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Catalyst and hydrogen ionizition

  1. Jun 24, 2011 #1

    Edi

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    How exactly does a catalyst (platinum.. with carbon.. something?) removes the electron from a hydrogen atom? (that means that quite a lot of energy was given to the hydrogen atom and the electron and proton separately (and the two particle system together) has more energy now.) Does the energy come form materials internal kinetic energy/ kinetic energy of the molecules/ atoms? .. as far as I know, the energy of particles at room temperature is way below what is necessary for hydrogen ionization..
    (Im talking about fuel cell here.)
    .. and if im talking about fuel cells here, then - why it is even necessary to have oxygen at "the other end" ? .. why cant hydrogen ion (proton) just re-connect with the electron and ... just stay hydrogen? In this case the energy to the "load" would come from.. internal kinetic energy of the materials aka heat.
    .. or what?
     
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  3. Jun 24, 2011 #2

    SpectraCat

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    [ATTN MODERATOR] Please move this thread to Chemistry

    Those are good questions .. the full answers are quite detailed. The short version is that it happens electrochemically ... that explains both the ionization of the H-atom (the electron flows into the less negative potential afforded by the metal catalyst), as well as the need for the oxygen reduction reaction at the cathode ... the electrical circuit must be completed for current to flow. The overall energy to drive the electrochemical reaction comes from the exothermicity of the 2H2 + O2 --> 2H2O reaction. The catalyst (typically platinum) is required to overcome the activation barrier to splitting the H2 molecule into two (surface-bound) H-atoms.

    This site may be helpful for a conceptual (although fairly non-technical summary): http://www.fueleconomy.gov/feg/fcv_PEM.shtml

    If you have further questions about the details, please feel free to ask them.
     
  4. Jun 25, 2011 #3

    Edi

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    hmm, so the catalyst, with its chemical properties, sucks up the proton (form hydrogen) and leaves out the electron.. ok, but what exactly is the chemical reaction?

    that catalyst reacted with the proton and changed its chemical composition.. then the proton is released again and reacts again an is released again until it reaches the other side, where it reacts with .. oxygen? and the electron it lost previously (but oxygen "wants" electrons, not positively charged particles .. ? ) and why does it keep reacting and self destructing in that one particular direction?
    What is the reaction for that "self destruction" where proton is released again? (assuming what I wrote above is at least remotely close)
     
  5. Jun 25, 2011 #4

    Edi

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    mm, and if the catalyst provides that "activation energy" spiting hydrogen, then after the split, the platinum catalyst molecule is in a lower energy state than before, right?
    I dont know the chemical reaction jet, but... assuming it is unstable and decays, then, after the proton is emitted, the molecule is in a higher energy level/ the level it was before, right... ? And now it is able to suck up another proton.. and do what catalyst do - reacts chemically, but is not consumed ..
     
  6. Jun 25, 2011 #5

    alxm

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    Nope, a catalyst's net energy doesn't change during the reaction (or it'd be a participant in the reaction rather than a catalyst). They don't 'provide' the activation energy, they just lower it.

    To make a mechanical analogy, a reaction is like a ball rolling over a hill (the reaction 'barrier'). As long as the valley on the other side of the hill is lower, it doesn't require any energy to get from one side of the hill to the other (neglecting friction, which is fine since it doesn't exist in the chemical context). But unless the ball's kinetic energy is large enough to take it over the hill, it can't get there.

    In that analogy, a catalyst is like a tunnel through the hill. It enables molecules with lower kinetic energy to get from one side to the other, but it doesn't expend any energy in doing so. It increases the rate of reaction, since a greater proportion of molecules will have sufficient energy to react, but it doesn't necessary speed up the reaction itself (the 'tunnel' isn't necessarily shorter than the direct route, it just has a lower elevation)
     
  7. Jun 25, 2011 #6

    Edi

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    ok, step by step.
    What exactly is "activation energy" and where does it apply?
    is that the energy required for hydrogen to react with oxygen or the energy to brake through that electrolyte (PEM) ?

    if it is the second .. then it kinda actually does make sense. As the catalyst lowers the energy for the proton to brake through but NOT for the electron. (due to the chemical interactions, which I still dont know the exact reactions..)
    Then the question remains - how does the electron get knocked off? Could I think of it like a car, that drove in a tree and the driver flew out, but the car stayed there? That is - there suddenly was something, that lets something pass, but something - to stop.
    So, in this case, electron would be knocked off due to the physical interactions, kinetic energy.. (witch increases as the H+ starts to get attracted by oxygen and moves faster.. witch is sort of what chemical energy is.. but I like to think in terms of physics.)
     
  8. Jun 25, 2011 #7

    SpectraCat

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  9. Jun 26, 2011 #8

    Edi

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    ok, so here you guys are talking about activation energy.. for hydrogen to react with oxygen?
     
  10. Jun 26, 2011 #9

    SpectraCat

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    That would be the overall activation barrier for the direct reaction of H2 with O2, so in the abstract sense, yes, that is the barrier that requires us to use a fuel cell (or a combustion reaction) to access the energy released in the reaction. However, that particular activation barrier is irrelevant to what actually happens in the fuel cell ... H2 and O2 never contact each other.

    The activation barrier I was referring to is the one for dissociating the H2 molecule into surface-bound H-atoms, so that the electrons can be separated from those H-atoms (i.e. they can be oxidized). The electron doesn't get "knocked off" the H-atoms .. what happens is that when the H-atoms are bound to the platinum catalyst, they are in a higher energy state, and the energy level can be lowered by oxidation of the H-atoms, so that the electron "naturally" (i.e. spontaneously) flows into the anode and the H+ ions are released.
     
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