CeO2âZrO2 Three-Way Catalysts

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Deactivation Mechanism of Pd/CeO2–ZrO2 Three-Way Catalysts Analyzed by Chassis-Dynamometer Tests and In-situ Diffuse Reflectance Spectroscopy Masato Machida, Ayumi Fujiwara, Hiroshi Yoshida, Junya Ohyama, Hiroyuki Asakura, Saburo Hosokawa, Tsunehiro Tanaka, Masaaki Haneda, Atsuko Tomita, Takeshi Miki, Katsuya Iwashina, Yoshinori Endo, Yunosuke Nakahara, Shigekazu Minami, Naohiro Kato, Yoshiyuki Hayashi, Hideki Goto, Masao Hori, Toyofumi Tsuda, Kazuya Miura, Fumikazu Kimata, and Kinichi Iwachido ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01669 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 8, 2019

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Deactivation Mechanism of Pd/CeO2–ZrO2 ThreeWay Catalysts Analyzed by Chassis-Dynamometer Tests and In-situ Diffuse Reflectance Spectroscopy Masato Machida,*,†,‡　Ayumi Fujiwara,† Hiroshi Yoshida,†,‡　Junya Ohyama,†,‡　Hiroyuki Asakura,§,‡ Saburo Hosokawa,§,‡ Tsunehiro Tanaka,§,‡ Masaaki Haneda,ǁ Atsuko Tomita,# Takeshi Miki,# Katsuya Iwashina,¶ Yoshinori Endo,¶ Yunosuke Nakahara,¶ Shigekazu Minami,∆ Naohiro Kato,∆ Yoshiyuki Hayashi,∆ Hideki Goto,∆ Masao Hori,∆ Toyofumi Tsuda,◊ Kazuya Miura,◊ Fumikazu Kimata,◊ and Kinichi Iwachido□ †

Division of Materials Science and Chemistry, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan ‡ Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 GoryoOhara, Nishikyo, Kyoto 615-8245, Japan § Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo, Kyoto 615-8510, Japan ǁ Advanced Ceramics Research Center, Nagoya Institute of Technology, Tajimi, Gifu 507-0071 Japan # Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya 463-8560, Japan ¶ Mitsui Mining & Smelting Co., Ltd., Ageo, Saitama 362-0021, Japan ∆ Umicore Shokubai Japan Co., Ltd., Chuo, Kobe, Hyogo 650-0047, Japan ◊ Suzuki Motor Corporation, Minami, Hamamatsu, Shizuoka 432-8611, Japan □ Mitsubishi Motors Corporation, Okazaki, Aichi 444-8501, Japan

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KEYWORDS Three-way catalyst, Deactivation, Chassis-dynamometer tests, Diffuse reflectance spectroscopy, Oxygen storage, Three-phase boundary ABSTRACT The thermal deactivation of engine-aged Pd/CeO2–ZrO2 three-way catalysts was studied by chassis-dynamometer driving test cycles with cold start and in situ diffuse reflectance spectroscopy (DRS). The extent of the catalyst deactivation after engine-aging at 800–1000 °C was correlated with the microstructural evolution, which was analyzed by X-ray diffraction, X-ray absorption spectroscopy, electron microscope, and chemisorption technique. This suggests that deactivation is caused by degradation of the catalytically active sites in the three-phase boundary (TPB) region, where Pd, CeO2–ZrO2, and the gas phase meet. The time-resolved in situ DRS revealed that the reoxidation of Pd metal under fluctuating air-to-fuel ratios was retarded relative to the reduction of Pd oxide. The retardation is attributable to the oxygen storage in CeO2–ZrO2. In the fresh catalyst with a high dispersion, most Pd was close to the TPB. Conversely, after engine-aging at elevated temperatures, the retardation effect was less pronounced with respect to Pd particle growth. Grown into large Pd particles, the Pd at sufficient distances from the TPB was no longer affected by the oxygen storage. Consequently, from the ratios of the initial rate constants of the Pd oxidation and reduction under fluctuating air-to-fuel ratio conditions, we can understand the quality and/or quantity of the TPB site in engine-aged catalysts. This measure provides a useful index of the extent of catalyst deactivation.

