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Comparison of precious metal doped and impregnated perovskite oxides for TWC application
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Comparison of precious metal doped and impregnated perovskite oxides for TWC application Sotirios A. Malamis a,b , Rachael J. Harrington a,c , Michael B. Katz a,d , David S. Koerschner a , Shuyi Zhang a , Yisun Cheng c , Lifeng Xu c , Hung-Wen Jen c , Robert W. McCabe c,∗ , George W. Graham a,∗ , Xiaoqing Pan a,∗ a
University of Michigan, Ann Arbor, MI 48109, USA University of Houston, Houston, TX 77204, USA c Ford Motor Company, Dearborn, MI 48121, USA d Naval Research Laboratory, Washington, DC 20375, USA b
a r t i c l e
i n f o
Article history: Received 25 October 2014 Received in revised form 21 November 2014 Accepted 27 November 2014 Available online xxx Keywords: Palladium (Pd) Rhodium (Rh) Lanthanum ferrate (LaFeO3 ) Calcium titanate (CaTiO3 ) Scanning transmission electron microscopy (STEM) CO oxidation
a b s t r a c t Four high-surface-area (50–60 m2 /g) perovskite-based powder catalysts, Pd-doped LaFeO3 , Pdimpregnated LaFeO3 , Rh-doped CaTiO3 , and Rh-impregnated CaTiO3 , were characterized by scanning transmission electron microscopy and CO oxidation measurements in fresh and redox-aged (14 h at 800 ◦ C) states. Both intrinsic catalytic activity and stability were significantly higher in Pd-doped LaFeO3 than in Rh-doped CaTiO3 under lean to stoichiometric reaction conditions. Activities of doped were initially lower than impregnated versions, though aging led to convergence in the catalytic performance for both systems. A mixture of metallic and cationic forms of Pd appear to contribute to activity in the Pd catalysts, whereas metallic Rh particles, which can easily segregate onto the surface of CaTiO3 in Rh-doped CaTiO3 upon aging, are the likely source of catalytic activity in the Rh catalysts. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Perovskites as catalysts have a history that spans several decades [1,2]. Interest in their possible application for automotive emissions control was initially stimulated by a study reported in Nature more than four decades ago [3]. Subsequent research has considered a variety of perovskites involving both base metals and Pt-group metals as dopants and supported particles [e.g., 4–6]. A little more than 10 years ago, researchers at Daihatsu Motor Company and their collaborators proposed a novel concept involving perovskites as hosts for precious metals in three-way catalysts (TWCs), according to which self-regeneration would naturally occur under normal TWC operating conditions [7,8]. The selfregeneration process was based on the idea that the precious metal should be stable as a dopant, substituting for the B-site cation in the perovskite under oxidizing conditions, but that is would tend
∗ Corresponding authors. Tel.: +1 734 763 6661; fax: +1 734 763 4788. E-mail addresses: [email protected] (R.W. McCabe), [email protected] (G.W. Graham), [email protected] (X. Pan).
to precipitate out of the perovskite as a metal nanoparticle under reducing conditions. Due to the oscillation in air-to-fuel ratio normally imposed to ensure its precise control around stoichiometry, the precious metal nanoparticle should, in principle, undergo alternating cycles of re-dissolution and precipitation, thus ensuring that the nanoparticles would not coarsen significantly. Considerable interest, created by reports that the self-regeneration process could significantly reduce precious metal usage in TWCs [9], led to a number of investigations of these and related systems [e.g., 10–14]. Relatively little effort has been expended to carefully probe the nature of the self-regeneration process, however, and only a few studies have been able to establish the fundamental relationship between structure and catalytic activity of these materials [12]. We thus recently used scanning transmission electron microscopy (STEM) to gain new insight into the self-regeneration process [15,16]. Here, we focus on structure–activity relationships, comparing our initial catalytic activity results from two of the most prominent systems [9], Pd-LaFeO3 and Rh-CaTiO3 , using CO oxidation as a probe reaction. Our primary goals were to (1) determine and contrast the intrinsic catalytic activities of Pd-doped LaFeO3 (LFO) and Rh-doped CaTiO3 (CTO), (2) compare their catalytic
http://dx.doi.org/10.1016/j.cattod.2014.11.028 0920-5861/© 2014 Elsevier B.V. All rights reserved.
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performances with those of Pd-impregnated LFO and Rhimpregnated CTO, respectively, and (3) assess the stabilities, or aging characteristics, of all four catalysts. Fracon of CO Converted
High-surface-area (50–60 m2 /g) powders of both pure perovskites, Pd-doped LFO, and Rh-doped CTO were provided by Cabot Corporation. Doping levels were nominally 5% of the Fe and Ti (Bsite) cations. Comparable levels of Pd (2.2 wt%) and Rh (2.12 wt%) were loaded onto the surfaces of the pure LFO and CTO powders by wet impregnation of tetraamminepalladium(II) nitrate and rhodium nitrate solution, respectively, to incipient wetness, followed by calcination in air (1 h at 600 ◦ C). Small samples of all four precious metal-containing powders, for STEM (JEOL 2100F) analysis, were aged in a quartz boat in a quartz tube furnace at 800 ◦ C under a flow of alternating reducing (1% H2 /N2 ) or oxidizing (2% O2 /N2 ) gas, each of 10 min duration, for a total of 14 h. Heating and cooling of the samples was performed under N2 , and the last half cycle at 800 ◦ C was 2% O2 /N2 . In addition, Rh-CTO samples were aged for 3 h, using the alternating gas treatment above. STEM specimens were prepared by simply dipping carbon grids into the powder samples. Measurements of the size of several (of order 100) metal and perovskite particles in the resulting STEM images were performed manually. Larger samples, for catalytic activity measurement, were mixed with alumina (50 wt%) from Sasol (TH100-150), pressed into pellets, crumbled and sieved (40–80 mesh), and then placed into a packed-bed quartz reactor, which was also used for in situ aging following the same 14 h protocol used for the STEM samples. XRD (Rigaku Minitab) was performed on some of the reactor samples before and after aging. For comparison, an Al2 O3 -supported Pd catalyst (1.4 wt% Pd on Sasol TH100-150 alumina) was also prepared and mildly aged, yielding a Pd dispersion of 14.4% as measured by CO chemisorption. The packed-bed quartz reactor used a vertical tube, 1 cm in diameter, with gas inlet at the top. Quartz wool plugs, approximately 0.75 cm long, were used to keep the catalyst bed, which was between 2 and 2.5 cm long, in place. Mass flow controllers were used to set and maintain the flow rates of CO, O2 , and N2 at a total flow rate of 2 L/min. The reactor system was also set up with a bypass channel, used to make inlet measurements of each species. When the flow was directed through the reactor, a pump was used to overcome the slight pressure drop created by the catalyst bed. The concentrations of CO and O2 were measured with an FTIR spectrometer and lambda sensor, respectively, and catalyst bed temperature was measured with a thermocouple contacting the bed at the gas exit. Using a LabView program to control the reactor, each catalyst sample was subjected to three cycles of data collection, designed to test its activity under varying gas composition, ranging between lean and slightly rich. Cycle 1 began by heating the reactor to 500 ◦ C under a flow of 10% O2 /N2 for 1 h. The reactor was then cooled to below 60 ◦ C, after which a series of light-off tests was conducted. A single light-off test involved feeding a fixed ratio of O2 and CO into the reactor and ramping the temperature of the reactor to between 300 and 400 ◦ C at 10 ◦ C/min. Once the limiting conversion was reached, the reactor was cooled to below 60 ◦ C, and the next test began. A series of five light-off tests was conducted in this first cycle, with the O2 concentration fixed at 0.8% and the CO concentration increasing in the order 0.3, 0.5, 1, 1.4, and 2%. Upon completion of Cycle 1, a second series of light-off tests, Cycle 2, was conducted in reverse order. Specifically, with the O2 concentration fixed at 0.8%, the first light-off test was performed with a CO concentration of 2%, followed by consecutive tests with progressively lower CO concentrations, ranging back down to 0.3%.
