Pt- and Pd-Promoted CeO2âZrO2 for Passive NOx ... - ACS Publications

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Pt- and Pd-promoted CeO-ZrO for passive NOx adsorber applications Yaying Ji, Dongyan Xu, Shuli Bai, Uschi M. Graham, Mark Crocker, Bingbing Chen, Chuan Shi, Deb Harris, Dave Scapens, and John Gerard Darab Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03793 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Industrial & Engineering Chemistry Research

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Pt- and Pd-promoted CeO2-ZrO2 for passive NOx adsorber applications Yaying Ji1, Dongyan Xu1, Shuli Bai1, Uschi Graham1, Mark Crocker1,2*, Bingbing Chen3, Chuan Shi3, Deb Harris4, Dave Scapens4, John Darab5 1

Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA

2

Department of Chemistry, University of Kentucky, Lexington, KY 40506, USA

3

Dalian University of Technology, Dalian, CN 116024, P.R. China

4

MEL Chemicals, Manchester, M27 8LS, United Kingdom

5

MEL Chemicals Inc., Flemington, NJ 08822, USA

*To whom correspondence should be addressed e-mail: [email protected] Tel.: +1 859 257 0295

Fax: +1 859 257 0220

ACS Paragon Plus Environment


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Abstract Pt- and Pd-promoted CexZr1-xO2 mixed oxides were characterized and investigated for passive NOx adsorber applications. XRD analysis revealed a phase transition from tetragonal to cubic with increasing cerium content in CexZr1-xO2, while H2 and CO chemisorption data in all cases indicated average Pt and Pd particle sizes of close to 2 nm. H2-TPR measurements revealed a shift of the Pt reduction peak to higher temperature with increasing Ce content, consistent with a corresponding increase in the degree of Pt oxidation. According to microreactor data, doping Ce into the ZrO2 lattice resulted in a significant improvement in low temperature (80-160 °C) NOx storage efficiency. DRIFTS measurements on Pt/CexZr1-xO2 showed that as Ce content increased, relatively more nitrite species were generated during NOx storage. However, oxidation of nitrite to nitrate during subsequent NOx-TPD – increasing the concentration of more thermally stable nitrate – also correlated with increased Ce content. The use of Pd as a promoter resulted in decreased NOx storage efficiency compared to Pt, although low-temperature NOx desorption behavior was improved. This is attributed to decreased formation of nitrate during NOx storage compared to Pt, as well as the lower activity of Pd for oxidation of nitrite to nitrate during subsequent NOx-TPD. In order to achieve more balanced NOx storage and desorption behavior, Ce0.2Zr0.8O2 was promoted with both Pt and Pd, resulting in superior overall NOx performance relative to its Pt and Pd analogs. After hydrothermal aging at 750 °C for 16 h, the co-promoted sample still maintained excellent NOx adsorption-desorption performance.

Keywords: passive NOx adsorber, NOx storage, low temperature, ceria, zirconia


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1. Introduction The abatement of NOx emissions from lean-burn engines during cold starts represents a major challenge to the automotive industry. The two mobile NOx aftertreatment systems presently in commercial application, selective catalytic reduction and lean NOx trap catalysts, are ineffective at the low exhaust temperatures associated with cold starts. Recently, a new concept has been proposed to reduce cold start NOx emissions, namely, the use of a passive NOx adsorber (PNA) device in combination with a urea SCR catalyst.1,2 In this system, the PNA adsorbs NOx emitted from the engine during cold starts, and then releases the NOx at higher temperatures, e.g., above 200 °C. At this point the SCR catalyst is sufficiently warm to function efficiently. As early as 1997, Cole et al.3 reported the application of a passive NOx adsorber to trap coldstart NOx in combination with three-way catalysts. In 2001, a Ford patent claimed the use of Ptpromoted γ-Al2O3 as a NOx adsorber for low-temperature lean exhaust aftertreatment.4 Recently, GM claimed a PNA system incorporating an external fuel injection system and air pump.5 In this system, if the vehicle shuts down without the PNA having reached its required regeneration (i.e., thermal desorption) temperature, then air and fuel are injected so as to raise the temperature of the PNA until NOx desorption occurs; the desorbed NOx is reduced to N2 by residual NH3 stored on the downstream SCR catalyst. Recently, Johnson Matthey reported a novel catalyst technology ‒ termed the Cold Start Concept, CSCTM ‒ for cold start emission control in which both hydrocarbon (HC) and NOx can be stored at low temperature.6,7 A significant portion of the stored HC/NOx is converted during the warm-up period and the rest of the HC/NOx is thermally released from the catalyst and then converted by the downstream catalyst components. Along similar lines, Yuichiro et al.8 from Honda developed a NOx trap – three-way catalyst (N-TWC) to reduce NOx emissions from gasoline engines during cold starts. In this system, a Pd/ZSM-5



