Article pubs.acs.org/JACS
In Situ Multimodal 3D Chemical Imaging of a Hierarchically Structured Core@Shell Catalyst Thomas L. Sheppard,†,‡ Stephen W. T. Price,§ Federico Benzi,‡ Sina Baier,‡ Michael Klumpp,∥ Roland Dittmeyer,†,⊥ Wilhelm Schwieger,∥ and Jan-Dierk Grunwaldt*,†,‡ †
Institute of Catalysis Research and Technology and ⊥Institute for Micro Process Engineering, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Engesserstraße 20, 76131 Karlsruhe, Germany § Science Division, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxon OX11 0DE, United Kingdom ∥ Institute of Chemical Reaction Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: A Cu/ZnO/Al2O3@ZSM-5 core@shell catalyst active for one-step conversion of synthesis gas to dimethyl ether (DME) was imaged simultaneously and in situ using synchrotron-based micro X-ray fluorescence (μ-XRF), X-ray diffraction (μ-XRD), and scanning transmission X-ray microscopy (STXM) computed tomography (CT) with micrometer spatial resolution. An identical sample volume was imaged stepwise, first under oxidizing and reducing atmospheres (imitating calcination and activation processes), and then under model reaction conditions for DME synthesis (H2:CO:CO2 ratio of 16:8:1, up to 250 °C). The multimodal imaging methods offered insights into the active metal structure and speciation within the catalyst, and allowed imaging of both the catalyst core and zeolite shell in a single acquisition. Dispersion of nanosized Cu species was observed in the catalyst core during reduction, with formation of a metastable Cu+ phase at the core−shell interface. Under DME reaction conditions at 1 bar, the coexistence of Cu0 in the active catalyst core together with partially oxidized Cu species was unraveled. The zeolite shell and core−shell interface remained stable under all conditions, preserving the bifunctional nature of the catalyst. These observations are inaccessible using standard bulk techniques like X-ray absorption spectroscopy (XAS) and XRD, demonstrating the potential of multimodal in situ X-ray CT for characterization of hierarchically designed materials, which stand to benefit tremendously from such 3D spatially resolved measurements.
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INTRODUCTION Core@shell composites in heterogeneous catalysis are a type of hierarchically structured material typically composed of two or more components, each of which may display individual catalytic behavior.1,2 With careful selection and tuning of each component, it is possible to generate bifunctional catalysts, which can perform multiple reactions simultaneously. One example is Cu/ZnO/Al2O3@ZSM-5, which exhibits bifunctionality through the Cu/ZnO/Al2O3 core active for methanol synthesis from syngas (CO/CO2/H2 mixtures), and the H-ZSM5 zeolite shell which can form dimethyl ether (DME) through methanol dehydration.2−4 DME is a valuable chemical product, considered as both a future sustainable energy source and a potential diesel substitute.5 The syngas feedstock for DME production can additionally be sourced from biomass, contributing to a CO2-neutral economy.6 A key advantage of bifunctional catalysts is the ability to synergize multiple catalytic functions in a single operation,2,4 improving process efficiency. For core@shell catalysts in particular, good performance can © 2017 American Chemical Society
depend on the stability and integrity of the hierarchical structure. This in turn may be influenced by physical, chemical, or mechanical changes which occur during catalyst operation, particularly for industrial processes with harsh operating conditions. To fully understand the relationship between catalyst structure and activity, it is therefore critical to analyze catalysts under in situ or operando conditions relevant to their real-life applications, including gas environment and temperature.7−12 In addition, measurements need to be performed with sufficient spatial resolution and on appropriate length scales in order to characterize the complex hierarchical structures present.13−15 In recent years, synchrotron-based X-ray imaging has developed as a flexible and powerful toolset for structural analysis of complex materials including catalysts with high spatial resolution.13,14 Synchrotron light sources can produce micro- or nanofocused hard X-rays with high brilliance and tunable energy. Received: March 3, 2017 Published: May 12, 2017 7855
DOI: 10.1021/jacs.7b02177 J. Am. Chem. Soc. 2017, 139, 7855−7863
Article
Journal of the American Chemical Society
in the literature. Previous in situ studies have mainly focused on standard XRD and XAS,10,39−43 but such bulk characterization methods do not offer the same spatial resolution nor sensitivity to dilute material phases as X-ray CT. Conversely the core has also been investigated using computational modeling,44−46 and advanced spatially resolved methods such as in situ transmission electron microscopy (TEM), although this is limited to thin slices and low gas pressures.11,15,47,48 With the added complexity of a microporous zeolite shell encapsulating the methanol synthesis core, the need to image noninvasively is essential for performing an accurate structural analysis. This is further evident from previous 2D TEM and X-ray ptychography studies on the same material which involved invasive sample preparation.15 The main goal in this study was to monitor in situ the behavior and speciation of active metal sites in the core, the influence of the shell on the core, and the structural integrity of the core−shell interface. This is especially relevant for bifunctional catalysts, whose function depends on the interplay and stability of the individual components. Furthermore, the aim was to demonstrate the power and applicability of in situ X-ray tomography for future studies of complex systems in the field of catalysis and solid-state chemistry.