1 0.8 Cycle 1 0.6
Cycle 2 Cycle 3
0.4 0.2 0 50
100
150
200 250 Temperature (°C)
300
350
400
(b)
Fresh Rh-doped CTO [CO] = 1.35% 1 Fracon of CO Converted
2. Materials and methods
(a)
Fresh Pd-doped LFO [CO] = 1.3%
0.8 Cycle 1 0.6
Cycle 2 Cycle 3
0.4 0.2 0 50
100
150
200 250 Temperature (°C)
300
350
400
Fig. 1. Light-off curves from Cycle 1, 2, and 3 tests performed on fresh (a) Pd-doped LFO and (b) Rh-doped CTO catalysts. [O2 ] was fixed at 0.8% for both.
Finally, a third series of light-off tests, Cycle 3, which was identical to Cycle 1, was conducted. Cycles 2 and 3 thus provide indications of the stability and reversibility, respectively, of the state of the catalyst. 3. Results Light-off curves (i.e., fraction of CO converted vs. temperature) for [CO] = 1.3% from Cycle 1, 2, and 3 tests performed on the fresh Pd-doped LFO catalyst are shown in Fig. 1(a). Aside from shifts of a few ◦ C to lower temperature of Cycle 2 curves for some of the CO concentrations, curves for all three cycles were found to be nearly identical at each CO concentration. The catalyst thus appears to be quite stable under these reaction conditions. Further, similar tests performed on pure LFO revealed that its light-off temperature (at which half of the CO is converted) is approximately 100 ◦ C higher than Pd-doped LFO, indicating that catalytic activity is primarily determined by Pd doping. For comparison, the light-off curves for [CO] = 1.35% from Cycle 1, 2, and 3 tests performed on the fresh Rh-doped CTO catalyst are shown in Fig. 1(b). Clearly, this catalyst is not stable in its initial state: within the time span of just one test, from the end of Cycle 1 to the end of the first test of Cycle 2, where the ratio [CO]/[O2 ] was 2.5 at the reactor inlet, the light-off temperature dropped by 150 ◦ C, and this difference was essentially retained throughout the entire series of Cycle 2 tests. As discussed below, the increase in activity is thought to be due to the reduction of Rh cations on the surface of the doped perovskite. Further, this rapid change in the state of the catalyst is not reversible under our test conditions, as indicated by the significant shift between Cycle 1 and Cycle 3 lightoff curves. Tests performed on pure CTO, which revealed that its light-off curve is shifted more than 150 ◦ C higher than the Cycle 1 curves of fresh Rh-doped CTO, indicate that the catalytic activity of the initial state is primarily determined by Rh.
Please cite this article in press as: S.A. Malamis, et al., Comparison of precious metal doped and impregnated perovskite oxides for TWC application, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.11.028
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Fresh Pd-impregnated LFO [CO] = 1.35%
CO Oxidaon Rate [O2] = 0.8%, [CO]=1.3% 0
Ln(TOF) [mol-CO/mol-sites]
Pd/Al2O3 Rh-doped CTO, Cycle 1
-1.5
Pd-doped LFO, Cycle 1
Ea=28.0 kcal/mol
Ea=16.7 kcal/mol
Ea=23.9 kcal/mol
-2 Pd-doped LFO, Cycle 3
0.8 0.6
0.4 0.2
Ea=15.2 kcal/mol
-2.5 -3 0.0016
Fracon of CO Converted
1
-0.5 -1
3
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0.0022
1/T (K-1)
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Fig. 3. Light-off curves from Cycle 1, 2, and 3 tests performed on the fresh Pdimpregnated LFO catalyst. [O2 ] was fixed at 0.8%.
Fig. 2. CO oxidation rates, expressed as turn over frequencies (TOFs), for fresh Pd-doped LFO and Rh-doped CTO, derived from Cycle 1 light-off tests. The TOF derived from Cycle 3 is also shown for Pd-doped LFO. The TOF for a mildly aged Al2 O3 -supported Pd catalyst, derived from the Cycle 3 light-off test, is shown for comparison.
CO Oxidaon Rate, Cycle 3 [O2] = 0.8%, [CO] = 1.3% -2.5
To present these results on a more quantitative basis, the rates of CO conversion, taken between 5 and 20% (in order to yield sufficiently high signal-to-noise data without grossly exceeding differential conversion conditions), were plotted vs. inverse absolute temperature for the Cycle 1 tests with [CO] ≈ 1.3% for both catalysts, as shown in Fig. 2. The rates, specified as a turn-over frequency (TOF = rate of CO converted per active site), were obtained by assuming that the active sites correspond to surface cations randomly populating B-sites at the nominal doping level (5%), that the surface exposes A and B-sites with equal probability, and by using the measured BET areas (58.9 m2 /g for Pd-doped LFO, 61 m2 /g for Rh-doped CTO). The same assumptions were made in plotting the rate of CO conversion for the Cycle 3 test with [CO] = 1.3% for the Pd-doped LFO catalyst. The TOF for CO oxidation over the Al2 O3 -supported Pd catalyst, measured under corresponding conditions (where Pd is expected to be in the form of metallic particles), is also plotted in Fig. 2. The apparent activation energy for the Al2 O3 -supported Pd catalyst, 28.2 kcal/mol, is in reasonable agreement with typical values for metallic Pd [17], whereas that for Cycle 1 of the fresh Rh-doped CTO catalyst, 24 kcal/mol, is somewhat lower. The apparent activation energies for the fresh Pd-doped LFO catalyst (Cycles 1 and 3) are clearly much lower (15–17 kcal/mol). Due to this difference in apparent activation energies, the TOF for the fresh Pd-doped LFO catalyst, where the Pd sites are presumably still part of the perovskite, would become comparable to that of the Al2 O3 -supported Pd catalyst at low temperature (∼160 ◦ C), assuming these rates could be extrapolated. The situation for the impregnated catalysts is somewhat different, depending on the metal and its perovskite support. In the Pd-LFO system, the light-off curves for the fresh Pd-impregnated LFO catalyst tend to rise more rapidly with increasing temperature than for the fresh Pd-doped LFO catalyst (Fig. 1(a)), as shown in Fig. 3. More significantly, the light-off temperatures are at least 80 ◦ C lower for the impregnated catalyst, indicative of higher catalytic activity, as might be expected in view of the fact that all (or most) of the Pd should be on the surface. A different dependence of light-off temperature on [CO] was also observed (supplemental data, Fig. A1). The aging treatment was found to have little effect on the light-off temperatures of the Pd-impregnated LFO catalyst, whereas those of the Pd-doped LFO catalyst were lowered by about 80 ◦ C (supplemental data, Fig. A1). Thus, the Pd-doped and
ln(rate) [mol-CO/mol-Pds]
Pd-impregnated LFO (fresh) -3
Ea=13 kcal/mol
Pd-impregnated LFO (aged) Ea=16.7 kcal/mol
-3.5 -4 -4.5
Pd/Al2O3 Ea=28.0 kcal/mol
Ea=13.5 kcal/mol
Pd-doped LFO (aged) -5 -5.5 0.002
0.0021
0.0022
0.0023
0.0024
0.0025
1/T (K-1)
Fig. 4. CO oxidation rates for aged Pd-doped LFO, fresh and aged Pd-impregnated LFO, and Al2 O3 -supported Pd catalysts, derived from Cycle 3 light-off tests.
