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Improvement of Air/Fuel Ratio Operating Window and Hydrothermal Stability for Pd-Only Three-Way Catalysts through a Pd−Ce2Zr2O8 Superstructure Interaction Zhiliang Zhang,† Yunzhao Fan,† Ying Xin,† Qian Li,† Ruirui Li,† James A. Anderson,‡ and Zhaoliang Zhang*,† †

School of Chemistry and Chemical Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan 250022, P. R. China ‡ Materials and Chemical Engineering Group, School of Engineering, University of Aberdeen, AB24 3UE Scotland, United Kingdom S Supporting Information *

ABSTRACT: The extremely severe and persistent haze problems in some parts of the world including China have prompted the implementation of increasingly stringent tailpipe regulations. This places increasingly higher performance requirements for three-way catalysts, and in particular a widening of the air/fuel (λ) ratio operating window to facilitate operation of the on-board diagnostic system. A new pathway is presented here by tuning the nanostructure of TWCs to improve their λ activities and hydrothermal stability. High-temperature reduction and a mild-temperature reoxidation treatment for alumina-modified ceria−zirconia brought about the formation of a cubic, fully oxidized, pyrochlore-like superstructure, Ce2Zr2O8. The combination of Pd and the Ce2Zr2O8 superstructure greatly improved the λ window for Pd-only three-way catalysts. X-ray powder diffraction (XRD), temperature-programmed reduction with H2 (H2-TPR) and high-resolution transmission electron microscopy (HRTEM) characterization confirmed the interaction between Pd and the Ce2Zr2O8 superstructure, which modifies the dynamic oxygen storage capacity in comparison to the conventional Pd−Ce(Zr)O2 interaction, due to higher low-temperature reducibility for the Ce2Zr2O8 superstructure than for Ce(Zr)O2. Furthermore, the retention of the Ce2Zr2O8 superstructure derived from the interaction with Pd results in superior λ and light-off performances after hydrothermal aging treatment at 1000 °C for 12 h in air containing 10% H2O.

1. INTRODUCTION The extremely severe and persistent haze problems in some parts of the world including China have prompted the implementation of increasingly stringent tailpipe regulations. This places much higher performance requirements for threeway catalysts (TWCs), and in particular a widening of the air/ fuel (λ) ratio operating window so as to ensure the work of onboard diagnostic (OBD) system. TWCs depend heavily on oxygen storage materials, normally CeO2−ZrO2 (CZ) oxides.1 This, thanks to the ability of CeO2 to store and release oxygen under lean and rich conditions, respectively according to the unique redox behavior between Ce4+ and Ce3+, permits TWCs to operate close to stoichiometric conditions so as to simultaneously eliminate CO, NOx, and hydrocarbons (HC) contained in exhaust streams.2 The oxygen storage capacity (OSC) of CZ determines, to a large extent, the overall performance of TWCs. Generally, two strategies have been adopted to improve OSC. One is doping with other metal ions. The composite oxide of Al2O3 and CZ (CZA) has been practically applied in TWCs as OSC materials since 2001.3,4 The higher OSC is contributed to the so-called “diffusion © XXXX American Chemical Society

barrier” of Al2O3. The other approach is redox (ro) treatment composed of high-temperature reduction treatment (e.g., > 900 °C in H2 atmosphere) with a mild-temperature reoxidation (e.g., 500−650 °C).5,6 The promoted OSC for CZ after rotreatment (CZ-ro) is related to the presence of the cubic, fully oxidized, pyrochlore-like superstructure, Ce2Zr2O8.1,5−7 OSC is often described as “total” OSC (TOSC) representing the amount of oxygen thermodynamically available at a given temperature, which is measured by temperature-programmed reduction with H2 (H2-TPR). Although TPR data may be useful to rapidly evaluate the potential OSC of candidate materials, a “dynamic” OSC (DOSC) provides a better simulation of the oscillations between lean (oxidizing) and rich (reducing) exhaust conditions during real operation and is therefore much more useful in the evaluation of the activity of the OSC material.8 Unfortunately, DOSCs are not often Received: March 17, 2015 Revised: May 29, 2015 Accepted: June 5, 2015

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DOI: 10.1021/acs.est.5b01361 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