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catalyst is used to trap NOx at low temperatures, the stored NOx being reduced at higher temperatures under suitable air-fuel ratio conditions. In addition, several other reports have recently appeared concerning the development of PNA catalysts. Olsson and co-workers,9 as well as workers at GM,10 reported significant NOx storage on Ag-promoted Al2O3 NOx adsorbers at 200 ºC. In a recent publication we reported Pt-promoted Al2O3-based PNA materials.11 It was found that Pt/Al2O3 possessed low NOx storage capacity, although it was significantly improved by doping Al2O3 with La2O3. Unfortunately, low-temperature NOx desorption efficiency decreased after this modification. Ceria-based materials have also been examined for PNA applications; Johnson Matthey has disclosed Pt/Pd-promoted CeO2-based PNA materials in recent patents,12,13 while Jones et al.14 recently reported the results of a study comparing the properties of Pt/CeO2 and Pd/CeO2 in low temperature NOx storage and desorption. CeO2 is widely used as a component in three-way catalysts, enabling rapid cycling of oxygen between the gas phase and the catalyst due to its unique redox properties. Among ceriacontaining materials, ceria-zirconia mixed oxides have attracted much attention due to their superior oxygen storage capacity (OSC) and high oxygen mobility, as well as their thermal durability. Consequently, Ce-Zr mixed oxides represent particularly interesting candidate materials for PNA applications. In the open literature, several research groups have reported NOx adsorption and desorption over Ce-Zr mixed oxides for different applications. Haneda et al. investigated Ce-Zr mixed oxides for NOx storage under lean conditions.15 By comparing two Ce-Zr solid solution preparation methods, it was found that a sol-gel method achieved higher NOx adsorption capacity at 200 °C than heterogeneous Ce-Zr mixed oxides prepared by co-precipitation. Thus, it was considered that homogeneous mixing of Ce and Zr ions in the solid solution was one of the important factors


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for the high NOx adsorption capacity. Adamowska et al.16 studied the interaction of NO and O2 with Ce0.62Zr0.38O2 for SCR applications. NOx-TPD profiles indicated three different NOx storage sites on Ce-Zr, which were dependent on degrees of support unsaturation and cerium oxidation states. Levasseur et al.17 studied NO2 adsorption on Ce-Zr mixed oxides and found that NO2 adsorption capacity was linked to the presence of reduced Ce and oxygen vacancies induced by the addition of Zr4+ cations to the structure. Azambre et al.18 utilized DRIFTS to study NO and NO2 adsorption on Ce-Zr mixed oxides. They found that NOx was stored on the support as nitrite at room temperature through electron transfer (i.e., donor-acceptor mechanism), the nitrite being gradually converted to nitrate upon extended exposure to NOx due to oxygen transfer. Atribak et al.19 studied NOx adsorption and desorption over Ce-Zr mixed oxides for soot combustion and found that the production of NO2 upon interaction of NO+O2 with the Ce-Zr catalysts strongly depended on the Ce content. Pt-promoted Ce-Zr mixed oxide was also applied to NOx storage and reduction by Le Phuc et al.20 The use of a reducing pretreatment led to an increase in NOx storage capacity at 300 and 400 °C, which was associated with improved Ce reducibility/oxygen mobility and NO oxidation rate. Very recently, Pd-promoted Ce-Pr-Zr mixed oxides were explored to mitigate NOx emissions during cold starts,21 and it was demonstrated that Zr- or Ce-rich mixed oxides exhibited the best storage capacity and thermal stability. Despite these studies, the potential of Ce-Zr mixed oxides for PNA applications at temperatures below 200 °C has been little studied. To fully explore the impact of Ce-Zr composition on NOx storage and desorption properties, in this work a series of Ce-Zr mixed oxides with different Ce/Zr molar ratios was prepared and evaluated by means of microreactor experiments. DRIFTS measurements were also undertaken to study the formation and evolution of surface species during NOx storage and desorption.



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2. Experimental 2.1. Catalyst preparation Ce-Zr mixed oxides with the composition CexZr1-xO2 were provided by MEL Chemicals and were prepared by a proprietary aqueous precipitation process used for the production of commercial materials; these are hereafter denoted as 20Ce-80Zr, 40Ce-60Zr, 60Ce-40Zr and 80Ce-20Zr (corresponding to x = 0.2, 0.4, 0.6 and 0.8, respectively). For comparison purposes, ZrO2 and CeO2 samples were also included in the study. Pt- or Pd-promoted Ce-Zr mixed oxides were prepared by means of incipient wetness impregnation (IWI) using aqueous solutions of tetraamine platinum (II) nitrate or tetraamine palladium nitrate to give a Pt or Pd loading of 1 wt%, followed by drying in a vacuum oven overnight and calcination at 500 ºC for 3 h. The resulting Pt samples are herewith designated as Pt-Zr, Pt-Ce, Pt-20Ce (Pd-20Ce), Pt-40Ce, Pt60Ce and Pt-80Ce. Ce0.2Zr0.8O2 promoted with both Pt (0.5 wt%) and Pd (0.5 wt%) was similarly prepared by co-impregnation using a mixed aqueous solution of tetraamine platinum (II) nitrate and tetraamine palladium nitrate. This co-promoted sample is designated as Pt-Pd-20Ce. 2.2. Catalyst characterization BET surface area and pore volume measurements were performed by nitrogen adsorption at -196 ºC using a Micromeritics Tri-Star 3000 system. Catalyst samples were outgassed overnight at 160 ºC under vacuum prior to the measurements. Pt dispersion was determined by means of pulsed H2 chemisorption at dry ice temperature (-78 oC) using a Micromeritics AutoChem II Analyzer. Around 200 mg of the catalyst was loaded into the reactor. After being reduced at 300 ºC in 10% H2/Ar for 30 min, the catalyst was heated up to 400 ºC (hold time 10 min) in flowing Ar to remove adsorbed H. Pulsed H2 chemisorption was initiated using a four-way valve after the


