adsorption - Physical Adsorption of Argon and Nitrogen on...

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Unformatted text preview: Physical Adsorption of Argon and Nitrogen on Graphite In this experiment you will record an adsorption isotherm of N 2 molecules and a second one of Ar atoms on a graphite substrate at a temperature of 77.4 K. A feature of the N 2 isotherm cor- responding to a known density of molecules on the graphite substrate allows one to calculate the total substrate area. With the substrate area thus determined, the Ar isotherm can be used to determine the density of Ar atoms at single (mono)layer completion, allowing for a determina- tion of the monolayer Ar-Ar spacing. This number can then be compared to the known nearest neighbor spacing of Ar atoms in the solid 3-D phase, and the Ar-Ar separation derived from the Lennard-Jones potential. 1 Introduction Adsorption is the process in which atoms or molecules of a substance in one phase (usually gas) become bonded to the surface of a second substance in a different phase (usually solid). Adsorption is to be distinguished from absorption, the process in which one substance interpenetrates the bulk volume of a second substance. There are many examples of adsorption in everyday life. Activated charcoal, which can have a surface area of up to several thousand square meters per gram, is capable of adsorbing many different substances. It is used to filter contaminants from drinking water, and is also used in gas masks to remove toxic substances from breathing air. Silica gel (SiO 2 ) packets, which adsorb water vapor, are often packed in with moisture sensitive merchandise. And adsorption is a key part of many catalytic processes, as many chemical reactions will not take place except in the presence of a particular kind of surface. For example, molecular hydrogen and oxygen can be mixed at room temperature with no effect. However, these molecules dissociate when adsorbed on a platinum surface, and will then combine (vigorously!!) to form water molecules. Before proceeding it is useful to present some definitions. In this write-up we will be discussing a system where a substance in the gas phase becomes bonded to the surface of a second, solid substance. Adsorbate: the substance originally in the gas phase which becomes bonded to the surface of a second, solid substance. Adsorber: the solid substance on the surface of which the adsorbate becomes bonded. The ad- sorber is also referred to as the adsorbent or, more commonly, the substrate. There are two broad types of adsorption processes, characterized by the strength of the bond between the adsorbate and substrate. In physical adsorption, or physisorption, the bonds between atoms or molecules being adsorbed and the substrate are due to dipole-dipole interactions, and are relatively weak. In chemical adsorption, or chemisorption, a chemical bond is formed between the adsorbate and the substrate. Such bonds, which involve the exchange or sharing of electrons between the adsorbate and the substrate, are much stronger. Typical physisorption binding energies are 50-500 meV per atom or molecule; chemisorption binding energies are typically an order of magnitude greater....
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adsorption - Physical Adsorption of Argon and Nitrogen on...

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