Sciencemadness Discussion Board

Reducing surface coating of CuO to metal

zerodan - 27-10-2021 at 07:54

Hello everyone,

I have prepared a layer of mixed CuO/Cu on glass and have been trying to come up with a good process to reduce the oxide back to metal to create a uniform copper surface.
So far I've considered the following

1 - Carbon reduction, like the one most commonly used in obtaining metals from oxides, C + MO => CO2 + M. The obvious problem is the high temperatures required are likely to damage the glass and without inert atmosphere the copper would get oxidized back to CuO in air at that temperature. I tried using a fuel rich flame from a butane torch but that didn't work.
2 - Catalytic oxidation of alcohol to aldehyde. This one was also attempted,
a copper strip was heated up in butane torch flame and removed until black layer of CuO formed, when submerged in 90% ethanol the CuO layer went away. The problem with this method is that the temperature required was very high, if the strip wasn't shy of red hot no reaction would occur. I'm afraid using this method with glass would yield visible deformation or cracks on rapid cooling.
3 - Oxalic acid. As far as I know oxalic acid could reduce CuO to Cu but I'm unsure of the chemistry, can there be side products such as copper oxalate or is it too weak of an acid for that? I don't have ascorbic acid.

If my concerns about 3 are void it would be the ideal method as it wouldn't require high temperatures (>100C) so I wouldn't have to worry about new CuO formation.
Please help me decide on the method or suggest other ideas if you have any

Metallophile - 27-10-2021 at 11:34

Would an alkali metal do anything? Or maybe NaK?

zerodan - 27-10-2021 at 11:39

Quote: Originally posted by Metallophile  
Would an alkali metal do anything? Or maybe NaK?


I'm sorry if this wasn't obvious from the beginning but I'm trying to find a practical solution not a theoretical one.
Even if I had any alkali metals on hand I wouldn't try making NaK in the first place.

fredsci93 - 27-10-2021 at 13:02

The best method would be to heat the glass (quite hot, ~700C) with a hydrogen torch, this will reduce the copper oxide in the process, then extinguish the hydrogen flame but continue the flow of hydrogen until the glass has cooled sufficiently, the hydrogen shields the copper layer from atmospheric oxidation. The only problem with this method is hydrogen torches aren’t super common and it involves high heat, however the heat is of no concern if you cool the glass slowly and evenly.

zerodan - 27-10-2021 at 13:22

Quote: Originally posted by fredsci93  
The best method would be to heat the glass (quite hot, ~700C) with a hydrogen torch, this will reduce the copper oxide in the process, then extinguish the hydrogen flame but continue the flow of hydrogen until the glass has cooled sufficiently, the hydrogen shields the copper layer from atmospheric oxidation. The only problem with this method is hydrogen torches aren’t super common and it involves high heat, however the heat is of no concern if you cool the glass slowly and evenly.


Yes I have considered it, forgot to add it to the list.
Like with most high temp methods the glass might get damaged.
I should have mentioned this is soda lime glass and unfortunately I cannot change that. That's why I was hoping for a clean chemical reduction that doesn't require high temps.

Also autoignition temperature of hydrogen is 585C.

brubei - 27-10-2021 at 13:47

Maybe cyanide dissolution of surface

fredsci93 - 27-10-2021 at 13:51

Good point, soda lime could easily get cracked, another potential method could be electrochemically reducing the layer by electrolysing it in a copper sulphate bath, the coper coat on the glass would be connected to negative and the positive would be connected to a copper plate (this could only work if the layer is conductive) however I'm not sure exactly how well this would work.

WGTR - 27-10-2021 at 15:11

CuO is very easy to reduce with hydrocarbons. The minimum temperature required is somewhere between 200°C and 300°C depending on how thin or finely divided it is. This is the temperature range commonly used for the dehydrogenation of alcohol to aldehyde over a copper catalyst.

Here is a simple experiment showing the oxidation and then reduction of lead oxide. Basically all you have to do is bubble an inert gas through alcohol, and use the vapors to reduce a hot oxide:

https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

Another thing you can try is to make a "gas bag" hooked up to a cardboard tube. This is what I did in this post:

https://www.sciencemadness.org/whisper/viewthread.php?tid=27...

A few mL of alcohol was added to the trash bag, and then the bag was filled with nitrogen gas and twisted to keep the gas inside (and oxygen outside). The cardboard tube was held securely in a vise. The hot glass slide (heated up to about 500°C) was placed into the tube and the tube was covered loosely with a glass crystalizing dish cover. Then the bag was quickly untwisted and squeezed, forcing nitrogen/alcohol vapors across the hot glass slide.

This setup worked well for reducing indium/tin oxide conductive coatings on glass. The conductivity went up at least 5x after this treatment.

Ascorbic acid as a reducing agent

Linus1208 - 27-10-2021 at 15:15

An aqueous solution of ascorbic acid should work for this purpose.

I did this multiple times with a suspension of Cu(OH)2 (CuO should act very similar).
The suspension first slowly turns from blue to green to a yellowy color, because of fine Cu2O particles form. As the Cu2O gets further reduced, the suspension gets a copper-brown color.
After letting it settle for a few ours and decanting it off and drying, there was a layer of fine copper powder sticking to the glass.