Pd-impregnated catalysts do not differ greatly in light-off characteristics after aging. Again, to present these results on a more quantitative basis, an Arrhenius plot of the rate of CO oxidation, but now normalized by the total number of Pd atoms in each catalyst, was constructed for the Cycle 3 tests with [CO] = 1.3%. As shown in Fig. 4, the rate for the aged Pd-impregnated LFO catalyst is actually slightly higher than that for the fresh Pd-impregnated LFO catalyst, which is comparable to that of the aged Pd-doped LFO catalyst. In addition, the apparent activation energies for all three catalysts are in the range of 13–17 kcal/mol, similar to that of the fresh Pd-doped LFO catalyst, but much lower than that of the Al2 O3 -supported Pd catalyst. In view of the similarity in apparent activation energies between the fresh Pd-doped LFO catalyst and the aged Pd-impregnated LFO catalyst with [CO] = 1.3%, the dependence on [CO] of both apparent activation energy and rate was examined in an effort to reveal possible differences between them. In the case of fresh Pd-doped LFO, the reaction order in [CO] was found to be positive, and the apparent activation energy relatively constant, whereas the order in [CO] was negative for aged Pd-impregnated LFO, and the apparent activation energy increased monotonically, from about 10 to 20 kcal/mol, as [CO] ranges from 0.3 to 2% (supplemental data, Fig. A2). In the Rh-CTO system, the convergence in performance between Rh-doped CTO and Rh-impregnated CTO catalysts occurs at an earlier stage, and the resulting behavior exhibits a qualitatively different character. Light-off curves for [CO] = 1.35% from Cycle 1, 2, and 3 tests performed on the fresh Rh-impregnated CTO catalyst, shown in Fig. 5, reveal that Cycle 1 and 3 tests produce quite similar results. Further, the light-off temperatures in Cycle 2 and 3
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Fresh Rh-impregnated CTO [CO] = 1.35%
Fracon of CO Converted
1 0.8 Cycle 1 0.6
Cycle 2 Cycle 3
0.4 0.2 0 50
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300
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Fig. 5. Light-off curves from Cycle 1, 2, and 3 tests performed on the fresh Rhimpregnated CTO catalyst. [O2 ] was fixed at 0.8%.
tests are close to those in the corresponding tests for the fresh Rhdoped CTO catalyst (Fig. 1(b)), in spite of the expectation that all (or most) of the Rh should be on the surface of the Rh-impregnated catalyst, whereas most of the Rh should be in solution (within CTO) in the Rh-doped catalyst. This was found to be the case at each [CO] tested (supplemental data, Fig. A3). The aging treatment was again found to have little effect on the light-off temperatures of the Rh-impregnated CTO catalyst, and except for Cycle 1, the same was true for the Rh-doped CTO catalyst (supplemental data, Fig. A3). In general, the light-off temperatures of both aged catalysts were higher than the fresh catalysts in Cycle 2 tests, but they were nearly the same in Cycle 3 tests. Interestingly, the light-off temperatures in Cycle 2 tests were consistently lower than those in Cycle 3 tests, whereas the opposite trend was observed in the Pd-LFO system. This likely reflects the differences in activity and stability between supported particles of PdO and Rh2 O3
Fig. 6. CO oxidation rates for fresh and aged Rh-doped and Rh-impregnated CTO catalysts, derived from Cycle 3 light-off tests.
that would have formed from supported metal particles during the oxidative treatment at the beginning of Cycle 3 tests. Once again, to present these results on a more quantitative basis, an Arrhenius plot of the rate of CO oxidation, normalized by the total number of Rh atoms in each catalyst, was constructed for the Cycle 3 tests with [CO] = 1.35%. As shown in Fig. 6, the rate for the aged Rh-impregnated CTO catalyst is higher than that of the fresh Rhimpregnated CTO catalyst, which is slightly higher than that of the aged Rh-doped CTO catalyst. The fresh, activated, Rh-doped CTO catalyst has the lowest rate, but its apparent activation energy is similar to the other Rh-CTO catalysts, all of which are comparable to that of metallic Rh [18]. Representative STEM images from both of the Rh-CTO catalysts, in various states, are shown in Fig. 7. The three images at the top are from Rh-doped CTO. The bright-field (BF) image on the left side of this series, typical of fresh Rh-doped CTO, provides an impression of the morphology and size of primary particles of CTO.
Fig. 7. STEM images of (a–c) Rh-doped CTO and (d–f) Rh-impregnated CTO catalysts at various stages of aging. The BF image (a) shows the fresh catalyst, while HAADF images (b) and (c) were obtained after aging for 3 and 14 h, respectively. Aging conditions for catalysts portrayed in HAADF images (d–f) correspond to those of (a–c).
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Rh Parcle Size Distribuons - Rh-doped CTO 40
(a) 35
Frequency (%)
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Rh 3hr Redox Rh 14hr Redox
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Rh Parcle Size Distribuons -Rh-impregnated CTO 90
(b) 80 70
Frequency (%)
60
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Parcle Size Range (nm)
Rh-doped CTO vs. Rh-impregnated CTO 45
(c)
40 35
Frequency (%)
30 25
Rh Impregnated 14hr Redox Rh Doped 14hr Redox
20 15 10 5 0
0 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
Parcle Size Range (nm) Fig. 8. Rh particle size distributions found in (a) Rh-doped CTO catalysts after 3 and 14 h of aging, (b) Rh-impregnated CTO catalysts in the fresh state and after 3 and 14 h of aging. A direct comparison of Rh-doped and Rh-impregnated CTO catalysts after 14 h of aging is shown in (c).
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CTO Parcle Size Distribuons 40
Calcined 3hr Redox
35
14hr Redox
Frequency (%)
30
25
20
15
10
5
0 0-5
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80
Parcle Size Range (nm) Fig. 9. CTO primary particle size distributions found in fresh and aged Rh-impregnated CTO catalysts.
particle size. In addition, XRD results, shown in Fig. 10(b), revealed that a significant fraction of the LFO had reacted with alumina to form LaAlO3 . This phase was also present in the sample aged at 800 ◦ C, though at much smaller concentration.
Pd-doped LFO Cycle 3, [CO] = 1.4%
(a)
Fracon of CO Converted
1 0.8 0.6
fresh
900 °C aged
800 °C aged 0.4 0.2 0 50
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200
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3500
Pd-doped LFO + Al2O3 (900 ° C aged)
(b)
3000 2500 Intensity (counts)
Few, if any, Rh particles appear since most of the Rh is dispersed throughout the perovskite matrix in cationic form, occupying Ti sites. The high-angle annular dark field (HAADF) images in the middle and on the right side of this series are from Rh-doped CTO samples that were aged under the alternating gas treatment for 3 and 14 h, respectively. The brighter features in these images arise from metallic Rh particles, though their precise environment (i.e., whether they are embedded within the CTO matrix or supported on a surface) is not always obvious. The three HAADF images at the bottom of Fig. 7 are from Rh-impregnated CTO samples, in states corresponding to the samples directly above them, i.e., fresh, aged 3 h, and aged 14 h (left to right). It is clear that many (if not most) of the Rh particles in these images are supported on the CTO surface. The Rh particle size distributions generated from a number of STEM images from both of the Rh-CTO catalysts are shown in Fig. 8. For Rh-doped CTO, the histograms in Fig. 8(a) suggest that the distribution is bi-modal, and that the average particle size in the larger size mode increases with aging time between 3 and 14 h. For Rh-impregnated CTO, the histograms in Fig. 8(b) suggest that there is little change in average particle size between 3 and 14 h. For ease of comparison, the distributions from the two catalysts after 14 h of aging are overlaid in Fig. 8(c). The similarity between the average particle size in the larger size mode in Rhdoped CTO and the average particle size in Rh-impregnated CTO suggests that the larger mode arises from Rh particles that are on the surface of CTO. Though relatively small in number, these particles actually account for more than 90% of the Rh that was originally in solution within the CTO, which maintained much of its original surface area after aging, as shown by the histograms in Fig. 9. Although neither of the metal-impregnated perovskite catalysts showed clear signs of losing activity due to the relatively short and mild aging treatment to which they were subjected here, the question of thermal stability of the perovskite is a critical one for TWC applications. In an effort to test the thermal limit of the Pd-LFO system, an additional 14 h of aging was performed on the aged Pd-doped LFO catalyst at a higher temperature, 900 ◦ C. As shown in Fig. 10(a), the activity at low conversion appears to have been retained, but a delay in complete conversion is clearly evident, suggesting possible diffusional limitations. In fact, STEM confirmed that sintering of LFO had occurred, leading to an increase in primary
2000
Pd-doped LFO + Al2O3 (800 ° C aged)
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Pd-doped LFO + Al2O3 (fresh)
0 20
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40
45
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60
65
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2θ (deg) Fig. 10. (a) Light-off curves from Cycle 3 tests performed on fresh and two aged Pddoped LFO catalysts. [O2 ] was fixed at 0.8% for all three. (b) XRD patterns obtained from the catalysts in (a).
