative to support sintering, Pd sintering has the more profound influence on deteriorated Pd-(Ce,Zr)Ox interaction, with the decline of DOSC. Notably, the reaction rates on the Arrhenius plots are similar for Pd/ SAA and Pd/HAA samples. For the Pd/SAA sample, Pd encapsulating by ceria-zirconia is avoided due to the surface prestabilization on the Pd/SA sample. For the Pd/HAA sample, Pd encapsulating and sintering may occur simultaneously due to Pd support on the high surface area HA sample. By comparison with the Arrhenius plots in Figure 8, it should be underlined that, under hydrothermal aging, Pd site evolution may be dominated by Pd sintering rather than by Pd encapsulation. As per the literature, encapsulation and sintering of active sites occur during accelerated high temperature aging.28 Under high temperature redox aging, strong metal support interaction (SMSI) including metal decoration appears on Rh/CeO2 catalyst,29,30 where CeO2 decorations on the top of Rh sites lead to OSC decrease. Surface collapse to bury the precious metal sites in the support as well as Rh inserting into the lattice of ceria also possibly occurs. However, because of the comparably larger


Figure 9. Comparison between surface area aging and Pd site aging with the effects on OSC performance. (a) OSCPd/HA-OSCPd/SA; (b) OSCPd/SAOSCPd/SAA; (c) OSCHA-OSCSA.

ion radius of Pd (1.375 Å), it is more difficult to bury the Pd sites by surface collapse than those of Rh (1.345 Å) at some level. Compared with the Pd/HAA and Pd/SAA samples, the similar Arrhenius plots indicate that Pd sites on HA and SA samples experience similar evolution during high temperature aging. In the case of aging the Pd at 800 °C for 5 h at 10% steam/air, the Pd surface area should greatly decrease due to Pd sintering. Hori et al. also suggested that the CO oxidation rate is dependent on the Pt surface area when it falls below 0.15 m2/g.10 In this regard, preventing the decrease of Pd surface area is of great importance for stable OSC results. Another thing is that some points of reaction rates in the Arrhenius plots deviate from the straight line at R ) 5%. Three kinds of active sites determine the DOSC performance: Ce sites, Pd sites, and Pd-(Ce,Zr)Ox interface. For the OSC of Pd sites, it has been consumed at R ) 1%. Hence, at high reducibility, i.e., R ) 5%, sites of Ce and Pd-(Ce,Zr)Ox interface are applied to the OSC model. At low temperatures, oxygen back-spillover on Pd-(Ce,Zr)Ox interface dominates the DOSC performance, where contribution of the Ce site is insignificant (see CO2 response peak in Figure 6). This is in contrast to the behavior at high temperatures; other than the oxygen back-spillover process, bulk oxygen migration on Ce sites becomes more and more important to DOSC performance (see CO2 response peak in Figure 6). To some extent, contributions of Ce sites and Pd(Ce,Zr)Ox interface to DOSC are varied with temperature. Thus, when Pd/Ce0.67Zr0.33O2 is in a higher reduced state at high temperature, reaction rates in Arrhenius plots (Figure 8) are more affected by bulk oxygen migration on Ce sites. Consistent with results of Figure 7, the effect of Pd promotion is weakened at high temperature. Thus deviations of reaction rates in the Arrhenius plots arise from the different contributions of Ce and Pd-(Ce,Zr)Ox interface to DOSC performance. 3.3. Effect of Extent of Surface Area on OSC. The effect of support sintering on OSC versus temperature is depicted in Figure 9. It is observed that the DOSC gap between Pd/HA and Pd/SA behaves a declining trend and attains a stable value at high temperature. For instance, at 300 °C, the DOSC gap between Pd/HA and Pd/SA is 180 µmol‚g-1; at 550 °C, this value falls below 50 µmol‚g-1. According to Hori et al., the concentrations are estimated to be 221 µmol‚g-1 on HA and 36 µmol‚g-1on SA.10 Interestingly, the DOSC gap at 300 °C is equal to the difference of the surface oxygen numbers between HA and SA (221 µmol‚g-1 - 36 µmol‚g-1 ) 185 µmol‚g-1;

