Al2O3 and ReâCo

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

EFFECTS OF Co PARTICLE SIZE ON THE STABILITY OF Co/Al2O3 AND ReCo/Al2O3 CATALYSTS IN A SLURRY-PHASE FISCHER-TROPSCH REACTOR Pooneh Ghasvareh, and Kevin J. Smith Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01981 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

EFFECTS OF Co PARTICLE SIZE ON THE STABILITY OF Co/Al2O3 AND Re-Co/Al2O3 CATALYSTS IN A SLURRY-PHASE FISCHER-TROPSCH REACTOR

Pooneh Ghasvareh, Kevin J. Smith* Department of Chemical & Biological Engineering University of British Columbia 2360 East Mall Vancouver BC V6T 1Z3 Canada

KEYWORDS Catalyst deactivation, Co particle size, carbon deposition, rate of carbon deposition, FischerTropsch synthesis, slurry phase reactor

ACS Paragon Plus Environment

1

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

ABSTRACT

The stability of a series of Co/Al2O3 and Re-Co/Al2O3 Fischer-Tropsch (FT) catalysts, with varying Co particle size, was measured in a continuous flow, stirred tank reactor operated at 220°C, 2.1MPa and a H2/CO = 2/1 synthesis gas for periods up to 190 h time-on-stream (TOS). Results showed that catalyst stability was dependent upon the Co particle size, degree-ofreduction (DOR) of the catalyst precursor and the CO conversion. At the chosen operating conditions, carbon deposition was the main cause of catalyst deactivation and the initial rate of carbon deposition per active Co site increased with increased Co particle size (dCo = 2 – 22 nm) when measured at approximately the same CO conversion level. On the 15wt% Co/Al2O3 catalyst the initial rate of carbon deposition increased with CO conversion (CO conversion ≤ 40%) whereas on the 1.2wt%Re-12wt%Co/Al2O3 catalyst, the initial rate of carbon deposition decreased with increased CO conversion (CO conversions > 60%) due to high concentrations of H2O and CO2 in the reactor.


2

Page 3 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

INTRODUCTION

Co catalysts lose up to 30-50% of their activity and hydrocarbon productivity within 9-12 months of operation in a typical Fischer-Tropsch (FT) reactor.1 Since the cost of Co and noble metal promoters present in commercial FT catalysts is high, the lifetime of the catalyst has a significant impact on the process economics.2 Poisoning,3-5 sintering, 6-11 carbon deposition 3, 12-16 catalyst re-oxidation17,

18

and the formation of Co support compounds

19-22

in the presence of

H2O, are the main causes of FT catalyst deactivation that have been identified in the literature.

Sintering, a consequence of metal crystallite migration or atomic migration,23-25 occurs at high temperatures and is promoted by the presence of H2O.20, 26, 27 Bezemer et al.27 showed that the dispersion of Co catalysts supported on carbon nanofiber (average Co particle size dCo = 5 nm) decreased by 77% when the catalyst was exposed to H2O (P = 20 bar, / = 1, T = 200220 0C and a relative humidity of 65%). It is also reported that sintering is more severe during FT synthesis with a H2/CO =1.11, 28, 29 It has been suggested that at P = 20 bar, / = 1, T=220 0C sintering is H2O assisted while at / = 2-4 other mechanisms such as Co oxidation or carbon deposition are more significant. It was also shown that sintering only occurs when the Co catalyst is exposed to a combination of high CO and high H2O partial pressures and accordingly, Co/Al2O3 catalysts with dCo ~6 nm

did not show any sintering in a H2/H2O

atmosphere at typical FT reaction temperature (T~230 0C).29 Sintering of smaller metal particles is more probable than larger ones because surface diffusion is more rapid for smaller crystallites.30,

31

However, a strong metal-support interaction (MSI) may reduce sintering.32,

33

The strength of the MSI depends on the metal/metal oxide34 and the size of the metal cluster,


3

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 39

with smaller metal particles generally having a stronger MSI.35-37 Hence the Co crystallite size impacts the diffusion of Co particles and the MSI, both of which can affect sintering.

Formation of Co-support compounds may also occur as a result of diffusion of Co particles into the support. Co3O4 and γ-alumina have isotopic crystal structures which assists in the migration of cobalt ions from the Co3O4 phase into the support during oxidative treatments. Spinel compounds such as CoAl2O4 and Co2Al2O4 result, which in H2 can only be reduced at temperatures > 800 °C.7 Smaller Co particles with higher surface diffusion rates in the presence of H2O are more prone to form Co aluminate compounds.20 More recently, Moodley et al.38 reported that cobalt aluminate formation during FTS under realistic operating conditions is not a major catalyst deactivation mechanism and that even at high H2O partial pressures ( = 10 bar, / = 2.2), only about 10% cobalt aluminate is formed.

Carbon deposition is another important catalyst deactivation mechanism that can occur on FT catalysts.1, 2, 39-41 Moodley et al.39 showed that the amount of polymeric carbon deposited on a PtCo/Al2O3 FT catalyst operated at 230 ºC, H2/CO~2 and 20 bar in a 100 barrel/day slurry bubble column reactor, increased with TOS. The increased amount of carbon accounted for long-term catalyst deactivation. Keyvanloo et al.1 also showed that catalyst deactivation rate increased with increased carbon deposition on Co/Al2O3-SiO2 catalysts, operated at low CO conversions (20-25%) to prevent Co oxidation and using catalysts with large Co crystallites to prevent sintering. Since the FT product selectivity and activity of Co catalysts is dependent on Co particle size,12, 42-47 the rate of carbon deposition and catalyst deactivation may also depend on Co particle size. The number of Co active sites on the catalyst surface depends on the catalyst


4

Page 5 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

degree-of-reduction (DOR) and the Co particle size, so that the number of active Co sites deactivated by carbon or non-reactive heavy hydrocarbons will also depend on these variables.

Finally, re-oxidation of Co catalysts has been reported as a likely deactivation mechanism in FT synthesis because the product H2O is a strong oxidizing agent that can oxidize Co particles.10, 48, 49

Van Steen et al.50 reported that small Co particles with dCo< 5 nm are prone to oxidation under

realistic FT conditions ( / < 1.5).

From the above summary it is clear that many of the reported catalyst deactivation mechanisms in the FT synthesis are related to the catalyst metal particle size. However, a study of the effect of Co particle size on the deactivation of Co-based FT catalysts has not, to our knowledge, been reported in the literature. In the present study, the deactivation rate of several Co/Al2O3 and ReCo/Al2O3 catalysts during FT synthesis is reported, with the catalysts prepared so that the Co particle size varied over a wide range (~1 to 22 nm). The FT catalyst activity was measured over a period of up to 190 h time-on-stream (TOS) using a slurry phase FT reactor. By combining the stability data with catalyst property data, the effect of Co particle size on the catalyst deactivation rate in the FT synthesis is elucidated.

