Extracting Hydrogen from water
mo at ccr.org
Mon Mar 22 16:17:28 CDT 2010
the original "jetpack" seen in various TV shows used hydrogen peroxide
as fuel (something like 60% or so - *very* strong). if you force H2O2
through a silver mesh, it gets very unhappy, turning into O2 + STEAM
with great vigor. the hot gasses went out the nozzles pointing down
and off you go! (insert "simple matter of programming" snark)
there are also designs that inject some other fuel, like ethanol,
into the dissociation reaction, resulting in
CO2 + REALLY REALLY REALLY HOT STEAM
do *not* try this at home!
as for how catalysts work, it's about as close to magic as
physical chemistry comes. that's an entire sub-specialty
of chemistry - "catalysts and surface reactions". i know just
enough to know that i don't know squat about it.
On 3/22/10 11:23 AM, Alex Fraser wrote:
> Reread both replies under the influence of Kenyan coffee. I get it.
> I was on a email list for people trying to build a rocket using hydrogen
> peroxide squirted over a catalyst for power. I found it fascinating. I
> never actually had an explanation of how a catalyst works. I wonder if
> the process could be affected by an electric field? Victory in this game
> seems to be not losing too badly.
> Philip Miller Tate wrote:
>> Close, but not quite there (no slight intended).
>> All of the energy (work) used to split H2O into H2 and O2 is 'stored'
>> as potential energy in the isolated gases, ready to be released upon
>> recombination. Unfortunately, the second law of thermodynamics demands
>> that some of the energy stored is released as heat, because the
>> product (a liquid) is less disordered than the reactants (two gases).
>> Disorder, aka entropy, must increase in both the splitting process and
>> the recombination process. In the splitting, the disorder is in the
>> materials - liquid produces gas. In the recombination, the disorder is
>> created in the surroundings as heat. If we just let all the energy out
>> as heat to escape throughout the Universe, we can release the lot.
>> However, if we extract the energy as useful work (we want an engine,
>> not just an explosion), some has to be released as heat anyway to the
>> environment. Hence, work out (usable energy) must be less than work
>> in, in this and all similar situations.
>> The suggestion of a catalyst is a typical misdirection. A catalyst
>> cannot improve the inherent efficiency of any process - it can only
>> increase the rate at which a chemical process occurs. Even so, there
>> is always a down side. No catalyst remains active forever, and will
>> eventually need replacing. (It may be possible to regenerate it, but
>> that will take more energy.) The most-recently announced fuel-cell
>> cars will have this problem, and with precious metal catalysts,
>> commonly platinum and/or palladium, replacement will be enormously
>> expensive. Furthermore, there may not be enough catalyst metals for
>> every car driver to have one...
>> Everything costs.
>> Phil M1GWZ
>> On 21 Mar 2010, at 19:25, Mike O'Dell wrote:
>>> splitting water into hydrogen and oxygen is simply a way
>>> of storing energy - the energy that went into splitting the
>>> molecule of water.
>>> the energy produced by the recombination of that hydrogen
>>> and oxygen is limited by the laws of thermodynamics to
>>> being *LESS* than the energy that went into splitting.
>>> what is going on here is mis-named in the headline;
>>> it is nothing like photosynthesis - it is simply electrolysis
>>> with the electricity coming from silicon PV cells.
>>> the contribution is a catalyst which improves the efficiency
>>> of water electrolysis. in normal circumstances, the process
>>> is not very efficient - it takes far more electrons to split
>>> the water molecule than will be produced in recombination.
>>> the catalyst acts by making it take less power to split
>>> the water molecules, so the round-trip loss is less.
>>> (round-trip meaning water to H2 and O2 and back to water)
>>> the net result is that for the same solar flux, more hydrogen
>>> is liberated than would be without the catalyst, but all of
>>> it is still subject to the laws of thermodynamics:
>>> (1) you can't win
>>> (2) you can't break even
>>> (3) you can't get out of the game
>>> all of this ignores energy required to store hydrogen.
>>> a kilogram of hydrogen is *LARGE* at standard temp and pressure,
>>> while a litre of hydrogen at STP is bupkis when it comes to energy
>>> generation in a perfectly-efficient fuel cell (of which there
>>> ain't none). so storing H2 requires further energy for compression,
>>> further reducing the round-trip efficiency.
>>> even rocket engines that burn H2 and O2 (eg, Saturn V second and third
>>> stage engines and the reincarnation, the S1X for Ares) can't get
>>> enough density with just liquid hydrogen. it's actually stored as
>>> a "liquid hydrogen slushy". NASA discovered how to use nickel as a
>>> catalyst to make LH2 start to freeze into slush without lower
>>> temps or higher pressure. this increased the density of the LH2
>>> sufficiently to make it a viable rocket fuel.
>>> Hydrogen is just hard.
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