# dissolution of Nishimura catalyst (Rh/Pt alloy)



## tensor9 (May 23, 2012)

Well, technically Nishimura catalyst refers to 45% Rh/19.5% Pt-oxide. But after hydrogenation, the actual catalyst is a finely divided Rh/Pt "alloy" (it may not technically be an alloy).

I was browsing the forums and noticed a thread on Rh dissolution. 85% sulfuric acid was suggested, which is good because I've looked up and down the literature trying to figure out how to dissolve Rh to no avail. 

Anyway, pure Rh is pretty resistant to aqua regia, but alloys of Rh and Pt are supposed to dissolve readily in AR, but I have had no such luck. Even the finely divided powder refuses to completely dissolve in AR. Any thoughts/ideas?


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## lazersteve (May 23, 2012)

Have you given HCl + Cl2 a shot? I've had very good success with this when dissolving Pt alloys, especially in the finely divided state.

You can make the chlorine in situ by several means of course. I prefer the methods that do not produce salt (NaCl) as a by product.

A little heat (60C or less) and stirring helps things along.

Steve


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## tensor9 (May 23, 2012)

I haven't tried chlorine yet. Maybe I'll git it a shot soon.


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## Lou (May 23, 2012)

If it is fine, just dissolve in hot sulfuric acid, dilute and filter through quartz wool. Give that another sulfuric treatment---no yellow/brown means the Rh is gone. Then proceed with Pt digestion.

Or if you have a tube furnace, blow the Pt out with chlorine at 450 and you'll be left with fairly pure rhodium.


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## tensor9 (May 23, 2012)

Is there any funny business with the sulfuric acid concentration? It be easier if I could just use 98%.


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## Lou (May 24, 2012)

You can use 98% but I don't find it to work as well.


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## freechemist (May 30, 2012)

Hi tensor9,

If dissolution in AR does not work, the HBr/H2O2 method, like described in my post about "_*De-NOxing of questionable need in Pt purification*_" could be worth a try.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=14330 

If this doesn't function, too, you could try to smelt the "alloy-powder" together with a mixture of very dry K2S2O7 and NaCl. This acid digestion procedure is published in:
"*Precious Metals 1995-Proceedings of 19th Conference-Lake Tahoe #126*" under the title "_Reinvention of Aqua Regia as a Mixture of Partially Molten Salts_".

A last alternative is, to prepare a new alloy by melting the powder together with additional Pt (if necessary) and Cu to produce a homogenous alloy of approximate composition: Pt 70%, Rh 15%, Cu 15%. This alloy, after "cornflaking" will completely dissolve in AR, usually without residue.


Regards, freechemist


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## tensor9 (May 31, 2012)

It is a very finely divided powder.

I tried 98% and slightly more dilute sulfuric acid with no luck. Then I added some 35% hydrogen peroxide to the sulfuric acid, still with no luck. Elevated temps of about 100 C were employed as well. It just laughs at me.

Perhaps I'll try the HBr/H2O2 method tomorrow.


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## freechemist (Jun 1, 2012)

The oxidant in the dissolution-processes for rhodium in hot sulfuric acid and/or a KHSO4-melt is S(VI). The reduction product is S(IV), SO2. After my experiences, H2O2 in hot, fairly concentrated sulfuric acid, is rapidly decomposed to water and gaseous elemental oxygen.


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## tensor9 (Jun 1, 2012)

Update:

The HBr/H2O2 worked wonderfully. I boiled the solution until bromine fumes were no longer present, but I didn't evaporate to dryness (I probably should have). I added ammonium bromide which resulted in a precipitate that I have yet to recover. The precipitate seems to be a finely divided brown/red solid. hrmmm

Side note. I switched to glass microfiber filter paper in the hopes that it holds up to all the strong acid better than cellulose, but man it holds onto the precipitate too well and scraping leads to glass fibers in the product.


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