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Ind. Eng. Chem. Res. 2010, 49, 11098–11100
Improving the Stability of Cobalt Fischer-Tropsch Catalysts by Boron Promotion Mark Saeys,*,† Kong Fei Tan,† Jie Chang,‡ and Armando Borgna*,‡ National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576, and Institute of Chemical and Engineering Sciences, 1 Pesek Road, Singapore 627833
Supported Co catalysts exhibit favorable activity and selectivity for Fischer-Tropsch synthesis (FTS) but deactivate slowly. To explore deactivation by carbon deposition, the stability of various forms of deposited carbon was evaluated using density functional theory (DFT). A surface carbide and graphene islands were calculated to be thermodynamically stable. Two forms of deposited carbon are also distinguished experimentally after 200 h of FTS. On the basis of this mechanistic insight, boron was proposed as a promoter to enhance the stability of Co catalysts. DFT calculations indicate that boron and carbon display similar binding preferences, and boron could selectively block the deposition of resilient carbon deposits. To evaluate the theoretical predictions, supported 20 wt % Co catalysts were promoted with 0.5 wt % boron and tested under realistic FTS conditions. Boron promotion was found to reduce the deactivation rate 6-fold, without affecting selectivity and activity. 1. Introduction Fischer-Tropsch synthesis (FTS) converts synthesis gas, a mixture of CO and H2, to linear hydrocarbons. Because FTS fuels meet stringent environmental requirements and the FTS feedstock can be produced from natural gas, coal, and renewable biomass, FTS has regained interest from industry and academia.1 Both Fe and Co-based catalysts are used industrially. While Fe-based catalysts are less expensive, Co-based catalysts show a higher activity and a higher selectivity toward paraffinic products.2,3 However, supported Co catalysts deactivate slowly under realistic FTS conditions and it is therefore desirable to improve their stability. To develop Co-based FTS catalysts with improved stability, a detailed understanding of the deactivation mechanism is required. Different mechanisms have been proposed, but carbon deposition has recently gained support.3,4 Moodley et al.3 studied the deactivation of supported Co catalysts in a pilot-scale 100 barrel/day slurry bubble column reactor under realistic FTS conditions of 230 °C, 20 bar, and H2/CO ) 2. After 55 days of reaction, the catalytic activity was reduced by 50%. Concurrently with the loss in activity, the gradual formation of resilient carbon deposits could be observed using temperature programmed hydrogenation (TPH). To further characterize the nature of the resilient carbon species, we studied the deactivation of a 20 wt % Co/γ-Al2O3 catalyst during FTS at 240 °C, 20 bar, and H2/CO ) 2.4 Both carbidic and polyaromatic carbon species could be identified on the Co catalyst after 200 h of reaction using a combination of characterization techniques.4 The characterization data were supported by density functional theory (DFT) calculations. The DFT calculations indicate that the formation of extended graphene islands is highly favorable under FTS conditions with a Gibbs free reaction energy of -116 kJ/ mol. The thermodynamic driving force to form a p4g surface carbide growing from step defects is slightly smaller at -96 kJ/ mol, but more favorable than the formation of narrow graphene strips at step sites. Recently, boron was proposed as a promoter to enhance the stability of Ni catalysts during steam reforming.5-7 Carbon and * To whom correspondence should be addressed. E-mail: chesm@ nus.edu.sg. Tel.: +65 6516 5826. Fax: +65 6779 1936 (M.S.). E-mail:
[email protected]. Tel.: +65 6796 3802. Fax: +65 6316 6182 (A.B.). † National University of Singapore. ‡ Institute of Chemical and Engineering Sciences.