(μmol [O] g−1). The concentration of CO2 was determined using a quadruple mass spectrometer (MS, OminiStar 200, Balzers). The evaluation of λ activity was performed at 400 °C in a fixed-bed quartz reactor. The feed stream was regulated using mass flow controllers and contained NO (0.1%), NO2 (0.03%), C3H6 (0.067%), C3H8 (0.033%), CO (0.75%), and O2 (0.085− 0.844%) with Ar as balance, and the space velocity was 43 000 h−1, referring to the catalyst volume (0.2 mL, 40−60 mesh) and to a gas flow rate at room temperature (25 °C). The contents of CO, NO, NO2, and total HC (C3H6 and C3H8) were recorded using an FTIR continuous gas analyzer (MKS MultiGas 2030, U.S.A.). The λ is defined as follows:

assessed with respect to CZ-ro and ro-treated CZA (CZA-ro, combining Al doping with ro-treatment) samples. Typical TWCs contain three components, including precious metals, such as Rh, Pt, and Pd, CZ as promoters, and Al2O3 as support.9−11 Pd-only TWCs have received considerable attention due to their better resistance to thermal sintering, lower cost, and higher activity for removing HC and CO than other precious metals.12−15 In this system, the OSC and DOSC are greatly promoted by the Pd−Ce(Zr)O2 interaction,10,16−18 which ensures TWCs working over a wider λ operating window. Considering the feasible reducibility and the presence of the Ce2Zr2O8 superstructure for CZ-ro, what happens if Pd was supported on a CZA-ro sample?19 In this work, the above two questions would be answered, and a new pathway is presented by tuning the nanostructure of the Pd-supported CZA TWCs to improve their λ activities and hydrothermal stability.

λ=

2O2 + NO + 2NO2 CO + 9C3H6 + 10C3H8

where λ = 1 is at stoichiometry, and the corresponding concentration of O2 is 7450 ppm.24 XRD patterns were recorded on a Rigaku D/max-2500/PC diffractometer employing Cu Kα radiation (λ = 1.5418 Å) operating at 50 kV and 200 mA. The Brunauer−Emmett− Teller (BET) surface area and pore structure were determined from N2 adsorption/desorption isotherms using a Micromeritics 2020 M instrument. Before N2 physisorption, the sample was outgassed at 300 °C for 5 h. HRTEM was conducted on a JEOL JEM-2010 and an FEI Tecnai G2 F20 transmission electron microscope operating at 200 kV. An inductively coupled plasma−atomic emission spectrometer (ICP−AES) experiments were carried out on an IRIS Intrepid IIXSP instrument from Thermo Elemental. H2-TPR experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor H2 consumption. A 50 mg sample was pretreated in situ at 500 °C for 1 h in a flow of O2 and cooled to room temperature in the presence of O2. TPR was conducted at 10 °C/min up to 900 °C in a 30 mL/min flow of 5 vol % H2 in N2. To quantify the total amount of H2 consumption, CuO was used as a calibration reference. Pd dispersion was measured by H2 − O2 titration in a quartz reactor with a TCD to monitor H2. Prior to H2 − O2 titration, a 50 mg sample was reduced in 5 vol % H2 + N2 at 500 °C for 2 h. N2 was passed over the sample for 1 h. After cooling to 120 °C, the O2 titration was performed, followed by flushing with N2, and then introduction of pure H2 pulses (10 μL). A stoichiometry of Pd/H2 = 2/3 was assumed.25

2. EXPERIMENTAL SECTION The atomic ratio of Ce and Zr in their complex oxides is 0.43/ 0.57.20 The weight ratio of CZ and Al2O3 was 50% (denoted 50CZA). The Ce, Zr, and Al complex oxides were prepared based on the procedure previously described.21 Typically, the mixed solution of Ce(NO3)3·6H2O, Zr(NO3)4·6H2O, and Al(NO3)3·9H2O in the desired Ce/Zr/Al ratio with H2O2 (Ce/H2O2 = 1/1.2, mole ratio) was dropped into the solution of NH4OH (25−28%) under vigorous stirring. After aging at 70 °C for 24 h, the precipitate was filtered and washed with water until pH = 7 and further washed with ethanol twice. Then the precipitate was dried at 100 °C overnight and finally calcined at 500 °C for 5 h in a muffle furnace. The fresh sample (50CZA-f) was thus obtained. The 50CZA-f sample was further heattreated under two conditions. One was at 1000 °C for 2.75 h, which is described as the aged sample (50CZA-a). The other was a high-temperature reduction treatment (1000 °C for 2.75 h in 4% H2 + N2) with a mild-temperature reoxidation (500 °C for 2 h in air flow), labeled as 50CZA-ro. The 1 wt % Pd-only TWCs were prepared by wet impregnation and an in situ reduction method using Pd(NO3)2 as the metal precursor and 50CZA as supports. The mixed solution of Pd(NO3)2 and poly(vinylpyrrolidone) (PVP K-30) was added into the suspension of 50CZA under a 5% H2 + N2 atmosphere. At 100 °C, NaBH4 was added dropwise. After cooling to room temperature, the suspension was evaporated while stirring at 90 °C until achieving a paste, which was then dried at 100 °C overnight and calcined at 500 °C for 2 h. The calcined sample was powdered and washed thoroughly with water until no Na+ was detected in the solution. After drying, the Pd-supported 50CZA catalysts were obtained and named as Pd/50CZA-f (fresh), Pd/50CZA-a (aged), and Pd/50CZA-ro (ro-treatment), respectively. Furthermore, a severe hydrothermal aging treatment was performed at 1000 °C for 12 h under air containing 10% H2O for Pd/50CZA-f, Pd/50CZA-a, and Pd/50CZA-ro, which is denoted as Pd/50CZA-f-a, Pd/ 50CZA-a-a, and Pd/50CZA-ro-a, respectively. DOSC measurements with CO/O2 pulses were obtained at 300−550 °C at 50 °C intervals.22,23 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 alternately with a pulsation frequency (i.e., the number of CO and O2 pulses per second) of 0.05 Hz. A DOSC value was the average value of 20 cycles calculated by integrating the CO2 formed during one CO/O2 cycle and was expressed as μmol of O per gram of catalysts

