Scientists explore complex metal-support interactions under redox conditions using our Climate system

Scientists explore complex metal-support interactions under redox conditions using our Climate system

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

For catalysts consisting of metal nanoparticles and an oxide support, understanding the synergistic reactions between the metal and support is paramount. In the case of reducible supports, the so-called strong metal-support interaction (SMSI) provides a means of tuning the chemisorption and catalytic properties of supported metal particles. SMSI involves the encapsulation of nanoparticles by a thin layer of partially reduced support material. The encapsulation is typically induced during high-temperature reductive “activation”, i.e., treatment in hydrogen. Notably, the direct imaging of this encapsulated state has mostly been achieved ex situ. Little is known about SMSI under catalytic working conditions, where the application of in situ electron microscopy is invaluable. While environmental transmission electron microscopy (TEM) is generally limited to chamber pressures of around 20 mbar, the DENSsolutions Climate Nano-Reactor can handle pressures 100 times higher, unlocking unprecedented research possibilities for the study of catalysts in their native environment.

In recent research performed at ScopeM, ETH Zurich, where our advanced Climate G+ system is installed, Hannes Frey, Arik Beck, Dr. Xing Huang, Prof. Dr. Jeroen Anton van Bokhoven and Prof. Dr. Marc Georg Willinger investigated the dynamic interplay between metal nanoparticles and oxide support under reaction conditions. More specifically, the scientists revealed the working state of a model catalyst, Pt–TiO, by directly observing the synergistic interactions related to SMSI between the platinum nanoparticles and titania support.

Switching to a redox-active H₂–O₂ mixture

Frey and his fellow collaborators first induced the classical SMSI state by heating the titania-supported platinum nanoparticles (NPs) in H₂. The nanoparticles were then transferred into an O₂ atmosphere via inert gas purging. Interestingly, this treatment resulted in the platinum NPs incurring a nonclassical oxidized SMSI state. After preparing the system, the researchers then exposed it to a redox active atmosphere through the addition of H₂ into the O₂ flow. With the Climate G+ system, the researchers were able to mix the gasses on the fly, while continuing with the high-resolution observation. 

In Figure 1 and Movie 1 below, the morphological change of the platinum NPs upon transition into the redox-active regime is presented. Increasing the partial pressure of H₂ in the Climate Nano-Reactor resulted in the gradual change in the encapsulated state of the Pt NPs. Moreover, this resulted in the ultimate disappearance of the overlayer as soon as the gas composition reached a set mixture of 60 mbar H₂ and 700 mbar O₂ after ~180 s. Once the overlayer was fully removed, the particle was observed to experience particle dynamics like restructuring and migration (shown in Figure 1E and 1F). 

Figure 1: Image series depicting the morphological changes observed when a titania-supported platinum nanoparticle in a nonclassical SMSI state is exposed to a redox-active atmosphere. The composition of the gas in the Nano-Reactor was gradually changed from 700 mbar O₂ to a mixture of 60 mbar H₂ plus 700 mbar O₂. t0 is the time at which the H₂ flow was turned on.

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.

Particle and interfacial dynamics in the redox-active regime

The researchers then set out to explore the response of a collection of nanoparticles to reaction conditions. They discovered that the degree of structural dynamics and mobility differed greatly among the nanoparticles. Whereas some NPs remained static and stationary, others incurred structural fluctuations and migrated across the substrate surface. The researchers decided to follow three representative cases of nanoparticles with different orientations. In all cases, it is observed that the redox chemistry at the interface is the driving force for particle reconstruction and migration. In Figure 2 below, the respective structural dynamics of the three selected nanoparticles are presented.

Figure 2: (A–C) Pt NP oriented with (111) planes perpendicular to the interface. (G to J) Pt NP oriented with (111) planes parallel to the interface. (K to N) Pt NP that has its (111) planes inclined toward the interface. The blue shapes indicate the respective positions of the Pt NP in the previous frames.