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1. INTRODUCTION Modern, state-of-the-art three-way catalysts (TWCs) are expected to efficiently convert NOx, CO, and hydrocarbons during a long operation period. The deactivation mechanism is especially important not only to estimate the lifetime, but also to develop highly durable catalysts. As is wellknown, deactivation is caused by thermal and/or poisoning effects.1-11 The most important deactivation factor is thermal sintering of the precious metals (Pd, Rh, and Pt) and the co-catalyst (CeO2-based oxides such as CeO2–ZrO2), which reduces the active surface area and the metal–cocatalyst interface.12-14 The CeO2–ZrO2 enables quick release and storage of oxygen in response to a fluctuating air-to-fuel ratio (A/F),12-28 compensating for the oscillation between the fuel-lean and fuel-rich exhaust conditions, and ensuring efficient catalytic conversion by the precious metals. Conversely, the precious metals increase the oxygen storage/release rates through spillover to/from CeO2–ZrO2.24,

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These phenomena can be rationalized by considering that the three-phase

boundary (TPB) of the precious metal, CeO2–ZrO2, and the gas phase provides the active sites. The resulting synergetic effects are essential for TWC performance under a dynamic lean–rich perturbation atmosphere.4-5, 30-32 As confirmed in both model and real catalyst systems, TPB loss contributes to the deactivation of the TWC. For instance, Martínez-Arias et al.33 reported that the CO and NOx conversion activity in a thermally aged Pd/CeO2–ZrO2/Al2O3 catalyst is lowered by the sintering of active components, reducing the interface between Pd and CeO2–ZrO2. Heo et al.34 studied the performance of a vehicle-aged TWC and concluded that weakening of the Pd–Ce interaction contributes to the TWC deactivation by decreasing the oxygen storage capacity (OSC) of the TWC. Some studies have proposed that the TPB on precious metals supported on CeO2-based oxides provides the active site of the TWC and related reactions, such as the water–gas shift reaction and CO oxidation.35-40

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According to these previous studies, TPB loss is associated with the extent of TWC deactivation. Nevertheless, the TPB site in a real TWC system is difficult to analyze, both qualitatively and quantitatively. We recently determined the time-resolved redox behavior of supported Pd catalysts under simulated lean/rich gas perturbations by in situ diffuse reflectance spectroscopy (DRS).41-42 Here the A/F was fluctuated between 14.1 (rich) and 15.0 (lean). Because the reflectance at the wavelength () of 450 nm is sensitive to the oxidation state of Pd, which is itself affected by the TPB, we can obtain the real-time information at the Pd–support interface. During rich-to-lean switching, Pd oxidation was significantly retarded on CeO2–ZrO2, because the excess gas-phase O2 was rapidly removed and stored in the CeO2–ZrO2 support. In contrast, the Pd on non-oxygen storage supports such as Al2O3 was immediately oxidized by the excess O2. Consequently, the time-resolved response reflects the oxygen storage functionality of the TPB. This approach is expected to be applicable to the Pd/CeO2–ZrO2 catalysts in real TWCs with different extents of interface interaction, and thus different degrees of thermal deactivation. In the present work, a full-scale honeycomb TWC containing a Pd/CeO2–ZrO2 catalyst was aged in an engine dynamometer at different temperatures, and its catalyst deactivation mechanism was investigated. The catalytic performance of the aged TWCs was examined in chassis-dynamometer test cycles. The engine- and chassis-dynamometer tests acquire larger amounts of information on the catalyst deactivation than laboratory-scale simulated model reactions.43-46 To better elucidate the mechanism of thermal deactivation, the as-obtained results were coupled with the results of various catalyst characterizations. The Pd redox dynamics were studied by time-resolved in situ DRS under simulated lean/rich perturbation conditions. The structural and microstructural alterations of the aged catalyst were characterized by X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), transmission electron microscope (TEM), chemisorption, and

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other methods to be described. After collecting and processing the results, the TPB deterioration was found to correlate with the catalyst deactivation.

2. EXPERIMENTAL SECTION 2.1. Preparation of Coated Honeycomb Catalysts. Supported Pd catalysts (2 wt% loading) were prepared by wet-incipient impregnation. The starting materials were Pd(NO3)2 (Tanaka Precious Metals, Japan) and CeO2–ZrO2 (CZ, 46.5 mol% Ce, 46.5 mol% Zr, 1.7 mol% La, and 5.3 mol% Nd, Brunauer–Emmett–Teller surface area (SBET)=67 m2 g−1, Daiichi Kigenso Kagaku Kogyo, Japan) or -Al2O3 (3 mol% La2O3 added, SBET=150 m2 g−1, Sasol Chemicals, USA). The impregnation was followed by drying and calcination at 600 °C for 3 h in air. Mixtures of catalyst powders (Pd/CZ or Pd/Al2O3), Al2O3 sol (Nissan Chemical Industries, Japan), and ion-exchanged water were ball-milled for 1.5 h into a homogeneous slurry. Monolithic honeycomb catalysts were prepared by dipping a cordierite honeycomb (diameter × length 105.7 mm × 114.0 mm, volume 1.0 L, cell density 600 cells in−2, 4.3 mil, NGK Insulators, Japan) into a slurry, followed by drying at 90 °C for 15 min. Finally, the honeycombs were calcined in air at 600 °C. The total amount of washcoat, containing Al2O3 sol (10 g L−1) and Pd (2 g L−1), was 110 g L−1.

