Nanoscale Structure of Oxidized and Reduced Rhodium-Loaded ZrO2

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Nanoscale Structure of Oxidized and Reduced Rhodium-Loaded ZrO2CeO2 Catalysts Jian Chen,† Hongying Jiang,‡ Hui Qian,† and Marek Malac*,† † ‡

National Institute for Nanotechnology, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada Toyota Motor Engineering and Manufacturing North America Inc., 1555 Woodridge Avenue, Ann Arbor, Michigan 48105, United States ABSTRACT: Transmission electron microscopy (TEM) was used to investigate microstructure and chemistry of Rh-loaded ZrO2CeO2 catalyst exposed to oxidizing and two types of reducing atmospheres. In the oxidized sample Rh was dispersed on the ceriazirconia support, while metallic Rh was precipitated in both H2 and CO reduced samples. The size distribution of Rh particles in the H2-reduced sample is narrower than in samples reduced in CO. Electron energy-loss spectroscopy (EELS) fine structure indicates that both Ce4+ and Ce3+ coexist in the oxidized sample while the reduced sample appears to be predominantly Ce3+. We discuss the orientation relations of Rh and the ceriazirconia support and the formation of superstructure in ceria-zirconia support in the reduced samples. We also briefly discuss the thermodynamics of the Rh particle reduction and on ordering in the yttrium/ lanthanum-doped ceriazirconia.

1. INTRODUCTION Automotive three way catalysts (TWC) are often based on Rh-loaded CeO2.1 They utilize the high activity and selectivity of CeO2 for reduction NO to N2.2 In TWC, cerium in ceria (CeO2) reverses between mixed +3 and +4 valence states resulting in high oxygen storage capacity (OSC).3 The application of pure ceria is limited due to its high reduction temperature and problems with sintering. Moreover, ceria reduction can result in ordering of oxygen vacancies and lowering of oxygen mobility. These drawbacks can be overcome by mixing ceria with other oxides such as zirconium oxide, yttrium oxide, and lanthanum oxide.4 Depending on the cerium content, the ceriazirconia solid solution exist in three different structures: monoclinic, tetragonal, and cubic.5 The presence of cubic ceria appears to result in high OSC.6 Adding yttrium and lanthanum oxide helps to stabilize the cubic ceria within the redox reaction temperature range. The samples presented in this study have a nominal composition of CeO2 30%, ZrO2 60%, Y2O3 5%, and La2O3 5%. We will refer to the support material as the CZYL. Investigations of Rh-loaded CeO2ZrO2 reported before4,5,711 include X-ray absorption near edge structure (XANES),4 Raman scattering,7 diffuse reflectance FT-IR,9 and X-ray photoelectron spectroscopy (XPS)10 measurements. Here we report new results on high-resolution transmission electron microscopy (HRTEM) imaging, nano beam diffraction (NBD), and electron energy loss spectroscopy (EELS) investigation of morphology and size distribution of Rh. To our knowledge we report for the first time the orientation relationship of individual Rh particles with CZYL support in reduced samples. Furthermore we investigate samples reduced in both H2 or CO atmosphere to bring additional insight to the differences in their properties.12 The use of NBD allowed us to determine structure and orientation of Rh particles and r 2011 American Chemical Society

their relationship with CZYL support at irradiation dose lower13 than in HRTEM. This can reduce the irradiation damage artifacts.

2. EXPERIMENTAL SECTION The chemical composition and sample designation of the a CZYL support is outlined in Table 1. Rh (2 wt %) was loaded into the CZYL powder using the conventional wet impregnation method by Rh(NH3)2(NO2)2 aqueous solution to obtain Rhloaded CZYL. The fresh Rh/CZYL catalyst was ground to powder using an agate mortar and pestle. The powder was then calcinated for 2 h in air at 773 K and pressed into pellets about 1 to 1.7 mm in diameter. They were then either oxidized or reduced in H2 or CO at 1223 K for 5 h. Samples that were further calcinated at 1223 K for 5 h are referred as oxidized sample (T3_O2) in the text below. Samples that were reduced at 1223 K for 5 h in either 5% of CO balanced with N2 (referred as T3_CO) or 10% of H2 balanced with N2 (referred to as T3_H2). The microscopy was carried in a 200-kV JEOL 2200 FS field emission (scanning) transmission electron microscope equipped with cryo polepiece and an in-column energy filter. EELS from individual nanoparticles were obtained in scanning TEM (STEM) mode. NBD was preformed in a Hitachi HF 3300 TEM, a 300-kV TEM/STEM equipped with a cold field emission gun. A JEOL ARM 200F with spherical aberration corrector of the probe-forming lens, was used to collect the energy-dispersive X-ray analysis (EDX) maps. Received: December 25, 2010 Revised: May 25, 2011 Published: June 17, 2011 14173

