- Email: [email protected]
γ-Al2O3 catalysts
Applied Catalysis B: Environmental 38 (2002) 271–281
Direct atomic scale analysis of the distribution of Cu valence states in Cu/␥-Al2 O3 catalysts Kai Sun a,∗ , Jingyue Liu b , Nigel D. Browning a a
Department of Physics (M/C273), University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607-7059, USA b Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO 63167, USA Received 5 December 2001; received in revised form 4 March 2002; accepted 7 March 2002
Abstract In this paper, the microstructure of a 1 wt.% Cu/␥-Al2 O3 catalyst that was reduced in a 4% hydrogen/argon atmosphere at temperatures of 523, 773 and 1073 K, is studied by Z-contrast imaging and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Results show that the copper species are well dispersed when the catalyst is reduced below 523 K. At 773 K, separated Cu(I) and Cu(0) species are found existing as ring-like and bulk-like particles. This appears to indicate that the copper has not been reduced to its metallic form due to the interaction between the copper oxide and the support. Large spherical particles having core-shell structures with Cu(I) in the shells and Cu(0) in the cores are generated when the catalyst is reduced at 1073 K. The formation of partially oxidized copper species upon reduction at 1073 K is attributed to the metallic copper interaction with the alumina support. This study also demonstrates that high-spatial resolution Z-contrast imaging and EELS performed simultaneously can provide unique information on the morphology and chemistry of metal species in supported metal catalysts. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cu/␥-Al2 O3 ; Reduction; EELS; Z-contrast imaging; STEM; Valence state; Metal–support interaction (MSI)
1. Introduction Carbon dioxide and nitrogen oxides, which have been identified as greenhouse gases, are all recognized as environmental pollutants [1–3]. Several approaches to reduce these gaseous emissions into the atmosphere have been suggested for both mobile and stationary sources, but the catalytic approach is the most effective to meet current and future requirements. Copper based catalysts, such as Cu/␥-Al2 O3 catalysts, can play an important role in these catalytic reactions. For example, they can be used for the direct ∗ Corresponding author. Tel.: +1-312-413-2790; fax: +1-312-996-4451. E-mail address: [email protected] (K. Sun).
decomposition of N2 O to N2 [2–4], the selective catalytic reduction of nitrogen oxides by hydrocarbons in an oxygen-rich atmosphere [5,6], and also for conversion of CO2 to CO by catalytic hydrogenation [7]. In addition, they can also be used in hydrogen fuel cells to generate energy for vehicles, thereby helping meet increasingly stringent legislation controlling emissions from internal combustion engines [8–10]. An important goal in catalyst research is the identification of the sites controlling activity and selectivity for a given reaction; elucidating the local structure of the active species as well as its dispersion in the catalytic reaction. For oxide supported metal catalysts, the morphology and oxidation states of metallic species as well as their interaction with substrates are key factors that determine the active sites and their
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 5 5 - 3
272
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
distribution. In the case of supported copper catalysts, copper species having different oxidation states have different catalytic properties. Cu(0) can act as the active site to dissociate CO2 [7], while Cu(I) can be used for photocatalytic decomposition of N2 O into N2 and O2 [4]. Understanding and controlling the formation of the copper species during the preparation of the catalyst is therefore important for the design of specific catalysts for particular uses. Information on the structural changes that occur during the reduction of the Cu/Al2 O3 catalysts has been obtained in the past using a multitude of different techniques. These include temperature-programmed reduction (TPR) [11], electron spin resonance (ESR) [12], X-ray photoelectron spectroscopy (XPS) [13], X-ray absorption spectroscopy (XAS) [12], etc. However, while these techniques can provide information on the overall distribution of the copper oxidation states, i.e. how much Cu(0) or Cu(I) exists, the information cannot be defined to a particular location, i.e. the surfaces or the interior of a nanoscale cluster. Moreover, although TPR is a relatively simple and widely used technique there are problems associated with the accuracy of characterizing Cu(0) and Cu(I) sites using CO adsorption [11]. Furthermore, XPS cannot separate Cu(0) and Cu(I) species [13] and catalysts containing less than 5% Cu showed only alumina by XRD [14]. To obtain detailed information on both the morphology and oxidation state of the metallic species in supported catalysts, it was suggested that the most efficient method should be the combination of atomic imaging and spectroscopic techniques [15]. By these methods, the morphologies of the supported metallic species can be directly revealed by imaging techniques while the oxidation states can be obtained by spectroscopic methods. Although high-resolution electron microscopy (HREM) is a powerful tool to determine structures, it is not suitable for imaging supported metal catalysts with metallic clusters smaller than 2 nm because the scattering from substrate (crystallized) masks the metallic cluster [16]. Additionally, it does not permit spectroscopic information to be obtained. However, metallic clusters with high Z (atomic number) in low Z supports can be easily resolved by Z-contrast imaging. In fact, Z-contrast imaging has been successfully used for directly imaging single atoms on carbon [17] and alumina [18]. This makes
Z-contrast imaging ideally suited to characterize the morphology of supported metallic catalysts [18–20]. Although the Z-contrast images provide unprecedented information on the structure of the metallic clusters, they do not contain electronic structure information. However, the experimental set-up of a STEM makes it possible for EELS to be performed simultaneously with Z-contrast imaging at the same atomic resolution [21,22]. EEL spectra contain, in addition to the chemical composition, information concerning the local electronic structure of materials [23]. Changes in the bonding type, the oxidation states of atoms or the structural arrangement of neighboring atoms can modify the distribution of unoccupied electronic states, and this is reflected in the features of EEL spectra. Therefore, by combining Z-contrast imaging with EELS, the optimum approach to characterizing active sites in supported metal clusters can be obtained at atomic spatial resolution [21]. In the present study, Z-contrast imaging together with EELS is used to characterize a Cu/␥-Al2 O3 catalyst and evaluate the microstructure/oxidation state changes that occur during reduction.
2. Experimental 2.1. Catalyst preparation ␥-Alumina powder with BET surface area and pore volume (measured by mercury porosimetry) of 160 m2 /g, and 1.3 ml/g, respectively, was used as the catalyst support. A 10% aqueous slurry of the alumina with the pH adjusted to 11 by sodium bicarbonate and an aqueous solution containing Cu as CuCl2 ·2H2 O to get 1.0 wt.% Cu loading on the support were prepared separately. The aqueous copper solution was added slowly into the slurry while it was continuously stirred by an impeller. The Cu salt was allowed to be adsorbed at room temperature for 30 min at which point the slurry was heated to 338 K, and held at this temperature for another 30 min to complete the adsorption of Cu. The slurry was then filtered, washed with deionized water and dried in an oven in air at 393 K overnight. This dried catalyst was then reduced in small portions in a tube furnace by a stream of 4% H2 /Ar (at a flow rate of 40 ml/min) for 2 h at various temperatures (523, 773 and 1073 K).
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
After a 2 h hold period, the tube was cooled down to room temperature in the hydrogen atmosphere. After this the flowing gas was switched to argon, and the catalyst unloaded. Specimens were prepared for electron microscopy analysis by placing the powders on holey-carbon-coated copper and molybdenum grids.