2.2. Catalyst Aging and Catalyst Performance Test. The full-scale honeycomb catalysts were tested in the fresh state and after aging in a gasoline engine at different temperatures (800 °C, 900 °C, and 1000 °C) for 40 h. Meanwhile, three gas feeds—a stoichiometric gas (A/F=14.6, 25 s), air (2.5 s), and a rich gas (A/F=12.0, 2.5 s)—were sequentially cycled at a flow rate of 178,000 L h−1 (Supporting Information, Figure S1). Hereafter,

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these catalyst samples are denoted as X-Y, where X and Y denote the catalyst composition and aging temperature, respectively. For example, Pd/CZ-1000 specifies the Pd/CeO2-ZrO2 catalyst after engine-aging at 1000 °C. The apparatuses for the catalytic tests were a chassis-dynamometer (FEB-DNR, Meidensha, Japan), a Mitsubishi Mirage-CVT gasoline vehicle with a 1.2 L engine, and a close-coupled TWC. Cold-start urban driving test cycles accorded with the New European Drive Cycle (NEDC). The NEDC is performed with a cold engine at the beginning of the cycle, and consists of two parts: the first part (780 s) simulating urban driving conditions (UDC), and the final part (400 s) run at higher speed. The latter part is called the extra urban driving cycle (EUDC). The concentrations of CO, NOx and total hydrocarbons (THC) in the effluent gas were analyzed online by a motor exhaust-gas analysis system (MEXA 9100, Horiba, Japan) equipped with nondispersive infrared detectors and a flame ionization detector. Using an engine dynamometer, the OSC was determined by measuring the differences between the transient signals from linear air/fuel sensors at 450 °C, which were installed near the inlet and outlet of the catalytic converter. The signals were detected while switching the gas feed between A/F=13.5 and 15.5 supplied at a space velocity (SV) of 30,000 h−1.

2.3. Characterization of Catalysts. The structure of the washcoated catalyst was elucidated through various characterization techniques. For this purpose, a small cylindrical catalyst fragment (diameter × length = 25 mm × 84 mm) was excised from the central portion of honeycombs after aging in a gasoline engine. A catalyst powder was then removed from the washcoated surface layer. After reduction treatment in 5% H2/N2 at 400 °C, powder XRD was performed under monochromated Cu–K radiation (40 mA, 45 kV, Cu tube, X’pert MPD, Spectris PLC, UK) (Supporting Information, Figure S2). The

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Pd metal loading was determined by X-ray fluorescence (EDXL300, Rigaku, Japan). The measurements closely agreed with the expected value (2 wt%). The N2 adsorption isotherms for the SBET calculation were obtained at −196 °C (Belsorp-mini, Microtrac-Bel, Japan). The mean crystallite sizes of Pd and CZ were determined by X-ray line-broadening analysis using the Scherrer's equation. After sample reduction in 5% H2 at 400 °C for 1 h, the amount of CO chemisorbed onto the Pd was determined by the pulsed injection method at 50 °C (Quadrasorb SI, Quantachrome, Austria). As CO also adsorbs onto CZ as the carbonate species, this method tends to overestimate the CO chemisorption. To prevent such overestimates, the CO chemisorption measurements were preceded by the CO2 pre-injection technique developed by Takeguchi et al.47 The metal dispersion is expressed as the CO/Pd ratio, defining the molar ratio between the chemisorbed CO and loaded Pd. When determining the Pd particle size, we assumed a chemisorption stoichiometry of Pd:CO=1:1 and a hemispherical particle shape. The microstructure of the aged catalysts after H2 reduction treatment (in 5% H2/N2 at 400 °C) was observed using highangle annular dark-field scanning TEM (HAADF–STEM) and energy-dispersive X-ray mapping analyses, performed on a Tecnai Osiris (FEI, USA) apparatus with an accelerating voltage of 200 kV. The Pd K-edge XANES analyses of the as-aged catalysts (without reduction treatment) were carried out on public beamlines, BL01B1 and BL14B2, of SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). The XANES experiment is detailed in Supporting Information (Figures S3 and S4).

2.4. In situ DRS Experiments. In situ DRS was performed as reported previously42 using an ultraviolet–visible-light (UV–vis) spectrometer (V-550, Jasco, Japan) equipped with a temperature-controlled gas flow cell (VHR-

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764A, Jasco, Japan) comprising a heating stage, a quartz window, and a gas flow system. Powder catalysts were pressed into pellets (diameter ~7 mm, thickness 1000 nm; Figure 1 (g, h)), but the primary Al2O3 particles were smaller than 50 nm. Figure 1(c, i) shows the Pd particle size distribution determined from the microscopic images. Although Pd particles in the fresh catalysts were mostly smaller than 10 nm, a spread of particle sizes in the range to 200 nm occurred in Pd/CZ-1000. A broader distribution was seen in Pd/Al2O3-1000. In addition to the significant particle growth of Pd, how aging affects the structure and intimate contact at the Pd– CZ interface remains to be elucidated. Based on the X-ray mapping results, most Pd is in contact with CZ even after thermal sintering. However, an exception is observed as in the enlarged images (Figure 1(d-f)), where an as-grown Pd particle pointed by an arrow is in contact with Al2O3 rather

