Influence of Fabrication Temperature and Time on Light Olefin

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Influence of Fabrication Temperature and Time on light olefins selectivity of Iron-cobalt-cerium Mixed Oxide Nanocatalyst for CO hydrogenation Tahereh Taherzadeh Lari, Ali Akbar Mirzaei, and Hossein Atashi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03171 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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Industrial & Engineering Chemistry Research

Influence of Fabrication Temperature and Time on light olefins selectivity of Iron-cobalt-cerium Mixed Oxide Nanocatalyst for CO hydrogenation Tahereh Taherzadeh Lari a1, Ali Akbar Mirzaei a, Hossein Atashi b a

Department of Chemistry, Faculty of Science, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran b Department of Chemical Engineering, Faculty of Engineering, University of Sistan and Baluchestan, P.O. Box 98164-161, Zahedan, Iran

1

Corresponding author: [email protected], [email protected] 1 ACS Paragon Plus Environment


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Abstract Fischer-Tropsch synthesis process is a polymerization reaction which various chain length of product can be produce from mixture of CO and H2 (synthesis gas). A solvothermal technique was used to synthesize magnetic iron-cobalt-cerium mixed oxides nanoparticle. Process variables such as reaction temperature and time can play an important role in a chemical process. The results showed that the optimal preparation time and temperature in order to light olefin (C2-C4) selectivity is 18 h and 120 0C. The effects of experimental variables including temperature (120, 150, 1800C) and reaction time (10, 14, 18 h) were investigated. The nanoparticles were characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), temperature programedreduction (TPR), Fourier infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray (EDS) and Xray photoelectron spectroscopy (XPS). Average crystalline size determined from XRD patterns ranged over 4.5‒8.3 nm. The results indicate that reaction temperature and time have marked effects on the magnetic properties of nanoparticles changing from a ferromagnetic behavior to a superparamagnetic one at a reaction temperature of 1200C. TPR profiles of the nanoparticles synthesized at different temperatures revealed that reducibility shifted to higher temperatures. XPS results verified Fe+3, Co+2 and Ce+3 of oxidation states.

Keywords: Fischer-Tropsch Synthesis, Iron-cobalt-cerium mixed oxide, Reaction Temperature and time, Light Olefin selectivity, Solvothermal method.

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1.1. Introduction The universe resurgence of interest in Fischer-Tropsch synthesis has been primarily driven by the problems of utilization of abandoned gas, diversification of sources of fossil fuels, and environmental concerns [1]. In 1922, Hans Fischer and Franz Tropsch proposed the process, which gave a mixture of hydrocarbons and aliphatic oxygenated compounds via reaction between carbon monoxide with hydrogen over heterogeneous catalysts [2]. Among VIII group transition metals, only both iron and cobalt based catalysts appear in the industrial scale for their economically feasible and highly activity. The iron-based catalyst attracted immensely attention not only due to its comparably cheap price but also controlled the light olefins or oxygenated hydrocarbons with high yield by tuning the reaction parameters. Compared to iron-based catalyst, the cobalt catalyst due to high activity, high selectivity into linear paraffins, high deactivation tolerance and also low water-gas-shift activity is attended as a right alternative in the lowtemperature FT process [3,4]. As the process parameters can play an important role on catalytic activity, average molecular weight of the hydrocarbon products and selectivity of FTS, study of the effect of reaction temperature, time and other parameters is necessary for process development, design and optimization [5]. As well as the reaction conditions of hydrocarbon products depends on type of catalyst, hence bimetallic catalysts, a mixture of two active catalysts, has higher selectivity into light olefins than either iron or cobalt [6]. A range of studies has been published on activity and selectivity of Fe-Co [7,8], which indicated the catalyst with 40%Fe/60%Co and 40%Fe/60%Co/15 wt.%SiO2/1.5 wt.% K as optimum molar ratio for conversion of syngas to ethylene and propylene, Fe-Mn [9], that illustrated the 50%Fe/50%Mn/5 wt.% Al2O3 as optimum catalytic performance for conversion of synthesis gas to light olefins, also it was found that the Fe-Co-Mn catalyst prepared by co-precipitation method has the better catalytic performance for CO hydrogenation [10], for Fe-Co-Ni catalyst supported on MgO calcined at 5000C for 6 h has shown the best catalytic performance [11]. It has been investigated that the addition of a metal rare-earth oxides as a promoter has beneficial effects on improving the initial activity, selectivity and thermal resistance of the catalyst. Ceria is the most significant features elements in industrial catalysis and also has a key component in catalyst formulation. It was reported that CeO2 was partially reduced to CeO2-x and formed new bi-nuclear active sites together with Co0, which decreased the adsorption ratio of H2 and CO, increased the catalyst activity and the selectivity for olefins and higher hydrocarbons. Actually addition of CeO2 as a promoter weakened the C-O bond and increased the dissociation and hydrogenation of CO [12,13]. As generally known, the catalyst at nano diameter increases surface into volume ratio thus increasing the number of surface metal atoms available for catalysis, which is directly causes catalytic activity. This material system is found as previously reported in the literature by Eshraghi et al [14]. The main objective of the current work is to investigate the influence of various preparation time and temperature on activity and selectivity of iron-cobaltcerium mixed oxide nanocatalyst prepared using solvothermal method at (270-3800C) range of operation temperature in a fixed-bed reactor in order to synthesis gas conversion to light olefins for the first time. Also the optimized reaction time and temperature due to light olefins selectivity is presented. More specifically, the effect of reaction temperature and time on particle size, crystal structure, magnetic properties, reducibility, and morphology will be examined by XRD, VSM, TPR, FTIR, SEM, EDS and XPS 3 ACS Paragon Plus Environment


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techniques. Markedly with changing reaction temperature and time the magnetic properties alter from ferromagnetic into superparamagnetic behavior.