dx.doi.org/10.1021/jp112272c | J. Phys. Chem. C 2011, 115, 14173–14179

The Journal of Physical Chemistry C

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Table 1. Composition and Sample Treatment Conditions chemical composition of support (wt %) CeO2

ZrO2

La2O3

Y2O3

Rh loading (wt %)

sample

treating conditions

T3_O2

oxidized in air, 1223 K, 5 h

29.3

60.5

5.0

5.2

2.0

T3_H2

10% H2/N2, 1223 K, 5 h

29.3

60.5

5.0

5.2

2.0

T3_CO

5% CO/N2, 1223 K, 5 h

29.3

60.5

5.0

5.2

2.0

Figure 1. (a) TEM image of sample T3_O2 and (b) broad beam SAD pattern (left) and its simulation (right). The two calculated patterns on the very right side are the ring pattern of CeO2 and ZrO2, respectively.

3. RESULTS AND DISCUSSION The overall morphology of the samples can be seen in Figure 1. Although Figure 1 shows the oxidized sample T3_O2, the overall morphology of the sample support changes little between the samples treated in various environments. The particle size of CZYL support varies from 3 to 17 nm peaking between 5 and 13 nm. The particles have well developed facets and clear lattice fringes with limited number of obvious defects. As we discuss in detail next, the Rh does not appear to precipitate in the oxidized sample whereas in the reduced sample the Rh appears to be precipitated in metallic form. We start by discussing oxidized sample followed by discussion of CO- and H2-reduced samples. We then briefly discuss thermodynamics of the reduction process and the structure of the CZYL support. 3.1. Oxidized Sample. Figure 1a shows the morphology of several CZYL particles in sample T3_O2. An example of broad beam selected area diffraction (SAD) pattern obtained a few micrometers in diameter beam is shown in Figure 1b. The right half of Figure 1b shows a simulated ring pattern of both ZrO2 and CeO2 obtained by Desktop Microscopist software. The left part (experimental) and the right part (simulation) of the ring pattern appear to match well. The reason we use the crystallographic data of ZrO2 and CeO2 for the ring pattern simulation is that the crystallographic data of CZYL is not available. The support matrix is composed mostly of ZrCe oxides. Figure 1b shows that the experimental diffraction rings are between the calculated ring of fluorite ZrO2 (red rings) and that of fluorite CeO2 (black rings) and the ratio of ring diameters in the sample agrees well with the ratios of ring diameters in CeO2 and ZrO2 calculations in Figure 1. This suggests that the CZYL support in our samples has same structure as CeO2 and ZrO2 and that the lattice parameters are between ZrO2 and CeO2. That indicates the formation of solid solution of ZrO2 and CeO2 constitutions if not taking into account the effect of YLa oxides due to their small contents in the support.

Figure 2. Bright-field STEM image of Rh in CZYL support and a typical EDX spectrum collected from a dark areas marked by arrows. The additional peaks in the EDX spectrum come from the microscope polepiece cryo box (Au) and the support grid (Cu).

We did not observe a ring that can be assigned to either Rh or Rh2O3, although the EDX does detect Rh signal. Imamura et al. compared the difference of CeO2, γ-Al2O3, and ZnO loaded with 0.1 wt % Rh and treated under the same calcination condition, respectively.14 They found Rh nanoparticles on the surface of γ-Al2O3 and ZnO, respectively, but did not find any Rh nanoparticles on the surface of CeO2. This suggests the agglomeration of Rh depends on the type of support rather than the loading amount of Rh. Taking into account the work of Imanura et al. on samples with loading significantly lower than ours and the extent of our SAD, NBD, and STEM investigations, it appears that Rh is finely dispersed in the CZYL support. The absence of Rh particles in Rh-loaded CeO2 was extensively studied.15,16 Extended X-ray absorption fine structure (EXAFS) study suggested 14174

dx.doi.org/10.1021/jp112272c |J. Phys. Chem. C 2011, 115, 14173–14179


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Figure 3. (a) HRTEM image of metal Rh particle with truncated octahedron morphology. The lower right inset shows the magnified image of the marked metal Rh. (b) NBD pattern of Rh nanoparticle. The two spots marked by white arrows are reflections from CZYL support.