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4. Discussion Both the Pd-LFO and Rh-CTO systems have been considered as self-regenerating catalysts, in which precious metal particle growth may be limited by a continuous process of metal dissolution into the perovskite under oxidizing conditions alternating with precipitation under reducing conditions. Evidence for this process was originally provided by X-ray absorption measurements performed on powder samples subjected to extended (1 h) treatments at 800 ◦ C under oxidizing (air) or reducing (10% H2 /CO) atmospheres [8,9]. Direct confirmation in the case Rh-CTO was subsequently provided by STEM studies performed on both model planar as well as powder samples, though these measurements revealed that the length scale for precious metal cation movement during this process was only a few nanometers [16], consistent with known bulk diffusion data. For perovskite powders with primary particle size of order 100 nm, most of the metal particles that precipitate and dissolve are thus contained entirely within the perovskite particle and unavailable for gas-phase catalytic reaction. In a TWC environment, the normal oscillation between oxidizing and reducing conditions occurs at a rate of ∼1 Hz, and the excursions from stoichiometry are typically a small fraction of those used in the treatments described above. The relevance of any of the observations above to an actual TWC is thus not apparent. On the other hand, these observations do not necessarily rule out the possibility that the process might be faster or more facile under real TWC conditions. Factors not considered, such as oxygen vacancy concentration in the perovskite, which could drastically alter the process and be affected by rapid cycling between oxidizing and reducing conditions, might become important, and if cation diffusion distances under such conditions were comparable to the size of the primary perovskite particle, metal particle segregation onto the surface of the perovskite might become possible. Although the two systems considered here were originally regarded as comparable, in terms of their self-regeneration ability [9], we find that they exhibit qualitatively different stabilities and aging behaviors under the redox conditions employed: according to our reaction kinetic measurements and STEM observations, the PdLFO system is the more stable, with an admixture of ionic species and supported metal particles accounting for its catalytic performance, while the Rh-CTO system rapidly evolves from Rh-doped CTO into a state resembling a typical supported metal catalyst, with most of the Rh ending up on the surface of CTO. The Pd-doped LFO catalyst, which has a relatively high intrinsic activity initially, as shown in Fig. 2, attains better catalytic performance (based on integral CO conversion) upon aging. Since this system does not lend itself to STEM, it is not possible to show that this improvement is related to any definite structural change, such as movement of Pd from the interior to the surface of LFO. Based on the low apparent activation energy, which persists with aging, there seems to be a dominant component of ionic character involved in its catalytic performance. (Such low apparent activation energy was also observed for CO oxidation over Pd-doped BaCeO3 catalyst under lean conditions [12].) The same can be said about Pd-impregnated LFO, though there are clearly differences in the kinetic behaviors between Pdimpregnated and Pd-doped LFO (supplemental data, Fig. A2). If it is surmised that some Pd moves from Pd-doped LFO onto the surface upon aging, thus accounting for the increase in integral CO conversion, then conversely, some of the Pd deposited onto the surface of Pd-impregnated LFO likely reacts and migrates into the perovskite matrix. Both processes are expected, according to the original literature on self-regenerating catalyst materials [8], and both have been observed, to relatively limited degrees, in the model planar catalyst [15,19]. Thus, both initial states (Pd-impregnated and Pd-doped LFO) are expected to evolve toward a common one,
7
consisting of a mixture of metallic Pd particles and isolated Pd cations on the surface of LFO. The presence of both forms of Pd in the aged catalysts examined here is supported by the similarity in their CO oxidation rates and their unusually low apparent activation energies. The connection between structure and activity is clearer in the case of the Rh-CTO system. Starting with a very low intrinsic activity (Fig. 2), the fresh Rh-doped CTO catalyst abruptly transforms (largely irreversibly) at the end of Cycle 1 into a much more active form, where the apparent activation energy looks very much like that of a supported metal catalyst. In previous investigations of Pt-doped CTO, using a novel gas cell for in situ STEM observations under 5% H2 /N2 at atmospheric pressure, it was found that Pt nanoparticles can precipitate within 1 min at 600 ◦ C [20], and similar experiments, performed more recently on Rh-doped CTO (S. Zhang, unpublished results), found precipitation may also occur just as rapidly at even lower temperature, 500 ◦ C. It is thus likely that some Rh nanoparticles form on the surface of the fresh Rh-doped CTO catalyst at the end of the last Cycle 1 test, where [CO] = 2%. Further, from in situ STEM observations performed under 20% O2 /N2 , also at atmospheric pressure, metal nanoparticle dissolution was found to occur on a much longer timescale. The implication of the in situ STEM work is thus that under the redox aging conditions employed here, Rh should tend to exist, on average, as Rh nanoparticles. The ex situ STEM measurements performed in this study establish that these nanoparticles tend to grow in size on a timescale of hours, as shown in Fig. 8. The difference in the particle size distributions between Rh-doped CTO and Rh-impregnated CTO (where most of the Rh is expected to be on the surface of CTO) after 3 h of aging (cf. Fig. 8(a) and (b)) suggests that there is a continuous, gradual expulsion of Rh from within CTO onto the surface, resulting in a bi-modal distribution of particle size after 14 h. The small particles, less than 5 nm in diameter, left inside CTO account for only a small fraction of the total Rh, and the rates of CO oxidation converge. Although this behavior does not closely conform to the ideal self-regeneration process, it is not yet known whether Rh particles supported on more conventional oxides, such as ZrO2 , would become even larger under our aging conditions. Interestingly, the initial active form of the Rh-doped CTO catalyst (following the Cycle 1 test) provides only 25% of the rate of CO oxidation compared with the aged catalyst, even though the dispersion of Rh in the doped CTO particles (initial average diameter of 20 nm) is similar to that of the Rh particles (final average diameter of 10 nm) that end up on the surface of CTO. A comparable discrepancy exists for the fresh and aged Rh-impregnated CTO catalysts, where the rates are similar, but the dispersions differ by a factor of 4 (Rh particles having initial average diameter of 2 nm versus final average diameter of 10 nm). It thus seems that the specific activity of small (≤2 nm in diameter) nanoparticles of Rh is depressed when supported on CTO.
5. Conclusions The intrinsic catalytic activities of Pd-doped LFO and Rh-doped CTO for CO oxidation are significantly different, as are their stabilities under lean to stoichiometric reaction conditions: Pd-doped LFO is the more active and stable. Whereas both metallic and cationic forms of Pd likely contribute to activity in aged Pd-doped LFO catalysts (which exhibit higher activity than fresh catalysts), metallic Rh particles, which can easily segregate onto the surface of CTO upon aging (if the CTO remains in high-surface-area form), are most likely the primary source of catalytic activity in the Rh-CTO
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system. Although aging led to the eventual convergence of catalytic performance in both systems considered in this study, precious metal impregnation provided better initial activity than doping. The much shorter time scale required for convergence in the case of Rh-CTO than for Pd-LFO probably reflects a greater asymmetry in the processes of precious metal segregation and re-dissolution in Rh-CTO than Pd-LFO. Sintering of either of these perovskites upon aging for up to 14 h at 800 ◦ C does not appear to be a major concern, but sintering and/or reaction with other oxides that might be present in a TWC washcoat, such as alumina, may readily occur by 900 ◦ C.
