I've mentioned it before, but never done it. Tonight I tried.
Apparatus:
I built a vaporizing chamber out of copper pipe sections silver soldered together. Two holes were drilled in the side, one to admit air and the other
proceeding to the catalyst tube. 1/4" copper tubes were placed into these holes, fitting tightly enough not to need solder. The catalyst tube was a
piece of 1/4" copper tubing bent into a tight coil of two turns. The end went into a 300ml RBF.
Use:
The vaporizer is filled with urea prills, the tubes installed and the urea melted by flame. The catalyst tube was heated to orange hot (about
700-800C) and air puffed through the apparatus, blown by mouth. When the vaporizer is heated strongly and air blown in, fumes blow out of the joints.
Most of the air goes through the catalyst tube.
When the tube is cold (below about 400C), white fumes are given off, presumably, urea not entirely decomposed. When the tube is hot, a clear gas is
produced. When this gas is trapped in an RBF, some moisture condenses on the walls and, varying with conditions (vaporizer and catalyst temperatures,
air supply, etc.), a light brown gas is sometimes present in the flask. The gas smells like NO2 (who would've guessed).
After some time of this, a few drops of sodium carbonate solution was added, but no efforvescence was produced. Also, some flakes (presumably, copper
oxide scale) had blown off from inside the catalyst tube, into the flask, and were not dissolved appreciably.
Conclusion:
Some NOx was produced. With an awful lot of optimization, it might be viable.
Timnot_important - 17-2-2008 at 03:25
Those white fumes could well be cyanic and/or cyanuric acids.
It takes fairly tight controls on the oxidation conditions to avoid too high a percentage of formation of N2 instead of NOx.7he3ngineer - 20-2-2008 at 21:45
Sorry to take a few days to bring up this post again, but have the bloody flu!
This process interests me, but there are some details that I'm unsure of.
In the prepublication section, Tim stated:
Quote:
I've been interested in seeing urea vapor and air passed over a copper catalyst to produce NO, and thus NOx, and thus HNO3.
Would I be right in saying that the aim of this process is to simply produce nitric acid by the standard process from ammonia, but taking this a step
backwards to produce the ammonia from urea (essentially CON2H4 -> NH3 -> NO -> NOx -> HNO3). Assuming this is right...
When urea is heated well over its melting point (132C), besides evolving ammonia, as not_important said, ammonium cyanate (a white sublimate left on
the walls?) and cyanuric acid are produced, in addition to biuret (NH2CONHCONH2 - although decomposes by 190C). Was the biuret reaction the intended
means for ammonia production?
ie, 2CON2H4 -> NH2CONHCONH2 + NH3
To avoid the ammonium cyanate, etc production, would it be better to instead heat the urea with an aqueous caustic alkali (say NaOH) to hydrolise the
urea to ammonia and to (hopefully) just be left with the corresponding carbonate (CO(ONa)2)?
Why the Cu catalyst? Was this to favour the biuret reaction rather than producing ammonium cyanate, etc (I could find nothing about this), or was this
some form of substitution for the platinum catalyst generally used to favour NO over N2 production from NH3?
Regarding the problems faced, there are really tight controls:
In the reaction 4NH3 + 5O2 -> 4NO + 6H2O (with catalyst - I'm sure it's Pt!), a ratio of about one volume of ammonia to nine of air (maybe 11 of
breath!) is necessary - are you sure this being achieved by blowing through the tube?
The transition from nitic oxide to nitrogen peroxide, should occur around 200 - 250C (too hot/cold? - didn't sound like this was a problem though).
I don't quite follow how you were trying to collect the NOx in the RBF, didn't you want to pass it through water at this stage (similar to axehandle's
setup) or is this to be done in a later continuation/attempt of the experiment.
I'm actually a little surprised that you were more interested in this than the Birkland-Eyde reactor (with the application of high currents, a nice,
big neon transformer and the like), is this because this method already so well explored and documented on this forum.
Josh12AX7 - 21-2-2008 at 00:16
I don't have an NST.
I was trying to burn urea in one step (not necessarily one chemical step) to NOx. I wasn't trying to collect the products in any specific way;
putting it into an empty flask would at least tell me if it's turning brown at all, which it did at times. I wasn't hoping to make practical amounts,
but I would've liked to see a big cloud of brown.
The copper tube was thoroughly oxidized I'm sure (flakes of scale blew out at one point), so it was an oxidizing atmosphere. Whether it was oxidizing
from free O2 or NOx or just because urea may not be a good reducing agent, I don't know.
It's my understanding that copper is a good (or good enough) catalyst for this, although I'm not exactly clear on how it's supposed to work if it
readily oxidizes to CuO under these conditions. Then the question is, is CuO? I don't know if I have anywhere near enough surface area, either. I
recall I have a few chunks of catalytic converter honeycomb on hand, maybe there's enough platinum in a strip of that. I could push a piece into a
copper pipe and solder it closed.
