Understanding catalyst behavior during in situ heating through simultaneous secondary and transmitted electron imaging
© Howe et al.; licensee Springer. 2014
Received: 5 September 2014
Accepted: 3 November 2014
Published: 14 November 2014
By coupling techniques of simultaneous secondary (SE) and transmitted electron (TE) imaging at high resolution in a modern scanning transmission electron microscope (STEM), with the ability to heat specimens using a highly stable MEMS-based heating platform, we obtained synergistic information to clarify the behavior of catalysts during in situ thermal treatments. Au/iron oxide catalyst 'leached' to remove surface Au was heated to temperatures as high as 700°C. The Fe2O3 support particle structure tended to reduce to Fe3O4 and formed surface terraces; the formation, coalescence, and mobility of 1- to 2-nm particles on the terraces were characterized in SE, STEM-ADF, and TEM-BF modes. If combined with simultaneous nanoprobe spectroscopy, this approach will open the door to a new way of studying the kinetics of nano-scaled phenomena.
KeywordsScanning transmission electron microscopy Scanning electron microscopy Catalyst Phase transformation In situ characterization
Our ability to image surface and bulk features of nanomaterials plays an important role in the field of nano-scaled materials research. Even more desirable (especially for the study of catalytic materials) is the capability to simultaneously image morphological and structural changes that occur on the surface and within the bulk during in situ heating. Scanning electron microscopy (SEM) is by far the most widely used technique for imaging the surfaces of materials. Standard SEMs used for imaging bulk materials (i.e., samples installed below the final imaging lens) do not have a resolution high enough to clearly reveal the smallest nanoparticles such as catalysts. SEMs capable of in-lens operation have given sub-nanometer image resolution at a relatively low accelerating voltage, but the majority lack the capability for transmission electron microscopy. A modern scanning transmission electron microscope (STEM), operating at 60 to 300 kV (e.g., the Hitachi HF-3300 STEM/TEM (Hitachi, Tokyo, Japan)), can routinely provide imaging in the 0.2-nm range in both TEM and STEM imaging modes. Bulk crystal lattice structure is imaged with a 0.1-nm information limit in bright-field (BF) TEM mode and about 0.2-nm resolution in BF and annular dark-field (ADF) STEM modes. SE imaging resolution is limited by specimen-beam interaction effects, but resolution at the 1-nm level is achieved, even at elevated temperatures as shown in the present study. We report herein a simultaneous SE and TE study of morphological evolution of an Au/iron oxide catalyst during in situ heating in an Hitachi HF-3300 STEM/TEM operated at 300 kV. This microscope permits rapid and facile switching between TEM and STEM operating modes. In addition to the standard ADF and BF detectors that are fitted on most STEMs, this instrument also comes equipped with a secondary electron (SE) detector. Because it has a cold-field emission electron gun with an energy spread of <0.4 eV, the small probe size (approximately 0.2 nm) and the high-efficiency SE detector lead to an SE imaging resolution at better than 0.5 nm. It was of special interest in the present study to characterize the ability to image in SE mode at high temperatures, where light emitted from the heating device would typically (in conventional SE mode using a standard heater in a SEM [1, 2]) cause the SE detector to be adversely affected and make high resolution imaging impossible. It was found that the particular characteristics of the Protochips Aduro™ (Protochips Inc., Raleigh, NC, USA) heating technology used in this study allows high resolution imaging in STEM-SE mode, even at temperatures as high as 700°C.
Environmental TEM and STEM have been used for catalysis research for decades [3–10]. Notably, Gai and Boyes pioneered the development of environmental cell and heating stage for in situ study of catalysts [3–5]. They achieved sub-Ångström resolution using the aforementioned setup in a TEM/STEM with double correctors [8–10]. Our work was carried out using a TEM/STEM without an aberration corrector. The goal of this work was to demonstrate the advantage of obtaining surface and bulk information of catalysts by simultaneous imaging using both transmitted (ADF, TE) and secondary electron signals (SE).
