Dear All,
I was carrying out CO pulse chemisorption dispersion with an AMI-200 catalyst test instrument. I hope to find the percent of metal dispersion on Pd nanoparticle catalysts.
One thing that worries me is that the area (or the TCD signal) is not monotonically increasing with the number of pulses like a normal S-curve. It starts high in the first pulse, have a dip in the 2nd and the 3rd pulse, and then gradually go all the way up to saturation (please see the attached file).
I'm not really sure what was happening during the measurement. I repeated the experiment and still see the similar curve. The quartz I used to hold the catalyst is new and clean, so contamination issues can probably be ruled out.
Please let me know if you've seen the similar thing before. What might be the explanation? Thank you!
My recipe of the CO chemisorption process:
Step 1: Raise temp to 250C and reduce ~80 mg of Pd/SrTiO3 catalyst under H2 for 30 mins. Then cool down to RT under He flow.
Step 2: 25 pulses of CO injection at RT.
It is because CO is not always an innocent adsorbate. CO induces morphology change of Pd clusters due to the formation of Pd carbonyls may lead to the change of adsorption profile from normal S curve. If you want to determine Pd dispersion, TEM combined with EXAFS are more reliable.
Probably you should check your pre-treatment technique first. YOu can run TPR profile for catalysts followed by Ar treatment and let gases pass through TCD and check at what temperature Pd is completely reducing. When running pulse, i advice you to do TPD follwed by pulse that will give the CO binding with Pd ( whether linearly bonded or bridge bonded)
Step 1 is correct. But you have to choose amount of catalyst and volume of CO pulse so that the first 2 or 3 pulses fully disappear. Then report what happens. You can conveniently read the many papers I have published on CO chemisorption over Pd.
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