Question about how H2 is formed with Acid/Metal Reactions

[AbeMichelson]

We all learned this:

Zn(s) + 2HCl(aq) --> ZnCl2(aq) + H2(g)

 

Recently I saw a paper that claimed hydrogenation of graphene using the reaction between HCl and Zn. Unfortunately, they did no studies on the mechanism of this reaction (just "it worked" instead of "why it worked"). Past studies have shown that H2 or H+ will not hydrogenate graphene. However hydrogen (as well as fluorine) radicals do react with graphene.

 

My question is: Do you form, even for a short time, hydrogen radicals in the reaction with Zn and HCl? I know in gas/solid phase you may see a radical on the surface which meets another to form H2. That mechanism seems less plausible at higher pressures even less for liquid/solid interface.

OR

Since they used graphene oxide (loaded with CO bonds), did the reaction occur there?

 

I now wonder what is happening on the surface of zinc metal during the acid reaction when that first H+ hits. It would be an interesting surface science question if no one has done it...

Edited January 8, 2014 by AbeMichelson

[Enthalpy]

Intuitively (so check other sources) I'd say:

Since H⁺ arrive separated at the zinc surface, atomic H must result at the metal surface. I expect the H atoms to move along the metal surface until they meet an other H atom, then form a H₂ molecule, and after some time leave the metal, for instance when enough H₂ molecules have met to form a bubble, and the bubble is big enough.

[John Cuthber]

http://en.wikipedia.org/wiki/Nascent_hydrogen

[AbeMichelson]

@ Enthalpy: yeah, that's what happens during catalysis. I should have known that when I posted this. I'm still wondering how the radicals make it to the graphene. The lifetime would be staggeringly small for a radical in a liquid. Maybe the zinc is in intimate contact with the graphene?

[Enthalpy]

How intimate would be enough? Single atoms of zinc? Nanodots? Or the graphene sheet just sits on zinc, and H⁺ flow through it? Nice, since graphene would prevent hydrogen atoms to first move at the zinc surface.

 

CO bonds are a more likely candidate to catch hydrogen.

 

What you might give a try: a hydrogen plasma. Low pressure, very low voltage - and low intensity and duration, because graphene contains few atoms...

 

An other direction: excite the pi bonds with light, within gaseous hydrogen. Widely delocalized pi are passive, but excitation gives them punch. Despite they absorb long wavelengths also, I'd go to hard UV (excimer lamps and the like, they're spreading presently) to shake the delocalized electrons locally enough.

http://en.wikipedia.org/wiki/Nascent_hydrogen

Fun. So the "nascent hydrogen" was just an electron transfer... Not a good explanation for graphene, since H⁺ is said not to operate, but H yes.

Edited January 10, 2014 by Enthalpy

[John Cuthber]

"Fun. So the "nascent hydrogen" was just an electron transfer... Not a good explanation for graphene, "

Odd.

I thought that graphene (or its oxide)- as a conductor, in contact with the zinc- was rather well placed to pick up electrons released from the zinc and, accordingly, be reduced to some (notionally free-radical) species that could then react with, for example, water to produce the sorts of things listed as products.

Meh.

Perhaps I will okok again when I sober up

[AbeMichelson]

@ enthalpy:

Yes, hydrogen plasma will work (In fact I have done that, even at higher pressures! ( http://www.sciencedirect.com/science/article/pii/S0008622311004696 )). The paper I was speaking of actually reported hydrogenated graphene (and judging by the Raman Spectra it looks legit) using only the zn+HCl reaction. I couldn't see a mechanism that made sense for it. The UV sounds interesting, not sure if anyone has tried that (from what I recall, graphene's bandgap is in the very deep UV, 190 nm or so...I could be wrong). The current on the graphene is plausable, but would also require intimate contact with the graphene to work. But the original question posed is pretty well answered with "yes, H radicals exist on the surface for short time scales that preclude being an efficient source of radicals for hydrogenation."

[Enthalpy]

If the graphene sheet can be laid on zinc, then H+ going through graphene will release H upon reaction with Zn, and this H will be prevented from grouping to H₂, hence stay available for other uses. That would be plausible to me.

 

190nm is just fine. Among the recent excimer lamps, you have:

Ar2 (126 nm), Kr2 (146 nm), Xe2 (efficient 172 nm), KrCl (222 nm) and many more.

List there http://old.iupac.org/goldbook/ET07372.pdf

Providers: Heraeus, Ushio, Yuasa and more. Affordable, depending on your criteria.

 

Excimer lamps are still a bit new, but with >10% efficiency, multi-kW power, and wavelengths where needed, I suppose they will open fully new paths to chemistry. Up to now, Hg lamps at 254nm needed indirect and inefficient methods like sensitizers. The new sources can excite one particular bond, hence be efficient and selective. Time to make PhD on the subject.