A team of experts develop a data synchronization method for operando gas and heating TEM via time delay calibration, enabled by the unique ability of the DENSsolutions Climate Nano-Reactor to perform nano-calorimetry.

Original article by Fan Zhang, Merijn Pen, Ronald G. Spruit, Hugo Perez Garza, Wei Liu, Dan Zhou

“The DENSsolutions Climate system is able to create an accurate working environment of what practical catalysts go through. Not only does the system facilitate the real-time observation of dynamic catalytic behavior, but it also enables the simultaneous monitoring of the performance of a catalyst by detecting the resulting products within the Nano-Reactor. By enabling this elaborate strategy of data synchronization in operando gas and heating TEM, Climate initiates the distinct functionality of correlating catalytic property with microstructural changes, which adds core value to what the technique of in situ TEM can reveal about a dynamic reaction.”

Prof. Dr. Wei Liu
Professor  |  Dalian Institute of Chemical Physics, CAS

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Via the DENSsolutions Climate system, a team of scientists uncover the dynamic interplay between platinum nanoparticles and titania support under reaction conditions.

Original article by H. Frey, A. Beck, X. Huang, J.A. van Bokhoven and M. G. Willinger

Movie 1: TEM Movie depicting the morphological changes observed when a titania-supported platinum nanoparticle in a nonclassical SMSI state is exposed to a redox-active atmosphere.

Movie 4: Image series of the Pt NP presented in Figure 2K–N) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Figure 3: Image series depicting the morphological changes of a Pt NP observed when switching from a redox-active to purely oxidizing environment at 600°C. t0 is the time at which the H₂ flow was set to zero.

“The DENSsolutions Climate System allows us to reveal the so-far unseen: Looking at not only how the gas-phase is changed in the presence of a catalyst, but also studying how the interaction between gas-phase and catalyst leads to the emergence of catalytic function. Direct real-space observation is essential for our understanding of working catalysts and the development of new processes that are urgently needed in view of climate change and limited natural resources.”

Prof. Dr. Marc-Georg Willinger
Professor  |  Technical University of Munich

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Using our Climate system, scientists are able to unveil the most effective method to obtain sintering-resistant metallic cobalt nanoparticles at lower temperatures.

Original article by Ofentse A. Makgae, Tumelo N. Phaahlamohlaka, Benzhen Yao, Manfred E. Schuster, Thomas J. A. Slater, Peter P. Edwards, Neil J. Coville, Emanuela Liberti, and Angus I. Kirkland

The Fischer-Tropsch synthesis (FTS) is an established catalytic chemical process that occurs when carbon monoxide and hydrogen are converted into hydrocarbons. These hydrocarbons can be further developed into high-quality fuels, lubricants as well as raw materials for the production of plastics, rubbers and industrial chemicals. In FTS, supported cobalt catalysts are commonly used, which are active in their reduced metallic state. Many factors can affect the reduction process of cobalt oxide (Co₃3O₄) to metallic cobalt, including the reduction temperature, the composition of the reducing gas, the catalyst promoter, catalyst support and particle size. Despite this, little is known about the effect of using H₂ versus syngas (H₂ & CO) as the reducing agent on carbon-supported cobalt catalysts.

In recent research performed at the David Cockayne Centre for Electron Microscopy and the electron Physical Science Imaging Centre (ePSIC), a number of scientists from highly renowned universities and institutes, including University of Oxford, Johnson Matthey, Diamond Light Source and Cardiff University, investigate the atomic structure and valence state of cobalt nanocrystals on carbon supports under syngas versus hydrogen reduction. Specifically, using ex situ and in situ high-resolution aberration-corrected analytical electron microscopy with our Climate system, the researchers were able to explore the effect of H₂ versus syngas (H₂ & CO) on the reducibility of cobalt oxide nanoparticles supported on hollow carbon spheres. By uncovering highly effective methods to obtain sintering-resistant metallic Co nanoparticles at lower temperatures on hollow carbon sphere supports, this work provides valuable insights that can inform important industrial processes.

Synthesis and reduction of cobalt oxide

Dr. Ofenste Makgae and his fellow collaborators first synthesized the Co₃3O₄ nanoparticles before preparing the hollow carbon sphere supports (HCSs). The synthesized Co₃3O₄ nanoparticles were then loaded on the surface of the HCSs to prepare the precursor catalyst.

