546629 Structure-Performance Relationships on Co-Based Fischer-Tropsch Synthesis: The Impact of H2-CO-H2 Activation Treatment

Monday, June 3, 2019: 5:48 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Nikolaos Tsakoumis1, Eleni Patanou2, Rune Myrstad3, Erling Rytter4 and Edd Blekkan4, (1)Department of Chemical Engineering, NTNU, Trondheim, Norway, (2)NTNU, Trondheim, Norway, (3)SINTEF Industry, Kinetics and Catalysis Research Group, Trondheim, Norway, (4)Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway

Structure-Performance relationships on Co-based Fischer-Tropsch Synthesis:
The impact of H2-CO-H2 activation treatment

Nikolaos E. Tsakoumis a, Eleni Patanou a, Rune Myrstad b, Erling Rytter a,b, Edd A. Blekkan a

a Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim

b SINTEF Industry, N-7465 Trondheim, Norway <>1.      Introduction

Fischer – Tropsch synthesis (FTS) is the heart of the gas-to-liquids (GTL) process which is widely used for the utilization of synthesis gas deriving from natural gas, coal and in the future also biomass1. Cobalt based catalysts are the most attractive choice for natural gas based FTS, because of their high activity, good stability and low water–gas shift activity2. FT catalyst activity and selectivity depend on several factors such as Co nanoparticle (NP) size, nature of the support, the presence of promoters, preparation methods and pre-treatment conditions3. FTS is a structure sensitive reaction4 where catalyst performance strongly depends on NP size. In addition, the crystallographic phase of cobalt appears to have an influence on catalyst performance5. The phase of Co nanoparticles can be altered by the application of multistep preparation procedures that include a carburization step6.

The present study has been performed on Re/Co/γ-Al2O3 and Co/α-Al2O3 catalysts and focuses on the effect of pre-treatment conditions on FTS. The pre-treatment includes a sequence of Reduction (H2) - Carburization (CO) - Reduction (H2) (RCR) on calcined Co-catalysts and is compared with a conventional one step H2 reduction. The goal of the work is to unravel relationships between nanoparticle structure and the effect of different types of carbon on the FTS performance. <>2.      Results

Catalysts were prepared by insipient wetness impregnation of Co and Re (if added) salts. All kinetic data reported are from lab scale fixed-bed reactors7. The materials prepared were extensively investigated using ex situ and in situ characterization methods. This included synchrotron based X-ray diffraction (XRD) and X-ray absorption near edge structure spectroscopy (XANES), thermo-gravimetric analysis (TGA) coupled with a mass spec (MS), H2 Chemisorption, Raman spectroscopy (RS), transmission electron microscopy (TEM) and 13C-NMR. The obtained average cobalt nanoparticle size was 12nm, far above the size sensitivity region. All the samples were reduced at 350°C prior to the carburization step. Subsequent carburization was performed at temperatures from 200°C to 300°C and eventually the final reduction step at temperatures from 200°C to 450°C. Air exposure of the carburized sample was evaluated for exploiting potential use of a carburized catalyst as a precursor.

As has been shown previously by several research groups the H2 reduction of Co3O4 NPs, found in the calcined catalyst, leads to a complex metallic Co0 structure consisting of both fcc-Co and hcp-Co where the fcc phase is dominant. After carburization of the reduced catalyst, under pure CO at 14 bar, the Co NPs adapt mainly a Co2C orthorhombic structure, while significant amount of carbon is co-produced. After the final H2 reduction step the Co2C decomposes to Co NPs with a hcp-Co rich configuration independent of the temperature conditions applied, whereas a small contribution from fcc-Co can be observed (Figure 1). Clearly, the application of RCR has both a positive and negative influence at CO consumption rate and C5+ selectivity, depending on the applied temperature conditions of each step and air exposure of the sample. On the contrary, catalyst deactivation behaviour is similar independent on the applied activation procedure (Figure 1). Co NPs retain their structure even after 120 h on stream.   