2.1. Experimental Section 2.1.1. Chemicals and materials All the chemicals were of analytical grade used without further purification. Iron nitrate (II) Nona hydrate (Fe(NO3)3.9H2O, 99%), Cobalt nitrate (II) Hegza hydrate (Co(NO3)2.6H2O, 99%), Cerium nitrate (III) Hegza hydrate (Ce(NO3)3.6H2O, 99%), Toluene (C7H8, 99%), Ethanol (C2H5OH, 99%) were purchased from Merck. Oleylamine (C18H37N, 70%) was purchased from Aldrich. 2.1.2. Sample Preparation and Description Iron-cobalt-cerium mixed oxide was prepared using the rapid solvothermal synthesis [15]. The preparation procedure can be briefly described as follows: 0.38 g (0.94 mmol) of iron nitrate, 0.32 g (0.91 mmol) of cobalt nitrate, and 0.5 g (1.15 mmol) of cerium nitrate were added into 50 ml of toluene containing 5.4 g (20.2 mmol) of oleylamine. The mixture was magnetically stirred vigorously for one hour at room temperature. The resulting mixture solution was subsequently transferred into an 80ml Teflon-lined autoclave and heated to 1800C. The autoclave was sealed and maintained at the given temperature for 18h before it was allowed to cool down to room temperature. The nanoparticles formed were precipitated in the excess ethanol and further isolated from each other by centrifugation. The resulting nanoparticles were finally transferred to an oven to be dried before calcination at 1000C and 5000C in air for 4h. 2.2. Sample Characterization X-ray diffraction (XRD) patterns of iron-cobalt-cerium mixed oxide were recorded in the range of 10-1100(2θ) at room temperature (2θ step =0.020 with a counting time of 0.35 sec per step) using an EXPERT PHILIPS X-ray diffraction system equipped with a CuK∝ radiation source (λ=1.54046 A0) operated at 40 kV and 30 mA. The data were refined using the Xpert software [15]. The XRD measurement error coefficient was 10%. The average diameter of the nanocrystalline domain was determined from the full width at half-maximum (FWHM) of the strongest reflection peak (111 reflection) using the Scherrer’s equation: D = (1) Where D denotes mean particle size; λ, X-ray wavelength; β, total width at half maximum; and θ, the scattering angle. The magnetic properties of iron-cobalt-cerium mixed oxide were studied in a vibrating sample magnetometer (magnetic analysis was taken from VSM Lab of Sistan & Baluchestan University). VSM measurements were accomplished by taking 0.02 g of the solid sample on the tips of the vibrating rod and analyzed at room temperature with a maximum magnetic field of 10000Oe. Various mixed oxide samples were subjected to temperature-programmed reduction (TPR) with hydrogen using an automated nanoparticle characterization system, which incorporated a thermal conductivity detector (TCD). The precursor (0.05 g) was loaded 4 ACS Paragon Plus Environment

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into the quartz TPR cell and the experiments were carried out at a heating rate of 5 °C /min. The reactive gas composition was H2 (5 vol.%) in Argon. The flow rate was fixed at 20 ml/min (STP). The total reactive gas consumption was measured during the TPR analysis. TPR measurements were carried out following activation after cooling the sample to 40 °C in an argon flow. The sample was then maintained at 50 0C under the argon flow in order to remove the remaining adsorbed oxygen until the TCD signal returned to the baseline. Subsequently, the TPR experiments were performed up to a temperature of 850 0C. FTIR spectra were collected in the region from 400 to 4000 cm-1 using a Perkin Elmer Spectrum TWO spectrophotometer. The nanoparticles were analyzed by dispersing powders in KBr pellets. The morphology and homogeneity of the samples were determined using a Scanning Electron Microscope (SEM MIRA II LMU/TESCAN EDS) with an accelerating voltage of 1500 kV. The amounts and types of iron, cobalt, and cerium elements were determined by Energy Dispersive X-ray (EDS) attached to the scanning electron microscope. For SEM analysis, the sample in ethanol was dispersed on an aluminum foil wrapped on the aluminum stub used for sample mounting. The sample was dried in air and the stub was mounted in the SEM chamber. The particle size was measured at a resolution of 200 nm with a magnification of 15 kx. The X-ray photoelectron spectrum (XPS) was recorded on a spectrometer (VestechGermany) with the Al Kα line radiation (1486.6eV) at (15kV 10mA) was used as the excitation source. The vacuum level during the experiment was 10-10 mbar. To control charging of the samples, the charge neutralizer filament was used during all experiments. The instrument was run in hybrid mode with pass energy of 97 eV for survey spectra and pass energy of 27eV for high-resolution spectra. Peak energies were calibrated to the adventitious carbon C 1s peak centered at 285eV. The accuracy in binding energy measurements was ±0.1 eV. The peaks were deconvoluted using SDP software (version 4.1) in order to peak fitting. 2.3. Research Catalytic Micro-Reactor Setup Fischer- Tropsch synthesis were performed in a stainless fixed-bed micro-reactor with an iner diameter of 12 mm. The catalyst (1.0 g) was well dispersed with asbestos and loaded in the center of reactor with thermocouple inside. Three mass flow controllers (Model 5850E, Brooks Instrument, Hatfield, PA, USA) were used to automatically adjust the flow rate of the inlet gases containing CO, H2 and N2 (with 99.99% purity). A mixture of CO and H2 (H2/CO =1, flow rate of each gas = 30 mlmin-1) was subsequently introduced into the reactor, which was placed inside a tubular furnace (Fig 1, Model ATU 150-15, Atbin). The reaction temperature was controlled by a digital program controller (DPC) and visually monitored by a computer through a thermocouple inserted into the catalytic bed. The catalyst is situ was pre-reduced under 2-bar pressure and H2 flow (with flow rate of 30mlmin-1) at 4000C for 48 h before the reaction started. In each test, 1.0 g of catalyst was loaded and all data was collected after the time of 24h to ensure steady state operation was attained.



