Unraveling Catalytic Reaction Mechanism by In Situ Near Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract

Probing surface chemistry during reactions closer to realistic conditions is crucial for the understanding of mechanisms in heterogeneous catalysis. Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is one of the state-of-the-art surfacesensitive techniques used to characterize catalyst surfaces in gas phases. This Perspective begins with a brief overview of the development of the NAP-XPS technique and its representative applications in identifying the active sites at a molecular level. Next, recent in situ NAP-XPS investigations of several model catalysts in the CO 2 hydrogenation reaction are mainly discussed. Finally, we highlight the major challenges facing NAP-XPS and future improvements to facilities for probing intermediates with higher resolutions under real ambient pressure reactions in heterogeneous catalysis.

Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems

“…X-ray photoelectron spectroscopy (XPS) is an important technique for characterizing the elemental composition of materials, based on measuring the element-specific energy of photoelectrons emitted from core levels. − Since their binding energy depends sensitively on the charge density on the atoms, even small changes in the chemical environment lead to a notable peak shift in the XP spectrum, which is referred to as chemical shift . For example, chemical bonds between atoms of different electronegativity in solids or molecules are polarized, making it possible to determine oxidation states or to distinguish bonds with different atoms, which has been documented in reference works and databases. , Therefore, XPS has become one of the most common methods for characterization in chemical engineering and surface chemistry, as it allows one to analyze bonding structures and the composition of surfaces and interfaces. − In addition, photoelectron spectroscopy proves to be very helpful in the development of novel molecular functional materials that can be synthesized through targeted chemical functionalization. , A prominent example is the tailoring of π-conjugated molecules, which are of great interest for use in the field of organic electronics. − An efficient way to adjust the molecular energy levels in organic semiconductors and also to control the polarity of the charge carriers is halogenation, especially fluorination, since the latter barely changes the size of the molecules . In the case of the prototypical pentacene (PEN), perfluorination (PFP) causes a significant increase in the C1s core-level binding energy and a splitting into sublevels associated with C–F and C–C/H sites (PEN: E B (C–C) = 284.5 eV; PFP: E B (C–C) = 286.5 eV, E B (C–F) = 288.1 eV) .…”

Vision 2050: Reaction Engineering Roadmap

“…Extending these types of in situ characterization studies to other commercial processes could lead to novel strategies for extending the catalyst lifetime. In this context, we envision that the use of techniques such as X-ray photoelectron spectroscopy (XPS) at conditions close to those encountered during catalytic reactions will find increasing use in the catalysis and reaction engineering community. , In summary, the spatiotemporal resolution of characterization techniques will need to be advanced to keep pace with advancements in concepts in dynamic catalysis and elucidating catalytic sites and their environments, both of which are gaining widespread traction in the catalysis and reaction engineering research community. The characterization of active sites and reaction intermediates in the liquid phase remains especially challenging, and the discovery of electrocatalysts for key oxidation and reduction reactions will rely on the advancement of techniques that can be applied efficaciously in the liquid phase.…”

State of the Surface During CO Hydrogenation over Ni(111) and Ni(211) Probed by Operando X-ray Photoelectron Spectroscopy

“…Product analysis of real catalysts agree with the predicted results from DFT calculations, , yet there are no atomistic observations of the chemical state of the catalyst and the adsorbates under working conditions. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is routinely used for mechanistic investigations of catalytic reactions at pressures of <2 mbar − but is hard to combine with pressures commonly used in industrial syngas applications. Utilizing the virtual pressure cell concept in the POLARIS instrument allows one to drastically increase the working pressure, making operando studies of catalyzed syngas reactions possible at semirealistic pressures .…”