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NH3 adsorption at high pressure

  1. May 10, 2007 #1
    Hi! Does anywone knows what happen with ammonia adsorption at pressures of 1-15 bar? thanks.:smile:
     
  2. jcsd
  3. May 10, 2007 #2

    chemisttree

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    According to the Langmuir adsorption isotherm theory, the fraction of adsorbed sites on a surface is increased as the pressure (or concentration) rises. Therefore as the pressure goes up, the adsorption goes up.
     
  4. May 10, 2007 #3
    Yes, O know. the problem is: vapor pressure of NH at 25C is 8 Bar. Do we have adsorption at 15Bar? secondly. When an adsorption is performed at very small pressures (below 0.5Torr = 0.000666 Bar) ammonia will recover gradually the different adsorption sites. iun accordance with this, different heats of adsortpion are released. firstly, the strongest sites are recoverd after that the lowest one. these heats are able to give information about surface energetics of the solid. When we work at so high pressures (more than 1Bar) are we able to have the same information? What really happen with ammonia. I think that all adsorption sites will be recoverd at the same time. i think that a multilayer will be frmed at the begining of the adsorption. What do you think?
     
  5. May 10, 2007 #4

    chemisttree

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    Yes.


    After populating all of the various sites (including the possibility of multilayer sites...) the application of a lower pressure will cause the more weakly bound sites to give up ammonia before the strongly bound sites will. You called this "...strongest sites are recoverd after that the lowest one." There is a heat of desorption associated with this but I don't know what it is. You also need some information about the substrate to answer that one... When even higher pressures are applied prior to the application of the lower (desorption) pressure there is a possibility that additional modes of adsorption may be observed. These will be extremely weakly bound ammonia and may be difficult to measure.

    I don't agree but I don't have any specific information for your system. It is likely that a monolayer will be formed over all of the sites at moderate to high pressure and at even higher pressures multilayers may form where space allows. Desorption would follow the general trend of multilayer > weakly bound > strongly bound in any case and should occur in definite stepwise (and measurable) fashion. Differences in the desorption isotherm after application of increasingly high pressure would indicate that something interesting is happening such as fracture, permanent surface change, reaction, etc...
     
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