Insights into the regulation of FeNa catalysts modified by Mn promoter and their tuning effect on the hydrogenation of CO2 to light olefins

Abstract

This study unravels the effect of Mn promoter on the surface enrichment and competition of promoters and the evolutions of the chemical and structural of Fe species during the reaction, and illustrates the regulation of FeMn and FexCy for CO2 conversion and hydrogenation of intermediates. With the contents of Mn promoter increasing from 0.5% to 10.0%, the selectivity of light olefins decreasing from 38.73% to 30.58% and the ratio of olefin to paraffin decreases from 4.97 to 3.56. The surface ratio of Mn/Fe and Na/Fe is much higher than the impregnation ratio, which confirms the surface enrichment of Mn and Na promoters. It turns out that the iron oxides are transformed into iron carbides in the bulks and oligomeric FexOy on the surface after reaction. The enrichment of Mn promoters leads to more surface Fe species near the Mn promoters for the strong adsorption of CO2 and decreases the surface actual density of FexCy sites for the hydrogenation of CO to hydrocarbons, which consequently decreases the conversion of CO2 and the FTY value. The increasing content of Mn promoters from 0.5% to 3.0% also increases the adsorption ability of H2 on the surface of iron carbides, which is possibly associated with the hydrogenation of intermediates and leading to decreasing of the ratio of olefins to paraffins ranging from C2-C4.