Sciencemadness Discussion Board

manganese dioxide particle size

Leafs - 30-7-2017 at 18:07

So I made some "active" MnO2. I'm wondering how fine should I mill this stuff considering it really isn't soluble in anything so presumably it would be best to get it to a really fine state? I thought perhaps it would break up into smaller pieces in solution but apparently this doesn't happen as last oxidation I noticed many chunks and my yield wasn't where it should've been.

Anyone with any experience working with this stuff and how to maximize reactivity/yields of this stuff... would be much appreciated. Thanks.

Boffis - 31-7-2017 at 01:18

By its very nature "active" Mn oxide occurs as clumps of fine particles that have a high surface area to mass ratio and are microporous. You don't want to mak the particles too fine or they become difficult to remove by filteration. The method you use to make it pretty much determines its characeristic. The type made by the thermal decomposition of manganese carbonate may appear granular but it has a very high surface area per unit weight and appears perfectly serviceable as prepared. The material made by the various wet processes is often so fine that it is hard to filter anyway. The dried filter cake may appear massive and chunky abd does not alway disperse readily but this doesn't appear to affect its activity.

Leafs - 31-7-2017 at 09:18

Quote: Originally posted by Boffis  
By its very nature "active" Mn oxide occurs as clumps of fine particles that have a high surface area to mass ratio and are microporous. You don't want to mak the particles too fine or they become difficult to remove by filteration. The method you use to make it pretty much determines its characeristic. The type made by the thermal decomposition of manganese carbonate may appear granular but it has a very high surface area per unit weight and appears perfectly serviceable as prepared. The material made by the various wet processes is often so fine that it is hard to filter anyway. The dried filter cake may appear massive and chunky abd does not alway disperse readily but this doesn't appear to affect its activity.


Thank you for that information. I think I might have to redo another batch because I already milled this pretty fine. I was thinking a celite pad would catch it.

Leafs - 1-8-2017 at 14:07

Quote: Originally posted by Boffis  
By its very nature "active" Mn oxide occurs as clumps of fine particles that have a high surface area to mass ratio and are microporous. You don't want to mak the particles too fine or they become difficult to remove by filteration. The method you use to make it pretty much determines its characeristic. The type made by the thermal decomposition of manganese carbonate may appear granular but it has a very high surface area per unit weight and appears perfectly serviceable as prepared. The material made by the various wet processes is often so fine that it is hard to filter anyway. The dried filter cake may appear massive and chunky abd does not alway disperse readily but this doesn't appear to affect its activity.


Hi I further looked into alternative methods to produce "active" MnO2 and the Manganese Carbonate method appears ideal but calls for stirring of the resulting black solid with 15% HNO3 in water... I'm not too sure what this step does exactly, and I'm thinking it could be skipped as none of the other methods call for anything like this. The appeal to me in the first place with this method is the fact it doesn't require filtration (without that step) etc and wont get everywhere. Would appreciate yours or anyone else's opinion who is more skilled in these matters.

The reference I'm referring to.

Quote:
Preparation of Active MnO2 by Pyrolysis of MnCO3.
Powdered MnCO3 was spread in a one-inch thick layer in a Pyrex glass and heated at 220-280 °C for about 18 h in an oven in which air circulated by convection. The initially tan powder turned darker at about 180 °C, and black when maintained at over 220 °C. No attempt was made to determine lower temperature or time limits, nor the upper limit of temperature. The MnO2 prepared as above was stirred with about 1 L of a solution made up of 15% HNO3 in H2O. The slurry was filtered with suction, the solid was washed on the Buchner funnel with distilled water until the washes were about pH 5, and finally was dried at 220-250 °C. The caked, black solid was readily crushed to a powder which retained its oxidizing ability even after having been stored for several months in a loosely stoppered container.

JJay - 1-8-2017 at 14:11

Wouldn't that step remove unreacted manganese carbonate?

Leafs - 1-8-2017 at 18:08

Quote: Originally posted by JJay  
Wouldn't that step remove unreacted manganese carbonate?


yeah google indicates manganese carbonate is soluble in dilute acid. I think I'm just going to overkill it and let it cook in the oven for 48 hours. It's a mini-oven so it shouldn't be too inefficient. I will also periodically break and mix up the solid.

edit: also getting mixed information as to the decomposition of manganese carbonate. some sources claim it decomposes to manganese (||) oxide, others say manganese dioxide. hmmmm
Quote:
The carbonate is insoluble in water but, like most carbonates, hydrolyses upon treatment with acids to give water-soluble salts. Manganese carbonate decomposes with release of carbon dioxide at 200 °C to give manganese(II) oxide: MnCO3 → MnO + CO.


[Edited on 2-8-2017 by Leafs]

[Edited on 2-8-2017 by Leafs]

Boffis - 2-8-2017 at 13:28

The decomposition of manganese II carbonate is complex. Mn II carbonate breaks down in the absense of oxygen to give MnO and CO2 but in the presence of oxygen and/or moisture it breaks down at lower temperatures to complex multivalent Mn oxides with variable stoichometery. It is possible that the activation is due to the leaching of Mn2+ from these complex oxides in a manor similar to the leaching of K from the oxides produced by such methods as the oxidation of Mn sulphate with KMnO4. I make activated Mn oxide from the potassium bearing (8-10% K) Mn oxide that I have left over from the used of potassium permanganate and KOH to oxidize various heterocyclic and hydrocarbons to carboxylic acids.