SiO2 catalysts

Fuel 90 (2011) 756–765

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Fischer–Tropsch synthesis: Support and cobalt cluster size effects on kinetics over Co/Al2O3 and Co/SiO2 catalysts Wenping Ma a, Gary Jacobs a, Dennis E. Sparks a, Muthu K. Gnanamani a, Venkat Ramana Rao Pendyala a, Chia H. Yen b, Jennifer L.S. Klettlinger b, Thomas M. Tomsik b, Burtron H. Davis a,⇑ a b

Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, USA NASA Glenn Research Center, 21000 Brookpark Rd., Cleveland, OH 44135, USA

a r t i c l e

i n f o

Article history: Received 11 December 2009 Received in revised form 11 October 2010 Accepted 13 October 2010 Available online 23 October 2010 Keywords: Fischer–Tropsch synthesis Water effect Cobalt catalyst Co cluster size effect Support effect

a b s t r a c t The influence of support type and cobalt cluster size (i.e., with average diameters falling within the range of 8–40 nm) on the kinetics of Fischer–Tropsch synthesis (FT) were investigated by kinetic tests employing a CSTR and two Co/c-Al2O3 catalysts having different average pore sizes, and two Co/SiO2 catalysts a prepared on the same support but having different loadings. A kinetic model rCO ¼ kP CO PbH2 = ð1 þ mP H2 O =P H2 Þ that contains a water effect constant ‘‘m” was used to fit the experimental data obtained with all four catalysts. Kinetic parameters suggest that both support type and average Co particle size impact FT behavior. Cobalt cluster size influenced kinetic parameters such as reaction order, rate constant, and the water effect parameter. In the cluster size range studied, decreasing the average Co cluster diameter by about 30% led to an increase in the intrinsic reaction rate constant k, defined on a per g of catalyst basis, by 62–102% for the c-Al2O3 and SiO2-supported cobalt catalysts. This increase was due to the higher active Co0 surface site density as measured by hydrogen chemisorption. Moreover, less inhibition by adsorbed CO and greater H2 dissociation on catalysts having smaller Co particles was suggested by the higher a and lower b values obtained for the measured reaction orders. Interestingly, irrespective of support type, the catalysts having smaller average Co particles were more sensitive to water. Comparing the catalysts having strong interactions between cobalt and support (Co/Al2O3) to the ones with weak interactions (Co/SiO2), the water effect parameters were found to be positive (indicating a negative influence on CO conversion) and negative (denoting a positive effect on CO conversion), respectively. No clear trend was observed for b values among the different supports, but greater a and a/b values were observed for both Al2O3-supported Co catalysts, implying greater inhibition of the FT rate by strongly adsorbed CO on Co/Al2O3 relative to Co/SiO2. For both supports, the order on PCO was always found to be negative (i.e., suggesting an inhibiting effect) and positive for PH2 for all four catalysts. The order of the reaction on PH2 was close to 0.5, suggesting that dissociated H2 is likely involved in the catalytic cycle. Finally, in the limited range of average pore diameters studied (13.5 and 18.2 nm), the average pore size of the Al2O3-supported Co catalysts displayed no observable impact on the reaction rate or water effect, suggesting either that the reaction is kinetically controlled, or that the pore size difference was not significant enough to elicit a measurable response. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Fischer–Tropsch synthesis (FT) provides a practical means of producing liquid fuels and chemicals from natural gas, coal, and biomass derived syngas. Investigating the kinetics of FT is important not only for optimizing process parameters, but also for shedding light on the catalytic mechanism. Two classifications of kinetic models have been used to describe the FT reaction rate – ⇑ Corresponding author. Tel.: +1 859 257 0251; fax: +1 859 257 0302. E-mail address: [email protected] (B.H. Davis). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.10.029

those based on proposed mechanisms and involving elementary steps, and those derived empirically. Conventional Langmuir–Hinshelwood–Hougen–Watson (LHHW) expressions based on three leading FT mechanisms, (i.e., carbide, enol and CO insertion) have been put forward by a number of authors [1–13], including Anderson et al., Dry et al., Huff et al., Ledakowicz et al., etc., as summarized in Table 1. With exceptions, most of the mechanisticallyderived models describe the kinetic behavior of iron catalysts, while the majority of expressions used in conjunction with cobalt and ruthenium catalysts have been empirically derived [14–26]. Note that in the earlier models for cobalt, water inhibition was

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W. Ma et al. / Fuel 90 (2011) 756–765 Table 1 Summary of mechanistic-based and empirical kinetic models of Fischer–Tropsch synthesis. Mechanistic kinetic models of FTS 2

1 2

rco = kPcoP H2 /(l + mPco)

3

0:5 0:5 2 /(1 + m1P 0:5 rCO = kPcoP H co + m2P H2 ) 2

0:5

3 rco = kP co P H2 /(l + m1P 0:5 co )

0:5 kP 0:5 co P H2 /(1

0:5 m1P 0:5 co +m2P H2 +

4

rco =

5 6 7 8

rco+H2 = kP H2 Pco/(Pco + m1Pco2) rco+H2 = kP H2 Pco/(Pco + m1P H2 O + m2Pco2) rco = kP H2 Pco/(l + mP H2 O /P H2 ) rco = kP H2 Pco/(l + mP H2 O )

+

m3Pco)

Empirical kinetic models of FTS

Catalyst

Mechamism

Co Fe

Carbide Carbide

Fe

Carbide

Fe

Carbide

Fe Fe Fe and Co Fe

Enol theory Enol theory Carbide and CO insertion Enol theory Reaction order a

b

Fe and Co Fe Fe & Co Co

1 to 1 0.65

0.5–2 0.6

1 2 3 4

rco = kP H2 rHC = kP H2 /Pco rco = kP coa ðP H2

5

rFT = kP aco P bH2 /(1 + mPco)2

Fe

1

0.5

6

rft = k(P aco P bH2 =P H2 O )/(l + mPcoP H2 /P H2 O )2

Fe

1

1.5

7

rco = kP aco P bH2 /(1 + mP H2 O /P H2 )