The reaction equations are the following:
2CuO + C6H8O6 = Cu2O + C6H6O6 + H2O
copper(II) oxide + ascorbic acid = copper(I) oxide + dehydroascorbic acid + water

Cu2O + C6H8O6 = 2Cu + C6H6O6 + H2O
copper(I) oxide + ascorbic acid = copper + dehydroascorbic acid + water

overall:
CuO + C6H8O6 = Cu + C6H6O6 + H2O

[Edited on 27-10-2021 by Linus1208]

Linus1208 - 27-10-2021 at 15:18

And a question just out of curiosity:
What exactly are you doing?

walruslover69 - 27-10-2021 at 15:21

why do you need to reduce the oxide to the metal instead of just starting with pure metal? does the reaction need to be cycled?

oxalic acid might work very well, if you expose the plate to a solution of oxalic acid, all the oxide should get converted to the oxalate, and then if you heated the plate to a more reasonably 300-400C it will probably decompose the oxalate to reduced copper metal.

zerodan - 28-10-2021 at 08:33

Quote: Originally posted by Linus1208  
An aqueous solution of ascorbic acid should work for this purpose.

I did this multiple times with a suspension of Cu(OH)2 (CuO should act very similar).
The suspension first slowly turns from blue to green to a yellowy color, because of fine Cu2O particles form. As the Cu2O gets further reduced, the suspension gets a copper-brown color.
After letting it settle for a few ours and decanting it off and drying, there was a layer of fine copper powder sticking to the glass.

The reaction equations are the following:
2CuO + C6H8O6 = Cu2O + C6H6O6 + H2O
copper(II) oxide + ascorbic acid = copper(I) oxide + dehydroascorbic acid + water

Cu2O + C6H8O6 = 2Cu + C6H6O6 + H2O
copper(I) oxide + ascorbic acid = copper + dehydroascorbic acid + water

overall:
CuO + C6H8O6 = Cu + C6H6O6 + H2O

[Edited on 27-10-2021 by Linus1208]


From my online research I was expecting a similar reactions to happen with oxalic acid. I'll attempt that in the future.
As for ascorbic acid I have vitamin C fizzing tablets, supposedly 1g L-ascorbic acid in 4g tablet, the rest is citric acid and sodium (bi?)carbonate. Is there a simple way to isolate ascorbic acid from citric? There doesn't seem to be any solubility data of ascorbic acid on wikipedia. I have most common solvents like ethanol, acetone, isopropyl alcohol, toluene, xylene.
Quote: Originally posted by walruslover69  
why do you need to reduce the oxide to the metal instead of just starting with pure metal? does the reaction need to be cycled?

oxalic acid might work very well, if you expose the plate to a solution of oxalic acid, all the oxide should get converted to the oxalate, and then if you heated the plate to a more reasonably 300-400C it will probably decompose the oxalate to reduced copper metal.


I chose to deposit copper by carboxylic acid salt decomposition (copper acetate).
Unfortunately this process produces traces of CuO that I'd ideally like to convert back to metal.

walruslover69 - 28-10-2021 at 08:59

You might be able to get away with thermal decomposition of copper oxalate directly onto the glass instead of copper acetate. I believe it yields fine particles of copper metal and CO2. Decomposition of oxalate salts is how a lot of micro and nano meter size metal particles are made.

for the separation of ascorbic and citric acid. It appears that ascorbic acid's solubility in ethanol is 2g/100ml while citric acid's is 62g/100ml so you should be able to precipitate the ascorbic acid out in ethanol.

zerodan - 28-10-2021 at 09:16

Quote: Originally posted by walruslover69  
You might be able to get away with thermal decomposition of copper oxalate directly onto the glass instead of copper acetate. I believe it yields fine particles of copper metal and CO2. Decomposition of oxalate salts is how a lot of micro and nano meter size metal particles are made.


Actually this was also considered, but like most oxalate salts copper oxalate is insoluble in water, (no info on ethanol so probably also a no) which is a hinderance as my process requires using a liquid solution.
Quote: Originally posted by walruslover69  

for the separation of ascorbic and citric acid. It appears that ascorbic acid's solubility in ethanol is 2g/100ml while citric acid's is 62g/100ml so you should be able to precipitate the ascorbic acid out in ethanol.


Sounds promising. I'll read up on that and try it on small scale.

Linus1208 - 1-11-2021 at 00:58

Quote: Originally posted by zerodan  

As for ascorbic acid I have vitamin C fizzing tablets, supposedly 1g L-ascorbic acid in 4g tablet, the rest is citric acid and sodium (bi?)carbonate. Is there a simple way to isolate ascorbic acid from citric?

I don't know, I found 100g if pure Vitamin-C powder in the grocery store so I bought that one.

German wikipedia lists 333g/l solubility for ascorbic acid, for citric acid I found very different values on different sources (Wikipedia: 1450 g/l, Merk: 800g/l) so I wouldnt be so sure about that.

Citric acid thermally decomposes at 175°C, ascorbic acid does so at 190°C so thermal decomposion theoretically could work, but it wouldn't really be a clean method, as the decomposition temperatures are very close to eachother.

So I guess seperating with ethanol would be the best choice.

[Edited on 1-11-2021 by Linus1208]

Fantasma4500 - 9-11-2021 at 08:01

if you have access to acetylene-oxygen torch you may try adjusting the flame to oxygen-negative, this will give a reducing flame
swap acetylene with hydrogen could also work really well

ascorbic acid can reduce copper sulfate into metallic copper, but im not quite sure if it will do anything to CuO. CuO is generally reactive and reacts quite fast with HCl, maybe take it from there and re-treat it? anyhow isnt CuO used in ceramics, that turns into Cu/Cu2O after being fired in oven?

BAV Chem - 24-2-2022 at 05:37

A hot solution of glucose and sodium hydroxide works for reducing Cu(I) oxide to Cu metal and it should work for Cu(II) as well. Made some crappy copper mirrors like that once.