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Fig 1. The experimental setup of fixed bed reactor (FBR) for Fischer-Tropsch synthesis over iron-cobalt-cerium mixed oxide nanocatalyst 1- Gas Cylinders, 2- Valve, 3- Pressure Gauge, 4- Mass Flow Controller (MFC), 5Mixing Chamber, 6- Thermocouple, 7- Tubular Furnace, 8- Fix Bed Reactor and Catalyst Bed (Reaction Zone), 9- Temperature Digital Program Controller (DPC), 10- Resistance Temperature Detector, 11- Condenser, 12- Trap, 13- Back Pressure Regulator (BPR), 14Flow Meter, 15- Control Panel, 16- Electrical Motor, 17- Air Pump, 18- Hydrogen Generator, 19- Gas Chromatograph, 20- Silica-Gel Column

2.4. Catalytic Selectivity Measurement The Fischer-Tropsch synthesis was performed with mixture of CO and H2, in the temperature range of 270-3800C, with H2/CO molar ratio of 1:1, the space velocity of 3600h-1 and at 2 bar of pressure. In each experiment, for reactor catalyst testing at each aging time to avoid of deactivation effect, fresh catalyst was loaded. An automatic backpressure regulator in order to adjust and modify the pressure range via the TESCOM software was used. Reactant and product streams were analyzed by on-line gas chromatography (Thermo ONIX UNICAM PROGC+) equipped with two thermal conductivity detectors (TCD) and one flame ionization detector (FID) with ability to analysis of a broad variety of gaseous hydrocarbon mixtures. One TCD used for the 6 ACS Paragon Plus Environment

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analysis of hydrogen (H2) and the other one used for all the permanent gases like N2, O2 and CO. The analysis of hydrocarbons was done by FID. The analysis of noncondensable gases, methane through C8 hydrocarbons is applied. The contents of the sample loop were injected automatically into an alumina capillary column. As well as helium (He) was employed as a carrier gas for optimum sensitivity. The calibration was performed by various calibration mixtures (CH4, C2H4, C2H6, C3H6, C3H8, n-C4H10, iC4H10, n-C5H12) and pure compounds obtained from Tarkib Gas Alvand Company of Iran. The operation condition and obtained data of each experiment are presented in below Tables. The CO conversion % is calculated according to the normalization method: CO conversion (%) =

) – ) )

× 100

(2)

3. Results and Discussion 3.1. Fischer-Tropsch synthesis Iron-cobalt-cerium mixed oxide nanocatalyst with 1:1:1 molar ratio was synthesized using sollvothermal method and the catalytic performance on Fischer-Tropsch synthesis under operation condition of (GHSV=3600cm-1, H2/CO=1, P=2 bar) was investigated. Effect of preparation time and temperature at various range of operation temperature (270-3800C) on light olefin selectivity was discussed (Table S1-S5). Furthermore the product yields of olefins presents in Table S6.

3.1.1. Effect of Time & Temperature preparation condition on selectivity The activity and selectivity of iron-cobalt-cerium mixed oxide nanocatalyst into light olefins as well as CO conversion are affected by preparation time and temperature. Selectivity data of iron-cobalt-cerium nanocatalyst under operation condition of (GHSV=3600 cm-1, H2/CO=1, P=2 bar) are reported in Table S1-S5. For sample prepared at 1800C and 18h (Fig 2.) showed that the highest CO conversion and the maximum value of propylene achieved at 2700C of operation temperature, while the ethylene and butylene value at 3800C and 3000C are the highest respectively. Generally the total selectivity into C2-C4 obtained at 3800C of operation temperature.



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Fig 2. Selectivity into light olefins on iron-cobalt-cerium mixed oxide nanocatalyst under 1800C and 18 h of preparation condition.

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For sample prepared at 1500C and 18h (Fig 3.) the highest CO conversion is at the lowest operation temperature of 2700C, despite at the highest operation temperature of 3800C, the ethylene value is maximum. The highest value for propylene and butylene was achieved at operation temperature of 350 and 3300C respectively. At 3300C of operation temperature the highest selectivity into C2-C4 was obtained.


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For sample prepared at 1200C and 18h (Fig 4.) the highest CO conversion is at lower operation temperature of 2700C, despite the maximum selectivity into ethylene is at the highest operation temperature of 3800C. Selectivity into propylene and butylene was obtained at 330 and 3500C of operation temperature. The total selectivity into C2-C4 is related to operation temperature of 3800C.

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For sample prepared at 14h and 1800C (Fig 5.) the highest CO conversion and ethylene value are at lowest and highest operation temperatures of 270 and 3800C respectively. The maximum selectivity into propylene and butylene are at 3500C and 3000C of operation temperature. Total selectivity into C2-C4 was obtained at 3300C of operation temperature.



