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Comparative study of supported catalyst particles by electron microscopy methods
ultramicr0sc0py
UltramicroscopyNorth_Hol 52land (1993)282-288
Comparative study of supported catalyst particles by electron microscopy methods M i n g - H u i Y a o a,,, David J. Smith
a
and A b h a y a K. D a t y e b
a Center for Solid State Science and Department of Physics, Arizona State Unit,ersity, Tempe, A Z 85287, USA b Department of Chemical and Nuclear Engineering, Unit,ersity of New Mexico, Albuquerque, NM 87131, USA Received 2 February 1993 Dedicated to Professor John M. Cowley on the occasion of his seventieth birthday
High-resolution transmission electron microscopy, high-resolution scanning electron microscopy and high-angle annular dark-field imaging were used to study the size distribution and surface structures of Pt model catalysts on various oxide supports. The relative merits of different electron microscopy methods for catalyst research were evaluated by comparing images recorded with microscopes of different type. It was concluded that H R T E M profile imaging was the most effective technique for direct observation of microstructure, especially the surface structure of supported particles, while H R S E M and H A A D F , respectively, were preferred for characterizing the surface topology of catalyst supports and the size distribution of supported particles. Using profile imaging, crystalline monolayers caused by high-temperature reduction on {111} surfaces of P t / T i O 2 could be recorded with atomic resolution. These overlayers help explain the drop in chemisorption ability due to high-temperature reduction, a p h e n o m e n o n usually referred to as strong metal support interaction. H R S E M showed the presence of surface steps on model TiO 2 and a concentration of larger Pt particles on these steps.
1. Introduction
The activity and selectivity of supported metal particle catalysts often depend on the particle size, shape, and surface structure. Electron microscopy can be used to relate these morphological features to catalysis, in particular to gain an understanding of catalytic performance and reaction mechanisms. Recent advances in instrumentation have made it possible to observe ultra-fine particles at high resolution with different electron microscopy techniques. Of these electron microscopy techniques, high-resolution transmission electron microscopy ( H R T E M ) has proven to be an invaluable tool in catalyst characterization [1]. Bright-field imaging can even resolve isolated atoms on some thin supports of weak contrast [2-4]. However, in plan * To whom correspondence should be sent.
view, the details of the structure and shape of sub-nanometer-sized particles are often obscured by overlapping contrast from the support. The profile imaging method can provide atomic-scale information about supported particles without the images being affected by contrast from the support [5], and there are numerous successful examples of supported particles being characterized by H R T E M profile imaging [1]. Moreover, it is even possible using profile imaging to determine very subtle changes on the surfaces of supported particles as a function of catalytic treatment, and thereby gain insight into catalytic behavior [6,7]. High-angle annular dark-field (HAADF) [8] imaging in the scanning transmission electron microscope (STEM) is another electron microscopy technique that is increasingly being used in recent years for catalyst study. This is because the detection sensitivity to high-atomic-number materials on relatively low-atomic-number supports can be
0304-3991/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved SSDI 0 3 0 4 - 3 9 9 1 ( 9 3 ) E 0 0 8 8 - P
M.H. Yao et aL / EM methods for the study of supported catalysts
improved greatly by collecting electrons incident upon a high-angle annular dark-field detector [9]. For example, on alumina supports, particles containing only three atoms have been detected by H A A D F [10]. The drawback of H A A F, however, is its relatively poor signal-to-noise ratio due to low scattering cross sections at high angle. As well as HAADF, other imaging modes are also available with the STEM, including bright-field (BF), dark-field (DF), and secondary electron imaging. The secondary electron (SE) signal carries surface-specific information, and the technique can therefore be used to study the surface topology of catalyst supports and supported particles. A knowledge of the surface topology of the support can provide better understanding of factors responsible for particle nucleation and growth and any interaction between the particles and their supports. In previous work, Pt particles as small as 3 nm in diameter on alumina supports have been observed by SE in STEM [11]. However, no ultra-high resolution (better than 1 nm) studies of catalysts using secondary electrons with either the scanning electron microscope (SEM) or the STEM appear to have been reported. This may have been due to limited SEM resolution in the past, as well as severe charging of the insulating catalyst supports. Although the resolution of SE imaging is somewhat limited by the delocalized nature of the secondary electron production process, recent developments of in-lens immersion systems and cold field emission sources of high brightness have made it possible to image specimen surfaces with secondary electrons at subnanometer resolution [12]. As well as its capability of giving surface topological information, the SEM also has a distinct advantage over TEM and STEM in terms of sample preparation and ease of location of areas suitable for microscopic observation. In this paper, we compare different electron microscopy techniques for the study of catalyst particles, in particular to identify which methods are best suited for certain aspects of catalyst characterization. W e have used H R T E M , H R S E M and H A A D F in a STEM to study surface structure changes and size distributions in
283
supported Pt on various supports as a function of high-temperature treatment. By comparing images from three different types of microscopes, we are able to evaluate the relative merits and demerits of the different electron microscopy techniques for catalyst research.
