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

by | Jul 23, 2020 | Catalyst research, Climate

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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New article about the Stream Liquid Heating system

New article about the Stream Liquid Heating system

by | Jul 16, 2020 | Stream

Published in Journal of Materials Chemistry C

Original article by J. Tijn van Omme, Hanglong Wu, Hongyu Sun, Anne France Beker, Mathilde Lemang, Ronald G. Spruit, Sai P. Maddala, Alexander Rakowski, Heiner Friedrich, Joseph P. Patterson and H. Hugo Pérez Garza.
We are proud to announce a new publication in the Journal of Materials Chemistry C, in which we collaborated with our customers to observe the temperature dependent etching behavior of silica particles inside the TEM. The paper discusses the design of the Stream system and how it allows to control the solution conditions inside the Nano-Cell. For this experiment, we were particularly interested in the comparison between in situ LPEM data and ex situ data from more traditional methods.

According to the reviewers

“In this manuscript, the authors provided a new design of MEMS based liquid flow system with a unique on-chip microfluidic channel and a microheater, which enables the quick replenishment of fresh solution and uniform heating of the liquid solution.”

Connecting in situ to ex situ

One of the most fundamental challenges that any microscopist experiences is the question whether the phenomenon you observe inside the microscope is representative of what happens outside. You can see interesting things happening inside the microscope, but if there is no link to the outside world, the knowledge is not so useful.

To solve this challenge, we design our products so that the user has full control of all the relevant parameters during the in situ experiment. In the Stream system, this relates to controlling the solution conditions. Especially temperature and concentration. The sample should experience the same conditions inside and outside the TEM. To achieve this, the Stream system has a flow channel that enables rapid replenishment of the solution to ensure continuous supply of fresh reactant species. Meanwhile, the microheater accurately controls the temperature.

Temperature control

Temperature is a highly important variable to control. For this reason, all our product lines include the possibility to manipulate temperature. In liquid, the speed of chemical reactions is often dictated by the temperature. Moreover, completely different reaction pathways can be found at different temperatures. During an in situ experiment, the increase in temperature can be used to trigger a phenomenon. Many people rely on the electron beam to induce the dynamics. However, it’s normally desirable to decouple the stimulus from the imaging. In other words, the beam is used for imaging, while the MEMS device supplies the heat to start a reaction.
We chose to design the MEMS device to generate a uniform temperature throughout the Nano-Cell. In other words, no temperature gradients are present that could lead to complications. This also allows to accurately measure and control the temperature of the liquid and the sample.

Temperature dependent etching kinetics of silica nanoparticles in-flask vs. in situ LPTEM, showing good similarity between both situations. Time = reaction time.

Silica nanoparticle experiment

To validate the effect of the combined flow channel and microheater, we looked at the etching process of silica nanoparticles in NaOH. This process is quite sensitive to temperature; increasing the temperature substantially accelerates the reaction kinetics. In-flask, the etching time in NaOH with pH 13.8 is reduced from ~500 to ~10 minutes when increasing the temperature from 20 to 60 °C. This was found by measuring the transmittance of the solution. The TEM allows us to observe this process in real time, at the nanoscale. In the Stream, we aimed to reproduce the reaction conditions from the in-flask experiment. In the flask, the bulk liquid acts as a large reservoir of available reactant species, while in the Nano-Cell, the space is much more confined. A constant flow was used to refresh the solution to make sure that the silica particles are etched by fresh reactants continuously.
We found very good similarity between the results obtained in-flask and in situ. In the Nano-Cell, the etching time reduced from 360 to 4 minutes for the same temperature increase from 20 to 60 °C. So in both cases, the same order of magnitude increase in etching rate is observed, indicating that the Nano-Cell meticulously mimics the situation outside the microscope. This was the most important finding from the paper. The e-beam seems to slightly accelerate the etching process, but the low dose imaging procedure ensured that the effect of the e-beam was reduced to a minimum.

“The most exciting part of the Stream holder is that the control it offers over temperature and flow means that we have access to a completely new phase space to observe dynamic processes, this will undoubtedly result in the discovery of new nanoscale phenomena and lead to innovations in materials synthesis.”
Dr. Joseph P. Patterson
Department of Chemistry and the department of Materials Science and Engineering,
University of California, Irvine, USA

Collaboration with customers

DENSsolutions actively participates in the scientific community. We work closely together with our customers to make sure that our products help them to generate impact. This study is a good example where our expertise in the design and engineering of the in situ system was combined with the expertise at TU Eindhoven and UC Irvine.

In Eindhoven they were already very experienced working with the silica nanoparticle samples and with the ex situ etching behavior at different temperatures. So when the MEMS devices for Stream Liquid Heating were launched, they proposed to run this experiment inside the microscope. We anticipated that one of the key parameters to control during the experiment would be the e-beam, as it could interfere with the etching process. Fortunately the groups at Eindhoven and Irvine have a thorough background in imaging soft matter, so we managed to adhere to a low dose imaging protocol to successfully minimize the beam effect.

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