Case 1

The first platinum NP that was considered was oriented with (111) planes perpendicular to the interface. In Movie 2 below, the recorded time series of this platinum NP at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂ is shown. Here, the NP developed pronounced structural dynamics that involved twin formation and shearing along (111) planes in an up-down motion, perpendicular to the interface. 

Movie 2: Recorded time series of the Pt NP presented in Figure 2A–C) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Case 2

The second Pt NP that was considered was oriented with (111) planes parallel to the interface. Movie 3 below shows the image series acquired for this nanoparticle, at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂. Here, a continuous step flow motion of the (111)-type facet in contact with the interface is observed.

Movie 3: Image series of the Pt NP presented in Figure 2G–J) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Case 3

The third platinum NP that was considered was oriented with (111) planes inclined towards the interface. Movie 4 below shows the image series acquired for this nanoparticle, at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂. In this case, the nanoparticle is observed to engage in redox chemistry-driven directional surface migration which is caused by restructuring at the interface.

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.

Retraction of H₂ and reformation of the oxidic SMSI overlayer

The next step for Frey and his fellow researchers was to switch the gas composition from a reactive feed back to a purely oxidizing environment by turning off the H₂ flow. This change in the gas composition resulted in the encapsulated state of the platinum NPs to be reestablished. In Figure 3 below, the morphological change of a Pt NP upon switching the gas from a redox-active environment to a purely oxidizing environment is presented. This switching of the gas composition led to the reformation of a classical particle overgrowth. It is seen that the nanoparticles first adopted a spherical morphology (Figure 3A–C). Then, as soon as H₂ was fully removed from the Nano-Reactor, the support material migrated onto the Pt NPs and the overlayer reformed (Figure 3D–F).

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.

Conclusion

The aim of this paper was to reveal the working state of a catalyst via direct observation and to study possible synergistic interactions related to SMSI between metal nanoparticles and a reducible oxide support. The use of our Climate system enabled the researchers to capture in exceptional detail the dynamic and complex metal-support interactions of Pt–TiO₂ under reactive catalytic conditions. This is especially thanks to the advanced capacity of the system to handle pressures of up to 2 bar. 

A key finding of this paper is that the stable configurations of static Pt particles exhibiting encapsulation were observed to exist either in pure H₂ (the classical SMSI state) or in pure O₂ (the nonclassical SMSI state), but not in an environment where both gases were simultaneously present. Indeed, the exposure to the redox-active environment led to the removal of the overlayer and the subsequent emergence of pronounced particle dynamics. Moreover, the in situ observations show that the particle restructuring and migration behavior is dependent on the relative orientation of the particle on the support, and therefore the configuration of the interface. Overall, these findings advance our comprehension of SMSI-induced encapsulation of metal nanoparticles, which can in turn help us better tune the chemisorption and catalytic properties of catalysts. 

Hanglong Wu portrait

“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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Climate helps uncover phase coexistence and structural dynamics of redox metal catalysts

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Hydrogen vs. syngas: The battle of the reducing gases

Hydrogen vs. syngas: The battle of the reducing gases

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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Climate helps uncover phase coexistence and structural dynamics of redox metal catalysts

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

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Climate helps uncover phase coexistence and structural dynamics of redox metal catalysts

Climate helps uncover phase coexistence and structural dynamics of redox metal catalysts

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

Marc Willinger TOC 1200x628

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.

Figure 2: Marc Willinger

Figure 1: The redox dynamics and structural analysis of Cu. A–L) show the in situ TEM observations of catalyst reshaping (A–D), sintering (E–H), and splitting (I–L). In M) the integrated SAED pattern and corresponding radial intensity profile are shown. N) shows the HRTEM image of a nanoparticle containing 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: Marc Willinger

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).

