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Visualizing the dynamic behaviours during carbon nanotube growth in extensive detail

by Olaf Weller | Sep 26, 2019 | Catalyst research, Chemistry, Climate

In situ TEM allows for the direct observation of catalytic processes at relevant pressure and temperature conditions

Original article by Xing Huang, Ramzi Farra, Robert Schlögl and Marc-Georg Willinger

Carbon nanotubes (CNTs) hold many promises, for example in energy storage, high-performance catalysis, photovoltaics, and biomedical devices. Although industrial-scale production of CNTs has been realised, the controllability over the diameter, length, and chirality of CNTs is still unsatisfactory. This is largely due to the lack of atomic information on growth dynamics of CNTs and molecular-level understanding of growth mechanisms. Recently, Huang et al. from FHI Berlin and ETH Zürich have performed an In Situ TEM study on CNT growth and disclosed the growth and termination dynamics of CNTs in atomic detail under relevant conditions.

The stability of the DENSsolutions Nano-Reactor and the possibility to introduce stimuli, like gas and heating, allowed for the live observation at the atomic-scale:

In Situ TEM video made using the DENSsolutions Climate system, showing Fe-catalyzed multiwalled carbon nanotube growth. Temperature: 800 °C, pressure: 178.65 mbar, diluted H₂ + C₂H₄.

Using In Situ TEM gas and heating, the researchers were able to reveal the influence of pressure and temperature on the growth of CNTs. Previous studies were contradictory about the active state of the catalyst. Now with real-time observations of CNT growth at relevant conditions, researchers were able to reveal not only the active phase of the catalyst but also the rich structural dynamics of the catalyst during the course of CNT growth.

In this study, the Nano-Reactor, the core of the DENSsolutions Climate In Situ TEM system, was used as a carrier for the Fe₂O₃ sample (precursor material for CNT growth), which was then heated in a diluted hydrogen gas flow as a pre-treatment. The In Situ experiment was started at 150 °C and was followed by a step wise increase up to 800 °C in a gas mixture of H₂, C₂H₄ and He.

To accelerate catalyst research, the DENSsolutions Climate system can be delivered with a dedicated Gas Supply system that enables instant switching between gases and precise control over the gas mixture ratio.

Between 450 °C and 650 °C, the reduction of Fe₂O₃ to Fe₃O₄ was accompanied by a collapse of larger particles into smaller ones.

Did you know?

Our Impulse control software comes with a drag & drop profile builder that allows you to design & automate your experiment. It features a wide choice of parameters which enable you to create profiles that suit any sample and application.

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“Thanks to the development of the MEMS-based gas flow Nano-Reactor, we are allowed to perform In Situ experiments inside the chamber of the TEM at relevant conditions. Using the Climate system from DENSsolutions, we have recently carried out a detailed In Situ study on the growth behaviors of CNTs at realistic conditions. On the basis of the real-time observations, we are able to reveal the active structure of the working catalyst and its dynamic re-shaping during the course of CNT growth. Extended observations further reveal three different scenarios for the growth termination of CNTs at the atomic-scale. The presented work provides important insights into understanding the growth and termination mechanisms of CNTs and may serve as an experimental basis for rational design and controlled synthesis of CNTs.”

First and corresponding author – Dr. Xing Huang, ETH Zürich

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Converting CO2 into a valuable energy carrier using a model In2O3 catalyst

Converting CO₂ into a valuable energy carrier using a model In₂O₃ catalyst

by Olaf Weller | Aug 29, 2019 | Catalyst research, Climate

New discoveries made possible by In Situ TEM gas and heating

Original article by Athanasia Tsoukalou, Paula M. Abdala, Dragos Stoian, Xing Huang, Marc-Georg Willinger, Alexey Fedorov and Christoph R. Müller. Published in the Journal of the American Chemical Society on 19 July 2019.

Artist impression of methanol synthesis via CO₂ hydrogenation using In₂O₃ catalyst (copyright of the American Chemical Society)

The direct hydrogenation of CO₂ to methanol shows promise to be an important technique to reduce the amount of greenhouse gases in the atmosphere and thereby mitigate the negative effects of climate change while producing an important energy carrier. In his contribution to this article, Dr. Xing Huang has used In Situ TEM techniques to assess the limits of In₂O₃ catalytic performance in CO₂ hydrogenation.

In Situ TEM Climate Nano-Reactor study

This catalyst research article by ETH Zürich provides a good demonstration on how in situ TEM experiments can add value to the study of reaction mechanisms. First, the structural changes in the catalyst material were studied with traditional TEM techniques before and after the CO₂ reduction. By using the Climate in situ TEM system, the researchers were able to see the dynamics of the reaction, even at much lower pressures (0.8 vs. 20 Bar) than in the original operando experiment.

In Situ TEM video made using the DENSsolutions Climate system, at 300 °C, showing the co-existence of crystalline and amorphous phases as well as the transformation into film structures of the In₂O₃ catalyst.

Amorphization of the catalyst material inside the Nano-Reactor takes places in a few minutes, compared to hours in the capillary reactor. Re-activation of the catalyst is made easy by the Climate Gas Supply System since the hydrogen plus carbon-dioxide gas flow can be replaced by an oxygen flow at any moment.

“Direct observation using in situ TEM clearly reveals that the structure of In₂O₃ nanoparticles is highly dynamic under reaction conditions and irradiation of electron beam. The In₂O₃ is observed to transform into a dynamic structure in which both crystalline and amorphous phases coexist and continuously inter-convert.

This observation is in line with the operando XAS-XRD study revealing formation of the amorphous In⁰/In₂O₃ phase with time on stream. To sum up, combination of in situ TEM with other in situ/operando techniques enables to build a direct relationship between the structure and the catalytic performance of the In₂O₃ catalyst in CO₂ hydrogenation to methanol.”

