SiO2 anoparticles, of varying size 9-55 nm, show significant co-catalytic activity for H-2-production from HCOOH by the homogenous Fe-II/P(CH2CH2PPh2)(3) catalyst. Particle size and specific surface area are shown to play key-role in this phenomenon. Larger nanoparticles have better co-catalytic effect. Arrhenius analysis reveals that the significant co-catalytic effect can is attributed to a lowering of the activation energy of the rate-limiting step by Delta Ea = 16-36 kJ/mol. This phenomenon is attributed to a thermodynamic promotion of HCOOH deprotonation by the SiO2 nanoparticles, accelerating in this way coordination of HCOOon the Fe-II atom of active catalyst, during catalysis. The surface Si-O-Si bridges are shown to be responsible for the promotion of this HCOOH deprotonation, while surface-adsorbed H2O retards the co-catalytic efficiency. (C) 2014 Elsevier B.V. All rights reserved.