In this study, we have selected gold nanoparticles on hematite support as the material of interest, because gold nanoparticles have generated considerable interest for catalytic applications for certain oxidation reactions and selective hydrogenation reactions [11–14]. Recently, a series of Au catalysts supported on Fe2O3 (hematite) were characterized with atomic resolution during elevated temperature treatments, using Protochips Aduro in situ heating technology in an aberration-corrected STEM (JEOL 2200FS, JEOL, Tokyo, Japan) fitted with a CEOS hexapole corrector (CEOS GmbH, Heidelberg, Germany) on the probe-forming lenses) [15–17]. In order to extend the understanding of the behavior of Au species during heating, we carried out this simultaneous SE and TE imaging study in a sample of the Au/Fe2O3 system selected from the prior study [17, 18].
The microscopy was carried out using the Hitachi HF-3300 TEM/STEM at Oak Ridge National Laboratory. This instrument is fitted with a SE detector (using a photomultiplier tube), in addition to both BF and high-angle annular dark-field (HAADF) detectors for STEM imaging, and a Gatan 2 k × 2 k Ultrascan CCD camera (Gatan Inc., Pleasanton, CA, USA) for conventional TEM imaging. The advantage of TEM recording during in situ heating is that it provides shorter exposure times (approximately 1 s) relative to the slower scans required by SE in the STEM modes. This allows higher accuracy and more reliable analysis of the atomic structure via computed diffractograms (sample drift and scan artifacts can strongly affect STEM diffractograms). The column vacuum was maintained at 4.8 × 10−6 Pa even during in situ heating.
The capability of SE imaging in the Hitachi HF-3300 at the high temperatures afforded by the Protochips Aduro heater system is an added benefit for complementary analysis of catalyst structure and behavior during elevated temperature treatments. The MEMS-fabricated Aduro heater devices have a nominal 500 × 500 μm2 thin ceramic membrane on the order of 100-nm thick supported over a window in a silicon chip. The ceramic membrane has a pattern of 6-micron diameter holes over which is suspended a holey carbon support film (e.g., C-flat, Protochips, Inc.). In this study, all heating experiments were conducted under high-vacuum conditions in the microscope column. The heater device fits onto a special-made specimen holder, with electrical leads to provide power from a Keithley 2611A source meter (Keithley Instruments, Inc., Cleveland, OH, USA). The profile of temperature as a function of input current is calibrated by the manufacturer, using an optical pyrometer while heating the device in a vacuum chamber, thus allowing estimation of temperature of the sample deposited on it, up to 1,200°C. Owing to its miniature heated volume, the heating chip has minimal thermal drift and a near instantaneous temperature response at a heating/cooling rate on the order of 106°C/s .
An Au/Fe2O3 (hematite) catalyst with nominal 2 wt% Au loading, acquired from World Gold Council (WGC Ref. No. 60C), was leached in sodium cyanide solutions at pH 12, to remove the weakly bound surface Au species, and was shown to contain 0.7 wt% Au after the leaching process . The TEM specimen was prepared by depositing dry powder onto an Aduro device and then simply shaking off the excess. Imaging experiments were conducted first at room temperature, and the sample was then heated for 10 min at 250°C for stabilization (prior work had indicated that heating at 250°C for up to 30 min did not measurably affect the morphology of Au species in the support) [16, 17]. The sample was imaged during further heating at target temperatures of 500°C, 600°C, and 700°C. It was estimated that it reached the targeted temperature in no more than 10 s. This estimation is derived from a separate study which used powdery materials with known melting point.
Results and discussions
Conventional TEM imaging (TEM-BF)
In the past, reduction of hematite to magnetite at elevated temperatures in vacuum was studied using a thermomagnetic analysis method. Shive and Diehl suggested that partial reduction to magnetite occurs on the surfaces of micron-sized hematite crystallites in a vacuum of 1.4 × 10−3 Pa starting at 350°C . Absalyamov and Mulyukov measured the value of saturation magnetization of submicron-grained hematite in a 1.4 × 10−3 Pa vacuum during heating and cooling up to 750°C . They attributed the sharp increase of saturation magnetization near 500°C to the reduction of hematite to magnetite and further suggested that nano-crystallites of hematite could not exist in vacuum at elevated temperatures. Neither group provided any direct evidence of such a phase transition, citing that the fraction of formed magnetite is too low to be detected by X-ray diffraction. Our in situ TEM study, for the first time, presents the direct evidence that the nano-sized hematite completely converts to magnetite at 500°C in a vacuum of 4.8 × 10−6 Pa. Our experimental condition was at higher vacuum than the previous two groups. The higher vacuum level may contribute to a more favorable condition for the reduction of hematite nanocrystals. Because this is an Au/Fe2O3 (hematite) material system, it is also worth exploring whether the presence of Au nanoparticles and finely dispersed species in the hematite lattice might play a role in the reduction of the hematite. In further work, similar heating experiments will be carried out using equivalent hematite powders without the presence of Au.
