The Alchemy of Zinc

Back in the day, alchemists apparently spent all their time attempting to turn base metals into gold. I say apparently, since that is the common perception of alchemists. In reality, most alchemists were devoted to what was often referred to as the “Great Work.” According to alchemical theories, much of what is created by nature is imperfect, and by applying the art of alchemy, natural substances can be brought to a higher state of perfection. Since gold was the height of perfection for metals, it was thought that base metals could be transmuted into gold merely by removing these imperfections. The Philosophers Stone wasn’t just about making gold or the “Elixer of Life”, it was a process/device/concept for bringing about perfection.

Color was very important to the alchemists, perhaps since it was one of the few clues with which they had to go on during their experiments. Colors and color changes were rigorously recorded and eventually incorporated into many of the alchemical theories. Certain color sequences were expected during the path towards perfection. In the transmutation of base metals into gold, for example, a color sequence of black to white to yellow to purple was thought to be required. Later theories redefined the sequence as black to white to red. While this fascination with colors may have led them astray on occasion, it has also led to a vast array of wonderful, full color illustrations, intricately drawn and full of alchemical symbolism. Here is one such example.

An experiment often touted as reminiscent of alchemy and demonstrating the relative ease of generating color changes involves the apparent conversion of a copper penny into silver or gold. Simply dissolve some Zn powder in warm NaOH solution, toss in a penny, and watch the penny turn silver as a coating of Zn forms on its surface. Heating the penny turns it gold as the Zn and Cu alloy to form brass. (My kids were duly impressed when I performed this experiment for them, informing me that it was “awesome”, but considering they said the same thing when I turned phenolphthalein red, the “awesomeness” bar may be set pretty low.)

But why does this work? I’ve coated pennies with silver and mercury before, but zinc is more electropositive than copper. Based on electrochemical potentials, zinc shouldn’t plate out on copper -- copper should be plating out on zinc. Apparently I’m not the only person to wonder about this, as web pages devoted to this effect can be found throughout the Internet, for example, here and here.

First of all, Zn dissolves in NaOH to make zincate ion and hydrogen.

Zn + 2OH^- + 2H₂O ----> Zn(OH)₄^2- + H₂

Okay, that’s straightforward chemistry. But how does the copper reduce the Zn(OH)₄^2-? The answer is that it doesn’t. Surprisingly, copper is not the reductant . Copper is not oxidized and it does not go into solution. Copper’s role is to create a galvanic cell with the undissolved portion of the zinc powder. Zinc does not plate out on the penny until the copper is in direct contact with zinc metal. It turns out that zinc metal is the reductant, which sounds bizarre, at least to me. Once the zinc and copper metals are in contact and the galvanic cell is created, the zinc begins to oxidize…

Zn + 4OH^- ----> Zn(OH)₄^2- + 2e^-

And its electrons are now available to reduce the zincate ion near the surface of the penny...

Zn(OH)₄^2- + 2e^- ---> Zn + 4OH^-

So the overall reaction is:

Zn + Zn(OH)₄^2- ----> Zn(OH)₄^2- + Zn

Basically there is no net reaction! Which means the zinc is essentially just migrating from the surface of the zinc powder to the surface of the penny. At first glance, this would appear to violate the laws of thermodynamics, but obviously it doesn’t. I assume there is some sort of entropy effect here, perhaps related to the galvanic cell, but I don’t know the exact cause. In any case, this reaction appears just as mystical as alchemy itself!

If anyone happens to know any more about this process, I’d be happy to hear from you.

Nano-gold Catalysts

Reports of nano-gold and its use as a catalytic material are beginning to pile up. Last week I mentioned nanogold particles in stained glass windows and their ability to oxidize organic pollutants. This week, two more articles showed up, here and here. Hmmm…. seems like we’re beginning to reach a critical mass here. Pretty good for a metal that hasn’t been considered a particularly good catalyst over the years. Perhaps I'll start paying more attention to this area. I’ve read articles about nano-particle based catalysts in the past and have generally been underwhelmed, but nano-gold catalysts may be about to change that view. And that’s because of two words that are often found in these reports.

Room temperature.

Or at least, low temperature.

The obvious reason for using nano-particles is their much larger surface area to mass ratio. Higher surface areas generally mean better catalysts, so the drive to make smaller and smaller catalyst particles has been going on for decades. Back before “nano” became a marketing term, I recall at least one set of researchers making platinum nano-particle catalysts to improve the efficiencies of automotive exhaust catalysts. They succeeded, but unfortunately, at the high operating temperatures (generally above 400^oC) to which these catalysts were subjected, the platinum nano-particles tended to move around and coalesce into much larger particles, losing much of their surface area and eventually not looking all that different from more crudely prepared catalysts. In fact, with these types of catalysts, research is focused less on the catalyst itself and more on the substrate which supports the catalyst -- changing its properties so as to limit the movement of the catalyst particles. This is always a bit of a balancing act since too much interaction between the catalyst and substrate can change the properties of the catalyst in undesirable ways. But if the reactions to be catalyzed take place near room temperature, or at least at lower temperatures, you stand a much better chance of maintaining the catalyst nanoparticles and hence their benefits.

Gold has traditionally been the forgotten metal, with all catalyst discussion centering on its neighbors on the periodic table (Pt, Pd, Rh, Re, Ru, and Ag). That looks like it's about to change. The big question to be answered: Why are nanogold particles so much better than bulk gold? Here are some possibilities to consider.

1. Larger surface areas. Well yes, you would expect some benefit here, but gold has been so unremarkable as a catalyst, I don’t believe a simple increase of surface area would account for this big of an effect. If it were that easy, everyone would just have dumped more gold into their catalyst formulations in order to increase the available surface area.

2. More defect sites. In many cases, catalysis only occurs at specific sites on the catalyst surface, such as the so-called "steps" as shown in the figure below. Perhaps nano-gold particles naturally have more of these locations.

3. More of the appropriate crystal faces. Different lattice faces such as (111) or (100) often have significantly different catalytic rate constants. See figure above. Perhaps nano-gold particles have more of the correct lattice orientations at the defect sites than bulk gold.

4. Different surface oxide properties. One of the reasons metals like Pt and Rh make such good catalysts is their ability to maintain surfaces which are relatively clean of oxygen atoms. Whereas the early transition metals tend to form oxides, the late transition metals favor the metallic state. Heat vanadium or molybdenum in air and you form the oxides. Heat platinum or its salts in air and you wind up with platinum metal. Keeping the surface free of oxygen atoms makes the metal more available for catalyzing the desired reactions. Perhaps smaller gold particles tend to have less oxygen on their surface.

Figure obtained from http://www.pnas.org/content/103/28/10577.full.pdf