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Letter
In-situ observation of Rh-CaTiO3 catalysts during reduction and oxidation treatments by transmission electron microscopy Sheng Dai, Shuyi Zhang, Michael B. Katz, George W. Graham, and Xiaoqing Pan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03604 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017
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In-situ observation of Rh-CaTiO3 catalysts during reduction and oxidation treatments by transmission electron microscopy Sheng Dai,1,§ Shuyi Zhang,1,2,§ Michael B. Katz,2,3,§ George W. Graham,1,2 Xiaoqing Pan1,4,* 1
Department of Chemical Engineering and Materials Science, University of California – Irvine, Irvine, CA 92697, USA 2
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
3
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA 4
Department of Physics and Astronomy, University of California – Irvine, Irvine, CA 92697, USA
ABSTRACT: Scanning transmission electron microscopy of high-surface-area Rh-doped CaTiO3 powder, conducted near atmospheric pressure at elevated temperature (250-700 °C) by means of an electron-transparent gas cell with sample heater, provides evidence for both cyclical precipitation-dissolution of Rh nanoparticles in response to redox cycling of the ambient gas and sintering of the powder.
KEYWORDS: catalyst self-regeneration, CaTiO3, Rh, in-situ STEM, embedded nanoparticles
INTRODUCTION Daihatsu’s ‘intelligent catalyst’ concept of a continuously self-regenerating three-way catalyst for automotive exhaust-gas treatment is based on the idea that a high dispersion of precious metal may be maintained through the combined effect of two opposing processes, precipitation and dissolution of precious-metal nanoparticles (NPs) that could occur for a precious-metal-doped perovskite oxide under the opposing influences of reducing and oxidizing gas conditions that exist during closed-loop feedback control of a typical gasoline-powered internal com-bustion engine.1 Confirmation that such processes do, indeed occur in a variety of such systems under relatively extreme treatment conditions was originally provided by ex-situ x-ray absorption measurements, though little was revealed by these measurements about the kinetics of these processes or the resulting catalyst morphology.1,2
In order to learn something about the effect of the reduction and oxidation treatments on morphology, we previously employed aberration-corrected scanning transmission electron microscopy (STEM) to study several of the proposed ‘intelligent catalyst’ systems.3 As in the case of the x-ray absorption measurements, our STEM measurements were performed after catalyst samples had been exposed to relatively extreme treatment conditions, 800 °C for 1 h in 760 Torr of 10%H2/N2 or air. The major finding of these studies was that most of the metal NPs that initially precipitate upon reduction are located within the perovskite oxide matrix, which makes them unavailable for gas-phase catalysis. It was also evident that the reverse process, dissolution of the metal NPs into the perovskite oxide upon re-oxidation, is generally not as facile as NP precipitation, depending on the metal, perovskite oxide, NP size, and its location (surface versus bulk).
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Here, we present in-situ STEM results, obtained with a novel electron-transparent gas cell, that provide additional, real time information about these processes in one of the ‘intelligent catalyst’ systems, Rh-doped CaTiO3 (CTO). EXPERIMENTAL DETAILS All experiments were performed on high-surface-area (50-60 m2/g) Rh-doped CTO powder, provided by Cabot Corporation. The Rh doping level in this material was nominally 5% of the Ti cations. A small quantity of this powder was suspended in methanol with sonication and drop cast onto the heater chip of the MEMS-based gas cell of a Protochips Atmosphere system, as described previously.4 In-situ STEM observations were carried out with two different versions of this system, an experimental one at Oak Ridge National Lab, using an aberration-corrected JEOL 2200FS STEM/TEM, operating at 200 kV,5 and a pre-commercial one at University of Michigan, using a JEOL 3100-R05 double Cs-corrected STEM/TEM, operating at 300 kV.6
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ready appear at 250 °C. When the temperature is raised to 355 °C, these regions become clearer, and several more sub-nanometer-size regions become apparent, as shown by the image at higher magnification in Fig. 1b. Both the number and clarity of the sub-nanometer regions increase when the temperature is raised to 500 °C, as shown in Figs. 1c and 1d. For the most part, the larger bright regions remain stable, but small changes in the number or distribution of the smallest bright regions appear to occur with the passage of time. Unfortunately, it was usually not possible to make in-situ STEM observations directly along a low-index CTO zone axis, due to the random orientation of the particles, but the high-resolution image shown in Figure 2, taken along the [001]pseudo-cubic direction of one of the CTO particles after 12 minutes of reduction at 500 °C, reveals that the sub-nanometer bright regions have no discernible structure. These features, as well as the occasional bright spots (marked by yellow arrows), are considered further below.