References
Acknowledgements
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Perovskite powders were kindly provided by M. Oljaca, of Cabot Corporation, and financial support was provided by both Ford Motor Company, through a Ford – University of Michigan Innovation Alliance grant, and National Science Foundation, through grants CBET-1159240, DMR-0907191, and DMR-0723032 (which funded the STEM in the Electron Microbeam Analysis Laboratory at University of Michigan). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2014.11.028.
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Comparison of precious metal doped and impregnated perovskite oxides for TWC application Sotirios A. Malamis a,b , Rachael J. Harrington a,c , Michael B. Katz a,d , David S. Koerschner a , Shuyi Zhang a , Yisun Cheng c , Lifeng Xu c , Hung-Wen Jen c , Robert W. McCabe c,∗ , George W. Graham a,∗ , Xiaoqing Pan a,∗ a
University of Michigan, Ann Arbor, MI 48109, USA University of Houston, Houston, TX 77204, USA c Ford Motor Company, Dearborn, MI 48121, USA d Naval Research Laboratory, Washington, DC 20375, USA b
a r t i c l e
i n f o
Article history: Received 25 October 2014 Received in revised form 21 November 2014 Accepted 27 November 2014 Available online xxx Keywords: Palladium (Pd) Rhodium (Rh) Lanthanum ferrate (LaFeO3 ) Calcium titanate (CaTiO3 ) Scanning transmission electron microscopy (STEM) CO oxidation
a b s t r a c t Four high-surface-area (50–60 m2 /g) perovskite-based powder catalysts, Pd-doped LaFeO3 , Pdimpregnated LaFeO3 , Rh-doped CaTiO3 , and Rh-impregnated CaTiO3 , were characterized by scanning transmission electron microscopy and CO oxidation measurements in fresh and redox-aged (14 h at 800 ◦ C) states. Both intrinsic catalytic activity and stability were significantly higher in Pd-doped LaFeO3 than in Rh-doped CaTiO3 under lean to stoichiometric reaction conditions. Activities of doped were initially lower than impregnated versions, though aging led to convergence in the catalytic performance for both systems. A mixture of metallic and cationic forms of Pd appear to contribute to activity in the Pd catalysts, whereas metallic Rh particles, which can easily segregate onto the surface of CaTiO3 in Rh-doped CaTiO3 upon aging, are the likely source of catalytic activity in the Rh catalysts. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Perovskites as catalysts have a history that spans several decades [1,2]. Interest in their possible application for automotive emissions control was initially stimulated by a study reported in Nature more than four decades ago [3]. Subsequent research has considered a variety of perovskites involving both base metals and Pt-group metals as dopants and supported particles [e.g., 4–6]. A little more than 10 years ago, researchers at Daihatsu Motor Company and their collaborators proposed a novel concept involving perovskites as hosts for precious metals in three-way catalysts (TWCs), according to which self-regeneration would naturally occur under normal TWC operating conditions [7,8]. The selfregeneration process was based on the idea that the precious metal should be stable as a dopant, substituting for the B-site cation in the perovskite under oxidizing conditions, but that is would tend
∗ Corresponding authors. Tel.: +1 734 763 6661; fax: +1 734 763 4788. E-mail addresses: [email protected] (R.W. McCabe), [email protected] (G.W. Graham), [email protected] (X. Pan).
to precipitate out of the perovskite as a metal nanoparticle under reducing conditions. Due to the oscillation in air-to-fuel ratio normally imposed to ensure its precise control around stoichiometry, the precious metal nanoparticle should, in principle, undergo alternating cycles of re-dissolution and precipitation, thus ensuring that the nanoparticles would not coarsen significantly. Considerable interest, created by reports that the self-regeneration process could significantly reduce precious metal usage in TWCs [9], led to a number of investigations of these and related systems [e.g., 10–14]. Relatively little effort has been expended to carefully probe the nature of the self-regeneration process, however, and only a few studies have been able to establish the fundamental relationship between structure and catalytic activity of these materials [12]. We thus recently used scanning transmission electron microscopy (STEM) to gain new insight into the self-regeneration process [15,16]. Here, we focus on structure–activity relationships, comparing our initial catalytic activity results from two of the most prominent systems [9], Pd-LaFeO3 and Rh-CaTiO3 , using CO oxidation as a probe reaction. Our primary goals were to (1) determine and contrast the intrinsic catalytic activities of Pd-doped LaFeO3 (LFO) and Rh-doped CaTiO3 (CTO), (2) compare their catalytic
http://dx.doi.org/10.1016/j.cattod.2014.11.028 0920-5861/© 2014 Elsevier B.V. All rights reserved.
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performances with those of Pd-impregnated LFO and Rhimpregnated CTO, respectively, and (3) assess the stabilities, or aging characteristics, of all four catalysts. Fracon of CO Converted
High-surface-area (50–60 m2 /g) powders of both pure perovskites, Pd-doped LFO, and Rh-doped CTO were provided by Cabot Corporation. Doping levels were nominally 5% of the Fe and Ti (Bsite) cations. Comparable levels of Pd (2.2 wt%) and Rh (2.12 wt%) were loaded onto the surfaces of the pure LFO and CTO powders by wet impregnation of tetraamminepalladium(II) nitrate and rhodium nitrate solution, respectively, to incipient wetness, followed by calcination in air (1 h at 600 ◦ C). Small samples of all four precious metal-containing powders, for STEM (JEOL 2100F) analysis, were aged in a quartz boat in a quartz tube furnace at 800 ◦ C under a flow of alternating reducing (1% H2 /N2 ) or oxidizing (2% O2 /N2 ) gas, each of 10 min duration, for a total of 14 h. Heating and cooling of the samples was performed under N2 , and the last half cycle at 800 ◦ C was 2% O2 /N2 . In addition, Rh-CTO samples were aged for 3 h, using the alternating gas treatment above. STEM specimens were prepared by simply dipping carbon grids into the powder samples. Measurements of the size of several (of order 100) metal and perovskite particles in the resulting STEM images were performed manually. Larger samples, for catalytic activity measurement, were mixed with alumina (50 wt%) from Sasol (TH100-150), pressed into pellets, crumbled and sieved (40–80 mesh), and then placed into a packed-bed quartz reactor, which was also used for in situ aging following the same 14 h protocol used for the STEM samples. XRD (Rigaku Minitab) was performed on some of the reactor samples before and after aging. For comparison, an Al2 O3 -supported Pd catalyst (1.4 wt% Pd on Sasol TH100-150 alumina) was also prepared and mildly aged, yielding a Pd dispersion of 14.4% as measured by CO chemisorption. The packed-bed quartz reactor used a vertical tube, 1 cm in diameter, with gas inlet at the top. Quartz wool plugs, approximately 0.75 cm long, were used to keep the catalyst bed, which was between 2 and 2.5 cm long, in place. Mass flow controllers were used to set and maintain the flow rates of CO, O2 , and N2 at a total flow rate of 2 L/min. The reactor system was also set up with a bypass channel, used to make inlet measurements of each species. When the flow was directed through the reactor, a pump was used to overcome the slight pressure drop created by the catalyst bed. The concentrations of CO and O2 were measured with an FTIR spectrometer and lambda sensor, respectively, and catalyst bed temperature was measured with a thermocouple contacting the bed at the gas exit. Using a LabView program to control the reactor, each catalyst sample was subjected to three cycles of data collection, designed to test its activity under varying gas composition, ranging between lean and slightly rich. Cycle 1 began by heating the reactor to 500 ◦ C under a flow of 10% O2 /N2 for 1 h. The reactor was then cooled to below 60 ◦ C, after which a series of light-off tests was conducted. A single light-off test involved feeding a fixed ratio of O2 and CO into the reactor and ramping the temperature of the reactor to between 300 and 400 ◦ C at 10 ◦ C/min. Once the limiting conversion was reached, the reactor was cooled to below 60 ◦ C, and the next test began. A series of five light-off tests was conducted in this first cycle, with the O2 concentration fixed at 0.8% and the CO concentration increasing in the order 0.3, 0.5, 1, 1.4, and 2%. Upon completion of Cycle 1, a second series of light-off tests, Cycle 2, was conducted in reverse order. Specifically, with the O2 concentration fixed at 0.8%, the first light-off test was performed with a CO concentration of 2%, followed by consecutive tests with progressively lower CO concentrations, ranging back down to 0.3%.
