Catalyst Activity Measurement, June -03
New Catalyst Activity Measurement Method
To directly measure the activity of a decomposition catalyst in rocket tests is time consuming and complicated.
There is a need of a quicker and simpler method.
One such simple method is called "drop test" and simply means that hydrogen peroxide is dropped on the catalyst .The activity is judged by observing how violent the reaction is, so it is not a quantitative measurement.
I have started using a simple quantitative method. It is very convenient and easy to use! The principle is based on volumetric measurement of formed oxygen gas at the reaction:
* 200 ml Hydrogen Peroxide is filled to a 1 liter Erlenmeyer flask (a conical flask with wide flat bottom).
* The temperature is adjusted to the desired level.
* A defined quantity of catalyst is added. (The quantity should be pre calculated to give a reasonable but not too fast reaction rate)
* A stopper, with an empty 3 liter plastic bag connected to it, is put tightly to the flask neck. At exactly the same time a stop watch is started.
* When the bag is filled with gas completely, the watch is stopped and the time of filling is noted.
CALCULATIONS
3 liters of oxygen gas is corresponding to 0.134 moles at 20 degree C and atmospheric pressure.
To get this quantity of oxygen, 2 x 0.134 moles or 9.1 grams of hydrogen peroxide is needed to be decomposed.
The catalyst activity, a, can than be calculated as follows:
a = 9.1 / (t x m)
were
t = the time for filling the plastic bag
m = quantity of catalyst added
Re-calculation of the measured activity to the expected activity at real "Rocket Conditions"
HP Conc:
So far I have used 35% conc. HP at the volumetric measurements.
Assuming the reaction is of the first order (=rate is directly proportional to the HP conc.), the reaction will be 87/35= 2.5 times as fast when using 87 % HP.
Temperature
I have done the measurements at 30 C. At real rocket fireing conditions the temperature will increase to about 650 C.
To re calculate between different temperatures one can use the Arrhenius Equation (Svante Arrhenius was a Swedish scientist born 1859 and a Nobel price winner in chemistry 1903):
k = A x e (exp. –Ea/RT)
I do not go into detail what the different factors and constants in this formula means, because I do not know the value of them anyway, at this time! The important thing is that one should be able to calculate them by running a few more measurements at different temperatures and concentrations and by comparing with more real rocket tests.
RESULTS
So far I have been running just a few measurements.
The most interesting one, was run on fresh MnO2 catalyst deposed on AlOx pellets:
The 3 liter bag was filled in 8.1 seconds when using 5 grams (= 5 ml) catalyst.
The activity can than be calculated to 0.22 gram H2O2/sec., gram catalyst.
The test was run with 35% HP. If using 87% HP, the activity would be expected to be 87/35 x 0.22 = 0.55 g/sec,g. With other units: 32.8 kg H2O2/liter catalyst, minute.
At 650 C as at real rocket fireing conditions the activity could be expected to be several times higher.
I was running a second measurement with the same catalyst. The activity had than dropped dramatically! The bag filled in 1 min and 10 seconds.
You can compare these results with the real rocket testings I did with this catalyst in May.
CONCLUSIONS
This catalyst has an activity, well above 30 kg H2O2/liter catalyst and minute, at the first time it is used.
Already at a second start, the activity is dramaticly lower -at least with my present HP quality. It is rocommended to replace the catalyst between each start.
Development Report, August-03
This article was updated on October 24th, 2004