Co

0.8 to 0.2

0.5–0.6

rco = kP aco P bH2 /(1 + mPco)

largely neglected or presumed to be negligible. However, H2O is an important product of the FT reaction, and its partial pressure can be significant, especially at high conversion levels where its impact on the rate is often found to be significant over both Fe and Co catalysts [6,24,27]. This underscores the need to include a water effect parameter in updated models that can adequately describe the kinetic behavior of water during FT. Recently a simple empirical model was developed to describe the water effect over supported cobalt catalysts [24–26]: a

r CO ¼ kP CO PbH2 =ð1 þ mPH2 O =PH2 Þ

ð1Þ

where k = k0 exp (Ea/RT) is the apparent kinetic rate constant, a and b are the apparent reaction orders for CO and H2, respectively, and m is defined to be a water effect parameter. The water effect on the FT rate has been investigated and confirmed by several groups who tested cobalt-based catalysts using water co-feeding techniques. Support type, ranging from supports that strongly interact with cobalt (e.g., Al2O3) to those with more moderate (e.g., TiO2) or weak (e.g., SiO2) interactions, plays a decisive role in determining the manner in which water behaves from an observable (i.e., or apparent) kinetic standpoint. When the amount of added water was less than 28–30% by volume, a reversible negative effect of water on the FT rate was observed over a 25%Co/Al2O3 catalyst; however, a slightly negative to negligible effect was observed over Co/TiO2, and somewhat surprisingly, a positive effect of water was discovered over SiO2 supported Co catalysts [24,27–40]. On the other hand, the irreversible influence of high volume percentages of co-fed H2O on FT behavior has been interpreted by several research groups to be due to changes in the oxidation state (e.g., cobalt oxide and cobalt-support compound formation) of Co based on characterization methods such as TPR, XPS, HRTEM, EXAFS and XANES. Goodwin et al. [41] studied the formation of Co–Al2O3 compounds over Co/Al2O3 catalysts using TPR, XRD and the Raman spectroscopy by adding 3% H2O during reduction and FT. They found that Co aluminate species (e.g. CoAl2O4) were formed when 3% H2O was added during reduction at 350 °C. The formation of this species changed the characteristics of Co catalysts; most noteworthy was a significant decrease in Co catalyst reducibility, which in turn decreased overall FT activity. Using HR-TEM, Soled et al. [42] studied the transformation of metallic Co to oxidized forms during FT. The TEM images of used

Co samples showed that the smaller Co particles disappeared from the HR-TEM images, indicating cobalt-support compound formation, which has been assumed to be caused by water-induced oxidation of very small Co crystallites during FT. Using the XPS technique, Schanke et al. [36] studied Co/Al2O3 catalysts after reduction and treatment with various (1–25%) H2O-containing mixtures. A clear sign of re-oxidation of cobalt metal to Co2+ or Co3+ was observed, which was likely to be the cause of the deactivation of Co catalysts during water co-feeding studies (H2O in feed >20 mol%) using a fixed-bed reactor. Rothaemel et al. [43] obtained similar findings using a relatively new steady-state isotopic transient kinetic analysis (SSITKA) technique. Under certain conditions, re-oxidation of Co by water during H2O co-feeding studies has also been suggested by other studies [27,34,40,43–50]. For example, water addition may contribute to reversible or irreversible loss of activity depending on the nature of the oxidized Co species formed, and also to accelerate catalyst deactivation during FT. Jacobs et al. [27,28,44,49] suggested that differences in the water effect due to the support type may depend on the strength of the interaction between the support and the Co particles. The relative strength of the interaction between Co and support was found to follow the order: Al2O3 > TiO2 > SiO2 [34,44]. Strong interactions resulted in a negative water effect, while weak interactions led to a positive water effect. More recently, Jacobs et al. [46,49] studied the impact of H2O co-feeding on unpromoted and Pt-promoted Co/Al2O3 catalysts using XANES. At added H2O levels greater than 25% by volume of the feed gas to a low cobalt loaded 0.5%Pt–15%Co/Al2O3 catalyst (Co cluster size <6 nm), formation of cobalt-support complexes was suggested. Based on changes in the EXAFS/XANES spectra of used catalyst samples withdrawn at key moments from the slurry reactor and solidified in the wax product, the observed formation of cobalt-support complexes could explain the catastrophic and irreversible loss in activity, as the catalyst did not recover its activity when water was switched off. Using the same methodology for 25%Co/Al2O3 (with >10 nm average Co cluster size), high levels of H2O (e.g., >25%) resulted in the re-oxidation of a fraction of cobalt to form CoO, which was re-reduced when the H2O was switched off. The oxidation–reduction cycle also appeared to lead, based on changes in Co–Co metal coordination in EXAFS, to growth in the average Co metal cluster size, leading to some irreversible deactivation from the oxidation– reduction cycle [51].

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Recently, the water effect studies of Co FT catalysts have been extended to look at other catalyst structural impacts (e.g., location of cobalt inside/outside pores and average Co cluster size). Dalai et al. [52] investigated the water effect for SiO2 supported 12.4– 20% Co catalysts having different pore sizes. Based on the results of adding 5–30% H2O at 210 °C, 2.0 MPa, 5–8 Nl/gcat/h in a 1-L CSTR, it was reported that pore size influences the effect of water during FT in a unique manner. The positive water effect during FT only occurred for wide pore SiO2, where a majority of the Co clusters were found to be small enough to reside in the SiO2 pores; on the other hand, the effect of water was found to be virtually negligible over a narrow pore SiO2, where the Co particles were measured to be too large to reside within the pores, and must therefore be present in a higher fraction on the exterior surface of the SiO2 support. The positive effect of water for the wide pore SiO2 supported cobalt catalyst was therefore explained in terms of water reducing mass transport limitations by displacing heavier hydrocarbons that would otherwise fill the pores. The water effect during FT was also recently linked to the support pore structure and/or cobalt particle size by Borg et al. [40,50]. They studied the effect of water over 0.5%Re–20%Co catalysts supported on narrow pore (7.4–12.5 nm) and wide pore (20.8 nm) Al2O3 at 210 °C (20 bar, H2/CO = 2.0). The pore characteristics and water partial pressure were found to determine the impact of water on the FTS rate. They observed a negative water effect on a narrow pore Co/Al2O3 catalyst and a positive water effect on a wide pore Co/Al2O3 at the same water partial pressure (PH2O/PH2 = 0.4). Furthermore, a permanent negative water effect was observed when higher water partial pressures (e.g., P H2 O =PH2 ¼ 0:7) were used. These have been proposed to be due to diffusion effects present in the pores that contribute to the difference between narrowand wide-pore supports. The metal particle size effect on the FT rate has been the subject of much investigation as well. The significance of metal particle size for the CO hydrogenation reaction was addressed by Boudart et al. when they initially studied CO hydrogenation over Fe/MgO catalysts with different Fe particle sizes [53,54]. The CO hydrogenation rate, based on CH4 produced per metal site, increased by a factor of two by increasing the Fe particle size from 1 to 17 nm. For supported cobalt catalysts, Bezemer [55] reviewed this subject, which Bartholomew et al. [56] and Yermakov et al. [57] pioneered. Particle size of Co has been linked to interactions between Co and the support. Stronger interactions between Co and support usually were found to lead to smaller Co particles (i.e., higher dispersions), but with lower reducibility [44] (i.e., only a fraction of the cobalt is reduced to Co0 after a standard activation treatment in H2). The impact of Co particle size on the FT rate was studied by Iglesia and coworkers. An increase in the cobalt-time-yield as the cobalt particle size decreased from 210 to 10 nm was reported [39,58]. This implies that the increase in the FT rate based on cobalt-time-yield (moles of CO converted/moles Co in the catalyst) is a consequence of an increase in the Co site density with decreasing Co particle size for constant Co loading. Another way of stating this is that constant Co-site time yield was observed when the molar CO consumption rate is defined on the basis of number of surface Co0 sites (TON). Beyond that point, Bezemer et al. [55] found that as the Co cluster size became even smaller, the Co size effect was reported to change, displaying an increasing trend of Co-site time yield with decreasing Co size. Bezemer et al. [55] investigated the particle size effect on turnover frequency in the Co range of 2.6–27 nm using 0.8–22%Co/carbon-nanofiber and reported that the CO rate based on g of Co increases with Co size from 2.6–8.5 nm and then decreases between 8.5–16 nm at typical FT conditions (220 °C, 35 bar, H2/CO = 2.0); no reaction data were reported when Co particles were larger than 16 nm. These data resulted in an increased TOF with Co particle size when Co size was below 8 nm; after that