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For sample prepared at 10h and 1800C (Fig 6.) the maximum CO conversion is at operation temperature of 2700C. The highest selectivity into both ethylene and propylene were achieved at operation temperature of 3300C. The maximum selectivity into butylene was obtained the same value at 300 and 3300C. The total selectivity into C2-C4 indicated at 3300C of operation temperature. As a result, the maximum selectivity into light olefins (C2-C4) was achieved at operation temperature of 380 for nanocatalyst prepared at 18h and 1200C. Therefore the optimal preparation time and temperature due to light olefin selectivity was obtained at 18h and 1200C.


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CO Conversion (%)


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Generally CO conversion increases with increasing temperature. In this study the CO conversion changes a little different. Since the being used catalyst doesn’t have base and the catalyst activity is high however the deactivation time is short, therefore after two days catalyst losing almost about 60% of its activity. Also at this time after about one or two reactor tests, catalyst is reaching to steady state (almost after 48 hours from time on stream). According to explained the catalyst feature, now as the data shows with increasing temperature the CO conversion increases. In fact, in the first test the CO conversion is high. After the first test at the beginning of the reaction given the passage of 48 hours time (after reduction) the catalyst loses significantly its activity. So that, despite the increase in temperature the CO conversion had decreased. In general it can be said that the CO conversion in reactor is a function of catalyst activity $), temperature (T) and etc. as follows; X = Ϝ $, Τ, … ) We can conclude that increasing temperature doesn’t effect on CO conversion, unless after the catalyst has reached a steady state, now the temperature increase showing its effectiveness. As a result, in initial tests reduction in catalyst activity has more dominant role than effects of temperature on CO conversion. E.g. for sample prepared at 1800C and 18 h, expecting that with increasing in temperature the CO conversion increases, while CO conversion decreases from a high at 2700C to 300-3300C and then at 350-3800C increases. In first measure in 270 degree of temperature as it shows from data, the CO conversion has the most amount of itself. After next 24 hours, the catalyst activity significantly decreased and reaches in steady state. After catalyst became stable, at this time with increasing temperature CO conversion is 11 ACS Paragon Plus Environment


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going to be increases. As a result, everywhere catalyst doesn’t have this behavior it means the catalyst doesn’t reach yet to steady state condition of its activity. Some researcher reported effect of operating temperature on activity and selectivity of catalysts e.g. Arsalanfar et al [16] studied effect of operating conditions on Fe-Co catalyst promoted by Mn prepared by co-precipitation method. The finding showed at 573 K the highest selectivity towards light olefins achieves. Also Feyzi et al [17] demonstrated preparation conditions of cobalt-iron nanocatalyst. The results indicated the best operating conditions is at 2500C due to light olefin selectivity. Comparatively some literature reported on Fe-Ce and Co-Ce bimetallic catalysts using Fischer-Tropsch synthesis e.g. Perez-Alonso et al [18] demonstrated Iron-Cerium mixed oxide systems prepared by different methods. The result showed that Fe-Ce bimetallic catalysts had higher selectivity to olefins than un-promoted Fe catalyst and catalyst prepared by physical mixture of Fe and Ce oxides. Also Dorner et al [19] studied C2-C5+ olefin production using ceria modified Fe/Mn/K catalysts. The finding indicates addition of ceria approximately 5% increase in olefin formation and also size of ceria particles is pivotal in tailoring the catalyst’s activity and selectivity. Arsalanfar et al [20] investigated structural characteristics of cobalt-cerium oxide catalysts used in Fischer-Tropsch synthesis. The finding shows preparation conditions are crucial importance and influencing the structure and morphology of catalysts. The selectivity of Fischer-Tropsch synthesis depends primarily on which transition metal (Fe or Co) is used. Iron based FT catalysts are active for water-gas-shift (WGS) reaction and therefore are suitable for low H2/CO feed. While the WGS activities on Co catalyst is negligible under normal FTS conditions. Thus the product selectivity of Fe catalyst depends both on FT activity and WGS activity, while Co depend mainly on FT activity. Therefore the product distribution changes differently with CO conversion for Co and Fe. For iron catalyst, the productivity depends on the CO conversion level. The reason being that the water gas shift (WGS) reaction, which competes with FTS becomes more significant at a higher CO conversion level. One comparison between iron and cobalt catalyst reported that the iron catalyst is more active at higher space velocity (lower conversion) and vise versa for cobalt catalyst. Iron catalyst is more productive at more sever conditions i.e., at higher space velocity and reactor pressure [3]. A general perception is that a cobalt catalyst produces heavier products than an iron catalyst and for the iron-based catalyst the pressure can be varied over a wide range without having a significant impact on product distribution. Ceria improves the catalyst dispersion, decreases the electron density of catalyst, and enhances the dissociation of CO on the surface of catalyst. It was shown that the addition of ceria increased the concentration of surface-active carbon species (-CH2-), probability of chain growth and long chain hydrocarbons selectivity. Also it is reported that ceria by forming new bi-nuclear active sites with Ce0 cause decreasing the adsorption ratio of H2 and CO results in increasing catalyst activity and selectivity for olefins and significantly decreases the methane amount [13]. On the other hand CeO2 as promoter fractured the CO bond and cause increasing the dissociation and hydrogenation of carbon monoxide. Also CeO2 decreases interaction between cobalt species improves reducibility and dispersion of catalyst then leads to increase active sites of cobalt, which is reported by Zhang et al [21]. 12 ACS Paragon Plus Environment