that Rh3+ is substituted for Ce4+ in the surface layer of CeO2 to form RhOCe.15 The formation of RhOCe, which prevents the formation of agglomerated Rh metal particles, is described as the anchor effect of CeO2.16,17 Therefore, it is possible that RhOCe at the surface layer does not form a crystal structure distinct from the ones already present in the sample and consequently does not result in diffraction or mass contrast.14,16 3.2. CO-Reduced Sample. Figure 2 shows a typical bright field STEM image of CO-reduced sample T3_CO. The dark areas marked by arrows are Rh nanoparticles as confirmed by EDX analysis. Figure 3a shows an example of Rh nanoparticle with cubooctahedral morphology. The cubooctahedral shape (i.e., a combination of a cube and an octahedron) is often observed shape for a face-centered cubic metal cluster.18 The existence of cubo-octohedral morphology suggests that the calcination under CO provides sufficient time and thermal energy for the Rh nanoparticle to form a low energy shape. As discussed above, NBD is suitable for investigation of the crystal structure and crystallographic orientation relation between the catalyst and support. Figure 3b shows an NBD pattern of the cubooctahedral Rh nanoparticle marked in Figure 3a. The NBD patterns revealed presence of metal Rh oriented with [011] zone axis parallel to the incident electron beam. The two reflections indicated by arrows are {111} reflections of CZYL support. The two arrowed spots are aligned with reflection of {111}Rh, suggesting the corresponding planes of Rh and CZYL are parallel to each other. Therefore the crystallographic orientation relationship between the metal Rh and CZYL support is (111)Rh//(111)CZYL, [011]Rh//[011]CZYL in this case. To our best knowledge crystallographic orientation between metal Rh and CZYL was not reported before, although such orientation relationship were reported in Rh/CeO2 and Au/CeO2.18,19 3.3. H2-Reduced Sample. Cs-corrected JEOL TEM ARM 200F was used to obtain EDX chemical maps for sample T3_H2. As shown in Figure 4a, Rh maps show high Rh counts in small areas, which can be attributed to nanoparicles of metal Rh. For comparison, the corresponding images for sample T3_O2 are displayed in Figure 4b. There is no obvious agglomeration of Rh in T3_O2 sample in Figure 4b. The orientation relationship between metal Rh and CZLY appears to be the same as in the CO-reduced sample. Figure 5a shows a Rh nanoparticle that is entirely supported by CZYL support. Figure 5b is a diffractogram of the same Rh particle.