Figure 3: Marc Willinger

Figure 3: Relation between structural dynamics of Cu and catalytic activity. A) shows the MS data collected at varied temperatures. B–D) show the sequential HRTEM images of particle reshaping/restructuring at 550°C. E,F) show the enlarged HRTEM images of the areas indicated by dashed rectangles in (B) and (D). G) shows an HRTEM image, and in H) an enlarged HRTEM image of the area indicated by the dotted rectangle in (G).

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: Marc Willinger

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 4: Marc Willinger

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.

Hanglong Wu portrait

“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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DENSsolutions Climate system takes home the microscopy today 2021 innovation award

DENSsolutions’ Climate system takes home the Microscopy Today 2021 Innovation Award

DENSsolutions becomes a consecutive two-time winner of the Microscopy Today Innovation Awards. This year, our Climate system is recognized as one of the 10 most game-changing microscopy innovations of 2021.

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Announcing the establishment of the DICP-DENS Microscopy Centre

Announcing the establishment of the DICP-DENS Microscopy Centre

From left to right: Wei Liu, Yu Xiao, Lijian Geng, Yan Jin, Dan Zhou, Xi Liu

We believe that it is now more important than ever to expand our efforts in enabling fundamental research in the fields of catalysis and sustainable energy. In line with the emphasis we place on multinational collaborations, we have partnered with the Dalian Institute of Chemical Physics (DICP) in China in order to accelerate these fields and achieve results together. To celebrate the establishment of the DICP-DENS in-situ electron microscopy technology application laboratory, an exciting ceremony was recently held at DICP in which numerous speakers took the stage to share their areas of expertise. 

The DICP-DENS collaborative application laboratory combines the extensive research capabilities of DICP, China’s leading and most influential catalysis research institute, with the advanced technology and outstanding research and development capabilities of DENSsolutions in the in-situ field. DENSsolutions will equip a complete Climate G+ system at the Xishan Lake Electron Microscope Center for in-situ atmosphere and heating TEM studies. The aim of this collaboration is to expand the frontiers of catalysis research and deepen our understanding of the energy conservation process. 

Opening ceremony

The event was hosted by Yan Jin, Deputy Director of the Energy Research Technology Platform of DICP. During the opening ceremony, both Yu Xiao, Director of Science and Technology Department of DICP, and Lijian Geng, Chairman and General Manager of ALTA Scientific delivered speeches.

Host Yan Jin opening the ceremony

Researcher Yu Xiao first welcomed the guests and expressed his enthusiasm about the collaboration between DICP and DENSsolutions for the realization of this application laboratory. He also relayed his hopes that this cooperation could develop in a long-term and stable manner, and that the results of this cooperation could be realized as soon as possible.

On behalf of ALTA Scientific and DENSsolutions, Lijian Geng then made an affectionate review depicting the lengthy history of the cooperation between the two parties, thanking those who made it possible. He expressed his gratitude to the many experts and professors who could not be present for the opening as well as to DENSsolutions CEO, Ben Bormans and CCO, Robert Endert for their continuous support.

Finally, DENSsolutions CTO Dr. Hugo Perez Garza delivered a digital speech, in which he expressed his excitement and gratitude on behalf of the DENSsolutions team for the trust that DICP has placed in us as a reliable partner. In his video, he signed the contract that formalizes the collaboration and expressed his confidence in a fruitful collaboration. This celebratory video is shown below.

Dr. Hugo Perez Garza delivering a digital celebratory speech 

Unveiling ceremony

After the opening event, researcher Yu Xiao, representing DICP, and Geng Lijian, representing DENSsolutions and ALTA Scientific, held an unveiling ceremony of the joint laboratory. This marked the official establishment of the DICP-DENS in-situ electron microscopy technology application laboratory.

Yu Xiao and Lijian Geng during the unveiling ceremony

Application seminar

In the second half of the conference, three speakers were invited to give talks during the application seminar. First, Professor Wei Liu gave a detailed introduction to the current configuration and construction of the Xishan Lake electron microscope platform of DICP and the team’s latest research results in the in-situ field. He also shared his thoughts and prospects on in-situ electron microscopy technology.