Figure 1. In-situ HAADF images of Rh-CTO powder in 760 Torr of 5%H2/Ar (labeled by R) at various temperatures: (a) 250 °C, (b) 355 °C (under a higher magnification), and (c, d) 500 °C.
RESULTS The series of high-angle annular-dark-field (HAADF) images shown in Figure 1 provide both an impression of the morphology of the Rh-doped CTO powder and an overview of the effect of temperature on what we believe to be the process of Rh NP precipitation during the initial reduction treatment in 760 Torr of forming gas (5%H2 in Ar). As shown by the image in Fig. 1a, the CTO particles are generally 10-20 nm across, and some nanometer-size regions of brighter intensity, presumably due to Rh, al-
Figure 2. Atomic-scale in-situ HAADF image of Rh-CTO powder in 760 Torr of 5%H2/Ar (labeled by R) after 12 minutes at 500 °C. (a) Large field of view shows diffuse subnanometer bright regions and occasional bright spots, marked by yellow arrows. (b) Enlarged image, containing the small rectangular box in (a), identifies Ca (blue) and Ti (magenta) columns. (c) Intensity along a line scanned from left to right within the rectangular box in (b).
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Similar phenomena occurred in another in-situ experiment, aimed at observing the effect of a full reductionoxidation cycle. In this case, the sample was first heated to 600 °C in 600 Torr of forming gas (4%H2 in Ar). Again, pre-existing nanometer-size regions of bright intensity, already apparent in the HAADF image of the as-prepared sample shown in Fig. 3a, become much clearer at 600 °C, and many smaller regions also appear, as shown in Fig. 3b. Upon subsequent heating to 600 °C in 600 Torr of pure O2, most of the smaller regions disappear within 10 minutes, as shown in Fig. 3c, and all are gone after 20 more minutes at 700 °C in 600 Torr of pure O2, as shown in Fig. 3d. In addition, two of the large regions of bright intensity appear to have coalesced (in the left CTO particle), and another appears to have gotten smaller (in the lower right CTO particle).
comparative images shown in Fig. 5. Another point of interest is that both transitions (changes in bright intensity regions from first oxidation to second reduction and from second reduction to second oxidation) took place faster (within a minute) than in the initial reductionoxidation cycle.
Figure 3. In-situ HAADF images showing a full reductionoxidation cycle of Rh-CTO powder at various temperatures and time lapses. (a) As-prepared sample. (b) Sample in 600 Torr of 4%H2/Ar (labeled by R) after 10 minutes at 600 °C. (c) Sample in 600 Torr of pure O2 (labeled by O) after 10 minutes at 600 °C, subsequent to the reduction treatment in (b). (d) Sample in 600 Torr of pure O2 after 20 additional minutes at 700 °C.
Another demonstration of the same behavior, made with a larger collection of CTO particles at lower magnification, is shown in Figure 4. Both reducing and oxidizing treatments were conducted in 760 Torr of forming gas (5%H2 in Ar) and pure O2, respectively, at 600 °C, and changes were observed to occur over several minutes of treatment time in each case. Here, however, significant changes in CTO particle size, shape, and arrangement were observed, in addition to the changes to regions of brighter intensity, as described before. Both the apparent sintering of CTO particles and a clear difference in the distribution of brighter intensity regions between this and a second reduction-oxidation cycle are apparent in the
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Figure 4. In-situ HAADF images showing a full reduction-oxidation cycle of a larger collection of Rh-CTO particles at various time lapses. (a-d) Sequential images of sample in 760 Torr of 5%H2/Ar (labeled by R) at 600 °C. (e-h) Subsequent sequential images of sample in 760 Torr of pure O2 (labeled by O) at 600 °C.
