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The need for studying catalyst under realistic conditions is emphasized both by academic and industrial research. Acquiring highly resolved local information from materials under realistic environments by means of Transmission Electron Microscopy (TEM) has been found to be essential in connecting microscopic and macroscopic properties of materials, e.g. relating catalytic performance with crystal structure and morphology.This study presents extensive characterization of NiGa and CuNi alloys during catalyst formation, alcohol synthesis, and accelerated aging experiments. The characterization platform consists of three complimentary in situ techniques: (1) Activity measurements based on a reactor connected to a gas chromatograph (GC), (2) In situ x-ray diffractometer (XRD) measurements based on a reactor cell connected to a mass spectrometer (MS), and (3) environmental TEM (ETEM) that allows for observation in a gaseous environment. By using heating holders, dynamic information about catalysts in their working state can be gained using a variety of TEM techniques.The presented platform successfully illustrates the capability of correlating the dynamic changes in structural phase and particle size distribution, measured both macroscopically (XRD) and microscopically (ETEM), with the catalytic activity.

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500{degree sign}C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.

Transmission electron microscopy (TEM) is extensively used in catalysis research. Recent developments in aberration correction allows imaging surface structures with unprecedented resolution. Using these correctors in conjunction with environmental TEM (ETEM), where imaging of materials can be done under gas exposure, dynamic phenomena such as sintering and growth can be observed with sub-Ångstrøm resolution. Metal nanoparticles contain the active sites in heterogeneous catalysts, which are important for many industrial applications including the production of clean fuels, chemicals and pharmaceuticals, and the cleanup of exhaust from automobiles and stationary power plants. Sintering, or thermal deactivation, is an important mechanism for the loss of catalyst activity. In order to initiate a systematic study of the dynamics and sintering of nanoparticles, various catalytic systems have been studied in atmospheres relevant for their operation. By studying growth patterns, we found that changes in the atmospheres changes heavily effects not only the rate of sintering but also the mechanism through which it occurs. In many cases, anomalously large particles where observed indicating that particle sintering is not solely governed by the mechanisms previously proposed. These results are divided into the different phases of the catalyst lifetime.

The Solid Oxide Fuel Cell (SOFC) is a promising part of future energy approaches due to a relatively high energy conversion efficiency and low environmental pollution. SOFCs are typically composed of ceramic materials which are highly complex at the nanoscale. TEM is routinely applied ex situ for studying these nanoscale structures, but only few SOFC studies have applied in situ TEM to observe the ceramic nanostructures in a reactive gas environment at elevated temperatures.The present contribution focuses on the reduction of an SOFC anode which is a necessary process to form the catalytically active Ni surface before operating the fuel cells. The reduction process was followed in the TEM while exposing a NiO/YSZ (YSZ = Y2O3-stabilized ZrO2) model anode to H2 at T = 250-1000⁰C. Pure NiO was used in reference experiments. Previous studies have shown that the reduction of pure NiO is a relatively rapid autocatalytic process. On the contrary, the reduction of NiO/YSZ is significantly slower, which indicates that the presence of YSZ inhibits the reduction of NiO. This study aims to obtain fundamental insight into this reduction mechanism and to explain the inhibitive influence of YSZ.A Titan E-Cell 80-300ST TEM was used for the in situ work in combination with the chip-based Aduro heating holder from Protochips. Since the chip-based heating holder does not allow internal temperature measurements, and the since the temperature of the chips are only calibrated in vacuum, part of the presentation will focus on temperature calibration.

