Re: by what chemical means/reaction can Carbon Dioxide be decomposed

You are right that carbon dioxide is very difficult to decompose. It is 
almost impossible to split it up directly into carbon and oxygen at any 
temperature.
However, it is possible to reduce it (remove the oxygen). One way is to 
pass carbon dioxide over heated carbon at a temperature above about 1100o 
Celsius. This makes carbon monoxide and is one of the important reactions 
happening in the Blast Furnace production of iron:
                                                  C + CO2  --> 2 CO

In the laboratory you can reduce carbon dioxide to carbon with a reactive 
metal. Plunging burning Mg into a gas jar of carbon dioxide makes white 
magnesium oxide and black carbon:
				   2 Mg +  CO2  --> 2 MgO  +  C

Hope that helps, Kevin.

Dan Berger adds:
CAUTION CAUTION!

Magnesium burns with a very hot, blindingly bright flame and is not easy to put 
out! Water and CO2 will both support the combustion of magnesium!