3. RESULTS AND DISCUSSION In agreement with conclusions in the literature,5 CZ-ro shows a lower reduction peak temperature and higher TOSC (defined as H2 consumption) in comparison to CZ-f on the basis of H2TPR results (Supporting Information, SI, Table S1). However, the kinetically controlled DOSC was not improved (SI Table S2a) as a consequence of the lower BET surface areas (SI Table S2b). The doping of Al can help stabilize the BET surface areas not only for the fresh, but also for the aged and ro-treated samples (SI Table S1).26 Furthermore, the TOSC (the H2 consumptions of per gram CeO2 are shown in SI Table S1) and DOSC (SI Table S2a) for the aged and ro-treated samples reached a maximum at 50CZA, suggesting that the optimum Al2O3 content in CZA is 50 wt %. For this reason, 50CZA was selected for further study. B


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Environmental Science & Technology Table 1. Properties of Pd/50CZA H2 consumption (μmol/50 mg) samples Pd/50CZA-f Pd/50CZA-a Pd/50CZA-ro

Pd content by ICP (%) 1. 01 0.97 0.92

BET area (m2/g) a

238.5 (34.4) 56.0 (21.2)a 52.6 (47.3)a

Pd dispersion (%) a

24.2 (10.9) 16.4 (9.5)a 16.8 (11.4)a

peak α

peak β b

21.9 (70) 13.7 (61)b 12.3 (65)b

0 6.58 (141)b 34.2 (143)b

Wd

total c

21.9 (15.1) 20.3 (12.3)c 46.5 (11.7)c

0.33 (0.20)a 0.14 (0.16)a 0.37 (0.24)a

After hydrothermal aging treatment at 1000 °C for 12 h in air containing 10% H2O. bPeak temperature (°C). cTheoretical H2 consumption based on ICP. dWidth of λ values when CO, HC, and NOx conversions all reach to 80% under rich or lean conditions.

