Formic Acid As a Hydrogen Storage Medium: Ruthenium-Catalyzed Generation of Hydrogen from Formic Acid in Emulsions

Formic acid is decomposed to H2 and CO2 in the presence of RuCl3 and triphenylphosphines in an emulsion. In situ formed ruthenium carbonyls, such as [Ru­(HCO2)2(CO)2(PPh3)2] (1), [Ru­(CO)3(PPh3)2] (2), and [Ru2(HCO2)2(CO)4(PPh3)2] (3), and a large cluster, involving a Ru12 core, were identified and structurally characterized from the reaction mixtures. The catalytic activity of the mono and binuclear complexes was also investigated and it was found that [Ru2(HCO2)2(CO)4(PPh3)2] (3) shows high stability even at elevated temperatures and pressures and its activity is 1 order of magnitude lower than those measured for the mononuclear complexes. It was also attempted to use [Ru­(HCO2)2(CO)2(PPh3)2] (1) as a catalyst for the hydrogenation of CO2 to formic acid under neutral conditions. Although the reduction of CO2 did not take place, the conversion of [Ru­(HCO2)2(CO)2(PPh3)2] (1) to an unexpected carbonate, [Ru­(CO3)­(CO)2(PPh3)2]·H2O was observed.