Pulse Chemical Adsorption Analysis

A Pulse Chemisorption analysis determines active surface area, percent metal dispersion, and active particle size by applying measured pulses of reactant gas to the sample, depending upon what technique is used. Pulse Chemisorption is performed in the same manner as TPD, except that the sample is dosed with the analysis gas using the injection loop until each active site has reacted. After the active sites have completely reacted, the discretely injected gas volumes emerge unchanged. If TPD is performed after Pulse Chemisorption, additional information about the distribution of active sites and the strength of active sites is collected.

The amount chemisorbed is the difference between the total amount of reactant gas injected and the amount that did not react with the active sites of the sample. The size of each pulse is determined by the loop, which is located on an electrically operated valve.

Loop Volume

It is preferred for the sample to require at least two doses of gas, but no more than ten doses, before the reaction ends — depending on the environment. Some factors that influence the number of doses required are sample size, the active surface area per unit of sample, and the size of the loop. If the pulse chemisorption analysis requires more doses or fewer doses, change either the sample size or the loop size.

Three loops of different sizes are provided with the analyzer — each loop must be calibrated prior to use. As an alternative to replacing the loop, the loop volume may be changed by adjusting its temperature. If the loop volume is changed by changing its temperature, results will be more accurate if the loop volume is recalibrated at the new temperature.

Loop Calibration

Before data from an analysis that uses an injection loop can be reduced, a loop calibration experiment must be performed. An independent loop calibration can be performed either before or after the analysis.

The following example assumes that the loop calibration will be performed after the analysis. A Loop Calibration step can be included in a pulse chemical adsorption analysis.

Preparation

Pretreatment Degas by flowing inert gas (such as helium, argon, or nitrogen) over the sample while ramping the temperature.
Analysis Pulse the loop (analysis) gas over the sample until the peak area remains constant.
Cold Trap No cold trap is needed during pretreatment. During the analysis, use the cold trap fixture with a dewar of water in place as a short delay path. This delay compensates for the flow disturbance caused by each loop injection.
Pressure regulator Gas cylinders should be set to a level between 10 and 15 psig (69 and 103.5 kPag).
Furnace temperature Select a temperature high enough to remove any contaminants or moisture, but not so high as to cause sintering or fusing of the sample. Ensure the Done step is set to return the sample temperature to ambient.

Procedure

  In the following example, Pulse Chemisorption is performed on a sample of Pt/Al2O3 with CO. Alter the analysis conditions to accommodate a different analysis.
  Before performing an analysis, ensure the sample and analyzer are adequately prepared using the instructions in Prepare for Analysis.
  1. Obtain the sample mass, 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. Complete the Sample Description tab. Enter the correct mass and complete the optional fields.
  2. Select the Analysis Conditions tab and insert the experiment steps listed below:
Insert Step Window Field Field Entry or Selection
Experiment New Experiment Experiment description Enter a description of the experiment
Type of analysis Pulse Chemisorption
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 Trap
Analysis valve Analyze
Loop valve Fill
Back pressure Prepare
Outputs Outputs Use default values
Peaks Peaks Use default values
Wait Wait Wait [n] minutes 5
Change Gas Flows Gas Flows Prep Gas Hydrogen
Carrier Gas Helium
Loop Gas None
Cold trap valve Bypass
Analysis valve Prepare
Loop valve Fill
Back pressure Prepare
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 120
Ramp rate 10.0
Hold time 30
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 220
Ramp rate 10.0
Hold time 30
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 350
Ramp rate 10.0
Hold time 120.0
Change Gas Flows Gas Flows Prep Gas Helium
Carrier Gas Helium
Loop Gas None
Cold trap valve Bypass
Analysis valve Prepare
Loop valve Fill
Wait Wait Wait [n] minutes 90
Temperature Ramp Temperature Ramp Ramp Type Return to ambient
Wait Wait Wait [n] minutes 30
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 35
Ramp rate 5.0
Hold time 10.0
Change Gas Flows Change Gas Flows Prep Gas None
Carrier/Reference Gas Helium
Loop Gas Carbon Monoxide
Cold trap valve Trap
Analysis valve Analyze
Loop valve Fill
Back pressure Prepare
Wait Wait Wait until baseline is stable Select this option
Start Recording Start Recording One measurement every 0.2 second
Wait Wait Wait [n] minutes 3
Start Repeat Start Repeat Sequence Repeat until peaks are equal or [n] times 20
Dose Dose Inject Loop Gas Select this option
Wait for [n] minutes 1.5

The application automatically inserts a Stop Repeat step when a Start Repeat step is inserted. Ensure that the Dose step is inserted within the Start/Stop Repeat loop.

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.

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 modify values as needed.
  2. 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.
  Calibration files can also be associated with a sample file after analysis using Calibration button in the Peak Editor.
  1. Read the cautionary window and make any necessary changes.
  2. 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.

 

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.

When the displayed sample temperature reaches the ambient temperature, open the furnace. Using gloves, remove the sample tube.

Generate the Loop Calibration Report

Open the Peak Editor and ensure that peaks are properly marked.

There are three possibilities for each dose of gas injected during Pulse Chemical Adsorption:

  • all of the gas is taken up by the sample,
  • some of the gas is taken up by the sample, or
  • none of the gas is taken up by the sample.

When pulse chemical adsorption is properly performed, there will be some injections of each type. When the data is viewed using the Peak Editor, however, only those injections in which some or none of the gas is taken up will appear as peaks. When all of the gas is taken up by the sample, none of it reaches the detector and, therefore, the peak area is zero. These types of peaks are detected automatically by the application and do not require marking.

  1. Go to File > Open > .SMP file.
  2. Select the sample file used with this analysis. Click OK.
  3. Select the Report Options tab.
  4. Enter the percent of active gas in the Active concentration field.
  5. On the Sample Information tab, click Active Metals. Verify that the Stoichiometry Factor and the % of Sample Weight values are correct. Click OK.
  6. Verify that the values for the physical injection volume, ambient temperature, and ambient pressure are accurate.
  7. Go to Reports > Start Report to generate the report.
 
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