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than CZ. This is unexpected considering Pd impregnation onto CZ in the present catalyst preparation, and the small content of Al2O3, which was added as a binder to be less than 10 wt% of the total washcoat layer. This result indicates that the migration of Pd from CZ to Al2O3 and thus decreasing amounts of the Pd–CZ interface occurred in the engine-aged catalysts. The catalytic performances of the as-aged honeycomb catalysts were evaluated in the cold-start UDC chassis-dynamometer test (Figure 2). The A/F at the catalyst inlet, the concentrations and conversion efficiencies of CO, NOx, and THC at the catalyst outlet, the catalyst temperature, and vehicle speed, are plotted as functions of time-on-stream from the cold start (0–200 s). The conversions efficiencies of NOx, CO and THC were maximal in the fresh catalyst and declined with increasing aging temperature. A similar tendency was observed after 200 s and in the EUDC (Figures S5 and S6 of Supporting Information, respectively). The decline in the catalyst performance was more obvious in Figure 2 than in Figures S5 and S6, simply because of the low exhaust temperature in the catalyst bed. This trend was especially clarified during the first 40 s, when the catalyst bed temperature was below 200 °C. In addition, the long acceleration periods (50–70 s and 120–150 s from the start in Figure 2) at high exhaust velocity exaggerated the differences between the fresh and aged catalysts. This difference also depended on the aging temperatures. The cumulative conversion efficiencies of NOx, CO, and THC observed between 40 s and 200 s from the cold start in Figure 2 are compared in Table 1. Clearly, conversion efficiencies decreased monotonically with increasing aging temperature and thus with increasing the Pd particle size. However, note that the conversion efficiencies of the reference catalyst (Pd/Al2O3-1000) were much lower than those of Pd/CZ-1000 (Table 1), despite the comparable Pd particle sizes in the two catalysts. Therefore, deactivation is unlikely to manifest as a simple size effect.

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The performances of the catalysts are also related to their OSCs. The OSC buffers the A/F fluctuation in the driving test cycles (see Figure 2, top panel). The OSC substantially decreased from 1.39 g in the fresh Pd/CZ catalyst to 0.83 g in the Pd/CZ-1000 catalyst (Table 1). The OSC of the Pd/Al2O3-1000 catalyst was much lower still (0.09 g). In the presence of precious metal catalysts, TWC activity is known to be greatly facilitated by the quick oxygen release and storage of CZ.21, 28 Because this function occurs near the TPB between Pd, CZ and the gas phase, the particle growths of Pd and CZ may affect their interface contact, reducing the TPB abundance. As indicated by Figure 1(d-f), the migration of Pd may also reduce the interface contact. The present results imply that the observed sintering of Pd/CZ affects the quality and/or quantity of TPB, and can hence index the extent of catalyst deactivation. As the engine-aged, the Pd content of the catalyst remained unchanged regardless of aging temperature (Supporting Information, Table S2). In addition, no obvious phosphorus and sulfur deposits appeared on the aged catalysts, probably because the test was run for only 40 h. Therefore, ruling out chemical poisoning effects, we attribute the observed deactivation to thermal evolution of the catalyst structure.

3.2. Time-Resolved DRS of Pd Redox under Lean–Rich Cycles. Figure 1 shows the dispersion of Pd particles in the CZ support after thermal aging, but it is an important challenge to clarify the extent of the TPB degradation. To elucidate the details of the observed deactivation, the redox behavior of Pd was analyzed by the in situ DRS technique. Previously,42 we reported that both Pd and Ce can alter the UV–vis absorption because two redox cycles (Pd2+(PdO)−Pd0(Pd metal) and Ce4+−Ce3+) are possible under lean–rich fluctuating conditions. The former (Pd redox) influences the optical reflectance at 350 ≤ λ ≤ 800 nm, whereas

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the effect of the latter (Ce redox) is limited above 500 nm.42,

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Therefore, the time-resolved

