Performing Pulse Chemisorption Experiments

The Micromeritics Flow Reactor configuration known as the MCCTC (Micro Catalyst Characterization and Testing Center) was designed in order to equip the FR Series with the capabilities of characterizing and performing activity testing of catalysts in-situ. 

The combination of the FR Series MCCTC configuration with a mass spectrometer gives the user a powerful tool for the characterization of catalysts, the performance of activity testing, and the re-characterization of the catalyst. This process demonstrates how the catalyst’s surface has changed after it has been subjected to industrial conditions. 

Pulse chemisorption constitutes one of the characterization techniques which the MCCTC option employs. This method is often used to measure the dispersion of active metals. This is possibly the courtesy of an extra gas feed through the Mass Flow Controller (MFC), together with a known volume loop (generally with a volume of 0.5 cc) used for gas dosing. 

This article outlines the experimental procedure and the method required to set up the FR Series Process@ software so that it can automatically perform the pulse chemisorption experiment. 

It also explains the MKS Cirrus2 Mass Spec’s Process Eye software for the collection of relevant data in an experiment and describes the method of using MicroActive from Micromeritics to import the mass spec data and determine results.

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Experimental Procedure

In this instance, pulse chemisorption is used to analyse the Platinum-Alumina (Pt-Al) reference material from Micromeritics. This is done by pulse chemisorption, using pure O as the active species to fill the known volume loop for dosing. During gas dosing, 10% H₂/Ar is used as the carrier gas as well as the feed in order to reduce the sample. 

1 g of the sample is used in this experiment; however, it is worth noting that as the sample mass is an input parameter for calculations, differing quantities of the sample are able to be used in this characterization method. 

The sample’s quality must always be considered, as too much of it will lead to a high gas adsorption rate which requires several gas doses, while not enough of it will mean that not enough gas is adsorbed to provide high-resolution data. 

Although diluted CO mixtures such as CO/He can be used, they need more doses because of the reduced quantity of the active species in the dosing loop. Additionally, it is possible to use H₂ and H₂ mixtures as the active species when combined with an inert carrier feed like He or N₂. 

Building a Session Table in Process@ to Perform the Pulse Chemisorption Experiment

The user can build a session table in order to program an automated experiment, courtesy of Process@. Any or all the system parameters (pressure, flows, temperatures, and so on) are able to be changed when moving from one session to another, and each session has a finite amount of time. 

This article will move on to explaining how to build session tables for automatic analysis of the Pt-Al reference material. 

The Pt-Al reference material ought to be reduced to an H2/Ar flow of below 50 ml per minute while being ramped to 400 °C at a rate of 10 °C per minute before gas dosing. When the sample reaches 400 °C, it should be maintained at this temperature for an extra half an hour in order to facilitate the sample’s complete reduction. 

Once the reduction is finished, the sample should then be returned to room temperature. Opening the hotbox door is the quickest way to achieve this result. The mass spec  begins recording once the sample has cooled, and CO (the active species) can be dosed to the sample using a 6-way valve with a known volume loop. The session table should generally resemble that shown in Figure 1. 

This session table brings up a few items of note. Firstly, manual operation (PIC01 MODE=1) is established for the pressure control valve (PCV) and the valve is completely open (PIC01 MV=80%). This guarantees that the entire analysis will be operated with the system at atmospheric pressure. Secondly, the door can be opened by setting DOOR STATUS to '1'. Similarly, it will close when it is set to '0'. 

Finally, Figure 2 sets up CONDITION 1. This is designed to save time. Although the system has 120 minutes to reach room temperature, when the reactor temperature gets to 30 °C or below the software automatically jumps to Session 4 in order to begin the collection of data (GC RUN = 1) and proceed with the analysis. 

Figure 1. A Process@ session table built to reduce and dose active species onto the catalyst.

Session 5 and Session 6 are both looped for a total of nine CO injections into the carrier stream. While the purpose of Session 6 is to refill the loop (Loop=0), the purpose of Session 5 is to inject the full loop into the carrier stream (Loop=1). Only one minute is necessary for each of these sessions.

As shown in Figure 3, the total of nine injections is achieved through CYCLE 1. Input '9' into the 'Repetitions' field and select 'Session 5' in order to build the cycle, then click 'Include Cycle', followed by 'OK'. The analysis is terminated by the final session, which stops the mass spec from acquiring data (GC RUN=1).

It is worth noting that the software’s logic requires that the 'GC RUN' parameter is set to '1' for both starting and stopping. Users can manipulate this to their advantage by continuing to set the GC RUN Parameter to '1'.

This allows users to collect multiple sets of data at varying times throughout the analysis duration. Data collection will begin with each odd iteration when setting GC RUN to '1', and the collection of data will end with each even iteration.

Figure 2. Creating a conditional jump to move the experiment forward once the reactor reaches 30 °C.

Figure 3. Editing the analysis conditions to include a nine-repetition cycle between Session 5 and Session 6.

Creating and Running an Automatic Sequence in Process Eye to Automatically Collect Data

It is possible to set up the Process Eye so that it automatically collects data when triggered. The interface which is used to transmit the trigger signal mentioned in the previous section (GC RUN=1) is the digital signal cable between the Micromeritics Flow Reactor (FR) and Cirrus2 Mass Spec.

Click 'Start' and 'Create/Edit Recipe' once Process Eye is open. At this point, either enter a name for a new recipe (as depicted in Figure 4) or choose an existing recipe, and then click 'OK'. After this, enter the active species’ atomic mass. Generally, the Faraday detector is used for this application with 'Skip on Saturation' selected.

