This invention generally refers to a new generation of fuel additives which can provide catalytic action to improve the combustion process of fossil fuels and to a catalyst among others containing an iron compound combined with an over-based magnesium compound with molecular size particles inside the combustion chamber. Such fuel additive catalysts are particularly useful for fuel oil combustion, natural gas combustion, stationary gas turbines, natural gas-fired reciprocating engines, diesel engines, gasoline engines and all stationary dual-fuel engines.