Metallus
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How to TOTALLY dissolve a solid
Hi there
I'm currently working with a team in a lab to test catalysts. We are trying to dissolve a catalyst composed mainly by Al, Zn and Cu. We analyzed this
thing with XRD and XRF and the latter showed that only 80% of it was composed by metal oxides. The guy told me that there was a 20% of "something" but
he didn't know what. We wanted to know exactly if there was more inside and so we tried to dissolve it in acids in order to later analyze it with AAS.
We thought that a simple acid digestion would have been enough, but it seems some inert material is in there. We thought of silicon dioxide or a
stable allumina and so we tried dissolving it in HF but with no results. We couldn't dissolve this thing.
The solid is an Al, Zn, Cu mixed oxide (by XRD patterns it looks like hydrotalcite phase) with, maybe, aluminium magnesium carbonate hydroxide
hydrate. The peak relative to the latter also fits with the graphite one.
We tried to dissolve it in:
1) hot aqua regia x4h under reflux: there was a very little bubbling in the beginning, most likely some carbonate decomposing to CO2 and H2O. The
other bubbling was HNO3 decomposing to red NOx fumes. A fine dark suspension was obtained. It got filtered and washed with water. I'll weigh it next
week.
2) Hot HNO3 x4h under reflux (EPA method 3050B): only a little bubbling was observed in the beginning just like above, then nothing at all. Even the
vapours were very clear and transparent, no formation of red NOx (so no oxidation occurring). This was followed by the addition of H2O2 30%, with
nothing much happening. The only bubbles were due to H2O2 decomposing at 100°C I think, since nothing happened when it was added to the cold
solution. Everything was evaporated and a viscous black "thing" was obtained (looked like some heavy MW carbon compound).
3) Hot HF for 2h: only an even smaller bubbling could be observed. Most of the fumes coming out were HF vapours. This was followed by the addition of
HNO3, with no reaction occurring, and later by H2O2 30%, that just decomposed. Everything was evaporated and a fine powder of the same color of the
starting material was obtained. It lost 60% by weight though.
4) Hot solution of saturated NaOH followed by the addition of H2O2 30% for 1h: nothing at all happened. The solution just got dirty and the H2O2
decomposed.
Apparently there is a 20% of "something else" but we are running out of options. We couldn't dissolve the solid even in those conditions. If it was
SiO2 or Al2O3, it should have dissolved in HF (1g in 15ml HF) and if it was any other metal oxide, the HCl/HNO3 mix should have dealt with it nicely
on its own but nothing. We can't get to dissolve this stuff.
Also, if there was some organic compound, it should have been destroyed under those conditions. We were working at ca 100°C, under reflux, for hours
with highly oxidizing substances.
We decided to try piranha solution on what remained from the HNO3 acid digestion (method 2) to see if it can get further dissolved, but we have little
hope in this one since the HNO3/H2O2 mix didn't give good results. We'll try this next week.
What else could this shit be? It's a dark gray pellet that was grinded finely. Our only hope was HF but not even that could scratch the thing (the HF
vapours scratched the fume hood glass tho, lol).
I had suggested them alkaline fusion but it looks like they never tried it in their life and weren't much convinced (they made me use HF tho, which is
more of a bitch).
We could try TGA on this thing, in order to see if there is some organic stuff in it. We didn't try organic solvents, so that could also be an option.
If it's some polymer, perhaps that would help a bit.
Do you have any suggestion on how to totally dissolve this stuff? (EPA method 3052B assures total dissolution of everything with a microwave digetion
in HNO3/HF mix at 180°C and 25bar but the microwave apparatus has been fucked up for 2 months already and I don't think they have appropriate vessels
for it anyways.
Thanks for your attention
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Magpie
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Quote: Originally posted by Metallus  |
I had suggested them alkaline fusion but it looks like they never tried it in their life and weren't much convinced (they made me use HF tho, which is
more of a bitch).
