Re: how does manganese dioxide increase the decomposition hydrogen peroxide

Hi Vincent

There is no clear answer to this question, but I can give you some links:

According to this article:  http://chemistry.stackexchange.com/questions/7759

"The surface of solid manganese dioxide provides a particulary favourable 
environment to catalyze the decomposition, though the mechanism is not 
understood very well."

The two mechanisms proposed for iron (III) catalysis could both work with 
MnO2. I'll summarise them for you:

First, a [FeOOH]+2 complex forms

now, either

HOO. radical forms and Fe is reduced to Fe+2
Fe+2 and H2O2 react to form OH. radicals and Fe+3
H2O2 and OH. radicals react to form HOO. radicals and H2O
HOO. radicals and Fe+3 react to form O2, H2O and Fe+2

or

[FeOOH]+2 complex forms an iron (V) complex [FeO]+3
The iron (v) complex oxidises H2O2 and forms Fe+3, H2O and O2

There is a list of possible reaction steps with MnO2 - many more steps 
than would occur under any one set of conditions. pH affects which 
intermediates are formed. If acidic, the intermediates are radicals but 
at neutral or alkaline pH the intermediates are anions.

To summarise - MnO2 reacts with H2O2 to form intermediates. These 
intermediates react to form MnO2 and the products H2O and O2. The 
reactions have lower activation energies than the reaction of H2O2 
without MnO2. Iron (II) is another catalyst that could be used.

   Note added by moderator:  In Cotton and Wllkerson, "Advanced Inorganic
Chemistry", (5th edition, p. 458) reference is made to the decomposition 
being studied by using peroxide with both oxygens being the oxygen-18 
isotope.  This has shown that the oxygen-oxygen bond of the peroxide is 
either not broken, or is reassembled during the decomposition process, 
as both oxygen-18's come out bonded together in an oxygen molecule.  
This puts constraints on possible mechanisms, which are, as noted above, 
still obscure at best.  Also, manganese dioxide may not be, especially 
at its surface, a nice, neat, stoichiometric compound. (same book, p. 706).