Article pubs.acs.org/IECR
Synergistic Effect of Fe−Co Bimetallic Catalyst on FTS and WGS Activity in the Fischer−Tropsch Process: A Kinetic Study Sonal, Kishore Kondamudi, Kamal K. Pant,* and Sreedevi Upadhyayula Department of Chemical Engineering, Indian Institute of Technology, Delhi, 110016, India S Supporting Information *
ABSTRACT: The kinetics of Fischer−Tropsch (FT) and water− gas shift (WGS) reactions were investigated through detailed experimentation over a laboratory prepared 10Fe/20Co/SiO2 catalyst. An investigation was undertaken to understand the prevailing mechanism of CO activation over a Fe−Co bimetallic catalyst. The mechanisms of CO adsorption and intermediate formation are different over an iron and cobalt catalyst, which finally affect the rate of CO consumption. The mechanism of CO consumption in the FT and WGS reactions over this catalyst has been studied in the light of the synergistic effect due to the presence of both Fe and Co phases. These different phase formations (active sites) were investigated using characterization techniques, namely, XRD, TEM, and SAED. The reaction kinetic study was performed at industrial relevant reaction conditions (T = 473−553 K, P = 1−3 MPa, GHSV = 1800−6600 mL/gcat-h, H2/CO molar ratio = 0.5−2.5) in a continuous fixed bed reactor. The models based on rate of CO consumption were derived using the Langmuir−Hinshelwood−Hougen−Watson (LHHW) and Eley−Riedel (ER) approach. Mechanistic models were based on carbide, enol, and carbide plus enol mechanisms, where both H2-assisted and -unassisted adsorption of CO were taken into consideration for the derivation. The selected models were validated with experimental data. Models based on the enol and carbide mechanisms were able to predict the rate of consumption of CO very well where the dissociation of CO was hydrogen assisted. Literature models were tested for the WGS reaction, and models based on formate mechanism fit the experimental data well.
1. INTRODUCTION
synergistically to achieve high C5+ selectivity and also activity toward a WGS reaction. Kinetic studies of the Fischer−Tropsch synthesis (FTS) and WGS reactions are a prerequisite for the optimization of the process. Also, a detailed understanding of the rate of the feed consumption or product formation is required to scale up the process. However, the complex reaction network of the FT synthesis makes it very challenging to model the kinetics and product selectivities of this process. Numerous literature reports on the reaction kinetics over iron6−25 and cobalt20,26−40 40 catalysts which are related to the rate of CO consumption are available. However, substantial diversity is observed among the developed models. This is due to the wide range of process conditions chosen for the reaction, catalyst pretreatment methods,41 catalyst composition, and structure.19,42 FT reactions are structure sensitive reactions, which are directly related to the catalyst selection and their structural properties. Therefore, a thorough kinetic study and understanding of the mechanism is required for derivation of more realistic and
Due to escalating demand of energy and dynamic fuel prices, there has been a keen interest for developing alternative technologies for producing energy. At the same time, the aim is to produce clean energy from economical fuel sources keeping environmental implications at the top of the priority list. The Fischer−Tropsch (FT) process utilizes coal and biomass to produce syngas, a mixture of carbon monoxide and hydrogen gas, and transmute it into feasible gaseous and liquid hydrocarbons and wax, which can be further transformed into hydrocarbons of lower molecular mass at elevated temperature and pressure in the presence of a catalyst. Several disparate catalysts have been analyzed to produce fruitful results, and among them cobalt (Co)- and iron (Fe)-based catalysts exhibit good activity and are used industrially.1−4 Co-based catalysts are typically more active than Fe-based catalysts and require lower reaction temperature. A supported bimetallic Fe/Co catalyst is considered to be a feasible option for conversion of low H2/Co syngas ratio.5 It has been observed that cobalt catalysts exhibit high selectivity and activity toward long chain paraffins and reluctance to a water−gas shift (WGS) reaction, whereas Fe catalysts promote a WGS reaction. Here, it is hypothesized that the Fe−Co bimetallic catalyst can work © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
November 22, 2016 March 30, 2017 March 31, 2017 March 31, 2017 DOI: 10.1021/acs.iecr.6b04517 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
While investigating the FT reaction over iron and cobalt bimetallic catalysts, it is important to check the possibility of H2O inhibition. Kinetic modeling of this reaction over bimetallic catalysts has been performed by very few authors. Mirzaei and co-workers have developed a model for the FT reaction over Fe−Co−Mn, Fe−Co−Ni, and Co−Ni catalysts.27,54−58 They developed a number of models for the FT reaction based on various monomer formations and carbon chain distribution pathways and concluded that the value of kinetic parameters and activation energy can be attributed to different catalyst preparation methods and active phases of compounded catalysts which imply that mechanism. The role of different phases of iron and cobalt and their impact on the CO adsorption and consumption are not discussed in detail in the literature. Therefore, use of a bimetallic Fe−Co catalyst requires a clear insight about the mechanistic aspect related to the model development. The present kinetic study of FT and WGS reactions over Fe−Co bimetallic catalysts will incorporate the synergistic effect of both the catalysts. The kinetics are performed to examine the dominant role of Co or Fe on the CO activation mechanism. The models for FTS were developed based on a different CO activation mechanism, and rate expressions were formed using different ratedetermining steps. A suitable model was obtained, and the kinetic parameters were chosen by a nonlinear regression method. The obtained results were compared with the reported results in the literatures. All experimental data were collected while the catalyst is at highest activity to ensure the precise kinetic data.
