Field-Induced Chemistry in Catalysis: High Pressure and High Fields
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ORIGINAL PAPER
Field‑Induced Chemistry in Catalysis: High Pressure and High Fields H. J. Kreuzer1
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract High electrostatic fields of cations in zeolites and other catalysts is shown to modify the elctronic level struture of molecules. As an example, nitrogen molecules become nitrogen oxide like. It is argued that this is relevant for the Haber–Bosch synthesis. It is also pointed out that fields of the order of volts per Angstrom are equivalent to pressures of GigaPascals. Keywords Fields · Catalysis · Kinetics
1 Introduction The use of high electrostatic fields in the field ion microscope to promote chemical reactions was first proposed by Inghram and Gomer [1], and developed further by Block [2]. Detailed accounts of the early work have been given by Block [3, 4]. In the field ion microscope [5, 6] and later in the atom probe microscope [7] a metallic field emitter tip is used with the apex sharpened to a radius of the order of less than 1 μm , even down to single atom tips [8]. Applying a voltage of several kilovolts to a counter electrode some 10 cm away will create a field in the vicinity of the tip E0 = V0 ∕(kf r0 ) where V0 is the applied voltage and r0 is the radius of curvature; the field factor kf accounts for the modification due to the shank of the tip. Analytical field distributions can be obtained when the tip and detector surface are approximated by paraboloids or hyperboloids. For the study of field-induced chemical reactions a larger tip radius is used to accomodate larger molecules. When an imaging gas such as helium or neon is added to the vacuum chamber the gas atoms are polarized and accelerated to the tip in the inhomogeneous field where they are ionized over the surface atoms and accelerated to an imaging screen: an atomically resolved image of the surface structure of the tip is produced. At the best imaging field for helium, about 4 V/Å, helium is actually adsorbed at low temperature on the surface of the tip with a binding energy of about 0.4 eV, up by a factor of * H. J. Kreuzer [email protected] 1
Department of Physics and Atmospheric Science, Halifax, NS B3H 4J5, Canada
100 from the binding energy in the absence of a field [9, 10]. This dramatic change from a pure Van der Waals interaction at zero field to weakly covalent bonding is the result of raising the 1s level of helium relative to the Fermi level of the metal by an amount eEB d where EB is the binding energy and d distance from the metal so that weak hybridization can occur. This is our first example of field-induced chemistry. A word on the quantum mechanical methods needed for such calculations. The first calculations to study fieldinduced chemistry of N2 on Fe were done with a simple tight-binding method [11]. The reason why this turned out to be sufficient is simple: Electric fields of the order of volts per Angstrom shift the atomic electron levels by amounts of the order of electronvolts. So a method that has an accuracy of, say, 0.1 eV is qu
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