MadSci Network: Chemistry

Re: How does KMnO4 and MnO2 work as catalyst in the decomposition of H2O2

Date: Sun Feb 1 11:34:02 2009
Posted By: Cesar Prado-Fdez, Secondary School Teacher, Science
Area of science: Chemistry
ID: 1229344672.Ch

Two different questions are being raised here: the oxidation
of hydrogen peroxide by permanganate ion and manganese acting as a catalyst

For the first question we have to say that the decomposition of H2O2 is a
redox reaction in which permangante acts as an oxidant, and it is consumed
by changing its oxidation state from (VII) to (II), taking 5 electrons from
the other substance involved in the reaction, the hydrogen peroxide.

If the reaction takes place in a acidic media the products will be that
Mn(II) specie (colourless and perfectly soluble in water) and oxygen gas.
In neutral or basic media the oxidation is not cpmplete and produces not
Mn(II) but Mn(IV) in the form of the oxide, MnO2 [2] which precipitates as
a brown solid.

So permanganate does not act as a catalyst but as a reagent.

That is on one side. 

But manganese can also act as a catalyst [2]. Catalysts work by decreasing
the activation energy of a chemical reaction, by forming an activated
complex with a lower energy compared to that of the reagents by themselves,
and being recovered to be used again and again as they are not consumed.
This complex can be formed by bonding of the catalyst with the reagent
(both as dissolved species, in homogenoeus media) or adsorption of the
reagent on the surface of the catalyst (this is, a solid catalyst for
heterogeneous catalysis). The changes on the electronic structure of the
reagent-catalyst complex compared with the solo-reagent make the reaction
with the other specie easier in energetic terms, or the catalyst bring the
two species closer together, or the reaction happens on a supported
surface... [3].

Manganese is specially good at this. It can be found to work as a catalyst
in Nature either by itself or, more commonly forming an organometallic
complex, the chlorophil being the more popular. The fact that it is such a
good catalyst and that it can easily form these complexes with organic
fragments (ligands) which can have different structures or substituents,
makes manganese the basis of many new and more specific catalyzers [4,5].

[3] Wei Li and S. Ted Oyama. J. Am. Chem. Soc., 1998, 120 (35), pp 9047–9052
[4]Jiro Tsuji. "Transition Metal Reagents and Catalysts: Innovations in
Organic Synthesis." John Wiley & Sons, Ltd. 2000.
CATALYSIS" VOL. 46. Academic Press. 2001.

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