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catalyst had been cooled to -78 ºC. During this measurement, 0.5 ml of 10 % H2/Ar was pulsed into the reactor every 2 min, the H2 signal at the reactor outlet being monitored with a thermal conductivity detector (TCD). H2 pulsing was terminated after the TCD signal had reached a constant value, i.e., the precious metal sites were saturated with H2. Assuming a 1:1 ratio of atomic hydrogen to surface Pt, the metal dispersion was calculated based on the amount of H adsorbed. For Pd and Pt-Pd bimetallic samples, CO pulsed chemisorption was performed to determine the average particle size. After the same pretreatment used in H2 pulsed chemisorption, the sample was cooled to 0 °C in an ice bath. During the measurement, 0.5 ml of CO was pulsed into the reactor every 2 min, the CO signal at the reactor outlet being monitored with a TCD. The metal dispersion was calculated based on the amount of CO required to fully saturate the metal surface and an assumed a 1:1 ratio of CO to surface metal sites. Temperature-programmed reduction (TPR) was performed using Micromeritics AutoChem II Analyzer. Ca. 150 mg of catalyst was loaded in the reactor and pretreated in 10% O2/N2 at 500 °C for 1 h. After cooling the sample to room temperature (RT), TPR was carried out in a 10% H2/Ar flow with a ramp of 10 °C/min from RT to 500 °C. The H2 signal during TPR was monitored using a TCD. X-ray powder diffraction was conducted on a Phillips X’Pert diffractometer using Cu-Kα radiation (λ = 1.540598 Å). Diffractograms were recorded between 5o and 90o (2Ɵ) with a step of 0.02°. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB250 spectrometer using Al-Kα radiation (1486.6 eV) as the X-ray source. The beam was monochromatized by a twin crystal monochromator, yielding a focused X-ray spot with a size of 500 µm, at 10 mA ×15 kV. The alpha hemi-spherical analyzer was operated in the constant energy mode with a pass energy of 50 eV. Charge compensation was achieved with



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a low energy electron flood gun and low energy argon ions from a single source. Curve fitting of the Pt4f peak was performed using Avantage software (Thermo Scientific). Both Pt 4f7/2 and Pt 4f5/2 bands were included in the fitting. High Resolution Transmission Electron Microscopy (HRTEM) and Electron Energy Loss Spectroscopy (EELS) were performed to determine the morphology, size and phase separation of the support materials and also analyze the size and location of the supported Pt and Pd particles. Samples were prepared from selected catalyst powders that were then collected on copper grids (200-mesh, Ted Pella Inc. Redding, CA). TEM imaging was performed using a JEOL 20100F field-emission gun transmission electron microscope (accelerating voltage of 200 keV and magnification ranging from 50-1000K). A symmetrical multi-beam illumination was used for high-resolution imaging (HRTEM) with a beam resolution of 0.5 nm. Images were recorded using a Gatan Ultrascan 4k x 4k CCD camera and data processing and analysis was done using Gatan Digital Micrograph software. Scanning transmission electron microscopy (STEM) was used including a Gatan imaging filter (GIF) and high angle annular dark field (HAADF) detector. Electron Energy Loss Spectroscopy (EELS) was performed in STEM mode using the 1 nm probe size. To locate the probe and collect the EELS spectrum from specific catalyst grains, a STEM image was first acquired at the Fischione HAADF Detector. Spectra were collected using a 1024-channel Gatan Image Filter with a spectral resolution estimated from the full width at half maximum of the zero-loss peak (~ 0.8 eV). EELS-spectra and EELS line scans across catalyst particles were recorded using an alpha of 30 mrad, and a beta of 6 mrad and recorded in STEM imaging mode using the Digi-scan software for data recording and processing. 2.3. NOx storage efficiency (NSE) and temperature-programmed desorption


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A microreactor loaded with ca. 150 mg of powder catalyst was employed to study the NOx adsorption and desorption properties of the catalysts. In all the cases, a total flow rate of 120 sccm was used, corresponding to a gas hourly space velocity (GHSV) of ca. 30,000 h-1. Effluent gases were analyzed using a mass spectrometer (QMS 200). Unless otherwise stated, the catalysts were first pretreated at the desired NOx storage temperature under lean gas containing 5% O2, 5% CO2 and 3.5% H2O until the samples were saturated (based on a comparison of the feed and effluent gas concentrations); typically this required 15 min. NOx storage was performed at three different temperatures (80, 120 and 160 ºC) by adding 300 ppm NO to the lean feed gas. After NOx storage for a specified period of time, the feed gas was switched to bypass mode and the NO flow was switched off. When the NO concentration had dropped to zero, the gas was redirected to the reactor and temperature-programmed desorption was carried out to study NOx desorption behavior using a ramp rate of 10 oC/min from the storage temperature up to 500 oC. NOx storage efficiency (hereafter denoted as NSE) is defined as the percentage of NOx fed to the reactor that is stored, while NOx desorption efficiency (hereafter denoted as NDE) is defined as the percentage of stored NOx that is desorbed.11 For the Pt-Pd-20Ce sample, hydrothermal aging was performed by exposing the sample to flowing gas containing 5% CO2, 3.5% H2O and 5% O2 at 750 oC for 16 h (balance N2, GHSV = 30,000 h-1). The NOx storage and desorption properties of the sample were subsequently evaluated as described above, using a NOx storage temperature of 120 oC.