response for Pd/CZ at 450 nm should be determined by the redox of Pd. Under the present engine-aging conditions, the Pd oxidation state was affected by the dynamic A/F fluctuations between the stoichiometric (A/F=14.6), air, and rich (A/F=12.0) atmospheres. According to the Pd K-edge XANES (Supporting Information, Figure S3), Pd/CZ-fresh existed in an oxide-like state (PdO), whereas the catalysts engine-aged at higher temperatures contained greater fractions of metallic Pd. However, the metallic Pd in these aged samples was almost completely reoxidized by heating in the presence of O2 at ≥500 °C (Supporting Information, Figure S4). This means that not only the surface but also the whole bulk of Pd particles in the aged catalysts can be reversibly reduced and reoxidized by switching between a lean and rich gas atmosphere. The metallic Pd phase was detected by XRD, whereas PdO was difficult to detect due to a low crystallinity and peak overlapping with the CZ phase. The XANES data provide the bulk oxidation states, but in situ DRS is sensitive to the surface oxidation state, enabling time-resolved acquisition at every second. Figure 3 compares the transient responses KM at  = 450 nm in Pd/CZ at a constant temperature of 450 °C, when the two gas feeds with A/F = 14.1 and 15.0 were switched every 600 s. The intensity of the fresh catalyst (Figure 3(a)) increased immediately after lean-to-rich switching, then dropped to its initial intensity after rich-to-lean switching. These transient responses correspond to the reduction of PdO to Pd metal and reoxidation back to PdO, respectively.42 Notably, the slopes of the transient responses depended on the switching state (being steep during lean-to-rich switching and gentle during richto-lean switching), suggesting that the oxidation of Pd metal in the fresh Pd/CZ catalyst proceeded considerably more slowly than the reduction of PdO. This behavior can be explained by the rapid oxygen uptake by CZ, which reduces the local O2 partial pressure near the Pd surface. In the engine-

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aged samples (Figure 3(b–d)), however, the slopes of the transient responses steepened in both switching regimes. To compare the apparent initial rate constants of Pd reduction (kL→R) and reoxidation (kR→L) during lean-to-rich and rich-to-lean switching respectively, these transient response data were subjected to a kinetic analysis as described in a previous study (Supporting Information, Figure S7).42 Figure 4(a) plots the rate constants kR→L and kL→R as functions of Pd particle size (values shown in Table 1), which was determined by CO chemisorption. In the fresh Pd/CZ catalyst, kR→L was smaller than kL→R (as mentioned above), because Pd reoxidation was strongly retarded by the oxygen storage. Both k values tended to increase with Pd particle growth. Their ratio (kR→L/kL→R) also increased (Figure 4(b)), reaching unity after aging at 1000 °C (Pd particle size: 158 nm). This result suggests that the retardation effect on Pd reoxidation became less intense as the Pd sintering progressed. Nevertheless, the kR→L/kL→R ratio (1.0) was much lower in the Pd/CZ-1000 catalyst than in the reference catalyst Pd/Al2O3-1000 (1.82) for similar Pd particle sizes (134 nm in the reference). This finding accords with the OSC in Pd/CZ-1000, which was lower than in Pd/CZfresh but significantly larger than in Pd/Al2O3-1000 (see Table 1). These results indicate that the kR→L/kL→R ratio is affected by the quality and/or quantity of the TPB conjoining the Pd, CZ and gas phases. This is contrast to the ratio observed for Pd/Al2O3, which was almost similar (~2) regardless of the Pd particle size. Figure 5 correlates the three transient responses by plotting the KM, catalytic conversion efficiency (CO, NO, and C3H6) and the gas-phase O2 concentration versus the time-on-stream during lean-to-rich switching. When the gas feed was switched from lean to rich, the transition between the steady-state CO and NO conversions behaved quite differently from that of the richto-lean transition. In the fresh Pd/CZ, the CO-conversion decay was delayed because the CO

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reacted with oxygen released by CZ. In contrast, the CO conversion in the Pd/CZ-1000 began decreasing immediately, and the consequent degradation of the TPB slowed the oxygen release rate. Although no obvious differences were observed in the O2 concentration profiles of the fresh and aged catalysts, the steeper KM slope for the aged Pd/CZ implies that the Pd reduction rate increased after sintering during the aging process. Figure 6 shows the transient responses during rich-to-lean switching, where the transition occurs between the two steady-state catalytic conversions. In the fresh Pd/CZ, the excess O2 in the lean gas feed was removed and stored by CZ, so the NOx conversion was slower than in the aged sample. Meanwhile, the much slower change in KM suggests a gentler reoxidation of Pd in the fresh Pd/CZ than in the aged Pd/CZ, because the oxygen stored by CZ could not accelerate the Pd oxidation. Consequently, the greater abundance of active metallic Pd enhanced the NOx conversion during the rich-to-lean transition. It should be noted that the steep drop of NOx conversion in Pd/CZ-1000 accords with the steep increase of O2 concentration (and hence the Pd oxidation). Similar correlations between the transient responses of KM, catalytic conversion efficiencies and gas-phase O2 concentrations during lean-to-rich and rich-to-lean switching were observed in the samples aged at different temperatures (Pd/CZ-800 and Pd/CZ-900) (see Figures S8–S11 of Supporting Information). The behaviors of these samples were intermediate between those of Pd/CZ-fresh and Pd/CZ-1000.