Digging into the world of photocatalysts by means of electron microscopy gives rise to new challenges. In order to study photocatalysts under working conditions, a holder capable of exposing a sample to visible light in situ during gas exposure in the ETEM has been developed at DTU Cen1. One of the major challenges when studying the effects of visible light in the electron microscope is to differentiate between the effect of visible light and that of high-energy electrons. New protocols for acquisition and robust interpretation of in situ data on photocatalysts are needed to minimize the effect of the electron beam without compromising image quality and information to exploit the full potential of in situ light stimuli.A major parameter degrading the image quality in ETEM is the gas-electron interactions. Even though high-resolution ETEMs have been around for more than a decade little has been done in trying to understand the effects of gas-electron interaction on the image formation2-4.The addition of aberration correction and monochromation to ETEM has improved the point resolution and reduced image delocalization allowing for a more direct interpretation of surfaces and interfaces. Recently Yoshida et al. reported transmission electron microscopy imaging of CO molecules adsorbed on Au nanoparticles5. However, when gas is present in the microscope column the electron wave is modified due to gas-electron interactions6, 7. Several factors have to be taken into account when combining high-energy electrons with a gaseous environment. The fast electrons are scattered both elastically and inelastically by the gas molecules resulting in distortions of the electron wave both above and below the sample, thus altering the incoming and the exit wave carrying the image information. Furthermore, the interaction between fast electrons and gas molecules leads to ionization of the gas molecules. A full understanding of both the interaction of fast electrons with gas molecules and the effect of gas on high‐resolution imaging is therefore necessary to take the next step towards quantitative ETEM.Here, the case study of in situ Cu2O photocorrosion is presented together with our ongoing work involving systematic measured images, diffraction patterns and energy-loss spectra acquired in gases in order to elucidate the influence of gas-electron interaction under imaging conditions suitable for in situ light experiments.

Semiconductor nanowires have been studied using electron microscopy since the early days of nanowire growth, e.g. [1]. A common approach for analysing nanowires using transmission electron microscopy (TEM) involves removing them from their substrate and subsequently transferring them onto carbon films. This sample preparation method is fast and usually results in little structural change in the nanowires [2]. However, it does not provide information about the interface between the nanowires and the substrate, who’s physical and electrical properties are important for many modern applications of nanowires. In particular, strain and crystallographic defects can have a major influence on the electronic structure of the material. In improved method for the characterization of such interfaces would be valuable for optimizing and understanding the transport properties of devices based on nanowires. Here, we systematically investigate the interface between a nanowire and its substrate using three complementary methods for assessing strain. Results obtained using high resolution TEM for geometric phase analysis (GPA), convergent beam elecron diffraction (CBED) and nanobeam electron diffraction (NBED) are compared and contrasted. GPA measurements were acquired at 300kV in an FEI Titan 89-300 while the two diffraction methods were applied in the same microscope at 120kV. The GPA analysis software developed by C.T. Koch and V.B. Özdöl was used [3]. For samples other than nanowires, previous comparisons of GPA with CBED and NBED [4,5] have shown a high degree of consistency. Strain has previously only been measured in nanowires removed from their substrate [6], or only using GPA [7]. The sample used for the present investigation was an InP nanowire grown on a Si substrate using metal organic vapor phase deposition (MOCVD). Lattice missmatch between Si and InP is 8%. The nanowire had a diameter of approximately 100 nm in the interface area. TEM samples were prepared using a tripod polishing technique, with Ar ion milling used as the final thinning step. The resulting sample was clean and virtually free from defects from sample preparation. Measurements using all three techniques were obtained from the same well defined region of the specimen. Energy dispersive X-ray spectroscopy (EDS) maps were also acquired from the same area. Preliminary results acquired using GPA are shown in Figure 1. Whereas the base of the wire show some strain, little strain is observed in the substrate. The influence of defects, interfacial layers and compositional variations on the GPA will be discussed.

Novel Ni5Ga3 and Pd2Ga catalysts for CO2 hydrogenation to methanol are prepared by impregnation of aqueous Ni-Ga or Pd-Ga solutions of metal nitrates into high surface area SiO2, followed by drying, calcinations and reduction of the precursor in a H2 flow. Steady state experiments are performed in a reactor at atmospheric pressure and stoichiometric CO2/H2 mixture, while reaction products are analyzed by gas chromatography. The results are compared to the highly optimized Cu/ZnO/Al2O3. The activity and selectivity of the novel catalysts is close to that of Cu/ZnO/Al2O3 and the equilibrium conversion to CH3OH is found to be higher. XRD and XRF are used to investigate the phase and composition of the supported catalysts at the 5 stages of testing, i.e. after drying, calcination, reduction, CO2 hydrogenation, rapid ageing. SEM and TEM images of the exact same locations are acquired after each of the 5 stages to monitor particle formation and investigate particle size and distribution. In the same way, model systems consisting of Ni5Ga3 and Pd2Ga nanoparticles supported directly on a silica membrane of an Au TEM grid are also characterized by SEM and TEM after each of the 5 stages. Nanoparticles formation is observed after calcination. Size distribution analysis reveals that the Pd2Ga nanoparticles have a diameter of 5-10 nm which does not change after reduction, methanol synthesis and rapid ageing. Furthermore, in situ ETEM is used to monitor the development of the materials system during synthesis and reaction.