STEM imaging results
The spatial resolutions of Figure 3A,B were determined with the SMART-J method [22, 23]. This method uses the Fourier transform (FT) of the image to separate the contribution of the signal (object) and the noise in the FT image. A resolution of 1.5 nm was determined for the SE image (Figure 3A), and a resolution of 1.6 nm was determined for the ADF image (Figure 3B). The particle sizes were measured from intensity line scans extracted from the image. The size was defined by the full-width-half-maximum (FWHM) of the intensity peak corresponding to the particle. An average size of 3.3 ± 0.4 nm was obtained after the analysis of five particles. No difference in size was observed between the SE and ADF images.
Secondary electron micrographs are formed using secondary electrons (SE) and backscattered electrons (BSE). In a recent study by Zhu et al. , conducted using an essentially identical secondary detector, the ratio of SE and BSE was found to be in the range of 85% to 90% (SE) to 15% to 10% (BSE) at 200 kV. We estimated the fraction of secondary electrons exceeds 95% from a test using an atomically smooth silicon nitride film of 50-nm thick: the film facing the electron incident beam was clean. On the opposite side, we deposited Li3FePO4. There was absolutely no detectable contrast from the SE image. This test demonstrated that the signals from Figure 3A and Figure 4 are mostly from secondary electrons emitted from less than 50 nm depth. We further calculated that at 700°C the SE image resolution was 1.1 nm.
In summary, we have demonstrated that 1.1-nm spatial resolution in secondary electron imaging can be achieved even during in situ heating up to 700°C using a conventional TEM/STEM. We also have shown that information transfer in TEM imaging is about 0.14 nm at 600°C. Such a combined SEM and TEM in situ study is useful for nanomaterials research because information from the surface via SEM imaging and bulk via TEM/STEM imaging can be simultaneously obtained. If combined with simultaneous nanoprobe spectroscopy (e.g., energy dispersive X-ray spectroscopy and energy-loss electron spectroscopy), this approach will open the door to a wide range of applications, such as studying the kinetics of nano-scaled phenomena.
Microscopy research at the Oak Ridge National Laboratory’s High Temperature Materials Laboratory was sponsored by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Propulsion Materials Program. This is a contributed paper and published as part of the Proceedings of the Microscopy and Microanalysis 2010, Portland, OR, USA, August, 2010.
- Gregori G, Kleebe HJ, Siegelin F, Ziegler G: In situ SEM imaging at temperatures as high as 1450 degrees C. J Electron Microsc (Tokyo) 2002, 51(6):347–352. 10.1093/jmicro/51.6.347View ArticleGoogle Scholar
- Seward GG, Prior DJ, Wheeler J, Celotto S, Halliday DJ, Paden RS, Tye MR: High-temperature electron backscatter diffraction and scanning electron microscopy imaging techniques: in-situ investigations of dynamic processes. Scanning 2002, 24(5):232–240.View ArticleGoogle Scholar
- Boyes ED, Gai PL: Environmental high-resolution electron microscopy and applications to chemical science. Ultramicroscopy 1997, 67: 219–232. 10.1016/S0304-3991(96)00099-XView ArticleGoogle Scholar
- Gai PL, Kourtakis K, Ziemecki S: In situ real-time environmental high resolution electron microscopy of nanometer size novel xerogel catalysts for hydrogenation reactions in nylon 6,6. Microsc Microanal 2000, 6(4):335–342.Google Scholar
- Boyes ED: High resolution in situ SEM of competitive particle sintering and other surface processes. Microsc Microanal 2002, 8: 408–409.Google Scholar
- Williamson MJ, Tromp RM, Vereecken PM, Hull R, Ross FM: Dynamic microscopy of nanoscale cluster growth at the solid-liquid interface. Nat Mater 2003, 2: 532–536. 10.1038/nmat944View ArticleGoogle Scholar