The researchers then initiated the reduction process of the Co₃3O₄ nanoparticles for both the ex situ and in situ analysis. For the ex situ analysis, the precursor was reduced via H₂ and syngas, respectively, at 350 °C at atmospheric pressure for 20 hours. For the in situ analysis, the catalysts were studied using the DENSsolutions Climate system in a probe-corrected JEOL ARM200F at ePSIC operated at 200 kV. Notably, the DENSsolutions Climate system is capable of real-time dynamic mixing of 3 gases over a temperature range from room temperature to 1000 °C. Via the Climate Gas Supply System, Dr. Makgae and his fellow collaborators were able to provide a 0.2mL/min flow of gas at a pressure of 500mbar for both the H₂ and syngas reduction. The temperature was ramped from room temperature to 150 °C under a N₂ flow to remove contaminants. Next, it was ramped to 350 °C at 10 °C/min under a reducing environment. Finally, the temperature was held at 350 °C for 2 hours for catalyst activation.

Ex situ post-reduction morphology analysis

The researchers then set out to analyze the overall morphology of the carbon support post reduction, to ensure that its integrity was not compromised during the reduction process. In Figure 1 below, (high-resolution) STEM-ADF images of the as-prepared Co₃3O₄ and ex situ syngas- and H₂-reduced HCS-supported nanoparticles are shown. The researchers were able to determine that the morphology of the HCSs remained intact after 20 hours of reduction at 350 °C.

Figure 1: STEM–ADF images of the Co nanoparticles (a) before and after the ex situ reduction in (b) syngas (H₂/CO = 2) and (c) H₂ at 350 °C. (d–f) are the corresponding high-resolution STEM–ADF images of the as-prepared Co₃3O₄, syngas-, and H₂-reduced nanoparticles.

EELS spectra and elemental maps

Aside from morphological changes, the oxidation state of cobalt also changes during the reduction process as a function of the reducing gas composition. Using STEM–EELS, the researchers were able to thoroughly investigate the Co-oxidation state changes. Figure 2 below compares the EELS spectra and elemental maps of single particles of the as-prepared Co₃3O₄, and the syngas-reduced and H₂-reduced nanoparticles. Via the EELS spectra analysis, the researchers determined that there was indeed a significant decrease in the oxidation state of Co.

Figure 2: EELS spectra of the as-prepared Co₃3O₄ (green), syngas- (orange), and H₂- (blue) ex situ reduced nanoparticles at 350 °C. EELS elemental maps of (b–d) as-prepared, (e–g) syngas-reduced, and (h–j) H₂-reduced Co₃3O₄ nanoparticles.

Importantly, the EELS spectra observations also show that the degree of reduction varies amongst the two methods (H₂ and syngas). To explore this further, the researchers collected additional spectra to compare the ex situ syngas-reduced and H₂-reduced nanoparticles at 350 °C, with the as-prepared Co₃3O₄ as a reference. From this further analysis, it is observed that the Co in the case of the syngas-reduced nanoparticles’ spectra is in a near-metallic state as compared to the H₂-reduced nanoparticles. These results are consistent with the elemental maps obtained, showing that a more intense Co signal (green) is observed in the elemental map from a syngas-reduced nanoparticle (Figure 2g), compared to a more diffuse Co signal in the elemental map of a H₂-reduced nanoparticle (Figure 2j). Ultimately, in the absence of air surface reoxidation, the results of the in situ STEM–EELS and ADF analysis show that syngas achieves a higher degree of Co reduction than H₂.

High-resolution STEM-ADF analysis

The next step for Dr. Makgae and his collaborators was to perform high-resolution STEM–ADF imaging in order to investigate the differences in the atomic structure associated with the observed differences in Co-oxidation states in the syngas- and H₂-reduced nanoparticles. They explored the reduction of the syngas- and H₂-reduced nanoparticles at two different temperatures, 350 °C and 600 °C.

1) At 350 °C

In Figure 3a–e below, the high-resolution STEM–ADF images of syngas-reduced nanoparticles (a–c) and H₂-reduced nanoparticles (d,e) are presented. The results were found to be consistent with the EELS data, showing that syngas reduction produces a majority of metallic Co nanoparticles, whereas H₂ produced particles with a majority of the Co in the 2+ oxidation state. 

Figure 3: High-resolution STEM–ADF images of ex situ (a–c) 350 °C syngas-reduced nanoparticles and (d,e) H₂-reduced nanoparticles.