273
the presence of copper in the catalyst (shown in Fig. 1d). The fact that no obvious copper species can be directly resolved from the Z-contrast image and EEL spectra indicates that copper species is highly dispersed within the catalyst when reduced at 523 K. 3.2. The Cu/γ -Al2 O3 catalyst reduced at 773 K
2.2. Catalyst characterization The analysis reported here was performed using a JEOL 2010F field emission STEM/TEM operating at 200 kV in the STEM mode. The oxygen partial pressure in the microscope column is about 5 × 10−8 Pa during the experiment (i.e. a highly reducing atmosphere). The lens conditions during operation were defined for a probe size of 0.2 nm, with a convergence angle of 13 mrad and a collection angle of 52 mrad [24]. The energy resolution (defined by the full width at half maximum of zero loss peak) of the energy-loss spectra is 1.2 eV at a dispersion of 0.3 eV per channel. A Gatan PEELS system and EL/P 3.3 software were used for EELS data collection and processing. A more detailed description of these techniques can be found elsewhere [25].
3. Results 3.1. The Cu/γ -Al2 O3 catalyst reduced at 523 K Fig. 1a and b show low-magnification Z-contrast and bright-field images taken from the Cu/␥-Al2 O3 catalyst reduced at 523 K. From the images we can see that the catalyst consists of mainly needle-like particles and some small irregular particles. Selected-area electron diffraction (SAED) analysis shows aluminalike ring patterns (not shown here). No separated copper species can be resolved from the Z-contrast images although there exists a great Z difference between Cu and Al and O. Moreover, EEL spectra collected from different particles do not show the expected copper L2,3 edges (see the inset shown in Fig. 1c). As the Cu M2,3 edges (onset: 74 eV) overlap with the Al L2,3 edges (onset: 78 eV), we could not identify whether or not the EEL spectrum shown in Fig. 1c contains the copper peaks. However, energy dispersive X-ray spectroscopy (EDS) collected while scanning the probe over an area of the specimen clearly shows
Fig. 2 shows results obtained from the catalyst reduced at 773 K. Here we can see two types of morphological particles, ring-like (R, Fig. 2a and c) and bulk-like (B, Fig 2b and c) particles, besides the needle-like particles. One interesting feature is that these bulk-like particles always coexist with ring-like particles, although they are not completely encapsulated by the ring-like particles (as indicated by arrows in Fig. 2b). Fig. 2c is a Z-contrast image with several particles marked as a and b (ring-like), c (bulk-like), and d (needle-like). Corresponding EEL spectra acquired from these particles are shown in Fig. 2d. These spectra were processed in the following manner: after the background of the spectrum was removed by fitting the pre-edge to a power law function of the form AE−r (where E is the energy loss, A and r are constants), they were smoothed [23] and normalized to the energy loss 1000 eV and were vertically shifted for clarity. As the white-line intensity of the Cu L2,3 edges is inversely proportional to the occupancy of the Cu 3d-orbitals, the intensity of the white-lines gives an accurate indication of the copper valence. For metallic copper, i.e. Cu(0), the white-lines are not present and the intensity in the spectrum corresponds to transitions to the relatively flat region of the density of states with d character. Upon oxidation, electrons are transferred from these orbits to oxygen atoms and the white-line intensity increases [26–28]. The white-line intensity can thus simply be used to fingerprint the oxidation state of the Cu atoms. Based on the L3 /L2 intensity ratios calculated from reference spectra CuO and Cu available in the EELS atlas accompanied with the EL/P software, which are 2.7 and 1.8, respectively, the valence states corresponding to the EEL spectra were obtained which are summarized in Table 1. As we can see, there is a mixture of Cu(0) and Cu(I) present in the metallic clusters. Copper species in the ring-like particles exists as Cu(I) state while in the bulk-like particles as Cu(0) state.
274
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 1. Low-magnification: (a) Z-contrast and (b) bright-field images of the catalyst reduced at 523 K; (c) EEL spectra collected from different parts of the specimen; (d) EDS spectrum acquired from the catalyst.
3.3. The Cu/γ -Al2 O3 catalyst reduced at 1073 K Table 1 Threshold (±1.2 eV), L3 /L2 ratios of Cu L2,3 edges shown in Fig. 2d and corresponding copper species Spectra
Threshold (eV)
L3 /L2 ratios
Copper species
a b c d
931 931 931 –
2.46 2.49 1.74 –
Cu(I) Cu(I) Cu(0) –
Several very large spherical particles were observed in the catalyst reduced at 1073 K. Fig. 3a and b show both low-magnification and high-resolution Z-contrast images of one such particle. The images clearly show this particle has a core-shell structure. To identify the structure of this particle, an EELS profile across the particle was acquired. Fig. 3c shows the positions (marked as 1, 2, 3, and 4) of the particle from which EEL spectra were collected and the corresponding
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
275
Fig. 2. (a) and (b) Low-magnification Z-contrast images of the catalyst reduced at 773 K; (c) and (d) Z-contrast images of ring-like and bulk-like particles in the catalyst and corresponding EEL spectra.
EEL spectra were shown in Fig. 3d. These spectra have been processed in the same manner as the previous spectra with the additional de-convolution with the low-loss spectrum to remove multiple scattering and these spectra were not smoothed (these last two steps are possible as the size of the particle allows longer acquisition times and therefore increased signal-to-noise). Based on the obtained L3 /L2 intensity ratios, spectra 1 and 2 are identified as Cu(I) (the spectrum 2 may indicate an immediate state between Cu(I) and Cu(0)) and the spectrum 3 was acquired from an overlapped area of Cu(I) and Cu(0) species and the spectrum 4 mainly metallic Cu(0) (the con-
tribution from Cu(I) layers is less). The images and EEL profiles clearly show that the large particles formed at this reduction temperature have a metallic core covered with a Cu2 O shell. The summary of this analysis is given in Table 2. The structures of these particles were also investigated by SAED. Fig. 4a and b show experimental and schematic SAED patterns taken from such a particle confirming that this particle consists of metallic copper and Cu2 O. From a detailed analysis of the SAED patterns, it appears there exists a defined orientation relationship between the core and shell: [0 0 1](2 0 0)Cu2 O//[0 0 1](2 0 0)Cu. This rela-
276
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 3. (a)–(c) Z-contrast images and (d) EEL spectra from a particle having a core-shell structure formed when the catalyst was reduced at 1073 K.
Table 2 Threshold (±1.2 eV), L3 /L2 ratios of Cu L2,3 edges shown in Fig. 3d and corresponding copper species
marked as 1 and 2 have different orientations to that of the core structure.