EXPERIMENTAL

The Co/Al2O3 and Re-Co/Al2O3 catalysts were prepared by a modified incipient wetness impregnation method in which the Co particle size is controlled by using a water-ethylene glycol


5

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 39

impregnating solution.53 Accordingly, the required amount of Co(NO3)2.6H2O (Sigma Aldrich 98%) was dissolved in aqueous - ethylene glycol (EG) (Sigma Aldrich reagent plus 99%), with the EG weight fraction varied from 0 to 0.90. The solution was added drop-wise to 2.5 g of the γAl2O3 (Sasol 99%, pore volume 0.525 ml/g) support that was sieved before use to provide particles with diameters (d) in the size range 90≤ d≤ 150 µm. The impregnated support was aged for ~ 2 h, dried in stagnant air at 110 ºC for 3 h and calcined in a 30 mL(STP)/min flow of air at 300 ºC for 16 h. The catalysts were subsequently reduced in 9.5 vol% H2 in Ar, by heating at a ramp rate of 5 ºC/min from room temperature to 600 ºC, with the final temperature held for 30 min. After cooling to room temperature, the catalysts were transferred to the reactor in 30 mL of squalene under Ar flow to limit exposure to air. The Re-Co/Al2O3 catalysts were prepared similarly, except that a perrhenic acid solution (HReO4) (Sigma Aldrich 75-80 wt% in H2O) was added to the Co(NO3)2.6H2O and water-EG solution, so that the Re:Co molar ratio was set to 0.03125 for all catalysts. The catalysts were prepared with Co loadings of 5 to 20 wt %, with varying amounts of EG in the impregnating solution. Herein the catalysts are identified as nCo/Al2O3(f) or mRe-nCo/Al2O3(f) where n is the Co loading (wt %), m is the Re loading (wt %) and f is the mass fraction of EG in the impregnating solution.

Catalyst surface area, pore volume and pore size were estimated from N2 adsorption/desorption isotherms measured at 77 K using a Micromeritics ASAP 2020 analyzer. Prior to N2 adsorption the samples were degassed at 350 ºC in vacuum (66 Pa) for 2 h to remove moisture or adsorbed compounds. A Micromeritics Autochem II 2920 was used for temperature-programmedreduction (TPR) analysis of the calcined catalyst precursors. Prior to reduction, the precursors were dried in 50 ml (STP)/min Ar at 120 ºC for 45 min. The TPR was done in a 10%H2/Ar flow


6

Page 7 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

at 50 mL(STP)/min with the sample heated at a ramp rate of 5 ºC/min from room temperature to 600 ºC. The catalysts remained at 600 ºC for 30 min before cooling to 55 ºC. The degree-ofreduction (DOR) of the catalyst was calculated from the H2 consumption assigned to the reduction of CoO, measured as the 3rd reduction peak of the TPR profile, using the stoichiometry of the reduction reaction (CoO+H2→Co+H2O).54-56 Hence the DOR (mol %) was calculated as = ( ⁄ ) × 100, where is the moles of H2 consumed per gram of catalyst in the reduction of CoO species (mol/g) and is the moles of Co loaded on the catalyst per gram of catalyst (mol/g). CO pulse chemisorption was done after the TPR in the same unit, to measure the dispersion of metallic Co. Following TPR and cooling to 55 ºC, the catalyst sample was flushed in a 50 mL(STP)/min flow of He before CO pulses were injected repeatedly into the He flow until the response from the TCD detector showed no further CO uptake by the sample after consecutive injections. The amount of CO absorbed was subsequently used to calculate the Co dispersion (D) using Equation (1): =

××

(1)

× 100

where D is the Co dispersion (%), is the number of moles of CO adsorbed per gram of catalyst (mol/g), MW is the molecular weight of the metal, wCo is the weight fraction loading of Co on the catalyst and σ is the stoichiometric number for CO adsorption on Co, which in the present study was taken as σ = 2. 57, 58,59 Equation (1) assumes that all the Co is in metallic form. However, since the precursor Co3O4 particles were only partially reduced, the Co size was estimated by accounting for the degree-of-reduction (DOR), determined from the TPR analysis, using Equation (2).53 =

! "

(2)

×


7

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 39

where dCo is the Co particle size in nm, D is the Co dispersion (%) and DOR (%) is the degree of reduction.

XRD patterns of the calcined and reduced/passivated catalysts, the used catalysts and the Al2O3 support were collected using a Bruker D8 Focus (LynxEye detector) with a Co Kα X-ray source of wavelength 1.7902 Å. The data were obtained between 2θ =10 to 80º. The reduced catalysts were transferred in squalene under Ar flow to limit oxidation during analysis. The phase identification was conducted after subtraction of the Al2O3 background. Powder diffraction files (PDF) were used for peak identification. Crystallite size estimates were made using the Scherrer equation. The peak at 2θ~52º was used to measure the Co particle size by de-convoluting the peaks at 2θ~52º and 53º, assigned to Co and Al2O3 respectively. Also, the peak at 2θ~43º was used to measure Co3O4 particle size (the XRD diffractograms are provided in Supporting Information Figure S1-S3). Elemental C and H analysis of the calcined, reduced and used catalysts was done using a Perkin-Elmer 2400 Series 2 CHNS/O analyzer. Prior to analysis the samples were crushed with a mortar and pestle. Samples of 1.5-2.5 mg were loaded into tin capsules and injected into the combustion column of the unit. The weight percentage of C and H was determined by calibration.

Note that many of the catalyst characterization methods were applied to the reduced and/or used catalysts. Since the used catalysts were covered in wax when removed from the reactor, they were washed in methylene dichloride (CH2Cl2) and then transferred to a glass microfiber filter in order to be washed using a Soxhlet extractor. The extractor was filled with CH2Cl2 and heated to


8

Page 9 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

80 ºC. The samples were washed for 2 h and then dried in stagnant air and heated to 350 oC under vacuum prior to analysis.

The catalyst performance was measured using a 300 mL slurry phase, stirred autoclave in which the synthesis gas is contacted with the catalyst and heavy liquid hydrocarbons under forced mixing (700 rpm) and controlled pressure and temperature. The catalyst, reduced ex-situ was transferred to the reactor in 30 mL of squalene under Ar flow to avoid exposure to air. An additional 65 mL of squalene, added to the reactor, acted as the initial slurry phase in the reactor. The reactor was purged firstly in N2 and then in the feed synthesis gas (H2 ~33 mL(STP)/min, CO~16.5 mL(STP)/min and N2~20 mL(STP)/min) overnight. Subsequently, the reactor was pressurized to ~ 2.1 MPa over a period of 5 to 6 h. The reactor was then heated to the desired temperature within 30 minutes. The products from the reactor were directed through a heated line (120 °C) to a hot gas condenser, held at ~120 °C. The gases from the condenser passed through a heated outlet and a back-pressure regulator to an automated gas sampling valve connected to a Shimadzu GC-2010 gas chromatograph (GC) equipped with a capillary column and a flame ionization detector (FID) for product analysis every 3 h. The hot gases leaving the GC sampling valve were subsequently directed to a cold condenser, held at < 2 ºC, to condense any remaining hydrocarbons. The gases leaving the cold condenser were analyzed every hour by a second Shimadzu GC-2014 with a dual differential thermal conductivity detector (TCD) and an automated sampling valve. The liquid products were recovered once every 24 h from both the hot and cold condensers and analysed by GC. The gas analysis data were time averaged over the 24 h period and combined with the liquid analysis. The mass balance calculations were based on the cumulative gas and liquid analysis over the 24 hour period of each liquid sample. All the


9

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 39

experiments were conducted for a continuous period of up to 190 h TOS and were stopped when the CO conversion and the mass of the liquid samples collected from the hot and cold condenser was constant for a 48 h period. All experiments were conducted at 2.1 MPa, 220 ºC and H2/CO=2 and varying gas hourly space velocity (GHSV) from 0.01 to 0.18 mol/(g.h). The overall carbon balance and mass balance for all the experiments was within ±4% and ±5%, respectively. Experiments conducted at stirrer speeds up 1000 rpm showed no impact on the measured reaction rates, CO conversion and product selectivity, confirming that external diffusion effects were minimal and the Weisz-Prater analysis confirmed that the measured rate data were not influenced by internal diffusion effects.