2. Experiment details Pt was deposited on TiO 2 and CeO 2 by impregnation using Pt acetylacetonate and H2PtCI 6 as precursors. The P t / T i O 2 samples were calcined in 10% 0 2 at 573 K and then reduced in flowing H 2 at 673 K. This was termed the L T R state of the catalyst. This catalyst was also reduced in 500 Torr of H 2 at 923 K (the H T R state). The P t / C e O 2 catalyst was calcined at 773 K in flowing air, and reduced at 873 K, respectively, in H 2. The catalyst was repeatedly cycled under oxidizing (773 K in 10% 0 2) and reducing (773 K in flowing H2) atmospheres to study the influence of pretreatment on catalytic activity [13]. Samples for electron microscope observations were prepared by dipping copper grids in dry powders of the supported Pt catalysts. The differences of Pt surface structure in high-temperature reduction (HTR) and oxidized state were very subtle. To prevent any contamination or modification of surface structure, no solvents were used at any stage in the sample preparation process. Moreover, since charging is often a problem in recording high-resolution images of oxide-supported catalysts, most samples observed were supported directly on copper grids without carbon film. We found that these samples generally showed better overall stability under the electron beam. H R T E M observations were made with a JEM4000EX microscope, operated at 400 kV, with an interpretable resolution of about 0.17 nm. H R S E M images were obtained with a Hitachi S-5000 in-lens field-emission scanning electron microscope with a tested resolution of 0.6 nm at 30 kV, and the capability of signal processing. Polaroid films were exposed from the H R memory of 1024 x 1024 pixels/10 bits. H A A D F images were obtained using a VG HB501 STEM
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Fig. 1. H R S E M image of P t / T i O 2 catalyst recorded at operating voltage of 30 kV and initial magnification of 180 k x .
microscope operated at 100 kV. Very-high magnifications, typically 500 k x to 1000 k x times, were used with all three microscopes in order to image the structure of the supported ultra-fine particles, especially to reveal subtle differences in their suface morphology after temperature treatments.
3. Results and discussions
Fig. 2. H R S E M image of P t / T i O 2 catalyst recorded at operating voltage of 30 kV and initial magnification of 1000 k x .
three micrographs but the apparent densities of Pt particles on the support surfaces are not identical. As might be expected, the H A A D F image shows the highest density of the three methods. This is partly due to the high sensitivity of H A A D F to Z-contrast but it is also due to the fact that particles on both sides of the support are imaged. In contrast to H A A D F , the density observed in the H R S E M image was about half or slightly less, mainly because only particles on the top surface of the support were recorded with the SE detector. In this respect, imaging by H R S E M
3.1. Comparison of images from different microscopes A typical low-magnification SE micrograph of the P t / T i O 2 catalyst is shown in fig. 1. The supported Pt particles range from 0.8 to 14 nm in size, while their TiO 2 supports range in size from 20 to 100 nm. The contrast and profile of the TiO 2 particles clearly revealed topological information in three dimensions about the spherical TiO 2 particles. T h r e e high-magnification micrographs of P t / T i O 2 imaged with H R S E M , H R T E M , and H A A D F are shown in figs. 2, 3 and 4, respectively. The micrographs in figs. 2 and 4 were originally recorded at 1000 k x , whereas the H R T E M image in fig. 4 was recorded at 500 k X. Pt particles as small as 1 nm are observable in all
Fig. 3. H A A D F image of P t / T i O 2 recorded on VG HB501 at 100 kV and 1000 k x .
