Fuel additives serve a number of important functions, but their impact on mileage and emissions is usually rather limited, especially so in modern engines. This, however, depends on the type of additive. For instance, organometallic additives containing platinum may have some emission impact.
I did a search here but found nothing.
Well, you didn't search hard enough. See http://www.dieselnet.com/tech/fuel_dies ... tml#comcat
, the entire section discusses "combustion catalysts".