RESULTS AND DISCUSSION

The beneficial effects of the EG and the Re promoter in reducing Co particle size and increasing the DOR, respectively, are shown by the data of Table 1 and the TPR profiles for the 5 wt% Co/Al2O3 catalysts, reported in Figure 1. Without EG in the impregnating solution, Figure 1 shows that the calcined precursor requires high temperature (> 450°C) to obtain a high DOR (75%), resulting in large Co crystallites (Table 1, 5Co/Al2O3(0), dCo = 22 nm). The addition of 0.70 weight fraction EG to the impregnating solution, results in a much lower DOR (3%) because of the very small Co3O4 crystallites present in the calcined precursor (3 ± 0.3 nm estimated from XRD, not shown) that have a strong MSI and that are difficult to reduce (Table 1, 5Co/Al2O3(0.7)). By adding 0.5 wt% Re to the 5Co/Al2O3(0.7) catalyst the DOR increases significantly (from 3% to 69%) while maintaining small Co particle size (dCo = 5 nm).


10

Page 11 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

By varying the EG content of the impregnating solution and the catalyst Co loading, Co/Al2O3 and Re-Co/Al2O3 catalysts with Co particles in the size range 1– 22 nm were prepared. The data of Table 1 show that the catalyst surface area decreases with increasing Co loading, consistent with the blockage of pores by Co particles. In several cases the average size of the Co particles is larger than the average pore diameter of the support (10 nm) and hence some of the Co may be located outside the pores. The Co particle size estimated from the CO chemisorption data, assuming a CO-Co stoichiometry of 1:2,57, 58, 59 is in reasonable agreement with the Co crystallite size estimated from XRD, as shown in Table 1 for the samples where Co crystallites were detectable by XRD indicating that the Co occurs as individual crystallites accessible to CO, supported on the Al2O3. The C content of the reduced fresh catalysts after reduction at 600 ºC was ≤ 0.05 wt%, indicating complete removal of the EG during the calcination and reduction process.

The rate of catalyst deactivation during FTS is known to depend on catalyst operating conditions.1 Measuring the catalyst deactivation rate in a CSTR reactor at approximately the same CO conversion level ensures that the catalysts are exposed to similar concentrations of H2O, heavy hydrocarbons and other components that may impact the deactivation rate. Since the DOR and Co loading varied among the catalysts, the GHSV had to be varied to identify conditions that resulted in similar CO conversion levels. The effect of space velocity on CO conversion is shown in Figures 2 and 3 and the product selectivity of the 15Co/Al2O3(0.1) with dCo=10 nm at varied space velocities, is reported in Figure S4 (Supporting Information). Table 2 summarizes the time-averaged values over the TOS period t = 48 to 190 h for all the catalysts that were tested at different space velocities. Several studies on the effect of water in FT


11

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 39

synthesis on Co/Al2O3 catalysts show that the C5+ selectivity increases and CH4 selectivity decreases with increased CO conversion or H2O concentration.60-63 Similar observations are made from the selectivity data of Table 2, except for the 1.2Re-12Co/Al2O3 catalyst operated at 82 % CO conversion, where the CH4 and CO2 selectivity increase significantly compared to the same catalyst operated at lower conversion (Table 2). The increase is probably due to the high / ratio (>1.5) that exists at high CO conversion, which results in surface oxidation of Co particles and increases the WGS reaction. It has been demonstrated theoretically that oxidation of Co particles smaller than 4-5 nm is possible at / = 1-1.5 in the FT synthesis.50 Oxidation of 5.6 nm Co particles at / > 0.5948 and of 12 nm Co particles at CO conversion of 50-75% has also been reported.49

Figures 2 and 3 show that for all the catalysts the CO conversion increases rapidly within about the first 10 h following the introduction of the synthesis gas to the reactor. This initial behavior reflects the hydrodynamic response of the reactor system to the change in feed flowrate and composition. (The hydrodynamic response time of the system is estimated at 2 h). For the 5Co/Al2O3(0.7) and 5Co/Al2O3(0.9) catalysts with small Co particles (dCo = 2 nm and dCo = 1 nm) and low DOR, the CO conversion continued to increase with TOS, up to the end of the experiment after a TOS of 190 h (Figure 2). However, for the Co/Al2O3 catalysts with larger Co particles and higher DOR operated at almost the same CO conversion level (5Co/Al2O3(0) at GHSV=0.04 mol/g.h and 15Co/Al2O3(0.1) at GHSV=0.16 mol/g.h), the CO conversion reached a maximum and then declined with TOS. Furthermore, for the 15Co/Al2O3(0.1) catalyst the maximum in CO conversion occurred at later times as the GHSV increased. Together these data suggest that the Co/Al2O3 catalysts undergo a slow reduction or surface reconstruction upon


12

Page 13 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

exposure to synthesis gas at elevated temperatures and pressures, as is reported in other studies. 22, 64-71, 72, 73

In the case of the 5Co/Al2O3(0.7) and 5Co/Al2O3(0.9) catalysts with small Co

particles (dCo = 2 nm and dCo = 1 nm) and low DOR, the CoOx was continuously reduced in the presence of the synthesis gas during the 180 h TOS. For the 15Co/Al2O3(0.1) catalyst with a higher degree of reduction (61%), a longer TOS is required to achieve the same level of reduction as the GHSV increases, and hence the maximum in CO conversion occurred at longer times. A similar maximum in CO conversion after longer periods of TOS, followed by a decline, has been observed in other studies. Fischer et al.68 and Welker69 reported a maximum CO conversion at 30-50 min TOS in a system with a hydrodynamic response time of about 10 min and this system reached steady state after approximately 10 h.68 Welker69 and Fischer et al.68 showed that by exposing the catalyst to CO at pressure between 1-3.3 bar and temperature between 170-190 º C for about 1-2 h before the reaction the maximum initial activity decreased noticeably, which supports the idea that the maximum initial activity of the catalyst is the result of reconstruction of freshly reduced catalyst when exposed to synthesis gas.

