Stream 3: Operando Microscopy 10:00 - 12:00 Thursday, 8th July, 2021 Sessions Conference Session Session Organiser Hannah Nerl

Functional materials cannot be studied reliably when removing materials from their reaction environment. Recent operando studies aim to address this by correlating structure and function of materials under working conditions. Significant technical advances in instrumentation have led to the development and improvement of a range of operando techniques with great impact across scientific fields. These operando approaches have already been shown to allow for the visualization and analysis of materials during synthesis, degradation or function in well-defined environments. Aside from electron microscopy, relevant examples of emerging and improved operando techniques include X-ray microscopy, scanning probe microscopy, light microscopy and atomic force microscopy. This session will contain contributed and invited talks and posters that aim to highlight recent technical advances in operando approaches and the resulting science while studying a range of materials including 2D materials, nanoparticles and catalysts.

10:00 - 10:30

31 Seeing is believing: atomic-scale imaging of catalysts under reaction conditions

Dr. Irene Groot Leiden Institute of Chemistry, Leiden University, Leiden, Netherlands

Abstract Text

The atomic-scale structure of a catalyst under reaction conditions determines its activity, selectivity, and stability. Recently it has become clear that essential differences can exist between the behavior of catalysts under industrial conditions (high pressure and temperature) and the (ultra)high vacuum conditions of traditional laboratory experiments. Differences in structure, composition, reaction mechanism, activity, and selectivity have been observed. These observations made it clear that meaningful results can only be obtained at high pressures and temperatures. Therefore, the last years have seen a tremendous effort in designing new instruments and adapting existing ones to be able to investigate catalysts in situ under industrially relevant conditions. In this talk, I will give an overview of the in situ imaging techniques we use to study the structure of model catalysts under atmospheric pressures and elevated temperatures. We have developed setups that combine an ultrahigh vacuum environment for model catalyst preparation and characterization with a high-pressure flow reactor cell, integrated with either a scanning tunneling microscope or an atomic force microscope. With these setups we are able to perform atomic-scale investigations of well-defined model catalysts under industrial conditions. Additionally, we combine the structural information from scanning probe microscopy with mass spectrometry measurements. In this way, we can correlate structural changes of the catalyst due to the gas composition with its catalytic performance. Furthermore, we use other in situ imaging techniques such as transmission electron microscopy, surface X-ray diffraction, and optical microscopy, all combined with mass spectrometry. In addition, we make use of near-ambient-pressure X-ray photoelectron spectroscopy to obtain chemical information on the model catalysts during reaction. Scientific cases that I will discuss are hydrodesulfurization of S-containing organic molecules on (Co-promoted) MoS2 and graphene growth on liquid copper.

Keywords in situ measurements, scanning probe microscopy, optical microscopy, model catalyst, hydrodesulfurization, graphene growth, surface science, heterogeneous catalysis 10:30 - 10:42

247 In-situ hydration of calcium sulfate and the phase transformation pathways of bassanite to gypsum

Dr Martha Ilett1, Dr Helen Freeman1, Dr Johanna Galloway1, Dr Zabeada Aslam1, Dr Ian McPherson2, Dr Oscar Céspedes1, Dr Yi-Yeoun Kim 1, Professor Fiona Meldrum1, Professor Rik Drummond-Brydson1 1University of Leeds, Leeds, United Kingdom. 2University of Warwick, Warwick, United Kingdom