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Fig. 4. HRTEM micrograph of Pt/TiO 2 recorded by JEM4000EX at 400 kV and 1000 k ×. could be considered as an advantage over H A A D F and H R T E M since, in the latter techniques, it is almost impossible to determine the side on which the supported particles are located. In SEM, the locations of the supported fine particles are uniquely determined. Moreover, particles can be imaged on catalyst supports of any thickness. Sample preparation and observation become much easier. It is no longer necessary to locate particles dangling on the edges of grid bars or carbon film as in T E M and STEM. The most important aspect of H R S E M for catalyst study is its ability to provide information about the surface morphology of the catalyst support. For example, steps and ledges on TiO 2 surfaces are visible in fig. 2, but they cannot be seen at all in figs. 3 and 4. It is also interesting to note in fig. 2 that larger Pt particles are more likely to be located in the vicinity of kinks on the support surfaces. Despite all of the positive attractions of imaging with H R S E M , it does have some drawbacks. Figs. 5 and 6 show a comparison of H R images of P t / C e O 2 taken from T E M and H R S E M . Small Pt particles can hardly be seen in fig. 5, whereas Pt particles of 1 nm are very clearly visible in fig. 6. Moreover, the resolution of the H R S E M is material-dependent since the SE generating processes are material-dependent. In the P t / T i O 2 samples, particles of 1 nm in size could be seen without much difficulty by SEM, as in fig. 2, whereas they appear to be invisible in the
Fig. 5. HRSEM image of Pt/CeO 2. Pt particles almost invisible. P t / C e O 2 sample. The visibility of small supported particles in SE imaging improves considerably when the support particles are large and the support surfaces are relatively smooth as in the case of the model P t / T i O z catalyst: The TiO 2 support consisted of spherical particles synthesized by a dry process [6]. On the other hand, the CeO 2 support was prepared by calcining the precipitate formed by reacting C e ( N O ) 3 with N H 4 O H . Another drawback of H R S E M in catalyst characterization is the absence of characteristic features such as lattice fringes which facilitate identification of metal crystallites in H R T E M . In comparison to H R S E M and H A A D F , H R T E M has much better resolution and image
Fig. 6. HRTEM image of Pt/CeO 2. Pt particles on well faceted CeO2 surfaces are epitaxial.
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quality. This superiority is clearly shown by a comparison of figs. 4 and 6 with figs. 2, 3 and 5. Although small Pt particles on TiO 2 and CeO 2 may have poor contrast, or even be invisible in plan view, they can be seen clearly in profile images. With H R T E M , one can study the structure and morphology of both support and supported particles, as well as any crystallographic relationship between the support and the supported particles. As examples, in figs. 4 and 6, one can see a twin boundary in the TiO 2 support, cuboctahedral Pt particles along [110] projections and an epitaxial relationship between Pt particles with the CeO 2 support. Most importantly, surfaces of the catalyst particles can be studied by H R T E M profile imaging with a resolution that cannot be approached by these other electron microscopic methods. 3.2. Effects o f temperature treatment on the surface structure
High-temperature treatment of metal particles under reducing as well as oxidizing atmospheres has been extensively studied [1]. Catalysts such as Rh and Pt exhibit the phenomenon known as the strong metal-support interaction (SMSI) [14] when supported on reducible oxides after hightemperature reduction in H 2. For certain reactions, SMSI causes a drop in catalytic activity, while the reactivity may be unaffected or even enhanced for others. Several models have been proposed [15-17] to explain the mechanism of SMSI. In recent years, there is increasing evidence to favor the "decoration model", which suggests that SMSI, at least in part, is due to the encapsulation of metal by an oxide overlayer species. There are several H R T E M reports about the overlayers on supported Rh and Pt particles [6,7,18-20]. As discussed earlier, H R T E M profile imaging is the preferred technique for observing directly the surface structure of supported particles on the atomic scale. Therefore, in order to better understand the mechanism of SMSI, we used H R T E M profile imaging for comparing the differences between Pt surfaces of P t / T i O 2 catalysts in H T R and oxidized states. Crystalline
Fig. 7. Two atomic-resolution micrographs of cuboctahedral Pt particles in HTR state along (110) projection. {111} surfaces are covered by crystalline monolayers.