Previous studies report that the presence of CoOx promotes the water gas shift (WGS) reaction and hence CO2 selectivity during FT synthesis.48, 74 Figure 4 shows that the CO2 selectivity is much higher for the catalysts with smaller Co particles (dCo = 2 nm and dCo = 1 nm) compared to the catalysts with larger Co particles (dCo ≥10 nm) at approximately the same CO conversion level, consistent with the lower DOR and hence the presence of CoOx species in the case of small Co particles. As the CO conversion increases with TOS for the 5Co/Al2O3 (0.7) and 5Co/Al2O3 (0.9) catalysts (Figure 2), the CO2 selectivity declines with TOS (Figure 4), suggesting that the CoOx particles are being reduced to metallic Co during the reaction. The reduction of small Co


13

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 39

particles during FT synthesis has been reported previously72 and by van de Loosdrecht et al.,75 where the CoOx content of a 20 wt% Co/Pt/Al2O3 catalyst with 6 nm Co particle size was shown to decrease after FT reaction using in situ XRD and XANES analysis of the catalyst.

The used catalyst properties, reported in Table 3, provide some insight into the possible causes of catalyst deactivation observed in Figures 2 and 3. For the 15Co/Al2O3(0.1) catalyst with 10 nm average Co particle size, the BET surface area decreased with increased CO conversion. The decrease may be the result of H2O, which promotes sintering of the support, or because of carbon deposition in the pores of the support. As shown in Figure S5 (Supporting Information), there is a general increased loss in the used catalyst BET surface area with increased C content of the used catalyst. Furthermore, the small change in BET surface area of the Re-Co/Al2O3 catalyst operated at high CO conversion (Table 1 and Table 3; corresponding to high amounts of H2O in the system), suggests that the reduction in BET surface area is not the result of sintering of the support. Similarly, the significant decrease in BET surface area of the 5Co/Al2O3(0) catalyst occurs despite a low CO conversion (14%) and water content ( / < 0.09) in the reactor, whereas the carbon content of the used catalyst is high (4.7 C wt%). Hence carbon deposition appears to be the main cause of the reduced BET surface area of the used catalysts at the reaction conditions of this study.

The carbon deposited on the catalyst can be classified by the temperature at which the deposited carbon hydrogenates during temperature-programmed hydrogenation (TPH).

Amorphous or

polymeric carbon hydrogenates at 430-450 ºC showing a broad peak and semi-ordered graphitic carbon hydrogenates at 550 ºC to 650 ºC showing a sharp peak.39,


41, 76

TPH of the used

14

Page 15 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5Co/Al2O3(0.7) and the 20Co/Al2O3(0.6) catalysts, recovered after reaction at the conditions summarized in Table 3, washed in CH2Cl2 and vacuum dried at 350 °C, was conducted in a fixed bed reactor. From the TPH profiles summarized in Figure 5, it is concluded that the carbon on the surface of the 5Co/Al2O3(0.7) catalyst generated at low CO conversion (17%), is mainly in the form of amorphous or polymeric carbon, whereas the carbon deposited on the surface of the 20Co/Al2O3(0.6) catalyst at higher CO conversion (35%), is mostly in the form of semi-ordered graphite. These data, together with the very low H content of the used catalyst (< 0.05 wt% as measured by CH analysis), confirms that the measured C content of the used catalyst is not the result of residual high molecular weight hydrocarbons but rather the result of graphitic, amorphous or polymeric carbon deposition. These carbon species are un-reactive under FischerTropsch conditions and deactivate the catalyst. Weststrate et al.71 proposed a mechanism for graphene formation during low temperature FT synthesis at 227 ºC in which step edges and defects play a significant role in the formation of graphene nuclei. Subsequently, the growth occurs by addition of C2Hx compounds.

Note that the data of Table 3 show that on the 15Co/Al2O3(0.1) catalyst, carbon deposition increased with increased CO conversion, but this was not the case on the 1.2Re-12Co/Al2O3(0) catalyst (dCo=11 nm) operated at high average CO conversion (82%). An increase in the amount of CO2 produced as a result of the WGS reaction can decrease the amount of carbon deposited by the reverse Boudouard reaction: C + CO2 2CO. Alternatively, the high amount of H2O present under these conditions may remove C by steam gasification.


15

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 39

To quantify the catalyst deactivation rate, empirical activity models taken from the literature (Supporting Information, Table S1), were fitted to the measured catalyst activity factor defined as: (')

%&

#($) = %& (' ∗ ) with $ ≥ $ ∗

(3)

where α(t) is the activity factor at time t, -rCO(t) is the CO consumption rate per gram of catalyst (mol/g.s) at time t and –rCO(t*) is the CO consumption rate per gram of catalyst (mol/g.s) at time t*. The definition of t* is usually taken as the time at which the reaction rate is maximum and free of deactivation. However, as reported by others and shown in Figures 2 and 3, the unsteady state behavior following the introduction of synthesis gas results in t* being defined differently in various studies so as to exclude this initial response from the longer term catalyst deactivation analysis.1, 68, 69 Hence in the present study, t* was set at approximately 10 h, since for most cases this corresponds to the time period beyond the initial maximum rate that is a consequence of the hydrodynamic response of the system and the initial catalyst stabilization following introduction of the synthesis gas. However, as shown in Figure 2, for some of the Co/Al2O3 catalysts, the maximum CO conversion occurred at much longer times (11≤t*≤45 h) because of the slow reduction of cobalt oxides in the presence of the synthesis gas, as already discussed.72,

73

For

these catalysts, t* was chosen as the time at maximum CO conversion. The CO consumption rate was calculated directly from the CSTR design equation as: −+ =

, .

(4)

0 where / is the feed molar flow (mol/s) of CO, 1 is the CO conversion and W is the catalyst

mass (g). The model equations of Table S1 are written in terms of a dimensionless deactivation rate constant kd and a dimensionless time variable θ, where

2 = ($ − $ ∗ )/3 and 3 is the

residence time of the feed gas in the reactor volume (τ = 0.8 h). The models predict zero activity


16

Page 17 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

at infinite time, consistent with the deactivation models for FT synthesis reported in the literature.10,

11, 33, 77, 78

Since the models in the present study were used to quantify the

deactivation rate in the time span of t* to t = 200 h, it was not necessary to add a limiting activity, as done in other studies.79, 80

The models of Table S1 were fitted to the activity profiles measured for the Co/Al2O3 catalysts with Co particle size 10, 13 and 22 nm and the Re-Co/Al2O3 catalysts with Co particle size of 2, 5 and 11 nm at different CO conversions (see Supporting Information, Figures S6-S10 but note that since the 5Co/Al2O3 catalysts with small Co size showed a net activation during the reaction (Figure 2), these results were not included in the analysis). The reciprocal power model with n = 0.5 and consistent with a deactivation mechanism associated with catalyst fouling or coking, i.e. #(2) = 1⁄(1 + 5" θ0.8 ) had the best fit to the catalyst activity profiles, although for the 1.2Re12Co/Al2O3(0) catalyst operated at an average CO conversion of 82%, the fit was poor because of oxidation of Co particles in the presence of high concentrations of H2O and CO2 (Supporting Information, Table S3). The estimated values of the deactivation rate constant kd are reported in Table 3 and these can be related to the carbon deposition on the catalyst. Assuming that coke formation occurs in parallel with the FT synthesis reaction,81-83 then the rate of coke formation with TOS can be written as: + = +0 × # (θ)