Co-author – Dr. Xing Huang, ETH Zürich

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Real‐Time Imaging of Nanoscale Redox Reactions over Bimetallic Nanoparticles

by Olaf Weller | Jul 25, 2019 | Catalyst research, Climate

Shu Fen Tan, See Wee Chee, Zhaslan Baraissov, Hongmei Jin, Teck Leong Tan, Utkur Mirsaidov
Published in Advanced Functional Materials on 16 July 2019

Publication

Video 1: In situ TEM movie of a Pt–Ni rhombic dodecahedron NP undergoing oxidation reaction in 20% O₂ / 80% N₂ gaseous environment at 400 °C inside a gas cell.

Video 2: In situ TEM movie of a Pt–Ni rhombic dodecahedron NP undergoing reduction reaction in 5% H₂ / 95% N₂ gaseous environment at 400 °C inside a gas cell.

This recent article from National University of Singapore about the catalyst activity of bimetallic nanoparticles clearly demonstrates the versatility of the Climate G+ gas & heating system that was used for the in situ TEM experiments.

Figure S4. In situ TEM images showing the evolution of Pt–Ni NPs (A) before and (B) after the oxidation (20% O₂ / 80% N₂) reaction followed by (C) the reduction (5% H₂ / 95% N₂) reaction at 400 °C inside a gas cell. Here, t = 0 s marks the start of 20% O₂ / 80% N₂ flow. At t = 133.1 s, the gas mixture was switched to 5% H₂ / 95% N₂.

Fast switching between oxidizing and reducing gas conditions allows direct observation of the morphological changes in the catalyst material. The video’s above also demonstrate the UHR image stability at in situ conditions.

Combining the images and video’s from this experiment with SAED, EDS and Mass Spectroscopy results gives valuable information about the areas where the catalyst is most active.

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Gas Analyzer supporting ex situ Catalyst experiments

by Olaf Weller | Jun 17, 2019 | Catalyst research, Climate, Gas analyzer

Technical Research Engineer Marien Bremmer MSc with the gas analyzer (blue) in the background

Our solutions not only allow for highly controllable in situ experiments, they also allow for ex situ experiments that might save you valuable in situ time. With this ex situ experiment, we were able to prove the performance of the catalyst before moving in to the TEM.

The experiment

We used palladium nanoparticles for our catalyst These particles were dropcasted inside a Climate MEMS based Nano-Reactor. For the gas supply we used the Climate G+ system which allows for up to 3 gases to be mixed. We loaded the system with oxygen and methane as reactive gases and measured carbon monoxide and carbon dioxide as reaction products.

Figure 1. Sample temperature (top) and gas partial pressure (middle & bottom) measured as a function of time.

Catalyst performance

First we used the gas analyzer combined with our accurate temperature control to measure the catalyst performance. The supply of reactants was kept at a constant level (figure 1 – middle graph) while we used our Impulse software to automatically ramp up the temperature of the Nano-Reactor from 300 to 700 °C in 60 seconds (figure 1 – top graph). As a result we measured the level of reactant gases dropping and the level of reaction products rising (figure 1 – bottom graph). We see the levels stabilizing when the temperature is constant.

High activity phase shifting

Figure 2. Gas mixture composition into the Nano-Reactor (top), partial pressures of gases flowing out of the Nano-Reactor (middle) and dissipated power by the Nano-Reactor heater (bottom) as a function of time.

During the next experiment, we kept the palladium sample at a constant temperature while increasing the concentration of methane (CH₄) from 5% to 10% (figure 2 – top graph). At around t = 300 seconds you can clearly see fluctuations in the level of reaction products (figure 2 – middle graph). Here we observe the catalyst shifting in and out of a high activity phase that is reached at elevated temperatures. When passing a certain temperature range, this high activity phase can be demonstrated by oscillations in the partial pressure of the gas reaction products. Also oscillations in the power dissipated by the heater (figure 2 – bottom graph) indicates a change of activity at the sample.
At t = 500 seconds we use our Impulse software to drop the level of methane in steps of 0,5%. Measuring the CO₂level with our gas analyzer we can clearly see the influence of the first drop in concentration. The CO₂production rate starts to more unstable. By dropping the concentration with another 0,5%, the frequency of the fluctuations increases and, after the third drop of concentration, the catalyst starts to shift back to its normal activity phase, stabilizing the CO₂production after the fourth drop.

High time resolution

Figure 3. Detailed results for partial pressure (top) and power dissipation (bottom) measurements.

We zoomed in at areas A and B and plotted the results from the gas analyzer as well as those from the power dissipated by our 4-point probe temperature control system (see figure 3). This allows us to correlate the two measurements. We see that the reaction gases are in counter phase of each other and that their extremes are in line with the tops of the measured power. This shows not only a very high stability in temperature control but also a very high time resolution.

Conclusions

Thanks to our high accuracy gas analyzer and heating control and measurement, you are able to do ex situ experiments that can give you valuable data. This data can lead to new discoveries or can be used to prepare your in situ experiment better.

Marien Bremmer who conducted the experiment commented:
“Using the Climate G+ in combination with the Gas Analyzer allows you to characterize your catalyst sample ex situ, finding the best gas and temperature conditions for your reaction, and with this data to go to the TEM to finalize your research with real in situ images and spectroscopy.”

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Gas Analyzer Application Note

Technical Research Engineer Marien Bremmer MSc with the gas analyzer (blue) in the background

Our solutions not only allow for highly controllable in situ experiments, they also allow for ex situ experiments that might save you valuable in situ time. With this ex situ experiment, we were able to prove the performance of the catalyst before moving in to the TEM.