- Liu L: Advanced electron microscopy characterization of nanostructured heterogeneous catalysts. Microsc Microanal 2004, 10: 55–76. 10.1017/S1431927604040310View ArticleGoogle Scholar
- Gai PL, Boyes ED: Advances in atomic resolution in situ environmental transmission electron microscopy and 1A aberration corrected in situ electron microscopy. Microsc Res Tech 2009, 72(3):153–164. 10.1002/jemt.20668View ArticleGoogle Scholar
- Walsh MJ, Pay ML, Gai PL, Boyes ED, Yoshida K: On the effect of atomic structure on the activity and deactivation of catalytic gold nanoparticles. Chem Cat Chem 2012, 4(10):1638–1644.Google Scholar
- Boyes ED, Ward MR, Lari L, Gai PL: ESTEM imaging of single atoms under controlled temperature and gas environment conditions in catalyst reaction studies. Ann Phys 2013, 525: 423–429. 10.1002/andp.201300068View ArticleGoogle Scholar
- Bond GC, Thompson DT: Status of catalysis by gold following an AURICAT workshop. Appl Catal A Gen 2006, 302: 1–4. 10.1016/j.apcata.2006.01.001View ArticleGoogle Scholar
- McEwan L, Julius M, Roberts S, Fletcher JCQ: A review of the use of gold catalysts in selective hydrogenation reactions. Gold Bull 2010, 43: 298–306. 10.1007/BF03214999View ArticleGoogle Scholar
- Moreau F, Bond GC: CO oxidation activity of gold catalysts supported on various oxides and their improvement by inclusion of an iron component. Catal Today 2006, 114: 362–368. 10.1016/j.cattod.2006.02.074View ArticleGoogle Scholar
- Thompson DT: A golden future for catalysis. Top Catal 2006, 38: 231–240. 10.1007/s11244-006-0021-xView ArticleGoogle Scholar
- Allard LF, Bigelow WC, Jose-Yacaman M, Nackashi DP, Damiano J, Mick SE: A new MEMS-based system for ultra-high-resolution imaging at elevated temperatures. Micros Res Tech 2009, 72: 208–215. 10.1002/jemt.20673View ArticleGoogle Scholar
- Allard LF, Borisevich A, Deng W, Si R, Flytzani-Stephanopoulos M, Overbury SH: Evolution of gold structure during thermal treatment of Au/FeOx catalysts revealed by aberration-corrected electron microscopy. J Electron Microsc 2009, 58: 199–212. 10.1093/jmicro/dfp016View ArticleGoogle Scholar
- Allard LF, Flytzani-Stephanopoulos M, Overbury SH: Behavior of Au species in Au/FeOx catalysts as a result of in situ thermal treatments, characterized via aberration-corrected STEM imaging. Microsc Microanal 2010, 16: 375–385. 10.1017/S1431927610013486View ArticleGoogle Scholar
- Howe JY, Allard LF, Bigelow BC, Overbury SH: Synergy of combined (S)TEM imaging techniques for the characterization of catalyst behavior during in situ heating. Microsc iMicroanal 2010, 16(S2):312–313. 10.1017/S1431927610061039View ArticleGoogle Scholar
- Shive PN, Diehl J: Reduction of hematite to magnetite under natural and laboratory conditions. J Geomagn Geoelectr 1977, 29: 345–354. 10.5636/jgg.29.345View ArticleGoogle Scholar
- Absalyamov SS, Mulyukov KY: The stability of hematite in small-size particles. Dokl Phys 2000, 45: 657–658. 10.1134/1.1342443View ArticleGoogle Scholar
- Russ JC: The image processing handbook 6th edn. Boca Raton: CRC Press; 2011.View ArticleGoogle Scholar
- Joy DC: SMART - a program to measure SEM resolution and imaging performance. J Microsc Oxford 2002, 208: 24–34. 10.1046/j.1365-2818.2002.01062.xView ArticleGoogle Scholar
- Kim J, Jalhadi K, Lee SY, Joy DC: Fabrication of a Fresnel zone plate through electron beam lithographic process and its application to measuring of critical dimension scanning electron microscope performance. J Vac Sci Technol B 2007, 25: 1771–1775. 10.1116/1.2787874View ArticleGoogle Scholar
- Zhu Y, Inada H, Nakamura K, Wall J: Imaging single atoms using secondary electrons with an aberration-corrected electron microscope. Nat Mater 2009, 8: 808–812. 10.1038/nmat2532View ArticleGoogle Scholar
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