3.3. Deactivation of TPB. As shown in Figure 4, the Pd particle size in the Pd/CZ samples was clearly related to the ratio of apparent initial rate constants kR→L/kL→R. Therefore, the retardation of Pd reoxidation can indicate the quality and/or quantity of TPB between the Pd, CZ, and gas phase. To correlate this parameter with the extent of catalyst deactivation, Figure 7 plots the catalytic performances shown in Table

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1 versus kR→L/kL→R. In Figure 7(a), the ordinate is the cumulative conversion efficiency of CO, NOx, and THC in the 40–200 s period of the cold-start chassis-dynamometer test (Figure 2). The slopes of the plots are negative, suggesting that the catalytic activity decreased as the TPB deteriorated. The data of the reference catalyst Pd/Al2O3-1000 (open circles) fall on the extrapolations from the Pd/CZ sample lines, corresponding to an extreme case in which the oxygen storage function is totally lost. Similar relationships were observed in high-temperature catalytic performance data in the UCD (200–780 s from cold start, Figure S5) and EUDC (Figure S6) parts (Figure 7(b), plotted for the NOx conversions only). The OSC during the A/F switching also tended to decrease with initially increasing kR→L/kL→R, but was negligibly changed by further increases of the ratio (Figure 7(c)). This finding can be rationalized if the present OSC measurement is not kinetically controlled, but partially reflects the amount of thermodynamically available oxygen. The observed increase of kR→L and kL→R during the sintering process of Pd/CZ (Figure 4(a)) can be explained as follows. Duprez et al.24, 29 revealed that oxygen storage and release proceed through various steps such as activation on precious metals, spillover, surface diffusion, and bulk diffusion. Among these steps, the main contributors to the catalytic reactions (activation by precious metal and simultaneous spillover to/from CZ), occur near the TPB. The TPB region locates around the perimeter of the Pd particle placed on the CZ (see Figure 8). When the Pd nanoparticles are highly dispersed as in the Pd/CZ-fresh sample (Figure 8(a)), many Pd species on the surface are nearby and accessible to the TPB, so the oxygen storage and release processes exert a strong impact that spreads over the entire Pd/PdO particle. This effect is especially prominent during rich-to-lean switching of the gas feed to the fresh Pd/CZ, where excess gas-phase O2 is removed by adsorption onto Pd followed by simultaneous spillover, which fills the vacancies on CZ. This rapid oxygen uptake by CZ reduces the local O2 partial pressure near the TPB, where the Pd oxidation should be

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strongly retarded. Similarly, during lean-to-rich switching, the PdO reduction is retarded by oxygen release from CZ, although the retardation effect is weaker than in Pd oxidation during rich-to-lean switching (c.f. Figures 5 and 6). When thermal aging causes significant growth of the Pd particles, the TPB per unit weight of Pd becomes very small, and affects only a limited part of Pd/PdO near the TPB (see Figure 8(b)). Conversely, the Pd species at sufficient distances from the TPB are unlikely to be affected by the oxygen storage, and can be quickly reoxidized upon the rich-to-lean switching. This may explain why Pd reoxidation was no longer retarded in the most deactivated Pd/CZ-1000, in which kR→L/kL→R was close to unity (Figure 4(b)). Aside from this mechanism of the TPB loss via Pd sintering, the migration of Pd also leads to the decrease in the active Pd–CZ interface. As shown in Figure 1(d-f), Pd supported on CZ can migrate and contact with Al2O3 during the aging at high temperatures. This effect cannot be completely eliminated even in the present catalyst configuration without Al2O3, because washcoating of a honeycomb substrate needs Al2O3 as a binder component. Based on the present results, we conclude that in situ DRS evaluations of TWCs under a fluctuating redox atmosphere provide the time-resolved kinetic information of the Pd redox reactions. The obtained data contain both qualitative and quantitative aspects of the TPB, reflecting not only the particle size but also the intimate contact at the Pd–CZ interface. The ratio of the Pd redox rate constants, kR→L/kL→R, represents the retardation effect of oxygen storage on Pd reoxidation, providing a useful index of TPB deterioration. The present analysis method is expected to assess the deactivation of various catalyst systems consisting of precious metals, oxygen storage materials and gas phase, which operate under real driving conditions.

4. CONCLUSIONS

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ACS Catalysis

Full-scale honeycomb Pd/CZ three-way catalysts were subjected to chassis-dynamometer driving tests. The deactivation, which was most extensive in the cold-start urban driving test cycles, increased monotonically as the temperature of engine-aging increased from 800 °C to 1000 °C. The observed deactivation accorded with the microstructural evolution, indicating that Pd and CZ particles grew by significant sintering, with concomitant decrease in the catalytically active TPB sites. In situ DRS under the lean–rich perturbation condition revealed that in the fresh catalyst, much of the Pd was highly dispersed and thus resided near the TPB. Consequently, the Pd reoxidation was strongly retarded by oxygen storage in the CZ. In contrast, in the high-temperature engine-aged Pd/CZ catalysts, where most of the Pd particles were enlarged, Pd distributed far from the TPB were no longer affected by the oxygen storage/release. Therefore, the retardation effect was much more subdued than in the fresh catalyst. To understand the TPB degradation, a kinetic analysis of the Pd redox was performed using time-resolved reflectance data. The ratio of apparent initial rate constants of the Pd redox, kR→L/kL→R, was significantly correlated with the catalyst performance observed in the chassis-dynamometer tests, suggesting that the proposal for determining the extent of catalyst deactivation is widely applicable.

