Promotion of iridium complex catalysts for HCOOH dehydrogenation by trace oxygen

Abstract

Ir complexes are important homogeneous catalysts for formic acid (FA) dehydrogenation. This paper reports that the activity of Ir complexes can be greatly improved through the activation by trace amounts of oxygen. After activation the activity of the heterodinuclear Ir–Ru catalyst increased 18-fold whereas for the mononuclear catalyst a 23-fold increase was observed. Oxygen is the key factor for the activation. But an excessive concentration of oxygen has a negative effect on the activity. There is an optimal concentration of H₂O₂ for the activation of Ir complex catalysts in HCOOH dehydrogenation. A very low concentration of oxygen (2.4 × 10^–6 M) is needed for the activation of the heterodinuclear Ir–Ru catalyst while the mononuclear catalyst requires the presence of oxygen in a much higher concentration (290 × 10^–6 M). From the results of the study it can be inferred that the activation of complex catalysts is due to the interplay of chemical and structural changes. These findings may be helpful in the attempts to improve the catalytic activity of homogeneous catalysts, which are widely used in formic acid dehydrogenation, CO₂ reduction and in other processes. In addition, this paper indicates that iridium complexes are excellent catalysts for the direct synthesis of H₂O₂ from the H₂ and O₂.