[cliverlong]

Hi,

  As you can probably see I'm on a bit of an electrochemistry trip at the moment.

  I'm trying to understand the electrolysis of water in the presence of dilute, strong acid or alkali.

   I found the cell equations and was surprised by the process to produce oxygen at the anode - I had never really thought about it.

   I have a problem understanding at the atomic level how the oxygen is formed

   I have drawn a couple of diagrams (below) and written the balanced equations.

   However, my diagrams require lots of molecules to come together in one place and rearrange their bonds and then separate.

   Now what I have read about organic mechanism is that such rearrangements normally happen by one atom displacing another or one ion adding in a nucleophilic/electrophilic way and the intermediate then falls apart to produce a different product. Multiple atom interactions are just too improbable to occur. I'm sure the same applies to inorganic mechanisms

   My point is, my proposed mechanisms seem far too complex. I have googled and I can't find a proposed mechanism of what happens to produce the oxygen. Does anyone have a link for that?

Thanks

Clive

[DevaDevil]

cliverlong, as an electrochemist (PhD student at the moment) I can say that the oxygen evolution reaction and the oxygen reduction reaction are still heavily debated for mechanism. In my research it seems reasonable to say that oxygen reduction goes through the intermediate of hydrogen peroxide.
In oxygen evolution what will happen is that the surface of the electrode (platinum in many cases) will first form surface hydroxides: Pt-OH, which at higher potentials will form surface oxides Pt-O. Now

In neutral/acidic media:

Pt* + H₂O --> Pt-OH + H⁺ + e^- (the Pt* stands for empty Pt surface atom)
Pt-OH --> Pt-O + H⁺ + e^-

and the last step may be Pt-O + Pt-O --> 2 Pt* + O₂, or it may be a simultaneous: Pt-O + Pt-OH --> Pt* + Pt-O₂ + H⁺ + e^-, with the oxygen then leaving the surface.

key is: water will first adsorb on the surface, form surface hydroxides and oxides, and then react to oxygen


In alkaline media there is a surplus of OH^-, which will adsorb on the electrode at lower potentials. so:

Pt* + OH^- --> Pt-OH + e^-
and then again
Pt-OH --> Pt-O + H⁺ + e^-
and Pt-O + Pt-O --> 2 Pt* + O₂, or  Pt-O + Pt-OH --> Pt* + Pt-O₂ + H⁺ + e^- with Pt-O₂ --> Pt* + O₂


I used Pt as electrode material in this case, as it is the most common. Other metals (or carbon) are possible, but may corrode because of the high potential of the oxygen evolution reaction.

[cliverlong]

cliverlong, as an electrochemist (PhD student at the moment) I can say that the oxygen evolution reaction and the oxygen reduction reaction are still heavily debated for mechanism. In my research it seems reasonable to say that oxygen reduction goes through the intermediate of hydrogen peroxide.
 << snip rest - to be absorbed (adsorbed? )

Talk about getting from the source !

Thanks !

Clive

[DevaDevil]

you are welcome. If you have future electrochem. questions, feel free to IM me on msn if you need quicker answers.

-Dennis