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2.4. Diffuse Reflectance Infrared Fourier Transform Spectroscopy DRIFTS measurements were performed to monitor the surface species involved in NOx adsorption and desorption. Measurements were performed using a Nicolet 6700 IR spectrometer equipped with a Harrick Praying Mantis accessory and MCT detector. The reaction cell was sealed with a dome equipped with two ZnSe windows and one SiO2 observation window. The temperature of the reactor cell was controlled and monitored by a K-type thermocouple placed beneath the reaction chamber. For each DRIFT spectrum an average of 115 scans was collected (requiring ca. 1 min) with a resolution of 4 cm-1. The spectrometer, as well as the outside of the reaction cell, were continuously purged with dry nitrogen to avoid diffusion of air into the system. Unless otherwise stated, catalyst samples (̴ 50 mg) were pretreated in situ in flowing 5% O2/Ar (120 sccm) at 500 ºC for 1 h in order to remove moisture and carbonate, after which background spectra were collected (using the same feed gas) in the range 500 ºC to 100 ºC at intervals of 50 ºC. NOx storage was carried out at 100 ºC for 30 min using a feed consisting of 5% O2 and 300 ppm NO (120 sccm). During NOx storage spectra were collected as a function of time. After 30 min of NOx storage, temperature programmed desorption (TPD) was performed in flowing 5% O2/Ar flow (120 sccm), the temperature being raised from 100 ºC to 500 ºC at a rate of 10 ºC/min. DRIFT spectra were recorded during TPD at intervals of 50 ºC. Absorbance spectra were obtained by subtracting background spectra from the spectra collected during NOx storage and desorption. To probe the changes in the properties of the supported metals on 20Ce-80Zr after aging, CO chemisorption was carried out on fresh and aged Pt-Pd-20Ce samples, in combination with DRIFTS. Samples were first reduced at 300 oC in a H2/N2 flow for 15 min, then purged with Ar at 400 oC for 10 min. After cooling to room temperature in Ar, samples were subsequently


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exposed to a CO/Ar flow until the metal surface was fully saturated with CO. For comparison purposes, Pt-20Ce and Pd-20Ce samples were included in the study.

3. Results 3.1. Characterization As shown in Table 1, the Ce-Zr mixed oxides used in this study generally showed an increase in BET surface area (SA) with increasing Ce content. Among these materials, the ZrO2 sample possessed the lowest specific surface area (72 m2/g), while Ce0.6Zr0.4O2 possessed the highest (112 m2/g). Further increase in the Ce content failed to increase this value. However, a consistent decrease in the average pore size with Ce content was observed, Pt-Zr displaying an average pore diameter of 19.2 nm while Pt-80Ce possessed the smallest diameter, ca. 8.5 nm. Less pronounced effects were observed for the pore volume, a ~30% decrease in pore volume being observed for Pt-80Ce as compared to Pt-Zr. In the case of the co-promoted sample, Pt-Pd-20Ce, negligible differences in the physical properties were observed relative to Pt-20Ce. Hydrothermal aging of Pt-Pd-20Ce resulted in significant changes, the specific surface area decreasing to slightly less than half of the value for the fresh sample. Simultaneously, the pore volume dropped by 30% and the mean pore diameter increased from 16.1 nm to 22.4 nm, indicative of a collapse of the smaller pores during aging. XRD patterns of the support materials are shown in Fig. 1. Pure ZrO2 mainly exhibited the monoclinic phase, as characterized by two major diffraction peaks located at 28.2o and 31.3o.19 However, upon replacing 20% of the Zr4+ ions in the ZrO2 lattice by Ce4+, the phase transformed from monoclinic to tetragonal. This is evidenced by the XRD pattern of 20Ce-80Zr which



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featured a strong band at ca. 30o and a weaker, broad band at 34.7o which can be respectively attributed to the tetragonal contributions of the (002) and (200) planes. Increase of the Ce content to 40 mol% resulted in the formation of a Ce0.5Zr0.5O2 solid solution exhibiting the cubic phase, as evidenced by a major diffraction peak at ca. 29.2o. Further increase of the Ce content to 60 mol% resulted in phase segregation, both the Ce0.5Zr0.5O2 and CeO2 cubic phases being detected in 60Ce-40Zr. For the Ce content of 80 mol% only the cubic CeO2 phase was detected, the observed reflections at 28.5o, 33.1o, 47.6o and 56.5o being typical of the cubic fluorite-type structure, corresponding to the (111), (200), (220) and (311) planes,19 respectively. This shift in structure from tetragonal to cubic with increasing Ce content has been observed previously by Bandosz and co-workers for Ce-Zr mixed oxides prepared by co-precipitation,17 with the coexistence of both phases implied at intermediate Ce content (40Ce-60Zr and 60Ce-40Zr). A shift of the diffraction peaks towards lower 2Ɵ values with increasing Ce content is related to changes in the lattice parameters due to the expansion of the lattice structure when Zr4+ ions are replaced by larger Ce4+ atoms. Indeed, as indicated in Fig. S1, a linear correlation between the (111) peak position and Zr content was obtained, in agreement with Bandosz et al.17 In all cases the diffraction peaks were broad, indicating the presence of small oxide crystallites; applying the Scherrer equation, average crystallite sizes ranging between 4.71 and 6.76 nm were calculated for the samples. According to H2 and CO chemisorption results (Table 1), Pt was highly dispersed on the supports, as evidenced by an average particle size of ca. 2 nm. Relative to the Pt samples, Pd-20Ce contained slightly larger metal particles, while the bimetallic Pt-Pd-20Ce sample exhibited a very similar mean particle size to Pd-20Ce (ca. 2.4 nm). After hydrothermal aging of Pt-Pd-20Ce, an increase in the mean metal particle size from 2.36 nm to 4.13 nm was observed.


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Results of the STEM-EELS investigation are summarized in Figure 2. The STEM images and EELS line profiles of Pd-20Ce (Fig. 2a-c) as well as Pt-20Ce (Fig. 2d-f) indicate that the support particles range from 5 to 25 nm and that individual crystallites agglomerate to form large clusters spanning up to half micron in size. The Pd and Pt catalyst particles are well dispersed on the support surface and range from 0.5 to 4 nm in size with the majority of grains being 2 nm (Fig. 2b and Fig. 2e).