Spectra
Threshold (eV)
L3 /L2 ratios
Copper species
1 2 3 4
931 931 931 931
2.38 2.07 1.91 1.7
Cu(I) Cu(I) Cu(I) + Cu(0) Cu(0)a
3.4. In situ heating the Cu/γ -Al2 O3 catalyst pre-reduced at 523 K
a
The contribution from the Cu(I) shell is less.
tion is also confirmed by HREM imaging as shown in Fig. 4c. Fig. 4d shows an HREM image from which we can see the oxidized surface layer is sometimes not continuous. The two domains in the surface layer
For comparison, the catalyst pre-reduced at 523 K was also in situ heated at 723 K in the microscope. A Gatan double-tilt heating holder was used, which gives a variable temperature range between 293 and 1273 K. The specimen drift obtained with this holder is less than 1 nm/min for temperatures below 773 K. Fig. 5a and b show Z-contrast images taken from the catalyst before heating and in situ heated at 723 K for 5 min, respectively. These results show that upon heating in
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
277
Fig. 4. (a) Experimental and (b) schematic SAED patterns of a particle having a core-shell structure indicating this particle consists of a Cu core and Cu2 O shell with [0 0 1](2 0 0)Cu2 O//[0 0 1](2 0 0)Cu; (c) and (d) HREM images showing different surface structures of two big particles.
the microscope at 723 K, small particles are formed. These particles grow larger with increased heating times as shown in the image in Fig. 5c (which was acquired after the specimen had been kept at 723 K for about 40 min). EEL spectra (shown in Fig. 5d) were acquired from different parts of one particle shown in Fig. 5c. Analysis of these EEL spectra (the obtained L3 /L2 intensity ratios for a and b are 2.44 and 1.7, respectively) indicates that the particle again contains metallic cores with a partially oxidized surface layer. This is an intriguing result as oxygen partial pressure in the microscope column is about 5 × 10−8 Pa during the experiment, i.e. it is a highly reducing atmosphere. This experiment clearly indicates that reduction at higher temperatures can result in partially oxidized states of copper. It should be noted that the in situ heating does not reproduce precisely the morphology of the Cu species in the catalyst reduced in H2 /Ar at 773 K.
4. Discussion The data presented demonstrate that no separated copper species were detected in the catalyst reduced below 523 K. Upon reduction at 773 K, Cu(II) species in the catalyst is reduced to Cu(I) and Cu(0) species, and the redistribution of these species occurs. At the temperatures higher than 1073 K, Cu(I) species still exists in shells of some big spherical particles having core-shell structures with Cu(0) in the cores. The physical characteristics of the catalysts during the reduction process are summarized in Table 3. Although there is much debate in the literature concerning the changes occurring in copper catalysts during reduction [29–31], nearly all the former studies report that the Cu(II) species should be completely reduced by hydrogen at higher temperatures. However, in present study we detected partially oxidized copper species (Cu(I)) in the catalyst reduced at 773
278
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 5. Z-contrast images taken from the pre-reduced copper catalyst at 523 K: (a) before heating and (b) during in situ heating at 723 K in the microscope for 5 min, respectively; (c) an enlarged Z-contrast image of the area indicated by an arrow in (b) taken after the specimen has been heated at 723 K for 40 min; (d) corresponding EEL spectra acquired from different parts shown in (c).
Table 3 Physical characteristics of the Cu/␥-Al2 O3 catalyst reduced at different temperatures Reduction temperatures (K)
Color
Morphology of copper species
Copper species
523 773 1073
Light-green Gray Pale-green
No separated Ring-like and bulk-like particles Big particles with core-shell structures
Cu(II) and/or Cu(I) Cu(I) and Cu(0) Cu(I) (Cu2 O) and Cu(0)
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
and 1073 K. It should be noted that we did not avoid exposure of the catalyst to air after the catalysts had been reduced in the catalyst and specimen preparation procedures. One therefore may think that the Cu(I) species in the reduced catalyst are formed due to the re-oxidization of Cu(0) when the catalyst was exposed to air as observed by Urban et al. [32]. However, they report that copper oxidation occurs at room temperature after about 3 min of exposure to air—longer than it takes to get the sample in the microscope. The possibility of re-oxidation of the catalyst due to air-exposure cannot be completely ruled out in the present study. However, Edelmann et al. recently studied the oxidation state of a Cu/␥-Al2 O3 catalyst during reduction under different environments by X-ray absorption near-edge spectroscopy (XANES). They found that the XANES features of the catalyst reduced in hydrogen resembled those of bulk Cu and these features did not change markedly for the catalyst after exposure to air at 298 K [33]. This means that for a similar system, the reduced metallic copper species did not react with the oxygen in air (we also observed metallic copper particles in some of the reduced catalysts that were exposed to air). Furthermore, the in situ heating experiment indicated that upon reduction in the microscope at high temperatures, partially oxidized copper species are obtainable. Based on these results, it is probable that the Cu(I) species in the reduced catalysts are formed by the interaction of copper species with the ␥-Al2 O3 support. The observed Cu(I) species in the catalyst reduced at 773 K are formed from the reduction of the Cu(II) species. Due to the strong interaction of the copper species with the support, Cu(II) can only be partially reduced to mostly Cu(I) at 773 K. This is consistent with the report made by Baker and coworkers [34]. In their in situ experiment, they found that Cu2 O is the primary state for particles on graphite reduced at 673 K. Furthermore, these particles show a toroidal-shaped structure that is very similar to the ring-like structures observed here. They attributed this formation to the presence of a large concentration of oxygen groups on the surface of the support that served to stabilize the formation of Cu2 O. The formation of the Cu(I) surface layers in the surfaces of the big particles in the catalyst reduced at 1073 K may be due to the interaction of metallic copper with the oxide support. It may adopt a similar
279
mechanism to that of the formation of metal sulfide layers on the surface of Ni, Co, or Fe metals on molybdenum disulfide supports during heating in hydrogen [35]. Upon reduction at higher temperatures, these metals take away S from the substrate MoS2 and as a result, metal sulfide layers formed in the surface of these metal particles. Such a mechanism is supported by the observations in several studies of the formation of a Cu2 O phase between metallic copper and alumina substrates. Rühle and coworkers systematically studied the interface structures of metallic copper on alumina single crystals [27,36,37] and concluded that the oxidation state of copper at the interface is nominally Cu(I), i.e. the bonds at the interface being preferentially established between Cu and O rather than Cu and Al. Of particular relevance to this study is that also indicated that the formation of Cu2 O at the interface is independent of oxygen partial pressure. Furthermore, Kelber et al. reported the wetting of copper to alumina is through Cu–O bonds at low coverage of copper on alumina [38]. Their results indicated that the presence of hydroxyl groups in the surface of alumina greatly enhances the binding of Cu to alumina surfaces, stabilizing Cu(I) adatoms over two-dimensional metallic islands. Given the propensity of Cu(0) to form bonds with oxygen, it may well be that the oxide surface is formed from an interaction with the support oxide. Once the partially oxidized layers are formed during heating in hydrogen, they act as a protective skin against further oxidation when exposed to air afterwards. Fig. 6 schematically shows the possible structure changes of the catalyst during heating in hydrogen. At the reduction temperatures lower than 523 K (shown in Fig. 6a), the Cu(II) species are still well dispersed in the alumina. With the increase of the reduction temperature (a temperature higher than 523 K but lower than 773 K), these Cu(II) species start to undergo nucleation and collect into small islands (shown in Fig. 6b). At 773 K, most of these Cu(II) species transform into Cu(I) with ring-like morphology while some are partly reduced to Cu(0) (shown in Fig. 6c). At reduction temperatures higher than 773 K, two competing processes act on the copper species. Copper species are reduced to metallic state by hydrogen, while at the same time, the metallic copper may interact (which may be enhanced due to the formation of hydroxyl groups in the surface of the support) with
280
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 6. Schematically representing of the microstructures of the catalyst: (a) at lower reduction temperatures than 523 K; (b) at intermediate reduction temperatures between 523 and 773 K; (c) at the reduction temperature of 773 K; (d) at temperatures higher than 773 K.