Next, DENSsolutions Senior Application Scientist Dr. Dan Zhou introduced in detail the leading advantages of the DENSsolutions Climate in-situ TEM gas and heating system and the latest research and development progress. She also shared some recent developments in application results.

Finally, Dr. Xi Liu from Shanghai Jiaotong University introduced his current application of in-situ aberration-corrected gas and heating TEM in heterogenous catalysis and the surface science of iron oxide reduction. He detailed the importance of the existence of in-situ TEM and explained that when combined with other characterization methods, in-situ TEM can have both super-high-resolution volume and surface characterization capabilities, thereby providing a basis for the establishment of new characterization methodology.

The three speeches during the application seminar deepened everyone’s understanding of in-situ technology and won a warm applause from the participants.

Wei Liu presenting the latest research results of the DICP research team in the in situ field

Dr. Dan Zhou giving a speech about the DENSsolutions Climate system

Xi Liu giving a speech about his current application of in situ TEM 

We are very excited to unravel the ample potential that this collaboration has in regards to advancing research in the field of catalysis and sustainable energy, and we hope to play a key role in the fight against climate change.

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Scientists find an alternate route towards CO2 reduction

Scientists find an alternate route towards CO2 reduction

In situ ETEM uncovers that deep-ultraviolet plasmons have the ability to drive endothermic reactions at room temperature

Original article by Canhui Wang, Wei-Chang D. Yang, David Raciti, Alina Bruma, Ronald Marx, Amit Agrawal and Renu Sharma

Plasmonic nanoparticles of certain metals, like gold, silver and aluminium, have the unique capability of harvesting and scattering light. These nanoparticles can harvest energy from a light source and transfer it to adsorbed gas molecules, ultimately reducing the temperature needed to drive the chemical reaction. Most of the reactions reported in research are exothermic, and only H-D bond formation has been successful at room temperature. However, for the first time in research, scientists from the NIST and IREAP in Maryland, U.S., find that endothermic reactions can be achieved at room temperature using localized surface plasmons (LSP) in the deep-UV range. Without the DENSsolutions Climate Air in situ system, this revolutionary finding would be awaiting its indispensable discovery. 

 

The DENSsolutions Climate system

When light excites the conduction electrons of certain metallic nanoparticles, it causes these electrons to undergo oscillation, generating localized surface plasmons (LSP). This resonant oscillation, called surface plasmon resonance (SPR), is essentially why these plasmonic nanoparticles have this exceptional ability to absorb and scatter light. 

In this in situ experiment, the reduction of CO₂ on a graphite sample to CO was realized at room temperature by exciting multiple LSP modes of aluminum nanoparticles using high-energy electrons. An ETEM is used to excite and characterize the aluminum LSP resonances and concurrently measure the spatial distribution of the carbon gasification around the nanoparticles in a CO₂ environment. Although this experiment took place in an ETEM, the ETEM was only used to provide an electron beam to generate the localized surface plasmons. It was the Climate Sample Holder that enabled the introduction of the CO₂ gas towards the sample.

In order to detect CO as a reaction product, the Climate Sample Holder containing the Climate Nano-Reactor was coupled to a gas chromatograph-mass spectrometer (GCMS). Four nanoreactor environments were analyzed, represented by the lines in the figure below: 1) Pure CO₂, 2) 0.01% CO added to the CO₂ gas flow, 3) Pure graphite heated at 900°C without aluminum nanoparticles, and finally 4) Illumination of aluminum on graphite in CO₂ at room temperature using an e-beam.

Detection of CO as a reaction product using the GCMS

Measurable CO was detected only in the latter three cases but not in pure CO₂. However, it was particularly in the last case where a CO-peak was realized when the electron beam was switched on to generate LSPs. Typically, a standard ETEM will produce CO concentrations that are far below the detection threshold of the GCMS. Yet, because the Climate Nano-Reactor in the Climate Sample Holder has a small volume, high pressure environment, the CO concentration in the CO₂ gas could rise high enough to enable the GCMS to detect it. This experiment demonstrates the unique stability and integrity of the Climate Nano-reactor over long periods of time. 