Figure 5. Comparison of HAADF images of Rh-CTO powder in 760 Torr of 5%H2/Ar (labeled by R) or pure O2 (labeled by O) at 600 °C taken at the conclusion of the first (a, b) and second (c, d) reduction-oxidation cycles (labeled by 1 and 2).
DISCUSSION As mentioned in the Introduction, we previously employed STEM to perform ex-situ studies of the effect of reduction and oxidation treatments on the morphology of several of the proposed ‘intelligent catalyst’ systems, in-
cluding Rh-doped CTO and Pt-doped CTO, which were mainly in the form of single crystalline thin (~50 nm) films deposited epitaxially onto the surface of (100) SrTiO3 or (110) LaAlO3 substrates.3 The examination of crosssectional specimens that were prepared after the films were treated allowed us to unambiguously establish that precious-metal NPs, ranging in size from less than one to several nanometers, are formed throughout the perovskite oxide by a 1 h treatment in 760 Torr of 10%H2 in N2 at 800 °C, and that significantly fewer, mostly larger NPs remain after a similar subsequent treatment in air. In addition, the existence of Pt NPs within CTO particles of a powder sample after the same reduction treatment was confirmed using sample rotation and through-sample focusing techniques. Consequently, we believe that the regions of brighter intensity in the images from our insitu observations are due to Rh NPs, some of which were initially present, probably located on the surface of CTO particles, but the majority of which formed during the reduction treatment. Further, although it cannot be proven from these images, it is likely that most of the Rh NPs that formed and subsequently disappeared during the oxidation treatment were located within the CTO particles. Based on the similarity in appearance, it is also likely that the brighter atomic-scale and nanometer-size features shown in Fig. 2 correspond to Rh analogues of Pt structures found within Pt-doped CTO, which according to our recent analysis of slightly off-axis images arise from arrays of single atoms located on Ti sites and incipient metallic clusters, respectively.7
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One of the conclusions we immediately draw from our in-situ observations is thus that Rh NP precipitation and dissolution processes in CTO can take place on the time scale of minutes, or less, at temperatures that are 200-400 °C lower than those used for the ex-situ studies. (A similar conclusion for the Pd-LaFeO3 system has also been reached via x-ray absorption measurements.8) The detailed steps involved in both processes are difficult to capture, however, due to our inability to orient the CTO particles, but even if we could, the particles tend to reorient readily during the treatments. This latter phenomenon, consistent with the tendency for CTO (and many other perovskite oxides) to easily sinter, may be influenced by the disruption of the CTO lattice caused by Rh NP precipitation and dissolution. The precipitation of any Rh NP within the perovskite oxide matrix, for example, necessarily involves displacement of some CTO, accompanied by a volume expansion. The resulting strain imposed by a precipitating NP on its encapsulating perovskite oxide lattice has been modeled in a related system, Ni-doped La0.4Sr0.4TiO3-δ, in order to explain the emergence of large (~30 nm) Ni NPs from the near-surface region of lowsurface-area material during reduction treatments (exsolution).9 Due to the smaller length scales involved in our system, the mass transport of Rh and CTO may be expected to alter the CTO particle orientation, shape, and size distribution, as observed in Figs. 4 and 5, and some degree of amorphization of the CTO, as suggested by qualitative changes in image contrast observed after the first reduction-oxidation cycle, may explain the more rapid kinetics observed during the second cycle. Asymmetry between precipitation and dissolution of Rh NPs, together with a tendency for the larger Rh (or Rh2O3) NPs on the surface of CTO particles to coarsen, as suggested by the images shown in Fig. 3d, would be expected to lead to the net expulsion of Rh from CTO with increasing number of reduction-oxidation cycles. This is, in fact what we have observed by ex-situ examination of Rhdoped CTO catalysts following accelerated aging,10 where most (~90%) of the Rh, with an average NP size of 10 nm, appears to be on the surface of the CTO particles, which increased in average size from 15 to 40 nm.11 It is interesting that the average Rh NP size in an Al2O3-supported Rh catalyst was found to double, changing from about 1.5 to 3 nm, after the same accelerated aging treatment.12 This obviously counters the argument that a higher Rh dispersion should be retained in the CTO-based catalyst than in a more conventional Al2O3-based one upon aging. CONCLUSION The present in-situ STEM observations provide new, real time information about the Rh NP precipitation and dissolution processes in CTO, which are central to the ‘intelligent catalyst’ concept. Although the observations do not lend much support to the idea that Rh NP coarsen-
ing can be limited through reduction-oxidation cycling, the conditions under which these observations were performed still do not reproduce those that would exist in the exhaust gas passing over a three-way catalyst in use, and it is thus possible that exposure to more realistic conditions might lead to a different outcome. The effects of much smaller variations in oxygen chemical potential, imposed at higher frequency could, in principle, be explored in future experiments using this in-situ STEM technique.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Author Contributions § These authors contributed equally.
