表面化学The Brunauer-Emmett-Teller isotherm.pptVIP

The Brunauer-Emmett-Teller isotherm Langmuir isotherm : monolayer adsorption BET isotherm: multilayers adsorption, based on following assumptions: 1. Each layer of adsorbate is treated as a langmuir monolayer and each layer must be completed before the next layer starts to form. 2. The heat of adsorption for the first layer, is characteristic of the adsorbate adsorbent system. 3. The heat of adsorption for subsequent layers is equal to the heat of condensation. Application of BET Determination of surface area Example: adsorption characteristics of nitrogen molecules on activated charcoal particles Experimental procedure Preparation of activated charcoal: Carefully remove the sidearm flask from the glass line Measure approximately 2 grams of activated carbon and transfer into the sidearm flask without contaminating the glass wool filter inside the sidearm flask Reattach the sidearm flask to the glass line, and begin pumping through the sidearm valve to reestablish the vacuum Attach the heating tape to the flask so that as much of the carbon is covered without overlaying the heating tape on itself. Power the heating tape on the Variac (Heat for 30 minutes) Remove heating tape from flask and allow to cool for 15 minutes Once cool, slowly open the valve connecting the sidearm flask to the glass line making sure that the vacuum in the glass line does not disturb the carbon in the flask (B) Helium Expansion into glassware to determine the volume of the line and the volume of the sidearm vessel containing the charcoal: Check to make sure that all relevant stop cocks are open for initial pumping Start the mech. pump and pump the glass line and both gas line to the regulators, (both the Helium and Nitrogen) – MAKE SURE THAT THE NEEDLE VALVES ON BOTH NITROGEN AND HELIUM TANKS ARE CLOSED. Fill the 1025 mL round bottom flask with a known pressure of He (200-800 Torr in ? 100 Torr increments). Throughout all experiments pressure should be read off the Digital Press