reliable rate models for the selected catalyst (Fe/Co/SiO2). In the present study, the CO adsorption and dissociation mechanism over the bimetallic catalyst has been used to explain the kinetics of the reaction. On the basis of the differences in CO adsorption and dissociation on each of these metallic phases, different kinetics models are derived. In the FT process, the rate of CO consumption is determined in the CO activation step, which includes CO adsorption, dissociation, and monomer formation. Also, this step is slow compared to the subsequent chain growth and desorption steps.43 The desorption step controls chain length, i.e., product distribution.20 Reaction starts with the adsorption of CO over the catalyst site, which can be dissociative or associative. It was observed that the cobalt surface favors dissociative adsorption of CO, which dissociates before the interaction with H2,20,33,35,37,39,40,44,45 whereas iron favors hydrogen-assisted CO adsorption.1,6,7,16,24,46,47 Recent results from DFT studies also support the fact that a corrugated or stepped cobalt surface facilitates direct CO dissociation, while involvement of H2 in the CO adsorption may be feasible over a low reactive surface such as iron carbide which is proved to be an active phase in FT synthesis process.48,49 van Dijk50 also concluded in his study over a cobalt catalyst that an oxygen free C1 intermediate forms on the surface which further hydrogenates to form long chain. However, there are still some ambiguities regarding the CO activation path. Okeson et al.51 found that the model based on unassisted CO dissociation fits the kinetic data of an iron-based catalyst. At the same time, Fazlollahi et al.27 found that the hydrogen-assisted CO dissociation is favorable over a cobalt-based catalyst. Keyser et al.29 tested a number of rate models based on the LHHW approach and empirical model from the literature and observed that the model based on the enolic mechanism explained the kinetics better than that of the carbide mechanism. The difference in the CO activation path over the Co and Fe catalyst leads to a path of determining the exact mechanism when the Fe−Co bimetallic catalyst is being used. The consideration of inhibition of H2O and CO2 is also an important factor in the rate kinetics FT process over a Fe−Co bimetallic catalyst. Due to the presence of an iron (WGS active catalyst) catalyst, it is required to check the probability of H2O and/or CO2 inhibition in the model derived for the reaction over the bimetallic catalyst. In the study over the iron catalyst, the rate of the CO consumption term is mainly presented as a function of concentration of CO and H2. However, there is a discrepancy about the addition of H2O and CO2 as inhibition. H2O inhibition is present in the models derived for the monometallic iron catalyst.6,12,16,19,24,52,53 Some authors16 tried to include CO2 inhibition, and a few models24,51 excluded both CO2 or H2O inhibition. It was observed from the detailed literature review that, due to its high WGS activity which causes a competitive environment for H2O and CO adsorption, models for iron-based catalysts include an H2O inhibition term. However, it needs to be emphasized that the CO2 inhibition was observed in the FT reaction, where alkali promoters were used to enhance WGS activity.16,24,47 In the present study, there is no alkali promotion, so the CO2 inhibition term can be neglected while deriving the kinetic rate expression for the FT reaction. The kinetics of cobalt- and iron-based catalysts can be differentiated on the basis of the former’s inactivity toward the WGS reaction. Many researchers neglected the WGS reaction while modeling the kinetics of FT synthesis using cobalt-based catalysts as H2O is not adsorbed on the catalyst active sites.