the alumina support that leads to the formation of big particles with metallic cores and partially oxidized shell structures (as shown in Fig. 6d). It should be noted at this point that in the study of oxide supported metallic catalysts by high-resolution electron microscopy techniques, beam damage is always a concern. In this regard, Lyman et al. showed that whilst it was possible to identify the valence state of Cu in reduced CuO/ZnO catalysts using a dedicated STEM, the electron beam may cause the copper oxide to undergo a reduction towards a lower valence state if a strong focused beam is used [39]. In the same vein, Long and Fetford-Long studied the reduction of CuO under electron beam irradiation using HREM and EELS [28] and found that it proceeds via at least two intermediate phases: Cu4 O3 and Cu2 O. It should be noted that both of these previous studies used much more intense electron doses than in the current experiments (the Schottky source in the JEOL 2010F has less current than the dedicated STEM and the dose in STEM is much less than in HREM). Furthermore, to decrease the electron beam irradiation effect on the specimen, all the EEL spectra were acquired with
lower magnification and using short acquisition times (no more than 10 s). During these acquisition times, there was no obvious beam damage visible either in the Z-contrast image or by comparison of two EEL spectra acquired consecutively from the same position on the particles. The results and discussion presented therefore do not originate from a beam damage effect.
5. Conclusion The microstructure of a Cu/␥-Al2 O3 catalyst during its heating in H2 /Ar has been studied by Z-contrast imaging and EELS. Based on the results presented here, the indication is that at reduction temperatures lower than 523 K, no detectable separated copper species are formed in the catalyst. At the reduction temperature of 773 K, separated copper particles started to form, exhibiting ring-like morphologies containing Cu(I), and bulk-like morphologies containing Cu(0), which indicates that the copper species have not been completely reduced to the metallic state (due to the interaction between the copper species
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
and the oxide support). When temperatures higher than 1073 K were employed, big spherical particles having core-shell structures with Cu(I) in the shells and Cu(0) in the core were formed.
Acknowledgements This research is sponsored by Monsanto Company. The JEOL 2010F STEM/TEM used in this research was partially funded by NSF through the Grant DMR-9601792 and is operated by the Research Resources Center at UIC. We thank Dr. Nabin Nag for providing the samples. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M.H. Thiemens, W.C. Trogler, Science 251 (1991) 932. G. Centi, S. Perathoner, Appl. Catal. A 132 (1995) 179. A. Dandekar, M.A. Vannice, Appl. Catal. B 22 (1999) 179. M. Matsuoka, W. Ju, K. Takahashi, H. Yamashita, M. Anpo, J. Phys. Chem. B 104 (2000) 4911. H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, Appl. Catal. B 16 (1998) 359. F. Radtke, R.A. Koeppel, E.G. Minardi, A. Baiker, J. Catal. 167 (1997) 127. C. Chen, W. Cheng, S. Lin, Catal. Lett. 68 (2000) 45. K.J. Choi, M.A. Vannice, J. Catal. 131 (1991) 22. J. Szanyi, D.W. Goodman, Catal. Lett. 21 (1993) 165. D.L. Trimm, Z.I. Önsan, Catal. Rev. 43 (2001) 31. O. Dulaurent, X. Courtois, V. Perrichon, D. Bianchi, J. Phys. Chem. B 104 (2000) 6001. R.M. Friedman, J.J. Freeman, J. Catal. 55 (1978) 10. A. Wolberg, J.L. Ogilvie, J.F. Roth, J. Catal. 19 (1970) 86. B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal. 94 (1985) 514. B.C. Gates, Chem. Rev. 95 (1995) 511. K. Heinemann, F. Soria, Ultramicroscopy 20 (1986) 1.
281
[17] M.S. Isaacson, J. Langmore, N.W. Parker, D. Kopf, M. Utlaut, Ultramicroscopy 1 (1976) 359. [18] P.D. Nellist, S.J. Pennycook, Science 274 (1996) 413. [19] M.M.J. Treacy, A. Howie, C.J. Wilson, Phil. Mag. A 38 (1978) 569. [20] J. Liu, J.M. Cowley, Ultramicroscopy 34 (1990) 119. [21] N.D. Browning, M.F. Chisholm, S.J. Pennycook, Nature 366 (1993) 143. [22] P.E. Batson, Nature 366 (1993) 727. [23] R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, 2nd Edition, Plenum Press, New York, 1996. [24] R.F. Klie, N.D. Browning, Appl. Phys. Lett. 77 (2000) 3737. [25] K. Sun, J. Liu, N.D. Browning, J. Catal. 205 (2002) 266. [26] R.D. Leapman, L.A. Gruncs, P.L. Fejes, Phys. Rev. B 26 (1982) 614. [27] C. Scheu, W. Stein, M. Rühle, Phys. Stat. Sol. (b) 222 (2000) 199. [28] N.J. Long, A.K. Fetford-Long, Ultramicroscopy 20 (1986) 151. [29] H.H. Voge, L.T. Atkins, J. Catal. 1 (1962) 171. [30] B.R. Strohmeier, D.E. Leyden, R. Scott Field, D.M. Hercules, J. Catal. 94 (1985) 514. [31] M. Fernández-Garc´ıa, I. Rodr´ıguez-Ramos, P. FerreiraAparicio, A. Guerrero-Ruiz, J. Catal. 178 (1998) 253. [32] J. Urban, H. Sack-Kongehl, K. Weiss, Z. Phys. D 36 (1996) 73. [33] A. Edelmann, W. Schießer, H. Vinek, A. Jentys, Catal. Lett. 69 (2000) 11. [34] J. Ma, N.M. Rodriguez, M.A. Vannice, R.T.K. Baker, J. Catal. 183 (1999) 32. [35] B.H. Upton, C.C. Chen, N.M. Rodriguez, R.T.K. Baker, J. Catal. 141 (1993) 171. [36] G. Dehm, C. Scheu, G. Möbus, R. Brydson, M. Rühle, Ultramicroscopy 67 (1997) 207. [37] U. Alber, H. Müllejans, M. Rühle, Micron 30 (1999) 101. [38] J.A. Kelber, C.Y. Niu, K. Shepherd, D.R. Jennison, A. Bogicevic, Surf. Sci. 446 (2000) 76. [39] C.E. Lyman, A. Ferretti, N.J. Long, in: D.B. Williams, D.C. Joy (Eds.), Analytical Electron Microscopy, San Francisco Press, San Francisco, CA, 1984.