1 0.8 Cycle 1 0.6
Cycle 2 Cycle 3
0.4 0.2 0 50
100
150
200 250 Temperature (°C)
300
350
400
(b)
Fresh Rh-doped CTO [CO] = 1.35% 1 Fracon of CO Converted
2. Materials and methods
(a)
Fresh Pd-doped LFO [CO] = 1.3%
0.8 Cycle 1 0.6
Cycle 2 Cycle 3
0.4 0.2 0 50
100
150
200 250 Temperature (°C)
300
350
400
Fig. 1. Light-off curves from Cycle 1, 2, and 3 tests performed on fresh (a) Pd-doped LFO and (b) Rh-doped CTO catalysts. [O2 ] was fixed at 0.8% for both.
Finally, a third series of light-off tests, Cycle 3, which was identical to Cycle 1, was conducted. Cycles 2 and 3 thus provide indications of the stability and reversibility, respectively, of the state of the catalyst. 3. Results Light-off curves (i.e., fraction of CO converted vs. temperature) for [CO] = 1.3% from Cycle 1, 2, and 3 tests performed on the fresh Pd-doped LFO catalyst are shown in Fig. 1(a). Aside from shifts of a few ◦ C to lower temperature of Cycle 2 curves for some of the CO concentrations, curves for all three cycles were found to be nearly identical at each CO concentration. The catalyst thus appears to be quite stable under these reaction conditions. Further, similar tests performed on pure LFO revealed that its light-off temperature (at which half of the CO is converted) is approximately 100 ◦ C higher than Pd-doped LFO, indicating that catalytic activity is primarily determined by Pd doping. For comparison, the light-off curves for [CO] = 1.35% from Cycle 1, 2, and 3 tests performed on the fresh Rh-doped CTO catalyst are shown in Fig. 1(b). Clearly, this catalyst is not stable in its initial state: within the time span of just one test, from the end of Cycle 1 to the end of the first test of Cycle 2, where the ratio [CO]/[O2 ] was 2.5 at the reactor inlet, the light-off temperature dropped by 150 ◦ C, and this difference was essentially retained throughout the entire series of Cycle 2 tests. As discussed below, the increase in activity is thought to be due to the reduction of Rh cations on the surface of the doped perovskite. Further, this rapid change in the state of the catalyst is not reversible under our test conditions, as indicated by the significant shift between Cycle 1 and Cycle 3 lightoff curves. Tests performed on pure CTO, which revealed that its light-off curve is shifted more than 150 ◦ C higher than the Cycle 1 curves of fresh Rh-doped CTO, indicate that the catalytic activity of the initial state is primarily determined by Rh.
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Fresh Pd-impregnated LFO [CO] = 1.35%
CO Oxidaon Rate [O2] = 0.8%, [CO]=1.3% 0
Ln(TOF) [mol-CO/mol-sites]
Pd/Al2O3 Rh-doped CTO, Cycle 1
-1.5
Pd-doped LFO, Cycle 1
Ea=28.0 kcal/mol
Ea=16.7 kcal/mol
Ea=23.9 kcal/mol
-2 Pd-doped LFO, Cycle 3
0.8 0.6
0.4 0.2
Ea=15.2 kcal/mol
-2.5 -3 0.0016
Fracon of CO Converted
1
-0.5 -1
3
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0.0022
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Fig. 3. Light-off curves from Cycle 1, 2, and 3 tests performed on the fresh Pdimpregnated LFO catalyst. [O2 ] was fixed at 0.8%.
Fig. 2. CO oxidation rates, expressed as turn over frequencies (TOFs), for fresh Pd-doped LFO and Rh-doped CTO, derived from Cycle 1 light-off tests. The TOF derived from Cycle 3 is also shown for Pd-doped LFO. The TOF for a mildly aged Al2 O3 -supported Pd catalyst, derived from the Cycle 3 light-off test, is shown for comparison.
CO Oxidaon Rate, Cycle 3 [O2] = 0.8%, [CO] = 1.3% -2.5
To present these results on a more quantitative basis, the rates of CO conversion, taken between 5 and 20% (in order to yield sufficiently high signal-to-noise data without grossly exceeding differential conversion conditions), were plotted vs. inverse absolute temperature for the Cycle 1 tests with [CO] ≈ 1.3% for both catalysts, as shown in Fig. 2. The rates, specified as a turn-over frequency (TOF = rate of CO converted per active site), were obtained by assuming that the active sites correspond to surface cations randomly populating B-sites at the nominal doping level (5%), that the surface exposes A and B-sites with equal probability, and by using the measured BET areas (58.9 m2 /g for Pd-doped LFO, 61 m2 /g for Rh-doped CTO). The same assumptions were made in plotting the rate of CO conversion for the Cycle 3 test with [CO] = 1.3% for the Pd-doped LFO catalyst. The TOF for CO oxidation over the Al2 O3 -supported Pd catalyst, measured under corresponding conditions (where Pd is expected to be in the form of metallic particles), is also plotted in Fig. 2. The apparent activation energy for the Al2 O3 -supported Pd catalyst, 28.2 kcal/mol, is in reasonable agreement with typical values for metallic Pd [17], whereas that for Cycle 1 of the fresh Rh-doped CTO catalyst, 24 kcal/mol, is somewhat lower. The apparent activation energies for the fresh Pd-doped LFO catalyst (Cycles 1 and 3) are clearly much lower (15–17 kcal/mol). Due to this difference in apparent activation energies, the TOF for the fresh Pd-doped LFO catalyst, where the Pd sites are presumably still part of the perovskite, would become comparable to that of the Al2 O3 -supported Pd catalyst at low temperature (∼160 ◦ C), assuming these rates could be extrapolated. The situation for the impregnated catalysts is somewhat different, depending on the metal and its perovskite support. In the Pd-LFO system, the light-off curves for the fresh Pd-impregnated LFO catalyst tend to rise more rapidly with increasing temperature than for the fresh Pd-doped LFO catalyst (Fig. 1(a)), as shown in Fig. 3. More significantly, the light-off temperatures are at least 80 ◦ C lower for the impregnated catalyst, indicative of higher catalytic activity, as might be expected in view of the fact that all (or most) of the Pd should be on the surface. A different dependence of light-off temperature on [CO] was also observed (supplemental data, Fig. A1). The aging treatment was found to have little effect on the light-off temperatures of the Pd-impregnated LFO catalyst, whereas those of the Pd-doped LFO catalyst were lowered by about 80 ◦ C (supplemental data, Fig. A1). Thus, the Pd-doped and
ln(rate) [mol-CO/mol-Pds]
Pd-impregnated LFO (fresh) -3
Ea=13 kcal/mol
Pd-impregnated LFO (aged) Ea=16.7 kcal/mol
-3.5 -4 -4.5
Pd/Al2O3 Ea=28.0 kcal/mol
Ea=13.5 kcal/mol
Pd-doped LFO (aged) -5 -5.5 0.002
0.0021
0.0022
0.0023
0.0024
0.0025
1/T (K-1)
Fig. 4. CO oxidation rates for aged Pd-doped LFO, fresh and aged Pd-impregnated LFO, and Al2 O3 -supported Pd catalysts, derived from Cycle 3 light-off tests.