(5)

where + is the the rate of coke formation per gram of catalyst (mol/g.h), +0 is the initial rate of coke formation (mol/g.h) and # (θ) is the coke formation activity factor that captures the rate of deactivation of the coke formation reaction. Assuming that the activity factor of the CO consumption to FT products is the same for the coke formation reaction,80 i.e. # (θ) = α(θ)


17

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 39

then the total amount of carbon deposited on the catalyst is easily calculated by integration of the equation: B

:; = +0 τ =0

> >?@A BC/

(6)

2

in which Cc is the total amount of carbon deposited on the catalyst in mol/g, as measured by C elemental analysis of the used catalyst. Equation (6) allows the initial rate of carbon deposition to be calculated as follows: +0 = :; × 5D ⁄[23(5D × Ɵ0.8 − ln(1 + 5D × Ɵ0.8 ))]

(7)

Equation (7) was used to calculate +0 as reported in the Supporting Information (Table S4) and the initial coke formation rate per active Co site (i.e. the initial carbon deposition turnover frequency, K /0) is readily determined by dividing +0 by the CO uptake data of Table 1. Hence, Figure 6 shows that the initial rate of carbon deposition per active Co site (K /0), measured at approximately the same CO conversion level such that the gas composition within the reactor is approximately the same, increases approximately linearly (correlation coefficient of R2=0.87) with increased Co particle size. Note that the data of Figure 6 are restricted to carbon deposition rates measured at average CO conversions of 14-40% and excludes the data obtained on the 1.2Re-12Co/Al2O3(0) catalyst at high CO conversion of 60 and 82%. Furthermore, an average value for K /0 is reported for the 15Co/Al2O3(0.1) catalyst, based on the carbon deposition rates measured at CO conversions of 25-37%. The calculated K /0 values for this catalyst suggest that the initial rate of carbon deposition increases with CO conversion (see Table S4, Supporting Information). At higher CO conversions (> 60%), K /0 decreases with increased CO conversion on the 1.2Re-12Co/Al2O3(0) catalyst. As discussed above, the decrease in carbon deposition rate is likely a result of the reverse Boudouard reaction or steam gasification of C


18

Page 19 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

because of the high concentrations of H2O and CO2 in the reactor at these high CO conversion levels.

The K /0 can also be compared to the initial total CO consumption rate per active site (i.e. the 0 initial CO consumption turn-over-frequency,K / , calculated by dividing the initial rate,

calculated from Equation (4), by the CO uptake). The comparison shows (Table S4) that the carbon deposition rate is at least 2-orders of magnitude slower than the total CO consumption 0 rate. Hence, although K / includes the rate of carbon deposition, the error in assuming that

the CO consumption rate represents the conversion of CO by the WGS and FT reactions that 0 occur in parallel to the carbon deposition reaction, is minimal. Finally we note that the K /

measured on the Co/Al2O3 and Re-Co/Al2O3 catalysts follow the same general dependence on 0 Co particle size reported in the literature12, 42-47, 84, 85 with the K / increasing with Co particle

size up to approximately dCo~ 10 nm.

XRD analysis of the Co/Al2O3 catalysts (see Table S5, Figures S1-S3, Supporting Information) confirmed that there was no significant Co sintering observed during the FT reaction of the Co/Al2O3 and Re-Co/Al2O3 catalysts with dCo ≥10 nm, suggesting that the growth in Co particles is not sufficiently significant to be detectable by XRD. For small Re-Co/Al2O3 catalysts with dCo≤5 nm no Co peak was detected, since small Co particles are prone to oxidation and oxidize even when covered in a layer of wax. However, the Co3O4 particle size on these catalysts did not change significantly after reaction (Table S5). These data confirm that even for the 0.3Re3Co/Al2O3(0.9) catalyst with the smallest particle size (dCo= 2 nm), no significant sintering occurred.


19

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

CONCLUSIONS

Deactivation of Co catalysts used for FT synthesis is dependent on Co particle size, DOR and CO conversion. At the conditions of the present study, carbon deposition was the main cause of catalyst deactivation. The initial rate of carbon deposition per active Co site increased with increased Co particle size (dCo = 2 – 22 nm) when measured at approximately the same CO conversion (< 40%). On the 15Co/Al2O3(0.1) catalyst the rate of carbon deposition increased with CO conversion when CO conversion was ≤40%. However, for the 1.2Re12Co/Al2O3(0) catalyst, the rate of carbon deposition decreased with increased CO conversion at high CO conversions (> 60%) due to high concentrations of H2O and CO2 in the reactor. The Co catalysts that had both a low DOR and small Co particles were activated during the first 200 h TOS of FT synthesis, and the activation is attributed to increased reduction of CoOx species present in these catalysts, by the synthesis gas.

ACKNOWLEDGMENT The financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.


20

Page 21 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

REFERENCES 1. Keyvanloo, K.; Fisher, M. J.; Hecker, W. C.; Lancee, R. J.; Jacobs, G.; Bartholomew, C. H. J. Catal 2015, 327, 33-47. 2. Saib, A. M.; Moodley, D. J.; Ciobîcă, I. M.; Hauman, M. M.; Sigwebela, B. H.; Weststrate, C. J.; Niemantsverdriet, J. W.; van de Loosdrecht, J. Catal. Today 2010, 154, 271-282. 3. Agrawal, P. K.; Katzer, J. R.; Manogue, W. H. J. Catal 1981, 69, 327-344. 4. Bartholomew, C. H.; Bowman, R. M. Applied Catal. 1985, 15, 59-67. 5. Madon, R.; Seaw, H. Cat. Rev. - Sci. Eng. 1977, 15, 69-106. 6. Bessell, S. Stud. Surf. Sci.Catal.1994, 81, 483-486. 7. Jacobs, G.; Zhang, Y.; Das, T. K.; Li, J.; Patterson, P. M.; Davis, B. H. Stud. Surf. Sci. Catal. 2001, 139, 415-422. 8. Kiss, G.; Kliewer, C. E.; DeMartin, G. J.; Culross, C. C.; Baumgartner, J. E. J. Catal. 2003, 217, 127-140. 9. Shi, L.; Chen, J.; Fang, K.; Sun, Y. Stud. Surf. Sci. Catal. 2007, 167, 97-102. 10. Tavasoli, A.; Malek Abbaslou, R. M.; Dalai, A. K. Applied Catal., A General 2008, 346, 5864. 11. Zhou, W.; Chen, J.; Fang, K.; Sun, Y. Fuel Process. Tech. 2006, 87, 609-616.