boron were calculated to show similar binding preferences on Ni catalysts, and small amounts of boron were hence proposed to selectively block the deposition of resilient carbon species.6 To evaluate whether this concept can be extended to supported Co catalysts, the stability of boron on Co surfaces was calculated using DFT. The calculations indicate that boron adsorption is stable under FTS conditions and mimics the adsorption of resilient carbon species. Next, the activity and stability of supported Co catalysts promoted with 0.5 wt % boron was tested for 200 h in a fixed bed microreactor. 2. Computational and Experimental Methods Boron and carbon binding energies were computed using periodic spin polarized DFT with the Perdew-Burke-Ernzerhof functional (DFT-PBE)8 as implemented in the Vienna Ab Initio Simulation Package (VASP).9 Co terraces were modeled with a three-layer Co(111) slab where the top two layers and the adsorbates were fully relaxed, while step sites were modeled using a p(2 × 8) and p(4 × 8) Co(111) unit cell where four rows of Co atoms were removed from the surface layer.4,10,11 To evaluate the stability of boron and carbon under realistic conditions, reaction free energies, ∆Grxn (500 K, 20 bar) with reference to a reservoir of CO, H2, H2O, and B2H6 were calculated for the following reactions: CO(g) + H2(g) T C* + H2O(g)
(1)
/2B2H6(g) T B* + 3/2H2(g)
(2)
1
Supported cobalt catalysts were prepared by aqueous slurry impregnation of a γ-Al2O3 support with a surface area of 380 m2/g with a cobalt nitrate precursor (Sigma-Aldrich, 98% purity) to produce Co loadings of 20 wt %, following a procedure described elsewhere.4 The boron promoter was introduced by a second slurry impregnation with boric acid (Sigma-Aldrich, 99% purity) to produce boron loadings of 0.5 and 2.0 wt %. The unpromoted and promoted catalysts were tested for 200 h at 240 °C, 20 bar, H2/ CO ) 2, and W/F ) 7.5 gcath/mol to evaluate the effect of boron on the activity, selectivity and stability. About 1.0 g of the catalyst with a particle size range between 212 and 300 µm was diluted with 18 g SiC with a similar particle size to minimize heat transfer limitations. The Co FTS catalysts were reduced in the fixed bed microreactor for 12 h under 50 NmL/min H2 at 500 °C and
10.1021/ie100523u 2010 American Chemical Society Published on Web 07/14/2010
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 1. Boron and Carbon Binding Energies and Gibbs Free Energy of Reaction, ∆Grxn (500 K, 20 bar) under FTS Conditions for Various Adsorption Geometries on Co Terrace and Step Surfaces
a Gibbs free energy for the CO(g) + H2(g) T C* + H2O(g) and for the 1/2B2H6(g) T B* + 3/2H2(g) reaction under FTS conditions.
atmospheric pressure. After reduction, the reactor was cooled to 120 °C before introduction of the synthesis gas. Products were analyzed online with an Agilent GC 6890 gas chromatograph equipped with a thermal conductivity detector and a flame ionization detector. A first order deactivation model, da/dt ) -ka,12 where the catalyst activity a(t) is the rate relative to the initial rate and k is the deactivation rate coefficient, was used to quantify deactivation during FTS. Hydrogen chemisorption was used to determine the particle size and dispersion of the Co-based catalysts. After reduction in H2 at 500 °C for 2 h, the H2 adsorption isotherms were recorded between 80 and 800 mbar at 25 °C in a Quantachrome Autosorb 1C instrument. The dispersion and particle size were calculated assuming a H:Co ratio of 1. The reactor average turnover frequency (TOF) was calculated using the number of active sites determined by H2 chemisorption. To characterize the nature of the boron promoter after reduction, X-ray photoelectron spectroscopy (XPS) was used. The procedure to prepare the XPS samples is described elsewhere.7 3. Results and Discussion Binding energies and thermodynamic stabilities under FTS conditions were calculated for different forms of deposited carbon and boron (Table 1). Carbon and boron atoms adsorb at the hcp hollow sites of the Co(111) surface with binding energies of -658 and -535 kJ/mol, respectively. Surface carbon is barely thermodynamically stable under FTS conditions at -4 kJ/mol. Surface boron is thermodynamically unstable under FTS conditions. Conversion of surface carbon to surface CH* or CH2* groups