monolayers on {111} surfaces, and some less ordered oxide layers on other surfaces of Pt, were found in the P t / T i O 2 samples after H T R [7]; Two typical cuboctahedral particles of Pt supported on TiO 2 in H T R state are shown in fig. 7 along (110) projection. The top layers on (111} P t / T i O 2 in H T R are distinctively different from the bulk and that of P t / T i O 2 in an oxidized state (fig. 8). The contrast from the monolayer was high, and the average periodicity in the monolayer was about the same as that of Pt in {111} plane. However, the spacing between the monolayer and the top (111) plane of Pt is about 15% larger than that of Pt (111). We believe that these black dots in the monolayer are Ti atoms, which migrated from the support and blocked certain atomic sites: they may be responsible for the substantial drop in n-butane hydrogenolysis activity [21]. The proposed structure model is shown
M.H. Yao et al. / EM methods for the study of supported catalysts
287
exhibits considerable suppression of hydrogenolysis activity after H T R [13]. No differences in structure can be detected between Pt particles in H T R and oxidized states on the CeO 2 support. It is likely that the mechanism for activity suppression for P t / C e O 2 is different from the encapsulation that seems to occur on P t / T i O 2 , and can be detected by H R T E M .
4. Conclusions
Fig. 8. P t / T i O 2 particle in oxidized state showing clean surfaces.
in fig. 9. Simulations of profile images will be used in attempts to confirm the atomic structure of this overlayer. No similar overlayers are observed on the P t / C e O 2 sample despite the fact that it also
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The techniques of H R T E M , H R S E M and H A A D F are all very useful for catalyst characterization. Because of its high sensitivity to atomic number, H A A D F is preferred for a statistical study of size distribution of supported heavy metal particles. H R S E M can also be used to study the size distribution of catalyst particles if the supported particles are greater than 1 nm and the support surface is relatively flat and smooth, but it is best suited to characterizing the surface morphology of the catalyst support. For direct observation of microstructure, especially the surface structure of fine particles, H R T E M profile imaging is the most effective technique. Surfaces of TiO 2 in H T R and oxidized states were compared by this method. Crystalline monolayers on {111} surfaces were found on P t / T i O 2 after H T R and it was proposed that they were primarily Ti suboxide monolayers that blocked certain reaction sites, and caused a drop of butane hydrogenolysis activity after H T R . The P t / T i O 2 sample in o x i d i z e d / L T R state showed clean surfaces in profile view, an observation that correlates well with the observed restoration of catalytic activity of P t / T i O 2 after oxidation [21].
(b) 11 surface
Fig. 9. (a) (110) projection of a perfect cuboctahedron containing 309 Pt atoms. {111} surfaces are covered by Ti monolayers. (b) Unit cell on {111} surface.
Acknowledgements The authors would like to thank Mr. A. Higgs and Mikhail Reilly for assistance with the HB-5 S T E M and Hitachi S-5000 F E S E M instruments. This research is partially supported by the Donors of the Petroleum Research Fund, grant 23995AC5, and Mobil R & D. The microscopy was conducted at the Center for High Resolution Elec-
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M.H. Yao et al. / EM methods ]'or the study of supported catalysts
tron Microscopy supported by NSF Grant DMR9115680.
References [1] A.K. Datye and D.J. Smith, Catal. Rev.-Sci. Eng. 34 (1992) 129. [2] E.B. Prestridge and D.J.C. Yates, Nature 234 11971) 345. [3] L.D. Marks, M.C. Hong, H. Zhang and B.K. Teo, Mater. Res. Soc. Symp. Proc. 111 (1988) 213. [4] M.H. Yao, D.J. Smith and I.E. Wachs, in: Proc. 49th Annual EMSA Meeting, 1991, p. 1030. [5] L.D. Marks and D.J. Smith, Nature 303 (19831 316. [6] E.J. Braunschweig, A.D. Logan, A.K. Datye and D.J. Smith, J. Catal. 118 (1989) 227. [7] M.H. Yao, D.J. Smith, D. Kalakkad and A.K. Datye, Z. Phys. D, in press. [8] A. Howie, J. Microscopy 17 (1979) 11. [9] M.M.J. Treacy and A. Howie, J. Catal. 63 (19801 265.