Abstract Text

The nucleation, growth, and phase transformation of calcium sulfate must be understood to improve the performance of construction materials, reduce scaling in industrial processes and better understand the natural environment. Recent studies suggest that classical nucleation theory does not apply to calcium sulfate systems where a multi-stage process occurs instead, involving several intermediate phases. Through a variety of in-situ experimental work, there is a growing body of evidence which shows an oriented-attachment-like crystallisation process. In this work we have used advanced in-situ electron microscopy to study the phase transformation in real time to understand the mechanistic processes involved. Both liquid cell (LC) and cryogenic (cryo) transmission electron microscopy (TEM) were used to monitor the phase transformation of bassanite (calcium sulfate hemihydrate CaSO4·0.5H2O) to gypsum (calcium sulfate dihydrate CaSO4·2H2O) during hydration in an aqueous, undersaturated calcium sulfate solution. An FEI Titan3 Themis G2 operating at 300 kV and equipped with a Gatan OneView CCD was used for in-situ electron microscopy studies, alongside Raman spectroscopy of both bulk and confined calcium sulfate systems. By collecting real-time images and videos using LCTEM we have been able to follow the aggregation and alignment of bassanite nanoparticles at the nanoscale. When coupled with Raman spectroscopy our results show there is a period where the two phases (bassanite and gypsum) co-exist. In addition, comparisons between LCTEM and cryo-TEM will allow us to evaluate any beam induced changes in LCTEM where it would be predicted these would be ‘frozen out’ in cryo-TEM. Early comparisons between the two techniques using a model nanoparticle system suggest beam induced dissolution observed in LCTEM and caused by changes in pH can be prevented using cryo-TEM. Ultimately, this work has allowed real time observation of the phase transformation of bassanite to gypsum alongside comparisons between LC and cryo TEM, which can advance the application of both techniques within materials science research.

Keywords

Calcium sulfate In situ TEM 10:47 - 10:59

205 Direct observation of the chemical dynamics of Pt nanoparticles in CO oxidation reaction by operando TEM

Dr. Milivoj Plodinec1,2, Dr. Hannah C. Nerl3, Dr. Thomas Lunkenbein2, Prof. Dr. Robert Schlögl2,4 1ETH Zurich, ScopeM, Zurich, Switzerland. 2Fritz-Haber Institute of the Max-Planck Society, Department of Inorganic Chemistry, Berlin, Germany. 3Humboldt-Universität zu Berlin, Department of Physics, Berlin, Germany. 4Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, Germany

Abstract Text

Introduction Up to date, regardless of our advances in synthesis and characterization methods, the empirical approach towards the discovery of new catalysts still prevails. Clearly, this is a very inefficient and time-consuming endeavor. The key point still missing is the knowledge of the structure-function relationships of catalysts. Without that, the often-claimed tailored design of catalysts partially remains in the domain of wishful thinking.1 Moreover, the capability to design novel and more efficient catalysts is limited by the lack of a detailed understanding of catalytic reactions on an atomic scale, particularly since catalyst dynamic active structures and surfaces under reaction conditions are mostly unknown.1,2 The only way to probe the catalyst working state is to use operando characterization techniques. Recently, many studies have shown that catalysts are metastable and dynamic systems, where the nature of the active state depends on the applied chemical potential and associated “chemical dynamics”, and the formation of transient active sites.1,3-5 Thus, the morphological and structural changes which relate to the induced surface and bulk transformations of the catalyst can be defined by the term chemical dynamics.1,3 Moreover, since the working catalysts are thought to be metastable, the active surfaces could be unstable at non-active conditions, which could lead to the detection of inactive structures by ex situ studies. Therefore, catalysts should be exclusively studied in their working state, under operando conditions. Methods and Materials For the experiment, we use a homebuilt gas feed and analysis system coupled with a quadrupole mass spectrometer (QMS, with a response time < 20s) and using commercially available gas flow TEM holder for the catalytic reactions inside the column of an aberration-corrected 300 kV FEI Titan (scanning) transmission electron (STEM).3 Using this system, we are able to correlate structural and morphological changes of catalysts with activity in the pressure between 20-1000 mbar, and in the temperature between 20-1000°C. The studied platinum catalysts were prepared either in situ by the thermal decomposition of tetraamineplatinum(II) nitrate 3,4 in synthetic air: 20%O2 in helium (He), at 400°C for 3h (Figure 1a) or ex situ by magnetron sputtering of Pt nanoparticles (NPs) directly on the top of SiNx membrane of microelectromechanical system (MEMS) chip (Figure 1b). The reaction conditions for CO oxidation were: temperature ramp 1°C/min and 2°C/min, pressure:700 mbar, flow rate: 20 mL/min, and gas feed: CO:O2:He = 1:5:19. Result and discussion In this study, we will show that using operando TEM approach we are able to visualize changes of Pt catalysts with different NPs shapes and sizes during different activity regimes. Moreover that we are able to directly correlate differences in conversion rates of CO and morphological changes of two differently prepared Pt catalysts (Figure 1). The TEM image of Pt NPs prepared by decomposition shows a broad size and shape distribution (Figure 1a and c) compared to NPs prepared by the sputtering method which shows a very narrow size distribution and spherical shape (Figure 1b and d). Ind addition, the sputter catalyst shows higher conversion, however the ignition point it is almost the same temperature, and appears to be independent of the Pt NPs shape and size (Figure 1c-1d). In addition, we used operando TEM to correlate real-time structural and morphological changes of the Pt catalyst with activity (Figure 2). Catalytic cycling inside the TEM and simultaneous recording of TEM images, selected area electron diffraction patterns (SAED), and catalytic conversion data show that chemical dynamics have different origins as well as impacts on activity. It can lead to morphological transformations and/or structural dynamics. Morphological transformations can be described as the formation of equilibrated surface facets which are detrimental to the catalytic activity (Figure 2b-c). These transformations are dominantly induced by gradients of the partial pressures. Structural dynamic is a result of frustrated phase transitions and occurs in bulk when the gradient of chemical potential is almost constant (Figure 2d). They are initiated by reactant diffusion through the Pt nanoparticles. This dynamic process was found to be beneficial for catalytic performance.

