Reduction of CO₂ by molecular hydrogen to formic acid and formaldehyde and their decomposition to CO and H₂O

Abstract

The reduction of carbon dioxide by hydrogen is catalyzed by K[Ru^III-(EDTA-H)Cl]·2H₂O in aqueous medium at milder pressures (1-4 atm CO₂ or H₂) and 40°C. The reduction of CO₂ gives formic acid and formaldehyde as the initial reaction products, which later decompose to give CO and H₂O as the final products. The rates of formation of formic acid and formaldehyde exhibited first-order dependence with respect to catalyst and dissolved CO₂ and H₂ concentrations, respectively. The rates of decomposition of formic acid and formaldehyde studied under the same reaction conditions also showed first-order dependence with respect to catalyst and substrate concentrations, respectively. The effect of temperature on the rates of formation and decomposition of formic acid and formaldehyde was also studied in the temperature range 30-50°C, and from the Arrhenius plots activation energies were evaluated. Based on the kinetic data, a mechanism is proposed for the formation of formic acid and formaldehyde and their decomposition to CO and H₂O, the end products of the reverse water-gas shift reaction.