The simulated diffraction pattern is shown in Figure 5c. In the diffractogram (Figure 5b), the circled spots mark the reflections that belong to metal Rh. The spots marked by squares are the reflections originating from CZYL support. Comparison between simulated diffraction patterns and the experimental diffractograms suggests the orientation relationship (111)Rh//(111)CZYL, [011]Rh//[011]CZYL. The extra reflections rather than marked in the diffractogram originate from the superstructure of CZYL, which will be discussed in detail in the last section of the manuscript. Moire fringes can be seen in Figure 5a as result of overlapping of Rh lattice planes with CZYL (see the arrowed magnified image of Rh nanoparticle as inset in the Figure 5b). The Moire fringes are the CZYL lattice fringes in the overlapping Rh/CZYL region with a spacing that is about twice that of the CZYL lattice. In the specific orientation relationship of (111)Rh//(111)CZYL, [011]Rh//[011]CZYL, the reticular planes facing each other are {111}Rh and {111}CZYL. We take the d-spacing of {111}CZYL to be 0.298 nm. The d-spacing for {111}Rh is known to be 0.220 nm. The lattice mismatch is thus about 26%, which is too large to be matched elastically. As a consequence, a dislocation network is created in the Rh phase for compensation. Figure 6 is the enlarged image of the metal Rh nanoparticle in Figure 3a. It shows that every two lattice planes of CZYL match three planes of Rh with the creation of one edge dislocation (marked by an arrow) at the interface. In Figure 6, we are looking perpendicular to the dislocation line therefore the Rh lattice planes associated with the edge dislocation will not match the plane of CZYL. So the contrast of the CZYL lattice plane is perturbed by the edge dislocation at the Rh-CZYL interface, resulting in Moire fringes with a double spacing of the CZYL lattice planes. 3.4. Comparison between Sample Reduced by CO and H2. Because H2 always presents in exhaust gas with at a ratio of 1:3 to CO,20 it is important to compare the Rh/CZYL under H2 and CO reduction conditions in order to tailor the catalysts for oxidation of CO or H2 as desired by given application. The size distribution of the metal Rh is one of the most important properties for catalysts because small particle size and narrow size distribution is needed to ensure high surface area at which the catalytic reaction can take place. Considering the high adsorptive capacity for H2 and CO by CZYL, the size distribution of Rh measured by standard chemisorption experiments using H2 and CO gases is not accurate.21 Electron microscopy, on the other hand, can provide reliable size distribution down to about 1 to 2 nm particle size (since the atomic number of Rh and the 14175



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Figure 4. Comparison between the dark field STEM image and EDX mapping of sample T3_H2 (a) and T3_O2 (b). The images are arranged in sequence of dark field STEM image, Zr mapping, Ce mapping, and Rh mapping for both samples.

Figure 5. (a) HRTEM image of sample T3_H2. The inset on the up left is the enlarged image of metal Rh particle marked by the yellow square. (b) The experimental diffractogram of the Rh particle. (c) Simulated diffraction pattern.

Figure 7 shows the distribution of metal Rh nanoparticle diameter in T3_H2 and T3_CO. The mean size of an individual Rh particle is obtained by averaging the largest diameter and smallest diameter of each (nonspherical) particle. In comparison to T3_CO in Figure 7b, the mean particle size of metal Rh in T3_H2 shifts toward smaller sizes. Moreover, the size distribution in T3_H2 appears narrower than that of T3_CO. As mentioned in the Experimental section, the Rh/CZYL catalyst was calcinated in air at 773 K for 2 h before the reduction treatment. There is no possibility for the existence of metal Rh under this condition; however, the amount of Rh oxide is too low to be detected by X-ray diffraction (XRD). According to the work of Music et al.,22 Rh2O3 is the main phase under such conditions. When Rh2O3 is reduced by CO or H2, eqs 1 and 2 describe the corresponding chemical reaction, respectively. The equations for the free energy of the two reactions are derived according to the thermodynamic data listed in a previous work23 Rh2 O3 ðsÞ + 3COðgÞ f 2RhðsÞ + 3CO2 ðgÞ

ð1Þ

ΔG ¼ 465861 10:28T ðJÞ ðT > 773 KÞ Rh2 O3 ðgÞ + 3H2 ðgÞ f 2RhðsÞ + 3H2 OðgÞ

ð2Þ

ΔG ¼ 365541 98:31T ðJÞ ðT > 773 KÞ

Figure 6. The magnified image of Rh nanoparticle shown in Figure 3 a. The arrows indicate edge dislocations at the interface.

elements in the CZYL support are very close, Rh particles smaller than about 1 or 2 nm may be difficult to detect and are subject to limits set by electron irradiation damage). In our case, the average size for Rh metal in sample T3_H2 obtained by chemisorption is 11 nm, which does not agree with the HRTEM observations.

where ΔGo is free energy (in Joules) of the reaction. The s and g in the brackets denote solid and gas states. T is the reducing temperature (in Kelvin). Figure 8 depicts the changes of free energy of the above reactions with the reduction temperature. When reduction temperature is lower than 1140 K, ΔGo for reaction 1 is lower than that of reaction 2. That suggests Rh2O3 reduced by CO is more thermodynamically favored compared to H2 from 773 to 1140 K. According to Williams et al., the reduction kinetics of Rh2O3 by H2 or CO is almost the same when the reduction temperature higher than 673 K.11 Moreover, according to Wootsch et al.,24 CO reduction preferentially occurred at the metal/support interface while reduction of H2 happened on the surface of the loaded Rh metal. When reduction happens at the interface of Rh2O3/CZYL, the reaction can weaken the strength of CeORh bonds, in turn lessening 14176



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Figure 7. Comparison of particle size distribution for sample T3_H2 (a) and sample T3_CO (b).