Figure 1. Effect of different morphologies and Pt particle size on CO oxidation rates studied by operando TEM. a) TEM image of Pt NPs obtained by thermal decomposition during CO oxidation at 500 °C. b) HAADF STEM image of Pt NPs prepared by magnetron sputtering during CO oxidation at 400 °C. Particle size distributions of the c) Pt NPs prepared by decomposition and CO conversation rate during temperature ramp, d) Pt NPs prepared by magnetron sputtering and CO conversion rate during 2°C/min. Reaction conditions in c) and d) are: pressure, 700 mbar; flow rate, 20 µL/min, gas feed, CO:O2:He = 1:5:19, temperature ramp 1 and 2°C/min, respectively. Figure 2. Morphological and structural changes of Pt NPs during catalytic activity in CO oxidation. TEM imaging a) during the 1st cycle at 362°C and b) after the 3rd cycle at 225°C. c) Increase of the ignition temperature during catalytic cycling. d) Evolution of radial profiles extracted from SAED patterns series acquired during catalyst cycling. Pristine – Pt NPs at 400 °C in synthetic air: 20%O2 in He, flow 20 μL/min, pressure 720 mbar. Reaction conditions: temperature ramp,1 °C/min; pressure, 700 mbar; flow rate, 20 μL/min, gas feed: CO:O2:He = 1:5.

Keywords

Operando electron microscopy, Heterogenous catalysis, Chemical dynamics, CO oxidation, Pt nanoparticles

References

1. Schlogl, R., Angewandte Chemie-International Edition, 2015. 54(11), 3465-3520. 2. Kalz, K.F., et al., ChemCatChem, 2017. 9(1), 17-29. 3. Plodinec, M., et al., Microscopy and Microanalysis, 2020. 26(2), 220-228. 4. Plodinec, M., et al.,. ACS Catalysis, 2020. 10(5), 3183-3193. 5. Vendelbo, S.B., et al., Nature Materials, 2014, 13, 884-890.

10:59 - 11:11

252 Gold Oxide Formation on TiO2/Au(111) Model Catalysts

Sabine Wenzel, Irene Groot Leiden Institute of Chemistry, Leiden, Netherlands

Abstract Text

Hydrogen produced from methanol has to be cleaned from traces of CO for its use in fuel cells [1]. Conventional CO oxidation catalysts such as platinum and palladium oxidize hydrogen as well and are thus not suitable. Alternative gold-based catalysts have been shown to selectively oxidize CO at low temperatures [2]. There is ample evidence for strong interactions between gold and typically used supports such as TiO2 [3]. However, the exact oxidation state of the active phase of gold remains under debate [4,5]. Additionally, there is evidence that water plays a role in the activity and might make the oxide support unnecessary [6]. Our set-up [7] allows for the controlled preparation and characterization of model catalyst surfaces in ultra- high vacuum combined with scanning tunneling microscopy at atmospheric pressures and elevated temperatures. A TiO2/Au(111) model catalyst was prepared via physical vapor deposition and exposed to CO oxidation reaction conditions. We present evidence for the formation of a surface gold oxide in this environment as can be seen in Figure 1. We will discuss the role that the titania nanoparticles, contaminants on the gold substrate, and the water background in the gas mixture play in the oxidation of the gold substrate. Our findings suggest that transfer of atomic oxygen from the titania nanoparticles to the gold substrate does not occur.