The synthesis of transportation fuels from sustainable resources requires new and better production paths. Our approach is to use biogas to synthesize alcohols, such as methanol or higher alcohols for fuel and other chemical products. For the production of methanol a reduction of processing temperature and pressure to lower the process cost and make the product more competitive is desired. Higher alcohols are in general favorable over methanol due to their high energy density and ease of use in current internal combustion engines. However, better catalysts for this reaction are needed to increase the poor yield of higher alcohols in present production routes [1].The Catalysis for Sustainable Energy Project (CASE) at the Technical University of Denmark aims at discovering new and improved catalysts based on density functional theory (DFT) and testing the chemical reactivity of the most promising candidates experimentally. Transmission electron microscopy (TEM) is used for microstructural characterization and provides feedback for both theory and synthesis.We have studied the catalysts close to their working conditions in an environmental transmission electron microscope (ETEM) equipped with a differential pumping system to confine a controlled gas flow around the specimen, allowing observation in a gaseous environment. Using heating holders, dynamic information about catalysts in their working state can be gained using a variety of TEM techniques in situ. [2,3]. Here, we present recent ETEM studies of CuNi and NiGa catalysts for alcohol synthesis using High-Resolution TEM (HRTEM), energy electron-loss spectroscopy (EELS), Energy-Dispersive X-ray Spectroscopy (EDX). Complementary observations have been done using in-situ X-Ray Diffraction (XRD). We focus on structural changes during the catalysts synthesis and activation in a reducing atmosphere at elevated temperature. Changes in phase and particle size distribution with respect to the temperature can be directly observed and correlated to catalytic activity and integral phase information from the in-situ XRD.

SOFCs/SOECs are typically composed of ceramic materials, which are highly complex at the nano-scale. Scanning and transmission electron microscopy (SEM and TEM) are routinely applied for studying these nano-scaled structures post mortem, but only few SOFC/SOEC studies have applied environmental TEM (ETEM). ETEM offer the possibility to record image series (movies) of the ceramic nanostructures with atomic scale resolution during exposure to a reactive gas environment at elevated temperatures. The present contribution focuses on the typical reduction preparation step for the state-of-the-art Ni/YSZ (YSZ = Y2O3-stabilized ZrO2) based anodes for SOFC and cathodes for SOEC. Specifically, the reduction of nickel oxide to form the catalytically active nickel surface is monitored directly at the nano- and atomic scale by using an ETEM. The reduction process was followed while exposing NiO/YSZ and pure NiO to H2 at temperatures from room temperature to ca. 800°C. The NiO/YSZ was prepared by crushing down a tape casted and sintered model anode/cathode into a fine powder. Previous studies based on averaging techniques have shown that the reduction of pure NiO is a relatively rapid process, while the reduction of NiO/YSZ is slower, which indicates that the presence of YSZ inhibits the reduction of NiO. In the presents in situ experiments the temperature dependent reduction profile are found similar for the both nano-scaled NiO and NiO/YSZ sample. The apparent inhibitive effect of YSZ on NiO reduction is therefore not caused by a direct interaction between NiO and YSZ, but is an indirect effect depending on the NiO being integrated in a macroscopic network of NiO/YSZ. A Titan E-Cell 80-300ST TEM was used for the in situ work in combination with the chip-based heating holder from Protochips which facilitated rapid temperature ramping for example from room temperature to 800°C in only 1 s. The ETEM results are compared to complementary averaging techniques such as thermo-gravimetric analysis (TGA) and X-ray diffraction analysis (XRD). The figure presents a TEM image series of NiO during exposure to 2 mbar H2 and constant temperature ramping rate of 1°C/min. The NiO observed in the first image at 320°C is dense. From the lower left corner a front of porous Ni is progressing until full reduction at 340°C.

