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Pre-Activated Silver Screens

Background

Pure solid silver wire screens have been the preferred catalyst for decomposing hydrogen peroxide in rockets and torpedoes for a long time. The reasons are several:

-    High activity per unit volume. Small, compact chambers 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 favourable in these respects.)
-    The screens are easily re-activated with a simple nitric acid wash, after de-activation.

Pure solid silver wire screens also have some draw backs:

-    Untreated screens have a "slow start". They need to be used for some time before they reach full activity. This is why we pre-activate the screens.
-    They are very sensitive to poisoning by the stabilizers in standard grades hydrogen peroxide.
-    They melt if the hydrogen peroxide concentration is higher than about 89%.


My first testings

When I first tested the activity of untreated solid silver wire screens I found that they were almost completely inactive! This result is not surprising, having in mind that a solid silver wire has a very small surface. It does not help much that silver is an active decomposing catalyst metal.

In literature and patents one can read about different activation procedures. The most common are nitric acid wash and treatment with samarium nitrate. I tested both methods with very poor results. The activity increase was almost none.

Still, untreated silver screens and screens treated as above are reported to be used in rockets. What I can find though, is that rocket motors in these cases are first tested in bench tests. The first test firings seem to result in so called wet starts with hydrogen peroxide not being completely decomposed. After some time the catalyst activity will pick up. My theory is that the surface is enlarged at high temperature and oxidizing atmosphere.


Development of a pre-activation procedure

To avoid this unpractical run-in period I have developed a pre-activation method.

The procedure is as follows:

-    First I oxidize the screens. A layer of black silver oxide particles, Ag2O, is formed.
-    Secondly I heat the screens to just below the melting point of silver. At this high temperature, silver oxide is not stable. The oxide decomposes to metallic silver particles and oxygen. The silver particles are sticky because the temperature is close to the melting point and a porous silver skeleton with a big surface is formed.

At professional catalyst development the so called BET surface in m2/gram is measured, to characterize the catalyst. I have not been able to do such a measurement yet, but it is clearly visible to the naked eye that the surface changes completely from silver shiny to pale opaque white.

The surface is still mechanically strong. It is not possible to wipe off any material even at quite tough mechanical brushing.


Test Results

The pre-activated silver screens were tested, using the method described here. There was a huge improvement of the activity!

The decomposition rate when using purified hydrogen peroxide with concentration 35% and starting from 25 to 30 oC, was around 5 g H2O2/cm2, minute with the pre-activated screens. Assuming that the decomposition rate is proportional to the HP concentration it will be around 12 g H2O2/cm2, minute for a cold start with Rocket Grade HP87. The screen is 32 x 32 Mesh with wire diameter 0.27 mm. The thickness is ~0.5 mm. Calculated per volume unit the decomposition rate (cold start) for a pile of screens will be ~ 240 g H2O2/cc, minute (1 cc = 1 ml).

The decomposition rate did not decrease much when the test was repeated several times. When the screen was washed with 25% HNO3 for one minute, the activity was fully restored.

The pre-activated catalyst was still sensitive to poisoning by stabilizers. When standard grade HP35 was used, the decomposition rate was only a fraction of the rate for purified HP35.


Update, January 2005

1. Activity
We have now gained much more experiance in using the pre-activated silver wire screens in real rocket applications. We have found that one can design the catalyst chamber for an activity of 200 to 250 kg Hydrogen Peroxide per liter of catalyst and minute, but the catalyst can take only about half of this flow at the first operation for reliable operation without wet starts. So the catalyst does not have the full capacity from start, despite the pre-activation. It still needs some run-in time. It is still a big improvment compared to un-treated silver screens though.

2. Study of the catalyst characteristics
Mario Böhme, a diploma engineer student in Darmstadt Germany, contacted me. He offered to help me to study my catalysts with modern methods available at his university.
Of cource I accepted his generous offer! Thank you Mario.

Below are a few of Marios comparing photos of the silver screens, before and after treatment, at different magnifications. The untreated screen is always in the uper part of the photo and the treated screen on the lower part.


Untreated and treated screen. 60 times magnification


Untreated and treated screen wire. 2000 times magnification


Untreated and treated screen wire, 10 000 times magnification