2. REACTION MECHANISM AND MODEL DEVELOPMENT Depending upon the different CO activation pathways19,20,24,52,59 over Co- or Fe-based catalysts, five sets of elementary reaction steps were written for CO consumption (Table 1). In all these, it was assumed that CO adsorbs either associatively or dissociatively, and the adsorption can be hydrogen assisted or direct adsorption. As assumed by Dry et al.9 hydrogen reacts in the gaseous phase, while some previous models7,19,29,47 suggested that it first adsorbs on surface and then reacts with already adsorbed carbon species. Therefore, both LHHW and EL approachs were incorporated for the model development. The following assumptions were made in order to derive the rate expression: (1) One molecule of CO is consumed in the formation of the −CH2− monomer. (2) In the −CH2− preparation steps, one step was a rate-determining step, and others are assumed to be quasi-equilibrium. (3) The adsorbed CO and H2 were in quasi-equilibrium with its gas phase concentrations. (4) Adsorbed CO and adsorbed hydrogen were the most abundant intermediate. (5) An isothermal condition was assumed throughout the reactor as it was operated at lower reactant conversion level. All the models were fitted separately, against the generated experimental data. Models were derived separately for the FT reaction as RFT = RCO − RWGS, where the experimental values of RFT were taken as the difference in rate of consumption of CO and the rate of formation of CO2. Detailed descriptions of the models are shown in Table S1 of the Supporting Information. A sample rate law derivation for model FT3RDS10 is shown below: K1
CO + ∗ ⇔ CO ∗ B
DOI: 10.1021/acs.iecr.6b04517 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 1. Elementary Reactions for Model Development model
no.
elementary reactions
FT-I CO dissociates unassisted H2 reacts as adsorbed H (carbide mechanism)
1 2 3 4 5
CO + * = CO* CO* + * = C* + O* H2 + 2* = 2H* C* + H* = CH* + * CH* + H* = CH2* + * CH2* + H* = CH3* + * O* + H* = OH* + * OH* + H* = H2O + 2*
6 7 FT-2 CO dissociates unassisted H2 reacts molecularly (carbide mechanism)
1 2 3 8 6 7
[∗] =
1 3 10 11 4 5
CO + * = CO* H2 + 2* = 2H* CO* + H* = HCO* + * HCO* + H* = C* + H2O* C* + H* = CH* + * CH* + H* = CH2* + * H2O* = H2O + *
FT-4 assisted dissociation of CO H2 reacts as adsorbed H (enol mechanism)
1 3 10 12 13 5 7
CO + * = CO* H2 + 2* = 2H* CO* + H* = HCO* + * HCO* + H* = HCOH* + * HCOH* + * = CH* + OH* CH* + H* = CH2* + * OH* + H* = H2O + 2*
1 3 14 15
CO + * = CO* H2 + 2* = 2H* CO* + H2 = HCOH* + * HCOH* + H2 = CH2* + H2O CH2* + H* = CH3* + *
[H ∗] =
rFT =
rFT =
[H ∗]2 PH2[∗]2
K3PH2 [∗] =
K1PCO 1 + K1PCO + K3PH2 K3PH2 1 + K1PCO +
K3PH2
k1K1PCO K3PH2 (1 + K1PCO +
K3PH2 )2
kPCOPH21/2 (1 + aPCO + bPH21/2)2
⎛ −Ea, i ⎞ k = k 0 exp⎜ ⎟ ⎝ RT ⎠
(1)
⎛ −ΔHi ⎞ ⎟ a = a0 exp⎜ ⎝ RT ⎠
(2)
⎛ −ΔHi ⎞ ⎟ b = b0 exp⎜ ⎝ RT ⎠
(3)
3. EXPERIMENTAL SECTION 3.1. Catalyst Characterization. The catalyst was prepared using the coprecipitation method. Catalysts were characterized using XRD and transmission electron microscopy (TEM). Powder X-ray diffraction (XRD) analyses of all the catalysts were performed using a Rigaku MiniFlex600. Scans were collected with a 2θ step size of 0.02 using a Cu Kα radiation source generated at 40 kV and 15 mA. Data were collected over a 2θ range from 10 to 80 °C, and phases were identified by matching experimental patterns to the pattern provided by ICDD library. Transmission electron microscopy (TEM) images and selected electron diffraction (SAED) analyses were performed on a FEI Tecnai TF20 instrument operated at an accelerating voltage of 200 kV. Samples were prepared by putting a drop of solution (catalyst dispersed in ethanol) of the sample onto the carbon-coated Cu grid. 3.2. Catalyst Performance Evaluation. A statistical design procedure was employed to suggest an experimental condition for the kinetic study. A total 37 numbers of runs were taken in a fixed bed reactor within a reaction condition proposed by a central composite design for four factors and two levels for each factor. Details of the experimental setup are shown elsewhere.60 Runs for the kinetics were carried out in a continuous flow