Direct atomic scale analysis of the distribution of Cu valence states in Cu/␥-Al2 O3 catalysts Kai Sun a,∗ , Jingyue Liu b , Nigel D. Browning a a
Department of Physics (M/C273), University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607-7059, USA b Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO 63167, USA Received 5 December 2001; received in revised form 4 March 2002; accepted 7 March 2002
Abstract In this paper, the microstructure of a 1 wt.% Cu/␥-Al2 O3 catalyst that was reduced in a 4% hydrogen/argon atmosphere at temperatures of 523, 773 and 1073 K, is studied by Z-contrast imaging and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Results show that the copper species are well dispersed when the catalyst is reduced below 523 K. At 773 K, separated Cu(I) and Cu(0) species are found existing as ring-like and bulk-like particles. This appears to indicate that the copper has not been reduced to its metallic form due to the interaction between the copper oxide and the support. Large spherical particles having core-shell structures with Cu(I) in the shells and Cu(0) in the cores are generated when the catalyst is reduced at 1073 K. The formation of partially oxidized copper species upon reduction at 1073 K is attributed to the metallic copper interaction with the alumina support. This study also demonstrates that high-spatial resolution Z-contrast imaging and EELS performed simultaneously can provide unique information on the morphology and chemistry of metal species in supported metal catalysts. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cu/␥-Al2 O3 ; Reduction; EELS; Z-contrast imaging; STEM; Valence state; Metal–support interaction (MSI)
1. Introduction Carbon dioxide and nitrogen oxides, which have been identified as greenhouse gases, are all recognized as environmental pollutants [1–3]. Several approaches to reduce these gaseous emissions into the atmosphere have been suggested for both mobile and stationary sources, but the catalytic approach is the most effective to meet current and future requirements. Copper based catalysts, such as Cu/␥-Al2 O3 catalysts, can play an important role in these catalytic reactions. For example, they can be used for the direct ∗ Corresponding author. Tel.: +1-312-413-2790; fax: +1-312-996-4451. E-mail address: [email protected] (K. Sun).
decomposition of N2 O to N2 [2–4], the selective catalytic reduction of nitrogen oxides by hydrocarbons in an oxygen-rich atmosphere [5,6], and also for conversion of CO2 to CO by catalytic hydrogenation [7]. In addition, they can also be used in hydrogen fuel cells to generate energy for vehicles, thereby helping meet increasingly stringent legislation controlling emissions from internal combustion engines [8–10]. An important goal in catalyst research is the identification of the sites controlling activity and selectivity for a given reaction; elucidating the local structure of the active species as well as its dispersion in the catalytic reaction. For oxide supported metal catalysts, the morphology and oxidation states of metallic species as well as their interaction with substrates are key factors that determine the active sites and their
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 5 5 - 3
272
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
distribution. In the case of supported copper catalysts, copper species having different oxidation states have different catalytic properties. Cu(0) can act as the active site to dissociate CO2 [7], while Cu(I) can be used for photocatalytic decomposition of N2 O into N2 and O2 [4]. Understanding and controlling the formation of the copper species during the preparation of the catalyst is therefore important for the design of specific catalysts for particular uses. Information on the structural changes that occur during the reduction of the Cu/Al2 O3 catalysts has been obtained in the past using a multitude of different techniques. These include temperature-programmed reduction (TPR) [11], electron spin resonance (ESR) [12], X-ray photoelectron spectroscopy (XPS) [13], X-ray absorption spectroscopy (XAS) [12], etc. However, while these techniques can provide information on the overall distribution of the copper oxidation states, i.e. how much Cu(0) or Cu(I) exists, the information cannot be defined to a particular location, i.e. the surfaces or the interior of a nanoscale cluster. Moreover, although TPR is a relatively simple and widely used technique there are problems associated with the accuracy of characterizing Cu(0) and Cu(I) sites using CO adsorption [11]. Furthermore, XPS cannot separate Cu(0) and Cu(I) species [13] and catalysts containing less than 5% Cu showed only alumina by XRD [14]. To obtain detailed information on both the morphology and oxidation state of the metallic species in supported catalysts, it was suggested that the most efficient method should be the combination of atomic imaging and spectroscopic techniques [15]. By these methods, the morphologies of the supported metallic species can be directly revealed by imaging techniques while the oxidation states can be obtained by spectroscopic methods. Although high-resolution electron microscopy (HREM) is a powerful tool to determine structures, it is not suitable for imaging supported metal catalysts with metallic clusters smaller than 2 nm because the scattering from substrate (crystallized) masks the metallic cluster [16]. Additionally, it does not permit spectroscopic information to be obtained. However, metallic clusters with high Z (atomic number) in low Z supports can be easily resolved by Z-contrast imaging. In fact, Z-contrast imaging has been successfully used for directly imaging single atoms on carbon [17] and alumina [18]. This makes
Z-contrast imaging ideally suited to characterize the morphology of supported metallic catalysts [18–20]. Although the Z-contrast images provide unprecedented information on the structure of the metallic clusters, they do not contain electronic structure information. However, the experimental set-up of a STEM makes it possible for EELS to be performed simultaneously with Z-contrast imaging at the same atomic resolution [21,22]. EEL spectra contain, in addition to the chemical composition, information concerning the local electronic structure of materials [23]. Changes in the bonding type, the oxidation states of atoms or the structural arrangement of neighboring atoms can modify the distribution of unoccupied electronic states, and this is reflected in the features of EEL spectra. Therefore, by combining Z-contrast imaging with EELS, the optimum approach to characterizing active sites in supported metal clusters can be obtained at atomic spatial resolution [21]. In the present study, Z-contrast imaging together with EELS is used to characterize a Cu/␥-Al2 O3 catalyst and evaluate the microstructure/oxidation state changes that occur during reduction.
2. Experimental 2.1. Catalyst preparation ␥-Alumina powder with BET surface area and pore volume (measured by mercury porosimetry) of 160 m2 /g, and 1.3 ml/g, respectively, was used as the catalyst support. A 10% aqueous slurry of the alumina with the pH adjusted to 11 by sodium bicarbonate and an aqueous solution containing Cu as CuCl2 ·2H2 O to get 1.0 wt.% Cu loading on the support were prepared separately. The aqueous copper solution was added slowly into the slurry while it was continuously stirred by an impeller. The Cu salt was allowed to be adsorbed at room temperature for 30 min at which point the slurry was heated to 338 K, and held at this temperature for another 30 min to complete the adsorption of Cu. The slurry was then filtered, washed with deionized water and dried in an oven in air at 393 K overnight. This dried catalyst was then reduced in small portions in a tube furnace by a stream of 4% H2 /Ar (at a flow rate of 40 ml/min) for 2 h at various temperatures (523, 773 and 1073 K).
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
After a 2 h hold period, the tube was cooled down to room temperature in the hydrogen atmosphere. After this the flowing gas was switched to argon, and the catalyst unloaded. Specimens were prepared for electron microscopy analysis by placing the powders on holey-carbon-coated copper and molybdenum grids.