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3.2. X-ray diffraction The XRD patterns of the synthesized iron-cobalt-cerium mixed oxides nanocatalyst at different temperatures and reaction times are shown in Figs. 7 and 8. In these samples the cubic structures of CeO2 (card no. 34-394) and Co3O4 (card no. 9-418) and rhombohedral structure of Fe3O4 (card no. 33-664) can be found. Here prominent Bragg reflections can be indexed as cubic and rhombohedral type structures of CeO2, Co3O4 and Fe3O4 with the corresponding diffraction peaks of (111), (311) and (104) respectively. Meanwhile, the good crystallization of the samples and this fact that the synthesized samples were grown more orderly in that particular direction can be proved by the sharp and strong XRD reflections [22]. Also, the XRD data is used to estimate the average crystallite size of the samples, which are given in Table S7. As it can be seen from the Figs. 7 and 8 and also from Table S7, the sharpest and the most intense peaks appear at 1500C of reaction temperature, while the peaks at 1200C and 1800C becomes broadening. The widest peak for the sample prepared at 1200C, indicates a small crystal size. These results demonstrated that the crystallization process has appeared at 1200C, which shows the particle size is small due to a decreasing in nucleation time. It has been reported that at 1500C of reaction temperature, the coalescence of smaller grains results in increasing average grain size of the nanoparticles [23]. XRD patterns of samples prepared at different reaction times are shown in Fig. 8. In this figure, we can observe that at shorter reaction time, the intensity of peaks is relatively low, while it would increase to the maximum intensity at 14 hour of reaction time, which is indicative of better crystallinity. It is clear that the relative crystallinity increase as the reaction time increases from 10 to 14 hour. Moreover, the average crystallite size of synthesized samples is estimated to be around 4.5-8.3 nm, and the corresponding values mean that the reaction temperature of 1500C and reaction time of 14 hour are suitable to maximize growth of the iron-cobalt-cerium mixed oxides which is most likely due to this fact, that the temperature and reaction time of the coalescence process enhance and leads to an increase on the grain size and it is in accordance with the experimental results in Ref. [23]. XRD pattern of used catalyst prepared at 1200C and 18h shown in Fig 9. From the XRD pattern cerianite CeO2 (card no. 04-0593), hematite Fe2O3 (card no. 24-0072) and magnetite Fe3O4 (card no. 01-1111) were found.



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Fig 7. X-ray diffraction patterns of iron-cobalt-cerium mixed oxides nanocatalyst prepared at various reaction temperature (180, 150 and 1200C).

Fig 8. X-ray diffraction patterns of iron-cobalt-cerium mixed oxides nanocatalyst prepared at different reaction time (14 and 10h).


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Fig 9. X-ray diffraction patterns of used iron-cobalt-cerium mixed oxides nanocatalyst in reactor. 3.3 Magnetic Measurements The magnetic properties of the synthesized iron-cobalt-cerium mixed oxides were characterized using VSM at room temperature. 3.3.1 Effect of reaction time The effect of reaction time on the magnetic properties of the samples was studied in the ranges from 14 to 18 hour at 1800C. The corresponding hysteresis loops are given in Fig. 10, which show that all the samples exhibit ferromagnetic behavior with different coercivity values. The values of saturation magnetization (Ms), residual magnetization (Mr), coercivity (Hc) and the residual magnetization ratio R (R=Mr/Ms) for various reaction times are listed in Table S8. The results showed that the saturation magnetization (Ms) did not exhibit a clear trend with heating time and took values between 7.768 and 10.077 emu g-1 Fig. 11. As can be seen from Table S8, the saturation magnetization is independent of average crystalline size that was obtained from XRD analysis. The Maximum saturation magnetization was 10.077 and obtained at 10 hour of reaction time as shown in Fig. 11. More saturation magnetization cause more magnetic areas. It causes less magnetic-crystalline anisotropy, which arises from the spin-orbit interaction [24]. The saturation magnetization, Ms and magnetic field, Hc versus reaction time are shown in Fig. 11. The highest value of coercivity was obtained for the largest particle size at 14 hour of reaction time, which was attributed to the nanoparticles agglomeration [24]. Fig. 12 presents the variations of coercivity and residual magnetization ratio with particle size. The Coercivity (Hc) and the residual magnetization (Mr) are between 49.47 until 1963 Oe and 2.449 until 4.052 emu g-1 respectively. As it was reported Hc < 100 Oe is soft ferromagnetic and Hc > 100 Oe is due to hard ferromagnetic [25]. Therefore, the nanoparticles prepared at 14 hour is hard ferromagnetic and at 10 and 18 hours of reaction time have soft ferromagnetic behaviors. It can be seen from Table S8, that the highest value of the residual magnetization (Mr) 15 ACS Paragon Plus Environment


and also the residual magnetization ratio (R=Mr/Ms) were obtained at 14 hour of reaction time, which show the particle size is maximum. In the other words, Mr/Ms ratio and the residual magnetization Mr depends on the particle size as it was reported [26].

Fig 10. Hysteresis loops of iron-cobalt-cerium mixed oxides nanocatalyst synthesized at different reaction time, A)18 h, B)14 h and C)10 h at 1800C.

2000

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Fig 11. Variation of saturation magnetization and coercivity in synthesized ironcobalt-cerium mixed oxides nanocatalyst with different reaction time. 2000 1800

Coercivity (Oe)

1600

0.6 Hc 0.5

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4.5

Fig 12. Variation of the coercivity and residual magnetization ratio of synthesized iron-cobalt-cerium mixed oxides nanocatalyst at different reaction time with particle size.

