Title : Passivation by atomic layer deposition to increase the lifetime of Co/Al2O3 catalysts in the Fischer Tropsch synthesis
Nowadays, Fischer-Tropsch synthesis (FTS) has regained interest motivated by the development of the biorefinery concept, which is focused on producing chemicals and fuels from biomass sources. Despite being discovered more than a century ago, FTS is still facing some challenges, which hinder its consolidation as a viable alternative to produce synthetic fuels from renewable sources. In this regard, deactivation is a major issue [10.1016/j.cattod.2010.02.077]. In this work, Atomic Layer Deposition (ALD) was used as a passivation technique in cobalt-based catalysts to prevent from deactivation during FTS. This technique has been proven to be feasible in other reactions [10.1021/ar300229c], but not in the FTS.
Starting from a Co/γ-Al2O3 reference (REF, Johnson Matthey), three catalysts have been passivated by Al2O3-ALD at different levels (3, 6 and 10 cycles -3ALD, 6ALD, 10ALD, respectively-), then characterized (N2 physisorption, XRD, TPR, Oxygen titration) and tested in the FTS (230 ºC, 20 barg, 8400 NmL/gh, H2/CO 2 (v/v)). Characterization results revealed that the number of ALD cycles did not affect the cobalt particle size, but led to important changes in reduction properties. In this sense, the higher the number of ALD cycles, the harder the cobalt particles to reduce. These differences were also observed in FTS tests, so that initial catalytic activity of 3ALD (A/AREF = 0.99) and 6ALD (A/AREF = 0.95) was practically the same as the reference, whereas it was lower in sample 10ALD (A/AREF = 0.71). After 100 h of experiment, activity of 3ALD was higher than that of the reference (A/AREF = 1.02), which confirmed the positive effect of ALD as a way to increase the resistance of Co-based catalysts against deactivation in the FTS. Extrapolation of 100 h results to one-year operation (8000 h) led to a significant increase in catalytic resistance against deactivation.
Comparison between REF and 3ALD-REF results and power fitting (a) / Extrapolation of fitted results to 8000 h operation (b). ???? = ????(%) ? ????????????????/100
To understand the deactivation phenomena and the prevention role of ALD, post-mortem characterization studies will be carried out. Sintering is one of the main causes of catalyst deactivation in the FTS [10.1016/j.cattod.2010.02.077], although other causes such as carbon deposition and cobalt re-oxidation cannot be ruled out.
Further studies will be focused in two pathways: optimize the ALD variables (temperature, number of cycles, ALD precursor) and evaluate the catalytic performance in FTS with real syngas mixtures (H2/CO/CO2). This work if part of the European project Heat to Fuel [www.heattofuel.eu], which aims to deliver cost-competitive biorefineries to produce synthetic fuels from biomass residues.