2) At 600 °C

The temperature was then increased to 600 °C to investigate the effect of temperature on reducing HCS-supported Co₃3O₄ nanoparticles under H₂ versus syngas. It is observed that both the H₂- and syngas-reduced nanoparticles exhibit an atomic structure consistent with cobalt (Figure 4a–d). Indeed, the results are consistent with the EELS data from the 600 °C reduced nanoparticles, which shows the presence of metallic cobalt. Conclusively, in order to achieve a complete reduction of Co₃3O₄ under H₂, the HCS-supported nanoparticles must be reduced at high temperatures. Importantly, it is observed that the high temperature of 600 °C resulted in severe sintering of the nanoparticles reduced in H₂ compared to syngas. The sinter-resistance in syngas-reduced nanoparticles is attributed to the encapsulation of nanoparticles with a carbon shell (indicated by the orange arrow in Figure 4b). This encapsulation prevents the coalescence of the nanoparticles at high temperatures. Interestingly, in the case of the H₂-reduced nanoparticles, a carbon shell is not observed (Figure 4d).

Figure 4: High-resolution STEM–ADF images of 600 °C ex situ (a,b) syngas-reduced nanoparticles and (c,d) H₂-reduced Co nanoparticles.

Conclusively, the presence of CO in syngas is observed to play a critical role in the conversion of Co₃3O₄ to metallic Co at 350 °C. In contrast, the full reduction of Co₃3O₄ to active metallic Co in H₂ is not achieved at 350 °C. Increasing the activation temperature to 600 °C does drive the complete reduction, however, significant particle sintering was still observed. Therefore, the presence of CO in a syngas mixture is indeed crucial to achieving complete Co₃3O₄ reduction at 350 °C.

Conclusion

This paper comprehensively illuminates the most effective method to obtain sintering-resistant metallic cobalt nanoparticles at lower temperatures under carbon supports. By conducting a thorough ex situ and in situ electron microscopy analysis using the DENSsolutions Climate system, Dr. Makgae and his fellow collaborators were able to observe under nanometer resolution how the reduction process under H₂ versus syngas affects the physicochemical properties of the reduced nanoparticles. The findings show that syngas achieves a higher reduction at an industrially relevant Fischer-Tropsch reduction temperature of 350 °C compared to H₂ on carbon supports. Importantly, lower reduction temperatures are more efficient as operating costs are reduced, which is a significant consideration for organizations involved in industrial processes. The discovery of such findings could not be possible without the application of in situ electron microscopy techniques, for which our Climate system has proved to be remarkably valuable.

“The DENSsolutions Climate system allowed us to directly evaluate the reducibility of Fischer-Tropsch catalysts under industrially relevant temperatures. Such observations provide a unique insight into the evolution of catalysts under activation conditions.

Dr. Ofentse Makgae
Post-doctoral Research Fellow  |  Lund University

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Using our Climate system, scientists are able to interrelate the atomic-scale structural dynamics of redox metal catalysts to their activity.

Original article by Xing Huang, Travis Jones, Alexey Fedorov, Ramzi Farra, Christophe Copéret, Robert Schlögl, and Marc-Georg Willinger

Metal catalysts have been extensively studied due to their critical role in industrial redox reactions. However, many gaps in research still remain, hampering the optimization of their design. Specifically, the behavior of metal catalysts under operating conditions and the relationship between structural dynamics and catalytic activity are still not fully understood. Indeed, an atomistic comprehension of the structure–activity relationship of working catalysts is essential for the optimization of their design. 

In their recently published paper, Dr. Marc-Georg Willinger from ETH Zurich, Dr. Xing Huang from Fuzhou University and fellow collaborators from the Fritz-Haber Institute of Max-Planck Society and the Max Planck Institute for Chemical Energy Conversion explore the phase coexistence and structural dynamics of redox metal catalysts. Using our Climate system, the researchers are able to achieve controlled gas-flow and imaging, obtaining atomic-level insights into the correlation between the structural and chemical dynamics and catalytic function. In light of today’s environmental challenges, the development of improved catalysts for more resource-efficient processes is becoming increasingly critical. Importantly, developing improved catalysts requires their direct observation during operating conditions. In this work, the authors have obtained atomic-scale insights into catalyst dynamics in various relevant redox reactions.