The EELS line profiles with corresponding spectrum images for the Pd

supported sample (Fig. 2b-c) and Pt supported sample (Fig. 2e and Fig. 2f) indicate that the support structure has not segregated into separate CeO2 and ZrO2 domains and generally represents a solid solution with Ce and Zr well dispersed throughout the support matrix. The study did not find individual CeO2 particles that had formed separate domains for the 20% doped support. The HRTEM imaging furthermore revealed that the support particles were highly crystalline. The STEM image in Figure 2b shows that there is some alignment of the individual crystals in the Ce0.2Zr0.8O2 support. The alignment resembles a ridge-valley structure that has been previously reported for a ceria support.22 The EELS line scans indicate that the Pd and Pt particles occur at discrete locations on the Ce0.2Zr0.8O2 support. There appears to be no homogeneous and sub-nanometer dispersion of the promoter. EELS line scans were placed strategically by starting at a catalyst support region and spanning ~ 25 nm across a series of neighboring support grains to reveal the presence of Pd and/or Pt particles at the support surface. Most notably, the Pd and Pt particles are typically at the interface where two support crystallites intersect (Fig. 2b and 2e). In contrast, the STEM imaging of the Ce0.6Zr0.4O2 support material revealed a granular morphology with significant particle separation between individual crystallites (Fig. 2g). Fig. 2h shows a magnified view of the same region and reveals the presence of individual CeO2 domains as part of the higher Ce doped support structure. An



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individual Pt nanoparticle of ~2 nm size is hosted on the surface of a ~15 nm CeO2 crystal. A corresponding EELS line scan that transects the CeO2 crystal and Pt nanoparticle is shown in Figure 2i and confirms the large domain size of the crystal (~ 20 nm wide) and intersection with a Zr-rich domain. These results indicate that the individual particles in the Ce0.6Zr0.4O2 support material do not represent a homogeneous solid solution and that the Pt nanoparticles are preferably located with segregated CeO2 crystals (Fig. 2h and 2i). XPS measurements were carried out on three Pt samples: Pt-Zr, Pt-20Ce and Pt-60Ce. From Table 1, the surface atomic ratio of Pt to (Ce+Zr) increased with Ce content, which is consistent with slightly enhanced Pt dispersion as determined by H2 chemisorption. Notably, the atomic ratio of Ce to Zr for Pt-20Ce (0.16) and Pt-60Ce (0.975) was smaller than the supposed bulk molar ratio (0.25 for Pt-20Ce and 1.5 for Pt-60Ce), indicative of Zr enrichment at the surface. From H2-TPR profiles shown in Fig. 3, a very small and extremely broad reduction peak was observed for the Pt-Zr sample between 100 and 200 °C, suggesting that a slow reduction occurred on this sample. However, a large, well-defined reduction peak was obtained for all of the Pt-promoted Ce-Zr-based samples. Notably, the reduction peaks shifted to slightly higher temperature with Ce content; as shown in the insert, the peak temperature shifted from ca. 110 °C to ~ 160 °C as the Ce content increased from 20 mol% to 80 mol%. Further, the area of the reduction peak also varied with Ce content. As shown in the insert, the relative peak area increased from 0.172 for Pt-20Ce to 0.260 for Pt-40Ce and then slightly decreased with further increase of the Ce content. It is well documented24 that the reduction peak corresponds to partial reduction of the surface support in addition to Pt, from which it follows that Pt-40Ce and Pt60Ce showed the most significant support reduction. As shown in Fig. 3, the incorporation of Pd modified the reduction behavior. For Pt-Pd-20Ce, the peak reduction temperature shifted to ca.


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60 °C as compared to the peak temperature of ca. 110 °C for Pt-20Ce. Moreover, the relative peak area decreased from 0.172 for Pt-20Ce to 0.117 for Pt-Pd-20Ce, indicating less reduction of the support surface. Interestingly, the reduction behavior of the Pd-20Ce sample was very different from that of the Pt-20Ce sample. Three reduction events were observed for Pd-20Ce, which corresponded to H2 uptake at 60 oC, 80 °C and 250 °C. Similar reduction peaks have been reported by Luo and Zheng,23 the peak at 60 oC being ascribed to reduction of PdO. Given the large size of the peak at 250 °C, it should be associated with reduction of the oxide surface, most likely triggered by the reduction of particularly stable PdO species. Additionally, the formation of Pd hydride might also contribute to H2 consumption in this temperature region. A peak at 80 o

C was observed by Luo and Zheng only after reduction and re-oxidation of PdO/Ce0.5Zr0.5O2, its

occurrence being explained by non-uniform size distribution of PdO and/or part of the PdO moving into the lattice of the Ce0.5Zr0.5O2 solid solution.23 In the present case the former explanation appears more likely, although we cannot rule out the possibility of a reduction event associated with the presence of PdO2 or Pd hydroxide. 3.2. Effect of Ce content Fig. 4 displays the NOx storage efficiency (NSE) of the four Ce-based samples as a function of storage time at three different storage temperatures. Plotted results correspond to cumulative values (percentage of NOx stored based on total NOx fed). Regardless of the temperature used for NOx storage, Pt-20Ce consistently showed the highest initial NSE (for the first two minutes of NOx storage), while Pt-80Ce possessed the lowest initial NSE. Pt-40Ce and Pt-60Ce ranked between these two samples. A significant increase in the initial NSE was observed with increasing storage temperature, the initial NSE ranging from typically ca. 30% at 80 °C to ca. 90% at 160 °C. As the storage time increased the NSE tended to decrease, this effect being most