Figure 1. Effect of the RCR activation protocol CO reaction rate (210 C, 20 bar, GHSV=15000 Nml/hg, H2/CO=2.1, TOS=24 h), C5+ selectivity (210 C, 20 bar, 46-48% CO conversion, H2/CO=2.1) and crystal structure. The reference catalyst, which underwent conventional H2 reduction activation, is denoted with green colour.


Figure 2. (a) Raman spectra of the calcined catalyst and carburized catalyst samples at 230°C and 300°C (325 nm excitation and 0.3 mW laser power). (b) Contour plot of differential XANES (ΔXANES) of the normalized spectra shows the slow development of a phase formed on Co2C; first minus last scan (up).

Carbon formation is evident after carburization treatments at all temperatures investigated (Figure 2). At 230°C a less H2 resistant carbon is produced than after  carburization at 300 ⁰C. More than 5 types of carbon were identified to be present with respect to their structure and resistance to hydrogenation, in addition to the Co2C that was detected by XRD. Atomic carbon remaining on cobalt lattice due to low temperatures of the final reduction step has a negative effect on FTS performance. The detrimental effect of more stable structures of carbon appears related to the secondary hydrogenation ability of the catalyst however; it merits further investigation since it apparently demonstrates both positive and negative aspects.

The effect of air exposure of the carburized sample was investigated. Apparently, although the bulk crystalline structure of the catalysts doesn’t change, minor structural changes occur as observed by in situ XANES (Figure 2). A clear performance loss is observed upon exposure to air for the catalyst carburized at low temperature (230⁰C). Our results clearly demonstrate that the application of RCR activation protocols combined with O2 passivation or air exposure with the idea of further use of the material as a precursor in CSTRs or other temperature limited applications probably are not applicable.  <>3.      Conclusions

·       H2 reduction of a calcined Re/Co/γ-Al2O3 catalyst lead to the formation of Co NPs with intergrown fcc-Co and hcp-Co, rich in stacking disorder. Fcc is the dominant phase. On the contrary, a three step RCR activation procedure gives hcp-rich Co NPs. Small fcc-Co is present depending on the applied conditions.  

·       Overall, the three step RCR activation procedure had a strong impact on the catalyst performance compared to the ordinary H2 reduction process. FTS results show a rather diverse behavior depending on the pretreatment temperature. Both exceptional and very poor performance was recorded.

·       Poor performance can be related partially to remaining carbon species. Carbon formation is evident after carburization treatments at all investigated temperatures 200°C to 300°C. Optimum carburization temperature lies between 200 and 265°C. The H2 resistant carbon structures formed has primarily a negative influence in catalyst performance, with only one minor positive aspect. 

·       Air exposure of Co2C leads to minor phase change of Co, observed by in situ XANES, that has an impact on catalyst performance.

·       A coverage dependence on the crystallographic phase for the surface species could also be extracted. <>References

(1) Dry, M. E. In Handbook of Heterogeneous Catalysis; 2008; pp 2965–2993.

(2) Iglesia, E. Appl. Catal. A Gen. 1997, 161, 59–78.

(3) Rytter, E.; Tsakoumis, N. E.; Holmen, A. Catal. Today 2016, 261 (2016), 3–16.

(4) Bezemer, G. L. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128 (12), 3956–3964.

(5) Ducreux, O.; Rebours, B.; Lynch, J.; Roy-Auberger, M.; Bazin, D. Oil Gas Sci. Technol. - Rev. l’IFP 2008, 64 (1), 49–62.

(6) Hofer, L. J. E.; Peebles, W. C. J. Am. Chem. Soc. 1947, 69 (10), 2497–2500.

(7) Patanou, E.; Tsakoumis, N. E.; Myrstad, R.; Blekkan, E. A. Appl. Catal. A Gen. 2018, 549, 280–288.

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