TY - JOUR
AU - Hirata,, Hirohito
AB - Abstract We introduce herein a combined environmental high-voltage electron microscope and a quadrupole mass spectrometer to detect product gas species associated with chemical reactions occurring in the microscope, which allows new operando experiments of, for instance, observing catalytic reactions by concurrent high-resolution transmission electron microscope (TEM) observation. We demonstrate the preliminary results of redox reactions, where the product gas species are unambiguously detected, associated with the expected structural transformations observed with TEM. reaction science high-voltage transmission electron microscope, quadrupole mass spectrometer, gas environmental cell, in situ observation, catalytic redox reaction, operando measurement A 1 MV transmission electron microscope (TEM), called the reaction science high-voltage scanning TEM (RS-HVSTEM) JEM 1000 K RS, was installed in 2010 in the high-voltage electron microscopy (HVEM) laboratory of Nagoya University. This machine is capable of conventional high-resolution TEM observation with bright/dark-field scanning imaging of thick samples, chemical analyses using a post-column-type energy filter (or electron energy loss spectrometer), 3D computer tomography, and other in situ observations/measurements under a reactive gas atmosphere [1]. The high penetration power of 1 MeV electrons allows a clear lattice fringe observation even at a gas (e.g. N2) pressure of 10 000 Pa. The new HVEM system has delivered a number of novel and outstanding achievements, particularly atomic resolution observations of catalytic reactions [2–4], where the surface structures of metal catalysts dynamically change under a reaction gas atmosphere at usually elevated temperatures. Although such observations should essentially be informative for the design of catalysts with better performance, no direct evidence has shown that the chemical reaction of interest really occurred and is associated with the actually observed structural changes at the same time. Thus, our new system combines the HVEM and a quadrupole mass spectrometer (QMS) [5] to identify the reaction gases in situ in the specimen chamber. Several attempts have been made to implement a mass spectrometer (residual gas analyzer) to a commercial TEM equipped with an environmental gas cell [6–9]; however, the direct detection of product gas species associated with a solid catalytic reaction in the microscope has yet to be reported. We describe herein the design of the RS-HVSTEM-QMS system and demonstrate the preliminary results obtained using this system. Figure 1 schematically shows the block diagram of the present system. The QMS system JEOL JMS-Q1500 (see details in [10]) was attached to the JEM1000K RS HVEM with the two valves implemented in the backside of the column gas unit, one (V1) between the gas unit and a turbo-molecular pump (TMP) or a scroll pump depending on the gas pressure inside the gas cell, and another (V2) between the gas unit and the QMS system. The gas molecules in the unit were evacuated from the gas cell through the 1/4-inch copper liner tube by the internal TMP of the QMS system backed up by a scroll pump. Fig. 1. View largeDownload slide Schematic diagram of the present HVEM-QMS system (see text for details). OL P. P.: objective lens pole piece; TMP: turbo-molecular pump; Scroll P: scroll pump. Fig. 1. View largeDownload slide Schematic diagram of the present HVEM-QMS system (see text for details). OL P. P.: objective lens pole piece; TMP: turbo-molecular pump; Scroll P: scroll pump. In the QMS mode, valve V1 is closed, while valve V2 is open to introduce the gas species in the gas unit chamber into the QMS. The base pressure of the specimen chamber is approximately 2 × 10−5 Pa with the gas cell retracted and 3 × 10−3 Pa with the gas cell inserted. The pressure is directly monitored with a crystal pressure gauge equipped at the end of the chamber [11]. The gas cell is differentially pumped through the upper and lower orifices (gray area, Fig. 1) to avoid the gas leakage to the microscope column (blue area, Fig. 1), also acting as the electron path. The QMS system was evacuated overnight before the measurement. It took 0.5–1 h until the pressure of the specimen chamber stabilized after the reaction gas was supplied. The QMS spectra were collected, and the gas species of interest in units of m/z (m: mass number of molecule, z: charge number) were monitored as functions of time in the selected ion monitoring (SIM) mode with a sampling rate of 1 s/channel and an ionizing voltage of 70 eV. Powder samples were ground and dispersed in ethanol in a mortar and pestle, then painted on a spiral tungsten filament in the sample heating holder [12]. Reaction gas was sprayed to the sample in the HVEM gas chamber. The actual gas partial pressure around the sample position was estimated using a calibration curve based on the crystal gauge value. The pressure estimation method is provided in detail in the Supplemental data. The sample was heated with Joule heating using a constant-voltage power supply. The temperature of which was measured in reference to the temperature–current calibration curve prepared by a number of reference experiments using standard samples. The gas pressure and temperature values are reproducible. We attempted to detect CO2 emission as a result of carbon nanotube (CNT) combustion assisted by a Pd nanoparticle catalyst in O2 atmosphere to verify the system. A mixture of a bundle of carbon nanotubes (CNTs) and Pd fine particles (Fig. 2) was heated in ~15 Pa of O2 gas. Figure 3 shows the QMS-SIM chart monitoring m/z 44 (CO2) and 16 (double charged (z = 2) O2 since the counts of m/z 32 were saturated). Before sample heating, the mass spectra were checked to confirm that no other gas species or fragments existed with m/z of 16 and 44, except for O2 and CO2, respectively. The m/z 16 signal can be used as a