where K1 and K3 are the adsorption coefficients of CO and H2, respectively, and ∗ represents a vacant site. Assuming the second step to be the RDS and all others at equilibrium, we have the following:
K3 =
K3PH2
where k is kinetic parameter group and can be considered as kinetic rate constant, and a and b are adsorption parameter groups (Table S2, Supporting Information). These parameters are temperature dependent and can be presented in the form of Arrhenius equations and expressions for equilibrium constant as shown below:
K3
[CO ∗] PCO[∗]
1 1 + K1PCO +
[CO ∗] = K1PCO[∗] =
H 2 + 2 ∗⇔ 2H ∗
K1 =
K3PH2 [∗] = 1
Hence
CO + * = CO* CO* + * = C* + O* H2 + * = 2H* C* + H2 = CH2* + * CH2* + H* = CH3* + * O* + H* = OH* + * OH* + H* = H2O + 2*
FT-3 assisted dissociation of CO with formyl(HCO−) formation H2 reacts as adsorbed H (enol+carbide mechanism)
FT-5 assisted dissociation of CO H2 reacts molecularly (enol mechanism)
[∗] + K1PCO[∗] +
−rCO = k1[CO ∗][H ∗]
While applying site balance, it is assumed that the CO and dissociated H2 occupy most of the sites, i.e., CO ∗ and H ∗ are the most abundant species on the surface. Other species were assumed to be negligible in the site balance [∗] + [CO ∗] + [H ∗] = 1 C
DOI: 10.1021/acs.iecr.6b04517 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research fixed-bed reactor made of stainless steel with an ID of 12.6 mm. Three mass flow controllers (Brooks, Model, 5850S) for CO, H2, and N2 were connected to adjust the flow rate. Two thermocouples were inserted, one into the catalyst bed to read the bed temperature and the other into the furnace to control the temperature of a reaction, and it was visually monitored by a computer. The entire unit was controlled using supervisory control and data acquisition system (SCADA) software provided by General Electric. Prior to the catalytic run, the catalyst was crushed, sieved (mesh size: 0.1−0.75 mm), and then mixed with SiC in order to avoid hot spots and to maintain uniform heat distributions within the catalyst bed. In each test, 3.0 g of catalyst was loaded, and the reactor operated for about 12 h to ensure steady-state conditions. Flow of the gas at the exit was measured with the help of a wet gas totalizer connected at the exit of the whole reactor system. Uncondensed gases at the exit of the reactor were analyzed in an online gas chromatograph (Nucon 5700) equipped with FID and TCD. The gases H2, CO, CO2, and CH4 were analyzed in GC-TCD after separation in a Carbosieve column, whereas the lower hydrocarbon C1−C5 were analyzed using FID after separation in a Porapak-Q column. The liquid products were analyzed in GC-MS equipped with a DB-5 column. Prior to the reaction, the catalyst was reduced in situ at atmospheric pressure under flow of H2 at 773 K for 5 h. Experiments were conducted with mixtures of H2, CO, and N2 in a temperature range of 473−553 K, H2/CO feed ratio of 0.5−2.5 (mol/mol), and pressure range of 1.0−3.0 MPa. The detailed experimental design and the related output are shown in Table S3 of the Supporting Information. In all of the experiments, the space velocities were between 1800 and 6600 mL/h-gcat h. The experimental reaction rate was determined as follows:
Table 2. Rate Expressions for FT Reaction (Present Study) model
%Selectivity =
FT2 RDS-8
rFT =
FT3RDS-10