21

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 39

12. Barbier, A.; Tuel, A.; Arcon, I.; Kodre, A.; Martin, G. A. J. Catal. 2001, 200, 106-116. 13. Borkó, L.; Horváth, Z. E.; Schay, Z.; Guczi, L. Stud. Surf. Sci. Catal. 2007, 167, 231-236. 14. Lee, D.; Lee, J.; Ihm, S. Applied Catal. 1988, 36, 199-207. 15. Ducreux, O.; Lynch, J.; Rebours, B.; Roy, M.; Chaumette, P. Stud. Surf. Sci.Catal. 1998, 125-130. 16. Niemelä , M. K.; Krause, A. O. I. Catal. Lett. 1996, 42, 161-166. 17. Schanke, D.; Hilmen, A.; Bergene, E.; Kinnari, K.; Rytter, E.; Ådnanes, E.; Holmen, A. Catal. lett. 1995, 34, 269-284. 18. Schanke, D.; Hilmen, A. M.; Bergene, E.; Kinnari, K.; Rytter, E.; Ǻdnanes, E.; Holmen, A. Energy Fuels 1996, 10, 867-872. 19. Jacobs, G.; Patterson, P. M.; Zhang, Y.; Das, T.; Li, J.; Davis, B. H. Applied Catal A, 2002, 233, 215-226. 20. Jacobs, G.; Das, T. K.; Patterson, P. M.; Li, J.; Sanchez, L.; Davis, B. H. Applied Catal. A, 2003, 247, 335-343. 21. Tehrani, S.; Irani, M.; Tavasoli, A.; Mortazavi, Y.; Khodadadi, A. A.; Pour, A. N. J. Natural Gas Chem. 2011, 20, 65-71. 22. Tsakoumis, N. E.; Voronov, A.; Rønning, M.; Beek, W. v.; Borg, Ø; Rytter, E.; Holmen, A. J. Catal. 2012, 291, 138-148.


22

Page 23 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

23. Karaca, H.; Safonova, O. V.; Chambrey, S.; Fongarland, P.; Roussel, P.; Griboval-Constant, A.; Lacroix, M.; Khodakov, A. Y. J. Catal. 2011, 277, 14-26. 24. Moulijn, J. A.; van Diepen, A. E.; Kapteijn, F. Applied Catal., A 2001, 212, 3-16. 25. Ruckenstein Eli; Dadyburjor, D. B. Rev. Chem, Eng. 2011, 1, 251. 26. Bartholomew, C. H. Applied Catal., A 2001, 212, 17-60. 27. Bezemer, G. L.; Remans, T. J.; van Bavel, A. P.; Dugulan, A. I. J. Am. Chem. Soc. 2010, 132, 8540-8541. 28. Sadeqzadeh, M.; Chambrey, S.; Hong, J.; Fongarland, P.; Luck, F.; Curulla-FerrÃ©, D.; Schweich, D.; Bousquet, J.; Khodakov, A. Y. Ind. Eng. Chem. Res. 2014, 53, 6913-6922. 29. Claeys, M.; Dry, M. E.; van Steen, E.; van Berge, P. J.; Booyens, S.; Crous, R.; van Helden, P.; Labuschagne, J.; Moodley, D. J.; Saib, A. M. ACS Catal. 2015, 5, 841-852. 30. Mirjalili, M.; Vahdati-Khaki, J. J. Phys. Chem. Solids 2008, 69, 2116-2123. 31. Palasantzas, G.; Vystavel, T.; Koch, S.; De Hosson, J Th M J. Appl. Phys. 2006, 99, 024307. 32. Figueiredo, J. L. Carbon formation and gasification on Nickel, In Progress in catalyst deactivation; Springer: Netherlands, 1982, 45-63 33. Tavasoli, A.; Trépanier, M.; Dalai, A. K.; Abatzoglou, N. J. Chem. Eng. Data 2010, 55, 2757-2763. 34. Bartholomew, C. H. Applied Catal., A General 1993, 107, 1-57.


23

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

35. Benfield, R. E. J. Chem. Soc. , Faraday Transactions 1992, 88, 1107-1110. 36. Müller, H.; Opitz, C.; Skala, L. J. Mol. Catal. 1989, 54, 389-405. 37. den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Frøseth, V.; Holmen, A.; Jong, K. P. J. Am. Chem. Soc. 2009, 131, 7197-7203. 38. Moodley, D. J.; Saib, A. M.; van de Loosdrecht, J.; Welker-Nieuwoudt, C. A.; Sigwebela, B. H.; Niemantsverdriet, J. W. Catal. Today 2011, 171, 192-200. 39. Moodley, D. J.; van de Loosdrecht, J.; Saib, A. M.; Overett, M. J.; Datye, A. K.; Niemantsverdriet, J. W. Applied Catal., A 2009, 354, 102-110. 40. Park, S.; Bae, J. W.; Jung, G.; Ha, K.; Jun, K.; Lee, Y.; Park, H. Applied Catal. A 2012, 413– 414, 310-321. 41. Peña, D.; Griboval-Constant, A.; Lancelot, C.; Quijada, M.; Visez, N.; Stéphan, O.; Lecocq, V.; Diehl, F.; Khodakov, A. Y. Catal. Today 2014, 228, 65-76. 42. Chu, W.; Chernavskii, P. A.; Gengembre, L.; Pankina, G. A.; Fongarland, P.; Khodakov, A. Y. J. Catal. 2007, 252, 215-230. 43. Fu, L.; Bartholomew, C. H. J. Catal. 1985, 92, 376-387. 44. Ho, S. W.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1990, 94, 6396-6399. 45. Lee, J.; Lee, D.; Ihm, S. J. Catal. 1988, 113, 544-548. 46. Martı́nez, A.; López, C.; Márquez, F.; Dı́az, I. J. Catal. 2003, 220, 486-499.


24

Page 25 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

47. Reuel, R. C.; Bartholomew, C. H. J. Catal. 1984, 85, 78-88. 48. Li, J.; Zhan, X.; Zhang, Y.; Jacobs, G.; Das, T.; Davis, B. H. Applied Catal., A 2002, 228, 203-212. 49. Lögdberg, S.; Boutonnet, M.; Walmsley, J. C.; Järås, S.; Holmen, A.; Blekkan, E. A. Applied Catal., A 2011, 393, 109-121. 50. Van Steen, E.; Claeys, M.; Dry, M. E.; van, d. L.; Viljoen, E. L.; Visagie, J. L. J. Phys. Chem B 2005, 109, 3575-3577. 51. Sun, S.; Fujimoto, K.; Yoneyama, Y.; Tsubaki, N. Fuel 2002, 81, 1583-1591. 52. Li, J.; Jacobs, G.; Das, T.; Zhang, Y.; Davis, B. Applied Catal. A, 2002, 236, 67-76. 53. Borg, Ø; Dietzel, P. D. C.; Spjelkavik, A. I.; Tveten, E. Z.; Walmsley, J. C.; Diplas, S.; Eri, S.; Holmen, A.; Rytter, E. J. Catal. 2008, 259, 161-164. 54. Borg, Ø; Eri, S.; Blekkan, E. A.; Storsæter, S.; Wigum, H.; Rytter, E.; Holmen, A. J. Catal. 2007, 248, 89-100. 55. Bao, A.; Liew, K.; Li, J. J. Mol. Catal. A:, Chem. 2009, 304, 47-51. 56. van, d. L.; Barradas, S.; Caricato, E. A.; Ngwenya, N. G.; Nkwanyana, P. S.; Rawat, M. A. S.; Sigwebela, B. H.; van Berge, P. J.; Visagie, J. L. Top. Catal. 2003, 26, 121-127. 57. Kadinov, G.; Bonev, C.; Todorova, S.; Palazov, A. J. Chem. Soc., Faraday Trans. 1998, 94, 3027-3031.