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significant for storage at 160 °C; however, in no cases was the sample fully saturated with NOx, while only slight differences in NSE were discerned for the four samples after 15 min of storage. As a reference, results for Pt-Zr are included in Fig. 4 for NOx storage conducted at 120 °C. PtZr exhibited much lower NSE compared to the Ce-based samples, showing that the introduction of Ce into the ZrO2 lattice significantly modified NOx storage behavior. Moreover, the Ce-Zr samples also showed superior NSE compared to Pt/CeO2 (“Pt-Ce”), as shown in Fig. S2, which compares Pt-Ce, Pt-Zr and Pt-20Ce. Turning to NOx desorption, NOx profiles during TPD are compared in Fig. 5. Regardless of the NOx storage temperature two NOx desorption events were observed during NOx-TPD, one occurring at ca. 200 °C and the other occurring at ca. 360 °C. In all cases the high temperature desorption peak was more intense than the low temperature one, although the low temperature peak became relatively more intense as the storage temperature increased. This is consistent with previous observations that the relative amount of NOx released at low temperature ( 80Ce-20Zr > 60Ce-40Zr > 40Ce-60Zr > CeO2 > Zr(OH)4. These results were attributed to the formation of oxygen vacancies upon the introduction of Zr4+ into the CeO2 structure, which can be formally expressed as: CeO2 + xZr4+ → Ce1-xZrxO2 → Ce1-xZrxO2-y + 1/2yO2

(1)

Such vacancies are clearly implicated in NOx storage, as demonstrated in a number of previous studies,17,20,31,32 e.g.: Ce3+-☐ + NO → Ce4+-NO-

(2)

Ce4+-NO- + Ce4+-O* → Ce4+-NO2- + Ce3+-☐

(3)

Ce3+-□ + NO2 → Ce4+-ONO-

(4)

Ce4+-ONO- + NO2 → Ce4+-NO3- + NO

(5),

where □ represents an oxygen vacancy. Against this background, the promoting effect of the precious metal can be readily understood. First, Pt and Pd sites can readily adsorb NO (as shown by the foregoing DRIFTS data), which can spill over onto the ceria surface with subsequent formation of nitrite (reactions (2) and (3)). Secondly, if oxidation of NO occurs on the metal, the resulting NO2 can react with the ceria surface to form nitrites and nitrates via reactions (4) and (5), respectively. As shown in Fig. 4, increasing the storage temperature significantly improved NSE, the 1 min NSE at 160 oC in each case being approximately three times higher than that at 80 oC. Similar


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behavior was observed previously for 1% Pt/Al2O3,11 the increased NSE at higher temperatures being attributed to the increased activity of Pt for NO oxidation, with subsequent NO2 storage as nitrate. DRIFTS results during NOx storage demonstrated that more NOx was stored as nitrite on Pt-60Ce than Pt-20Ce. Several studies have demonstrated that NO can be stored directly on CeO2 as nitrite, without the pre-requisite for NO oxidation to NO2.14,16,18,33 In contrast, on ZrO2 NOx is stored mainly as nitrate.19,28 Hence, it follows that increasing the Ce content of the support should favor nitrite formation. Additionally, it is well known that NO oxidation is very sensitive to the Pt particle size and oxidation state.34-36 Since similar Pt dispersions were obtained for Pt-20Ce and Pt-60Ce, NO oxidation activity should be mainly related to the oxidation state of Pt, oxidized Pt showing poor NO oxidation activity due to poor NO adsorption.11 During DRIFTS/NOx-TPD, in all cases the nitrate bands present were observed to initially increase in intensity, as previously seen for Pt/Al2O3.11 In the case of Pt-promoted CeO2-ZrO2, the oxidation of nitrite to nitrate during TPD is expected due to the existence of mobile oxygen. Additionally, at high coverages nitrite disproportionation may occur, with simultaneous nitrate formation and NO release.37 Compared to Pt-20Ce, Pt-60Ce showed more significant growth in the nitrate bands with increasing temperature which may be explained by two factors. First, relatively more nitrite species were formed on Pt-60Ce than Pt-20Ce during NOx storage, therefore, more nitrates were generated through oxidation or disproportionation of nitrite on Pt60Ce than on Pt-20Ce during TPD. Second, improved oxygen mobility in the 60Ce-40Zr solid solution relative to 20Ce-80Zr may have favored the oxidation of nitrite to nitrate. This suggests that the formation of nitrite during NOx storage isn’t necessarily beneficial with respect to NOx desorption behavior. Indeed, this was confirmed by the microreactor data which showed a lower NOx desorption efficiency (< 350 °C) for Pt-60Ce as compared to Pt-20Ce.



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NO chemisorption on Pt (or Pd) is described in terms of a donor-acceptor mechanism, involving electron transfer from the NO-5σ molecular orbit to the metal and back-donation of metal delectrons to the 2π* level.38 Therefore, the Pt oxidation state can be predicted from the formation and band intensity of Pt-NO species by DRIFTS. The observation of an intense Pt-NO band for Pt-Zr indicated that Pt possessed significant metallic character, while the low intensity of the PtNO band for Pt-60Ce suggests that Pt was mainly present in an oxidized state. According to DRIFTS, the degree of Pt oxidation followed the ranking: Pt-Zr < Pt-20Ce < Pt-60Ce, which is in accordance with XPS results (Table 1) showing that the percentage of metallic Pt decreased with Ce content (67% for Pt-Zr vs. 33% for Pt-60Ce). Besides its effect on catalyst activity for NO oxidation to NO2 (or nitrite oxidation to nitrate), the Pt oxidation state can also influence the kinetics of NOx adsorption. Two mechanisms can be proposed for NOx adsorption on Ce-Zr mixed oxides: (i) Pt-assisted NOx adsorption, involving the adsorption of NOx on Pt, followed by spillover of either NO or NO2 onto the CeO2-ZrO2 surface, and (ii) adsorption of NOx directly from the gas phase onto the surface of the mixed oxide.19,39 At low temperature (80 and 120 oC), the Pt-based NOx adsorption pathway significantly contributes to the total NOx adsorption, which explains why Pt-20Ce was superior to Pt-60Ce (the latter containing more oxidized Pt), especially for short storage periods (1-2 min). The slower kinetics of NOx storage was also demonstrated by the slower growth of the NOx bands on Pt-60Ce than on Pt-20Ce. However, at higher temperatures (e.g., 160 °C), the contribution of direct NO adsorption on the support becomes significant due to the improved oxygen mobility. Indeed, as demonstrated in Fig. S3, a significant improvement in NSE was obtained on the unpromoted 20Ce-80Zr when storage temperature increased from 120 °C to 160 °C. However, the Pt-20Ce consistently achieved significantly higher NSE than the unpromoted 20Ce-80Zr. Therefore, the Pt-assisted