the TOF remain stable, which is consistent with the results of Iglesia et al. [39,58]. C5+ selectivity was observed to increase greatly with increasing Co particle size up to 16 nm. Most recently, Borg et al. [59] studied Co particle size (3–18 nm) effects with 10–30% Co/Al2O3 catalysts under the conditions of 210 °C, 20 bar, H2/CO = 2.0. According to the FT data in a micro-fixed-bed reactor, they reported the best Co particle size range for production of C5+ was 7–8 nm, which is likely consistent with the results on the effect of pore size during FT [60,61] because Co with average size of 8.0 nm was produced in the optimal pore size range 8.0– 12.6 nm (I.D). Therefore, according to these studies, the metal particle size effect on FT behavior is a complicated issue. Even though many studies deal with Co size effects on both FT activity and HC selectivity, further investigation is needed to place the conclusions on a firmer footing. There are few reports, if any, regarding either the sensitivity of the Co particle size to the kinetic water effect or the quantifying of a water effect kinetic parameter. In this work, we examine how the support type and structure, as well as the average Co cluster size, impact kinetic parameters using the CAER kinetic model (Eq (1)), which includes a water effect parameter. In this manner, the kinetic influence of water was investigated over two different Co/c-Al2O3 catalysts, differing in terms of support type and treatment, and having slightly different average pore and average Co cluster sizes, and two Co/SiO2 catalysts, having the same support but differing in terms of cobalt loading and average Co cluster size. 2. Experimental 2.1. Catalyst preparation In this study, Puralox HP14/150 Al2O3 (Sasol), treated Degussa

c-Al2O3 and PQ-SiO2 CS-2133 were used as catalyst supports, and cobalt nitrate was used as the precursor. The Co/Al2O3 and Co/ SiO2 catalysts were prepared by a slurry impregnation method. With this method, which adheres in part to a patented procedure [62], the ratio of the volume of loading solution used to the weight of alumina was 1:1, such that approximately 2.5 times the pore volume of solution was used to prepare the catalyst. Multiple impregnations were needed to achieve the final loading for the four catalysts: 25%Co/HP14/150 c-Al2O3, 25%Co/treated Degussa c-Al2O3, 15%Co/ and 25%Co/SiO2. Between impregnation steps, the catalyst was dried under vacuum in a rotary evaporator at 333 K and the temperature was slowly increased to 373 K. After the final impregnation/drying step, the catalyst was calcined at 623 K for 4 h under flowing air. 2.2. Catalyst characterization 2.2.1. BET measurement BET measurements for the catalysts were conducted using a Micromeritics Tri-Star system to determine the loss of surface area with loading of the metal. Prior to the measurement, samples were slowly ramped to 160 °C and evacuated for 24 h to approximately 50 mTorr. 2.2.2. Hydrogen chemisorption with pulse re-oxidation Hydrogen chemisorption measurements were performed using a Zeton Altamira AMI-200 unit, which utilizes a thermal conductivity detector (TCD). The sample weight was always 0.220 g. The catalyst was activated at 350 °C for 10 h using a flow of pure hydrogen and then cooled under flowing hydrogen to 100 °C. The sample was then held at 100 °C under flowing argon to prevent physisorption of weakly bound species prior to increasing the temperature slowly to the activation temperature. At that temperature, the catalyst

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was held under flowing argon to desorb the remaining chemisorbed hydrogen so that the TCD signal returned to the baseline. The TPD spectrum was integrated and the number of moles of desorbed hydrogen determined by comparing to the areas of calibrated hydrogen pulses. Prior to experiments, the sample loop was calibrated with pulses of nitrogen in helium flow and compared against a calibration line produced from gas tight syringe injections of nitrogen under helium flow. After TPD of hydrogen, the sample was reoxidized at the activation temperature by injecting pulses of pure oxygen in helium referenced to helium gas. After oxidation of the cobalt metal clusters, the number of moles of oxygen consumed was determined, and the percentage reduction calculated assuming that the Co0 reoxidized to Co3O4. While the uncorrected dispersions are based on the assumption of complete reduction, the corrected dispersions, which are reported in this work, include the percentage of reduced cobalt as follows.

catalyst deactivation. After each set of experiments at a fixed H2/ CO ratio was completed, reaction conditions were returned to the reference baseline conditions of 220 °C, 2.03 MPa atm, H2/ CO = 2.5, N2% = 12.5, 10 or 25 Nl/g cat/h in order to account for catalyst deactivation. To approach steady state, each data point was recorded after holding conditions for 8–24 h to ensure that enough reactor volume exchanges had occurred. Total mass closure during the kinetics experiment was 100 ± 3%. 2.5. Product analysis Inlet and outlet gases were analyzed on-line by a Micro GC equipped with four packed columns. The liquid organic and aqueous products were analyzed using a HP5890 GC with capillary column DB-5 and a HP5790 GC with Porapak Q packed column, respectively. The reactor wax withdrawn periodically was analyzed by a high temperature HP5890 GC employing an alumina clad column.