Redox reactions

Copper is a popular transition metal used as an active component in redox catalysts for many reactions, including CO₂ reduction and water gas shift reaction (WGSR). However, an atomistic description of the state of copper under redox conditions in these catalysts remains unrealized. In this publication, Dr. Willinger and his fellow collaborators present a detailed, high-resolution study of copper during the hydrogen oxidation reaction (HOR), revealing fundamentals of catalyst dynamics under reactive conditions. Beyond the elementary hydrogen oxidation reaction, the researchers extend the observed dynamic behavior to more relevant redox reactions and other metal catalysts. Specifically, they explore the redox dynamics of copper and palladium in the active state during methanol oxidation and methane oxidation reactions, respectively.

Hydrogen oxidation reaction on copper

Studying HOR offers the opportunity to obtain detailed atomistic insights into the reaction between the catalyst and the gas phase. HOR was chosen because it is the most elementary redox reaction and yields only water as a product, thereby reducing the complexity related to potential electron-beam-induced processes. In this study, the researchers systematically assess the effects of temperature and gas-phase conditions. Moreover, they explore how the chemical potential of the gas phase defines the phase, size, and shape of catalyst particles, driving the system into a nonequilibrium dynamic state during catalysis. Due to the simultaneous detection of catalytic conversion, they are able to relate directly the observed dynamics and surface structures to catalytic activity. 

1) Redox dynamics and structural analysis of Cu

Copper nanoparticles of 50–200 nm were first loaded in the Climate Nano-Reactor. In situ TEM images of the copper nanoparticles were then recorded. It was found that the particles exhibit rich structural dynamics, which are associated with reconstruction and random motion, as well as particle sintering and red-ox induced splitting. The figure (A–L) and movie below depict these structural dynamics. Shown in M) is the integrated SAED pattern and corresponding radial intensity profile. The in situ SAED revealed dynamically appearing and disappearing diffraction spots, and confirmed the presence of metallic copper and Cu₂O as the sole oxide phase. The constant competition between oxide growth and reduction are reflected in the in situ SAED and the observed structural dynamics. Indeed, the structural dynamics are a consequence of chemical dynamics, characterized by phase coexistence and continuous interconversion between Cu⁰ and Cu₂O. The high-resolution imaging in Figure 1N) confirms this, showing a metallic head coherently interfacing with an oxide tail.

Movie 1: The redox dynamics of copper showing catalyst reshaping, sintering and splitting.

2) Effects of temperature and gas-phase composition

The authors then sought out to explore the effect of temperature on the observed redox dynamics. First, the temperature was decreased from 500 to 300°C, while maintaining a H₂/O₂ ratio of 10/1. During the temperature decrease, the researchers observed the growth of oxide dendrites, which reflects the increasing oxidation potential. Simultaneously, due to the slower kinetics of the redox reaction at lower temperatures, a reduction of the structural dynamics was observed. During heating from 300 to 750 °C, the system passes through a regime of increased dynamics (550 °C) that are characterized by translational motion and restructuring due to oxide growth and reduction, until it finally reaches a state that is less dynamic and dominated by metallic copper at 750°C. This is shown in the figure below (A–D). The reconstructed HRTEM images taken at 300 and 750°C are shown in 2E) and 2F), respectively. Next, integral SAED (Figure 2G,H) was performed to investigate phase analysis, revealing the relation between phase composition and temperature. It has been previously demonstrated that the trend in the oxide content reflects the decreasing chemical potential of oxygen with increasing temperature. However, the authors observe a notable exception of this general trend at around 550 °C, which is mostly due to the effect of water that is produced at substantial rate and contributes to the redox dynamics.

Figure 2: Chemical potential versus structural dynamics of Cu. A–D) In situ TEM observation of dynamics at 300–750 °C and a H₂/O₂ ratio of 10/1. E,F) show the reconstructed HRTEM images taken at 300 and 750 °C. G) and H) show the normalized radial profiles extracted from the integrated SAED patterns and subsequent radial intensity profile, respectively. I–L) In situ observation of copper dynamics during decreasing H₂/O₂ ratio from 10/1 to 5/1 at 500 °C.

Next, the researchers explored the influence of gas-phase composition on the structural dynamics, gradually decreasing the H₂/O₂ ratio from 10/1 to 5/1 at 500 °C. It was found that the relative increase of the oxygen partial pressure leads to a transformation of initially spherical nanoparticles into elongated particles with a head–tail structure. This is depicted in Figure 2I–L above. At the same time, the average particle size declines due to an increased rate of particle splitting, until a new size regime and dynamic equilibrium is established. Conclusively, the real-time observations under varying gas-phase composition and temperature show a clear effect of the gas-phase chemical potential on the average particle size. The in situ observations show clearly that redox dynamics make particles mobile, thereby considerably increasing the rate of sintering as compared to thermal sintering; yet the sintering under redox conditions is balanced by particle splitting, such that a certain size distribution is established as a function of reaction conditions.

