DECOMPOSITION CATALYSTS

A catalyst that decomposes the hydrogen peroxide into oxygen and water is instrumental to any rocket. It doesn't matter how potent your fuel is if you don't have a good catalyst to decompose it! The reaction that you are looking for is of course:

2H2O2 (liquid) -> 2H2O (steam) + O2 (gas)

yielding three moles of gas for every two moles of hydrogen peroxide, and a great deal of heat to help the reaction along as well.

Hydrogen peroxide rockets were first developed in Germany at the time of WW II. The first decomposition catalyst used was a liquid spray: -Calcium permanganate solution.

The technology was further improved by British, American and other scientist after WW II and since then silver wire screen catalysts that are cut into circles and stacked/piled to a cylinder, have been the preferred catalyst for decomposing hydrogen peroxide. The reasons are several:

- The solid and fixed catalyst bed is a reliable, simple and robust design solution.
- High activity per unit volume. Small, compact catpacks can be used.
- High mechanical strength. No dust or debris even at high flow velocity.
- The piled screens configuration gives a turbulent flow at a moderate pressure drop with even distribution and fast mass transfer. (Monolithic channel configuration or pellets are less favorable in these respects.)

Peroxide Propulsion is producing two types of silver catalysts:


Pure solid silver wire screen catalyst



Pre-activated
The catalyst is pre-activated by enlarging the surface area. This is achieved by
oxidizing the silver to silver oxide , followed by reduction back to silver metal
by heat treatment.
20 x 20 mesh
Wire diameter 0.35 mm (0.014")
The screen is 0.7 mm thick (= 2 wire diameters)
Silver purity: >99.9 % Ag

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Silver plated stainless steel wire screen catalyst



20 x 20 mesh
Wire diameter 0.35 mm
The screen is ~0.75 mm thick, incl. the silver layer, 2 x ~25 um thick.
The screens are thoroughly cleaned before the electro plating. A thin layer of nickel
plating is applied before the silver plating in order to improve the bond between the
layers and the SS material.
The silver plating is done at high current density to create a “rough”, porous silver
layer with a large surface area.
Finally the screens are heat treated in order to improve the mechanical strength of the silver layer.

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The silver plated stainless steel wire screens have the highest decomposition capacity, because the plated layer is more porous and has a larger surface area in contact with the hydrogen peroxide. They are also somewhat lower in cost because the costly silver layer is only 20-30 um thick


Pure solid silver wire catalyst at the inlet end of the catpack.

We recommend to install the pure solid silver wire screens (15 -20 screen circles) at the inlet end of the catalyst package. This is because pure solid silver wire screens can better withstand the “hit” by high velocity liquid drops and gas before complete vaporization. The liquid micro drops are very hard and will cause abrasion.

If the catalyst has such a porous structure like plated screens have, there is a mechanism of liquid gasifying inside the pores, making the pores to burst because of the expansion. The plated silver layer would quickly strip off. Pure solid silver wires can better resist this effect.

Finally the silver will in fact also oxidize to AgO and Ag2O in the inlet end as long as the temperature is still low because hydrogen peroxide is a strong oxidizer! (At a test it took only 12 minutes to totally corrode a 0.25 mm diameter solid silver wire when dispersed in a beaker with liquid rocket grade peroxide! The wire simply disintegrated and “disappeared”! A colloidal silver oxide, AgO, dispersion was all that remained of the silver wire!) – A thin layer of plated silver would corrode much quicker!


Silver plated SS screens in the remaining outlet part of the catpack

As soon as the tough conditions at the inlet end of the catpack is fading out, we recommend to use the more active silver plated SS screens. Typically ~100 screens are needed to completely decompose the peroxide and avoid wet/”rough” starts.

The temperature becomes very high when the decomposition reaction becomes more and more complete towards the outlet end of the catpack. Silver oxide particles coming from the inlet end of the catpack where they have been formed, will then reduce back to metallic silver and precipitate/stick to the screens at the outlet end!

At >130 oC: 2 AgO → Ag2O + ˝ O2
At >330 oC: Ag2O → 2 Ag + ˝ O2


Inspections of the catpack screens

The oxidation/reduction phenomena makes silver material to migrate from inlet to outlet and may gradually block the screens at the outlet end and make the flow resistance/pressure drop to increase. We recommend one should therefore take apart and inspect the screens as often as practically possible. Partly blocked screens can be moved to a new section/position with less precipitation/clogging and partly stripped screens can be moved to a section with Ag precipitation.
Oxidized and blackish screens at the inlet end can be washed with Nitric Acid or heat treated (> 330 oC) to restore the activity. By making such rearrangements and treatments one can extend the lifetime of the catpack.
It can be a good idea to have some screens in spare so the worst effected screens can be scraped and replaced if found necessary at the inspections. Let´s say 10 spare screens or so, of both types.


