SteveF Please go back and reread the links I have posted on catalysts.
--------------QUOTE---------------------
Catalysts don't change the output, they speed up the reaction. If the output is altered, it's not a catalyst.
----------------------------------------
Many chemical reactions have a variety of reaction products.
A catalyst can be used to drive the bulk of the reaction products towards a preferred output.
In JBII's case the reaction products can be driven towards the diesel range reducing the shorter chained hydrocarbons.
Hence we get more diesel and less gasoline and gasses.
The purpose of a catalyst is to alter the output of a reaction.
It may not create new chemical species from that reaction, but it sure can alters the outputs.