3.3.2. Effect of temperature The effect of reaction temperature on the magnetization of the produced iron-cobaltcerium mixed oxides nanocatalyst was studied from 120 to 1800C for 18h. The coercivity (Hc), saturation magnetization (Ms), residual magnetization (Mr) and residual magnetization ratio R(R=Mr/Ms) of nanoparticles synthesized at different temperatures are shown and also listed in Fig. 13 and Table S8. The respective hysteresis loops of samples at 180 and 1500C of reaction temperatures, which demonstrate the ferromagnetic behavior, while the sample with 1200C of reaction temperature trend to have superparamagnetic behavior. In this work, as the reaction temperature of iron-cobalt-cerium mixed oxides decreases from 1800C to 1200C, the residual magnetization (Mr) decreases and tends to be zero. As seen in Table S8 and Fig.14, the higher reaction temperature, leads to the greater residual magnetization (Mr). It is well known that when the particle size becomes smaller and results decrease in (Mr), it is easier to be thermally activated to overcome the magnetic anisotropy [27]. This suggests that the samples gradually tend to demonstrate superparamagnetic behavior by the decrease in reaction temperature at 1200C. Therefore, an increase in the reaction temperature leads to monotonic increase in the residual magnetization (Mr). Moreover, at Superparamagnetic sample, the hysteresis behavior vanishes and the magnetization direction of the particles simply follows the direction of



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the applied magnetic field. The magnetization of superparamagnetic sample is weak due to trend to the residual magnetization (Mr) to become zero [28]. From Fig. 15 the Maximum value of saturation magnetization (Ms) was achieved at 1800C of reaction temperature, while the highest value of coercivty (Hc), which behaves as hard ferromagnetic and is attribute to the sample prepared at 1500C. It has shown that the higher saturation magnetization, leads to the lower magnetic-crystalline anisotropy and for the samples prepared at different reaction temperature, Ms is independent in particle size [29]. Also, as it known from Fig. 15, the coercivity and the residual magnetization ratio have direct portion into the particle size. The average crystalline size obtained from XRD analysis was in range of 4.5-8.1 nm and for the sample prepared at 1500C has the maximum size. Fig. 16 and Table S8 present the variations in residual magnetization ratio R (R= Mr/Ms) and coercivity with changes in particle size. Clearly, R and Hc increase with increasing particle size. It is evident that changes in coercivity and residual magnetization ratio are correlated with variations in particle size, which in turn, depends on reaction temperature. The data shows that coercivity increases with temperature rapidly to reach its maximum value of 1318Oe at 150 0C before it declines with reducing reaction temperature (Fig. 15) It is well established that particles with a smaller size have single-domain nanocrystallites. This shows that the particles are mostly smaller than the single domain size. The larger particles precipitated at higher temperatures because of their multidomain nanocrystallites, which are larger than the single domain ones. Therefore, as particle size grows towards the single domain size, coercivity increases and reaches its maximum value at the single domain size because of the coherent rotation of the spins. However, the formation of domain walls becomes predominant as the particle becomes larger than the single domain size. Moreover, the reduction in coercivity occurring at 1800C is because of the motion of the walls [30]. Recent studies have revealed that the properties of magnetic materials greatly influenced by many factors. The relation between particle size and coercivity is controversies because of various data reported. In general, the coercivity of magnetic materials depends on size, structure and crstallinity and as the particle size increases, the coercivity increases [31]. Jacob and Abdul Khadar [32] reported contrast, which the sample with smaller grain size of hematite showed higher value of coercivity. Since several studies reported the effect of reaction temperature and time on nanoparticle size and magnetic properties, (e.g) Briceno et al [33] studied synthesis variable on particle size and magnetic properties of CoFe2O4 nanoparticles prepared by precipitation method. Magnetic properties of nanoparticles show strong dependence on particle size, which calculated according to Scherrer equation and the size of particles was observed to be increasing linearly at higher annealing temperature. The data shows by increasing at reaction temperature from 200 to 8000C, saturation magnetization (Ms) increased from 7.84 emu/g to 44.6 emu/g. The maximum value of coercivity achieved 0.087T at 6000C and then decreases with annealing temperature and lead to ferromagnetic behavior. Effect of reaction time data from 2-6 hour shows that the corresponding saturation magnetization (Ms) values did not exhibit a clear trend with heating time and take values between 4.023 and 0.790 emu/g. The results show that nanoparticles prepared at different reaction time have superparamagnetic behavior due to zero value of coercivity (Hc) and residual magnetization (Mr). Kiatphuengporn et al [34] studied CO2 hydrogenation and 18 ACS Paragon Plus Environment

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selective conversion to light hydrocarbons over Fe/MCM-41 catalysts with magnetic field-enhanced before and after reduction process. The result showed that magnetic field help facilitate the reactant adsorption and surface reaction over the magnetized Fe catalysts and leads to outstanding catalytic activities, decrease of apparent activation energy and also increase of selectivity to hydrocarbons.

Fig 13. Hysteresis loops of iron-cobalt-cerium mixed oxides nanocatalyst synthesized at different reaction temperature, A)1800C, B)1500C and C)1200C.

3.5 3 Mr (emu g-1)

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2.5 2 1.5 1 0.5 0 120

150

Temperature (0C)


180


Fig 14. Residual magnetization (Mr) of synthesized iron-cobalt-cerium mixed oxides nanocatalyst as a function of reaction temperature. 2000 1500

9

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7

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500 4 0

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Ms emu g-1

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-500 1 -1000

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Fig 15. Variation of saturation magnetization and coercivity in synthesized ironcobalt-cerium mixed oxides nanocatalyst with different reaction temperature.