a

HC on all samples reach 100% under λ ≥ 1 conditions. However, they descend with decreasing λ value under λ < 1 conditions. On the contrary, the NOx (NO and NO2) conversions reached 100% under λ ≤ 1 conditions, but decreased as expected with increasing λ value under λ > 1 conditions. In both cases, Pd/50CZA-ro shows the highest activity. The W value, which represents CO, HC, and NOx conversions all above 80% under rich and lean conditions, normally described as the air/fuel ratio operating window, is 0.37 (Table 1), higher than that of Pd/50CZA-f (the fresh sample) and those in the literature.24 This is significant considering the much lower BET surface area of Pd/50CZA-ro in comparison to Pd/50CZA-f (Table 1). However, the BET surface areas of Pd/50CZA-a and Pd/50CZA-ro are similar (Table 1), but the former has the lowest W value, namely, the lowest λ activity. The two facts combined strongly suggest that the surface area did not play an important role. The XRD patterns of Pd/50CZA-f, Pd/50CZA-a, and Pd/ 50CZA-ro (Figure 2a) show that no Pd species were detected because of either the high dispersion or the overlap of the diffraction peaks with those features of the tetragonal Ce0.5Zr0.5O2 (JCDPS 38-1436) phase.27 Additionally, weak features due to the γ-Al2O3 (JCDPS04-0880) phase were observed, which limited the degradation of surface areas, as mentioned above.3,4 However, the CeAlO3 phase, a species that has no reducibility,19,26 was never present, confirming that 50% Al2O3 in CZA is optimum from the perspective of Al2O3 and CZ interactions.3 XRD results are consistent with STM elemental mapping taking Pd/50CZA-ro as an example, which shows Pd dispersed over the CZ and Al2O3 (Figure 2b1). In particular, the Pd/50CZA-ro sample clearly shows the presence of the cubic, fully oxidized, pyrochlore-like superstructure, Ce2Zr2O8 (JCDPS 70-4048), as shown in Figure 2c.1,5−7 In addition to the fact that the residual pyrochlore Ce2Zr2O7.04 phase can be distinguished, the (111) and (222) reflections, characteristic of the superstructure in the Digital Diffraction Patterns (DDP) from fast Fourier transformation (FFT) analysis were observed.5−7 The Ce2Zr2O8 phase in the Pd/50CZA-ro is derived from 50CZA-ro prior to Pd loading (Figure 2a inset). However, as shown in SI Figure S1 (SI Table S2a), 50CZA-ro shows a lower DOSC than the 50CZA-f, even though the former has a higher TOSC and higher lowtemperature reducibility than that of the latter (SI Table S1). This suggests that TOSC cannot be effectively utilized in dynamic conditions for 50CZA-ro due to the low surface area resulting from the high temperature reduction treatment (SI Table S2b).2 It is postulated that the addition of Pd overcomes this issue. Figure 3 shows the H2-TPR profiles of Pd/50CZA-f, Pd/ 50CZA-a, and Pd/50CZA-ro. Pd/50CZA-ro shows two dominant low-temperature reduction features with maxima at 65 (α peak) and 143 °C (β peak). A similar reduction behavior was observed for Pd/50CZA-a. The low reduction temperature

As shown in Table 1, the Pd content determined by ICP is nearly the same as the nominal value of 1%. However, Pd dispersions by H2 − O2 titration for both Pd/50CZA-ro (16.8%) and Pd/50CZA-a (16.4%) were lower than that for Pd/50CZA-f (24.2%) due to much lower BET surface areas (Table 1). Figure 1 shows the conversion curves of CO, HC, and NOx as a function of λ at 400 °C for Pd/50CZA-f, Pd/50CZA-a, and Pd/50CZA-ro. The left and right sides of the theoretical stoichiometric value (λ = 1) represent lean and rich oxygen, respectively. It is reasonable that the conversions of CO and

Figure 1. Conversion curves of CO (a), HC (b), and NOx (c) at 400 °C for Pd/50CZA-f, Pd/50CZA-a, and Pd/50CZA-ro as a function of λ. C


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Figure 2. (a) XRD patterns of Pd/50CZA-f, Pd/50CZA-a, and Pd/50CZA-ro; (b) STM images and EDS mapping for Pd/50CZA-ro (b1) and Pd/ 50CZA-ro-a (b2); (c) HRTEM and FFT images (inset) of Pd/50CZA-ro showing the coexistence of Ce0.5Zr0.5O2, Ce2Zr2O7.03, and the Ce2Zr2O8 superstructure.

suggest that the α peak is ascribed to the reduction of PdO species dispersed on the surface of the supports.28 This is

and similar H2 consumption, which is close to the theoretical H2 consumption based on ICP measurements (Table 1), D


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Figure 3. H2-TPR profiles of Pd/50CZA-f, Pd/50CZA-a, and Pd/ 50CZA-ro.