dotted line in Figure 9). It is inferred that the DOSC gap between the Pd/HA and Pd/SA samples at 300 °C arises from the surface oxygen number difference at the Pd-(Ce,Zr)Ox interaction. Increased by the temperature, the DOSC difference between Pd/ HA and Pd/SA samples (curve a in Figure 9) falls sharply below the value of 185 µmol‚g-1. In contrast, in the absence of Pd support, the DOSC gap between HA and SA samples increases evidently with the temperature and has a higher value than 185 µmol‚g-1 above 400 °C (curve c in Figure 9). By comparison with Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2, the DOSC gap between HA and SA is minimized by the Pd support, where the influence of the extent of surface area is not obvious. It is consistent with the Arrhenius plots shown in Figure 8; when the reduction degree is at 1% and 5%, reaction rates exhibit less sensitivity to the specific surface area. As for the influence of surface area, Pd support induces more active sites on the surface and compensates the DOSC difference caused by surface oxygen concentration, accelerating more bulk oxygen involved in the DOSC process by oxygen back-spillover. This is also confirmed by the H2-TPR that by supporting Rh on low surface area of ceria-zirconia, similar TPR curves are present with Rhsupported high surface area samples due to the interaction between support and precious metal.11 Similarly, CO-TPR profiles of Pd/CeO2 show a low reduction peak at 70 °C,4 which is lower than the reduction temperature of pure ceria-zirconia (above 350 °C), where the mechanism of CO2 formation is described as CO is adsorbed on the precious metal and oxidized by the oxygen from ceria, where oxygen back-spillover to the Pd sites on the Pd-(Ce,Zr)Ox interface is the rate-determining step.31 Therefore, Pd support can minimize the OSC gap by surface oxygen concentration. The OSC gap between Pd/HA and Pd/SA arises from the surface oxygen concentration at the Pd-(Ce,Zr)Ox interface. Considering the effect of Pd sintering, the DOSC gap between Pd/SA and Pd/SAA (curve b in Figure 9) shows a higher value than 185 µmol‚g-1 between Pd/SA and Pd/HA and remains stable while the temperature is elevated. This is an indication, compared with what support sintering induces, that Pd sintering induces more decrease of OSC and less evolution of OSC with temperature. In this regard, Pd site deterioration on the surface exhibits a stronger effect on the Pd-(Ce,Zr)Ox interface than the support due to the formation of bulk metal particle by the atrocious oxidation which would be reduced with difficulty, and thus deteriorates the Pd-(Ce,Zr)Ox interaction.32 Thus, compared with the Pd sites, the effect of the decreased number of surface oxygens is not as important as Pd morphology. 4. Conclusions Comparison of OSC of CO-He pulse and CO-O2 measurements on Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 samples revealed that oxygen storage/release of ceria-zirconia was strongly promoted by the Pd support, showing as the high extent of the CO2 reduction peak and gradient CO2 slope on Pd/Ce0.67Zr0.33O2 samples. Under CO pulse measurement, Pd promotion to oxygen reduction was limited by the reducibility. At low reducibility, Pd promotion is effective; at high reducibility, after reducibility of ca. 12%, Pd/Ce0.67Zr0.33O2 and Ce0.67Zr0.33O2 show the same OSC behavior. It is inferred that different rate-determining steps are involved in the reduction process. Bulk oxygen migration was the rate-determining step when reducibility was high. However, Pd-(Ce,Zr)Ox interaction and PdOx reduction were corroborated to be constructive for CO oxidation at low reducibility. Under dynamic CO-O2 measurement, Pd promotion is varied by the temperature: Pd promotion is more

