Correlated operando electron microscopy and photoemission spectroscopy in partial oxidation of ethylene over nickel
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Date
2023
Publication Type
Doctoral Thesis
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Abstract
The solid-gas interface in heterogeneous catalysis has been shaping our society over the past 120 years. With finite fossil fuel reserves, we, as scientists, need to explore alternative methods to meet the escalating energy demands until a complete transition to renewable energy sources is achieved. Syngas production from underutilized light hydrocarbons presents a promising solution, providing a dual benefit of upgrading hydrocarbon sources to valuable products and reducing greenhouse gas emissions. However, existing heterogeneously catalyzed syngas production technologies collectively suffer from catalyst deactivation, either through coking or oxidation. Achieving and maintaining balance in the reaction conditions proves to be a non-trivial task, requiring precise control over the reaction conditions.
This thesis investigates the partial oxidation of hydrocarbons, with a focus on ethylene's partial oxidation to syngas using oxygen in a self-sustained oscillation mode, catalyzed by polycrystalline nickel foils and nickel nanoparticles. The primary objective of this work is to investigate the structure-surface orientation-performance relationships, as a function of the environmental conditions. To shed light on the complex phenomena governing the solid-gas interface, a combination of integral, near ambient pressure X-ray photoelectron spectroscopy (NAPXPS), online mass spectrometry (MS), and localized, environmental scanning electron microscopy (ESEM), and environmental transmission electron microscopy (ETEM) was employed. The results presented in this work were enabled by the development of a complete set of operando accessories compatible with UHV-multi bar pressure conditions, including a five-channel gas-feeding system, a laser-based high-temperature heating stage, and a software platform for environmental, imaging, and spectral dataset synchronization (Chapters 2 and 3).
During the ethylene partial oxidation in self-sustained oscillation mode, the dynamically changing structure of the nickel catalyst was demonstrated, for the first time under stoichiometric reactant ratios (Chapter 4). Under the studied experimental conditions, the high activity state corresponded to a metallic nickel surface, while during the low activity state, the surface was covered with nickel oxide. The origins of surface species during the activity states were identified, with carbon species being directly connected to catalytic activity, while the presence of oxygen species being inversely correlated with catalytic activity. The state transitions are attributed to the depletion of the CxHy(ads.) reservoir by the presence of water vapors in the gas phase. Moreover, under stoichiometric reactant feeds, mbar pressure regimes, and a catalyst temperature of 950oC, virtually complete ethylene conversion was recorded, alongside the continuous formation of Ni3C. The complex catalytic behavior was attributed to a multi-step mechanism, whose reaction rates are influenced by the dynamic nature of the catalyst. During the active state, the total oxidation of ethylene dominates the first step, and a mixture of reverse water gas shift (RWGS), dry reforming of ethylene (DRE), and wet reforming of ethylene (WRE) is involved in the second step. The variation in reaction kinetics is directly visualized in Chapter 5, as redox chemical waves propagate across the surface with velocities that are specific to the surface orientation. Those waves give rise to complex coupling phenomena through which the entire catalytic surface synchronizes. Moreover, by investigating the temporal evolution of (111), (112), and (113) surfaces, crystallographic surface orientation-dependent dynamics were observed during the collective high activity state and the transition between the high and low activity state. Inaccessible from integral characterization techniques, these dynamics range from the higher stability of the metallic state on the (111), to the surface energy minimization via surface reconstructions or CxHy(ads.) coverage on the (112), to the presence of CxHy(ads.) species with lifetimes shorter than 90 milliseconds on the (113). Furthermore, the oscillatory behavior of nickel nanoparticles at atmospheric pressure, stoichiometric reactant flow, and a catalyst temperature of 950oC was investigated in Chapter 6. Under those conditions, the bulk Ni-NiO transitions are completed in less than 160 milliseconds. The high temporal and spatial resolution imaging revealed that atomic steps on (200) index planes of NiO are essential for the stability of the oxide state, while the selected area electron diffraction revealed the presence of a metastable Ni3C phase.
In conclusion, the work presented in this thesis not only offers new insights into the complex interplay between the catalyst structure, surface orientation, and performance but also has the goal of motivating other scientists to explore the fascinating field of chemical dynamics.
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Examiner : van Bokhoven, Jeroen Anton
Examiner : Willinger, Marc Georg
Examiner : Artiglia, Luca
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ETH Zurich
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Subject
Scanning Electron Microscopy; X-ray photoelectron spectroscopy (XPS); Operando characterization; syngas; partial oxidation; self sustained oscillations; Nickel
Organisational unit
03746 - Van Bokhoven, Jeroen A. / Van Bokhoven, Jeroen A.
02891 - ScopeM / ScopeM
Notes
Funding
ETH-36 18-2 - Bridging scales in catalysis: Investigation of hydrogen spill-over by a combination of in situ methods (ETHZ)