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increases the stability to -18 and -17 kJ/mol. Subsurface carbon is only 2 kJ/mol more stable than surface carbon, and the driving force for carbon to diffuse to the subsurface and bulk octahedral sites is smaller for Co than for Ni.6 Subsurface boron is 10 kJ/mol more stable than surface boron at low concentrations. However, the stability of subsurface boron increases for high concentrations. A monolayer of subsurface boron is calculated to induce a surface reconstruction due to boron-boron interactions, as was also observed for the Ni(111) surface.10 The reconstruction greatly enhances the stability of subsurface boron to -53 kJ/mol under FTS conditions. Carbon and boron atoms also bind strongly at step sites at low step coverage, but the stability reduces with step coverage. Diffusion of both carbon and boron into the step sites is calculated to induce a p4g clock reconstruction, forming a surface carbide4 and a surface boride (Table 1). Both structures are calculated to be very stable at -96 and -59 kJ/mol and are more favorable than subsurface carbon and boron. In a p4g clock reconstruction, the Co surface atoms undergo small displacements, creating 3- and 4-fold hollow sites. The driving force for this reconstruction is the increased binding energy at the new 4-fold hollow sites. Displacement of the boron atoms by carbon seems unlikely. Although carbon is more stable than boron at the step and p4g sites, the energy cost to move boron atoms to the terraces is significantly higher than the energy gained by carbon adsorption at the step or p4g sites. Carbon can also form a very stable polyaromatic graphene overlayer. Large graphene islands are 20 kJ/mol more stable than the p4g surface carbide. On Ni catalysts, graphene sheets have been found to nucleate and grow from step defects.13,14 Here, we evaluated the stability of narrow graphene strips growing out of the step sites. Calculations indicate that hydrogen termination of the remaining carbon edges is highly favorable.4 The stability of narrow, hydrogen terminated graphene strips is significantly lower than the stability of large graphene islands, and increases from -76 kJ/mol for a three carbon atom wide strip to -80 and -82 kJ/mol for five and seven carbon atoms wide strips.4 The calculations hence indicate that boron is most stable at step and at p4g clock sites of the Co surface. This suggests that, similar to the effect for a Ni catalyst, the introduction of small and controllable amounts of boron might remove adsorption sites for the nucleation and growth of resilient carbon deposits. To evaluate the effect of boron promotion, boron promoted Cobased catalysts were prepared. Introducing a monolayer of boron atoms on the 10 nm Co particles of our 20 wt % Co/γ-Al2O3 would require approximately 0.3 wt % boron. However, a large fraction of the boron atoms, introduced as boric acid, can be expected to bind to the γ-Al2O3 support. To confirm that some of the boron atoms are indeed associated with the Co particles, a 20 wt % Co/ γ-Al2O3 catalyst was promoted with 2.0 wt % boron and reduced under H2 at 500 °C. The boron 1s XPS spectrum in Figure 1 clearly shows the formation of a shoulder at 188 eV, characteristic of Co boride.15 A similar shoulder is also observed for lower boron loadings, but the XPS signal becomes weaker. To confirm that the reduced boron atoms are indeed associated with the Co particles, the γ-Al2O3 support was also loaded with 2.0 wt % B and subjected to the same reduction procedure. No shoulder could be observed in the boron 1s XPS data in the absence of Co (Figure 1). Next, the effect of 0.5 wt % boron on the stability, activity, and selectivity of a 20 wt % Co/γ-Al2O3 catalyst was evaluated under typical FTS conditions (Table 2). Introduction of 0.5 wt % boron had a limited effect on the catalyst reducibility and reduced the hydrogen uptake by less than 10%. Higher boron concentrations have a more significant effect on the catalyst reducibility and on the hydrogen chemisorption data. The boron promoted and the 4
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200 h of FTS. Boron atoms bind strongly at step sites and at a p4g clock reconstruction. The addition of a small and controllable amount of a boron promoter might hence be able block the adsorption and growth of resilient carbon species. XPS characterization of a boron promoted Co catalyst after reduction indicates the formation of a Co boride, though much of the boron remains as boron oxide and is likely associated with the γ-Al2O3 support. Promotion of a 20 wt % Co/γ-Al2O3 catalyst with 0.5 wt % boron was found to reduce the deactivation rate coefficient 6-fold, without affecting the activity and C5+ selectivity in a 200 h experiment. Acknowledgment Financial and experimental support from the Agency for Science, Technology and Research (A-Star project 062-101-0035) and from the National University of Singapore are gratefully acknowledged. Figure 1. Boron 1s XPS spectra for a 20 wt % Co/γ-Al2O3 catalyst and for the γ-Al2O3 support, both promoted with 2.0 wt % boron: experimental signal (solid line) and after deconvolution (bold line) and (gray line). Table 2. Effect of 0.5 wt % Boron Promotion on the Dispersion and the Catalytic Performance of a 20 wt % Co/γ-Al2O3 FTS Catalyst unpromoted
0.5 wt % boron
H2 chemisorption Co particle diameter dispersion H2 uptake
(nm) 10.5 (%) 9.8 (µmol/gcat) 161
11.4 8.7 148
