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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MECHANISM (The effect of water on the Fischer-Tropsch reaction mechanism and kinetics over bimetallic Co-based catalysts: Theoretical and experimental studies)

Teaser

Summary

The Fischer-Tropsch Synthesis (FTS) is currently an industrial catalytic reaction that converts a gas mixture of carbon monoxide and hydrogen (syngas) into liquid hydrocarbons (synthetic fuels) and water. The FTS reaction provides the most economic path for the synthesis of liquid fuels free of sulphur and with lower soot emissions than conventional fuels produced in the oil refineries. H2O is formed during the FTS and influences rates and selectivities of reaction products. This project addresses the roles of void structure, Co nanoparticle size, extent of reduction of Co oxide precursors and experimental conditions (P, T) on the magnitude of these rate and selectivity enhancements by the presence of water (added on purpose or increases in concentration with increasing residence time).
The pore size of support plays an important role on the effect of water on the FTS rates. Catalysts with large pore size shows positive effect of water on the FTS rates, while catalysts with medium and small pore size show negligible effect. In the case of medium and small pore size catalysts, the condense intrapore liquid H2O phase may already exist, while large pore size catalyst, requires higher H2O pressures for intrapore condensation. These rates enhancements caused by H2O (large pore size) are independent of the Co nanoparticle size and the extent of reduction. In all cases (small, medium and large pore size), CH4-selectivity decreases and C5+ selectivity increases with increasing water partial pressure.

Work performed

Cobalt-based catalysts (M1â€“M2/support) were prepared by the incipient wetness impregnation method, with different pore size of support (3.3-11.6 nm), Co loading (10-30 wt%), noble metal loading (0.5-1 wt%) and support (SiO2, Î³-Al2O3 and La2O3-Î³-Î‘l2O3). The Co-based catalysts were characterized by TEM, PXRD, BET, H2-chemisorption, O2-titration and H2-TPR techniques. The catalysts were tested towards FTS reaction with H2O present in the synthesis gas feed stream. The effect of: (i) void structure, (ii) Co nanoparticle size, (iii) extent of reduction of Co precursors and (iv) experimental conditions (temperature, pressure and mean residence time) were investigated. It was found that the pore size of support and the experimental conditions plays an important role on the effect of water on the FTS rates.

Final results

SSITKA-MS, SSITKA-DRIFTS and other transient isotopic experiments will be used to estimate the effect of water partial pressure, Co nanoparticle size, extent of reduction of Co oxide precursors, reaction temperature and time on stream on the concentration of active intermediates and inactive species formed during FTS. The experiments will be conducted using 13CO isotope gas in the feed stream. The surface coverage of active intermediate species and of inactive ones will be correlated with the catalytic performance of the various Co-based catalysts to be investigated. It is expected that the concentration of inactive carbonaceous deposits (e.g., various types of adsorbed -CxHy and Co2C) to increase with time on stream, while the concentration of active intermediate species to decrease or remain constant. According to the MECHANISM results obtained until now, it is expected that H2O partial pressure will not affect the concentration of active intermediate species and of the inactive ones. On the other hand, the Co particle size, the extent of reduction and reaction temperature is expected to affect the concentration of active and inactive carbonaceous species.
This project is important from an economic point of view since the price of crude oil is rising frequently (production of synthetic fuels) and the development of more active and selective FTS catalysts would offer an alternative given the environmental advantages of fuels derived from FTS. The results of this project are considered very important and strongly linked to the European energy needs. It is also expected to stimulate other European scientific teams to investigate more closely the effect of water on FTS reaction rates and product selectivity.