In Situ Investigations of Chemical Reactions on Surfaces by X-Ray Diffraction at Atomospheric Pressures
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S. Ferrer, M.D. Ackermann, and E. Lundgren Abstract Catalytic reactions occurring at metal surfaces and nanoparticles have been an established research field for decades, yielding information on adsorption sites and reaction pathways under ultrahigh-vacuum conditions. Recent experimental developments have made it possible to perform well-controlled in situ surface x-ray diffraction measurements from single-crystal surfaces and nanoparticles under industrially relevant conditions. In this way, a new understanding of atomic-scale processes at surfaces and nanoparticles occurring during catalytic reactions under realistic conditions has been gained. In particular, the identification of the formation of thin oxides on model catalysts and their role in oxidation reactions demonstrates the importance of in situ probes under relevant conditions.
Introduction During the past 40 years, gas–surface interactions between a gas and a solid surface have been investigated by carefully exposing low-index single-crystal surfaces in ultrahigh-vacuum (UHV) to small amounts of atomic or molecular gases. This has led to detailed knowledge on adsorption sites, dissociation processes, and reaction pathways. However, the large differences between these simple UHV model systems and more real-world non-single-crystal surfaces exposed to high pressures often prevent extrapolation of this knowledge to more realistic situations (materials and pressure gaps).1 Whereas most traditional electron- or ion-based surface science techniques are 1010
limited to pressures of up to a few millibars, surface x-ray diffraction (SXRD) is able to determine structural parameters at surfaces under high-pressure operating conditions of several bars. The reason for this highly attractive property is the negligible attenuation of the x-ray beam due to the low interaction between high-energy x-rays and gases. Recent experimental advances have made it possible to exploit this property in order to simultaneously probe the surface structure and composition of single crystals and nanoparticles using in situ SXRD and the gas-phase composition using mass spectrometry in a gas mixture at atmospheric pressures and elevated sample temperatures.
Figure 1a shows a photograph of the experimental equipment that has enabled recent advances in the understanding of catalytic reactions under realistic conditions2 using SXRD. The chamber consists of a 360° beryllium window brazed to the stainless steel cylindrical walls in a way that the edges of the Be cylindrical tube are inserted in a cylindrical groove. This allows for operation of the chamber under external atmospheric pressure when the chamber is in vacuum, and under internal pressure when the chamber is pressurized. The beryllium window is practically transparent to hard x-rays because of its low atomic number. Sample surfaces can be prepared in UHV with traditional techniques of surface science, such as ion etching and annealing, and then pressurized from 10−10 bar to 5 bar in order to study surface structures of the adso
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