273
the presence of copper in the catalyst (shown in Fig. 1d). The fact that no obvious copper species can be directly resolved from the Z-contrast image and EEL spectra indicates that copper species is highly dispersed within the catalyst when reduced at 523 K. 3.2. The Cu/γ -Al2 O3 catalyst reduced at 773 K
2.2. Catalyst characterization The analysis reported here was performed using a JEOL 2010F field emission STEM/TEM operating at 200 kV in the STEM mode. The oxygen partial pressure in the microscope column is about 5 × 10−8 Pa during the experiment (i.e. a highly reducing atmosphere). The lens conditions during operation were defined for a probe size of 0.2 nm, with a convergence angle of 13 mrad and a collection angle of 52 mrad [24]. The energy resolution (defined by the full width at half maximum of zero loss peak) of the energy-loss spectra is 1.2 eV at a dispersion of 0.3 eV per channel. A Gatan PEELS system and EL/P 3.3 software were used for EELS data collection and processing. A more detailed description of these techniques can be found elsewhere [25].
3. Results 3.1. The Cu/γ -Al2 O3 catalyst reduced at 523 K Fig. 1a and b show low-magnification Z-contrast and bright-field images taken from the Cu/␥-Al2 O3 catalyst reduced at 523 K. From the images we can see that the catalyst consists of mainly needle-like particles and some small irregular particles. Selected-area electron diffraction (SAED) analysis shows aluminalike ring patterns (not shown here). No separated copper species can be resolved from the Z-contrast images although there exists a great Z difference between Cu and Al and O. Moreover, EEL spectra collected from different particles do not show the expected copper L2,3 edges (see the inset shown in Fig. 1c). As the Cu M2,3 edges (onset: 74 eV) overlap with the Al L2,3 edges (onset: 78 eV), we could not identify whether or not the EEL spectrum shown in Fig. 1c contains the copper peaks. However, energy dispersive X-ray spectroscopy (EDS) collected while scanning the probe over an area of the specimen clearly shows
Fig. 2 shows results obtained from the catalyst reduced at 773 K. Here we can see two types of morphological particles, ring-like (R, Fig. 2a and c) and bulk-like (B, Fig 2b and c) particles, besides the needle-like particles. One interesting feature is that these bulk-like particles always coexist with ring-like particles, although they are not completely encapsulated by the ring-like particles (as indicated by arrows in Fig. 2b). Fig. 2c is a Z-contrast image with several particles marked as a and b (ring-like), c (bulk-like), and d (needle-like). Corresponding EEL spectra acquired from these particles are shown in Fig. 2d. These spectra were processed in the following manner: after the background of the spectrum was removed by fitting the pre-edge to a power law function of the form AE−r (where E is the energy loss, A and r are constants), they were smoothed [23] and normalized to the energy loss 1000 eV and were vertically shifted for clarity. As the white-line intensity of the Cu L2,3 edges is inversely proportional to the occupancy of the Cu 3d-orbitals, the intensity of the white-lines gives an accurate indication of the copper valence. For metallic copper, i.e. Cu(0), the white-lines are not present and the intensity in the spectrum corresponds to transitions to the relatively flat region of the density of states with d character. Upon oxidation, electrons are transferred from these orbits to oxygen atoms and the white-line intensity increases [26–28]. The white-line intensity can thus simply be used to fingerprint the oxidation state of the Cu atoms. Based on the L3 /L2 intensity ratios calculated from reference spectra CuO and Cu available in the EELS atlas accompanied with the EL/P software, which are 2.7 and 1.8, respectively, the valence states corresponding to the EEL spectra were obtained which are summarized in Table 1. As we can see, there is a mixture of Cu(0) and Cu(I) present in the metallic clusters. Copper species in the ring-like particles exists as Cu(I) state while in the bulk-like particles as Cu(0) state.
274
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 1. Low-magnification: (a) Z-contrast and (b) bright-field images of the catalyst reduced at 523 K; (c) EEL spectra collected from different parts of the specimen; (d) EDS spectrum acquired from the catalyst.
3.3. The Cu/γ -Al2 O3 catalyst reduced at 1073 K Table 1 Threshold (±1.2 eV), L3 /L2 ratios of Cu L2,3 edges shown in Fig. 2d and corresponding copper species Spectra
Threshold (eV)
L3 /L2 ratios
Copper species
a b c d
931 931 931 –
2.46 2.49 1.74 –
Cu(I) Cu(I) Cu(0) –
Several very large spherical particles were observed in the catalyst reduced at 1073 K. Fig. 3a and b show both low-magnification and high-resolution Z-contrast images of one such particle. The images clearly show this particle has a core-shell structure. To identify the structure of this particle, an EELS profile across the particle was acquired. Fig. 3c shows the positions (marked as 1, 2, 3, and 4) of the particle from which EEL spectra were collected and the corresponding
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
275
Fig. 2. (a) and (b) Low-magnification Z-contrast images of the catalyst reduced at 773 K; (c) and (d) Z-contrast images of ring-like and bulk-like particles in the catalyst and corresponding EEL spectra.
EEL spectra were shown in Fig. 3d. These spectra have been processed in the same manner as the previous spectra with the additional de-convolution with the low-loss spectrum to remove multiple scattering and these spectra were not smoothed (these last two steps are possible as the size of the particle allows longer acquisition times and therefore increased signal-to-noise). Based on the obtained L3 /L2 intensity ratios, spectra 1 and 2 are identified as Cu(I) (the spectrum 2 may indicate an immediate state between Cu(I) and Cu(0)) and the spectrum 3 was acquired from an overlapped area of Cu(I) and Cu(0) species and the spectrum 4 mainly metallic Cu(0) (the con-
tribution from Cu(I) layers is less). The images and EEL profiles clearly show that the large particles formed at this reduction temperature have a metallic core covered with a Cu2 O shell. The summary of this analysis is given in Table 2. The structures of these particles were also investigated by SAED. Fig. 4a and b show experimental and schematic SAED patterns taken from such a particle confirming that this particle consists of metallic copper and Cu2 O. From a detailed analysis of the SAED patterns, it appears there exists a defined orientation relationship between the core and shell: [0 0 1](2 0 0)Cu2 O//[0 0 1](2 0 0)Cu. This rela-
276
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 3. (a)–(c) Z-contrast images and (d) EEL spectra from a particle having a core-shell structure formed when the catalyst was reduced at 1073 K.
Table 2 Threshold (±1.2 eV), L3 /L2 ratios of Cu L2,3 edges shown in Fig. 3d and corresponding copper species
marked as 1 and 2 have different orientations to that of the core structure.