[10] M.S. Isaacson, D. Kopf, M. Ohtsuki and M. Utlaut, UItramicroscopy 4 (1979) 101. [11] J. Liu and J.M. Cowley, Scanning Microsc. 2 (1988) 65, 1957. [12] T. Nagatani, S. Saito, S. Sato and M. Yamada, Scanning Microsc. 1 (1987)901. [13] D. Kalakkad, A.K. Datye and H.J. Robota, Appl. Catal. B 1 (1992) 191. [14] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 11978) 170. [15] S.J. Tauster and S.C. Fung, J. Catal. 55 119781 29. [16] J.M. Herrmann and P. Pichat, J. Catal. 78 (19821 425. [17] J.A. Horsley, J. Am. Chem. Soc. 55 (1979) 29. [18] J. Santos, J. Phillips and J.A. Dumesic, J. Catal. 81 (19831 147. [19] R.T. Baker, E.B. Prestridge and L.L. Murrell, J. Catal. 79 (1983) 348. [211] L. Wang, G.W. Qiao, H.Q. Ye, K.H. Kuo and Y.X. Chert, in: Proc. 9th Int. Congr. on Catalysis (The Chemical Institute of Canada, Ottawa, 1988) p. 1253. [21] K.J. Blankenburg and A.K. Datye, J. Catal. 128 (1991) 186.
UltramicroscopyNorth_Hol 52land (1993)282-288
Comparative study of supported catalyst particles by electron microscopy methods M i n g - H u i Y a o a,,, David J. Smith
a
and A b h a y a K. D a t y e b
a Center for Solid State Science and Department of Physics, Arizona State Unit,ersity, Tempe, A Z 85287, USA b Department of Chemical and Nuclear Engineering, Unit,ersity of New Mexico, Albuquerque, NM 87131, USA Received 2 February 1993 Dedicated to Professor John M. Cowley on the occasion of his seventieth birthday
High-resolution transmission electron microscopy, high-resolution scanning electron microscopy and high-angle annular dark-field imaging were used to study the size distribution and surface structures of Pt model catalysts on various oxide supports. The relative merits of different electron microscopy methods for catalyst research were evaluated by comparing images recorded with microscopes of different type. It was concluded that H R T E M profile imaging was the most effective technique for direct observation of microstructure, especially the surface structure of supported particles, while H R S E M and H A A D F , respectively, were preferred for characterizing the surface topology of catalyst supports and the size distribution of supported particles. Using profile imaging, crystalline monolayers caused by high-temperature reduction on {111} surfaces of P t / T i O 2 could be recorded with atomic resolution. These overlayers help explain the drop in chemisorption ability due to high-temperature reduction, a p h e n o m e n o n usually referred to as strong metal support interaction. H R S E M showed the presence of surface steps on model TiO 2 and a concentration of larger Pt particles on these steps.
1. Introduction
The activity and selectivity of supported metal particle catalysts often depend on the particle size, shape, and surface structure. Electron microscopy can be used to relate these morphological features to catalysis, in particular to gain an understanding of catalytic performance and reaction mechanisms. Recent advances in instrumentation have made it possible to observe ultra-fine particles at high resolution with different electron microscopy techniques. Of these electron microscopy techniques, high-resolution transmission electron microscopy ( H R T E M ) has proven to be an invaluable tool in catalyst characterization [1]. Bright-field imaging can even resolve isolated atoms on some thin supports of weak contrast [2-4]. However, in plan * To whom correspondence should be sent.