%D ¼ ð# of Co0 atoms on surface  100%Þ=ðtotal # Co0 atomsÞ 2.6. Kinetic data processing and parameter calculation

%D ¼ ð# of Co0 atoms on surface  100%Þ=½ðtotal # Co atomsÞ  ðfraction reducedÞ

2.2.3. Temperature programmed reduction Temperature programmed reduction (TPR) profiles of fresh catalyst samples were obtained using a Zeton Altamira AMI-200 unit. Calcined fresh samples were first heated and purged in flowing argon to remove traces of water. TPR was performed using 30 cm3/ min of a 10%H2/Ar mixture referenced to argon. The ramp rate was 5 °C/min from 50 to 1100 °C, and the sample was held at 1100 °C for 30 min. 2.3. Catalyst pretreatment In a typical test, catalyst (10 g) was ground and sieved to 80– 325 mesh before loading into a fixed-bed reactor for 10 h of ex-situ reduction at 350 °C and atmospheric pressure using a gas mixture of H2/He with a molar ratio of 1:3. The reduced catalyst was then transferred to a 1-L continuously stirred tank reactor (CSTR) under the protection of N2 inert gas, which was previously charged with 315 g of melted Polywax 3000. The transferred catalyst was further reduced in situ at 230 °C at atmospheric pressure using pure hydrogen for another 10 h before starting the FT reaction. 2.4. Kinetic design and CSTR testing The kinetic experiments over the two Co/Al2O3 catalysts were carried out using a 1-L CSTR employing three sets of conditions (see Tables 1s–2s in Appendix A, Supplementary data). The first set varied H2 partial pressure between 0.51–1.27 MPa while maintaining reaction temperature (220 °C) and CO partial pressure (0.51 MPa) constant, the latter achieved by using N2 (12.5– 50 vol.%.) as balancing gas. The second set varied CO partial pressure between 0.32–0.81 MPa while keeping constant both the reaction temperature (220 °C) and H2 partial pressure (0.81 MPa). Temperature was varied in the third and last set (205–220 °C) while holding constant the H2 and CO partial pressures (0.81 and 0.32 MPa, respectively). Throughout the test, total reaction pressure was kept at 2.03 MPa. Four H2/CO ratios in the range of 1.0–2.5 and four space velocities in the range of 3.3– 35 Nl/g-cat/h were used in the experiment. In the first and second parts of the experiment, for each H2/CO ratio space velocity was adjusted in a decreasing order since higher H2O partial pressures (i.e., obtained at higher CO conversion levels) tend to accelerate

Catalyst deactivation was taken into account when processing the kinetic data. Each catalyst aging curve considered only the data recorded at the baseline condition, recalling that a return to baseline conditions was made following the completion of a kinetic data set at each H2/CO ratio. The catalyst activity at baseline conditions for the two 25%Co/Al2O3 catalysts can be found in Appendix A, Supplementary data (Fig. 1s). This accounting procedure, defined on an extent of reaction basis, made it possible to correct CO conversion at each group of kinetic conditions to a fresh catalyst basis using a correction factor, fj, recognizing that the decline in conversion at a point in time is related to the number of moles of CO that have reacted on the catalyst surface.

X COi ;fresh ¼ X COi ;exp ð1 þ fj Þ where fj is the correction factor at time tj, the time associated with the completion of a set of kinetic conditions. This correction factor can be obtained from the CO conversion levels, X0, Xi obtained at the baseline conditions at time tb0 and tbi, respectively, prior to starting kinetic conditions, along with the number of moles of CO that were converted in that interval, denoted nbi. All that remains is to determine the total accumulated moles of CO converted at time tj, the time at which the kinetic conditions were completed, denoted nj. Thus, the term fj was calculated from the following equation:

fj ¼ ðX 0  X i Þ=X i  nj =nbi where nbi is the total number of moles of CO converted over the baseline interval from time tb0–tbi; X0 and Xi represent the CO conversions during the baseline interval at time tb0 and tbi, respectively; and nj is the total number of moles of CO converted between tb0 and tj, where tj is the time associated with the completion of a set of kinetic conditions. A simple way to obtain the partial pressures in kinetic equation (1) is to assume that gases such as CO, H2, CO2, N2, CH4 and light hydrocarbons, C2–C5, are insoluble in the liquid in the reactor, since these gases have very limited solubilities in liquid hydrocarbons. The hydrocarbons, C4–C30, in the liquid phase condensed in the cold trap and warm trap were assumed to reach vapor–liquid equilibrium in the reactor. These assumptions are reasonable and sufficient to obtain kinetic results for discussing the kinetic water effect based on the kinetic model (1). Simply put, the partial pressure of component i (CO, H2, N2, CO2, CH4, H2O, C2–C30) in the vapor phase of the reactor was expressed by Dalton’s Law:

Pi ¼ PT  ni =

X ðni Þ

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W. Ma et al. / Fuel 90 (2011) 756–765

where PT is the total pressure of the reaction system, and ni is molar flow rate of component i. Calculation of the kinetic parameters (a, b, k, m, and Ea) in Eq. (1) was made using a graphical approach, and details of the calculations are provided in Appendix A, Supplementary data (Fig. 2s, a–f). 3. Results and discussion 3.1. Catalyst characterization Results of surface area measurements by adsorption of N2 at 77 K (Table 2) show that the surface areas and average pore diameters of Puralox HP14/150 c-Al2O3 and treated Degussa c-Al2O3 supported 25%Co catalysts were 108 and 77 m2/g, and 13 and 18 nm, respectively, as provided in Table 2. A weight percentage loading of 25% cobalt is equivalent to 33.3% by weight Co3O4. If the c-Al2O3 is the only contributor to the area, then the area of the Co/HP14/150 Al2O3 catalyst should be 0.67  152.6 m2/ g = 101.7 m2/g, which is within experimental error of the measured value. Likewise, for the treated Degussa c-Al2O3 catalyst, the areas of the catalysts should be 0.67  138.9 m2/g = 92.6 m2/g, respectively. Thus, the predicted areas were somewhat greater than the measured values in the case of 25%Co/treated Degussa c-Al2O3. These findings suggest that some Co oxide particles blocked a fraction of the pores of the catalyst. In line with this, the average pore diameters for the 25%Co/treated Degussa c-Al2O3 catalyst were slightly higher than the support. In the cases of the PQ SiO2-supported cobalt catalysts, the anticipated areas were 0.80  351.9 = 281.5 m2/g for 15%Co/SiO2 and 0.67  351.9 m2/ g = 235.8 m2/g for 25%Co/SiO2. While the measured value for the 25%Co/SiO2 catalyst is within experimental error of the expected value, the measured value of the 15%Co/SiO2 catalyst is somewhat lower than expected, suggesting that a fraction of the Co3O4 clusters were large, enough to cause some pore blocking. Chemisorption results indicated that the reduction degree of Co supported on the two types of c-Al2O3 were similar (55–59%), but slightly smaller Co clusters were present on the treated Degussa c-Al2O3 (8.6 vs. 12.3 nm) relative to those supported on HP14/ 150 c-Al2O3. For the PQ SiO2-supported catalysts, the 25% Co loading resulted in a lower Co reduction degree (70% vs. 80%) and a smaller average Co cluster size compared with the 15%Co/SiO2 catalyst (27 vs. 38 nm, respectively). This is in agreement with the BET results, where it was suggested that a fraction of the Co3O4 particles for the 15%Co/SiO2 catalyst were large enough to cause some pore blocking. In all cases, with SiO2 supported Co catalysts, in spite of higher surface areas, larger Co clusters were formed with higher extents of Co reduction upon activation in H2 relative to the Al2O3 supported Co catalysts. This indicates that, despite the much higher surface area of the silica support, it was the weaker interaction between the SiO2 support and the Co oxides that led to the formation of larger Co clusters exhibiting reduction behavior