25

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

58. Rygh, L.; Ellestad, O.; Klaeboe, P.; Nielsen, C. Phys. Chem. Chem. Phys. 2000, 2, 18351846. 59. Yang, J.; Frøseth, V.; Chen, D.; Holmen, A. Surf. Sci. 2016, 648, 67-73. 60. Borg, Ø; Storsæter, S.; Eri, S.; Wigum, H.; Rytter, E.; Holmen, A. Catal. Lett. 2006, 107, 95102. 61. Hilmen, A. M.; Schanke, D.; Hanssen, K. F.; Holmen, A. Applied Catal., A 1999, 186, 169188. 62. Storsæter, S.; Borg, Ø; Blekkan, E. A.; Tøtdal, B.; Holmen, A. Catal. Today 2005, 100, 343347. 63. Dalai, A. K.; Davis, B. H. Applied Catal., A 2008, 348, 1-15. 64. Banerjee, A.; van Bavel, A. P.; Kuipers, H. P.; Saeys, M. ACS Catal. 2015, 5, 4756-4760. 65. Banerjee, A.; Navarro, V.; Frencken, J. W.; van Bavel, A. P.; Kuipers, H. P.; Saeys, M. J. Phys. Chem. lett. 2016, 7,1996-2001 66. Ciobîcă, I. M.; van Helden, P.; van Santen, R. A. Surf. Sci. 2016, 653, 82-87. 67. Corral Valero, M.; Raybaud, P. The Journal of Physical Chemistry C 2014, 118, 2247922490. 68. Fischer, N.; van Steen, E.; Claeys, M. J. Catal. 2013, 299, 67-80.


26

Page 27 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

69. Welker, C. A. Ruthenium based Fischer–Tropsch synthesis on crystallites and clusters of different sizes. Department of Chemical Engineering, PhD Thesis, University of Cape Town, Cape Town. 2007. 70. Weststrate, C. J.; Ciobîcă, I. M.; Saib, A. M.; Moodley, D. J.; Niemantsverdriet, J. W. Catal. Today 2014, 228, 106-112. 71. Weststrate, C. J.; Kızılkaya, A. C.; Rossen, E. T. R.; Verhoeven, M. W. G. M.; Ciobĭcâ, I. M.; Saib, A. M.; Niemantsverdriet, J. W. J. Phys. Chem. C. 2012, 116, 11575-11583. 72. Saib, A. M.; Borgna, A.; van, d. L.; van Berge, P. J.; Niemantsverdriet, J. W. Applied Catal., A 2006, 312, 12-19. 73. Tsakoumis, N. E.; Dehghan-Niri, R.; Rønning, M.; Walmsley, J. C.; Borg, Ø; Rytter, E.; Holmen, A. Appl. Catal., A 2014, 479, 59-69. 74. Newsome, D. S. Catal. Rev.- Sci. Eng. 1980, 21, 275-318. 75. Van de Loosdrecht, J.; Balzhinimaev, B.; Dalmon, J. ; Niemantsverdriet, J. W.; Tsybulya, S. V.; Saib, A. M.; van Berge, P. J.; Visagie, J. L. Catal. Today 2007, 123, 293-302. 76. Xu, J.; Bartholomew, C. H. J. Phys. Chem. B 2005, 109, 2392-2403. 77. Qin, Q.; Ramkrishna, D. Ind. Eng. Chem. Res. 2004, 43, 2912-2921. 78. Eschemann, T. O.; de Jong, K. P. ACS Catal. 2015, 5, 3181-3188. 79. Argyle, M. D.; Frost, T. S.; Bartholomew, C. H. Top. Catal. 2014, 57, 415-429.


27

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

80. Eliason, S. A.; Bartholomew, C. H. Applied Catal. , A 1999, 186, 229-243. 81. Froment, G.; Bischoff, K. Chem. Eng. Sci. 1961, 16, 189-201. 82. Froment, G. F.; Bischoff, K. B. Chem. Eng. Sci. 1962, 17, 105-114. 83. Forzatti, P.; Lietti, L. Catal. Today 1999, 52, 165-181. 84. Bezemer, G. L.; van Laak, A.; van Dillen, A. J.; de Jong, K. P. Stud. Surf. Sci. Catal. 2004, 147, 259-264. 85. Lisitsyn, A. S.; Golovin, A. V.; Kuznetsov, V. L.; Yermakov, Y. I. J. Catal. 1985, 95, 527538. 86. Fogler, S., H. Elements of Chemical Reaction Engineering; Prentice Hall: New York, 2005, 634-684


28

Page 29 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1. The properties of the reduced Co/Al2O3 and Re-Co/Al2O3 catalysts used in FT synthesis

SBET

dpore DOR

CO uptake

dCo XRD

CO

Catalyst

m2/g

nm

%

µmol/g

5Co/Al2O3(0.7)

170

9

3

7.3

-

2

5Co/Al2O3(0.9)

186

9

2

10.1

-

1

5Co/Al2O3(0)

186

8

75

14.0

21±0.8

22

15Co/Al2O3(0.1)

161

9

61

70.3

9±0.8

10

20Co/Al2O3(0.6)

155

8

41

51.3

12±0.7

13

0.3Re-3Co/Al2O3(0.9)

185

9

55

61.7

-

2

0.5Re-5Co/Al2O3(0.7)

152

11

69

56.4

-

5

1.2Re-12Co/Al2O3(0)

123

11

85

74.6

8±0.7

11


nm

29

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 30 of 39

Table 2. Effect of space velocity on averagea CO conversion and product selectivity of Co/Al2O3 and Re-Co/Al2O3 Co/Al2O3 catalysts operated at 220°C, 2.1MPa and H2/CO = 2/1 Product Distribution Catalyst

5Co/Al2O3(0.9)

5Co/Al2O3(0.7)

15Co/Al2O3(0.1)

1.2Re-12Co/Al2O3(0)

a b

dCo

GHSV

CO conversion

PH2O/PH2

CH4

C5+

CO2 Selectivityb

nm

mol/(g.h)

%

-

wt%

wt%

mol %

1

0.01

21

0.13

17.3

73.7

3.2

0.04

4

0.02

28.8

51.8

7.2

0.01

17

0.10

19.6

72.2

5.2

0.04

4

0.02

46.9

38.6

12.2

0.04

37

0.31

8.2

81.8

1.8

0.07

33

0.25

9.3

81.7

0.7

0.08

30

0.23

10.1

78.5

0.6

0.16

25

0.18

11.1

81.2

0.7

0.05

82

1.69

16.6

77.7

17.9

0.08

60

0.85

6.4

87.6

2.2

0.12

40

0.34

10.4

82.2

2.4

2

10

11

averaged from TOS data from t = 48 to 190 h mole CO2/mole CO converted


30

Page 31 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Table 3. The properties of used Co/Al2O3 and Re-Co/Al2O3 catalysts after FT synthesis and the catalyst deactivation rate constant Reduced Catalyst dCo

Deactivation rate constanta

Used Catalyst TOS Avg. CO conv.