AUTHOR INFORMATION Corresponding Author * Masato Machida, Professor Division of Materials Science and Chemistry, Faculty of Advanced Science and Technology, Kumamoto University,

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ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan, Fax: 096-342-3651 E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Engine-aging conditions, chassis-dynamometer test results, XRD, XAFS, kinetic analysis of transient DRS responses, catalytic activity, reaction conditions, Pd contents. This information is available free of charge via http://pubs.acs.org

ACKNOWLEDGMENTS A part of this work was supported by the MEXT program, “Elements Strategy Initiative to Form Core Research Center” (since 2012), which is run by MEXT (Ministry of Education Culture, Sports, Science and Technology), Japan. XAS experiments were performed at a public beamline BL01B1 and BL14B2 in SPring-8 with the approval of JASRI (Proposal No. 2017B1457, and 2018B1786).

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ACS Catalysis

Table 1 Characterization and catalytic performance different temperatures. Catalyst Aging a SBET b Pd Pd 2 −1 (m g ) particle particle size c size d (nm) (nm) Pd/CZ fresh 61 12 ‒

a

results of 2 wt% Pd/CZ catalysts aged at Crystallite size e (nm) Pd CZ

Conversion f (%) NOx CO THC

OSC g (g)

–

12

98

100

86

1.39

800 °C

35

44

‒

–

17

77

83

76

0.87

900 °C

26

78

71±24

71

24

68

76

74

0.83

1000 °C

17

158

87±37

107

45

50

66

65

0.83

Pd/Al2O3 1000 °C

70

134

110±60

−

−

26

46

46

0.09

Aged in an engine dynamometer under stoichiometric-lean–rich gas cycles at the specified

temperature for 40 h (Supporting Information, Figure S1). b

BET surface area of Pd-loaded catalysts.

c

Calculated from the CO/Pd ratio assuming a chemisorption stoichiometry of Pd:CO = 1:1 and a

hemispherical particle. The CO chemisorbed to loaded Pd was determined by the pulse injection technique at 50 °C after reduction treatment in 5% H2 at 400 °C. d

Volume-weighted mean particle size and standard deviation, calculated by histogram analysis of

the HAADF–STEM/X-ray mapping images. The reduction treatment in 5% H2 at 400 °C. e

Calculated by X-ray line-broadening analysis using the Scherrer equation. The reduction

treatment in 5% H2 at 400 °C. f

Cumulative conversion during the 40–200 s period of the cold-start urban driving test cycles

shown in Figure 2. g

Oxygen storage capacities per full-scale honeycomb catalyst at 450 °C.

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ACS Catalysis

Figures

(a)

20

(b)

(c)

Pd/CZ-1000

Fraction (%)

15

(e)

(d)

5

60 120 180 240 Pd particle size (nm)

300

15 45 75 105 135 165 195 225 255 285 315

0

(f)

70 nm

70 nm

(g)

10

0

700 nm

700 nm

70 nm

20

(h)

(i)

Pd/Al2O3-1000

15

Fraction (%)

10 5

700 nm

700 nm

0 0

60 120 180 240 Pd particle size (nm)

300

15 45 75 105 135 165 195 225 255 285 315

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

Figure 1. HAADF–STEM, X-ray mapping images, and Pd particle size distribution of (a–f) Pd/CZ1000 and (g–i) Pd/Al2O3-1000. The mapping images are highlighted by (b, h) Pd (red), (e) Pd (red), Ce(green), and Al(pink), (f) Ce(green) and Al(pink). The particle shown by an arrow is Pd in contact with Al2O3 rather than CZ. The samples were reduced in 5% H2/N2 at 400 °C.

26


THC (ppmC) Temp.(C)

Pd/CZ-1000 Pd/CZ-900 Pd/CZ-800 Pd/CZ-fresh 100 0

4000 3000 2000 1000 0

100 0

8000 6000 4000 2000 0

Conversion efficiency (%)

A/F

6.0 4.0 2.0 0

NOx (ppm)

16.0 15.2 14.4 13.6 12.8 12.0

100 0

600 500 400 300 200 100 0 60 30 0

TWC inlet

TWC bed

Vehicle speed (km/h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

CO (%)

Page 27 of 34

0

40

80

120

160

200

Time on stream (s)

Figure 2. The urban driving cycles (UDC) with cold start (0–200 s) according to the NEDC standard for the catalytic performance test using a chassis-dynamometer. Honeycomb catalysts were tested before and after engine-aging at 800 °C, 900 °C, or 1000 °C for 40 h.