“Controlled gas-flow and imaging – coupled with on-line mass spectroscopic analysis of the gas-phase composition as enabled by the Climate system – is essential for studies on the behavior of active catalyst and allows us to correlate observed structural and chemical dynamics to catalytic function.” – Dr. Marc-Georg Willinger, ETH Zurich

3) Relation between structural dynamics and catalytic activity

After investigating the gas-phase and temperature-induced dynamic processes, the researchers then sought out to explore the relationship between the observed structural dynamics and catalytic activity. The MS data is presented in Figure 3A) below, showing the formation of water and simultaneous consumption of oxygen. This ultimately confirms the catalytic activity of copper. A notable increase in water production and oxygen consumption is observed between 500 and 600°C, which is also the same range in which the intense structural dynamics occurred. In Figure 3B–D), the sequential HRTEM images of particle reshaping/restructuring at 550°C is presented (H₂/O₂ ratio of 10/1). This is also shown in the movie below. Although challenging, the researchers were still able to capture the thin oxide monolayer existing on the surface of the metallic portion of the particles (see Figure 3E,F). Interestingly, the surface oxide layer is observed even at 750°C. The structural features of the monolayer oxide imaged on various facets can be observed in Figure 3G,H).

Movie 2: HRTEM movie showing particle reshaping and restructuring at 550°C

Methanol oxidation reaction on copper

After investigating the particle dynamics for hydrogen oxidation on copper, the researchers then set out to assess the generality of the phenomena described above. They first investigated the state of copper under conditions of methanol oxidation, a catalytic reaction that is relevant to industrial synthesis of formaldehyde. The figure below shows in situ TEM images of copper nanoparticles recorded at 600 and 500°C (Figure 4A–C and 4E–G), respectively. The dynamic behavior observed involves reshaping, sintering, and splitting of particles, similar to what was observed in the case of hydrogen oxidation. A shift to a more oxidized state with decreasing temperature was observed and verified by in situ SAED (Figure 4D,H). The redox dynamics are most pronounced at around 500°C under the chosen 1:1 ratio of MeOH and O₂.

Figure 4: Structural dynamics of Cu in methanol oxidation reaction. A–H) show TEM images and SAED patterns of Cu recorded in situ during methanol oxidation at 600 °C (A–D) and at 500 °C (E–H), respectively.

Methane oxidation reaction on palladium

Next, they investigated methane oxidation on palladium, a transition metal that is much harder to oxidize than copper. In the figure below, the structural dynamics related to catalytic activity in methane oxidation on palladium is presented. As in the case of copper, structural dynamics evolve when palladium is driven toward the Pd/PdO phase boundary. In an ~2:1 ratio of CH₄ and O₂, the catalyst remains relatively static at 350°C and shows coexistence of Pd and PdO as evinced by the in situ SAED. The system evolves to a highly dynamic state at 550°C. The MS data recorded simultaneously with TEM observation reveal a pronounced formation of CO₂ and consumption of CH₄ and O₂ under these conditions (see Figure 5I).

Figure 5: Structural dynamics related to catalytic activity in methane oxidation on Pd. A–H) In situ TEM images and SAED patterns of Pd recorded during methane oxidation at 350°C (A–D) and 550°C (E–H), respectively. I) shows the MS data recorded during in situ TEM observation of Pd in methane oxidation.

Conclusion

Via the above in situ studies of copper and palladium catalysts using our Climate system, the researchers show that catalytic activity goes hand-in-hand with redox processes of the metal catalyst. This paper evinces that the associated dynamics sensitively depend on reaction temperature and gas-phase composition. Importantly, only direct observation could reveal the interplay between metal and oxide phases and relate it to the onset of catalytic activity. This is precisely where our advanced in situ solutions come into play, enabling the direct observation of phenomena while it occurs. We are proud of the role that our Climate system has played in making this research possible and strive to continue enabling groundbreaking research in the future.

“The DENSsolutions Climate System allows us to reveal the so-far unseen: Looking at not only how the gas-phase is changed in the presence of a catalyst, but also studying how the interaction between gas-phase and catalyst leads to the emergence of catalytic function. Direct real-space observation is essential for our understanding of working catalysts and the development of new processes that are urgently needed in view of climate change and limited natural resources.”

Dr. Marc-Georg Willinger
Group Leader |  ETH Zurich

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