Pd-impregnated catalysts do not differ greatly in light-off characteristics after aging. Again, to present these results on a more quantitative basis, an Arrhenius plot of the rate of CO oxidation, but now normalized by the total number of Pd atoms in each catalyst, was constructed for the Cycle 3 tests with [CO] = 1.3%. As shown in Fig. 4, the rate for the aged Pd-impregnated LFO catalyst is actually slightly higher than that for the fresh Pd-impregnated LFO catalyst, which is comparable to that of the aged Pd-doped LFO catalyst. In addition, the apparent activation energies for all three catalysts are in the range of 13–17 kcal/mol, similar to that of the fresh Pd-doped LFO catalyst, but much lower than that of the Al2 O3 -supported Pd catalyst. In view of the similarity in apparent activation energies between the fresh Pd-doped LFO catalyst and the aged Pd-impregnated LFO catalyst with [CO] = 1.3%, the dependence on [CO] of both apparent activation energy and rate was examined in an effort to reveal possible differences between them. In the case of fresh Pd-doped LFO, the reaction order in [CO] was found to be positive, and the apparent activation energy relatively constant, whereas the order in [CO] was negative for aged Pd-impregnated LFO, and the apparent activation energy increased monotonically, from about 10 to 20 kcal/mol, as [CO] ranges from 0.3 to 2% (supplemental data, Fig. A2). In the Rh-CTO system, the convergence in performance between Rh-doped CTO and Rh-impregnated CTO catalysts occurs at an earlier stage, and the resulting behavior exhibits a qualitatively different character. Light-off curves for [CO] = 1.35% from Cycle 1, 2, and 3 tests performed on the fresh Rh-impregnated CTO catalyst, shown in Fig. 5, reveal that Cycle 1 and 3 tests produce quite similar results. Further, the light-off temperatures in Cycle 2 and 3
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Fresh Rh-impregnated CTO [CO] = 1.35%
Fracon of CO Converted
1 0.8 Cycle 1 0.6
Cycle 2 Cycle 3
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Fig. 5. Light-off curves from Cycle 1, 2, and 3 tests performed on the fresh Rhimpregnated CTO catalyst. [O2 ] was fixed at 0.8%.
tests are close to those in the corresponding tests for the fresh Rhdoped CTO catalyst (Fig. 1(b)), in spite of the expectation that all (or most) of the Rh should be on the surface of the Rh-impregnated catalyst, whereas most of the Rh should be in solution (within CTO) in the Rh-doped catalyst. This was found to be the case at each [CO] tested (supplemental data, Fig. A3). The aging treatment was again found to have little effect on the light-off temperatures of the Rh-impregnated CTO catalyst, and except for Cycle 1, the same was true for the Rh-doped CTO catalyst (supplemental data, Fig. A3). In general, the light-off temperatures of both aged catalysts were higher than the fresh catalysts in Cycle 2 tests, but they were nearly the same in Cycle 3 tests. Interestingly, the light-off temperatures in Cycle 2 tests were consistently lower than those in Cycle 3 tests, whereas the opposite trend was observed in the Pd-LFO system. This likely reflects the differences in activity and stability between supported particles of PdO and Rh2 O3
Fig. 6. CO oxidation rates for fresh and aged Rh-doped and Rh-impregnated CTO catalysts, derived from Cycle 3 light-off tests.
that would have formed from supported metal particles during the oxidative treatment at the beginning of Cycle 3 tests. Once again, to present these results on a more quantitative basis, an Arrhenius plot of the rate of CO oxidation, normalized by the total number of Rh atoms in each catalyst, was constructed for the Cycle 3 tests with [CO] = 1.35%. As shown in Fig. 6, the rate for the aged Rh-impregnated CTO catalyst is higher than that of the fresh Rhimpregnated CTO catalyst, which is slightly higher than that of the aged Rh-doped CTO catalyst. The fresh, activated, Rh-doped CTO catalyst has the lowest rate, but its apparent activation energy is similar to the other Rh-CTO catalysts, all of which are comparable to that of metallic Rh [18]. Representative STEM images from both of the Rh-CTO catalysts, in various states, are shown in Fig. 7. The three images at the top are from Rh-doped CTO. The bright-field (BF) image on the left side of this series, typical of fresh Rh-doped CTO, provides an impression of the morphology and size of primary particles of CTO.
Fig. 7. STEM images of (a–c) Rh-doped CTO and (d–f) Rh-impregnated CTO catalysts at various stages of aging. The BF image (a) shows the fresh catalyst, while HAADF images (b) and (c) were obtained after aging for 3 and 14 h, respectively. Aging conditions for catalysts portrayed in HAADF images (d–f) correspond to those of (a–c).
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Rh Parcle Size Distribuons - Rh-doped CTO 40
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CTO Parcle Size Distribuons 40
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Parcle Size Range (nm) Fig. 9. CTO primary particle size distributions found in fresh and aged Rh-impregnated CTO catalysts.
particle size. In addition, XRD results, shown in Fig. 10(b), revealed that a significant fraction of the LFO had reacted with alumina to form LaAlO3 . This phase was also present in the sample aged at 800 ◦ C, though at much smaller concentration.