Ceramic platinum catalyst



In addition to the two silver catalysts, we are also selling a ceramic platinum catalyst.
12 x 12 mesh
The screens are 1.4 mm thick

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This catalyst is best suited if the hydrogen peroxide concentration is above 90%, as silver catalysts may melt at the temperature generated by high concentration peroxide. The catalyst is not poisoned by phosphorous containing stabilizers present in technical grades hydrogen peroxide as is the case with silver catalysts. The activity is also higher, but the durability is not as long as solid silver wire catalysts.

The catalyst is made of a stainless steel wire screen, 12 x 12 mesh. The screen is plasma coated with a quite solid and high strength ceramic coating. The bond between the plasma coating and the steel is very strong.

A porous ceramic wash coating layer is applied on top of the plasma coating and platinum is applied. The wash coating has a strong bond to the plasma coating, but the porous layer still tends to wear off over time at usage.


The experience so far is that it is more active than solid silver catalysts and at least as active as silver plated SS screens. It also gives a lower pressure drop.

We have studied the surface of the catalyst under a microscope. There is a large surface area present, which is clearly seen under 2,000 times magnification.

Ceramic Pt catalyst 100 times magnification


Ceramic Pt catalyst 2000 times magnification

Our palladium catalyst has been developed for us by Catator


Sizing of the catalyst package

We have prepared an excel program, here, as a tool for sizing. For those of you who prefer words before theoretical formulas we present a more descriptive sizing method in the following:

1. Decide what thrust the rocket needs to have, Thrust = F
For example a Rocketbelt needs to have enough thrust to lift the gross weight of the belt filled with fuel plus your own weight, let´s say 150 kg, so the thrust needs to be minimum 150 kgf (The thrust force needed to lift the mass 150 kg is called 150 kilopond, kp, or 150 kgf or 150 x 9,91 Newton or 2.2 x 150 poundforce, lbf. -Units is a hassle!)

If you instead are going to build a rocket that is moving horizontally, like for a drag racing car, it is more interesting to calculate what thrust force is needed to achieve a certain acceleration. We have prepared a small excel program for making this calculation, that you can download here.

If expressed with words:
The thrust in Newton equals the mass that is going to be accelerated in kg, times the acceleration in meters per seconds square.

F (Newton) = m (kg) x a (m/s2)


2. Calculate the fuel consumption

From the rocket thrust one can easily calculate the fuel consumption by dividing with the Specific Impulse, ISP for rocket grade hydrogen peroxide.

ISPRGHP 90%, 20 bar = 130 seconds (= 130 kgf per kg/second or 130 lbf per lb/sec. Unit hassle again!)

Fuel flow, W = Rocket thrust, F/ISP

For example a rocket belt: W = F/ISP = 150 kgf/130 = 1.15 kg P90/sec = 69 kg/min


3. Calculate the diameter of the catalyst package

Our recommended cross section area load of peroxide flow across the cat pack, L, is 0.014 kg/mm2, minute (= 20 lbs/inch2, minute)

Cross section area of the cat pack in mm2, A = Fuel flow (kg/min)/Load

In the example with the rocketbelt: A = 69/0.014 = 4929 mm2

Diameter of the cat pack, D = √ (A * 4/π)

In the example with the rocketbelt: D = √(4929 * 4/3.14) = 79 mm (= 3.1 “)

4. Length/Depth of the catalyst package

With this load, L, our recommended depth/length of the pack will always be ≥ 3 inch (≥ 75mm, ≥ 100 screens) for a small rocket as well as for a big rocket. Only the diameter, but not the length will vary!

If the load is higher than 0.014 the flow restriction/pressure drop will be too high and the length is needed to be increased. If the load is lower the depth of the catalyst needs to be very less and the flow distribution over the cross-section area will not be even and efficient. At Load=0.014 the pressure drop is around 30 psi/inch = 2bar/inch. Totally 3 inch length gives around 90 psi = 6 bar pressure drop.

This article was updated on March 19th, 2010