By further combining hard X-ray imaging with computed tomography (CT), 3D sample volumes can be rendered noninvasively, revealing features which may be otherwise undetectable by bulk methods such as X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), or by 2D imaging.16,17 Noninvasive imaging is highly desirable in the context of sensing, functional materials, and especially catalysis, allowing intact samples to be investigated in their normal physical state without cutting or sectioning. Various tomographic acquisition modes are possible depending on experimental needs, including XRD-CT,8,18 X-ray fluorescence (XRFCT),17,19 X-ray absorption near edge structure (XANESCT),19−21 or pair distribution function (PDF-CT)18,22 among others. Detailed studies often rely on combining acquisition modes to gain complementary information;17,19,23−26 however the practicality of acquiring such data should be carefully considered. For example, due to the specialized nature of synchrotron beamlines, it is not always possible to record complementary data at the same time or even in the same facility. As a result, it can be problematic to measure with multiple techniques on the same exact sample volume; thus differences in structure and composition between particles can have an impact on the results obtained.25,26 Ideally, it is advantageous from a scientific point of view to perform correlative measurements on an identical sample volume, providing the greatest accuracy and comparability between acquisition modes.17,19,27 By extension, while several high quality ex situ tomography studies have provided “before and after” analysis of catalyst bodies after various treatments;28−30 clearly it is optimal to monitor such processes as they occur using in situ imaging methodologies. For this purpose, modern in situ cells allow realistic gas and temperature conditions to be applied while retaining free rotation capabilities necessary for tomographic measurement, offering unprecedented insight into catalysts at work.19,22,31 In the field of catalysis and materials science, previous tomography studies have typically focused on thermal treatment or structural rearrangement (quasi) in situ.22,32,33 Studies of in situ catalytic reactions are only beginning to emerge, but have mostly been performed at lower resolution on mm-sized samples,16,23 at ambient temperature,19 or with single acquisition modes.20 Hence, hierarchically structured materials such as core@shell catalysts remain unexplored up to now, demanding multimodal imaging at the microscale or below in order to accurately resolve the various material components and structures present. Here we present a multimodal 3D structural analysis of a bifunctional Cu/ZnO/Al2O3@ZSM-5 core@shell catalyst, active for one-step synthesis of DME from syngas via methanol. In situ μ-XRF-CT, μ-XRD-CT, and scanning transmission X-ray microscopy (STXM-CT) were performed simultaneously under sequential oxidation, reduction, and model DME synthesis conditions. This enabled mapping of the crystalline structure and elemental distribution of an intact core@shell catalyst grain with effective spatial resolution of 2 μm. These complementary techniques allowed us to unravel structural features of both the core and zeolite shell in a single measurement. The same catalyst volume was measured throughout, which is crucial to allow direct and accurate comparison of the catalyst structure under different chemical environments. The catalyst itself consists of a Cu/ ZnO/Al2O3 methanol synthesis core of the type used in industry and to which numerous studies have been addressed.34−38 However, the nature of zinc as a promoter and its precise role in the mechanism, the role of CO and CO2 as reactants, and especially the state of copper in the active catalyst are still debated