that was closer to bulk-like Co3O4 reduction (i.e., more facile reduction). Conversely, it was the stronger interaction between the Al2O3 support and the Co oxide species that led to a higher fraction of the cobalt remaining unreduced after activation (i.e., less facile reduction) but, more importantly, the stabilization of much smaller Co clusters. By comparing the H2 TPD data, which represent a direct correlation with the active site densities, it is evident that the gains in active site densities by the smaller cobalt clusters on Co/Al2O3 catalysts outweighed the losses from the lower extents of reduction, in comparison with the Co/SiO2 catalysts. This should dispel the notion that catalysts with greater ease of reduction automatically should be preferable to those having lower extents of reduction, since one must also carefully consider, perhaps even more so, the resulting average Co cluster size. 3.2. Stability test of structured 25%Co/Al2O3 catalysts To examine the effect of Al2O3 support type and structure on Co catalyst activity and stability, reactor tests with the Puralox HP14/ 150 and treated Degussa c-Al2O3 supported 25%Co catalysts were conducted using a 1-L CSTR at the same reaction conditions, 220 °C, 280 psig, H2/CO = 2.5 and 10 Nl/g cat/h (Fig. 1). Initial CO conversion at 25 h for the 25%Co/treated Degussa c-Al2O3 was 72%, which is double that observed with 25%Co/Puralox HP-14/ 150 c-Al2O3. This indicates that the 25%Co/treated Degussa c-Al2O3 is much more active based on CO converted per g of catalyst. This should mainly be due to a higher Co0 surface site density, since the catalyst possesses smaller Co clusters. During the next 150 h of testing, the 25%Co/treated Degussa c-Al2O3 catalyst deactivated rapidly. Analysis of the partial pressure of water in the reactor for the two runs showed that the measured values of PH2 O =P H2 were about 0.18 for the 25%Co/Puralox HP-14/150 c-Al2O3, and 0.8 for the 25%Co/treated Degussa c-Al2O3 catalyst, which likely indicates that the faster deactivation observed on the 25%Co/treated Degussa c-Al2O3 was associated with higher partial pressure of water inside the reactor. To confirm this, a second FT run over the more active catalyst, 25%Co/treated Degussa c-Al2O3, was conducted at lower water partial pressure (Fig. 1). It was found that the catalyst is quite stable at a low P H2 O =PH2 of 0.12, resulting from a low CO conversion of 15% during 175 h of testing. Since these two runs used the same catalyst (i.e., 25%Co/ treated Degussa c-Al2O3), but the two runs had significant differences in the H2O partial pressure in the reactor resulting from the different conversion levels, these observations imply that the partial pressure of water inside the reactor is one of main factors affecting Co/Al2O3 catalyst stability. The fact that it displayed poor stability at high partial pressures of water is consistent with the perspective that water may oxidize a fraction of the Co (e.g., smaller cobalt particles) during FT [27,28,34,40,46,49,50], either to cobalt oxide or to an irreversible cobalt-support compound. The Co particle size dependency of the water effect is not clearly reflected

Table 2 Results of BET surface area, porosity, and hydrogen chemisorption with pulse re-oxidation. Description

N2 physisorption

Chemisorption with pulse re-oxidation

H2 desorbed O2 pulsed Reductiona Uncorrected Uncorrected Corrected Corrected Co Average BET surf Pore Disp., % Cluster size, volume pore area lmol/gcat lmol/gcat Degree, % Disp., % Co Cluster nm cm3/g Diameter, nm m2/g size, nm Puralox HP-14/150 c-Al203 25 wt.%Co/HP14/150 c-Al203 Treated Degussa c-Al203 25 wt.%Co/treated Degussa c-Al2O3 PQ-SiO2 CS-2133 15 wt.%Co/PQ-SiO2 25 wt.%/Co/PQ-SiO2 H2 reduction at 350 °C.

152.6 108.2 138.9 76.9 351.9 263.1 225.5

0.92 0.43 0.81 0.36 2.37 1.04 0.76

20.7 13.5 17.6 18.2 25.8 15.4 13.4

97.7

1554

55.0

4.6

22.4

8.4

12.3

149.6

1653

58.5

7.1

14.6

12.1

8.6

38 74.2

1436 2121

80 ± 3 68 ± 3

3 3.5

34.5 29.5

3.5 4.7

38.4 27.0

761

W. Ma et al. / Fuel 90 (2011) 756–765

Fig. 1. CO conversion as a function of time on 25%Co/HP14–Al2O3 and 25%Co/ c-Al2O3 catalysts (220 °C, 2.03 MPa, 10–25 Nl/gcat/h, H2/CO = 2.5, N2 = 12.5%).

Fig. 2. Kinetic rate constant and water effect constant as a function of Co cluster size on Al2O3 and SiO2 supported catalysts.

by these two stability tests even though a different average Co cluster size was present on the two 25%Co/Al2O3 catalysts. However, the results to be discussed suggest that the kinetic influence of water is particle size dependent.

3.3. Support and Co cluster size effects on kinetics 3.3.1. Support and Co particle size effects on reaction orders a and b The empirical kinetic model represented by Eq. (1) to describe the kinetic effect of water first appeared in reference [24], and

