Exploring the benefits beyond the pre-reduction in methane of the Cr/SiO2 Phillips catalyst: The molecular structure of the Cr sites and their role in the catalytic performance

•CrVI/SiO2 Phillips catalyst was pre-reduced in methane at 400 °C.•Ethylene polymerization occurs without induction time and with higher activity.•Formic acid and hydrogen are released during the pre-reduction.•CrVI sites are reduced to CrII (23%), hexacoordinated CrIII (33%), CrIV(=O) (44%).•CrIII sites do not take part to the catalysis.•Both CrII and CrIV(=O) are accessible by incoming molecules.•Most of the credits for the improved catalytic activity go to CrIV(=O).