rFT
FT3RDS-11
rFT =
FT3RDS-4
rFT =
FT3 RDS-5
rFT =
FT4 RDS-5
rFT =
FT4 RDS-13
rFT =
FT5 RDS-14
rFT =
FT5RDS-15
rFT = kPCOPH22
PH2O(1 + aPCO + bPH21/2)3 kPCOPH2 PH2O(1 + aPCO + bPH21/2)3 kPCOPH5/2 2 PH2O(1 + aPCO + bPH21/2)3
kPCOPH1/2 2 (1 + aPCO + bPH21/2)2
kPCOPH2 (1 + aPCO + bPH21/2)2 kPCOPH2 K H2O(1 + aPCO + bPH21/2)3 kPCOPH3/2 2 PH2O(1 + aPCO + bPH21/2)3 kPCOPH3/2 2 PH2O(1 + aPCO + bPH21/2)
kPCOPH1/2 2 (1 + aPCO + bPH21/2)
kPCOPH2 1 + aPCO + bPH21/2
R2
%MARR
0.01
35.63
0.01
29.5
0.02
37.4
0.98
2.9
0.87
9.6
0.33
10.5
0.012
29.1
0.02
33.6
0.91
7.7
0.017
30.8
0.15
45.8
Nexp
1 Nexp
∑ riexp ,j
(5)
i
∑i
cal 2 (riexp , j − ri , j )
Nexp
∑i
2 (riexp ,j − σ)
(6)
⎞1/2
Nexp
∑
1 %MARR = Nexp
(r jexp ,i
i Nexp
∑ i=1
−
2⎟ r jcal ,i ) ⎟
⎠
riexp − rical × 100 riexp
(7)
(8)
where rexp and rcal express the experimental and calculated i i conversion rate for ith data point, respectively. Here, Nexp represents the number of experimental data points with pure error variance σ. In addition to the statistical significance, the parameters should also satisfy the physicochemical law, i.e., the physical meaning of the parameters confirmed by testing the following law.36,61−63 I. Kinetic rate constant k should follow the Arrhenius equation with activation energy Ea,i > 0. II. Being an exothermic process, the enthalpy of adsorption should follow −ΔHad, i° > 0
Nexp i
rFT =
⎛ 1 RMSE = ⎜⎜ N ⎝ exp
Total moles of CO converted
cal 2 ∑ [riexp , j − ri , j ] j = FTS or WGS
FT1 RDS-5
Nexp
4. MODEL EVALUATION The initial parameters of the models (Table 2) were estimated by minimizing the objective function (eq 4) using a genetic algorithm tool. These parameters were used as an initial guess for the fminicon function in MATLAB to determine the global minimum. Discrimination among the models was achieved on the basis of statistically significant values measured by F values, R squared values (R2), root-mean-square error (RMSE), and mean absolute relative residual (% MARR). Statistical significance of the models indicated that model F values should be greater than a critical F value at a certain probability level for the rejection of null hypothesis. The value of R2 and RMSE are the measures of goodness of fit. Smaller RMSE and higher R2 values indicate how accurately models can predict kinetic behavior. FObj =
rFT =
R2 =
Moles of carbon in product Ci × 100
kPCOPH3/2 2
FT1RDS-4
σ=
Rate of conversion of CO Fractional conversion of CO × Input molar flow rate of CO = Weight of catalyst
rate expression
III. Adsorption entropy should follow36,62
(4) D
DOI: 10.1021/acs.iecr.6b04517 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research 0 < − Sad, i° < Sg, i°
observed in the XRD pattern. The presence of the Fe−Co phase in the same XRD pattern showed the nonreducibility of the Fe−Co phase. The XRD analysis of used catalyst introduced a new iron carbide phase (2θ = 22.21°, 42.36°, 63.22°, and 79.39°) which forms upon exposer of CO to the catalyst. The presence of cobalt oxide confirmed the reoxidation of cobalt metal due to water formation. All the three catalysts were subjected to HRTEM analysis. The TEM image (Figure 2) of the fresh calcined catalyst shows that the size range of the crystal size lies within the range of 5− 25 nm. From the TEM image it was observed that the lighter portion is silica, and the dark spots are metal particles. Separations in the dark spots are observed in Figure 2b, which are due to changes in the phases of the metals. In the used catalyst (Figure 2c), an agglomeration of metal crystal can be seen in the form of a bigger dark spot. The diffraction patterns of different metal phases are shown in the selected area electron diffraction (SAED) images. The images are of different metal-rich selected areas of freshly calcined catalyst, which confirm the different phases present in the catalyst. The planes shown in the SAED images match with the planes shown in the XRD pattern of the same catalyst. 