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You might try this alkaline fusion from Holtzclaw et al, College Chemistry with Qualitative Analysis, 7th ed:
"Fusion with Sodium Carbonate Silicates and certain oxides and calcined salts may not be taken into solution by treatment with acid or Na2CO3
solution or by reduction with zinc. Fusion with Na2CO3 is effective with many of these substances. Transfer the residue remaining after zinc
reduction of silver halides to a small nickel crucible. Add to the residue 100mg of anhydrous Na2CO3, about half as much K2CO3, and a few mg of
NaNO3. Place the cruible in a small clay triangle and heat it in the hot flame of a Meker burner until the mixture fuses. Cool the crucible, add 1
ml of water, and warm the mixture until the solid mass has disintegrated. Separate the mixture and reserve the solution. Treat the residue with 8
drops of 6M HNO3 and warm in the hot water bath for a few minutes. Separate and add this nitric acid solution to the original solution. Analyze the
solution for cations."
The single most important condition for a successful synthesis is good mixing - Nicodem
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deltaH
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I once assisted with the analysis of chromite rich mineral samples. They needed to be digested completely into solution for AAS analysis, but the
conventional HF/HNO3/HClO4 methodology failed.
What worked well was digesting it in molten sodium peroxide in zirconium crucibles over a torch flame, swirling with tongs. I don't have the exact
procedure available anymore, but you aught to be able to find it in the literature.
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deltaH
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BTW, is this a low-temperature water gas shift or methanol from H2/CO/CO2 catalyst?
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WGTR
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Fusion with sodium bisulfate in a test tube, or with molten alkali hydroxide in a nickel crucible (or something). I dissolved calcined
TiO2 in sodium bisulfate here:
https://www.sciencemadness.org/whisper/viewthread.php?tid=27...
Later in the thread, I ran some EDS spectra of glass:
https://www.sciencemadness.org/whisper/viewthread.php?tid=27...
This latter thing is part of my job, and I generally don't do freebies with that. It would have to go through my employer.
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Metallus
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Yes, more precisely for the high temperature shift.
EDIT
Anyways, I just did a bit of research and in some catalysts of this type it is stated that they add an inert material like MgAl2O4.
If this was the case, how am I supposed to dissolve it? I'm a fan of alkaline fusion since when I saw it destroying glass at a faster rate than HF,
but these guys just throw sulfuric and nitric acid at everything, hoping that it dissolves. How do I get them to understand that this is actually a
viable method?
Once one of them was trying to remove fused V2O5 stuck in a test tube. I told him to try with some diluite HCl and he was like "huh, I'll see". Not
much convinced, he asked an older dude who told him "Try sulfuric and nitric acid to oxidize it and remove it"... but what the fuck, V2O5 is already
in its maximum oxidation state, it's non-sense using an oxidizing solution... and in fact it didn't work. The guy later tried HCl and wonder what...
[Edited on 20-3-2015 by Metallus]
[Edited on 20-3-2015 by Metallus]
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Fulmen
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I was thinking fusing it with sodium carbonate, boric acid or perhaps a borate. I know borate is used for xrf-analysis of cement and it's raw
materials, I think it was either lithium thetraborate or metaborate.
We're not banging rocks together here. We know how to put a man back together.
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Sulaiman
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I am a novice so maybe this is rubbish,
if I wanted an inert substrate I might use something like asbestos,
I do not know how to test for that but you may.
Can you get a spark spectrum, that may give a clue to the composition?
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deltaH
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Low-temperature WGS  wow, thought those were traditionally iron-based and
copper used for the low-temperature shift. Crap... I'm getting old 
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Metallus
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| Quote: Originally posted by deltaH | | Low-temperature WGS wow, thought those were traditionally iron-based and
copper used for the low-temperature shift. Crap... I'm getting old
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Catalysts for the High temperature shift are traditionally iron/chromium based.
We are trying to synthetize a chromium free catalyst containing copper that doesn't suffer the sintering at the high temperature. In the past they've
actually managed to produce such chromium free active catalyst, we are just trying to improve it further. But ye, if you look into literature, you'll
find out that there have been developments in the WGS.
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