Spectra
Threshold (eV)
L3 /L2 ratios
Copper species
1 2 3 4
931 931 931 931
2.38 2.07 1.91 1.7
Cu(I) Cu(I) Cu(I) + Cu(0) Cu(0)a
3.4. In situ heating the Cu/γ -Al2 O3 catalyst pre-reduced at 523 K
a
The contribution from the Cu(I) shell is less.
tion is also confirmed by HREM imaging as shown in Fig. 4c. Fig. 4d shows an HREM image from which we can see the oxidized surface layer is sometimes not continuous. The two domains in the surface layer
For comparison, the catalyst pre-reduced at 523 K was also in situ heated at 723 K in the microscope. A Gatan double-tilt heating holder was used, which gives a variable temperature range between 293 and 1273 K. The specimen drift obtained with this holder is less than 1 nm/min for temperatures below 773 K. Fig. 5a and b show Z-contrast images taken from the catalyst before heating and in situ heated at 723 K for 5 min, respectively. These results show that upon heating in
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
277
Fig. 4. (a) Experimental and (b) schematic SAED patterns of a particle having a core-shell structure indicating this particle consists of a Cu core and Cu2 O shell with [0 0 1](2 0 0)Cu2 O//[0 0 1](2 0 0)Cu; (c) and (d) HREM images showing different surface structures of two big particles.
the microscope at 723 K, small particles are formed. These particles grow larger with increased heating times as shown in the image in Fig. 5c (which was acquired after the specimen had been kept at 723 K for about 40 min). EEL spectra (shown in Fig. 5d) were acquired from different parts of one particle shown in Fig. 5c. Analysis of these EEL spectra (the obtained L3 /L2 intensity ratios for a and b are 2.44 and 1.7, respectively) indicates that the particle again contains metallic cores with a partially oxidized surface layer. This is an intriguing result as oxygen partial pressure in the microscope column is about 5 × 10−8 Pa during the experiment, i.e. it is a highly reducing atmosphere. This experiment clearly indicates that reduction at higher temperatures can result in partially oxidized states of copper. It should be noted that the in situ heating does not reproduce precisely the morphology of the Cu species in the catalyst reduced in H2 /Ar at 773 K.
4. Discussion The data presented demonstrate that no separated copper species were detected in the catalyst reduced below 523 K. Upon reduction at 773 K, Cu(II) species in the catalyst is reduced to Cu(I) and Cu(0) species, and the redistribution of these species occurs. At the temperatures higher than 1073 K, Cu(I) species still exists in shells of some big spherical particles having core-shell structures with Cu(0) in the cores. The physical characteristics of the catalysts during the reduction process are summarized in Table 3. Although there is much debate in the literature concerning the changes occurring in copper catalysts during reduction [29–31], nearly all the former studies report that the Cu(II) species should be completely reduced by hydrogen at higher temperatures. However, in present study we detected partially oxidized copper species (Cu(I)) in the catalyst reduced at 773
278
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 5. Z-contrast images taken from the pre-reduced copper catalyst at 523 K: (a) before heating and (b) during in situ heating at 723 K in the microscope for 5 min, respectively; (c) an enlarged Z-contrast image of the area indicated by an arrow in (b) taken after the specimen has been heated at 723 K for 40 min; (d) corresponding EEL spectra acquired from different parts shown in (c).
Table 3 Physical characteristics of the Cu/␥-Al2 O3 catalyst reduced at different temperatures Reduction temperatures (K)
Color
Morphology of copper species
Copper species
523 773 1073
Light-green Gray Pale-green
No separated Ring-like and bulk-like particles Big particles with core-shell structures
Cu(II) and/or Cu(I) Cu(I) and Cu(0) Cu(I) (Cu2 O) and Cu(0)
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
and 1073 K. It should be noted that we did not avoid exposure of the catalyst to air after the catalysts had been reduced in the catalyst and specimen preparation procedures. One therefore may think that the Cu(I) species in the reduced catalyst are formed due to the re-oxidization of Cu(0) when the catalyst was exposed to air as observed by Urban et al. [32]. However, they report that copper oxidation occurs at room temperature after about 3 min of exposure to air—longer than it takes to get the sample in the microscope. The possibility of re-oxidation of the catalyst due to air-exposure cannot be completely ruled out in the present study. However, Edelmann et al. recently studied the oxidation state of a Cu/␥-Al2 O3 catalyst during reduction under different environments by X-ray absorption near-edge spectroscopy (XANES). They found that the XANES features of the catalyst reduced in hydrogen resembled those of bulk Cu and these features did not change markedly for the catalyst after exposure to air at 298 K [33]. This means that for a similar system, the reduced metallic copper species did not react with the oxygen in air (we also observed metallic copper particles in some of the reduced catalysts that were exposed to air). Furthermore, the in situ heating experiment indicated that upon reduction in the microscope at high temperatures, partially oxidized copper species are obtainable. Based on these results, it is probable that the Cu(I) species in the reduced catalysts are formed by the interaction of copper species with the ␥-Al2 O3 support. The observed Cu(I) species in the catalyst reduced at 773 K are formed from the reduction of the Cu(II) species. Due to the strong interaction of the copper species with the support, Cu(II) can only be partially reduced to mostly Cu(I) at 773 K. This is consistent with the report made by Baker and coworkers [34]. In their in situ experiment, they found that Cu2 O is the primary state for particles on graphite reduced at 673 K. Furthermore, these particles show a toroidal-shaped structure that is very similar to the ring-like structures observed here. They attributed this formation to the presence of a large concentration of oxygen groups on the surface of the support that served to stabilize the formation of Cu2 O. The formation of the Cu(I) surface layers in the surfaces of the big particles in the catalyst reduced at 1073 K may be due to the interaction of metallic copper with the oxide support. It may adopt a similar
279
mechanism to that of the formation of metal sulfide layers on the surface of Ni, Co, or Fe metals on molybdenum disulfide supports during heating in hydrogen [35]. Upon reduction at higher temperatures, these metals take away S from the substrate MoS2 and as a result, metal sulfide layers formed in the surface of these metal particles. Such a mechanism is supported by the observations in several studies of the formation of a Cu2 O phase between metallic copper and alumina substrates. Rühle and coworkers systematically studied the interface structures of metallic copper on alumina single crystals [27,36,37] and concluded that the oxidation state of copper at the interface is nominally Cu(I), i.e. the bonds at the interface being preferentially established between Cu and O rather than Cu and Al. Of particular relevance to this study is that also indicated that the formation of Cu2 O at the interface is independent of oxygen partial pressure. Furthermore, Kelber et al. reported the wetting of copper to alumina is through Cu–O bonds at low coverage of copper on alumina [38]. Their results indicated that the presence of hydroxyl groups in the surface of alumina greatly enhances the binding of Cu to alumina surfaces, stabilizing Cu(I) adatoms over two-dimensional metallic islands. Given the propensity of Cu(0) to form bonds with oxygen, it may well be that the oxide surface is formed from an interaction with the support oxide. Once the partially oxidized layers are formed during heating in hydrogen, they act as a protective skin against further oxidation when exposed to air afterwards. Fig. 6 schematically shows the possible structure changes of the catalyst during heating in hydrogen. At the reduction temperatures lower than 523 K (shown in Fig. 6a), the Cu(II) species are still well dispersed in the alumina. With the increase of the reduction temperature (a temperature higher than 523 K but lower than 773 K), these Cu(II) species start to undergo nucleation and collect into small islands (shown in Fig. 6b). At 773 K, most of these Cu(II) species transform into Cu(I) with ring-like morphology while some are partly reduced to Cu(0) (shown in Fig. 6c). At reduction temperatures higher than 773 K, two competing processes act on the copper species. Copper species are reduced to metallic state by hydrogen, while at the same time, the metallic copper may interact (which may be enhanced due to the formation of hydroxyl groups in the surface of the support) with
280
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
Fig. 6. Schematically representing of the microstructures of the catalyst: (a) at lower reduction temperatures than 523 K; (b) at intermediate reduction temperatures between 523 and 773 K; (c) at the reduction temperature of 773 K; (d) at temperatures higher than 773 K.
