Thieves target catalytic converters because of the precious metals they contain. 4 x 4 and high clearance vehicles like vans are particularly vulnerable.
Things to consider
if a catalytic converter is a bolt on type it is possible to have the bolts welded on
fit protective coverings on catalytic converters, these can make it much more difficult for thieves
have your catalytic converter etched or forensically marked and put stickers in the windscreen to say this has been done