Photocatalysts are of fundamental interest for sustainable energy research [1]. By means of transmission electron microscopy (TEM) it is possible to obtain insight into the structure, composition and reactivity of photocatalysts. Such insight can be used for their further optimization [2]. We have constructed a specimen holder capable of shining light onto samples inside the TEM. The holder contains a laser diode and an optical system that guides light onto a sample with maximum power transmission. The source can be changed and tuned, in principle spanning the whole visible and UV spectrum. The device can be used inside an environmental TEM (ETEM) allowing specimens to be analyzed during exposure to a controlled gas atmosphere and illumination. The holder is presently being used to study a variety of photoreactive materials and structures, including photocatalysts, photonic devices and solar cells. For example, electron holography is being used to study p-n junctions both in the presence and in the absence of light in order to assess electron beam induced charging and discharging effects during laser light exposure [3]. Here, we present results from ETEM studies of light-induced phenomena that include metal nanoparticle photodeposition, light-driven particle discharging and photodegradation. We concentrate on phase transitions of Cu2O nanocubes under visible light exposure in the presence of water vapor, which we study in situ. Cu2O is an active photocatalyst for water splitting under visible light irradiation, but it undergoes photodegradation in an aqueous environment [4]. [1] Herrmann, J. M., Top. Catal. 2005, 34, (1-4), 49-65. [2] Tsujimoto, M., S. Moriguchi, et al., J. Electron. Microsc. (Tokyo) 1999, 48, (4), 361-366. [3] Shindo, D., K. Takahashi, et al., J Electron Microsc. (Tokyo) 2009, 58, (4), 245-249. [4] Nagasubramanian, G., A. S. Gioda, J. Electrochem. Soc. 1981, 128, (10), 2158-2164.

The mechanisms controlling the growth of an oxide film during oxidation are subject to controversies at intermediate length scales (20-1000 nm) [1]. Relating rate-controlling mechanisms and resulting structural changes, which is essential to the understanding of oxidation processes, has proved challenging under these conditions.Here, nickel particles are oxidized under 3.2 mbar of O2 inside an environmental TEM (ETEM) equipped with a post-column filter [2]. Images, diffraction patterns and core-loss electron energy-loss spectra are acquired to monitor the structural and chemical evolution of Ni during oxidation, whilst increasing the temperature up to 600 °C.Nucleation of NiO on Ni is observed to occur rapidly at room temperature before the introduction of O2 in the environmental cell (in the vacuum of the microscope). It involves the formation of randomly orientated oxide domains of a few nanometres in size. These domains impinge and cover the particles surface. As the temperature increases under O2, the NiO film grows and creates irregular structures composed of many crystallites. The reaction kinetics are inferred by EELS using different techniques analyzing changes in shapes of the Ni L2,3 white lines [3]. The results indicate that the oxidation process is diffusion-controlled, similarly to results from the literature that were obtained at larger oxide thicknesses [1]. Pores are observed to form at the Ni/NiO interfaces, resulting in the loss of metal/oxide contact (Fig. 1). These observations illustrate that the outward diffusion of Ni2+ ions through NiO is the dominant mass transport mechanism under these conditions (in opposition to O2/O2- transport). Imagesalso indicate that the NiO film might rupture in some regions, a process that should enable some inward diffusion of O2 and therefore inward growth of NiO. An activation energy for Ni oxidation comparable to the ones found in the literature is determined from our EELS data.By using ETEM, we are able to relate the structural and chemical changes occurring at the nanoscale during the oxidation of Ni particles with O2 at high temperature, providing new insights into oxidation/corrosion processes.