ACKNOWLEDGMENT We are grateful to M. Oljaca, of Cabot Corporation, for providing the sample of Rh-CTO and to the National Science Foundation, for providing financial support through grants CBET-1159240 and DMR-0723032.
REFERENCES (1) Nishihata, Y.; Mizuki, J.; Akao, T.; Tanaka, H.; Uenishi, M.; Kimura, M.; Okamoto, T.; Hamada, N. Nature 2002, 418, 164-167. (2) Tanaka, H.; Taniguchi, M.; Uenishi, M.; Kajita, N.; Tan, I.; Nishihata, Y.; Mizuki, J.; Narita, K.; Kimura, M.; Kaneko, K. Angew. Chem. Int. Ed. 2006, 45, 5998-6002. (3) Katz, M. B.; Zhang, S.; Duan, Y.; Wang, H.; Fang, M.; Zhang, K.; Li, B.; Graham, G. W.; Pan, X. J. Catal. 2012, 293, 145-148. (4) Zhang, S.; Chen, C.; Cargnello, M.; Fornasiero, P.; Gorte, R. J.; Graham, G. W.; Pan, X. Nat. Commun. 2015, 6, 7778. (5) Allard, L. F.; Overbury, S. H.; Bigelow, W. C.; Katz, M. B.; Nackashi, D. P.; Damiano, J. Micros. Microanal. 2012, 18, 656-666. (6) Zhang, S.; Cargnello, M.; Cai, W.; Murray, C. B.; Graham, G. W.; Pan, X. J. Catal. 2016, 337, 240-247. (7) Zhang, S.; Katz, M. B.; Zhang, K.; Du, X.; Graham, G. W.; Pan, X. Manuscript in preparation. (8) Tanaka, H.; Uenishi, M.; Taniguchi, M.; Tao, I.; Narita, K.; Kimura, M.; Kaneko, K.; Nishihata, Y.; Mizuki, J. Catal. Today 2006, 117, 321-328. (9) Oh, T. S.; Rahani, E. K.; Neagu, D.; Irvine, J. T. S.; Shenoy, V. B.; Gorte, R. J.; Vohs, J. M. J. Phys. Chem. Lett. 2015, 6, 5106-5110. (10) Malamis, S. A.; Harrington, R. J.; Katz, M. B.; Koerschner, D. S.; Zhang, S.; Cheng, Y.; Xu, L.; Jen, H-W.; McCabe, R. W.; Graham, G. W.; Pan, X. Catal. Today 2015, 258, 535-542. (11) The aged catalyst actually exhibited a bimodal distribution of NP sizes, where one mode, characterized by an average NP size of 1-3 nm, contained most of the NPs but only about 10% of the Rh mass, and the other mode, characterized by an average NP size of 10 nm, contained the rest of the NPs which accounted for about 90% of the Rh mass. Most of the large NPs appeared to be located on the surface of CTO particles.
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(12) Cai, W. Private communication, University of Michigan, 2014. Unpublished.
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