Temperature Programmed Desorption Analysis

  This topic provides an example of how to perform a Temperature Programmed Desorption analysis using NH3 on calcium oxalate under helium with a 10 °C/min temperature ramp. Make the appropriate modifications for the material being analyzed.

Temperature Programmed Desorption (TPD) analyses determine the quantity, type, and strength of active sites available on the surface of a catalyst from measurement of the amount of gas desorbed at various temperatures.

After the sample has been outgassed, reduced, or otherwise prepared, a steady stream of analysis gas flows over the sample and reacts with the active sites. (Alternatively, Pulse chemical adsorption can be used to react with active sites.) Programmed desorption begins when the temperature is ramped linearly over time while a constant stream of inert carrier gas passes over the sample.

At a certain temperature, the heat will overcome the activation energy, breaking the bond between the adsorbate and adsorbent. The adsorbed species will then desorb. If different active metals are present, they usually will desorb the reacted species at different temperatures. The desorbed molecules enter the stream of inert carrier gas and are swept to the detector where the detector response is proportional to the gas concentrations. The quantity of desorbed species, combined with the stoichiometry factor, and the temperature at which pre-adsorbed species desorb, yield the quantity and strength of active sites, respectively.

If TPD is performed after coverage of the active sites by flow or pulse chemical adsorption, additional information about the distribution of active sites and the strength of active sites is collected.

Preparation

Pretreatment Degas by flowing inert gas — such as helium, argon, or nitrogen — over the sample while ramping the temperature.
Analysis

Choose a gas mixture from the following table based on the needed results. Flow the gas over the sample, then ramp the temperature beginning at ambient.

Cold Trap Not required
Needed Results Carrier Gas Analysis Gas
Active Metal Surface Area H2 Chemisorption argon hydrogen
Active Metal Surface Area CO Chemisorption helium carbon monoxide
Acidity helium ammonia (15% maximum NH3, blended in helium)
  Before performing an analysis, ensure the sample and analyzer have been adequately prepared.

Procedure

These instructions analyze zeolite with ammonia (use 15% maximum NH3 blended in helium) at 3 to 20 ºC/min. Adjust the values in the example to accommodate the analysis you are performing.

  1. Obtain the sample weight, then install the loaded sample tube and thermocouple on the analyzer. Close the furnace around the sample tube.
  2. Create a sample file for this analysis.
  1. Go to File > New Sample.
  2. Complete the Sample Information window. Enter the correct mass and complete the optional fields as needed.
  3. Select the Analysis Conditions tab and insert the following experiment steps:
Insert Step Window Field Field Entry or Selection
Experiment New Experiment Experiment description Enter a description of the experiment
Type of analysis Other
Zones TCD Detector

Block zone

100

Filament

175
Valve Zones

Cold trap

110

Analysis

110

Loop

110

Back pressure

110
Gas Flows Prep Gas Helium
Carrier Gas Helium
Loop Gas None
Rate 50 cm3/min
Cold trap valve Bypass
Analysis valve Prepare
Loop valve Fill
Back pressure Prepare
Outputs Outputs Use default values
Peaks Peaks Use default values
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 350
Ramp rate 50.0
Hold time 10.00
Experiment New Experiment Experiment description Enter a description of the experiment
Type of analysis Temperature Programmed
Desorption
Zones TCD Detector

Block zone

100

Filament

175
Valve Zones

Cold trap

110

Analysis

110

Loop

110

Back pressure

110
Gas Flows Prep Gas Helium
Carrier Gas Helium
Loop Gas None
Rate 50 cm3/min
Cold trap valve Bypass
Analysis valve Prepare
Loop valve Fill
Back pressure Prepare
Outputs Outputs Use default values
Peaks Peaks Use default values
Temperature Ramp Temperature Ramp Ramp Type Return to ambient
Wait Wait Wait for Operator Add sample and setup cold trap.
Change Gas Flows Gas Flows Prep Gas Ammonia (15% maximum NH3 blended in helium)
Carrier Gas Helium
Loop Gas Ammonia (15% maximum NH3 blended in helium)
Rate 50 cm3/min
Cold trap valve Bypass
Analysis valve Prepare
Loop valve Fill
Back pressure Prepare
Wait Wait Wait [n] minutes 60
Change Gas Flows Gas Flows Prep Gas None
Carrier Gas Helium
Loop Gas Ammonia (15% maximum NH3 blended in helium)
Cold trap valve Bypass
Analysis valve Analyze
Loop valve Fill
Back pressure Prepare
Wait Wait Wait [n] minutes 30
Wait Wait Wait until baseline is stable Select this option
Start Recording Start Recording One measurement every 1.0 seconds
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 500
Ramp rate 3.0
Hold time 0.0

The application automatically inserts a Stop Recording step when a Start Recording step is inserted. Ensure that the Temperature Ramp step is inserted within the Start/Stop Recording loop.

Wait Wait Wait [n] minutes 30
Temperature Ramp Temperature Ramp Ramp Type Return to ambient
Repeatedly insert the steps between the double lines in this table. Use a different ramp rate for each repetition; after ramping at 3°/min. and 5°/min., use 10, 15, and 20°/min. This shifts the peak as a function of the Heat of Desorption.
Termination Termination Return to ambient temperature Select this option
Leave detector enabled after analysis Option is not selected
Zones Not applicable
Gas Flows Not applicable
  1. Select the Report Options tab and set the values. Click Save then click Close.
  1. Start the analysis.
  1. Go to Unit > Sample Analysis. From the Files list box, select the sample file created in the previous step. Edit the file as needed. Click Next.
  2. From the drop-down list, select the calibrations associated with each experiment in the sample file (if any). For this example, select None. Click Next.
  3. Read the cautionary window and make any necessary changes.
  4. Click Start to start the analysis.
 

When the analysis ends, the furnace begins to lower the sample to room temperature. To speed cooling of the sample, use the optional CryoCooler or KwikCool. Remove the sample tube when the analysis is complete.

 

Use the cotton gloves provided in the accessory kit to remove the sample tube before it has cooled. Rubber gloves may be used to handle the sample tube when it has cooled.

 
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