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ACS Catalysis

0.04

lean

rich

lean 0.04

lean

0.03

0.03

0.02

0.02

0.01 0

lean

0.01 0

-0.01 0

300

600

900

-0.01

1200

0

300

Time on stream (s)

0.04

rich

(b)

KM

KM

(a)

lean

rich

600

900

1200

Time on stream (s) lean

0.04

(c)

0.03

0.03

0.02

0.02

KM

KM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

0.01

lean

rich

lean

(d)

0.01 0

0

-0.01

-0.01 0

300

600

900

1200

0

Time on stream (s)

300

600

900

1200

Time on stream (s)

Figure 3. In situ DRS transient intensity responses at  = 450 nm in the (a) Pd/CZ-fresh, (b) Pd/CZ800, (c) Pd/CZ-900 and (d) Pd/CZ-1000 catalysts during lean-to-rich and rich-to-lean switching of two simulated gas feeds equivalent to A/F = 15.0 (lean) and 14.1 (rich) measured at 450 °C.

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k ×103 (s-1)

1.5

(a)

1.0

k L→R

0.5 k R→L

0.0 0

50

100

150

200

Pd particle size (nm)

2.0

k R→L/k L→R

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Pd/Al2O3

(b)

1.5 1.0

Pd/CZ

0.5 0.0 0

50

100

150

200

Pd particle size (nm) Figure 4. Plots of (a) kR→L and kL→R and (b) their ratio (kR→L/kL→R) versus Pd particle size in Pd/CZ catalysts after engine-aging at different temperatures. The open circle in (b) corresponds to the reference catalyst Pd/Al2O3-1000.

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ACS Catalysis

lean

rich

O2 conc. (%)

Conversion (%)

100

KM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

C3H6

80

CO

60 40

Pd/CZ-fresh Pd/CZ-1000

NO

20 0 0.4 0.3


0.2 0.1 0.0 0.04 0.03 0.02 0.01 0 0

100

200

300

400

500

600

Time on stream (s)

Figure 5. (top) Catalytic conversion of CO, NO, and C3H6, (middle) the gas phase O2 concentration, and (bottom) the DRS transient response at  = 450 nm, in the Pd/CZ-fresh and Pd/CZ-1000 catalysts during lean-to-rich switching (A/F = 15.0 → 14.1) at 450 °C.

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lean

rich

O2 conc. (%)

Conversion (%)

100

KM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

C3H6

80


60 40

CO

NO

20 0 0.4 0.3


0.2 0.1 0.0 0.04 0.03 0.02 0.01 0 0

100

200

300

400

500

600

Time on stream (s)

Figure 6. (top) Catalytic conversion of CO, NO, and C3H6, (middle) the gas phase O2 concentration, and (bottom) the DRS transient response at  = 450 nm, in the Pd/CZ-fresh and Pd/CZ-1000 catalysts during rich-to-lean switching (A/F = 14.1 → 15.0) at 450 °C.

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ACS Catalysis

Conversion (%)

100

(a)

80

CO THC

Pd/CZ

60

NOx

40

Pd/Al2O3

20 0.0

NOx Conversion (%)

100

0.5

1.0 k R→L/k L→R

(b)

1.5

2.0

EUDC Pd/CZ

80

UDC 60

Pd/Al2O3 40 0.0 1.5

OSC (g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5

1.0

1.5

2.0

k R→L/k L→R

(c)

1.0

0.5

0.0 0.0

0.5

1.0

1.5

k R→L/k L→R

Figure 7. Plots of (a) NOx, CO, and THC conversion efficiencies in the 40–200 s period from cold start in the UDC chassis-dynamometer test (Figure 2), (b) NOx conversion efficiency in the 200– 780 s period in the UDC and EUDC tests (Figures S5 and S6), and (c) OSC versus kR→L/kL→R for the Pd/CZ catalysts after engine-aging at different temperatures. Open circles in (a) and (b) correspond to the reference catalyst Pd/Al2O3-1000.

32


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Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(a) fresh

O2

CO O

Pd Pd VO

CZ

rich

lean

lean

rich

VO

VO

Pd

CO2

PdO

CZ

VO

TPB

(b) aged

O2

CO

O2

Pd

CO2 CO PdO

Pd VO

CO2 O

Pd PdO

VO

CZ

CZ

Figure 8. Schematic of the Pd−CZ interfaces in the (a) fresh and (b) aged catalysts. Oxygen vacancies in CZ near the TPB are labeled Vo. For simplicity, only O2 and CO are shown as representative oxidizing and reducing gases, respectively.

33


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

TOC.

in situ DRS O2

TPB VO

engine aging

O2 O2

Pd

CO2 CO PdO

Pd

Pd Pd VO

CeO2-ZrO2

VO

Pd PdO

CO2 O VO

CeO2-ZrO2

34


CeO2âZrO2 Three-Way Catalysts

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