SBET

dpore

C

dCo (XRD)

kd

nm

h

%

m2/g

nm

wt%

nm

-

5Co/Al2O3(0.7)

2

185

17

147

8

2.8

-

activated

5Co/Al2O3(0.9)

1

188

21

173

8

2.3

-

activated

5Co/Al2O3(0)

22

145

14

126

9

4.7

23±0.7

1.4E-02±1.9E-04

15Co/Al2O3(0.1)

10

191

37

112

7

7.6

12±1.6

8.6E-02±5.5E-04

10

165

33

135

7

4.6

13±1.6

5.7E-02±2.8E-04

10

163

30

157

7

4.4

12±0.6

3.5E-02±2.6E-04

10

137

25

157

7

2.7

11±0.6

3.2E-02±2.6E-04

20Co/Al2O3(0.6)

13

162

35

101

8

6.5

11±0.6

8.9E-03±1.3E-04

0.3Re-3Co/Al2O3(0.9)

5

190

24

163

10

3.4

3.5 E-02±3.1E-04

0.5Re-5Co/Al2O3(0.7)

2

190

30

141

10

4.9

4.0E-02± 3.9E-04

1.2Re-12Co/Al2O3(0)

11

233

82

135

8

4.1

9±1.5

1.2E-02±3.6E-04

11

139

60

111

9

5.9

8±1.0

2.2E-02±1.7E-04

11

166

40

109

-

6.0

8±1.6

3.6E-02±4.8E-03

Catalyst

a

>

#(2) = >?@

-.N LM


31

Energy & Fuels

5Co/Al2O3(0) DOR = 75% dCo = 22 nm

H2 consumption, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

5Co/Al2O3(0.7) DOR = 3% dCo = 2 nm

0.5Re-5Co/Al2O3(0.7) DOR = 69% dCo = 5 nm 0

100

200

300

400

500

600

700

800

900

o

Temperature, C

Figure 1. TPR profiles for 5Co/Al2O3 catalysts prepared without EG (5Co/Al2O3), in the presence of 70wt % EG (5Co/Al2O3(0.7)) and the Re-promoted catalyst prepared with 70 wt% EG (0.5Re-5Co/Al2O3(0.7).


32

Page 33 of 39

30 20 5Co/Al2O3(0.9), GHSV=0.01 mol/g.h

10

5Co/Al2O3(0.7), GHSV=0.01 mol/g.h

0

5Co/Al 2O3(0), GHSV=0.04 mol/g.h

20 10 0

CO conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20

15Co/Al2O3(0.1), GHSV=0.16 mol/g.h

10 0 40 20

15Co/Al O (0.1), GHSV=0.08 mol/g.h 2

3

0 40 20

15Co/Al O (0.1), GHSV=0.04 mol/g.h 2

3

0 60 20Co/Al O (0.6), GHSV=0.04 mol/g.h

40

2

3

20 0 0

20

40

60

80 100 120 140 160 180 200 220

Time-on-stream (h) Figure 2. CO conversion for Co/Al2O3 catalysts as a function of TOS measured at 220 °C, 2.1MPa and H2/CO =2


33

Energy & Fuels

100 75 0.3Re-3Co/Al2O3(0.7), GHSV=0.05 mol/g.h

50 25 0 75

0.5Re-5Co/Al2O3(0.7), GHSV=0.05 mol/g.h

50

CO conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

25 0 75

1.2Re-12Co/Al2O3(0), GHSV=0.12 mol/g.h

50 25 0


75 50 25 0 75


50 25 0 0

20

40

60

80

100

120 140 160

180 200

Time-on-stream (h) Figure 3. CO conversion for Re-Co/Al2O3 catalysts as a function of TOS measured at 220 °C, 2.1MPa and H2/CO =2


34

Page 35 of 39

12 5Co/Al2O 3(0), 22 nm 20Co/Al 2O3(0.6), 13 nm

10

15Co/Al 2O3(0.1), 10 nm 5Co/Al2O3(0.7), 2 nm

CO2 Selectivity, mol/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5Co/Al2O3(0.9), 1 nm

8 6 4 2 0 0

40

80

120

160

200

Time-on-stream, h Figure 4. The CO2 selectivity versus time on stream for small and large Co particles, Average conversion 20±5%


35

Energy & Fuels

2.2 2.0 1.8 1.6

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 200

400

600 o

Temp, C Figure 5. TPH profile for used catalysts, Solid line: 20Co/Al2O3(0.6) with dCo=13 nm after reaction at average CO conversion of 35%; Dashed line: 5Co/Al2O3(0.7) with dCo=2 nm after reaction at average CO conversion of 17%


36

Page 37 of 39

-4

3.5x10

-4

3.0x10

-4

-1

4.0x10

5Co/Al2O3(0) 20Co/Al2O3(0.6)

o

Initial coke deposition rate, TOFC, s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

15Co/Al2O3(0.1) 0.5Re-5Co/Al2O3(0.7) 0.3Re-3Co/Al2O3(0.9)

2.5x10

-4

2.0x10

-4

1.5x10

-4

1.0x10

-4

5.0x10

-5

1.2Re-12Co/Al2O3(0)

0

5

10

15

20

25

dCo, nm

Figure 6. Initial coke deposition rate per active site, K /0 versus dCo for Re-Co/Al2O3 and Co/Al2O3 catalysts with different Co particle sizes. All the experiments were conducted at 220 º C and 2.1 MPa


37

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 39

ASSOCIATED CONTENT Supporting Information

Effect of GHSV on product selectivity, CO conversion versus TOS, reduction in BET surface as a result of carbon deposition, comparison between experimental data and reciprocal power law fit, empirical catalyst deactivation models, estimated parameters for the empirical models for Co/Al2O3 and Re-Co/Al2O3 catalysts, example calculations to estimate initial rate of carbon deposition per active Co site, Co and Co3O4 particle sizes before and after the reaction measured by XRD. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Kevin J. Smith, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors contributed equally. Funding Sources This work has been done by financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC).


38

Page 39 of 39

FT products CO2 H2O

Syn Gas

coke Co C

dCo C Al2O3

C

Al2O3 -4

-1

4.0x10

5Co/Al2O3(0)

-4

3.5x10

20Co/Al2O3(0.6)

o

Initial coke deposition rate, TOFC, s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

15Co/Al2O3(0.1)

-4

3.0x10

0.5Re-5Co/Al2O3(0.7) 0.3Re-3Co/Al2O3(0.9)

-4

1.2Re-12Co/Al2O3(0)

2.5x10

-4

2.0x10

-4

1.5x10

-4

1.0x10

-5

5.0x10

0

5

10

15

20

dCo, nm ACS Paragon Plus Environment

25

Al2O3 and ReâCo

Recommend Documents

Al2O3 and ReâCo