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] (<b>1</b>), [Ru­(CO)₃(PPh₃)₂] (<b>2</b>), and [Ru₂(HCO₂)₂(CO)₄(PPh₃)₂] (<b>3</b>), 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] (<b>3</b>) 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] (<b>1</b>) 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] (<b>1</b>) to an unexpected carbonate, [Ru­(CO₃)­(CO)₂(PPh₃)₂]·H₂O was observed