Coercivity (Oe)

2000 1800 1600 1400 1200 1000 800 600 400 200 0 -200 -400 -600

0.5 Hc

0.45 0.4

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0.35 0.3 0.25 0.2 0.15

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0

Particle size (nm) Fig 16. Variation of the coercivity and residual magnetization ratio of synthesized iron-cobalt-cerium mixed oxides nanocatalyst at different reaction temperature with particle size. 20 ACS Paragon Plus Environment

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3.4. Temperature programmed reduction The temperature-programmed reduction (TPR) technique was used to investigate the reducibility of nanoparticles and determine the types of metal oxide species present in the samples. Hydrogen-TPR profiles of nanoparticles synthesized at various temperatures and reaction times are shown in Fig 17. (A-D). The phases of nanoparticles characterized using XRD were found to be CeO2 (cubic), Co3O4 (cubic) and Fe2O3 (rohombohedral). It has been reported that ceria has two reduction peaks centered around on 4850C and 8000C [35]. Also Co3O4 and Fe2O3 indicate two peaks of hydrogen consumption were centered at 3320C and 350-6000C of cobalt oxide respectively, while for iron oxide the first occurs at 3480C and the second peak at 6210C [36,37]. For all the catalysts, the TPR profiles revealed multiple overlapping peaks resulting from different reduction steps. It can be seen that at 150 and 1200C of reaction temperature (A and B) the reduction occurs at three steps. The TPR profiles of iron-cobalt-cerium mixed oxides prepared at reaction times of 1500C and 1200C are at 3460C, 412-5300C, 7830C and 3380C, 4900C, 7750C respectively. The peaks at 3460C and 3380C (Figs. 17.A and B) for both 1500C and 1200C of reaction time associated with the reduction of Fe2O3 → Fe3O4 and Co3O4 → CoO. The broad and sharp peak at 412-5300C and 4900C is due to the reduction of Fe3O4 to Fe0, CoO to Co0 and CeO2 → Ce2O3. The last small reduction peak at ~ 770-7800C is corresponds to Ce2O3 → Ce0. Each of above peaks, which have reduced at higher temperatures, is due to the decrease in the mixed oxides dispersion and increase in aggregation of them [38]. The TPR profiles of nanoparticles obtained at 14 and 10 h of reaction times showed four reaction peaks. It can be seen from (Figs 17. C and D) multiple overlapping peaks arise from different reduction steps. In the present study, four different peaks are observed in the TPR profile of iron-cobalt-cerium mixed oxides synthesized at 14 h and 10 h of reaction times at 3330C, 4040C, 5360C, 6340C and 2360C, 4000C, 5370C, 7670C respectively. For both 14 and 10 h of reaction times (Figs. 17. C and D) the first peak centered at ~ 3300C is related to the reduction of Fe2O3 to Fe3O4 and Co3O4 to CoO. The second intense peak at 400-4040C is due to the reduction of Fe3O4 to FeO and also the surface reduction of CeO2 to Ce2O3. The third shoulder peak at ~ 5350C corresponds to the reduction of Fe3O4 → FeO → Fe0. The fourth reduction peak at about 640-7600C is attributed to the bulk reduction of cerium by elimination of O2- anions of lattice Ce2O3→ Ce0 [35].



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Fig 17. The TPR profiles of iron-cobalt-cerium mixed oxides nanocatalyst synthesized at different temperature and reaction time (A-18h, 1500C, B-18h, 1200C, C-14h, 1800C and D-10h, 1800C) 22 ACS Paragon Plus Environment

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3.5. FTIR In order to understand the nature of the oleylamine linkage in the samples and also to characterize the surface of the samples, the FTIR spectra of the synthesized nanoparticles were recorded for various experimental conditions. The resultant FTIR spectra are given in Fig 18. The presence of a band at ~ 3400 cm-1 for all the samples indicates the stretching vibration of N‒H because of the absorption of –NH2 group on the surface of the metal mixed oxides [39]. The bands in the range 2300-2900 cm-1 in Fig. 18 are related to C-H stretching vibration of oleylamine alkyl chain [40]. The vibration seen at 1600 cm-1 is attributed to the bending vibration of oleylamine C=C group which surrounds the nanoparticles. The absorption bands at 1384 cm-1 and ~ 1100 cm-1 are created by the bending stretching of –CH3 group and C‒H of oleylamine, respectively [39]. The high frequency bands in the range of 560-660 cm-1 in Fig 18 (B-E) are related to M‒O metal ion stretching vibration [40].



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Fig 18. FTIR spectra of iron-cobalt-cerium mixed oxides nanocatalyst synthesized at different temperature and reaction time (A=18h, 1800C; B=18h, 1500C ; C=18h, 1200C ; D=14h, 1800C ; E= 10h, 1800C)


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3.6. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) Fig 19. Shows the SEM images for iron-cobalt-cerium mixed oxides synthesized at various temperatures and reaction times. It reveals that the lower reaction temperature at 1200C is due to the larger particle size while at the maximum reaction time of 18h, there is a smaller particle size (Table S9). By reducing the reaction time, particles are more accumulate and their size are larger. Also, SEM micrograph indicates that the samples consist of approximately spherical shaped and more uniform morphology of nanoparticles. In addition, their particle sizes are in the range of 25-80 nm. The larger particle sizes of the nanoparticles, as compared to the average sizes obtained from XRD pattern (Table S7), can be attributed to the existence of agglomeration caused by the effect of the relatively stronger interaction among magnetic particles, such as van der Waals forces and magnetic dipolar interaction [41]. It is well known that variation in particle size of nanoparticles may result in marked changes in their magnetic behavior, therefore SEM analysis is in agreement with VSM data. These variations are related to the coercive force and remanence [29]. The EDS spectra of these nanoparticles prepared at various temperature and reaction time are also illustrated in Fig 20. It showed the presence of the Fe, Co, Ce and O (Table S9 and Fig 20, A-E), which confirms that these materials comprised of oxidic phases of iron, cobalt and cerium.