view, the details of the structure and shape of sub-nanometer-sized particles are often obscured by overlapping contrast from the support. The profile imaging method can provide atomic-scale information about supported particles without the images being affected by contrast from the support [5], and there are numerous successful examples of supported particles being characterized by H R T E M profile imaging [1]. Moreover, it is even possible using profile imaging to determine very subtle changes on the surfaces of supported particles as a function of catalytic treatment, and thereby gain insight into catalytic behavior [6,7]. High-angle annular dark-field (HAADF) [8] imaging in the scanning transmission electron microscope (STEM) is another electron microscopy technique that is increasingly being used in recent years for catalyst study. This is because the detection sensitivity to high-atomic-number materials on relatively low-atomic-number supports can be
0304-3991/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved SSDI 0 3 0 4 - 3 9 9 1 ( 9 3 ) E 0 0 8 8 - P
M.H. Yao et aL / EM methods for the study of supported catalysts
improved greatly by collecting electrons incident upon a high-angle annular dark-field detector [9]. For example, on alumina supports, particles containing only three atoms have been detected by H A A D F [10]. The drawback of H A A F, however, is its relatively poor signal-to-noise ratio due to low scattering cross sections at high angle. As well as HAADF, other imaging modes are also available with the STEM, including bright-field (BF), dark-field (DF), and secondary electron imaging. The secondary electron (SE) signal carries surface-specific information, and the technique can therefore be used to study the surface topology of catalyst supports and supported particles. A knowledge of the surface topology of the support can provide better understanding of factors responsible for particle nucleation and growth and any interaction between the particles and their supports. In previous work, Pt particles as small as 3 nm in diameter on alumina supports have been observed by SE in STEM [11]. However, no ultra-high resolution (better than 1 nm) studies of catalysts using secondary electrons with either the scanning electron microscope (SEM) or the STEM appear to have been reported. This may have been due to limited SEM resolution in the past, as well as severe charging of the insulating catalyst supports. Although the resolution of SE imaging is somewhat limited by the delocalized nature of the secondary electron production process, recent developments of in-lens immersion systems and cold field emission sources of high brightness have made it possible to image specimen surfaces with secondary electrons at subnanometer resolution [12]. As well as its capability of giving surface topological information, the SEM also has a distinct advantage over TEM and STEM in terms of sample preparation and ease of location of areas suitable for microscopic observation. In this paper, we compare different electron microscopy techniques for the study of catalyst particles, in particular to identify which methods are best suited for certain aspects of catalyst characterization. W e have used H R T E M , H R S E M and H A A D F in a STEM to study surface structure changes and size distributions in
283
supported Pt on various supports as a function of high-temperature treatment. By comparing images from three different types of microscopes, we are able to evaluate the relative merits and demerits of the different electron microscopy techniques for catalyst research.
2. Experiment details Pt was deposited on TiO 2 and CeO 2 by impregnation using Pt acetylacetonate and H2PtCI 6 as precursors. The P t / T i O 2 samples were calcined in 10% 0 2 at 573 K and then reduced in flowing H 2 at 673 K. This was termed the L T R state of the catalyst. This catalyst was also reduced in 500 Torr of H 2 at 923 K (the H T R state). The P t / C e O 2 catalyst was calcined at 773 K in flowing air, and reduced at 873 K, respectively, in H 2. The catalyst was repeatedly cycled under oxidizing (773 K in 10% 0 2) and reducing (773 K in flowing H2) atmospheres to study the influence of pretreatment on catalytic activity [13]. Samples for electron microscope observations were prepared by dipping copper grids in dry powders of the supported Pt catalysts. The differences of Pt surface structure in high-temperature reduction (HTR) and oxidized state were very subtle. To prevent any contamination or modification of surface structure, no solvents were used at any stage in the sample preparation process. Moreover, since charging is often a problem in recording high-resolution images of oxide-supported catalysts, most samples observed were supported directly on copper grids without carbon film. We found that these samples generally showed better overall stability under the electron beam. H R T E M observations were made with a JEM4000EX microscope, operated at 400 kV, with an interpretable resolution of about 0.17 nm. H R S E M images were obtained with a Hitachi S-5000 in-lens field-emission scanning electron microscope with a tested resolution of 0.6 nm at 30 kV, and the capability of signal processing. Polaroid films were exposed from the H R memory of 1024 x 1024 pixels/10 bits. H A A D F images were obtained using a VG HB501 STEM
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Fig. 1. H R S E M image of P t / T i O 2 catalyst recorded at operating voltage of 30 kV and initial magnification of 180 k x .
microscope operated at 100 kV. Very-high magnifications, typically 500 k x to 1000 k x times, were used with all three microscopes in order to image the structure of the supported ultra-fine particles, especially to reveal subtle differences in their suface morphology after temperature treatments.