Novelty in findings

This novel finding not contributes highly on an academic front, paving the way for scientists to explore other industrially relevant chemical processes initiated by plasmonic fields at room temperature, but also globally by providing an alternate route for CO₂ reduction. Aluminium, Earth’s most abundant metal, presents itself as an ideal candidate for channelling energy from light to perform large-scale CO₂ reduction. This common and inexpensive metal therefore has ample potential to aid in the relentless fight against climate change.

Dr. Mihaela Albu

“The Climate Nano-Reactor proved to be essential for taking the reaction product of this LSP experiment out of the ETEM specimen chamber. Due to its unique low-volume, high-pressure design, the CO concentration in the carrier gas was still high enough to be detected ex situ.”

Ronald Marx
Senior Technical Consultant | DENSsolutions

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In-situ imaging provides detailed insights on the dynamics of SMSI induced overlayer formation on catalyst particles

In-situ imaging provides detailed insights on the dynamics of SMSI induced overlayer formation on catalyst particles

Enabled by DENSsolutions Climate system in correlation with other TEM characterization techniques

Original article by Arik Beck, Xing Huang, Luca Artiglia, Maxim Zabilskiy, Xing Wang, Przemyslaw Rzepka, Dennis Palagin, Marc-Georg Willinger & Jeroen A. van Bokhoven.

Noble metal nanoparticles stabilized on oxide supports are an important class of catalysts that are used in many applications such as fuel cells, exhaust gas treatment and energy conversion. It is well known that an interaction occurs between the nanoparticles and their oxide support which affects the catalytic activity called ‘strong metal-support interaction’ (SMSI). SMSI is a surface phenomena in which the migration of partially reduced oxide species, from the oxide support, covers the nanoparticle and thereby alters the chemisorption and catalytic properties. It can give rise to desired synergistic effects and increased selectivity. Now, using in situ TEM combined with other analytical techniques and theoretical modelling, researchers at ETH Zurich have been able to create a real time view of the SMSI phenomena.

Controlling the sample environment

Reductive pre-treatment of catalysts by heating, resulting in SMSI, has been known to alter the selectivity of oxide supported nanoparticles since the late 1970’s. However, the exact influence of the different parameters like temperature and gas concentration were still unknown. But now, thanks to the DENSsolutions Climate G+ system, researchers are able to determine the immediate effect of these parameters in increasing detail. The Climate G+ system provides a nano-reactor, containing the catalyst sample, that can be placed in any TEM* and gives the researcher unprecedented control over the sample environment in terms of temperature and gas parameters.

The in situ TEM experiments performed for this research required multiple switching between hydrogen and oxygen environments at 600 °C. This made the Climate G+ system, that is used on the JEM-ARM 300F at ETH Zurich, ideal for this research.

Evolution and dynamic structural changes of the overlayer in SMSI. A platinum particle on a titania support in the first exposure to H2 at 600 °C (a,b) and the subsequent atmosphere change to O2 at 600 °C (c), a switch to H2 (d) and then a switch to O2 again (e), and interpretation of the phenomena based on the combined results of in situ transmission electron microscopy, in situ X-ray  photoemission spectroscopy, and in situ powder X-ray diffraction (f–j). Insets for c–e show a magnified image of the overlayer structure observed. Scale bar is 5 nm.

Correlative techniques

In situ TEM, using gas and heating, is a powerful characterization technique to obtain atomistic, real time, information about the SMSI phenomena. To derive a holistic view of SMSI and the role of hydrogen and oxygen within this process. The in situ TEM results have been combined with ambient pressure XPS and in situ powder XRD experimental results. Finally, theoretical density functional theory (DFT) modelling was used to support the conclusions about how SMSI actually works.

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