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Fig 19. SEM images of synthesized iron-cobalt-cerium mixed oxides nanocatalyst different temperature and reaction time variables (A=18h, 1800C), (B=18h, 1500C), (C=18h, 1200C), (D=14h, 1800C), (E= 10h, 1800C)


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Fig 20. EDS spectra of prepared iron-cobalt-cerium mixed oxides nanocatalyst at different temperature and reaction time variables (A=18h, 1800C), (B=18h, 1500C), (C=18h, 1200C), (D=14h, 1800C), (E= 10h, 1800C)



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3.7. X-ray photoelectron spectroscopy (XPS) To further study of the metal oxidation states and binding energy on the base of particle surface characteristics were determined by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the samples prepared at 1800C of temperature and 18,14 hour of time were studied. The survey XPS spectra of these metal oxide samples in a wide energy range are presented in Fig. 21 and 22. The positions of XPS peaks were corrected using the C 1s core level taken at 285eV as a binding energy reference. From the survey spectra, we can clearly observe the respective peaks for C 1s, O 1s, Fe 2p, Co 2p and Ce 3d which confirms the elements to be present in the system. The XPS high-resolution spectra are presented in Figs. 23 (A, B, C) and 24 (E, F, G). The corresponding spectrum for Fe 2p is presented in Figs. 23 (A) and 24 (E) From the figure, the signals for Fe 2p1/2 and Fe 2p3/2 are observed at 725 and 711 eV respectively which are characteristic binding energies for the Fe+3 state [42]. The spectrum of Co 2p reflects two signals at about 790807 and 780eV. These peaks represent the Co 2p1/2 and Co 2p3/2 for the Co+2 state, confirming the valence state of cobalt to be Co+2 [42]. The spectra representing the Ce+3 binding energy for 3d state are shown in Figs 23 (B) and 24 (F). The peaks representing the Ce 3d3/2 and Ce 3d5/2 for the Ce+3 state are seen at 902 and 886 eV respectively (Figs 23 (C) and 24 (G)) [43]. The O 1s peak (not shown) of samples were similar is often believed to be composed of two peaks, related to two different chemical states of oxygen. The main peak corresponding to oxidic O-2 (M-O) with binding energy at 531 eV was accompanied with a weak shoulder at higher binding energy of 533eV. This component is usually related to a small amount of (OH-) caused by moisture in the air or non-stoichiometric surface oxygen [44,45]. A change in the oxidation state brings along a change in the arrangement of electrons and thereby leads to changing the properties of a system.

Fig 21. Survey XPS spectrum of iron-cobalt-cerium mixed oxides nanocatalyst prepared at 1800C temperature and 18h time.


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Fig 22. Survey XPS spectrum of iron-cobalt-cerium mixed oxides nanocatalyst prepared at 1800C temperature and 14h time.

Fig 23. High-resolution XPS spectra of iron-cobalt-cerium mixed oxides nanocatalyst prepared at 1800C temperature and 18h time, displaying the binding energy and chemical sates of (A) Fe 2p, (B) Co 2p and (C) Ce 3d.



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Fig 24. High-resolution XPS spectra of iron-cobalt-cerium mixed oxides nanocatalyst prepared at 1800C temperature and 14h time, displaying the binding energy and chemical sates of (E) Fe 2p, (F) Co 2p and (G) Ce 3d.


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Acknowledgments The authors would like to thank and appreciate by ministry of science & research, research department of Sistan & Baluchestan University specially VSM lab for vsm characterization presented in this paper, as well as Iranian National Petrochemical Company (INPC) for financial supports.

Conclusions Fischer-Tropsch synthesis converts a mixture of CO and H2 that can be produced from natural gas to liquid fuels. A variety of preparation conditions like reaction temperature and time affects on catalyst performance and selectivity. Iron-cobalt-cerium mixed oxides nanocatalyst under different reaction temperatures and times have been successfully prepared by a simple solvothermal procedure. The results showed that the optimal preparation time and temperature in order to light olefin (C2-C4) selectivity was at 18h and 1200C. The magnetic properties are influenced by temperature and reaction time and the synthesized samples exhibit ferromagnetic and superparamagnetic behaviors. The VSM measurnment of samples indicate that the saturation magnetization is independent on particle size and Ms decreases with the increase of reaction time. The maximum value of coercivity and residual magnetization ratio obtained at 14h of reaction time and 1500C of reaction temperature. The variations of the particle size and morphology and linkage nature of oleylamine are confirmed by XRD, SEM and FTIR analyses. From XRD and VSM measurements observed that high crystalline magnetic nanoparticles were obtained at 1500C and 14h of reaction temperature and time. It was concluded from FTIR spectra that oleylamine surrounded the surface of nanoparticles. Comparing SEM with XRD data represent the larger particle size because of the agglomeration. Also, the micrographs of SEM showed that the higher temperature and reaction time achieved the smaller particle size. The XRD spectra and EDS data confirmed the formation of oxidic phases of nanoparticles. From TPR, it was observed that the reduction peaks of sample synthesized at range of 120 to 1800C of reaction temperature reduced into higher temperatures due to decrease in dispersion and increase in aggregation of mixed oxide. Also, the reduction steps of samples achieved at different temperatures and reaction times indicate multiple overlapping peaks.

“ Supporting Information “ The contents of Tables listing: S1, S2, S3, S4, S5, S6, S7, S8 and S9 Supplied.



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Graphical Abstract


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Influence of Fabrication Temperature and Time on Light Olefin

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