3. Results and discussions
Fig. 2. H R S E M image of P t / T i O 2 catalyst recorded at operating voltage of 30 kV and initial magnification of 1000 k x .
three micrographs but the apparent densities of Pt particles on the support surfaces are not identical. As might be expected, the H A A D F image shows the highest density of the three methods. This is partly due to the high sensitivity of H A A D F to Z-contrast but it is also due to the fact that particles on both sides of the support are imaged. In contrast to H A A D F , the density observed in the H R S E M image was about half or slightly less, mainly because only particles on the top surface of the support were recorded with the SE detector. In this respect, imaging by H R S E M
3.1. Comparison of images from different microscopes A typical low-magnification SE micrograph of the P t / T i O 2 catalyst is shown in fig. 1. The supported Pt particles range from 0.8 to 14 nm in size, while their TiO 2 supports range in size from 20 to 100 nm. The contrast and profile of the TiO 2 particles clearly revealed topological information in three dimensions about the spherical TiO 2 particles. T h r e e high-magnification micrographs of P t / T i O 2 imaged with H R S E M , H R T E M , and H A A D F are shown in figs. 2, 3 and 4, respectively. The micrographs in figs. 2 and 4 were originally recorded at 1000 k x , whereas the H R T E M image in fig. 4 was recorded at 500 k X. Pt particles as small as 1 nm are observable in all
Fig. 3. H A A D F image of P t / T i O 2 recorded on VG HB501 at 100 kV and 1000 k x .
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Fig. 4. HRTEM micrograph of Pt/TiO 2 recorded by JEM4000EX at 400 kV and 1000 k ×. could be considered as an advantage over H A A D F and H R T E M since, in the latter techniques, it is almost impossible to determine the side on which the supported particles are located. In SEM, the locations of the supported fine particles are uniquely determined. Moreover, particles can be imaged on catalyst supports of any thickness. Sample preparation and observation become much easier. It is no longer necessary to locate particles dangling on the edges of grid bars or carbon film as in T E M and STEM. The most important aspect of H R S E M for catalyst study is its ability to provide information about the surface morphology of the catalyst support. For example, steps and ledges on TiO 2 surfaces are visible in fig. 2, but they cannot be seen at all in figs. 3 and 4. It is also interesting to note in fig. 2 that larger Pt particles are more likely to be located in the vicinity of kinks on the support surfaces. Despite all of the positive attractions of imaging with H R S E M , it does have some drawbacks. Figs. 5 and 6 show a comparison of H R images of P t / C e O 2 taken from T E M and H R S E M . Small Pt particles can hardly be seen in fig. 5, whereas Pt particles of 1 nm are very clearly visible in fig. 6. Moreover, the resolution of the H R S E M is material-dependent since the SE generating processes are material-dependent. In the P t / T i O 2 samples, particles of 1 nm in size could be seen without much difficulty by SEM, as in fig. 2, whereas they appear to be invisible in the
Fig. 5. HRSEM image of Pt/CeO 2. Pt particles almost invisible. P t / C e O 2 sample. The visibility of small supported particles in SE imaging improves considerably when the support particles are large and the support surfaces are relatively smooth as in the case of the model P t / T i O z catalyst: The TiO 2 support consisted of spherical particles synthesized by a dry process [6]. On the other hand, the CeO 2 support was prepared by calcining the precipitate formed by reacting C e ( N O ) 3 with N H 4 O H . Another drawback of H R S E M in catalyst characterization is the absence of characteristic features such as lattice fringes which facilitate identification of metal crystallites in H R T E M . In comparison to H R S E M and H A A D F , H R T E M has much better resolution and image
Fig. 6. HRTEM image of Pt/CeO 2. Pt particles on well faceted CeO2 surfaces are epitaxial.