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NOx storage should still dominate for Pt-promoted samples. Moreover, Pt also improved NOx desorption behavior. As shown in Fig. S3, Pt-20Ce displayed much less unwanted NOx release below 200 oC than the unpromoted 20Ce-80Zr. With respect to the function of the precious metal in NOx storage and desorption, our results show that Pt affords superior NSE in comparison to Pd. Given that DRIFTS measurements reveal that more nitrate was formed on Pt-20Ce relative to Pd-20Ce, the improved NSE of the Pt sample can be attributed to the fact that Pt is a highly effective NO (and nitrite) oxidation catalyst. However, Pd improved NOx desorption behavior below 350 °C, which can be ascribed to two reasons based on the DRIFTS results: (1) comparatively more NOx was stored as less thermally stable nitrite on Pd-20Ce than on Pt-20Ce during NOx storage; and (2) according to DRIFTS data (Fig. 9) oxidation of nitrite to nitrate occurred to a lesser extent on Pd-20Ce than on Pt-20Ce during subsequent NOx-TPD. Hence, the nitrite species formed during NOx storage mainly underwent decomposition with release of NOx rather than being oxidized to more stable nitrate species. Overall, these findings are very similar to the results of our recent study concerning NOx storage and release from Pt/CeO2 and Pd/CeO2.14 In order to realize optimal performance it is necessary to achieve a balance between NOx storage and desorption behavior, which can be accomplished by incorporating both Pt and Pd as promoters. During NOx storage the initial NSE of Pt-Pd-20Ce fell between the values of the Pt and Pd samples, NOx being stored as both nitrate and nitrite. During subsequent TPD, Pt-Pd20Ce behaved similarly to Pd-20Ce, fairly minimal growth of the nitrate bands being observed in DRIFT spectra. No doubt, the overall NOx performance of Pt-Pd-20Ce can be further improved by tuning the Pt to Pd ratio. Further, Pd incorporation alleviated the metal particle sintering during aging, as demonstrated by the fact that the Pt particle size on 20Ce-80Zr increased from



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ca. 2 nm before aging to 9.6 nm after aging (data not shown), whereas only a small growth in metal particle size was observed for Pt-Pd-20Ce (3.0 nm vs 4.1 nm) after aging. This finding is consistent with results reported by Graham et al.,40 the inhibition of Pt particle sintering being ascribed to formation of a Pt-Pd alloy. Moreover, Kaneeda et al. reported significant improvement in the NO oxidation activity of supported Pt after aging under lean conditions upon adding Pd.41 To understand the origin of the improved NSE for Pt-Pd-20Ce after aging, combined CO chemisorption/DRIFTS measurements were performed on both fresh and aged samples. The samples were first reduced at 300 oC in a 10% H2/N2 flow for 20 min then purged with Ar at 400 o

C. CO chemisorption was carried out at room temperature in a 0.5% CO/Ar flow. As shown in

Fig. 11, two CO absorption regions can be assigned based on the literature:24,

42-44

(1) CO

adsorption on the support (i.e., carbonate region) below 1700 cm-1 and (2) CO adsorption on the metal sites between 2200 and 1700 cm-1, which is discussed below. Pt-20Ce exhibited five CO absorption bands which were related to CO adsorbed on different Pt sites, whereas Pd-20Ce mainly showed two CO bands. For Pt-Pd-20Ce, the bands observed between 2200 and 1700 cm-1 showed mixed features, indicating that CO was adsorbed on both Pt and Pd sites. Moreover, a new feature at 1880 cm-1 appeared on Pt-Pd-20Ce, which is believed to be related to CO adsorbed on a Pt-Pd alloy.40 However, this new band disappeared after aging. The overall CO absorption spectrum was similar to that for Pd-20Ce, particularly as evidenced by the observation of a band at ca. 1927 cm-1 which was also observed for the Pd sample. Moreover, the bands between 2200 and 1700 cm-1 were less intense after aging. Evidently, Pd became the dominant component for CO adsorption on Pt-Pd-20Ce after aging, although the contribution of CO adsorbed on Pt sites cannot be ruled out from the spectra. This finding infers that partial


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segregation of the more abundant Pd atoms from the Pt-Pd alloy took place during aging. Further, XPS analysis (Table 1) showed a decrease in the atomic Ce/Zr ratio after aging, indicating that the surface is less Ce-rich after aging. Simultaneously, the Pt/(Ce+Zr) ratio increased from 0.0032 to 0.0075, implying that Pt became more exposed after aging. Indeed, this surface enrichment of Pt is consistent with the improved NOx storage efficiency observed for the aged Pt-Pd-20Ce.

