The NH3-TPD of commercial silica (S.A= 223m2/g) has been run on a Micromeritics 2900 AutoChem II Chemisorption Analyser to determine total acidity. However, there was no peak observed post-run.
The experimental conditions are as follows:
The material was initially pre-treated by heating to 400 ºC under a stream of argon (30 mL/min) at a rate of 20 °C/min for 30 min and then cooled down to 90 ºC under the same stream of argon. A 5 % NH3 in helium gas mixture was then passed through the system and allowed to adsorb onto the surface of the catalyst for 30 minutes. Helium gas (30 mL/min) was then passed through, while the temperature was ramped from 100 °C to 700 °C at a rate of 10 °C/min. The amount of ammonia desorbed was monitored using a TCD.
Why are no peaks observed? is there a possibility the surface hydroxyl groups (if any?) are too weak to interact with NH3 ?
At 400C you are already well into dehydroxylation region. You need to remove any water hydrogen bonded to the silanols of course to allow ammonia to do the same, but this should be complete by 150C (though water directly bonded to the silicon will need a higher temperature). This paper is informative in this regard: https://psec.uchicago.edu/library/photocathodes/SiO2_OH_model.pdf
A TGA on your as-received (as-synthesized) silica would be useful to understand when the various water groups are liberated.
Good luck.
Thanks for the response Martin A Thomas. TGA-DSC indicated a weight loss occurring at approx. 100 °C which was ascribed to physisorbed water. Thereafter the broad weightloss between 400 - 800 °C was the region where dehydroxylation was thought to occur.
Cheslin Moodley it is the water that comes off at 100C that will leave surface hydroxyls to which ammonia should be able to hydrogen bond (that wekaly bound water was on those hydroxyls). From the TGA the sample is well into dehydroxylation by 400C... and a dehydroxylated sample won't adsorb NH3. Try your NH3 TPD experiment again, with a fresh aliquot, and pre-treat by heating to 100 ºC under a stream of argon (30 mL/min) at a rate of 20 °C/min for 30 min and then cool down to 90 ºC before continuing with adsorbing NH3 etc.
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