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Few, if any, Rh particles appear since most of the Rh is dispersed throughout the perovskite matrix in cationic form, occupying Ti sites. The high-angle annular dark field (HAADF) images in the middle and on the right side of this series are from Rh-doped CTO samples that were aged under the alternating gas treatment for 3 and 14 h, respectively. The brighter features in these images arise from metallic Rh particles, though their precise environment (i.e., whether they are embedded within the CTO matrix or supported on a surface) is not always obvious. The three HAADF images at the bottom of Fig. 7 are from Rh-impregnated CTO samples, in states corresponding to the samples directly above them, i.e., fresh, aged 3 h, and aged 14 h (left to right). It is clear that many (if not most) of the Rh particles in these images are supported on the CTO surface. The Rh particle size distributions generated from a number of STEM images from both of the Rh-CTO catalysts are shown in Fig. 8. For Rh-doped CTO, the histograms in Fig. 8(a) suggest that the distribution is bi-modal, and that the average particle size in the larger size mode increases with aging time between 3 and 14 h. For Rh-impregnated CTO, the histograms in Fig. 8(b) suggest that there is little change in average particle size between 3 and 14 h. For ease of comparison, the distributions from the two catalysts after 14 h of aging are overlaid in Fig. 8(c). The similarity between the average particle size in the larger size mode in Rhdoped CTO and the average particle size in Rh-impregnated CTO suggests that the larger mode arises from Rh particles that are on the surface of CTO. Though relatively small in number, these particles actually account for more than 90% of the Rh that was originally in solution within the CTO, which maintained much of its original surface area after aging, as shown by the histograms in Fig. 9. Although neither of the metal-impregnated perovskite catalysts showed clear signs of losing activity due to the relatively short and mild aging treatment to which they were subjected here, the question of thermal stability of the perovskite is a critical one for TWC applications. In an effort to test the thermal limit of the Pd-LFO system, an additional 14 h of aging was performed on the aged Pd-doped LFO catalyst at a higher temperature, 900 ◦ C. As shown in Fig. 10(a), the activity at low conversion appears to have been retained, but a delay in complete conversion is clearly evident, suggesting possible diffusional limitations. In fact, STEM confirmed that sintering of LFO had occurred, leading to an increase in primary
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4. Discussion Both the Pd-LFO and Rh-CTO systems have been considered as self-regenerating catalysts, in which precious metal particle growth may be limited by a continuous process of metal dissolution into the perovskite under oxidizing conditions alternating with precipitation under reducing conditions. Evidence for this process was originally provided by X-ray absorption measurements performed on powder samples subjected to extended (1 h) treatments at 800 ◦ C under oxidizing (air) or reducing (10% H2 /CO) atmospheres [8,9]. Direct confirmation in the case Rh-CTO was subsequently provided by STEM studies performed on both model planar as well as powder samples, though these measurements revealed that the length scale for precious metal cation movement during this process was only a few nanometers [16], consistent with known bulk diffusion data. For perovskite powders with primary particle size of order 100 nm, most of the metal particles that precipitate and dissolve are thus contained entirely within the perovskite particle and unavailable for gas-phase catalytic reaction. In a TWC environment, the normal oscillation between oxidizing and reducing conditions occurs at a rate of ∼1 Hz, and the excursions from stoichiometry are typically a small fraction of those used in the treatments described above. The relevance of any of the observations above to an actual TWC is thus not apparent. On the other hand, these observations do not necessarily rule out the possibility that the process might be faster or more facile under real TWC conditions. Factors not considered, such as oxygen vacancy concentration in the perovskite, which could drastically alter the process and be affected by rapid cycling between oxidizing and reducing conditions, might become important, and if cation diffusion distances under such conditions were comparable to the size of the primary perovskite particle, metal particle segregation onto the surface of the perovskite might become possible. Although the two systems considered here were originally regarded as comparable, in terms of their self-regeneration ability [9], we find that they exhibit qualitatively different stabilities and aging behaviors under the redox conditions employed: according to our reaction kinetic measurements and STEM observations, the PdLFO system is the more stable, with an admixture of ionic species and supported metal particles accounting for its catalytic performance, while the Rh-CTO system rapidly evolves from Rh-doped CTO into a state resembling a typical supported metal catalyst, with most of the Rh ending up on the surface of CTO. The Pd-doped LFO catalyst, which has a relatively high intrinsic activity initially, as shown in Fig. 2, attains better catalytic performance (based on integral CO conversion) upon aging. Since this system does not lend itself to STEM, it is not possible to show that this improvement is related to any definite structural change, such as movement of Pd from the interior to the surface of LFO. Based on the low apparent activation energy, which persists with aging, there seems to be a dominant component of ionic character involved in its catalytic performance. (Such low apparent activation energy was also observed for CO oxidation over Pd-doped BaCeO3 catalyst under lean conditions [12].) The same can be said about Pd-impregnated LFO, though there are clearly differences in the kinetic behaviors between Pdimpregnated and Pd-doped LFO (supplemental data, Fig. A2). If it is surmised that some Pd moves from Pd-doped LFO onto the surface upon aging, thus accounting for the increase in integral CO conversion, then conversely, some of the Pd deposited onto the surface of Pd-impregnated LFO likely reacts and migrates into the perovskite matrix. Both processes are expected, according to the original literature on self-regenerating catalyst materials [8], and both have been observed, to relatively limited degrees, in the model planar catalyst [15,19]. Thus, both initial states (Pd-impregnated and Pd-doped LFO) are expected to evolve toward a common one,
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consisting of a mixture of metallic Pd particles and isolated Pd cations on the surface of LFO. The presence of both forms of Pd in the aged catalysts examined here is supported by the similarity in their CO oxidation rates and their unusually low apparent activation energies. The connection between structure and activity is clearer in the case of the Rh-CTO system. Starting with a very low intrinsic activity (Fig. 2), the fresh Rh-doped CTO catalyst abruptly transforms (largely irreversibly) at the end of Cycle 1 into a much more active form, where the apparent activation energy looks very much like that of a supported metal catalyst. In previous investigations of Pt-doped CTO, using a novel gas cell for in situ STEM observations under 5% H2 /N2 at atmospheric pressure, it was found that Pt nanoparticles can precipitate within 1 min at 600 ◦ C [20], and similar experiments, performed more recently on Rh-doped CTO (S. Zhang, unpublished results), found precipitation may also occur just as rapidly at even lower temperature, 500 ◦ C. It is thus likely that some Rh nanoparticles form on the surface of the fresh Rh-doped CTO catalyst at the end of the last Cycle 1 test, where [CO] = 2%. Further, from in situ STEM observations performed under 20% O2 /N2 , also at atmospheric pressure, metal nanoparticle dissolution was found to occur on a much longer timescale. The implication of the in situ STEM work is thus that under the redox aging conditions employed here, Rh should tend to exist, on average, as Rh nanoparticles. The ex situ STEM measurements performed in this study establish that these nanoparticles tend to grow in size on a timescale of hours, as shown in Fig. 8. The difference in the particle size distributions between Rh-doped CTO and Rh-impregnated CTO (where most of the Rh is expected to be on the surface of CTO) after 3 h of aging (cf. Fig. 8(a) and (b)) suggests that there is a continuous, gradual expulsion of Rh from within CTO onto the surface, resulting in a bi-modal distribution of particle size after 14 h. The small particles, less than 5 nm in diameter, left inside CTO account for only a small fraction of the total Rh, and the rates of CO oxidation converge. Although this behavior does not closely conform to the ideal self-regeneration process, it is not yet known whether Rh particles supported on more conventional oxides, such as ZrO2 , would become even larger under our aging conditions. Interestingly, the initial active form of the Rh-doped CTO catalyst (following the Cycle 1 test) provides only 25% of the rate of CO oxidation compared with the aged catalyst, even though the dispersion of Rh in the doped CTO particles (initial average diameter of 20 nm) is similar to that of the Rh particles (final average diameter of 10 nm) that end up on the surface of CTO. A comparable discrepancy exists for the fresh and aged Rh-impregnated CTO catalysts, where the rates are similar, but the dispersions differ by a factor of 4 (Rh particles having initial average diameter of 2 nm versus final average diameter of 10 nm). It thus seems that the specific activity of small (≤2 nm in diameter) nanoparticles of Rh is depressed when supported on CTO.
5. Conclusions The intrinsic catalytic activities of Pd-doped LFO and Rh-doped CTO for CO oxidation are significantly different, as are their stabilities under lean to stoichiometric reaction conditions: Pd-doped LFO is the more active and stable. Whereas both metallic and cationic forms of Pd likely contribute to activity in aged Pd-doped LFO catalysts (which exhibit higher activity than fresh catalysts), metallic Rh particles, which can easily segregate onto the surface of CTO upon aging (if the CTO remains in high-surface-area form), are most likely the primary source of catalytic activity in the Rh-CTO
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system. Although aging led to the eventual convergence of catalytic performance in both systems considered in this study, precious metal impregnation provided better initial activity than doping. The much shorter time scale required for convergence in the case of Rh-CTO than for Pd-LFO probably reflects a greater asymmetry in the processes of precious metal segregation and re-dissolution in Rh-CTO than Pd-LFO. Sintering of either of these perovskites upon aging for up to 14 h at 800 ◦ C does not appear to be a major concern, but sintering and/or reaction with other oxides that might be present in a TWC washcoat, such as alumina, may readily occur by 900 ◦ C.
References
Acknowledgements
[10]
[1] [2] [3] [4] [5] [6] [7] [8] [9]
[11]
Perovskite powders were kindly provided by M. Oljaca, of Cabot Corporation, and financial support was provided by both Ford Motor Company, through a Ford – University of Michigan Innovation Alliance grant, and National Science Foundation, through grants CBET-1159240, DMR-0907191, and DMR-0723032 (which funded the STEM in the Electron Microbeam Analysis Laboratory at University of Michigan). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2014.11.028.
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