Decomposition Catalysts. Aug.-03
MnO2 CATALYSTS
The decomposition catalyst I have used up to now is a MnO2 catalyst. This is a low cost catalyst and it is robust: - High stability hydrogen peroxide can be used. Variations of this catalyst have been used since the days of the Second World War.
It is prepared by impregnating a porous support material - in my case aluminum oxide pellets -with a permanganate solution. The permanganate is highly active as a catalyst, but because it is soluble in the hydrogen peroxide it will be washed out from the support material at use. Therefor the impregnated catalyst is heat-treated. The permanganate is than reduced to solid manganese oxide, MnO2, which is also an active catalyst.
The MnO2 catalyst is not as active as silver catalysts ?see below. The catalyst bed has to be made bigger. The pressure drop is also a problem ?especially if the particles brake.
These drawbacks can at least partly be avoided by using a monolithic shaped body as a support material. These bodies have many narrow parallel channels, like an automobile catalyst. Development work with this type of catalyst has been reported, but to my knowledge it has yet not been used in practical applications.
Permanganate solutions have been used as a liquid catalyst. In some of the Walter engines, like Me163, a permanganate solution was contacted with hydrogen peroxide in the decomposition chamber. In some cases the calcium salt was selected, rather than the potassium salt, which is a common and cheap chemical. The reason is that the calcium salt is much more soluble.
SILVER CATALYSTS
After the Second World War, silver catalysts took over as the most common technical solution. They are still used in torpedoes. They were also used in the Bell Rocket Belt and in the Intora Firebird helicopter. These catalysts are more sensitive to poisoning and therefor demand very pure hydrogen peroxide, but they have a higher activity. They also have other important advantages. like being mechanically strong and leaving a clean exhaust gas.
There are two types:
* Pure silver wire screens.
* Silver plated screens of nickel wire or some other metal wire.
Preparation of silver plated nickel wire screens.
I have now prepared and tested silver plated nickel wire screens.
The nickel screens are 28 x 28 mesh, with a wire diameter of 0.19 mm
The silver plating is made electrolytically - see drawing on the top of this page. This procedure is also known as galvanizing. The electrolytic cell has a silver anode and the nickel screen is the cathode. The electrolyte is a solution of silver cyanide.
The procedure is similar to what silver smiths are doing when they are silver plating jewelry or cutlery, but with one important difference: The silver smith want a solid, bright and shiny surface, but the catalyst need to have a rugged, porous surface. This is mainly controlled by the current density. To get the right kind of surface for a decomposition catalyst one need a high current density - at least 4 amps/dm2.
The plating structure does not get enough mechanical strength, because of its porosity. Therefor the plated screens are needed to be heat treated at 800 oC, before use. (The melting point for silver is 925 oC). I haven?t acquired an oven, because one of my neighbors who happens to be a potter, is kind enough to let my use her electric pottery oven. Thank you, Anna Lena!
Test Results
So far I have not used the catalyst in real rocket firings. Instead they have been tested, using the quantitative volumetric method, described in the June development report.
Using extra pure HP
When using extra pure 33% conc. hydrogen peroxide at 25 oC starting temperature and 2.5 cm x 2.5 cm squares of the silver plated screens, the 3 liter plastic bag was typically filled in 20 to 24 seconds. This result corresponds to decomposing 3.6 to 4.4 g H2O2/minute and cm2 of silver screen. Assuming that the activity is proportional to the HP conc., the activity would be 2.6 times higher at 85% HP or 9.4 to 11.4 g H2O2/minute, cm2
When running the test with the same piece of screen a second time, the activity was typically only about half of the activity at the first run.
At a third test run with the same piece of catalyst the activity was only a small fraction (~5 %) of the original starting activity.
Using high stability HP
When using high stability 35% conc. hydrogen peroxide, the activity was low already at the first test: ~0.3 g H2O2/minute, cm2. Re-calculated to 85% HP it would be ~0.75 g H2O2/minute,cm2
At repeated testing of the same piece of catalyst, the activity became even lower -even if the decrease was not so dramatic.
At testing with high stability HP the catalyst pieces turned black, while when using extra pure HP they staid white!
Calculations and Conclusions
Capacity
The initial capacity with extra pure HP85 was found to be 9.4 to 11.4 g H2O2/minute, cm2.
Example: If punching 1" discs, with a cross section area of 4.9 cm2, 100 such discs would have an area of 490 cm2 and be capable of decomposing 4.6 to 5.6 kg H2O2/minute. This is for a cold slam start - as soon as the catalyst pick up temperature the capacity will be much higher.
This result is well in line with what has been reported in literature for this kind of catalyst.
Life Time
The tests shows that extra pure HP is needed for this kind of catalyst.
The life time is needed to be verified at real rocket firings, but already now an estimation based on the performed tests can be made:
At each activity test, the piece of catalyst was contacted with 200 ml HP. The test pieces had an area of 2.5 x 2.5 = 6.25 cm2. In the example above, the pile of 100, 1" diameter discs have an area of 490 cm2. This catalyst pile would be effected in the same way if contacted with the equal amount of HP per cm2, which would be 15.7 liters of hydrogen peroxide or about 4 minutes of operation.
My preliminary conclusion is that the expected life time of this catalyst is a few minutes only, when using my quality of extra pure hydrogen peroxide. To receive the real long lifetimes of 30 minutes and more that have been reported in the literature, the hydrogen peroxide probably needs to be ultra pure!
Update January 2005
Mario Böhme, a diploma engineer student in Darmstadt, Germany, contacted me. He offered himself to caractarize my catalysts, using modern methods on his university. Of cource I accepted his genereous offer! Thank you, Mario.
Below a few of Marios photos of the silver plated screens, with different magnifications.
This article was updated on July 18th, 2007