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In certain chemical reactions, catalysts can be used to decrease the activation energy of those reactions. For example, platinum is a catalyst for hydrogen/oxygen reactions, so, therefore, the resulting gases have more energy leading to a higher ISP. Right?

If so, then I understand why there aren't platinum catalysts in the reaction chamber of the engine as it would melt the platinum. However, what would be the issue with suspending the platinum within the hydrogen fuel to form a colloid, therefore removing that aspect of the problem?

The video linked is an interesting watch too

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    $\begingroup$ Not all of us know - I don't for example :) Can you back that up the statement "platinum decreases the activation energy of hydrogen/oxygen reactions so therefore the resulting gases have more energy leading to a higher ISP" with a link - something I and others here can read and study? Thanks! $\endgroup$ – uhoh Oct 3 '16 at 10:03
  • $\begingroup$ The issue? How about cost? $\endgroup$ – Hobbes Oct 3 '16 at 10:46
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    $\begingroup$ @uhoh Thanks for this! I don't know why I included that sentence. I've included a paper and a great video by university of Nottingham. $\endgroup$ – Boris Deletic Oct 3 '16 at 11:15
  • $\begingroup$ @Hobbes Platinum costs US30,000 dollars per kilogram. The colloid that you would create would consist of nano particles (2nm in diameter) of platinum. With 1kg you could create a lot of these nano particles, I haven't looked at any numbers yet, however, I believe that that you could feasibly create a colloid in the space shuttle tank of 1.5 million litres with 10kg of platinum. That's $300,000, not too ridiculous if it improves the performance of the rocket significantly. $\endgroup$ – Boris Deletic Oct 3 '16 at 11:26
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    $\begingroup$ And is a colloid even maintainable in the H2 environment, i.e. won't the platinum (even nanoparticles) settle on the bottom of the tanks? $\endgroup$ – Jan Doggen Oct 3 '16 at 12:40
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Short answer: no, it won't increase the performance of the rocket.

Platinum (or other catalysts) can be used in fuel cells, where the purpose is to get a chemical reaction at (relatively) lower temperatures. That means without having to burn the gases, in other words. (It achieves this by reducing the "activation energy" of the reaction as the question correctly states.)

Combustion however does not need a catalyst. Once ignited the conditions within fire provide enough energy to get the reaction to proceed, no catalyst is needed.

So adding platinum to the fuel isn't going to have any overall effect on the combustion. (If you were adding something that actually combusted, that would be a different matter - but then it would just be a different sort of fuel. Similarly if you can show the platinum itself is combusting - which I doubt - then that might add some heat.)

It's fairly well-known chemistry that catalysts reduce the activation energy of the reaction, but not the overall heat (enthalpy, if you like) of reaction.

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    $\begingroup$ Thanks for the answer, I had no idea that catalysts don't change the enthalpy of a reaction, but it makes sense now. You can't get something from nothing. $\endgroup$ – Boris Deletic Oct 3 '16 at 22:39
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It is possible that the platinum would improve the performance, but not in the way you're thinking. Metal particles in rocket fuels have been used experimentally as a way of increasing the density impulse. This is discussed in chapter 12 of Ignition: an Informal History of Liquid Rocket Propellants by John D. Clark (available in PDF on the Internet if you look in the right places, but I'm not sure how officially so I won't link to them myself).

Mercury does work well in certain applications: its toxicity is a big problem, but its low melting and boiling points, plus its monoatomicity, are very favourable. Solid metal in suspension, however, has a tendency to settle; it was tried as far back as 1929 in Germany, and revived in the late 1950s in the US as part of a push to use thixotropic gels as propellant in order for the US Navy to mitigate the risk of propellant leakage, but when Clark wrote his book in 1972 he noted that

And there have been other aluminized fuels, but none of them are ready for operational use. In my own opinion, it will be a long time before they are operational, if they ever are.

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  • $\begingroup$ If you need density, Al seems like a suboptimal choice. Wouldn't lead outperform it? Or maybe is it Al's oxide coating that prevents it from reacting with fuel and lead doesn't have that? $\endgroup$ – Daerdemandt Oct 4 '16 at 9:33
  • $\begingroup$ The idea with Hg was purely to vaporise it, but the Al was actually being used as fuel. One of the problems was that it came with impurities, and when it was finely divided and dispersed throughout the propellant those impurities were able to react with the propellant, generating gas bubbles which affected the flow. $\endgroup$ – Peter Taylor Oct 4 '16 at 10:59

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