Figure 10. CZYL support in sample T3_H2. The two insets are diffractograms of the two areas close to the inset (a) or marked with yellow square in (b).

Figure 8. The correlation between the free energy and reduction temperature for reactions 1 and 2.

Figure 9. The HRTEM image of CZYL support in T3_O2. The insert in (a) is the diffractogram of the largest single CZYL crystal in the image. The diffractograms in (b) are obtained from nearby crystallites.

the anchoring effect of Rh to Ce. This can explain why the mean size of Rh is larger and the size distribution is wider for

CO-reduced metal Rh nanoparticles compared to that of H2reduced Rh. 3.5. The CZYL Support. Depending on the content, ZrO2doped CeO2 exhibits three equilibrium polymorphs: monoclinic, tetragonal, and cubic structure.25 If we only take into account the composition of CeO2 and ZrO2, the composition point in the current investigation is within monoclinic plus fluorite-type cubic region. Because monoclinic phase is not desired for OSC, Y2O3, and La2O3 were added to the composition to shift the phase region from monoclinic to tetragonal plus fluorite-type cubic region.26 Figure 9 shows HRTEM images of two areas in sample T3_O2. The diffractogram shown as inset in Figure 9a depicts the {111} and {002} reflections of the [110] orientation for a fluorite structure. However, as shown in the up and low-left insets of Figure 9b, additional reflections emerged besides the reflections from the fluorite structure. Figure 10 shows the HRTEM image of sample T3_H2. The insets show diffractrograms of the CZYL particles. Compared to the diffractrograms of the oxidized sample (see Figure 9), there are two subsets of intensity maxima in the diffractograms in this Figure. The first set of maxima contains reflections only from the fluorite subcell (see reflections marked in white in the insets). The second set of diffraction maxima (see reflections marked in green in the insets) appear at 1/2{111} positions of fluorite structure. They corresponds to the supercell of pyrochlore superstructure. Pyrochlore structure is related to fluorite, AO2. Most pyrochlore have IIIIV composition with the general formula A3+2B4+2O7. Their structure may be described as an ordered cubic closepacked array of cations with the larger A3+ and the smaller B4+ 14177



Figure 11. Ce-M4,5 EELS spectra (white lines) of sample T3_O2 (a) and T3_H2 (b). The sample was treated at 1123 K.

cations located, respectively, at the eight-coordinated 16d and six-coordinated 16c sites in the space group F-d3m. They are ordered in alternate ( Æ110æ rows in every other {002} planes. Oxygen anions occupy 7/8 of the tetrahedral sites between the cations. Among the anions, six locate at the two B4+ and two A3+coordinated 48f position, and one resides at the four B4+coordinated 8b site. An unoccupied interstitial site (i.e., oxygen vacancy) at 8a is surrounded by four B4+ cations.2731 For the composition studied in the current investigation, the valence of cerium cation could be 3 or 4, and the valence of zirconium cation is always 4. Thus Y3+, La3+, and Ce3+ occupy A sites, while Zr4+ and Ce4+ take B sites in the structure. Among the reflections displayed in the two insets in Figure 10, those comply with the conditions where h+k, k+l, and l+h are multiple of four (i.e., h+k = 4n, k+l = 4n, and l+h = 4n) carry information on the fluorite subcell. Of these intensity maxima, the subset that further obeys the condition that h, k, and l are even but not a multiple of four receive no contribution from the anions (i.e., h, k, l = 4n+2). They only depend on the total cation scattering in the A and B sites. The reflections that further obey the conditions whereby h+k+l is equal to a multiple of four and individual indices are not equal to a multiple of four come from the contribution of the array of anions (oxygen ions).32 According to the reflection restriction, the 111 reflections are contributed by both ordered cations and ordered anions, while the 222 reflections are contributed by cations only. However, the 022 reflections, which usually exist in the ordered pyrochlore,29 are missing in the diffractograms in Figure 10. According to the reflection restriction, the absence of 022 reflections suggests that anions disordering happens in the microstructure. The meaning of anion disordering is the anions (i.e., oxygen ions) randomly occupy the 48f, 8b, and 8a site. As described in the pyrochlore crystal structure above, 8a site is unoccupied in ordered pyrochlore, thus the random occupancy of 8a site upon anion disordering also means the disordering of oxygen vacancy. Obviously, the disordering of oxygen vacancy is good for the mobility of oxygen which can lead to the significant improvement of OSC for CZYL based catalysts compared to other support materials. Figure 11 shows energy-loss near edge structure (ELNES) of Ce-M4,5 of the support in Rh/CZYL before and after reduction by H2 atmosphere, respectively. The EELS spectra were obtained in STEM mode. The white lines of Ce-M4,5 for sample T3_O2 are located at 884.0 and 901.5 eV are separated by 17.5 eV (see spectrum a) in Figure 11). They are followed by lower intensity, broader maxima. The energy separation of the white lines reflects transitions of 3d core electrons to unoccupied 4f-shell. In comparison

