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The Analytical Technique
The ChemiSorb 2720 and 2750 both utilize the dynamic (flowing gas) technique of analysis. The quantity of gas adsorbed from the gas stream by the sample is monitored by a downstream thermal conductivity detector (TCD). The temperature and pressure at which adsorption/desorption occurs is either known or monitored. The instruments can be used to study physical or chemical adsorption. Preparation usually is accomplished by flowing either an inert or chemically active gas over the sample. After preparation, another gas is selected for analysis. Prep and carrier gases typically used to allow both physical and chemical adsorption experiments are He, Ar, N2, He/N2mixtures, H2, and O2, some serving as both prep and carrier.
Any of a number of reactive gases such as anhydrous NH3, CO2, CO, H2, N2O, O2, and H2S can be used to react with the active surface. A series of injections of a known quantity of reactive gas is injected into an inert gas stream that passes through the bed of catalysts. Downstream from the reactor is a detector, which determines the quantity of reactive gas that is removed from each injection. Chemisorption tests ideally are made with the sample at a temperature such that only chemisorption occurs. The active surface of the sample is saturated when the detector indicates that the total quantity of subsequent injections passes through the sample bed without any loss. The sum of the injected quantity minus the quantity of gas that passed without adsorption equals the quantity adsorbed.
Unlike physical adsorption, the injected gas chemically adsorbs only on the active surface and not on the support. Thus, the number of gas molecules required to cover the active surface area, once determined, leads directly to the active surface area. Applying the stoichiometry factor for metal reaction yields the number of accessible atoms of active metal. Furthermore, using the total quantity of active metal per gram of catalyst material (determined from the manufacturing formula) leads to the determination of the percent dispersion of active metal. Using the information gathered plus the density of the metal, the size of the metal crystallite can be estimated if it is assumed that these particles have uniform geometry of known volume-to-area ratio.
The surface area of granulated and powdered solids or porous materials is measured by determining the quantity of a gas required to form a monomolecular layer on a sample. Physical adsorption tests typically are performed at or near the boiling point of the adsorbate gas; N2 being most common with a liquid N2 bath being used to maintain the analysis temperature. Under these conditions, a nitrogen and helium mixture of 30 volume percent nitrogen achieves the partial pressure condition most favorable for the formation of a monolayer of adsorbed nitrogen at atmospheric pressure. Under such specific conditions, the area covered by each gas molecule is known within relatively narrow limits. The area of the sample is thus calculable directly from the number of adsorbed molecules, which is derived from the gas quantity at the prescribed conditions, and the area occupied by each. Additionally, atmospheric pressure and ice water temperature may establish appropriate conditions for an n-butane and helium mixture. Other gases at other conditions are also usable.
Chemical adsorption is an interaction much stronger than physical adsorption. In fact, the interaction is an actual chemical bond where electrons are shared between the gas and the solid surface. While physical adsorption takes place on all surfaces if temperature and pressure conditions are favorable, chemisorption only occurs on certain surfaces and only if these surfaces are clean. Chemisorption, unlike physisorption, ceases when the adsorbate can no longer make direct contact with the surface; it is therefore a single layer process.
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