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pronounced at low temperature and, conversely, Pd promotion is less effective at high temperature. Concerning the effect of Pd deterioration and ceria-zirconia sintering, Pd-(Ce,Zr)Ox interaction was weakened by both factors after high temperature hydrothermal aging. Compared with ceria-zirconia sintering, Pd deterioration exerts more influence on the evolution of the Pd-(Ce,Zr)Ox interface, resulting in the OSC decrease. According to Arrhenius plots, more apparent activation energy arose from Pd deterioration than from ceria-zirconia sintering. As far as the Pd site deterioration was concerned, Pd sintering dominated the Pd evolution rather than Pd encapsulating during high temperature hydrothermal aging. Compared with Pd deterioration, the extent of the surface area exerts an inferior influence on OSC. By surface area decrease, one reason for the OSC decrease is that there is less oxygen contact with Pd sites at the Pd-(Ce,Zr)Ox interface, resulting in less oxygen participation in the oxygen backspillover process. In view of recent results, variation of specific surface area was not determinate of Pd-(Ce,Zr)Ox interaction. Acknowledgment The authors are grateful to the National Basic Research Program (also called 973 Program) of China (No. 2004CB719503) and the Program of Introducing Talents of Discipline to Universities of China (No. B06006) for financial support. The Program for New Century Excellent Talents in University (NCET-06-0243) and Key Program of Natural Science Foundation of Tianjin (No. 07JCZDJC01600) are also appreciated for their steady financial support. Note Added after ASAP Publication. Because of a production error, an author name was omitted and his affiliation was printed incorrectly in the version of this paper that was published on the Web October 31, 2007. The correct version of this paper was posted to the Web November 6, 2007. Literature Cited (1) Kasˇpar, J.; Fornasiero, P. Nanostructured materials for advanced automotive de-pollution catalysts. J. Solid State Chem. 2003, 17, 19. (2) Kasˇpar, J.; Fornasiero, P.; Hickey, N. Automotive catalytic converters: current status and some perspectives. Catal. Today 2003, 77, 419. (3) Kasˇpar, J.; Fornasiero, P.; Graziani, M. Use of CeO2-based oxides in the three-way catalysis. Catal. Today 1999, 50, 285. (4) Zhu, H.; Qin, Z.; Shan, W. Pd/CeO2-TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents. J. Catal. 2004, 225, 267. (5) Wang, X.; Gorte, R. J.; Wagner, J. P. Deactivation Mechanisms for Pd/Ceria during the Water-Gas-Shift Reaction. J. Catal. 2002, 212, 225. (6) Hickey, N.; Fornasiero, P.; Kasˇpar, J. Effects of the Nature of the Reducing Agent on the Transient Redox Behavior of NM/Ce0.68Zr0.32O2 (NM ) Pt, Pd, and Rh). J. Catal. 2001, 200, 181. (7) Fernańdez-Garcıá, M.; Martıńez-Arias, A.; Salamanca, L. N. Influence of Ceria on Pd Activity for the CO+O2 Reaction. J. Catal. 1999, 187, 474. (8) Martıńez-Arias, A.; Coronado, J. M.; Catalun, R. Influence of Mutual Platinum-Dispersed Ceria Interactions on the Promoting Effect of Ceria for the CO Oxidation Reaction in a Pt/CeO2/Al2O3 Catalyst. J. Phys. Chem. B 1998, 102, 4357. (9) Nibbelke, R. H.; Nievergeld, A. J. L.; Hoebink, J. H. B. J. Development of a transient kinetic model for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst. Appl. Catal., B: EnViron. 1998, 19, 245. (10) Hori, C. E.; Brenner, A.; Simon, K. Y. N. Studies of the oxygen release reaction in the platinum-ceria-zirconia system. Catal. Today 1999, 50, 299. (11) Fornasiero, P.; Fonda, E.; Di Monte, R. Relationships between Structural/Textural Properties and Redox Behavior in Ce0.6Zr0.4O2 Mixed Oxides. J. Catal. 1999, 187, 177.

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ReceiVed for reView December 11, 2006 ReVised manuscript receiVed July 26, 2007 Accepted September 17, 2007 IE061590Y

Influence of Pd Morphology and Support Surface Area on Redox

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