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EXPERIMENTAL SECTION
Catalyst Preparation. The Cu/ZnO/Al2O3@ZSM-5 core@shell catalyst was prepared via a two-step hydrothermal synthesis approach described previously.3 To summarize, a Cu/ZnO/Al2O3 catalyst (approximately 6:3:1) was ground and sieved to a size of 80 to 100 μm. The shell was synthesized via a 2-step hydrothermal synthesis approach, including in situ silicalite-1 seeding, followed by secondary growth of a ZSM-5 zeolite shell. The resulting core@shell catalyst consisted of 80 to 100 μm grains of Cu/ZnO/Al2O3 core, encased in a ZSM-5 zeolite shell approximately 5 μm thick, as characterized by SEM (SI Figure S1.1). In Situ Measurement Procedure. Several grains of as-prepared catalyst were placed in the in situ microreactor19 (quartz capillary, 0.4 mm diameter, 0.04 mm wall thickness, SI Figure S1.2), which was then mounted on the beamline stage and centered over the rotation axis. The particle of interest was selected based on the presence of no overlapping particles in x, y, or z dimensions. The following conditions were applied sequentially: (i) fresh catalyst ex situ (20% O2/N2, 6 mL min−1, RT); (ii) calcination (20% O2/N2, 6 mL min−1, 400 °C); (iii) reduction/ activation (5% H2 in He, 6 mL min−1, 250 °C); (iv) DME synthesis conditions (16:8:1 H2:CO:CO2, 2% in He balance, 250 °C). Each condition was maintained for 2 to 3 h before commencing image acquisition. Heating was performed using a custom-made hot air blower, controlled by a K type thermocouple. Gases were supplied via mass flow controllers (Alicat). Online mass spectrometry (Cirrus Minilab LM92) was used to verify and monitor the appropriate gas conditions throughout the experiments. Image Acquisition and 3D Rendering. Figure 1 outlines the experimental methodology for in situ catalyst treatment combined with tomographic imaging used in this study. Hard X-ray tomography data was recorded at beamline I18 at Diamond Light Source operating at a fixed energy of 17 keV, selected by a Si(111) double crystal monochromator. The beam was focused to a 1.8 (H) × 2.6 (V) μm (fwhm) spot on the sample using Kirkpatrick-Baez mirrors.49 The sample was rastered across the beam in a translate−rotate data collection scheme, similar to first generation CT with a pencil beam. 81 translation steps at each rotation angle and 101 rotations (in 2° steps) were used to generate one “slice”; the sample was then translated vertically by 5 μm and the measurement repeated. A nonisotropic voxel size was chosen to minimize the radiation dose at each measurement point and reduce the potential for beam damage. An angular range of 200° (rather than 180°) was used to compensate for the microreactor vertical support which blocked a 20° wedge from the beam (SI Figure S1.2). In total, 8 slices were measured under each condition, encompassing a volume of 160 × 7856
DOI: 10.1021/jacs.7b02177 J. Am. Chem. Soc. 2017, 139, 7855−7863
Article
Journal of the American Chemical Society
probe for metal species. Cu and Zn components in the core were rendered individually by integrating the partial fluorescence yield (PFY) of the Cu and Zn Kα lines and plotting the relative intensity per pixel, indicating the number of absorbing species in each 2 × 2 μm2 area. In this way the distribution of Cu and Zn species in the core could be visualized under sequential in situ conditions (Figure 2). Excluding the fresh catalyst state, which
Figure 1. Overview of the experimental method: (a) simultaneous tomography data collection and (b) in situ reaction conditions during measurement. 160 × 40 μm3, or approximately 40% of the total particle volume. The same catalyst volume (vertical motor error
