Design, Synthesis, and Mechanistic Evaluation of Iron-Based Catalysis for Synthesis Gas Conversion to Fuels and Chemicals

This project extends previously discovered Fe-based catalysts to hydrogen-poor synthesis gas streams derived from coal and biomass sources. These catalysts have shown unprecedented Fischer-Tropsch synthesis rates and selectivities for feedstocks consisting of synthesis gas derived from methane. During the first reporting period, we certified a microreactor, installed required analytical equipment, and reproduced synthetic protocols and catalytic results previously reported. During the second reporting period, we prepared several Fe-based compositions for Fischer-Tropsch Synthesis and tested the effects of product recycle under both subcritical and supercritical conditions. During the third and fourth reporting periods, we improved the catalysts preparation method, which led to Febased FT catalysts with the highest FTS reaction rates and selectivities so far reported, a finding that allowed their operation at lower temperatures and pressures with high selectivity to desired products (C5+, olefins). During the fifth reporting period, we studied the effects of different promoters on catalytic performance, specifically how their sequence of addition dramatically influenced the performance of these materials in the FischerTropsch synthesis. We also continued our studies of the kinetic behavior of these materials. Specifically, the effects of H2, CO, and CO2 on the rates and selectivities of FischerTropsch Synthesis reactions led us to propose a new sequence of elementary steps on Fe and Co Fischer-Tropsch catalysts. More specifically, we were focused on the roles of hydrogen-assisted and alkali-assisted dissociation of CO in determining rates and CO2 selectivities. During this sixth reporting period, we have studied the validity of the mechanism that we propose by analyzing the H2/D2 kinetic isotope effect (rH/rD) over a conventional ironbased Fischer-Tropsch catalyst Fe-Zn-K-Cu. We have observed experimentally that the use of D2 instead of H2 leads to higher hydrocarbons formation rates (inverse kinetic isotopic effect). On the contrary, primary carbon dioxide formation is not influenced. These experimental observations can be explained by the two CO activation pathways we propose. During this reporting period, the experimental kinetic study has been also complemented with periodic, self-consistent, DFT-GGA investigations in a parallel collaboration with the group of Manos Mavrikakis at the University of WisconsinMadison. These DFT calculations suggest minimal energy paths for proposed elementary steps on Fe(110) and Co(0001) surfaces. These calculations support our novel conclusions about the preferential dissociation of CO dissociation via H-assisted pathways on Fe-based catalysts. Unassisted CO dissociation also occurs and lead to the formation of CO2 as a primary oxygen scavenging mechanism after CO dissociation on Fe-based catalysts. Simulations and our experimental data show also that unassisted CO dissociation route is much less likely on Co surfaces and that hydrocarbons form exclusively via H-assisted pathways with the formation of H2O as the sole oxygen rejection product. We have also started a study of the use of colloidal precipitation methods for the synthesis of small Fe and Co clusters using recently developed methods to explore possible further improvements in Fischer-Tropsch synthesis rates and selectivities. We have found that colloidal synthesis makes possible the preparation of small cobalt particles, although large amount of cobalt silicate species, which are difficult to reduce, are formed. The nature of the cobalt precursor and the modification of the support seem to be critical parameters in order to obtain highly dispersed and reducible Co nanoparticles.