consistent with the low surface areas and Pd dispersions for Pd/ 50CZA-ro and Pd/50CZA-a (Table 1). In contrast, Pd/ 50CZA-f shows only the α peak at higher reduction temperature and which possesses a much larger H 2 consumption than the theoretical value, suggesting a strong interaction between Pd and Ce(Zr)O2 in 50CZA-f, leading to support reduction. This may be attributed to the higher surface area and Pd dispersion (Table 1). The β peak is initially attributed to the reduction of Ce4+ at the PdCe(Zr)O2 interface due to hydrogen activation by the metal and consequent migration to the support.29 However, the more than four times larger H2 consumption of the β peak for Pd/ 50CZA-ro than for Pd/50CZA-a cannot be explained. Furthermore, the total H2 consumption for Pd/50CZA-ro is more than twice as large as that for Pd/50CZA-f and Pd/ 50CZA-a. We recall that the only difference among Pd/50CZAro, Pd/50CZA-f, and Pd/50CZA-a in the texture is the presence of the Ce2Zr2O8 superstructure in Pd/50CZA-ro. Therefore, an additional PdCe2Zr2O8 superstructure interaction is proposed as being responsible for the significant H2 consumption, because the Ce2Zr2O8 superstructure shows higher low-temperature reducibility in comparison to normal Ce(Zr)O2.5 As shown in Figure 4a, ca. 5 nm Pd nanoparticles and the Ce2Zr2O8 superstructure are directly observed by HRTEM. In accordance with the XRD results, the Pd particles exhibit small sizes. Figure 5 shows the collected DOSC data and the corresponding transition curves at various temperatures, with alternate dynamic pulses of 4% CO/1% Ar/He (10 s) and 2% O2/1% Ar/He (10 s) under 0.05 Hz given in the SI Figures S2−S4. In comparison with supports (50CZA-f, 50CZA-a, and 50CZA-ro in SI Figure S1), DOSCs of all Pd-supported samples are significantly improved, confirming the key role of Pd during the fast transitions between reduction and oxidation environments. At each temperature, the DOSCs decrease in the sequence: Pd/50CZA-ro > Pd/50CZA-f ≫ Pd/50CZA-a. Furthermore, TOSCs using the COHe pulse (TOSCs− CO) were also conducted at 400 °C.23 In this procedure, all are similar to DOSC measurements except that they employed alternating CO (4% CO/1% Ar/He at 300 mL/min for 5 s) and He (300 mL/min for 20 s) pulses. The TOSCs−CO was calculated by integrating CO2 formed during all COHe pulses, which are as follows: 1981.9, 1654.9, and 906.2 μmol[O]/g for Pd/50CZA-ro, Pd/50CZA-f, and Pd/50CZAa, respectively. On the basis of DOSCs, the practical efficiencies

Figure 4. HRTEM images of Pd/50CZA-ro (a) and Pd/50CZA-ro-a (b). FFT images corresponding to the Ce2Zr2O8 superstructure are shown in insets.

Figure 5. DOSC of Pd/50CZA-f, Pd/50CZA-a, and Pd/50CZA-ro with dynamic pulses of 4% CO/1% Ar/He and 2% O2/1% Ar/He under 0.05 Hz.

in terms of use of active oxygen were 86%, 81%, and 70%, which follows the sequence of the λ activity, confirming the high DOSCs derived from the PdCe2Zr2O8 superstructure interaction, rather than Pd or the Ce2Zr2O8 superstructure separately, were responsible for the high λ activity for Pd/ 50CZA-ro. E


Article

Environmental Science & Technology The improvement of λ operating window was reserved after severe hydrothermal aging treatment at 1000 °C for 12 h in air containing 10% H2O.30 As shown in Figure 6 and Table 1, the

the general concept that high-temperature reoxidation of the Pd-free CZ-ro would destroy the Ce2Zr2O8 superstructure therein.6 Finally, as H2O is always present in exhaust gas, the λ activity was checked in the presence of 10 vol % H2O for Pd/50CZA-fa and Pd/50CZA-ro-a (SI Figure S6). Although NO x conversion was slightly decreased at λ > 1, CO conversions were significantly increased at λ Pd/50CZA-f-a > Pd/50CZA-a-a, which is different from that of Pd/50CZA-f > Pd/50CZA-ro > Pd/ 50CZA-a, suggesting that the PdCe2Zr2O8 superstructure interaction (Figure 2b2) can hinder sintering. This is demonstrated by the lowest light-off temperatures for CO, HC, and NOx for Pd/50CZA-ro-a, even though the light-off performance of Pd/50CZA-ro was inferior to Pd/50CZA-f (SI Figure S5 and Table S3). The superior TWC performance for Pd/50CZA-ro-a compared with Pd/50CZA-f-a can be attributed to the retention of the Ce2Zr2O8 superstructure and the small size of PdO particles (Figure 4b). The interaction even prevents a small portion of PdO from being reduced to Pd. However, the Ce2Zr2O8 superstructure in Pd/50CZA-ro-a cannot be determined using a bulk characteristic technique such as XRD. As shown in Figure 2a, a segregated Ce-rich cubic Ce0.75Zr0.25O2 (JCDPS 28-0271) phase was only observed in Pd/50CZA-ro-a. This is an unprecedented progress considering

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Corresponding Author

*Tel/Fax: +86 531 89736032; e-mail: [email protected]. cn. Notes

The authors declare no competing financial interest. F


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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21477046 and 21277060).

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