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with sample T3_O2, the Ce-M4,5 edges of sample T3_H2 is different in energy loss and shape (see spectrum b) in Figure 11). The main peaks are at 882.0 and 899.7 eV, respectively. Furthermore, the Ce-M4,5 appears to have shoulders (marked by arrows) at the lower energy-loss side in sample T3_O2. According to Garvie et al.,33,34 these shoulders can be assigned to Ce3+. In the experiment, the EELS spectrum collection time is 2 s. The corresponding electron dose is 780 e/(Å2). There are reports on the electron-beam-induced reduction of Ce4+,34,35 but we found the changes in the spectrum became noticeable only when the acquisition time was beyond 32 s. The corresponding electron dose is 1.25 104 e/(Å2). In the in situ TEM study of CeO2ZrO2 loaded with Pt, Arai et al. found that only when the electron dose was higher than 9.0 105 e/(Å2), Ce4+ in CeO2ZrO2 was reduced to Ce3+.36 Thus it is unlikely that Ce3+ in the CZYL support is from the reduction of Ce4+ by electron beam irradiation. The relative intensity I(M4)/I(M5) for the while lines is associated with the 4f-shell occupancy; thus it can be used to determine the valence of cerium cations.3335 Because the change in the white line intensity of Ce is pronounced, we can obtain I(M4)/I(M5) simply by removing background beneath the M4,5 edges and integrating the M4 and M5 intensities as recommended.37 The observed ratio of white line intensity in sample T3_O2 and T3_H2 is 1.68 to 1.47, respectively. This indicates that more Ce4+ was reduced to Ce3+ in the reduced sample compared to the oxidized sample.

4. CONCLUSIONS We investigated the microstructural and chemical changes of Rh/CZYL treated in oxidizing and reducing atmosphere. STEM and HRTEM fail to locate any Rh or Rh oxide nanoparticle in the oxidized sample; however, nanoparticles with very high Rh content are observed in H2 - or CO-reduced samples. NBD identifies they are metal Rh. Compared to those in CO-reduced sample, the morphologies of metal Rh nanoparticles in H2reduced sample are similar, but its size distribution is narrower and the mean size is larger. NBD and experimental diffractogram analysis reveal one orientation relationship ((111)Rh//(111)CZYL, [011]Rh//[011]CZYL) between metal Rh and CZYL exists in the reduced samples. While Ce4+ and Ce3+ coexist in the oxidized sample, more Ce4+ has been reduced to Ce3+ for sample reduced by H2. Oxygen vacancy disordering was found in the support in the H2-reduced sample, which is good for the oxygen storage capacity (OSC) of the catalyst. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 780-641-1662. Fax: 780-641-1601.

’ ACKNOWLEDGMENT We would like to thank Dr. Okunishi and Dr. Kawasaki at JEOL Ltd. for analysis using Cs-corrected JEOL ARM 200F. Toyota Engineering and Manufacturing North America and National Research Council National Institute for Nanotechnology are acknowledged for financial support for this project. 14178



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Nanoscale Structure of Oxidized and Reduced Rhodium-Loaded ZrO2

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