was later used by Das et al. [25] to effectively define the water effect for a Co/Al2O3 catalyst. This model has been found to be superior to mechanistically-derived CO consumption models for describing the CO conversion rate over Co-based catalysts [63]. Using the same kinetic method and model, we obtained kinetic parameters (a, b, k and m) for the 25%Co/HP14/150 c-Al2O3 and 25%Co/treated Degussa c-Al2O3 catalysts, as shown in Table 3. The kinetic parameters for 25%Co/SiO2 and 15%Co/SiO2 reported in [63] are also listed in Table 3 in order to more effectively discuss the influences of support and average Co cluster size on kinetic parameters. Reaction orders a and b correspond to the partial pressures of CO and H2 in equation (1). The values of a and b calculated using the empirical kinetic model (1) for the two Co/SiO2 catalysts are quite similar, (e.g., a, 0.19 to 0.22 and b, 0.5–0.6), but larger differences for both a and b occur over the two Co/Al2O3 catalysts, (e.g., a, 0.24 to 0.60 and b, 0.5–0.83). It is important to note that the signs on the a and b orders of reaction were the same for all the catalysts tested in this work. The differences in the a and b values may reflect differences in adsorption of CO and H2 on the Co-based catalysts resulting from support and/or Co particle size effects. In most of the mechanistically-derived models, it was assumed that H2 participates in the FT reaction in either a molecular or molecularly adsorbed state. Thus, the order of reaction for PH2 in the mechanistically-derived models (Table 1) is usually the integers 1 or 2. However, in our work, the values of b observed for the two Co/SiO2 catalysts, as well as for the 25%Co/treated Degussa c-Al2O3 catalyst, were 0.5, implying that adsorbed H2 was likely dissociated on the catalyst surface before taking part in the FT reaction; b is somewhat larger, 0.83, over the Co/Puralox HP-14/150 c-Al2O3, but did not achieve unity to imply the involvement of molecularly adsorbed H2. In fact, without knowing which elementary step is rate-determining and the true catalytic mechanism, one can only suggest and not definitively conclude how the molecules adsorb and react in the cycle, according to a kinetic treatment. Secondly, the a values of all four catalysts were negative, indicating that CO adsorption is much stronger than hydrogen. As a result, CO adversely affects the FT reaction rate. This is consistent with the work of Yang [17], Wang [19] and Zennaro and Boudart [20b], and partly agree with the mechanistically-derived FT models [3–13], where the FT rate was proportional to PCO (order is between 1 and 1). On the other hand, the a value for the 25%Co/Puralox HP-14/150 c-Al2O3 was found to be more negative than that of either Co/SiO2 catalyst (0.60 vs. 0.19 to 0.22), which seems to suggest that HP14–Al2O3 helps in some manner to retain more adsorbed CO on the catalyst surface than SiO2. However, the value of a over the 25%Co/treated Degussa c-Al2O3 catalyst, 0.24, was close to that of either Co/SiO2 catalyst, so such a conclusion is only tentative, at best. The impact of strong CO adsorption indicates that more active sites are occupied by CO on Co/Al2O3 catalysts, which tends to suppress the FT reaction. One possibility is that the stronger interaction between Co and Al2O3

Table 3 Summary of kinetic parameter values.a a

rCO ¼ kP CO PbH2 =ð1 þ mPH2 O =PH2 Þ Catalyst

25%Co/HP14/150 c-Al2O3 25%Co/treated Degussa c-Al203 15%Co/PQ-SiO2b 25%Co/PQ-SiO2b a b

Reaction order a

b

0.60 0.24 0.22 0.19

0.83 0.50 0.60 0.51

a, b, k and m are obtained at 220 °C. Kinetic values from [63].

a/b

Activation Energy Ea, kJ/mol

Water effect constant, m

Reaction rate constant, k mol/g-cat/h/MPa(a+b)

0.82 0.48 0.37 0.37

75.8 90.6 85.9 93.7

0.28 0.99 0.33 1.11

0.0205 0.0334 0.0187 0.0381

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than between Co and SiO2 [34,42] may influence the reaction order of PCO. On further comparing a and b values between the Co/Al2O3 and Co/SiO2 catalysts, it was found that the a values are closer to zero over both the 25%Co/treated Degussa cAl2O3 and the 25%Co/SiO2 catalyst, which have smaller Co particles compared to their 25%Co/HP-14/150 c-Al2O3 (8.6 vs. 12.3 nm), and 15%Co/ SiO2 (27 vs. 38.4 nm) counterparts; also, b was found to be smaller. This finding may imply that Co particle size affects CO and H2 adsorption as well. With decreasing Co size, inhibition of the FT rate by strongly held CO on Co active sites appears to decrease, and adsorbed H2 appears to favor dissociation. This is likely consistent with the results on Co-based catalysts reported by Kuznetsov et al. [64], which pointed out that finely dispersed Co has lower affinity for adsorption of CO. Co particle size effects during FT may be linked to other kinetic parameters as well (e.g., k and m, to be discussed below). The ratio of the coefficient of a to b for the two Co/SiO2 and the two Co/Al2O3 catalysts (ranging from 0.37 to 0.82), agree well with the values reported in the literature (ranging from 0.3 to 0.74) [17–20,25,26]. It is interesting that a/b ratios are more negative over Al2O3 supported Co catalysts (0.48 to 0.82) than over SiO2 supported catalysts (both approximately 0.37), again suggesting more inhibition of adsorbed CO with the influence of Al2O3. It should be noted that a and b were obtained using the same H2/CO feed ratios (1–2.5) in all four kinetic experiments. The ratio of a/b varied with catalysts, and larger values of a and b were obtained over the 25%Co/Puralox HP14/150 Al2O3, also suggesting that reaction orders of a and b for PCO and P H2 are a function of catalyst type. 3.3.2. Support and Co particle size effects on k and STY Apparent reaction rate constants can be used as an effective means to benchmark working catalyst activity. Apparent kinetic rate constants, calculated on a per g of catalyst basis, are listed in Table 3. In terms of the k values, 25%Co/SiO2 was the most active catalyst, followed by 25%Co/treated Degussa c-Al2O3, 25%Co/ HP14/150 c-Al2O3 and 15%Co/SiO2. Therefore, the apparent reaction rate constant suggests that support (both type and structure) directly impacts FT behavior. To gain insight into how the particle size impacts the reaction rate constant, a correlation between reaction rate constant and Co cluster size for both Al2O3 and SiO2 supported catalysts was plotted, as displayed in Fig. 2. Over both supports, a significant particle size effect on the reaction rate was observed. As the particle size decreased from 38.4 to 27 nm

Fig. 3. Change of kinetic rate constants with Co cluster size.