5.2. FT Reaction and CO Consumption. In the FT process, there are complex network of the reactions which require a kinetic rate model that can explain the experimental results based on mechanism. Eleven different CO consumption models were derived (Table 2) and validated. The models containing H2O in their denominator did not fit the data, which is evident by lower values of R2. This also means that the water inhibition did not significantly affect the FT rate. The result corroborates the Mirzaei and co-workers55−58 results, where the H2O inhibition term was not present in their models for FT kinetics over Fe−Co−Mn, Fe−Co−Ni, and Co−Ni bimetallic catalysts. Water inhibition was not evident in most of the models derived for the cobalt-based catalysts.35,39,40,44,45 Krishnamurthy et al.64 and Botes65 studied the effect of water on the kinetics of the FTS reaction over a cobalt-based catalyst and found that the water does not inhibit the reaction. In the case of iron-based catalysts, the inhibition of water was not significantly at a low CO conversion level.7,24,47 Very recently, Ma et al.66 reviewed the previous kinetic studies done in the last three decades over iron, and the water inhibition on the FTS rate was insignificant. The three models FT3RDS10, FT3RDS11, and FT4RDS13 adequately fit the kinetic data and were able to predict the rate of FT synthesis. However, among the three models, FT3RDS10 was the most suitable for the parameter estimation with a high correlation coefficient (R2) of 0.98 and the lowest %MARR value. Also, the highest F value of the same shows the statistical significance of the model. The parity plot (Figure 3) compared the rate of CO consumption calculated experimentally (RFT) with the model predicted rate at all four temperatures. Their data fit the model prediction within ±10% variation. It is important to note that the model predicts the formyl formation step (CO* + H* → HCO* + *) as RDS in the case of an Fe-CO bimetallic catalyst. Also, the dominant mechanism over the catalyst includes CO adsorption with the assistance of H and HCO formation, which ends up forming C and H2O and further interaction of adsorbed C and H results in −CH2− formation. Several kinetic studies over iron- and cobalt-based catalysts reveal different contradictory results about the CO activation path. Zhou et al.24 assumed that the hydrogen-assisted CO hydrogenation
41.8 < − Sad, i° < 51.4 + 0.0014 × ΔHad, i °′
where Sad,i° is entropy of adsorption of ith species at standard state. Here, Sg,i° is the standard entropy in the gaseous phase, and ΔHad,i° is enthalpy of adsorption at the standard state.
5. RESULTS AND DISCUSSION 5.1. Physicochemical Properties of Catalyst. While investigating the kinetics of the FT reaction, it is important to relate the CO consumption with the CO adsorption and −CH2− formation mechanism. These are directly related to the presence of different active sites of the catalyst. In the case of Fe−Co bimetallic catalysts, phases of iron and cobalt metals (Co3O4, CoO, Co°, Fe3O4, and Fe2O3) were present. In addition, formation of an Fe−Co bimetallic phase was also observed by some researchers. Upon exposer of CO, the carbide phase of iron also forms and was considered to be an active FT phase. Detailed characterization of fresh and spent catalysts in the present study confirmed the formation of these phases. XRD analysis of freshly calcined, reduced, and spent catalyst (Figure 1) showed the formation and disappearance of
Figure 1. XRD analysis of fresh, reduced, and spent catalyst.
