Properties of carbon dioxide absorption and reduction by sodium borohydride under atmospheric pressure

An innovative method has been developed to reduce carbon dioxide into formate under atmospheric pressure and ambient temperature, in which, sodium borohydride solution was used as a reducing agent. The effects of various factors on carbon dioxide reduction were investigated experimentally, from which, the optimal experimental conditions were obtained with 80% of ethanol content, 0.44 mol L−1 of sodium borohydride concentration, 318 K of reaction temperature and 9.0 of solution pH. And the average absorption efficiency of carbon dioxide was achieved at about 54.1%. According to the analysis results of products, the comparison of electrode potentials and the relevant references, formate was determined as the main reduction product; [BHi(OH)4−i]−1 (i = 4, 3, 2, 1) and [BHj(HCO2)(OH)3−j]−1 (j = 3, 2, 1, 0) were considered as the vital active intermediates for carbon dioxide reduction. The thermodynamic parameters such as enthalpy, Gibbs free energy and equilibrium constant were calculated at about −7.84 × 102 kJ mol−1, −5.78 × 102 kJ mol−1 and 95.0, respectively, indicating that this reduction was feasible in thermodynamics. Meanwhile, the kinetics of carbon dioxide reduction was investigated, from which, the reaction order of 1.03 and the apparent activation energy of 11.1 kJ mol−1 were obtained, respectively.