over Co/SiO2 and from 12.3 to 8.6 nm over Co/Al2O3, the kinetic rate constant increased by 102% and 62%, respectively. This result should be consistent with a higher activity resulting from the higher Co0 surface site density due to smaller clusters [43,65]. It is noted that the interactions of Co with two types of Al2O3 supports were similar, since the extents of Co reduction between the two catalysts were close, 55–59%. Thus, the main difference in their rate constants should be mainly due to a difference in Co cluster size. A correlation of the reaction rate constant with Co cluster size without taking into account the support effect is shown in Fig. 3. Clearly, the reaction rate constant is not directly related to the Co cluster size if one does not also take into account the support effect. But if considering the catalysts of Co on SiO2 and Al2O3 supports separately, we noticed the cobalt size effect on each type of supports: the smaller Co, the larger the reaction rate constant. This suggests that it is necessary to specify the support type when studying the Co cluster size effect in FT. The cobalt site time yields (STYs) at 220 °C in this work were observed to decrease from 77 to 55.3  103 s1 and from 198.0 to 186.0  103 s1 as the Co particle size increased from 8.6 to 12.3 over Al2O3 and 27.0 to 38.4 nm over SiO2, respectively. These STYs are consistent with a decrease from 51 to 36.6  103 s1 and from 126 to 118  103 s1 with particle size at a lower temperature of 210 °C for the Co/Al2O3 and Co/SiO2, respectively. And, they are also consistent with a decrease from 34 to 24.4  103 s1 and from 78 to 73  103 s1 at 200 °C, as well as from 22.6 to 16.2  103 s1 and from 58 to 49  103 s1 at 190 °C, for the Co/Al2O3 and Co/SiO2 catalysts, respectively. This assumes that the Co STYs at different temperatures are proportional to the ratio of the kinetic rate constant (k) at the two temperatures (Co STY)T1/ (Co-STY)T2 = k1/k2 = exp[Ea/Rg(1/T1  1/T2)]). Thus, the STYs vary over a small range for both types of catalysts, Co/Al2O3 and Co/ SiO2, but the Co STYs over all the catalysts used in this work could not be correlated with Co particle size over the entire range of 8.6– 38.4 nm. Recent FT work about the influence of Co particle size on catalyst activity was reported by Iglesia et al. [58,39]. Using a micro fixed-bed reactor under the conditions of H2/CO = 2.0, 20 atm, XCO = 50–60% (except for reactor type, conditions that are similar to this study), they found that the CO conversion rate for Co catalysts, on a per g catalyst basis, increases with decreasing Co size when the average Co diameter is greater than 10 nm, but that the Co-site time yield (STY) remained virtually constant. For example, at 200 °C, the Co STYs were reported to vary between 16 and 32  103 s1. Thus, our STYs data for the two Co/Al2O3 catalysts are essentially consistent with the data reported by Iglesia et al. [58,39]. More recently, Borg et al. [59] optimized Co size in the range of 3–18 nm for the FT rate and HC selectivity at 210 °C by using 26 different Co/Al2O3 catalysts. They found that the Co STYs on Co/Al2O3 at 210 °C varied between 31 and 63  103 s1. Thus, our STYs on for Co/Al2O3 catalysts are also consistent with that study. Feller et al. [66] studied cobalt cluster size effects on FTS using Zr modified 8%Co/SiO2 catalysts. It was reported that Co STY (i.e., turnover frequency) over the Co/Zr–SiO2 catalysts were in the range of 6–55  103 s1 at 190 °C and 5 atm. Interestingly, the Co STY obtained over Co/SiO2 catalysts employed in this study are also close to the results of Feller et al. [66]. Recently, Bezemer and co-workers [55] studied the Co particle size effect with 12 Co/ Carbon fiber catalysts using a micro-fixed-bed reactor. They reported that the Co STY increased with Co cluster size up to 8 nm (1.4–34  103 s1) at 210 °C, 350 psig, and H2/CO = 2.0. Above this Co diameter, they found that the Co STY became constant, 22  103 s1. Apparently, the Co STYs for the catalysts having larger Co particles (>8 nm) from the study by Bezemer et al. [55] were slightly lower than those obtained in the current study, and also lower than those found in the work of Iglesia et al. [58,39]

W. Ma et al. / Fuel 90 (2011) 756–765

and Borg et al. [59]. The differences in the Co STYs among the studies is not very clear, and further work is required in this area. Activation energies for the four Co catalysts with the same size (sieved) catalyst particles, 80–325 mesh, in this study were determined to be 75–94 kJ/mol, which are close to the range of 80– 120 kJ/mol for Co/TiO2 [20a], Co–Mg/SiO2 [1], Co–Cu/Al2O3 [17] and Co/SiO2 and Co/Al2O3 [7,20b], as reported in the open literature. 3.3.3. Support and Co particle size effects on m In the empirical Eq. (1), the denominator can be obtained by assuming ‘‘effective” active sites are mainly covered by dissociated H2 and adsorbed water if the inhibition of the CO conversion rate by CO adsorption is considered separately (by a). The parameter m in the equation represents an adsorption parameter for the water adsorption term and can be used together with P H2 O =PH2 to describe the extent to which water affects the overall CO consumption rate. At a given CO conversion, a higher m will result in larger ðmPH2 O =PH2 Þ values, indicating a greater sensitivity by the catalyst to water. Note that m can be a negative or a positive number, which implies a positive or a negative impact on the CO conversion rate, respectively. The m values of the four Co catalysts calculated from our kinetic data are summarized in Table 3. Negative m values are obtained over the two Co/SiO2 catalysts, indicating a positive impact on the CO conversion rate, while positive values were observed for the three Co/Al2O3 catalysts, demon-

763

strating a negative impact on CO conversion. These findings are in good agreement with the positive water effect observed for Co/SiO2 and negative effect found with Co/Al2O3 in water co-feeding studies [29–37]. In addition, higher absolute m values were obtained for the 25%Co/treated Degussa c-Al2O3 and 25%Co/SiO2 catalysts, which possessed smaller Co clusters (Table 2), indicating a greater sensitivity to water compared with the corresponding catalysts having larger Co clusters on average (i.e., 25%Co/Puralox HP14/ 150 c-Al2O3, and 15%Co/SiO2 catalysts). Moreover, it is interesting to consider that kinetic model (1) may be derived from the mechanistic kinetic model [67], r CO ¼ kPCO PH2 =ðK 1 P H2 O þ K 2 P H2 Þ, where k is the reaction rate constant and K1 and K2 are the H2O and H2 adsorption equilibrium constants, respectively, and assuming that CO and H2 adsorption occur on the catalyst. Thus, the water effect constant m in Eq. (1) could be a combined term of the H2 adsorption equilibrium constant K2 and the water desorption equilibrium constant K1. If this is the case, the overall apparent m would be determined by the desorption heat of water and the adsorption heat of hydrogen:

DH ¼ DHads;H2 þ DHdes;H2 O

ð2Þ

It has been reported that the heat of H2 adsorption is related to the metal particle size, with smaller metal particles exhibiting a higher heat of adsorption [67]. If this is true, it could lead to a larger m = m0 exp(DH/Rg/T) for smaller Co particles, which is consistent with our current kinetic results. Since CO adsorption affects the overall CO consumption rate (taken into account sep-

Fig. 4. Change of experimental and theoretical values of P H2 O =P H2 with CO conversion (a) 25%Co/Puralox HP14/150 c-Al2O3, (b) 25%Co/treated Degussa c-Al2O3, (c) 15%Co/ SiO2 and (d) 25%Co/SiO2.