The development of transportation fuels from sustainable resources requires new and better production paths. One approach involves the use of biogas to synthesize alcohols, such as methanol or higher alcohols for fuel. For the production of methanol a reduction of processing temperature and pressure to lower the process cost and make the product more competitive is desired. Higher alcohols are in general favourable over methanol due to their high energy density and ease of use in current internal combustion engines. However, the yield of higher alcohols in present production routes is poor prompting for the development of better catalysts [1]. The Catalysis for Sustainable Energy Project (CASE) at the Technical University of Denmark aims at discovering new and improved catalysts based on density functional theory and on testing the chemical reactivity of the most promising candidates experimentally. Transmission electron microscopy (TEM) is used for microstructural characterization and provides feedback for both theory and synthesis. TEM is a powerful tool for characterizing of catalysts. However, conventional TEM does not provide dynamic information about catalysts in their working state. We have recently installed an environmental transmission electron microscope (ETEM) equipped with a differential pumping system to confine a controlled flow of gas around the specimen, allowing observation in a gaseous environment (FEI Titan E-cell, monochromated, objective lens aberration corrector, Gatan imaging filter). In combination with a heating holder, this microscope allows catalysts to be studied using a variety of TEM techniques in situ in a reactive environment approaching the working conditions of the catalysts [2,3]. Here, we present recent ETEM studies of newly synthesized catalysts for alcohol synthesis. Using High-Resolution TEM, electron diffraction, energy electron-loss spectroscopy (EELS), Energy Dispersive X-ray spectroscopy (EDX) and High-Angle Annular Dark Field (HAADF) investigations we have studied the structural changes of these catalysts. Complementary obeservations have been done using Extended X-ray Absorption Fine Structure (EXAFS). The experimental findings have been correlated with the observed changes in the chemical activity of the catalysts. Systematic studies involving variation in temperature, gas pressure and composition were performed and related to structural changes in the specimen. Representative TEM images of a CuSn based catalyst for synthesis of higher alcohols are shown in Figure 1. The CuSn particles are observed to sinter during the reduction leading to a decreased activity of the catalyst. Figure 2 shows the distribution of Co and Mo in a Co/MoS2 catalyst during treatment in 1.5 mbar H2 at 600K. The Co phase is seen to segregate from MoS2. The EDX scans in Figure 2 b) and c) indicate this segregation.

The increasing use of environmental transmission electron microscopy (ETEM) in materials science provides exciting new possibilities for investigating chemical reactions and understanding both the interaction of fast electrons with gas molecules and the effect of the presence of gas on high‐resolution imaging. A gaseous atmosphere in the pole‐piece gap of the objective lens of the microscope alters both the incoming electron wave prior to interaction with the sample and the outgoing wave below the sample. Whereas conventional TEM samples are usually thin (below 10‐20 nm), the gas in the environmental cell fills the entire gap between the pole pieces and is thus not spatially localized. By using an FEI Titan ETEM equipped with a monochromator and an aberration corrector on the objective lens, we have investigated the effects on imaging and spectroscopy caused by the presence of the gas. State‐of‐the‐art aberration corrected TEMs provide electron micrographs with high spatial resolution. The apparent interpretability of such images encourages microscopists to analyze data more quantitatively. Such an analysis requires a detailed knowledge of the entire path and propagation of the electrons along the microscope column. The effects of gas on the electron wave in the objective lens are not well understood and needs further attention. Imaging samples with a simple geometry, such as gold particles on a flat graphene substrate and analyzing the variations in contrast, provides a means for understanding the issues involved with imaging in the presence of a gas. Furthermore, electron energy‐loss spectroscopy can tell us about the local chemical environment in the vicinity of the sample. Using a differentially pumped FEI Titan 80‐300 Titan ETEM, high‐resolution TEM micrographs and electron energy‐loss spectra were acquired from Au/graphene samples in vacuum and in a hydrogen atmosphere at pressures up to 700Pa. The gases were introduced into the environmental cell using digitally controlled mass flow controllers, providing accurate and stable control of the pressure in the cell. The loss of beam intensity when traversing the pole piece gap was measured by recording the signal outside the sample region on the pre‐GIF CCD camera (see Figure 1 left). The effects on high resolution imaging were investigated by imaging gold nanoparticles below 5nm in diameter (see Figure 1 middle and right). We will present results from imaging in various elemental as well as di‐molecular gases and their effect on imaging and spectroscopy in the environmental transmission electron microscope.

Energy-filtered transmission electron microscopy images are acquired during the reduction of a NiO-YSZ composite in H-2 up to 600 degrees C. Temperature-resolved quantitative information about both chemistry and structure is extracted with nm spatial resolution from the data, paving the way for the development of detailed reduction models.