5. Conclusions Pt- and Pd-promoted Ce-Zr mixed oxides were studied for passive NOx adsorber applications. Doping Ce cations into the ZrO2 lattice significantly improved NOx storage efficiency at short storage times. Compared to Pd, the use of Pt as a promoter significantly improved NOx storage capacity due to its superior NO oxidation activity. However, Pd significantly improved low temperature NOx desorption behavior, as demonstrated by DRIFTS measurements showing that NO was principally stored as nitrite and that during subsequent TPD the nitrite species mainly underwent decomposition with release of NOx rather than being oxidized to more stable nitrate species. Overall, 0.5%Pd-0.5%Pd/Ce0.2Zr0.8O2 exhibited superior overall NOx performance over its Pt and Pd analogs as a consequence of improved NSE (relative to 1%Pd/Ce0.2Zr0.8O2) and improved low temperature NDE (relative to 1%Pt/Ce0.2Zr0.8O2). The co-promoted catalyst also performed well after hydrothermal aging, the decreased NDE of the aged sample being compensated by improved NSE. Consequently, the amount of NOx that could be stored at 120 °C and thermally released below 350 °C was the same for the fresh and aged samples, albeit the decreased NDE of the aged sample may be undesirable from the standpoint of repeated storage-desorption cycles. Hence, for practical application, careful attention would need to be



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paid to NDE at the peak exhaust temperature of the given drive cycle, such that the PNA could be effectively regenerated over multiple cycles.

Supporting Information Figures showing the correlation between the (111) diffraction peak position and Zr content of the Ce-Zr mixed oxides; the comparison of NOx storage efficiency for Pt-20Ce, Pt-Zr and Pt-Ce; and the comparison of NOx storage and desorption behavior for bare and Pt-promoted 20Ce80Zr.

Acknowledgements The authors thank Shelley Hopps for XRD measurements and Dr. Dali Qian for curve fitting of XPS data. Drs. Christine Lambert and Joe Theis of Ford Motor Co. are thanked for helpful discussions. This project was funded by the National Science Foundation and the U.S. Department of Energy (DOE) under award no. CBET-1258742. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.

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Table 1. Physical properties of the catalysts Catalyst

BET SA (m2/g)

Vp (cm3/g)

Mean pore radius (nm)

Mean metal Particle size (nm)

Pt0/(Pt0+Ptn+) atomic ratioa

Pt/(Ce+Zr) atomic ratioa

Pt-Zr Pt-20Ce Pt-40Ce Pt-60Ce Pt-80Ce Pd-20Ce Pt-Pd-20Ce Pt-Pd-20Ce (aged)

72.2 88.8 107.2 111.9 109.7 -86.4 41.2

0.346 0.330 0.352 0.328 0.234 -0.349 0.231

9.588 7.421 6.557 5.871 4.264 -8.065 11.219

1.99 1.92 1.98 1.88 2.34 2.40 3.04 4.13

0.68 0.57 -0.33 -----

0.0107 0.0142 --0.0170 --0.0032 0.0075

a

Ce/Zr atomic ratioa.b --0.166 (0.25) --0.975 (1.5) --0.202 (0.25) 0.174 (0.25)

From XPS data. Corresponding data for Pd could not be determined due to overlap of the Pd and Zr lines. Bulk Ce/Zr ratio given in parentheses

b


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Table 2. DRIFTS assignments of adsorbed NOx species Surface species NO adsorbed on Pt NO adsorbed on Pt Bridging bidentate nitrate Chelating bidentate nitrate Monodentate nitrate Monodentate nitrite Bidentate nitrite

Band positions (cm-1) 1791 1776 ~1610 1225-1170 1590-1550 1300-1260 1530-1500 1290-1250 1470-1375 1206-1065 1314-1265 1203-1176


Assignments ν(N=O) ν(N=O) νas(NO3-) split νas(NO3-) split νas(NO3-) split ν(N=O) ν(N-O) ν(N=O) ν(N-O)


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Figure Captions Fig. 1. Powder X-ray diffraction patterns of the support materials Fig. 2. High resolution STEM images and EELS line profiles of Pd and Pt nanoparticles hosted on 20Ce-80Zr (a-f) and Pt nanoparticles hosted on 60Ce-20Zr (g-i). Fig. 3. TPR profiles of Pt- and Pd-promoted materials Fig. 4. NOx storage efficiency (based on cumulative NOx fed) measured at 80 °C, 120 °C and 160 °C Fig. 5. NOx-TPD profiles obtained after NOx storage at 80 °C, 120 °C and 160 °C Fig. 6. NOx desorption efficiency (a) and amount of NOx desorbed below 350 °C (b) after NOx storage at three different temperatures Fig. 7. Comparison of NOx adsorption and desorption for samples containing 20Ce-80Zr (NOx stored at 120 °C for 15 min); a: NSE (based on cumulative NOx fed); b: NOx-TPD; c: NDE; d: amount of NOx desorbed below 350 °C Fig. 8. DRIFT spectra collected during NOx storage at 100 °C Fig. 9. DRIFT spectra collected during NOx-TPD (NOx stored at 100 °C in NO/O2 flow for 30 min). Fig. 10. Comparison of DRIFT spectra for fresh and aged Pt-Pd-20Ce (top: during NOx storage at 100 °C; bottom: during subsequent NOx-TPD in 5% O2/Ar flow). Fig. 11. Comparison of DRIFT spectra collected after CO chemisorption at 25 °C.


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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Figure 7.



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Figure 8.


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Figure 9.



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Figure 10.


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Figure 11.



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338x190mm (96 x 96 DPI)


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