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arately), its effect on the overall m should not necessarily be based on the equation for DH above, even though CO exhibits stronger adsorption than either H2 or water (>2–10 times) on Fe and Co catalysts [5,68–70]. In this study, the inhibition of the rate due to the strong adsorption of CO is, as discussed previously, demonstrated by the negative reaction order of a, ranging from 0.19 to 0.60. The particle size impact on the water effect of FT (i.e., smaller Co particle size displaying a greater sensitivity to water) in this study is consistent with our earlier FT results over Co/SiO2 catalysts calcined by air and nitric oxide [63]. A much faster deactivation trend was observed with the NO calcined 15–25%Co/SiO2 catalysts, which exhibited, on average, a smaller Co cluster size relative to their air calcined 15–25%Co/SiO2 (14–19 vs. 27–38.5 nm) counterparts. The faster deactivation rate observed with the NO calcined Co/SiO2 catalysts, possessing on average smaller Co clusters, has been ascribed to more rapid oxidation of Co caused by the higher partial pressure of water, a direct result of higher activity relative to the air calcined Co/SiO2 catalysts. The pore size of the support has been reported to play a decisive role in influencing the FT rate and hydrocarbon selectivities. An optimized wide pore size range of 8–12 nm for the highest CO rate and C5+ selectivities has been reported in a number of studies [50,60,61,71–77]. Therefore, the pore size of the support could be a possible factor to impact the water adsorption constant m according to these previous reports. However, in our current study, the pore size effect on FT for the small catalyst particles (80–325 mesh/0.04–0.18 mm) that we used appears to be insignificant. As discussed, the BET results (Table 1) indicate that the Al2O3 supports consist of meso-pores (2–100 nm). Comparing the BET surface area and pore volume before and after loading 25%Co, BET surface area and pore volume decreased by 29% and 53%, respectively, for the Puralox HP14/150 c-Al2O3 and by 45% and 56% for the treated Degussa c-Al2O3, suggesting a greater fraction of smaller particles residing in the pores of the treated Degussa c-Al2O3. For the Co/SiO2 catalysts, the same trend was observed, with the 25% Co/SiO2 catalyst having a greater fraction of smaller particles within the pores. Thus, it can be speculated that more FT product is formed in the pores of the 25%Co/treated Degussa c-Al2O3 and 25%Co/SiO2 due to a greater fraction of smaller Co clusters being located within rather than external to the pores. This assumption is also supported by a much higher overall kinetic rate for 25%Co/treated Degussa c-Al2O3 and 25%Co/SiO2 relative to 25%Co/Puralox HP-14/150 -Al2O3 and 15% Co/SiO2; similar results were reported by Dalai et al. [52] when they studied the water co-feeding effect on FT over two Co/SiO2 catalysts having different pore sizes. Therefore, assuming the catalyst pores had mass transport limitations for syngas and FT products, it is anticipated that the experimental P H2 O =PH2 would be less than the theoretical value of P H2 O =PH2 obtained on the basis of the stoichiometry of the FT reaction (CO + (1 + m/2n) H2 ? 1/n CnHm + H2O), and a greater deviation of PH2 O =P H2 between experimental and calculated values should occur for the 25%Co/treated Degussa c-Al2O3 and 25%Co/SiO2 due to the higher CO consumption rates being restricted by CO and H2 transfer through an intraparticle water-rich liquid phase stabilized by capillary effects. However, this was not seen for our four kinetic runs. Fig. 4(a–d) show all experimental and calculated PH2 O =PH2 numbers over the four catalysts. Below 60% CO conversion, the experimental and theoretical values of PH2 O =PH2 with CO conversion agree with each other for all four runs. These results imply that the FT reaction over our catalysts at the conditions used is kinetically controlled. Transport effects within the catalyst pores can be neglected due to the fact that small catalyst particles were used; therefore, the impact on m by the pore size of the support for our situation can be disregarded.

Krishnamoorthy et al. [37] investigated the effects of water on rate and selectivity of FT on Co/CS-2133–SiO2 using an in situ infrared spectroscopy technique. The support they used is identical to ours, and the catalyst particle size (0.18–0.10 mm) was only slightly larger than what we used. They observed that water does not influence the density or structure of adsorbed CO intermediates or the number of exposed Co atoms, which bind CO at nearmonolayer coverages during FT. Furthermore, they found that CO transport restrictions from intrapellet regions as pores filled with liquid products are not important during FT due to the observation of constant intensity for the bands corresponding to adsorbed CO. The previous discussion regarding the experimental and theoretical values of PH2 O =P H2 are consistent with the conclusion drawn by Krishnamoorthy et al. [37]. Another recent study regarding the catalyst particle size effect on FT was performed using three 0.5%Ru–20%Co catalysts supported on wide and narrow pore c-Al2O3 by Rytter et al. [78] at typical FT conditions. It was found that selectivity remained constant for particles less than 0.4 mm, suggesting that catalyst particles only displayed mass transfer limitations when they were greater than 0.4 mm. The catalyst particle size we used is 10 times smaller than this value; thus, our conclusion of neglecting the pore size effect on m is also consistent with the results of Rytter et al. [78].

4. Conclusions The support effect and Co cluster size effect on the kinetics of the Fischer–Tropsch synthesis reaction were investigated by performing four kinetic runs over two Co/Al2O3 and two Co/SiO2 catalysts. The kinetic results demonstrated that the FT reaction exhibits some structure sensitivity for the kinetic effect of water with respect to support type and Co cluster size. Major conclusions are summarized below. Calculation of kinetic parameters for Co/Al2O3 and Co/SiO2 catalysts demonstrate a support effect. For each support type (i.e., c-Al2O3, SiO2), the activity of the catalysts based on a reaction rate constant defined on a per g of catalyst basis was higher for the catalysts having smaller Co particle size. The Al2O3 support likely enhances the adsorption of CO relative to the SiO2 support, resulting in a greater inhibition of the reaction rate, as indicated by a more negative value of reaction order for the partial pressure of CO, a. Kinetic results also suggested that adsorbed H2 was dissociated into surface bound H atoms, H-s, before taking part in the FT reaction. The support effect was also confirmed by the observation of a negative water effect for the Co/Al2O3 catalyst and a positive one for Co/SiO2. The Co STYs on Co/Al2O3 and Co/SiO2 were found to be different, and the number was greater for the Co/SiO2 catalysts relative to Co/Al2O3. This result is inconsistent with the literature [55] and [58] and further study is needed to confirm the discrepancy. Co particle size influences catalyst activity and kinetic parameters. In the particle size range studied, 8.6–38.4 nm, the kinetic rate constant defined on a per g of catalyst basis increased by 62% as Co clusters decreased from 12.3 to 8.6 nm on Al2O3, and by 102% from 38.4 to 27.0 nm on the SiO2. The Co turnover frequency range 55– 77 s1 (220 °C) for 25%Co/Al2O3 is in agreement with the literature [59]. Co cluster size was found to impact the water effect parameter. For both Co/Al2O3 and Co/SiO2 catalysts, smaller Co clusters were found to be more sensitive to water, as indicated by a larger m. Smaller Co clusters tended to diminish the adverse CO adsorption effect on the FT rate, and increase the probability for H2 dissociation on the Co catalyst surface. Diffusion effects of FT products and feed gas molecules in catalyst pores were not significant. Thus, the effect of pore size on

W. Ma et al. / Fuel 90 (2011) 756–765

kinetic parameters for the small catalyst particles of 80–320 mesh used in this work can be excluded, in agreement with others. Acknowledgments This work was supported by NASA contract, #NNX07AB93A and the Commonwealth of Kentucky. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fuel.2010.10.029. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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