It is vital that the Accuracy setting is properly configured in order for high-resolution results to be generated. Readings are more accurate when this parameter is increased, however, they require more time. Similarly, the reading is less accurate when the parameter is decreased, however, they can be more frequently recorded.

A good compromise for this application is the selection of setting '4'. This makes the Cycle Time '0.100', which means that Process Eye will record data every 0.1 seconds. This is shown in Figure 5. The optional addition of further mass readings for more complex applications is available to users who click the green plus icon, however, this is not necessary for the pulse chemisorption application.

Figure 4. The window which appears when clicking 'Start Create/Edit Recipe' in the Process Eye software.

Figure 5. Adjusting the Recipe Settings.

Figure 6 shows the outcome when 'Start Automatic Sequence' is selected. The recipe which was newly created needs to be selected from the Recipe Name menu. The name of the new data file which is created during the analysis will be the string entered into the Save As field. Both Filament Enabled and Export as Test should be selected.

The Process Eye software starts the automatic sequence when 'Begin Automatic Sequence' is chosen. It will then be ready to receive the trigger signal from the FR and begin recording data, as shown in Figure 7.

It is worth noting for applications that are more complex than it is possible to run multiple recipes in sequence when they are combined with the multiple trigger signals mentioned previously.

The automatic sequence will run the recipes in an infinite loop if 'Restart After Sequence Completion' is selected. This needs to be stopped manually by the user by clicking the 'Abort Automatic Sequence' or 'Stop Automatic Sequence'.

Figure 6. Building the Automatic Sequence in Process Eye.

Figure 7. The Automatic Sequence waiting to receive a trigger signal to begin data acquisition.

In the Session Table, Session 4 represents when the mass spec is initially triggered to begin data acquisition. There are two reasons why this lasts for 10 minutes. Firstly, it enables a steady baseline to be established by the mass spec. Secondly, it allows the active species to flush the loop.

Figure 8 displays what the automatic sequence looks like when the active species has been injected into the carrier stream. The first injection was performed here at the tenth minute. The active species took approximately 40 seconds to travel from the loop, initially to the reactor via the catalyst bed and subsequently through the exhaust to the mass spec connection.

Subsequent injections should have larger peaks than the initial injection, as some of the active species will be chemically adsorbed by the catalyst. However, because a logarithmic scale is a default that is used for the y-axis, the peaks may potentially seem to be the same size.

In Figure 8, just before the nine-minute mark, part of the live data plot disappears. If this happens, the data is still being successfully written to the data file. Once the analysis is complete, the second trigger signal is sent to the mass spec to stop the recording of data. This completes the experiment and saves the data file in a format that is able to be imported by MicroActive.

Figure 8. Live recording of data during the pulse chemisorption experiment.

Importing the Mass Spec Data with MicroActive and Calculating Results

MicroActive’s versions 4.04 and higher include the capability of importing mass spec files and using this data for calculations. To do this, select the 'File' menu, and then click 'Import'. After this, browse to the Process Eye data folder. The type of file must be specified in the bottom right-hand corner. As shown in Figure 9, for files generated by Process Eye, MKS file ought to be selected.

The user can either add data to an existing sample file or create new files when importing data into MicroActive. This example shows a new sample being created. As shown in Figure 10, in order to create a new file, the user has to enter the sample mass, sample name, and the operator.

Figure 9. Selecting the mass spec data file to import into MicroActive.

Figure 10. Entering the sample information into the .SMP file.

The percentage of active metal (catalyst) in the sample needs to be specified for a pulse chemisorption experiment. This is achieved by clicking the 'Active Metals' button. As seen in the Active Metals table in Figure 11, the Pt-Al reference material of 0.5% Platinum needs to be entered.

Following this, the Analysis Conditions need to be entered in the manner shown in Figure 12. At this point, the description Mass Spec is automatically entered. Pulse Chemisorption should be selected for the type of analysis. It is optional whether or not the user enters the carrier gas and its flow rate, however, it is useful information to have as a reference.

In order for the appropriate stoichiometric factor to be applied in calculations, the analysis gas needs to be selected. One must select 'Loop injection' and then enter the calibrated loop volume.

Keeping the loop temperature at 0.0 degrees centigrade ensures that the full entered loop volume which was entered in the previous step is used in the calculations. Lastly, the entered atmospheric pressure and ambient temperature values ought to remain as defaults.

Figure 11. The Active Metals table.

Figure 12. Entering the Analysis Conditions into the sample file.

Under the Report Options menu, 'Pulse Chemisorption' should be selected. As shown in Figure 13, the number of peaks used for saturation can be edited. In this instance, the last five peaks are used. Adsorption occurs during the first two to three injections when using about 1 g of the Pt-Al material with a 0.5 cc injection loop and pure CO as the active species.

The number of peaks that are used for saturation ought to be edited to reflect the analysis conditions. 'Peak Editor-Mass Spec' needs to be chosen from the drop-down menu at the bottom of the sample file window after the sample information, report options, and analysis conditions have been edited.

If you click 'Find All Peaks', MicroActive automatically finds and integrates the peaks generated by the gas injections during the experiment. As displayed in Figure 14, the peak information will be shown in a table on the window’s left-hand side.

Figure 13. Editing the Pulse Chemisorption Report Options.

Figure 14. Using the Peak Editor in MicroActive.

To generate the pulse chemisorption report, click 'Save' followed by 'Preview'. As shown in Figure 15, the dispersion will be calculated and reported. Note that if the final sample mass is not entered, the dispersion values will be low.

During the analysis’ reduction phase, the sample will lose some mass through water loss. The mass of the final sample can be input into the analysis file and the report can subsequently be regenerated.

Figure 15. The Pulse Chemisorption report.

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.