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M.H. Yao et al. / EM methods for the study of supported catalysts
quality. This superiority is clearly shown by a comparison of figs. 4 and 6 with figs. 2, 3 and 5. Although small Pt particles on TiO 2 and CeO 2 may have poor contrast, or even be invisible in plan view, they can be seen clearly in profile images. With H R T E M , one can study the structure and morphology of both support and supported particles, as well as any crystallographic relationship between the support and the supported particles. As examples, in figs. 4 and 6, one can see a twin boundary in the TiO 2 support, cuboctahedral Pt particles along [110] projections and an epitaxial relationship between Pt particles with the CeO 2 support. Most importantly, surfaces of the catalyst particles can be studied by H R T E M profile imaging with a resolution that cannot be approached by these other electron microscopic methods. 3.2. Effects o f temperature treatment on the surface structure
High-temperature treatment of metal particles under reducing as well as oxidizing atmospheres has been extensively studied [1]. Catalysts such as Rh and Pt exhibit the phenomenon known as the strong metal-support interaction (SMSI) [14] when supported on reducible oxides after hightemperature reduction in H 2. For certain reactions, SMSI causes a drop in catalytic activity, while the reactivity may be unaffected or even enhanced for others. Several models have been proposed [15-17] to explain the mechanism of SMSI. In recent years, there is increasing evidence to favor the "decoration model", which suggests that SMSI, at least in part, is due to the encapsulation of metal by an oxide overlayer species. There are several H R T E M reports about the overlayers on supported Rh and Pt particles [6,7,18-20]. As discussed earlier, H R T E M profile imaging is the preferred technique for observing directly the surface structure of supported particles on the atomic scale. Therefore, in order to better understand the mechanism of SMSI, we used H R T E M profile imaging for comparing the differences between Pt surfaces of P t / T i O 2 catalysts in H T R and oxidized states. Crystalline
Fig. 7. Two atomic-resolution micrographs of cuboctahedral Pt particles in HTR state along (110) projection. {111} surfaces are covered by crystalline monolayers.
monolayers on {111} surfaces, and some less ordered oxide layers on other surfaces of Pt, were found in the P t / T i O 2 samples after H T R [7]; Two typical cuboctahedral particles of Pt supported on TiO 2 in H T R state are shown in fig. 7 along (110) projection. The top layers on (111} P t / T i O 2 in H T R are distinctively different from the bulk and that of P t / T i O 2 in an oxidized state (fig. 8). The contrast from the monolayer was high, and the average periodicity in the monolayer was about the same as that of Pt in {111} plane. However, the spacing between the monolayer and the top (111) plane of Pt is about 15% larger than that of Pt (111). We believe that these black dots in the monolayer are Ti atoms, which migrated from the support and blocked certain atomic sites: they may be responsible for the substantial drop in n-butane hydrogenolysis activity [21]. The proposed structure model is shown
M.H. Yao et al. / EM methods for the study of supported catalysts
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exhibits considerable suppression of hydrogenolysis activity after H T R [13]. No differences in structure can be detected between Pt particles in H T R and oxidized states on the CeO 2 support. It is likely that the mechanism for activity suppression for P t / C e O 2 is different from the encapsulation that seems to occur on P t / T i O 2 , and can be detected by H R T E M .
4. Conclusions
Fig. 8. P t / T i O 2 particle in oxidized state showing clean surfaces.
in fig. 9. Simulations of profile images will be used in attempts to confirm the atomic structure of this overlayer. No similar overlayers are observed on the P t / C e O 2 sample despite the fact that it also
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The techniques of H R T E M , H R S E M and H A A D F are all very useful for catalyst characterization. Because of its high sensitivity to atomic number, H A A D F is preferred for a statistical study of size distribution of supported heavy metal particles. H R S E M can also be used to study the size distribution of catalyst particles if the supported particles are greater than 1 nm and the support surface is relatively flat and smooth, but it is best suited to characterizing the surface morphology of the catalyst support. For direct observation of microstructure, especially the surface structure of fine particles, H R T E M profile imaging is the most effective technique. Surfaces of TiO 2 in H T R and oxidized states were compared by this method. Crystalline monolayers on {111} surfaces were found on P t / T i O 2 after H T R and it was proposed that they were primarily Ti suboxide monolayers that blocked certain reaction sites, and caused a drop of butane hydrogenolysis activity after H T R . The P t / T i O 2 sample in o x i d i z e d / L T R state showed clean surfaces in profile view, an observation that correlates well with the observed restoration of catalytic activity of P t / T i O 2 after oxidation [21].
(b) 11 surface
Fig. 9. (a) (110) projection of a perfect cuboctahedron containing 309 Pt atoms. {111} surfaces are covered by Ti monolayers. (b) Unit cell on {111} surface.
Acknowledgements The authors would like to thank Mr. A. Higgs and Mikhail Reilly for assistance with the HB-5 S T E M and Hitachi S-5000 F E S E M instruments. This research is partially supported by the Donors of the Petroleum Research Fund, grant 23995AC5, and Mobil R & D. The microscopy was conducted at the Center for High Resolution Elec-
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tron Microscopy supported by NSF Grant DMR9115680.
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