reference curve of the gas pressure inside the gas cell, correlating well with the introduced O2 pressure monitored by the crystal gauge. Fig. 2. View largeDownload slide TEM image of moving Pd nanoparticles dispersed on CNTs and heated in O2 atmosphere. Fig. 2. View largeDownload slide TEM image of moving Pd nanoparticles dispersed on CNTs and heated in O2 atmosphere. Fig. 3. View largeDownload slide QMS-SIM chart monitoring m/z 44 (black line: CO2) and m/z 16 (red line: double charged O2) during heating of the Pd/CNT system in O2 atmosphere. The green line shows the sample temperature as a function of time. Fig. 3. View largeDownload slide QMS-SIM chart monitoring m/z 44 (black line: CO2) and m/z 16 (red line: double charged O2) during heating of the Pd/CNT system in O2 atmosphere. The green line shows the sample temperature as a function of time. The Pd particles started to move around in CNTs at approximately 473 K, which is much lower than the onset temperature for direct carbon combustion (Supplemental Video 1), unambiguously showing that the Pd particles catalyzed the oxidation of CNTs into CO2 gas on their surfaces when m/z 44 was detected. With no heater current, the m/z 44 curve promptly decayed. Good correlation was obtained between the TEM image and the QMS spectra without significant delay on the CO2 detection onset with respect to the start of the Pd particle motion (Fig. 3). Note that the detected gas was emitted from the entire sample, not only in the TEM field of view. Supplemental Fig. 1 shows the entire SIM chart. The second feasibility test attempted to detect oxygen emission associated with the thermal reduction of metal oxide nanoparticles in vacuum. The sample was ZrO2-supported Rh nanoparticles, which were fully or partly surface oxidized in ambient atmosphere depending on their size. The sample was mounted as before and gradually heated. Figure 4 shows the SIM chart monitoring m/z 44 (CO2) and 32 (O2). Supplemental Fig. 2 presents the entire SIM chart. Interestingly, the oxygen emission associated with the reduction was not detected, although unambiguous CO2 emission was observed to be synchronized with the onset of transformation from Rh oxide to metallic Rh at around 473 K, as observed by TEM (Fig. 5). The CO2 emission promptly decayed after the heater current was turned off, suggesting that the decomposed oxygen was so active as to immediately react likely with the surrounding hydro-carbon (possible sources: vacuum grease vapor in the sealing parts and residual organic solvent involved in the sample synthesis) into stable CO2. The multi-peak feature of the CO2 emission curve suggested that the reduction reaction depended on the particle size actually observed in the TEM observation. Fig. 4. View largeDownload slide QMS-SIM chart monitoring m/z 44 (black line: CO2) and m/z 32 (red line: O2) during heating Rh oxide particles in vacuum. The green line shows the sample temperature as a function of time. Fig. 4. View largeDownload slide QMS-SIM chart monitoring m/z 44 (black line: CO2) and m/z 32 (red line: O2) during heating Rh oxide particles in vacuum. The green line shows the sample temperature as a function of time. Fig. 5. View largeDownload slide (a) Surface-oxidized Rh nanoparticle supported by a ZrO2 particle at room temperature in vacuum. (b) Metallic Rh nanoparticle as a result of surface reduction at 473 K in vacuum. Each phase is confirmed by the electron energy loss spectrum (not shown). Fig. 5. View largeDownload slide (a) Surface-oxidized Rh nanoparticle supported by a ZrO2 particle at room temperature in vacuum. (b) Metallic Rh nanoparticle as a result of surface reduction at 473 K in vacuum. Each phase is confirmed by the electron energy loss spectrum (not shown). In summary, the new HVSTEM with the QMS detected gas emissions in typical solid redox reactions with good correlation with the reaction onset temperatures and structural changes in the TEM images associated with the reactions. The present system should significantly extend the capabilities of the RS-HVSTEM for future applications. Note that the detection of oxygen is always challenging because atomic oxygen is highly reactive. Another difficulty in the present system would be how the amounts of gas species detected are quantified. The system provides a direct correlation between the structural changes in atomic resolution and gas consumption/emission involved in the chemical reaction of interest and enables more detailed reaction kinetics as a function of temperature and time by the temporal intensity changes of the detected gas species in realistic catalytic reactions. Funding Grants-in-Aid for Scientific Research on Innovative Areas ‘Nano Informatics’ from the Japan Society of the Promotion of Science and partly by Nagoya University microstructural characterization platform as a program of ‘Nanotechnology Platform’ of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (25106004) and Kiban-kenkyu A from the Japan Society of the Promotion of Science and partly by Nagoya University microstructural characterization platform as a program of ‘Nanotechnology Platform’ of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (26249096) References 1 Tanaka N , Usukura J , Kusunoki M , Saito Y , Sasaki K , Tanji T , Muto S , and Arai S ( 2013 ) Development of an environmental high-voltage electron microscope for reaction science . Microscopy 62 : 205 – 215 . 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Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Environmental high-voltage S/TEM combined with a quadrupole mass spectrometer for concurrent in situ structural characterization and detection of product gas molecules associated with chemical reactions
JF - Microscopy
DO - 10.1093/jmicro/dfy141
DA - 2019-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/environmental-high-voltage-s-tem-combined-with-a-quadrupole-mass-0Z5gx6rO6A
SP - 185
VL - 68
IS - 2
DP - DeepDyve
ER -