Fig. 1: a) 120 nm x 120 nm image of the as-prepared TiO2/Au(111) model catalyst. b) 120 nm x 120 nm image of the same surface after exposure to 1 bar of 4 O2 + 1 CO for one hour showing the surface gold oxide. c) 10 nm x 10 nm zoom of the marked region in b) showing the squared unit cell of the surface gold oxide.

Keywords

References

[1] Dhar et al., J. Electrochem. Soc. 1987, 134, 12, 3021. [2] Haruta, The Chemical Record 2003, 3, 75–87. [3] Palomina et al., ACS Sustainable Chem. Eng. 2017, 5, 10783. [4] Min et al., Chem. Rev. 2007, 107, 2709. [5] Klyushin et al., ACS Catal. 2016 6 , 3372. [6] Kettemann et al., ACS Catal. 2017, 7, 8247. [7] Herbschleb et al., Rev. Sci. Instrum. 2014, 85, 083703. 11:16 - 11:46

129 Dynamics of nanostructure surfaces and ways to approach them

Professor Thomas Willum Hansen, Pei Liu, William Bang Lomholdt, Matthew Helmi Leth Larsen, Cuauhtémoc Núñez Valencia, Professor Jakob Schiøtz Technical University of Denmark, Kgs. Lyngby, Denmark

Abstract Text

In situ electron microscopy is a unique method for imaging materials in their operating state, monitoring growth phenomena at the atomic scale and observing the dynamic changes of particularly surfaces [1]. Data collected under operating conditions tells a lot more about the performance and stability of a material than that acquired under high vacuum conditions. In situ electron microscopy investigations have been performed through modification to the microscope column by inserting pressure-limiting apertures. Over the last decade, the use of electron transparent membranes has been more mainstream. Whereas the former requires microscope modifications, it sets no restrictions on the sample holders. The latter makes use of a dedicated holder that can be moved between different microscope installations. However, the electron beam can have a significant influence on the structure of materials, particularly under non-vacuum conditions. With recent development of camera technology, the use of in situ imaging has become even more interesting. The latest generation of cameras provide high sensitivity and high frame rates making it possible to detect dynamic phenomena such as particle migration and mobility of atomic columns with a higher temporal resolution than previously while keeping the electron dose rate at a minimum. Even at low electron dose rates, we can detect individual atomic columns and track them in time. The recent developments facilitate the analysis of nanoscale dynamics of materials. Particularly in catalysis, the surface structure and how the surface configuration changes with changing surroundings have attracted tremendous attention and has been the topic of several investigations using a plethora of experimental approaches [2, 3]. We have studied mainly the surfaces of gold nanoparticles supported on cerium dioxide under condition relevant for CO oxidation. Using environmental transmission electron microscopy, gold nanoparticles were subjected to hydrogen, oxygen and carbon monoxide containing atmospheres at elevated temperatures. Several types of structural changes were observed and an attempt at categorizing these events was made [4]. In presence of a hydrogen gas a concerted motion of surface atoms was observed. Entire layers at the {100} facet moves in a concerted fashion to a different location on the nanoparticle and reappears later, as seen on Figure 1. The process is observed continuously during image recording. As the temperature is increased, the process occurs faster. Under similar conditions, the topmost layer of the {111} facet was observed to glide parallel to the surface, still in a concerted fashion, as seen on Figure 2. In this work, we will show our observation of different changes observed in gold nanoparticle samples and our attempt at understanding the underlying dynamic processes and the effect of the electron beam. We will show recent results and discuss the progress of our work and future possibilities.

Keywords

TEM, Nanoparticles, ETEM, Dynamics, Machine learning