Insight into Mn enhanced short-chain olefins selectivity in CO2 hydrogenation over Na-CuFeO2 catalyst

Abstract

In this presentation, Mn-promoted Na-CuFeO2 catalysts were prepared with a two-step method, hydrothermal synthesis of Na-CuFeO2 followed by impregnation of Mn. The developed material is explored for single-step CO2 hydrogenation towards short-chain olefins through a modified Fischer-Tropsch route. The addition of the appropriate amount of Mn over Na-CuFeO2 significantly limits the C5+ hydrocarbon formation while improving the short-chain olefins selectivity in the product stream. Among the different compositional catalysts, 2.5% Mn/ Na-CuFeO2 demonstrated excellent catalytic performance with a CO2 conversion of 36.6% with high C2-C4 olefins selectivity (35.7%), high O/P ratio (3.9), and a space–time yield of around 4.2 mmol gcat−1 h−1 at 320 °C temperature, 20 bar pressure and 3600 ml gcat−1 h−1 space velocity. The H2-TPR, XPS, and CO2-TPD of the catalysts indicate that adding the manganese increases the basicity which increases the CO2 conversion as well as reducibility of the catalyst and thus facilitates the formation of Hägg carbide (Fe5C2), the active catalysts site. DFT calculations were performed on the CuFeO2(102), Mn4O4-CuFeO2(102), and Na-Mn4O4-CuFeO2(102) surfaces, demonstrating that the MnO promotional effect lowers the energy barrier for oxygen vacancy generation. Also, a kinetic estimation of the experimental data under varying reaction conditions illustrates the promotional effect of Mn in lowering the activation energy of the direct CO2 hydrogenation compared to the unpromoted catalyst. The values of activation energy and rate constants at temperature 320 °C are 28.483 kJ/mol & 1.69 × 10−4 mol (min gcat bar1/3)−1 and 29.358 kJ/mol & 3.35 × 10−5 mol (min gcat bar1/3)−1, respectively for the Mn-promoted and unpromoted Na-CuFeO2 catalysts. The activation energy values derived from the experimental data presented herein are in good agreement with that reported (25–50 kJ/mol) in literature for iron catalysts.