Re: Manganese (IV) oxide is used in the decomposition of hydrogen peroxide.

Paul,
  A catalyst is a species that speeds up a chemical reaction without itself 
being consumed.  In any chemical reaction the reactants are converted into 
products.  Nearly all chemical reactions require energy for them to occur, 
because there is an energetic barrier that reactants must overcome for them 
to be converted into the products.  You can think of this as a hill that 
must be climbed, which represents the energy than must be supplied in order 
to break bonds in the reactants.  Once enough energy is supplied to make it 
to the top of the hill, a position called the transition state, the 
reactants can then spontaneously fall downhill, either back to the 
reactants or on to form the products.  The distance to the top of this 
hill, which is the energy needed for a reaction to occur, is called the 
activation barrier to the reaction.

Catalysts work by lowering the height of this hill - or, in chemical terms, 
by lowering the activation barrier.  There are a variety of ways in which 
catalysts can accomplish this.  Metal ions, like the manganese in manganese 
oxide, are strong Lewis acids.  Lewis acids are atoms that draw electron 
density toward them.  In the presence of hydrogen peroxide, which has the 
molecular structure H-O-O-H, the manganese coordinates to one of the two 
oxygen atoms and draws electron density from it.  This weakens the bond 
between the two oxygen atoms, which is the bond that is broken when 
hydrogen peroxide decomposes.  This makes it easier for the bond to break, 
and thus lowers the activation barrier for the decomposition.