the alumina support that leads to the formation of big particles with metallic cores and partially oxidized shell structures (as shown in Fig. 6d). It should be noted at this point that in the study of oxide supported metallic catalysts by high-resolution electron microscopy techniques, beam damage is always a concern. In this regard, Lyman et al. showed that whilst it was possible to identify the valence state of Cu in reduced CuO/ZnO catalysts using a dedicated STEM, the electron beam may cause the copper oxide to undergo a reduction towards a lower valence state if a strong focused beam is used [39]. In the same vein, Long and Fetford-Long studied the reduction of CuO under electron beam irradiation using HREM and EELS [28] and found that it proceeds via at least two intermediate phases: Cu4 O3 and Cu2 O. It should be noted that both of these previous studies used much more intense electron doses than in the current experiments (the Schottky source in the JEOL 2010F has less current than the dedicated STEM and the dose in STEM is much less than in HREM). Furthermore, to decrease the electron beam irradiation effect on the specimen, all the EEL spectra were acquired with
lower magnification and using short acquisition times (no more than 10 s). During these acquisition times, there was no obvious beam damage visible either in the Z-contrast image or by comparison of two EEL spectra acquired consecutively from the same position on the particles. The results and discussion presented therefore do not originate from a beam damage effect.
5. Conclusion The microstructure of a Cu/␥-Al2 O3 catalyst during its heating in H2 /Ar has been studied by Z-contrast imaging and EELS. Based on the results presented here, the indication is that at reduction temperatures lower than 523 K, no detectable separated copper species are formed in the catalyst. At the reduction temperature of 773 K, separated copper particles started to form, exhibiting ring-like morphologies containing Cu(I), and bulk-like morphologies containing Cu(0), which indicates that the copper species have not been completely reduced to the metallic state (due to the interaction between the copper species
K. Sun et al. / Applied Catalysis B: Environmental 38 (2002) 271–281
and the oxide support). When temperatures higher than 1073 K were employed, big spherical particles having core-shell structures with Cu(I) in the shells and Cu(0) in the core were formed.
Acknowledgements This research is sponsored by Monsanto Company. The JEOL 2010F STEM/TEM used in this research was partially funded by NSF through the Grant DMR-9601792 and is operated by the Research Resources Center at UIC. We thank Dr. Nabin Nag for providing the samples. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M.H. Thiemens, W.C. Trogler, Science 251 (1991) 932. G. Centi, S. Perathoner, Appl. Catal. A 132 (1995) 179. A. Dandekar, M.A. Vannice, Appl. Catal. B 22 (1999) 179. M. Matsuoka, W. Ju, K. Takahashi, H. Yamashita, M. Anpo, J. Phys. Chem. B 104 (2000) 4911. H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, Appl. Catal. B 16 (1998) 359. F. Radtke, R.A. Koeppel, E.G. Minardi, A. Baiker, J. Catal. 167 (1997) 127. C. Chen, W. Cheng, S. Lin, Catal. Lett. 68 (2000) 45. K.J. Choi, M.A. Vannice, J. Catal. 131 (1991) 22. J. Szanyi, D.W. Goodman, Catal. Lett. 21 (1993) 165. D.L. Trimm, Z.I. Önsan, Catal. Rev. 43 (2001) 31. O. Dulaurent, X. Courtois, V. Perrichon, D. Bianchi, J. Phys. Chem. B 104 (2000) 6001. R.M. Friedman, J.J. Freeman, J. Catal. 55 (1978) 10. A. Wolberg, J.L. Ogilvie, J.F. Roth, J. Catal. 19 (1970) 86. B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal. 94 (1985) 514. B.C. Gates, Chem. Rev. 95 (1995) 511. K. Heinemann, F. Soria, Ultramicroscopy 20 (1986) 1.
281
[17] M.S. Isaacson, J. Langmore, N.W. Parker, D. Kopf, M. Utlaut, Ultramicroscopy 1 (1976) 359. [18] P.D. Nellist, S.J. Pennycook, Science 274 (1996) 413. [19] M.M.J. Treacy, A. Howie, C.J. Wilson, Phil. Mag. A 38 (1978) 569. [20] J. Liu, J.M. Cowley, Ultramicroscopy 34 (1990) 119. [21] N.D. Browning, M.F. Chisholm, S.J. Pennycook, Nature 366 (1993) 143. [22] P.E. Batson, Nature 366 (1993) 727. [23] R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, 2nd Edition, Plenum Press, New York, 1996. [24] R.F. Klie, N.D. Browning, Appl. Phys. Lett. 77 (2000) 3737. [25] K. Sun, J. Liu, N.D. Browning, J. Catal. 205 (2002) 266. [26] R.D. Leapman, L.A. Gruncs, P.L. Fejes, Phys. Rev. B 26 (1982) 614. [27] C. Scheu, W. Stein, M. Rühle, Phys. Stat. Sol. (b) 222 (2000) 199. [28] N.J. Long, A.K. Fetford-Long, Ultramicroscopy 20 (1986) 151. [29] H.H. Voge, L.T. Atkins, J. Catal. 1 (1962) 171. [30] B.R. Strohmeier, D.E. Leyden, R. Scott Field, D.M. Hercules, J. Catal. 94 (1985) 514. [31] M. Fernández-Garc´ıa, I. Rodr´ıguez-Ramos, P. FerreiraAparicio, A. Guerrero-Ruiz, J. Catal. 178 (1998) 253. [32] J. Urban, H. Sack-Kongehl, K. Weiss, Z. Phys. D 36 (1996) 73. [33] A. Edelmann, W. Schießer, H. Vinek, A. Jentys, Catal. Lett. 69 (2000) 11. [34] J. Ma, N.M. Rodriguez, M.A. Vannice, R.T.K. Baker, J. Catal. 183 (1999) 32. [35] B.H. Upton, C.C. Chen, N.M. Rodriguez, R.T.K. Baker, J. Catal. 141 (1993) 171. [36] G. Dehm, C. Scheu, G. Möbus, R. Brydson, M. Rühle, Ultramicroscopy 67 (1997) 207. [37] U. Alber, H. Müllejans, M. Rühle, Micron 30 (1999) 101. [38] J.A. Kelber, C.Y. Niu, K. Shepherd, D.R. Jennison, A. Bogicevic, Surf. Sci. 446 (2000) 76. [39] C.E. Lyman, A. Ferretti, N.J. Long, in: D.B. Williams, D.C. Joy (Eds.), Analytical Electron Microscopy, San Francisco Press, San Francisco, CA, 1984.