various phases during the whole process. This interpretation of XRD results were based on the standard XRD pattern of pure Co3O4 (JCPDS 74-1657), Fe2O3 (JCPDS 84-0307), FeCo (JCPDS 49-1567), CoO (89-2803), Co (JCPDS 89-7373), and Fe3C (JCPDS 89-2005). The XRD pattern of the calcined catalyst showed distinct peaks of Co3O4, Fe2O3, and Fe−Co phases. The reflections at 2θ = 19.7°, 31.6°, 37.3°, 39°, 45.2°, 59.8°, and 65.7° can be assigned to different diffraction planes of cubic cobalt oxide. Likewise, reflections at 2θ = 33°, 39°, and 56.14° can be assigned to the hematite phase of iron oxide (Fe2O3). However, the lower intensity of these peaks suggests a small crystalline size which indicates a well-dispersed iron phase. A new phase was identified as an FeCo bimetallic phase at 2θ = 45° and 65°. These peaks overlapped with cobalt oxide peaks in the XRD pattern indicating a good distribution of sites. The XRD pattern of the reduced catalyst showed some peaks of metal Co at 2θ = 42.12° and 45.24°. Some partly reduced CoO phase were also observed in the pattern at 2θ = 65.5° and 75.5°. Due to the very less stable metallic iron phase, it was not E
DOI: 10.1021/acs.iecr.6b04517 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. TEM image of (a) fresh, (b) reduced, and (c) spent catalyst. SAED pattern of fresh catalyst.
alkali Mn promotion the dominant mechanism over an Fe/ Mn/K/Al2O3 catalyst was unassisted dissociation of CO, where the reaction of adsorbed C and adsorbed H was considered as the rate-determining steps. Atashi et al.69 derived the rate equation for a Co−Mn/TiO2 catalyst using the carbide mechanism and observed that the reaction of adsorbed CO with gaseous H2 (Eley−Rideal) was RDS. Fazlollahi et al.27 observed that over a Co/Ni/Al2O3 catalyst CO dissociated with the help of adsorbed H, and Keyser et al.29 observed that over a Co/Mn bimetallic catalyst the reaction rate equation for the FT reaction based on the enolic mechanism gave results which were marginally better than results based on the carbide mechanism. Although, the classical study33,39,45,51 of kinetics for a cobalt catalyst emphasizes that the CO unassistedly dissociates, recent studies over a bimetallic catalyst supports the fact that the CO cannot dissociate alone over a bimetallic
occurs over an iron catalyst. The form of reacting hydrogen, i.e., dissociative hydrogen or molecular hydrogen, was the matter of investigation. They concluded that it is difficult to discriminate between the model describing the dissociative and associative hydrogen models; however, in a wide range of experimental conditions, the model incorporating dissociative hydrogen as a reactant was well fitted. VanderLaan and Beenacker,19 in their kinetic study over an Fe/Cu/K catalyst, observed that models based on the enol+carbide mechanism showed good results for their data, where RDS steps were surface reactions between undissociated H2 and undissociated adsorbed CO and between adsorbed formyl and dissociated H2. Davis et al.67 and Botes et al.7 also concluded that over an iron catalyst the reaction proceeds through an oxygen-containing intermediate, where adsorbed hydrogen reacts with undissociated CO and forms adsorbed HCO. However, Fazlollahi et al.68 argued that due to F
DOI: 10.1021/acs.iecr.6b04517 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 5. Effect of partial pressure of H2 on rate.
Figure 3. Calculated rate vs experimental rate for FT3RDS10.
trend of the CO partial pressure effect is well in agreement with the literature data.20,24,70 Zhou et al.24 observed that, at low concentration of CO, (PCO < 0.8 MPa) order of reaction is positive with respect to CO concentration over an Fe-based catalyst, suggesting that at this point the surface was not fully covered. Dry et al.9 also observed that at high partial pressure of CO the rate of FT synthesis is zero with respect to PCO over a K-promoted iron catalyst. Fazlollahi et al.27 observed the same trend for a cobalt catalyst where the FT rate became almost constant when PCO increased above 0.6 MPa. While reviewing the comparative study for iron and cobalt catalysts, Botes et al.71 used kinetic models for iron24 and cobalt44 to compare the effect of increasing partial pressure of CO on the FT rate over both the catalysts. The observation revealed the negative order dependence of the FT rate with respect to CO partial pressure over a Co catalyst. However, over an iron catalyst, the model predicted a significantly positive order of reaction. Recently, Mousavi et al.72 also used their generalized model for both iron and cobalt catalysts and reported similar conclusions to Botes and co-workers. It can be observed that the increasing trend of the FT rate with PCO in this study (Fe−Co bimetallic catalyst) is a combination of the behaviors of both iron- and cobalt-based catalysts. The kinetic rate equation (FT3RDS10) predicted the influence of hydrogen partial pressure on the FT rate. At very low H2 partial pressure (