catalytic performance maximum TOF maximum CO conversion deactivation rate coefficient chain growth probability R C1/C5+ selectivity
(s-1) (%) (h-1) (%)
37 × 10-3 96 -1.7 × 10-3 0.70 24/60
36 × 10-3 93 -2.7 × 10-4 0.71 22/61
unpromoted catalyst show a similar initial activity and selectivity during the reaction tests (Table 2). The maximum TOF of 37 × 10-3 s-1 can be compared with a value of 25 × 10-3 s-1 obtained by extrapolating the TOF reported for a 15 wt % Co/γ-Al2O3 catalyst at 215 °C and 8.2 bar to our reaction conditions, using a power law kinetic model proposed by Ribeiro et al.16 The C5+ selectivity of 60% and the chain growth probability of 0.70 are slightly lower than values reported by Oukaci et al.17 for comparable reaction conditions. After 25 h, the unpromoted Co catalyst begins to deactivate with a first order deactivation rate coefficient of -1.7 × 10-3 h-1. After 200 h, the average TOF has reduced to 25 × 10-3 s-1. The decrease in activity over 200 h is comparable to the activity loss reported in literature.3 Promotion of the catalyst with 0.5 wt % boron does not affect the maximum activity or the maximum CO conversion, but reduces the subsequent deactivation rate 6-fold to -2.7 × 10-4 h-1. After 200 h of reaction, the promoted catalyst retains 92% of its maximum activity, while the activity of the unpromoted catalyst has reduced to below 70%. It should be noted that promotion with 0.5 wt % does not affect the C5+ selectivity or the chain growth probability. 4. Conclusions The relative stability of different forms of deposited carbon and of boron on Co catalysts was calculated using DFT. The formation of extended graphene islands and of a p4g clock reconstruction, both growing from steps sites, was calculated to be thermodynamically favorable under FTS conditions. Two types of resilient carbon could also be detected on a 20 wt % Co/γ-Al2O3 catalyst after
Literature Cited (1) Dry, M. E. Practical and theoretical aspects of the catalytic FischerTropsch process. Appl. Catal., A 1996, 138, 319. (2) Iglesia, E. Design, synthesis and use of cobalt-based Fischer-Tropsch Synthesis catalyst. Appl. Catal., A 1997, 161, 59. (3) Moodley, D. J.; van de Loosdrecht, J.; Saib, A. M.; Overett, M. J.; Datye, A. K.; Niemantsverdriet, J. W. Carbon deposition as a deactivation mechanism of cobalt-based Fischer-Tropsch synthesis catalysts under realistic conditions. Appl. Catal., A 2009, 354, 102. (4) Tan, K. F.; Xu, J.; Chang, J.; Borgna, A.; Saeys, M. Carbon deposition on Co catalysts during Fischer-Tropsch synthesis: a computational and experimental study. J. Catal. 2010, in press, doi: 10.1016/ j.jcat.2010.06.008. (5) Chen, L.; Lu, Y.; Hong, Q.; Lin, J.; Dautzenberg, F. M. Catalytic partial oxidation of methane to syngas over Ca-decorated-Al2O3-supported Ni and NiB catalyst. Appl. Catal., A 2005, 292, 295. (6) Xu, J.; Saeys, M. Improving the coking resistance of a Ni-based catalyst by promotion with subsurface boron. J. Catal. 2006, 242, 217. (7) Xu, J.; Chen, L.; Tan, K. F.; Borgna, A.; Saeys, M. Effect of boron on the stability of Ni catalyst during steam methane reforming. J. Catal. 2009, 261, 158. (8) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. ReV. Lett. 1996, 77, 3865. (9) Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metal. Phys. ReV. B 1993, 47, 558. (10) Xu, J.; Saeys, M. First principles study of the effect of carbon and boron on the activity of a Ni catalyst. J. Phys. Chem. C 2009, 113, 4099. (11) Andersson, M. P.; Abild-Pedersen, F. Carbide induced reconstruction of monoatomic steps on Ni(111) - A density functional study. Surf. Sci. 2007, 601, 649. (12) Froment, G. F.; Bischoff, K. B. Chemical reactor analysis and design, 2nd ed.; John Wiley & Sons: New York, 1990. (13) Abild-Pedersen, F.; Nørskov, J. K.; Rostrup-Nielsen, J. R.; Sehested, J.; Helveg, S. Mechanisms for catalytic carbon nanofiber growth studied by ab initio density functional theory calculations. Phys. ReV. B 2006, 73, 115419. (14) Bengaard, H. S.; Nørskov, J. K.; Sehested, J.; Clausen, B. S.; Nielsen, L. P.; Molenbroek, A. M.; Rostrup-Nielsen, J. R. Steam reforming and graphite formation on Ni catalyst. J. Catal. 2002, 209, 365. (15) Kukula, P.; Gabova, V.; Koprikova, K.; Trtik, P. Selective hydrogenation of unsaturated nitriles to unsaturated amines over amorphous CoB and NiB alloys doped with chromium. Catal. Today 2007, 121, 27. (16) Ribeiro, F. H.; Schach von Wittenau, A. E.; Bartholomew, C. H.; Somorjai, G. A. Reproducibility of turnover rates in heterogeneous metal catalysis: Compilation of data and guidelines for data analysis. Catal. ReV.Sci. Eng. 1997, 39, 49. (17) Oukaci, R.; Singleton, A. H.; Goodwin, J. G. Comparison of patented Co-FT catalysts using fixed bed and slurry bubble column reactors. Appl. Catal., A 1999, 186, 129.
ReceiVed for reView March 8, 2010 ReVised manuscript receiVed June 25, 2010 Accepted June 25, 2010 IE100523U
